Professor William Gelletly
Academic and research departmentsDepartment of Physics.
The β decay of 208Hg into the one-proton hole, one neutron-particle 20881Tl127 nucleus was investigated at CERN-ISOLDE. Shell-model calculations describe well the level scheme deduced, validating the proton-neutron interactions used, with implications for the whole of the N>126, Z
The spins and parities of low-lying states in 72 Br populated in the beta decay of 72 Kr have been 14 studied via conversion electron spectroscopy. The measurements were carried out at ISOLDE using 15 a mini-orange spectrometer with Si(Li) and HPGe detectors for electrons and γ-rays detection. 16 Results of the conversion coefficients corresponding to transitions de-exciting 12 levels in 72 Br are 17 reported. The multipolarities of the transitions are deduced and the spins and parities of the levels 18 involved are discussed. From the multipolarities of the most intense transitions to the ground state, 19 the spin and parity of the 72 Br ground state have been definitely established as 1 +. The spin of 20 the 101.2-keV isomeric state is determined to be 3 −. The level scheme is compared with mean-field 21 and shell-model calculations and oblate deformation for the 72 Br ground state is deduced. No E0 22 transitions have been found in 72 Br. E0 transitions in the neighbouring isobaric nuclei, 72 Se and 23 72 Ge, have also been studied. 24
The half-life of the yrast I π = 2+ state in the neutron-rich nucleus 188W has been measured using fast-timing techniques with the HPGe and LaBr3:Ce array at the National Institute of Physics and Nuclear Engineering, Bucharest. The resulting value of t1/2 = 0.87(12) ns is equivalent to a reduced transition probability of B(E2; 2+ 1 → 0+ 1 ) = 85(12) W.u. for this transition. The B(E2; 2+ 1 → 0+ 1 ) is compared to neighboring tungsten isotopes and nuclei in the Hf, Os, and Pt isotopic chains. Woods-Saxon potential energy surface (PES) calculations have been performed for nuclei in the tungsten isotopic chain and predict prolate deformed minima with rapidly increasing γ softness for 184–192W and an oblate minimum for 194W.
The beta decays of 86Br and 91Rb have been studied using the total absorption spectroscopy technique. The radioactive nuclei were produced at the IGISOL facility in Jyvaskyla and further purified using the JYFLTRAP. 86Br and 91Rb are considered high priority contributors to the decay heat in reactors. In addition 91Rb was used as a normalization point in direct measurements of mean gamma energies released in the beta decay of fission products by Rudstam et al. assuming that this decay was well known from high-resolution measurements. Our results shows that both decays were suffering from the Pandemonium effect and that the results of Rudstam et al. should be renormalized.
A multinucleon transfer reaction between a thin self-supporting 78198Pt target and an 850 MeV 54136X6 beam has been used to populate and study the structure of the N=80 isotone 56136Ba. Making use of time-correlated γ-ray spectroscopy, evidence for an Iπ=(10+) isomeric state has been found with a measured half-life of 91±2 ns. Prompt-delayed correlations have also enabled the tentative measurement of the near-yrast states which lie above the isomer. Shell-model calculations suggest that the isomer has a structure which can be assigned predominantly as (vh 11/2)10+-2. The results are discussed in terms of standard and pair-truncated shell-model calculations, and compared to the even-Z N=80 isotones ranging from 50130Sn to 68148Er. A qualitative explanation of the observed dramatic decrease in the B(E2:10+→8+) value for the N=80 isotones at 136Ba is given in terms of the increasing single-hole energy of the h11/2 neutron configuration as the proton subshell is filled. The angular momentum transfer to the binary fragments in the reaction has also been investigated in terms of the average total γ-ray fold versus the scattering angle of the recoils.
The quenching of the experimental spectroscopic factor for the proton decay of 151mLu from the short-lived d3=2 isomeric state has been a long standing problem. In the present work, the proton energy value and half-life of this isomer were remeasured to be 1295(5) keV and 15.4 0.8 s, respectively, in an experiment at the Accelerator Laboratory of the University of Jyvaskyla. The re ned experimental data can resolve the discrepancy in the spectroscopic factor with the WKB approximation. It is also found that the proton formation probability extracted from the present measurements is much larger than that from the adopted data before, indicating no signi cant hindrance for the proton decay of 151mLu.
