Paul Sellin received his PhD in Nuclear Physics from University of Edinburgh (UK) in 1992 in the field of semiconductor nuclear detectors. His current research interests at the University of Surrey include the development and characterisation of radiation detectors and detector materials for applications in nuclear physics, medical imaging, and security detection. His research group focuses on the characterisation and development of new detector materials, including plastic and organic scintillators for mixed field neutron/gamma detection, including digital instrumentation and SiPM readout for neutron/gamma sensitive scintillators. Other interests include radiation-hard materials for extreme radiation applications where high dose rate and/or high temperature capability is required and the application of detector technology to nuclear security science, including new modalities for hazardous material detection and identification.
Areas of specialism
University roles and responsibilities
- Visiting Professor, Department of Medical Physics and Biomedical Engineering, UCL London
- Visiting Professor, Department of Physics, University of Wollongong, Australia.
My research interests cover the development of radiation detectors and the physics of materials for new detector devices. This work covers a range of topics, including:
Organic materials for use as radiation sensors and organic electronics, including thick film semi-conducting polymers and organic crystals.
High-Z compound semiconductor materials (eg. GaAs, InP, CdTe, CdZnTe) for X-ray and nuclear medicine imaging detectors.
New semiconductor materials for use in extreme conditions, (eg. diamond, silicon carbide, gallium nitride) where high dose rate and/or high temperature capability is required.
- Nanostructured scintillators for high efficiency gamma and neutron spectroscopy, with applications in nuclear security and environmental monitoring.
Development of Nanocomposite Scintillators
A DTRA-funded project to explore the use of nanoparticles for use in scintillating materials
My current teaching responsibilities are focused on second year undergraduate programmes in solid state physics, and general physics laboratory classes. I current teach the following modules:
- Second Year Solid State Physics Lecture Course
- Second Year Physics Laboratory Classes
- Masters Research Projects in Detector Physics
Flexible radiation dosimeters have been produced incorporating thick films (>1 μm) of the semiconducting polymer poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide) substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of -10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm3 have been achieved. Increasing the operating voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of the device (
Although the use of semi-insulating silicon carbide material for radiation detection purposes has been previously demonstrated, its use in practical applications has been inhibited by space charge stability issues caused by defect concentrations within the material, the so called polarisation effect, by which the count rate and resultant spectrum changes with irradiation time. This is a result of the charge carriers generated during irradiation filling deep level defects within the material, causing space charge buildup and de-activating that trap level until the trapped charge is re-emitted. Consequently, the time dependence of the polarisation effect has been determined by a combination of parameters that can be influenced during operation, namely the incident radiation intensity, ambient light, temperature and bias. The material properties have also been considered through the use of materials with different defect capture cross sections, concentrations and energy level. A thorough characterisation of the alpha irradiation induced polarisation phenomenon in semi-insulating silicon carbide has been conducted to demonstrate that stable operation detectors are in fact possible with this material. The effects were compared to single crystal diamond and polycrystalline diamond, which are known to exhibit similar polarisation issues. The polarisation rate as an effect of incident flux, bias and temperature was determined, with the depolarisation rate as a function of ambient light and bias also demonstrated. Consequently it has been shown that stable operation can be maintained for detectors made from semi-insulating SiC material of active thickness 350 μm at incident alpha radiation fluxes of < 0.7 alphas per second per mm2 with high operating biases (> ±400 V). Furthermore, polarisation can be suitably managed or reduced through the use of light illumination and elevated temperatures (373 K).
The presence of carbon atoms in silicon carbide and diamond makes the materials ideal candidates for direct fast neutron detectors. Furthermore the low atomic number, strong covalent bonds, high displacement energies, wide band gap and low intrinsic carrier concentrations make these semiconductor detectors potentially suitable for applications where rugged, high temperature, low gamma sensitivity detectors are required, such as Active Interrogation, Electronic Personal Neutron Dosimetry and Harsh Environment Detectors. A thorough direct performance comparison of the detection capabilities of semiinsulating silicon carbide (SiC-SI), single crystal diamond (D-SC), polycrystalline diamond (D-PC) and a self-biased epitaxial silicon carbide (SiC-EP) detector has been conducted and benchmarked against a commercial silicon PIN (Si-PIN) diode, in a wide range of alpha (Am-241), beta (Sr/Y-90), ionising photon (65keV to 1332keV) and neutron radiation fields (including 1.2MeV to 16.5MeV mono-energetic neutrons, as well as neutrons from AmBe and Cf-252 sources). All detectors were shown to be able to directly detect and distinguish both the different radiation types and energies by using a simple energy threshold discrimination method. The SiC devices demonstrated the best neutron energy discrimination ratio (Emax[n=5MeV] / Emax[n=1MeV] ~5), whereas a superior neutron/photon cross sensitivity ratio was observed in the D-PC detector (Emax[AmBe] / Emax[Co-60] ~16). Further work also demonstrated that the cross sensitivity ratios can be improved through use of a simple proton-recoil conversion layer. Stability issues were also observed in the D-SC, D-PC and SiC-SI detectors while under irradiation, that being a change of energy peak position and/or count rate with time (often referred to as polarisation effect). This phenomenon within the detectors was non-debilitating over the time period tested (>5h) and as such, stable operation was possible. Furthermore, the D-SC, self-biased SiC-EP and a semi-insulating SiC detector were shown to operate over the temperature range -60C to +100C.
The use of organic materials as radiation detectors has grown, due to the easy processability in liquid phase at room temperature and the possibility to cover large areas by means of low cost deposition techniques. Direct charged-particle detectors based on solution-grown Organic Semiconducting Single Crystals (OSSCs) are shown to be capable to detect charged particles in pulse mode, with very good peak discrimination. The direct charged-particle detection in OSSCs has been assessed both in the planar and in the vertical axes, and a digital pulse processing algorithm has been used to perform pulse height spectroscopy and to study the charge collection efficiency as a function of the applied bias voltage. Taking advantage of the charge spectroscopy and the good peak discrimination of pulse height spectra, an Hecht-like behavior of OSSCs radiation detectors is demonstrated. It has been possible to estimate the mobility-lifetime value in organic materials, a fundamental parameter for the characterization of radiation detectors, whose results are equal to lscoplanar¼ (5 .5 6 0.6 ) 106 cm2 /V and lssandwich¼ (1 .9 6 0.2 ) 106 cm2 /V, values comparable to those of polycrystalline inorganic detectors. Moreover, alpha particles Time-of-Flight experiments have been carried out to estimate the drift mobility value. The results reported here indicate how charged-particle detectors based on OSSCs possess a great potential as low-cost, large area, solid-state direct detectors operating at room temperature. More interestingly, the good detection efficiency and peak discrimination observed for chargedparticle detection in organic materials (hydrogen-rich molecules) are encouraging for their further exploitation in the detection of thermal and high-energy neutrons. VC 2016 AIP Publishing LLC.
Cadmium zinc telluride (CdZnTe) is a leading sensor material for spectroscopic X/-ray imaging in the fields of homeland security, medical imaging, industrial analysis and astrophysics. The metal-semiconductor interface formed during contact deposition is of fundamental importance to the spectroscopic performance of the detector and is primarily determined by the deposition method. A multi-technique analysis of the metalsemiconductor interface formed by sputter and electroless deposition of gold onto (111) aligned CdZnTe is presented. Focused ion beam (FIB) cross section imaging, X-ray photoelectron spectroscopy (XPS) depth profiling and current-voltage (IV) analysis have been applied to determine the structural, chemical and electronic properties of the gold contacts. In a novel approach, principal component analysis has been employed on the XPS depth profiles to extract detailed chemical state information from different depths within the profile. It was found that electroless deposition forms a complicated, graded interface comprised of tellurium oxide, gold/gold telluride particulates, and cadmium chloride. This compared with a sharp transition from surface gold to bulk CdZnTe observed for the interface formed by sputter deposition. The electronic (IV) response for the detector with electroless deposited contacts was symmetric, but was asymmetric for the detector with sputtered gold contacts. This is due to the electroless deposition degrading the difference between the Cd- and Te-faces of the CdZnTe (111) crystal, whilst these differences are maintained for the sputter deposited gold contacts. This work represents an important step in the optimisation of the metal-semiconductor interface which currently is a limiting factor in the development of high resolution CdZnTe detectors.
In response to the Fukushima Daiichi Nuclear Power Plant accident, there has occurred the unabated growth in the number of airborne platforms developed to perform radiation mapping—each utilising various designs of a low-altitude uncrewed aerial vehicle. Alongside the associated advancements in the airborne system transporting the radiation detection payload, from the earliest radiological analyses performed using gas-filled Geiger-Muller tube detectors, modern radiation detection and mapping platforms are now based near-exclusively on solid-state scintillator detectors. With numerous varieties of such light-emitting crystalline materials now in existence, this combined desk and computational modelling study sought to evaluate the best-available detector material compatible with the requirements for low-altitude autonomous radiation detection, localisation and subsequent high spatial-resolution mapping of both naturally occurring and anthropogenically-derived radionuclides. The ideal geometry of such detector materials is also evaluated. While NaI and CsI (both elementally doped) are (and will likely remain) the mainstays of radiation detection, LaBr3 scintillation detectors were determined to possess not only a greater sensitivity to incident gamma-ray radiation, but also a far superior spectral (energy) resolution over existing and other potentially deployable detector materials. Combined with their current competitive cost, an array of three such composition cylindrical detectors were determined to provide the best means of detecting and discriminating the various incident gamma-rays.
Numerous techniques and equipment have been developed to provide a capability for the detection of special nuclear materials (SNM), but due to the necessary security measures surrounding these materials alternate, or proxy, neutron sources are often utilised in their stead. In this paper we report the neutron and gamma pulse shape discrimination (PSD) response of plastic scintillator to mixed neutron/gamma beams produced from two radionuclide neutron sources, and also from an SNM source of weapons-grade plutonium. We discuss the suitability of using radionuclide sources, with appropriate shielding configurations as proxy sources for SNM. A 3 n-discrimination level has been achieved for an SNM source at a low-level energy threshold of 220 keVee when a shielding conguration of 5 cm of lead was implemented. Varying amounts of lead and HDPE shielding were also investigated with the 3 limit being reached by 240 keVee. This work shows that an AmBe neutron source serves as an appropriate SNM proxy achieving a comparable value for gure of merit above 1 MeVee. For energies below 1 MeVee down to 100 keVee a closer approximation of the expected FoM for SNM can be attained when using 252Cf as a proxy source or by utilising an ____enhanced" AmBe source with the addition of a further low energy ray source.
The edge surfaces of single crystal CdTe play an important role in the electronic properties and performance of this material as an X-ray and γ-ray radiation detector. Edge effects have previously been reported to reduce the spectroscopic performance of the edge pixels in pixelated CdTe radiation detectors without guard bands. A novel Technology Computer Aided Design (TCAD) model based on experimental data has been developed to investigate these effects. The results presented in this paper show how localized low resistivity surfaces modify the internal electric field of CdTe creating potential wells. These result in a reduction of charge collection efficiency of the edge pixels, which compares well with experimental data.
Semiconducting polymers have previously been used as the transduction material in x-ray dosimeters, but these devices have a rather low detection sensitivity because of the low x-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the x-ray attenuation. Specifically, bismuth oxide (Bi O ) nanoparticles (Z=83 for Bi) are added to a poly(triarylamine) (PTAA) semiconducting polymer in the active layer of an x-ray detector. Scanning electron microscopy (SEM) reveals that the Bi O nanoparticles are reasonably distributed in the PTAA active layer. The reverse bias dc currentvoltage characteristics for PTAABi O diodes (with indium tin oxide (ITO) and Al contacts) have similar leakage currents to ITO/PTAA/Al diodes. Upon irradiation with 17.5keV x-ray beams, a PTAA device containing 60wt% Bi O nanoparticles demonstrates a sensitivity increase of approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the x-ray stopping power of the active volume of the diode. Because the Bi O has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated. © 2012 IOP Publishing Ltd.
We report on the retrofitting of a standard DP2 environmental radiation monitor replacing the photomultiplier tube with a silicon photomultiplier (SiPM). The use of a SiPM has several advantages for a hand-held radiation monitor, including convenient low voltage operation and physical robustness. The SiPM is used to replace the existing photomultiplier tube, and we report the detection efficiency and alpha/beta discrimination performance of the modified probe compared to an unmodified version.
Real time imaging of the electric field distribution in CZT at low temperature has been carried out using the Pockels electro-optical effect. CZT detectors have been observed to show degraded spectroscopic resolution at low temperature due to so-called ‘polarization’ phenomena. By mounting a CZT device in a custom optical cryostat, we have used Pockels imaging to observe the distortion of the electric field distribution in the temperature range 240K - 300K. At 240K the electric field has a severely non-uniform depth distribution, with a high field region occupying ∼10% of the depth of the device under the cathode electrode and a low field in the remainder of the device. Using an alpha particle source positioned inside the vacuum chamber we have performed simultaneous alpha particle transient current (TCT) measurements. At low temperatures the alpha particle current pulses become significantly shorter, consistent with the reduced electron drift time due to a non-uniform electric field. These data provide useful insights into the mechanisms which limit the spectroscopic performance of CZT devices at reduced temperature.
Semiconducting polymer X-radiation detectors are a completely new family of low-cost radiation detectors with potential application as beam monitors or dosimeters. These detectors are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. However, their x-ray photocurrents are typically low as, being composed of elements of low atomic number (Z), they attenuate x-rays weakly. Here, the addition of high-Z nanoparticles is used to increase the x-ray attenuation without sacrificing the attractive properties of the host polymer. Two types of nanoparticles (NPs) are compared: metallic tantalum and electrically insulating bismuth oxide. The detection sensitivity of 5 µm thick semiconducting poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene) diodes containing tantalum NPs is four times greater than that for the analogous NP-free devices; it is approximately double that of diodes containing an equal volume of bismuth oxide NPs. The x-ray induced photocurrent output of the diodes increases with an increased concentration of NPs. However, contrary to the results of theoretical x-ray attenuation calculations, the experimental current output is higher for the lower-Z tantalum diodes than the bismuth oxide diodes, at the same concentration of NP loading. This result is likely due to the higher tantalum NP electrical conductivity, which increases charge transport through the semiconducting polymer, leading to increased diode conductivity.
We investigate the performance of Fourier-based neutron/gamma Pulse Shape Discrimination (PSD) algorithms applied to plastic scintillators that are coupled to silicon photomultipliers (SiPM). The detector acquired data from a mixed fast neutron and gamma field which was emitted from an AmBe source. Pulses produced from the detector were fully digitised for off-line analysis with the algorithms. We describe the performance of two Fourier-based PSD algorithms, Fourier Gradient Analysis (FGA) and Fourier Area Analysis (FAA), and compare their performance to the Charge Comparison Method (CCM). To compare the algorithms’ PSD performance the figure of merit (FoM) was calculated at various energies for each of the algorithms. The CCM analysed the pulses in the time domain whereas the other two algorithms processed the pulses within the frequency domain. Moreover, the detector was tested with different acquisition record lengths, in order to determine any impact on algorithm performance. It was determined that the FAA algorithm provided the best overall performance achieving a FoM of 1.57(1) at 1 MeVee with a 1.6 µs record length. Furthermore, the detector was tested using different load resistors which allowed the decay time of the pulses to be optimised. The influence of SiPM pulse decay time on the performance of the PSD algorithms is also presented.