This contribution summarizes an experiment performed at GSI (Germany) in the neutron-rich region beyond N=126. The aim of this measurement is to provide the nuclear physics input of relevance for r-process model calculations, aiming at a better understanding of the third r-process abundance peak. Many exotic nuclei were measured around 211Hg and 215Tl. Final ion identification diagrams are given in this contribution. For most of them, we expect to derive halflives and and β-delayed neutron emission probabilities. The detectors used in this experiment were the Silicon IMplantation and Beta Absorber (SIMBA) detector, based on an array of highly segmented silicon detectors, and the BEta deLayEd Neutron (BELEN) detector, which consisted of 30 3He counters embedded in a polyethylene matrix. © 2013 AIP Publishing LLC.
A multinucleon transfer reaction between a thin self-supporting Pt-198(78) target and an 850 MeV Xe-136(54) beam has been used to populate and study the structure of the N=80 isotone Ba-136(56). Making use of time-correlated gamma-ray spectroscopy, evidence for an I-pi=(10(+)) isomeric state has been found with a measured half-life of 91+/-2 ns. Prompt-delayed correlations have also enabled the tentative, measurement of the near-yrast states which lie above the isomer. Shell-model calculations suggest that the isomer has a structure which can be assigned predominantly as (nuh(11/2))(10+)(-2). The results are discussed in terms of standard and pair-truncated shell-model calculations, and compared to the even-Z N=80 isotones ranging from Sn-130(50) to Er-148(68). A qualitative explanation of the observed dramatic decrease in the B(E2:10(+)-->8(+)) value for the N=80 isotones at Ba-136 is given in terms of the increasing single-hole energy of the h(11/2) neutron configuration as the proton subshell is filled. The angular momentum transfer to the binary fragments in the reaction has also been investigated in terms of the average total gamma-ray fold versus the scattering angle of the recoils.
The positive-parity yrast states in the 89Rb, 92Y, and 93Y nuclei were studied using gamma-ray spectroscopy with heavy-ion induced reactions. In the multinucleon transfer reactions 208Pb+90Zr (590 MeV) and 238U+82Se (505 MeV), several gamma-ray transitions were identified in these nuclei by means of coincidences between recoiling ions identified with the PRISMA spectrometer and gamma rays detected with the CLARA gamma-ray array in thin target experiments. Level schemes were subsequently determined from triple gamma coincidences recorded with the GASP array in a thick target experiment, in the reactions produced by a 470 MeV 82Se beam with a 192Os target. The observed level schemes are compared to shell-model calculations.
The TIARA+VAMOS+EXOGAM set-up has recently been installed at GANIL to study the single-particle structure of exotic nuclei. The unique characteristics of the TIARA array, combined with the large acceptance spectrometer VAMOS and the high efficiency Germanium detector array EXOGAM, has allowed high-resolution measurements of transfer reactions in inverse kinematics using low intensity exotic beams. We will describe the experimental set-up, data analysis and preliminary results of the first experiments using a 24Ne beam from SPIRAL, concentrating in particular on the performance of VAMOS that has been used to detect the heavy fragments after the (d,p), (d,d) and (d,t) reactions. © 2006 American Institute of Physics.
Gamma decays from excited states up to J(pi)=6() in the N=Z-2 nucleus Ni-54 have been identified for the first time. Level energies are compared with those of the isobars Co-54 and Fe-54 and of the cross-conjugate nuclei of mass A=42. The good but puzzling f(7/2) cross-conjugate symmetry in mirror and triplet energy differences is analyzed. Shell model calculations reproduce the new data but the necessary nuclear charge-dependent phenomenology is not fully explained by modern nucleon-nucleon potentials.
High spin states in the neutron rich 188Os and 190Os nuclei have been populated using the 82Se + 192Os deep-inelastic reaction. The level schemes are extended up to spin I ≈21. The observed new structures are tentatively interpreted as fragments of rotational bands built on multi-quasiparticle configurations.
A compact, quasi-4π position sensitive silicon array, TIARA, designed to study direct reactions induced by radioactive beams in inverse kinematics is described here. The Transfer and Inelastic All-angle Reaction Array (TIARA) consists of 8 resistive charge division detectors forming an octagonal barrel around the target and a set of double-sided silicon-strip annular detectors positioned at each end of the barrel. The detector was coupled to the γ-ray array EXOGAM and the spectrometer VAMOS at the GANIL Laboratory to demonstrate the potential of such an apparatus with radioactive beams. The reaction, well known in direct kinematics, has been carried out in inverse kinematics for that purpose. The observation of the ground state and excited states at 7.16 and 7.86 MeV is presented here as well as the comparison of the measured proton angular distributions with DWBA calculations. Transferred l-values are in very good agreement with both theoretical calculations and previous experimental results obtained in direct kinematics.
The β decay of 207Hg into the single-proton-hole nucleus 207Tl has been studied through γ-ray spectroscopy at the ISOLDE Decay Station (IDS) with the aim of identifying states resulting from coupling of the πs−11/2, πd−13/2, and πh−111/2 shell model orbitals to the collective octupole vibration. Twenty-two states were observed lying between 2.6 and 4.0 MeV, eleven of which were observed for the first time, and 78 new transitions were placed. Two octupole states (s1/2-coupled) are identified and three more states (d3/2-coupled) are tentatively assigned using spin-parity inferences, while further h11/2-coupled states may also have been observed for the first time. Comparisons are made with state-of-the-art large-scale shell model calculations and previous observations made in this region, and systematic underestimation of the energy of the octupole vibrational states is noted. We suggest that in order to resolve the difference in predicted energies for collective and noncollective t=1 states (t is the number of nucleons breaking the 208Pb core), the effect of t=2 mixing may be reduced for octupole-coupled states. The inclusion of mixing with t=0,2,3 excitations is necessary to replicate all t=1 state energies accurately.
To study the β-decay properties of some well known delayed neutron emitters an experiment was performed in 2009 at the IGISOL facility (University of Jyväskylä in Finland) using Total Absorption -ray Spectroscopy (TAGS) technique. The aim of these measurements is to obtain the full β-strength distribution below the neutron separation energy (Sn) and the γ/neutron competition above. This information is a key parameter in nuclear technology applications as well as in nuclear astrophysics and nuclear structure. Preliminary results of the analysis show a significant γ-branching ratio above Sn. © Owned by the authors, published by EDP Sciences, 2014.
206Hg was populated in the fragmentation of an E∕A = 1 GeV 208Pb beam at GSI. It was part of a campaign to study nuclei around 208Pb via relativistic Coulomb excitation. The observation of the known isomeric states confirmed the identification of the fragmentation products. The isomeric decays were also used to prove that the correlations between beam identification detectors and the AGATA γ-ray tracking array worked properly and that the tracking efficiency was independent of the time relative to the prompt flash.
We investigate the decay of 87,88Br and 94Rb using total absorption γ -ray spectroscopy. These important fission products are β-delayed neutron emitters. Our data show considerable βγ intensity, so far unobserved in high-resolution γ -ray spectroscopy, from states at high excitation energy. We also find significant differences with the β intensity that can be deduced from existing measurements of the β spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for 235U fission at cooling times in the range 1–100 s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for γ emission from neutron unbound states populated in the daughter nucleus. The γ branching is compared to Hauser-Feshbach calculations, which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for 94Rb, although much smaller, hints of the need to increase the radiative width γ by one order of magnitude. This increase in γ would lead to a similar increase in the calculated (n,γ ) cross section for this very neutron-rich nucleus with a potential impact on r process abundance calculations.