We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications
Diagnostic studies have been performed on a 70 μm thick diamond film with a view to its development as a possible field-assisted moderator (FAM) for slow positron emission. The film was coated on one side with a 200 nm gold electrode and on the other with a similar thickness of gold in the form of a fine mesh (10 μm lines, 40 μm spaces) in the central part of the diamond surface and a solid edge for electrical contact. A potential difference of up to 300 V could be held across the film with a leakage current of ∼ 10 nA. Although positron re-emission was observed from the diamond surface, the re-emitted fraction was observed to decrease when a bias of 300 V was applied across the film. This negative result is interpreted in terms of the drifting of thermalised positrons to the gold mesh lines, a conclusion supported by Doppler broadening measurements. The potential for the development of a diamond-film-based positron beam tagger is discussed. © 2002 Elsevier Science B.V. All rights reserved.
We have studied the effects of electrode fabrication and detector capacitance on the time resolution of large area electronic grade polycrystalline chemical vapour deposited diamond sensors that are suitable for time of flight measurements of heavy ions at relativistic velocities. Sensors were prepared both in house, with Al or Au metal contacts, and commercially fabricated with Au/diamond-like carbon contacts. He, Ar and a mixture of Ne and O beams at 16.3, 33.5 and 21-23 MeV/u, respectively were used on these devices whilst arranged in transmission geometry. Signal processing only began over one meter away from the sensors. The present approach, where we have large-area/large capacitance multi-strip detectors with processing electronics at some distance from the target, is compatible with anticipated space limitations in particle- identification and tracking setups at existing and planned nuclear fragmentation facilities. In a systematic study under these conditions, we demonstrate that the time resolution is limited by detector capacitance and energy deposition in the sensors. An intrinsic time resolution σ = (44±5) ps was achieved for a diamond detector of ∼ 14 pF capacitance. We conclude that, once further refinements are made, a large area time of flight detection system using polycrystalline diamond detectors would be able to provide time resolutions better than 40 ps, approaching the requirement for particle-identification in relativistic fragmentation experiments, such as those at the facility for antiproton and ion research, FAIR. © 2012 IOP Publishing Ltd and Sissa Medialab srl.
The benefits of neutron detection and spectroscopy with carbon based, wide band gap, semiconductor detectors have previously been discussed within the literature. However, at the time of writing there are still limitations with these detectors related to availability, cost, size and perceived quality. This study demonstrates that lower quality materials—indicated by lower charge collection efficiency (CCE), poor resolution and polarisation effect—available at wafer scale and lower cost, can fulfil requirements for fast neutron detection and spectroscopy for fluxes over several orders of magnitude, where only coarse energy discrimination is required. In this study, a single crystal diamond detector (D-SC, with 100% CCE), a polycrystalline diamond (D-PC, with ≈4% CCE) and semi-insulating silicon carbide (SiC-SI, with ≈35% CCE) have been compared for alpha and fast neutron performance. All detectors demonstrated alpha induced polarisation effects in the form of a change of both energy peak position and count rate with irradiation time. Despite these operational issues the ability to detect fast neutrons and distinguish neutron energies was observed. This performance was demonstrated over a wide dynamic range (500–40,000 neutrons/s), with neutron induced polarisation being demonstrated in D-PC and SiC-SI at high fluxes.
In this letter, pronounced hysteresis loops were observed in single-walled carbon nanotube-based field-effect transistors (CNTFETs). The shift in threshold voltage was found to increase with increasing gate voltage sweep ranges. A significant enhancement in the charge storage stability over 14 days was obtained at room temperature after a two-stage hydrogen and air annealing process was applied to the CNTFETs. The passivation of interface traps by annealing in hydrogen and the removal of physisorption solvent molecules by annealing in air are suggested to be responsible for the improvement of the charge storage stability.
The potential for high-resolution alpha-particle energy spectrometry in high-temperature, high-radiation environments using thin-window 4H-SiC radiation detectors has been demonstrated. 238Pu alpha-particle peaks separated by only 42.7 keV have been completely resolved using a SiC Schottky detector with a 400 AÃÂ¿ titanium Schottky contact (entrance window). The observed FWHM for the 238Pu 5499.2-keV alpha particle peak is 20.6 keV or 0.37%. Factors affecting the observed alpha-particle energy resolution in SiC detectors will be discussed.
Diamond has been regarded as a promising radiation detector material for use as a solid state ionizing chamber for decades. The parameters degrading the charge transport from what is expected from an ideal crystal are still not completely understood. Recently, synthetic chemical vapor deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of synthesized material independent of grain boundaries. We have studied the charge transport of a synthetic single crystal diamond with alpha-particle induced charge transients as a function of temperature and established the presence of a shallow hole trap with an activation energy of 0.29 +/- 0.02 eV in some parts of the detector. Ion beam induced charge imaging has been used to study the spatial variations of the charge transport in a synthetic single crystal diamond. Pulses influenced by the shallow hole trap had their origin close to the substrate/CVD interface of the sample. They could be clearly distinguished from pulses affected by reduced charge carrier velocities due to polarization phenomena, which varied systematically with the growth direction of the CVD diamond material.
We report on the pulse shape discrimination (PSD) performance of plastic scintillators manufactured by Eljen Corporation and Amcrys. In this study we investigate the fast neutron and gamma performance of the plastic scintillators when coupled to the SensL J-series silicon photomultiplier (SiPM) and read out with fast waveform digitisers with an ADC resolution of 14-bits and a sample rate of 500 MS/s. The investigation observes a significant PSD performance increase for the SensL J-series SiPM in comparison to the previous C-series, and also for the latest variants of plastic scintillator from both suppliers. Analysis was performed using a Synchronous Charge Integration Pulse Shape Discrimination (PSD) algorithm which was applied to data acquired from a mixed fast neutron/gamma radiation field from an AmBe neutron source. The collected pulses were processed offline with the energy and PSD parameters calculated. The quality of the PSD performance was characterised by a common figure of merit (FoM). The best n- separation was found by the newer Eljen EJ-276 scintillator with a FoM value of 3.03 ± 0.03 at an energy of 1.5 MeV gamma equivalent. The Amcrys UPS-113NG material achieved a FoM value of 2.60 ± 0.04.
Recently, a new family of low-cost x-radiation detectors have been developed, based on semiconducting polymer diodes, which are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. To test their potential for radiotherapy applications such as beam monitors or dosimeters, as an alternative to the use of solid-state inorganic detectors, we present the direct detection of 6 MV x-rays from a medical linear accelerator using a thick film, semiconducting polymer detector. The diode was subjected to 4 ms pulses of 6 MV x-rays at a rate of 60 Hz, and produces a linear increase in photocurrent with increasing dose rate (from 16.7 to 66.7 mGy s(−1)). The sensitivity of the diode was found to range from 13 to 20 nC mGy(−1) cm(−3), for operating voltages from −50 to −150 V, respectively. The diode response was found to be stable after exposure to doses up to 15 Gy. Testing beyond this dose range was not carried out. Theoretical calculations show that the addition of heavy metallic nanoparticles to polymer films, even at low volume fractions, increases the x-ray sensitivity of the polymer film/nanoparticle composite so that it exceeds that for silicon over a wide range of x-ray energies. The possibility of detecting x-rays with energies relevant to medical oncology applications opens up the potential for these polymer detectors to be used in detection and imaging applications using medical x-ray beams.