A total absorption spectrometer, dedicated to the study of very short-lived atomic species, has been built and installed at the CERN/ISOLDE mass separator. The beta decay of the neutron-deficient Kr-74 nucleus has been studied using this new device. The Gamow-Teller strength distribution has been observed over most of the Q(EC) window, and a total strength of 0.69(3) g(A)(2)/4pi has been measured for states with excitation energy below 3 MeV. Shape mixing in the Kr-74 ground state is inferred from a comparison of the experimental strength distribution with self-consistent, deformed, quasiparticle-random-phase-approximation calculations.
In order to determine the Gamow‐Teller strength distribution for the N = Z nucleus 72Kr an experiment was performed with a Total Absorption Gamma Spectrometer. To fully accomplish this task it is crucial to determine the multipolarity of the low energy transitions as the spin‐parity of the daughter ground state has been debated. This is done by experimental determination of the conversion coefficients. Preliminary results for the multipolarity and conversion coefficients of the transition connecting the isomeric state at 101 keV with the 72Br ground state are presented.
We report here on the first results obtained from the study of the 24Ne(d, p)25Ne reaction performed with SPIRAL beam at GANIL using the new TIARA+V AMOS+EXOGAM setup. © 2007.
We have developed a new experimental setup based at the GANIL/SPIRAL facility in Caen, France to measure one-nucleon transfer reactions in inverse kinematics in order to study the evolution of the single particle structure of exotic nuclei. The setup couples together three state-of-the-art detection systems: the TIARA Si array, the large-acceptance magnetic spectrometer VAMOS and the high-efficiency segmented Ge γ-ray array EXOGAM. In a first experiment, the 24Ne(d,p)25Ne reaction has been studied to probe the N=16 shell closure. Details of the setup, data analysis and preliminary results are presented. © 2006 American Institute of Physics.
Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δn=0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z=50 (for N ˃ 82) and Z = 82 (for N ˃ 126). The level of forbiddenness of the Δn=1v1g9/2 → π0g7/2 transition has been investigated from the β decay of the ground state of 207Hg into the single-proton-hole nucleus 207Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0g-17/2 state in 207Tl, an upper limit of 3.9 x 10-3% was obtained for the probability of this decay, corresponding to log ft ˃ 8.8 within a 95% confidence limit. This is the most stringent test of the Δn=0 selection rule to date.
Fragmentation products from a 92Mo beam on a natural nickel target have been used to study structural properties of the very neutron deficient nuclei around N∼Z∼40. We present the first observation of isomeric decays in the Tz=1 systems 3674Kr, 3980 Y and 4184Nb. The isomer in 74Kr is interpreted as the hindered decay from an excited 0+ state, confirming the prediction of prolate/oblate shape coexistence in this nucleus. Transitions from states below an isomer in the N=Z nucleus 4386Tc have also been tentatively identified, making this the heaviest N=Z system for which decays from excited states have been observed. In addition, we have obtained the first conclusive evidence for the existence of the Tz = -1/2 isotopes 3977Y, 4079Zr and 4283Mo. The data for 3977Y is of particular interest in light of the reported instability in the lighter odd-Z, Tz = -1/2 systems 69Br and 73Rb.
The preliminary results from the RISING Stopped Beam Isomer Campaign are presented, with specific focus on results of the initial experiment to investigate isomeric decays along the N=Z line around A similar to 80-90 following the projectile fragmentation of a Ag-107 primary beam at an energy of 750 MeV per nucleon. A description of the technical aspects behind the design of the RISING array is presented, together with evidence for previously unreported isomeric decays in Tc-87,Tc-88 and the N=Z nuclei Nb-82(41) and Tc-86(43).
decays from heavy, neutron-rich nuclei with A∼190 have been investigated following their production via the relativistic projectile fragmentation of an E/A=1 GeV 208Pb primary beam on a ∼2.5 g/cm2 9Be target. The reaction products were separated and identified using the GSI FRagment Separator (FRS) and stopped in the RISING active stopper. γ decays were observed and correlated with these secondary ions on an event-by-event basis such that γ-ray transitions following from both internal (isomeric) and β decays were recorded. A number of discrete, β-delayed γ-ray transitions associated with β decays from 194Re to excited states in 194Os have been observed, including previously reported decays from the yrast Iπ=(6+) state. Three previously unreported γ-ray transitions with energies 194, 349, and 554 keV are also identified; these transitions are associated with decays from higher spin states in 194Os. The results of these investigations are compared with theoretical predictions from Nilsson multi-quasiparticle (MQP) calculations. Based on lifetime measurements and the observed feeding pattern to states in 194Os, it is concluded that there are three β−-decaying states in 194Re.
In a nuclear reactor, the decay of fission fragments is at the origin of decay heat and antineutrino flux. These quantities are not well known while they are very important for reactor safety and for our understanding of neutrino physics. One reason for the discrepancies observed in the estimation of the decay heat and antineutrinos flux coming from reactors could be linked with the Pandemonium effect. New measurements have been performed at the JYFL facility of Jyväskylä with a Total Absorption Spectrometer (TAS) in order to circumvent this effect. An overview of the TAS technique and first results from the 2009 measurement campaign will be presented. © Owned by the authors, published by EDP Sciences, 2013.
Picosecond lifetimes of medium spin states in Lu-165 were measured for the first time. The reaction used to populate the nucleus of interest was La-139(Si-30,4n)Lu-165 at a beam energy of 135 MeV. The beam was provided by the XTU-tandem accelerator of Laboratori Nazionali di Legnaro, Italy. By using the differential decay curve method, lifetimes of 19 states in four different rotational bands were obtained. Therefrom the B(E2) values and the transitional quadrupole moments were deduced. The obtained Q(t) for the different bands are compared with total Routhian surface (TRS) calculations and particle-rotor-model calculations. The TRS calculations predict different axial symmetric shapes for the bands built on the 9/2(-), 9/2(+), and 1/2(-) configurations, with a gamma softness for the 9/2(-) configuration. This band has also been studied using the particle-rotor model, the results of which, however, are consistent with a triaxial shape with a gamma value of -15(p).
The positive-parity yrast states in the Rb-89, Y-92, and Y-93 nuclei were studied using gamma-ray spectroscopy with heavy-ion induced reactions. In the multinucleon transfer reactions Pb-208+Zr-90 (590 MeV) and U-238+Se-82 (505 MeV), several gamma-ray transitions were identified in these nuclei by means of coincidences between recoiling ions identified with the PRISMA spectrometer and gamma rays detected with the CLARA gamma-ray array in thin target experiments. Level schemes were subsequently determined from triple-gamma coincidences recorded with the GASP array in a thick target experiment, in the reactions produced by a 470 MeV Se-82 beam with a Os-192 target. The observed level schemes are compared to shell-model calculations.
We report the results of an experiment in which we studied the near-yrast states in selenium isotopes approaching N=50 following their population in multinucleon transfer reactions between a Se-82 beam and a Os-192 target. The level schemes for Se-80,Se-82 derived from the current work are compared with restricted-basis shell-model calculations and pair-truncated shell-model calculations. These provide a good description of the yrast sequences in these nuclei using a basis space limited to excitations in the nu(p3/2, p1/2, p1/2) and pi(f5/2, p3/2, p1/2) orbitals.
Isomeric ratios have been measured for high-spin states in Po84198,200,206,208, At85208,209,210,211, Rn86210,211,212,213,214, Fr87208,211,212,213,214, Ra88210,211,212,214,215, and Ac89215 following the projectile fragmentation of a 1 AGeV U beam by a Be target at GSI Helmholtzzentrum für Schwerionenforschung. The fragments were separated in the fragment separator (FRS) and identified by means of energy loss and time-of-flight techniques. They were brought to rest at the centre of the RISING gamma-ray detector array and intensities of gamma rays emitted in the decay of isomeric states with half-lives between 100 ns and 40 μs and spin values up to 55/2ℏ were used to obtain the corresponding isomeric ratios. The data are compared to theoretical isomeric ratios calculated in the framework of the abrasion-ablation model. Large experimental enhancements are obtained for high-spin isomers in comparison to expected values. © 2013 Elsevier B.V.