An important factor for the high performance of light-harvesting devices is the presence of surface trappings. Therefore, understanding and controlling the carrier recombination of the organic–inorganic hybrid perovskite surface is critical for the device design and optimization. Here, we report the use of aluminum zinc oxide (AZO) as the anode to construct a p–n junction structure MAPbBr3 nuclear radiation detector. The AZO/MAPbBr3/Au detector can tolerate an electrical field of 500 V·cm–1 and exhibit a very low leakage current of ∼9 nA, which is 1 order of magnitude lower than that of the standard ohmic contact device. The interface state density of AZO/MAPbBr3 contact was reduced from 2.17 × 1010 to 8.7 × 108 cm–2 by annealing at 100 °C under an Ar atmosphere. Consequently, a photocurrent to dark current ratio of 190 was realized when exposed to a green light-emitting diode with a wavelength of 520 nm (∼200 mW·cm–2). Simultaneously, a high X-ray sensitivity of ∼529 μC·Gyair–1 cm–2 was achieved under 80 kVp X-ray at an electric field of 50 V·cm–1. These results demonstrate the use of surface engineering to further optimize the performance of MAPbBr3 detectors, which have many potential applications in medical and security detection with low radiation dose brought to the human body.
We report the first demonstration of a solid-state, direct-conversion sensor for thermal neutrons based on a polymer/inorganic nanocomposite. Sensors were fabricated from ultra-thick films of poly(triarylamine) (PTAA) semiconducting polymer, with thicknesses up to 100 μm. Boron nanoparticles were dispersed throughout the PTAA film to provide the neutron stopping power arising from the high thermal neutron cross-section of the isotope Boron-10. To maximize the quantum efficiency of the sensor to thermal neutrons, a high volume fraction of homogeneously dispersed boron nanoparticles was achieved in the thick PTAA film using an optimized processing method. Thick active layers were realized using a high molecular weight of the PTAA so that molecular entanglements provide a high cohesive strength. A non-ionic surfactant was used to stabilize the boron dispersion in solvent and hence suppress the formation of agglomerates and associated electrical pathways. Boron nanoparticle loadings of up to ten volume percent were achieved, with thermal neutron quantum efficiency estimates up to 6% resulting. The sensors' neutron responses were characterized under a high flux thermal neutron exposure, showing a linear correlation between the response current and the thermal neutron flux. Polymer-based boron nanocomposite sensors offer a new neutron detection technology that uses low-cost, scalable solution processing, and provides an alternative to traditional neutron sensors that use rare isotopes, such as Helium-3.
A digital timing method aiming to minimize the time walk caused by the depth-dependent pulse shape variations in CdTe detectors has been developed. Detector pulses are digitized at the preamplifier stage and a full digital process is carried out to deduce and correct the time walk according to the interaction depth. A time resolution of 6.52 ns FWHM at an energy threshold of 150 keV with a CdTe detector (10×10×1 mm3) is achieved, which is close to the intrinsic resolution of the detector. The method improves the time resolution with no loss of detection efficiency and it is easy to implement. It is confirmed that the slow mobility and the short lifetime of the holes are major obstacles for further improvement in the timing performance of the CdTe detectors. The method is applicable to any semiconductor detector.
A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2.2 × 10−5 cm2 V−1 s−1 (averaged for field strengths between 3 × 104 and 4 × 105 V cm−1), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.
A Cadmium Telluride (CdTe) detector has been developed for multiple-radioisotope SPECT imaging. The 22 cm detector has 8080 pixels on a 250 mm pitch and a three-side buttable design so that it can be tiled into larger arrays. The detector is termed hyperspectral as it measures the energy of every photon that interacts in the CdTe to give fully spectroscopic information from 5-200 keV in each pixel. The detector has been tested for applications in multipleradioisotope SPECT imaging using a 1mm diameter pinhole configuration and standard phantom test objects containing Tc-99m, I-123 and Ga-67. The detector has an average pixel energy resolution (FWHM) of 0.75% at the I-123 photopeak of 159 keV. We demonstrate the system's capability of resolving spatial features of 2 mm, although the spatial resolution of the detector is limited only by the pixel size and pinhole magnification factor. These characteristics are superior to alternative detectors currently in use in clinical SPECT systems. When imaging multiple radioisotopes simultaneously, we show that there is very little cross-talk between adjacent photopeaks, leading to superior image contrast. The detector is also capable of resolving fluorescence x-rays from the radioactive source, which could be used to improve image count statistics or derive information about the attenuation properties of the source. The performance presented here, and the ability to tile the detector modules to create a clinically useful field of view, makes this technology a strong candidate to be used in future solid-state SPECT cameras. © 2012 IOP Publishing Ltd and Sissa Medialab srl.
A pixellated germanium Compton camera is currently being developed for imaging 511 keV sources in nuclear medicine. It was built by ORTEC and consists of two planar Ge detectors (the scatter and the absorption detector) housed within the same cryostat. The scatter and absorption detectors have 152 and 25 4×4 mm2 pixels respectively. The readout electronics was developed at Daresbury Laboratory UK and consists of 15 GRT4 VME cards and a PowerPC. The system is controlled by a PC running MIDAS software. This paper reports the current status of camera development. The pixel energy resolution has been measured to give an average of ∼0.5% at 356 keV using a NIM module and ∼2% using a simple digital algorithm. In addition, a centroiding algorithm that takes into consideration the induced charge from the surrounding pixels, is being implemented to improve the intrinsic spatial resolution of the camera, which is restricted by the size of the pixel (4mm), to a target value of ∼1mm; so far a X-Y position sensitivity of 2mm has been achieved. The depth of interaction can be provided by analysis of the pulse rise time, theoretical predictions anticipate a value of the order of 0.5mm. Currently these parameters are determined offline using simple algorithms, but it is the intention to develop the algorithms and then implement them on the FPGAs available on the GRT4 cards for online analysis. © 2004 IEEE.
The electrical characteristics and fast neutron response of a High Temperature Chemical Vapour Deposition (HTCVD) grown semi-insulating bulk SiC wafer has been measured. Current - Voltage measurements demonstrated a low leakage current in the region of 10 to 10 A with a bulk resistivity of at least 10 - 10 Ω.cm. Alpha particle spectroscopy measurements demonstrated an electron charge collection efficiency of up to 90% with reasonable reproducibility of the acquired spectra. Evidence of (incident particle) rate dependent polarisation was seen following a constant applied bias combined with alpha irradiation over a period of time (order of tens of minutes). The ability of the wafer to detect fast neutrons was demonstrated and a comparison drawn with the MCNPX simulated response of a bulk SiC device. Comparing the MCNPX simulated response of a bulk SiC device to that of a silicon device suggests a superior ability to detect fast neutrons with an intrinsic efficiency 1.7 times that of silicon. © 1963-2012 IEEE.
Conventional K-edge subtraction imaging is based around the acquisition of two separate images at energies respectively below and above the K-edge of a contrast agent. This implies increased patient dose with respect to a conventional procedure and potentially incorrect image registration due to patient motion. © 2012 IEEE.
We describe the mode of operation of a detector for direct photon-electron conversion at room temperature, made of epitaxially grown GaAs. Contrary to bulk grown materials, epitaxial layers are free of defects, i.e. exhibit long lifetimes and high carrier mobilities, and have uniform electronic properties. However, the depleted zone is of limited extension, consequence of the level of the residual doping impurities, which are not compensated by defects. These detectors are adapted to X-ray imaging, in particular for low energy medical applications such as mammography, because of the availability of large areas (up to 4 inches in diameter), standard technological processes for making pixellated detectors and cost. However, charges in the neutral region can be collected by diffusion and we shall present data allowing to illustrate and evaluate this effect. Finally photocurrent measurements obtained under medical conditions demonstrate that, for the detector used, only a small fraction of the photocurrent originates from diffusing charges. They also show how a 120 μm thick GaAs epitaxial detector competes with a 0.5 mm thick CdZnTe detector.
The T20 analyzing powers have been measured for the 120Sn(7Li, 8Be → 2α) 119In and 120Sn(7Li, 6Li* → α + d) 121Sn transfer breakup reactions, using a 70 MeV beam. The data exhibit excellent agreement with the results of coupled reaction channels calculations, providing an important test of these calculations when applied to the transfer breakup reaction mechanism.