Preliminary results of the data analysis of the beta decay of 94Rb using a novel - segmented- total absorption spectrometer are shown in this contribution. This result is part of a systematic study of important contributors to the decay heat problem in nuclear reactors. In this particular case the goal is to determine the beta intensity distribution below the neutron separation energy and the gamma/beta competition above. © 2013 AIP Publishing LLC.
The nuclear structure of neutron-rich N > 126 nuclei has been investigated following their production via relativistic projectile fragmentation of a E/A = 1 GeV (238)U beam on a Be target. The preliminary analysis indicates the presence of previously unreported isomeric states in the N = 128 isotones (208)Hg and (209)Tl.
We report on the preliminary results from a study of the decay of the I-pi = 8(+) T-1/2 = 2 mu s isomer in Pd-96 performed as part of the Stopped-Beam RISING campaign within the Rare Isotope Investigation at GSI (RISING). The Pd-96 ions were produced following the projectile fragmentation of a 750 MeV per nucleon Ag-107 primary beam. The reaction products were separated and identified by the in-flight method using the GSI Fragment Separator. The residues of interest were stopped in a perspex stopper surrounded by an array of 15, seven-element germanium Cluster detectors. One of the goals of the current work is to investigate the population of high-spin states produced projectile fragmentation reactions using isomeric ratio measurements to infer information on the angular momentum population distribution. In this short contribution the method and results of determining the isomeric ratio for the I-pi = 8(+) microsecond isomer in Pd-96 nucleus are presented.
Heavy neutron-rich nuclei were populated via relativistic energy fragmentation of a E/A=1 GeV 208Pb beam. The nuclei of interest were selected and identified by a fragment separator and then implanted in a passive plastic stopper. Delayed rays following internal isomeric decays were detected by the RISING array. Experimental information was obtained on a number of nuclei with Z=73-80 (Ta-Hg), providing new information both on the prolate-oblate transitional region as well as on the N=126 closed shell nuclei.
High-spin states in the proton-rich nucleus 45V have been identified for the first time. A comprehensive gamma-ray decay scheme has been established following an experiment performed at the Vivitron accelerator at IReS Strasbourg by using the Euroball gamma-ray detector array coupled to the Neutron Wall and Euclides detector arrays. The natural (negative-) parity scheme is identified up to the f7/2 band termination in addition to a positive-parity collective structure based on a d3/2 particle-hole excitation. Comparison of this scheme with that of the mirror partner, 45Ti, has yielded detailed information on the variation of Coulomb energy as a function of excitation energy and angular momentum. This is the first time that such an analysis has been performed for a collective structure built on a cross-shell excitation. Comparison of the observed Coulomb energies with those predicted by large-scale shell-model calculations is presented. In this case, unusually, the calculations do not fare as well as for heavier nuclei in the shell. In addition, stark differences between the two nuclei are observed for the decay intensities of the parity-changing E1 decays that de-excite the positive-parity deformed bands.
Relativistic energy projectile fragmentation of Pb-208 has been used to produce a range of exotic nuclei. The nuclei of interest were studied by detecting delayed gamma rays following the decay of isomeric states. Experimental information on the excited states of the neutron-rich N = 126 nucleus, Pt-204, following internal decay of two isomeric states, was obtained for the first time. In addition, decays from the previously reported isomeric I=27h and I=(49/2)h states in Tb-148 and Gd-147, respectively, have been observed. These isomeric decays represent the highest spin discrete states observed to date following a projectile fragmentation reaction, and opens further the possibility of doing 'high-spin physics' using this technique.