We have fabricated a new experimental pixel array using 2mm-thick CdZnTe. The trial arrays have been bump-bonded to the Rockwell PICNIC readout IC which provides low noise read out of pixel signals. First measurements are presented from the detector characterisation, which in particular, demonstrate that a very high bond yield (>99%) was achieved. It is envisaged that these detectors will be suitable for future X-ray astronomy and planetary missions as well as ground based applications such as non-destructive testing, threat detection and baggage scanning.
Measurements and simulations have been carried out using bulk and epitaxial SiC detectors. Samples were irradiated to fluences of around 1014 hardrons/cm2. Material of thickness 40um gave a charge collection efficiency of 100% dropping to around 60% at 100μm thickness. Detailed MEDICI simulations incorporated the main defect levels in SiC, the vanadium center, Z-center and a mid-gap level as measured by deep level transient spectroscopy and other techniques. Calculated recombination currents and charge collection efficiencies at varying fluences were comparable to experimental data. The study suggests that SiC detectors will operate up to fluences around 10 16/cm2 as required by future particle physics experiments.
The pulse pile-up effect can significantly degrade the spectroscopic performance of scintillation radiation detectors at high counting rates. This paper reports on a digital pulse processing method for shortening the duration of scintillation pulses, thereby alleviating the pulse pile-up effect. The method operates based on replacing the decay-time constant of the scintillation pulses with a shorter decay-time constant. The details of the digital algorithm are presented and the performance of the method at a high counting rate of 795 kHz is experimentally examined with a NaI(Tl) detector. The effects of the pulse shortening on the spectroscopic performance of the system are also discussed.
Existing inorganic materials for radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, lack of tissue equivalence toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA) diodes have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. A single layer of PTAA is deposited on indium tin oxide (ITO) substrates, with top electrodes selected from Al, Au, Ni, and Pd. The choice of metal electrode has a pronounced effect on the performance of the device; there is a direct correlation between the diode rectification factor and the metal-PTAA barrier height. A diode with an Al contact shows the highest quality of rectifying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo K alpha X-radiation over long time scales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. Diodes with a low band gap, 'Ohmic' contact, such as ITO/PTAA/Au, show a slow transient response. This result can be explained by the build-up of space charge at the metal-PTAA interface, caused by a high level of charge injection due to X-ray-induced carriers. These data provide new insights into the optimum selection of metals for Schottky contacts on organic materials, with wider applications in light sensors and photovoltaic devices.
Contrast-enhanced digital mammography (CEDM) is an alternative to conventional X-ray mammography for imaging dense breasts. However, conventional approaches to CEDM require a double exposure of the patient, implying higher dose and risk of incorrect image registration due to motion artifacts. A novel approach is presented, based on hyperspectral imaging, where a detector combining positional and high-resolution spectral information (in this case based on Cadmium Telluride) is used. This allows simultaneous acquisition of the two images required for CEDM. The approach was tested on a custom breast-equivalent phantom containing iodinated contrast agent (Niopam 150®). Two algorithms were used to obtain images of the contrast agent distribution: K-edge subtraction (KES), providing images of the distribution of the contrast agent with the background structures removed, and a dual-energy (DE) algorithm, providing an iodine-equivalent image and a water-equivalent image. The high energy resolution of the detector allowed the selection of two close-by energies, maximising the signal in KES images and enhancing the visibility of details with low surface concentration of contrast agent. DE performed consistently better than KES in terms of contrast-to-noise ratio of the details; moreover, it allowed a correct reconstruction of the surface concentration of the contrast agent in the iodine image. Comparison with CEDM with a conventional detector proved the superior performance of hyperspectral CEDM in terms of the image quality/dose trade-off.
Herein, we report the details of the synthesis and crystal growth of a lead-free perovskite derivative Cs2TeI6 crystal, which reveals long-term environmental stability. The synthesis process is optimized to obtain pure Cs2TeI6 polycrystals, and to eliminate the formation of residual CsI resulting from high vapor pressure. Subsequently, the Cs2TeI6 single crystal with dimensions of Φ10 mm × 55 mm is grown by the vertical Bridgman method. The as-grown 0-D structural Cs2TeI6 crystal exhibits a high resistivity of 9.92 × 1011 Ω cm, and resulting mobilities of 4.03 ± 0.33 cm2 V−1 s−1 and 9.40 ± 0.45 cm2 V−1 s−1 for electrons and holes, respectively. The Au/Cs2TeI6/Au device shows a good response for optoelectronic and X-ray detection, with a relatively high X-ray sensitivity of 27.8 μC Gy−1 cm−2 and a low detection limit of 72.5 nGy s−1.
Purpose The development of novel detectors for dosimetry in advanced radiotherapy modalities requires materials that have a water equivalent response to ionizing radiation such that characterization of radiation beams can be performed without the need for complex calibration procedures and correction factors. Organic semiconductors are potentially an ideal technology in fabricating devices for dosimetry due to tissue equivalence, mechanical flexibility, and relatively cheap manufacturing cost. The response of a commercial organic photodetector (OPD), coupled to a plastic scintillator, to ionizing radiation from a linear accelerator and orthovoltage x‐ray tube has been characterized to assess its potential as a dosimeter for radiotherapy. The radiation hardness of the OPD has also been investigated to demonstrate its longevity for such applications. Methods Radiation hardness measurements were achieved by observing the response of the OPD to the visible spectrum and 70 keV x rays after pre‐exposure to 40 kGy of ionizing radiation. The response of a preirradiated OPD to 6‐MV photons from a linear accelerator in reference conditions was compared to a nonirradiated OPD with respect to direct and indirect (RP400 plastic scintillator) detection mechanisms. Dose rate dependence of the OPD was measured by varying the surface‐to‐source distance between 90 and 300 cm. Energy dependence was characterized from 29.5 to 129 keV with an x‐ray tube. The percentage depth dose (PDD) curves were measured from 0.5 to 20 cm and compared to an ionization chamber. Results The OPD sensitivity to visible light showed substantial degradation of the broad 450 to 600 nm peak from the donor after irradiation to 40 kGy. After irradiation, the spectral shape has a dominant absorbance peak at 370 nm, as the acceptor better withstood radiation damage. Its response to x rays stabilized to 30% after 35 kGy, with a 0.5% difference between 770 Gy increments. The OPD exhibited reproducible detection of ionizing radiation when coupled with a scintillator. Indirect detection showed a linear response from 25 to 500 cGy and constant response to dose rates from 0.31 Gy/pulse to 3.4 × 10−4 Gy/pulse. However, without the scintillator, response increased by 100% at low dose rates. Energy independence between 100 keV and 1.2 MeV advocates their use as a dosimeter without beam correction factors. A dependence on the scintillator thickness used during a comparison of the PDD to the ionizing chamber was identified. A 1‐mm‐thick scintillator coupled with the OPD demonstrated the best agreement of ± 3%. Conclusions The response of OPDs to ionizing radiation has been characterized, showing promising use as a dosimeter when coupled with a plastic scintillator. The mechanisms of charge transport and trapping within organic materials varies for visible and ionizing radiation, due to differing properties for direct and indirect detection mechanisms and observing a substantial decrease in sensitivity to the visible spectrum after 40 kGy. This study proved that OPDs produce a stable response to 6‐MV photons, and with a deeper understanding of the charge transport mechanisms due to exposure to ionizing radiation, they are promising candidates as the first flexible, water equivalent, real‐time dosimeter.