The beta decays of 86Br and 91Rb have been studied using the total absorption spectroscopy technique. The radioactive nuclei were produced at the Ion Guide Isotope Separator On-Line facility in Jyväskylä and further purified using the JYFLTRAP. 86Br and 91Rb are considered to be major contributors to the decay heat in reactors. In addition, 91Rb was used as a normalization point in direct measurements of mean gamma energies released in the beta decay of fission products by Rudstam et al. assuming that this decay was well known from high-resolution measurements. Our results show that both decays were suffering from the Pandemonium effect and that the results of Rudstam et al. should be renormalized. The relative impact of the studied decays in the prediction of the decay heat and antineutrino spectrum from reactors has been evaluated
β-decay properties of fission products are very important for applied reactor physics, for instance to estimate the decay heat released immediately after the reactor shutdown and to estimate the ν flux emitted. An accurate estimation of the decay heat and the ν emitted flux from reactors, are necessary for purposes such as reactors operation safety and non-proliferation. In order to improve the precision in the prediction for these quantities, the bias due to the Pandemonium effect affecting some important fission product data has to be corrected. New measurements of fission products β-decay, not sensitive to this effect, have been performed with a Total Absorption Spectrometer (TAS) at the JYFL facility of Jyväskylä. An overview of the TAS technique and first results from the 2009 campaign will be presented. © Owned by the authors, published by EDP Sciences, 2014.
The β-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with β-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb and Bi in the mass region N&126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the β-decay strength distribution. This provides important constraints to global theoretical models currently used in r-process nucleosynthesis.
Background: Previous measurements of Beta-delayed neutron emitters comprise around 230 nuclei, spanning from the 8He up to 150La. Apart from 210Tl, with a minuscule branching ratio of 0.07%, no other neutron emitter is measured yet beyond A = 150. Therefore new data are needed, particularly in the heavy mass region around N=126, in order to guide theoretical models and to understand the formation of the third r-process peak at A 195. Purpose: To measure both, Beta-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb and Bi isotopes beyond N = 126. Method: Ions of interest are produced by fragmentation of a 238U beam, selected and identifed via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) is used to measure ion-implants and -decays. An array of 30 3He tubes embedded in a polyethylene matrix (BELEN) is used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time-correlations. The latter are analyzed with an analytical model and results for the half-lives and neutron-branching ratios are derived using the binned Maximum-Likelihood method. Results: Twenty new Beta-decay half-lives are reported for 204
Understanding the β-decay properties of fission products is very important for nuclear reactor monitoring & safety. Indeed, the β-decay of fission products is a source of power during nuclear reactor operation and after, this activity is the main contributor to the decay-heat representing ∼7-8% of a reactor nominal power. β-decay is also at the origin of the antineutrinos coming from nuclear reactor. The study of antineutrinos is mandatory for nuclear safeguard policies based on antineutrino detection. But there are still some discrepancies in the calculation of the decay-heat and antineutrino spectra. One reason of the distortions observed seems to be linked to the Pandemonium effect. In order to improve the accuracy in the prediction of these quantities, new measurements of fission products β-decay properties, not sensitive to this effect, have been performed with a Total Absorption spectrometer (TAS). An overview of the TAS technique and first results from the latest experiment will be presented. © 2013 IEEE.
A novel method of deducing the deformation of the N=Z nucleus Sr-76 is presented. It is based on the comparison of the experimental Gamow-Teller strength distribution B(GT) from its beta decay with the results of quasi-random-phase approximation calculations. This method confirms previous indications of the strong prolate deformation of this nucleus in a totally independent way. The measurement has been carried out with a large total absorption gamma spectrometer, "Lucrecia," newly installed at CERN-ISOLDE.
The very neutron deficient Z = N + 1 nuelei 7739Y, 7940Zr, and 8342Mo have been observed for the first time following the fragmentation of a 92Mo beam. In contrast, no evidence was found for the existence of 8141Nb and 8543Tc. The observation of 7739Y is of particular interest in light of the instability of the odd-proton, Z = N + 1 systems, 6935Br, 7337Rb, 8141Nb, and 8543Tc, and may be explained as a consequence of the shape polarizing effect of the highly deformed, prolate Z = N = 38 core. The experimental results are discussed within the framework of the shell-correction model, which together with proton-decay calculations allow an estimate of the proton separation energies of these highly exotic systems to be calculated.
Neutron-rich nuclei were populated in a relativistic fission of U. Gamma-rays with energies of 135 keV and 184 keV were associated with two isomeric states in Pd and Ru. Half-lives of 0.63(5) μs and 2.0(3) μs were deduced and the isomeric states were interpreted in terms of prolate deformed single-particle states.