The ability of coherent x-ray scatter to provide the molecular structure of breast tissues could add a new dimension in x-ray breast imaging capable of tracking the molecular structural changes during disease progression and (if improving the sensitivity to low-contrast lesions without increasing the radiation dose. Work is under way to build a laboratory prototype dual-sensor breast-imaging scanning system, which combines the diagnostic information from both the transmitted primary and the forward scattered x-rays. This required the design and development of a coherent x-ray scatter detection system based on a high-resistivity multi-element 2D Si-pad array, a multi-channel low-noise pulse processing front-end electronics chip, the XA1.3, and a new DAQ system. Results on the characterization and optimization of the detector-readout electronics-DAQ system and its performance to measure diffraction signatures are presented.
In this work we investigate the potential use as a thermal neutron detector of cerium-doped gadolinium aluminium gallium garnet (GAGG:Ce) coupled to a silicon photomultiplier (SiPM). The response to thermal neutrons has been measured, with two strong low energy neutron-indicative peaks clearly identifiable below 100 keV and additional γ peaks at higher energies. The neutron-related peaks are produced by a combination of contributions from excited states of the two isotopes 156Gd and 158Gd which can be clearly resolved in our GAGG scintillation detector. In particular, two peaks due to neutron-induced γ-ray emission are observed at approximately 82 keV and 260 keV, with best achieved energy resolutions of 24.1 ± 0.2% and 22.7 ± 0.7% respectively. Three different scintillator volumes (0.1 cm3, 0.4 cm3, and 1 cm3) were investigated and the respective results for each configuration will be presented. Our findings show that a GAGG-SiPM based detector can be used as a compact, efficient thermal neutron detector in a low γ-ray contamination environment.
We present measured light outputs from a series of oleic acid and alkyl functionalized CdSeS/ZnS quantum dots in toluene and water-based suspensions. The colloidal nanocrystal samples were studied under UV and X-ray excitation, and in the presence of radioactive isotopes. Low-concentration (1 mg/ml) samples as small as 1 ml exhibited a measurable response to radiation exposure.
We have investigated the use of digital data acquisition techniques to analyze the performance of pulse shape discrimination from a liquid scintillation detector in mixed neutron/gamma radiation fields. Three digital pulse shape discrimination methods were explored, applied to pulses digitized from a liquid scintillator using a high-speed waveform digitizer. The various features of these digital discrimination techniques are discussed and quality of the resulting n/gamma pulse shape discrimination is compared. The digital approach is useful with regard to developing a compact neutron monitor that is capable of fast neutron spectroscopy in the presence of strong mixed n/gamma radiation fields.
The alpha spectroscopy performance and electric current stability of 4H-silicon carbide Schottky devices with 50 μm epitaxial layer was examined at temperatures between 300 to 500 K at 50 K intervals. An activation energy of 5.98 ±0.64 meV was extracted from temperature dependent resistivity measurements. The Schottky barrier height decreases from 1.33 eV at 300 K to 1.11 eV at 500 K and the ideality factor increases from 1.17 at 300 K to 1.79 at 500 K. The reverse bias leakage currents stabilizes faster at higher temperatures. The charge collection efficiency is above 90% for temperatures up to 500 K. Pulse height spectra collected for 24 hours at constant voltage and temperature show improvements with time within the first 8 hours and remained stable for the remainder of the acquisition time. The peak width of the alpha spectra reduces significantly with increasing temperature at applied bias voltages below 50 V, which indicates that leakage currents are not the limiting factor in those conditions even at 500 K in our set up. So far, the devices indicate reasonable stability for extended periods of operation and highlight possible applications in harsh radiation media.
The ability of coherent X-ray scatter to provide the molecular structure of breast tissues could add a new dimension in X-ray breast imaging capable of tracking the molecular structural changes during disease progression and of improving the sensitivity to low-contrast lesions without increasing the radiation dose. Work is under way to build a laboratory prototype dual-sensor breast-imaging scanning system, which combines the diagnostic information from both the transmitted primary and the forward scattered X-rays. This required the design and development of a coherent X-ray scatter detection system based on a high-resistivity multielement two-dimensional (2-D) Si-pad array, a multichannel low-noise pulse processing front-end electronics chip, the XA1.3, and a new DAQ system. X-rays in the energy range of 17-45 keV can be detected with a FWHM energy resolution of 1-3 keV. Results on the characterization and optimization of the detector-readout electronics-DAQ system and its performance to measure diffraction signatures of most commonly used breast-equivalent materials of interest are presented.
A significant reduction in the electrical percolation threshold is achieved by locking carbon nanotubes (CNTs) in a predominantly hexagonally close-packed (HCP) colloidal crystal lattice of partially plasticized latex particles. Contrary to other widely used latex processing where CNTs are randomly distributed within the latex matrix, for the first time, we show that excluding CNTs from occupying the interior volume of the latex particles promotes the formation of a nonrandom segregated network. The electrical percolation threshold is four times lower in an ordered segregated network made with colloidal particles near their glass transition temperature (T(g)) in comparison to in a random network made with particles at a temperature well above the T(g). This method allows for a highly reproducible way to fabricate robust, stretchable, and electrically conducting thin films with significantly improved transparency and lattice percolation at a very low CNT inclusion which may find applications in flexible and stretchable electronics as well as other stretchable technologies. For instance, our technology is particularly apt for touch screen applications, where one needs homogeneous distribution of the conductive filler throughout the matrix.
Silicon Photomultipliers (SiPM) are solid-state pixelated photodetectors. Lately these sensors have been investigated for Time of Flight Positron Emission Tomography (ToF-PET) applications, where very good coincidence time resolution of the order of hundreds of picoseconds imply spatial resolution of the order of cm in the image reconstruction. The very fast rise time typical of the avalanche discharge improves the time resolution, but can be limited by the readout electronics and the technology used to construct the device. In this work the parameters of the equivalent circuit of the device that directly affect the pulse shape, namely the quenching resistance and capacitance and the diode and parasitic capacitances, were calculated. The mean rise time obtained with different preamplifiers was also measured.
The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility-lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal. (c) 2007 American Institute of Physics.
Precise detection of low-dose X-radiation using purely organic direct detectors is vital for tissue-equivalent dosimeters and safety control in medical radiation treatment, but it still remains a challenge. Here, we report a promising organic radiation detector based on 4-hydroxycyanobenzene (4HCB, C7H5NO) single crystals. Plate-like 4HCB single crystals up to 18 × 15 × 1.2 mm3 in size are obtained by an optimized solvent evaporation method, thanks to the clarification of the two-dimensional nucleation growth mechanism. After post surface treatment, the leakage current of the 4HCB detector is no larger than 0.1 pA under an electric field of 600 V cm−1. The fabricated detectors show a capability of detecting 241Am 5.49 MeV α particles with a well resolved full energy peak. The calculated hole mobility (μh) and hole mobility lifetime product (μτ)h are 3.40 cm2 V−1 s−1 and 8.50 × 10−5 cm2 V−1, respectively. Simultaneously, under a 50 kVp X-ray beam, a detection limit as low as 0.29 μGyair s−1 with a high sensitivity of 10 μC Gyair−1 cm−2 is achieved in the bias range of 40–100 V, contributing to a superior X-ray imaging capability with a spatial resolution of 0.9 lp mm−1 at a low-dose rate (below 150 μGyair s−1) of exposure.
We report a study of pulse shapes of a radiation detector with a sandwich structure fabricated from chemical vapor deposited (CVD) polycrystalline diamond. The pulse shapes were recorded at room temperature using 5.486 MeV alpha particles from 241Am source. Only "fast" component was observed in the electron predominated pulses, whereas both "fast" and "slow" components were obtained in the hole predominated pulses, suggesting that electron charge drift is prompt and no detrapping occurred. In contrast, hole charge drift is slower than expected and trapping-detrapping took place during hole travel process. © 2005 Materials Research Society.
We investigated the feasibility of long-drift-time CdZnTe (CZT) gamma-ray detectors, fabricated from CZT material produced by Redlen Technologies. CZT crystals with cross-section areas of 5 5 mm2 and 6 6 mm2 and thicknesses of 20-, 30-, 40-, and 50-mm were configured as 3D position-sensitive drift detectors and were read out using a front-end ASIC. By correcting the electron charge losses caused by defects in the crystals, we demonstrated high performance for relatively thick detectors fabricated from unselected CZT material. VC 2016 AIP Publishing LLC.
Large area detectors capable of operating with high detection efficiency at energies above 30 keV are required in many contemporary X-ray imaging applications. The properties of high Z compound semiconductors, such as CdTe, make them ideally suitable to these applications. The STFC Rutherford Appleton Laboratory has developed a small pixel CdTe detector with 80×80 pixels on a 250 µm pitch. Historically, these detectors have included a 200 µm wide guard band around the pixelated anode to reduce the effect of defects in the crystal edge. The latest version of the detector ASIC is capable of four-side butting that allows the tiling of N×N flat panel arrays. To limit the dead space between modules to the width of one pixel, edgeless detector geometries have been developed where the active volume of the detector extends to the physical edge of the crystal. The spectroscopic performance of an edgeless CdTe detector bump bonded to the HEXITEC ASIC was tested with sealed radiation sources and compared with a monochromatic X-ray micro-beam mapping measurements made at the Diamond Light Source, U.K. The average energy resolution at 59.54 keV of bulk and edge pixels was 1.23 keV and 1.58 keV, respectively. 87% of the edge pixels present fully spectroscopic performance demonstrating that edgeless CdTe detectors are a promising technology for the production of large panel radiation detectors for X-ray imaging
Cadmium zinc telluride (CdZnTe) is now established as a popular choice of sensor for the detection of γ-rays and hard x-rays, leading to its adoption in security, medical and scientific applications. There are still many technical challenges involving the deposition of high-quality, uniform metal contacts on CdZnTe. A detailed understanding of the interface between the bulk CdZnTe and the metal contacts is required for improvements to be made. To understand these complex interfaces, a range of complementary materials characterization techniques have been employed, including x-ray photoelectron spectroscopy depth profiling, focused ion beam cross section imaging and energy dispersive x-ray spectroscopy. In this paper a number of Redlen CdZnTe detectors with asymmetric anode/cathode contacts have been investigated. The structures of the contacts were imaged and their compositions identified. It was found that the two stage electroless indium/electroless gold deposition process on 'polished only' surfaces formed a complex heterojunction on the cathode, incorporating compounds of gold, gold-tellurium, tellurium oxide (of varying stoichiometry) and cadmium chloride up to depths of several 100 nm. Trace amounts of indium were found, in the form of an indium-gold compound, or possibly indium oxide. At the surface of the CdZnTe bulk, a thin Cd depleted layer was observed. The anode heterojunction, formed by a single stage electroless gold deposition, was thinner and exhibited a simpler structure of gold and tellurium oxide. The differing (asymmetric) nature of the anode/cathode contacts gave rise to asymmetric current-voltage (I-V) behaviour and spectroscopy. © 2013 IOP Publishing Ltd.
The reduction in availability and inevitable increase in cost of traditional neutron detectors based on the 3He neutron capture reaction has resulted in a concerted effort to seek out new techniques and detection media to meet the needs of national nuclear security. Traditionally, the alternative has been provided through pulse shape discrimination (PSD) using liquid scintillators. However, these are not without their own inherent issues, primarily concerning user safety and ongoing maintenance. A potential system devised to separate neutron and gamma ray pulses utilising the PSD technique takes advantage of recent improvements in silicon photomultiplier (SiPM) technology and the development of plastic scintillators exhibiting the PSD phenomena. In this paper we present the current iteration of this ongoing work having achieved a Figure of Merit (FoM) of 1.39 at 1.5 MeVee.
Infrared (IR) transmission spectra and IR microscopy images were measured to evaluate the sliced CdZnTe crystals grown under different starting charges using modified vertical Bridgman method. Upon comparing the corresponding electric properties and charge transport performance, IR absorption within the wave-number range from 500 to 2500 cm-1 was potentially attributed to the free carrier absorption caused by the ionized impurities. The size and density of Te particles were not sensitive to IR transmission spectra over the same wave-number range. However, the electric field was modified around isolated Te particles, in such a way that the impurities gettering in the Te inclusions. With respect to the high resistive CdZnTe crystals, IR transmission measurements demonstrated that the mean transmittance is higher than 60% in the wave-number region from 500 to 4000 cm-1. IR microscopy shown the typical diameters of Te particles present in the material were in the range of 6-9 μm, and the density of the particles was 1-4×105 cm-3. The obtained electron mobility lifetime product (μτ)e value was in the range of 1-3×10-3 cm2V-1 by using well-known alpha particle spectra at room temperature. The fabricated CdZnTe thin planar detector showed the typical energy resolution was approximately 5.7% for the 59.5 keV peak at room temperature, without any additional signal processing.
Diamond is considered as tissue equivalent, due to its low atomic number, which makes it a particular attractive material for medical radiation dosimetry applications. X-ray dosimeters made of natural single crystal diamonds are commercially available and routinely used, with typical gain values of about 0.5. The selection of suitable natural diamonds for this purpose is time and cost intensive. We report an X-ray induced current study in synthetic single crystal diamonds. Similar to natural diamond dosimeters, the devices had to be pre-irradiated before use to achieve a reproducible performance. We compare two chemical vapour deposited (CVD) samples grown by Element Six Ltd (UK). D1 was a high purity sample contacted in a sandwich structure with an asymmetric contact pair. Sample D2 was cut vertically and contained some nitrogen rich layers. It was contacted with two Ohmic contacts, which partly covered the high temperature/high pressure substrate material. Particle spectroscopy suggests better charge transport in D1 compared to D2, most likely caused by the lower purity. In contrast to this, large gain values up to 6 x 104 and a sensitivity of similar to 30 mC/Gy were measured at field strengths of 2 kV/cm in D2, and only 7 mu C/Gy in D1, despite their comparable volumes. We deduce that the gain observed in these devices is affected by the electrical properties of the metal-diamond contacts. The response time of the high gain device was in the order of minutes, which is longer than expected by a purely photoconductive process. Long persistent currents have been reported before in diamond under UV irradiation and modifications of gain and response time by surface treatment of UV detectors are known in the literature, highlighting the influence of the surface and contact interface on the device operation. Our results indicate that synthetic single crystal diamond provides a promising material for high sensitivity tissue-equivalent X-ray dosimeters. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The Rutherford Appleton Laboratory has built a small pixel detector for spectroscopic measurements of high energy X-rays using CdTe and CdZnTe. The detector has an array of 20×20 pixels on a 250μm pitch with each pixel bonded to a separate channel on an application specific integrated circuit (ASIC). Each channel in the ASIC contains a charge preamplifier, leakage current compensation circuit, shaping amplifier and peak hold circuit. In recent years there has been an increase in the availability of high quality CdTe and CdZnTe material and the contacting technology required for low leakage current small pixel devices. The energy resolution and stability of the X-ray performance of 1mm thick CdTe with Aluminum Schottky contact pixels and 2mm thick CdZnTe grown by travelling heater method (THM) are measured. The CdTe detectors had an energy resolution of 1 to 1.1keV at 75keV. The THM CdZnTe had an energy resolution of 1.3keV at 75keV. The stability of the performance was measured over a 12 hour exposure with the detectors biased to -500V and held at 25°C. The CdZnTe exhibited stable performance whereas the CdTe suffered from bias induced polarization, the onset of which was delayed by cooling the detectors to 12°C.
Diamond is known for its extreme hardness which may allow it to operate as a particle detector in high fluence environments even after absorption of large radiation doses. We present a study of the deterioration of the charge collection efficiency (CCE) due to neutrons produced by U-235 fission, with irradiation fluences up to 1 x 10(16) n cm(-2). The planar devices were fabricated by thermal evaporation of Au onto approx. 300 mu m thick high purity chemical vapour deposited diamond produced by Element Six Ltd., UK. The detector performance was investigated as a function of bias voltage at room temperature using Am-241 alpha-particles and minimum ionising particles (MIPs) of a Sr-90 source. At low fluences up to 2 x 10(13) n cm(-2), the detectors reach the initial saturated signal amplitude after irradiation. However, the signal is less stable and deteriorates due to polarisation. This effect can be reduced by initial priming with X-rays. No peak could be distinguished in the detector response in the unprimed state after 10(16) n cm(-2) with bias voltages up to 1000 V (equivalent to 32 kV cm(-1)). However, a peak at about 18% CCE could be recovered after priming. (C) 2010 Elsevier B.V. All rights reserved.
We have studied the effects of irradiation induced damage on the detector response of a synthetic single crystal diamond radiation detector. Before introducing radiation damage, the spatial variation of the detector response was investigated using a highly focused 2.6 MeV proton beam. A very uniform response close to 100% charge collection efficiency (CCE) over the whole contact area was found at applied electric field strengths as low as 0.4 V μm–1. At lower biases, time dependent polarisation phenomena were observed and clearly reduced the average signal amplitude. Subsequently, the 2.6 MeV proton beam has been used to introduce radiation damage within selected areas. The ion beam induced charge imaging was repeated to probe the modified regions. Even at an applied electric field of 2.6 V μm–1, no signal above the analogue threshold of the system was observed in the areas which had received a dose larger then 5 × 1014 cm–2, whereas more than 90% CCE was reached in the area with 1012 protons cm–2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We report the x-ray photocurrent response of a coplanar chemical vapor deposition diamond detector fabricated using a metal-less graphitic ohmic contact. Ion implantation of 70 keV boron ions to a dose of 2x10(16) cm(-2) was performed through a patterned photoresist to produce a coplanar graphitic contact structure. The device photocurrent showed a fast response to pulsed x-ray irradiation, and showed no evidence of photocurrent persistence that is observed in devices fabricated using metal Schottky contacts. The graphite-contact device also showed no extrinsic photoconductivity when illuminated with white light.
Simultaneous dual-tracer brain imaging has the potential to shorten patient pathways in the diagnosis of neurodegenerative diseases, but the poor spectral resolution of conventional gamma cameras limits the utility of this technique. Solid state detectors offer improved capability to distinguish between two radioisotopes, but the technology has yet to be fully evaluated in the field of scintigraphic neuroimaging. We present imaging results for a new small-pixel CdTe detector in simultaneous dual-radioisotope scintigraphy of a brain phantom containing Tc-99m and I-123. Quantitative comparison is made with images of the same phantom obtained using a conventional gamma camera. We show that the CdTe detector offers improved scatter rejection and greatly reduced cross-talk between the energy windows. In addition, the new detector is able to resolve low-energy fluorescence x-rays from the source, which could be incorporated into SPECT reconstruction algorithms. Details of the planned development of the detector into a clinical demonstrator are discussed. © 2012 IEEE.
Recent improvements in the growth of wide-bandgap semiconductors, such as cadmium zinc telluride (CdZnTe or CZT), has enabled spectroscopic X/γ-ray imaging detectors to be developed. These detectors have applications covering homeland security, industrial analysis, space science and medical imaging. At the Rutherford Appleton Laboratory (RAL) a promising range of spectroscopic, position sensitive, small-pixel Cd(Zn)Te detectors have been developed. The challenge now is to improve the quality of metal contacts on CdZnTe in order to meet the demanding energy and spatial resolution requirements of these applications. The choice of metal deposition method and fabrication process are of fundamental importance. Presented is a comparison of two CdZnTe detectors with contacts formed by sputter and electroless deposition. The detectors were fabricated with a 74 × 74 array of 200 μm pixels on a 250 μm pitch and bump-bonded to the HEXITEC ASIC. The X/γ-ray emissions from an 241Am source were measured to form energy spectra for comparison. It was found that the detector with contacts formed by electroless deposition produced the best uniformity and energy resolution; the best pixel produced a FWHM of 560 eV at 59.54 keV and 50% of pixels produced a FWHM better than 1.7 keV . This compared with a FWHM of 1.5 keV for the best pixel and 50% of pixels better than 4.4 keV for the detector with sputtered contacts.
An Application Specific Integrated Circuit (ASIC) has been developed at the Rutherford Appleton Laboratory (RAL) to study the small pixel effect in spectroscopic CdTe and CdZnTe detectors. The PIXIE ASIC consists of four arrays of 3 × 3 channels flip chip bonded directly to the detector pixels. The active circuitry of each channel is a charge sensitive preamplifier and an output buffer which is multiplexed directly off chip. Each of the four detector arrays has a different anode geometry. The HEXITEC series of small pixel detectors developed at RAL have demonstrated energy resolutions of ~1 keV per pixel for both CdTe and CdZnTe, however, charge sharing events account for between 30-40% of the total count rate and can lead to degradation of the spectroscopy if not corrected for. The PIXIE ASIC will be used to study the effect of anode geometry on charge sharing and other aspects of the small pixel effect.
A pixellated CdTe detector system comprising 2x2 detector modules has been developed for high energy spectroscopic X-ray imaging applications and has an active area of 16 cm. The detector modules are made from the HEXITEC 80x80 ASIC and 1 mm thick CdTe with Al-Schottky contacts. The CdTe has 250 μm pitch pixels with an outer guard ring on the same pitch. The single HEXITEC 80x80 detectors have an average energy resolution (FWHM) of 800 eV at 59.9 keV. Limitations in the multiple module DAQ system mean that the energy resolution of the pixels in the 2x2 detector array is 2.0 keV at 59.9 keV. The spacing between the tiled detector modules is 150 μ m which results in an inactive area equivalent to 3 pixels, including the guard ring on the edge of the detectors. The modular detector configuration demonstrates the potential to create large area detector arrays in the future. © 1963-2012 IEEE.
Background and purpose: Measurement of dose delivery is essential to guarantee the safety of patients under-going medical radiation imaging or treatment procedures. This study aimed to evaluate the ability of organic semi conductors, coupled with a plastic scintillator, to measure photon dose in clinically relevant conditions, and establish its radiation hardness. Thereby, proving organic devices are capable of being a water-equivalent, mechanically flexible, real-time dosimeter. Materials and methods: The shelf-life of an organic photodiode was analyzed to 40 kGy by comparison of the charge-collection-efficiency of a 520 nm light emitting diode. A non-irradiated and pre-irradiated photodiode was coupled to a plastic scintillator and their response to 6 MV photons was investigated. The dose linearity,dose-per-pulse dependence and energy dependence was characterized. Finally, the percentage depth dose (PDD)between 0.5 and 20 cm was compared with ionization chamber measurements.Results:Sensitivity to 6 MV photons was (190 ± 0.28) pC/cGy and (170 ± 0.11) pC/cGy for the non-irra-diated and pre-irradiated photodiode biased at −2 V. The response was independent of the dose-per-pulsebetween 0.031 and 0.34 mGy/pulse. An energy dependence was found for low keV energies, explained by theenergy dependence of the scintillator which plateaued between 70 keV and 1.2 MeV. The PDD was within ± 3%of the ionization chamber. Conclusion: Coupling an organic photodiode with a plastic scintillator provided reliable measurement of a rangeof photon energies. Dose-per-pulse and energy independence advocate their use as a dosimeter, specifically image-guided treatment without beam-quality correction factors. Degradation effects of organic semi conducting materials deteriorate sensor response but can be stabilized.