### Dr Paul Stevenson

### Biography

Paul Stevenson studied physics at the University of Oxford last millennium, worked for one year at the University of Tennessee in Knoxville, and has been at the University of Surrey since 2000.

### Areas of specialism

### University roles and responsibilities

- MPhys Research Year Coordinator
- Head of Nuclear Theory Group

### My qualifications

### Affiliations and memberships

### Courses I teach on

### Supervision

# Postgraduate research supervision

### My publications

### Publications

states,

time-dependent Hartree-Fock approximation applied to nuclear giant monopole

resonances in the small amplitude regime. The problem is spatially unbounded as

the resonance state is in the continuum. The practical requirement to perform

the calculation in a finite-sized spatial region yields an artificial boundary,

which is not present physically. The question of how to ensure the boundary

does not interfere with the internal solution, while keeping the overall

calculation time low is studied. Here we propose an absorbing boundary

condition scheme to handle the conflict. The derivation, via a Laplace

transform method, and implementation is described. An inverse Laplace transform

required by the absorbing boundaries is calculated using a method of non-linear

least squares. The accuracy and efficiency of the scheme is tested and results

presented to support the case that they are a effective way of handling the

artificial boundary.

density functional studies of fission have previously concentrated on adiabatic approaches based on constrained

static calculations ignoring dynamical excitations of the fissioning nucleus and the daughter products.

Purpose: We explore the ability of dynamic mean-field methods to describe induced fission processes, using

quadrupole boosts in the nuclide 240Pu as an example.

Methods: Following upon the work presented in Goddard et al. [Phys. Rev. C 92, 054610 (2015)], quadrupoleconstrained

Hartree-Fock calculations are used to create a potential energy surface. An isomeric state and a state

beyond the second barrier peak are excited by means of instantaneous as well as temporally extended gauge boosts

with quadrupole shapes. The subsequent deexcitation is studied in a time-dependent Hartree-Fock simulation,

with emphasis on fissioned final states. The corresponding fission fragment mass numbers are studied.

Results: In general, the energy deposited by the quadrupole boost is quickly absorbed by the nucleus. In

instantaneous boosts, this leads to fast shape rearrangements and violent dynamics that can ultimately lead to

fission. This is a qualitatively different process than the deformation-induced fission. Boosts induced within a

finite time window excite the system in a relatively gentler way and do induce fission but with a smaller energy

deposition.

Conclusions: The fission products obtained using boost-induced fission in time-dependent Hartree-Fock are

more asymmetric than the fragments obtained in deformation-induced fission or the corresponding adiabatic

approaches.

of freedom is a well?established prism with which to understand atomic nuclei. Self?consistent mean?field models provide one tool to understand nuclear shapes, and their link to other nuclear properties and observables. We present examples of how the time?dependent extension of the mean?field approach can be used in particular to shed light on nuclear shape properties, particularly looking at the giant resonances built on deformed nuclear ground states, and at dynamics in highly-deformed fission isomers. Example calculations are shown of 28Si in the first case, and 240Pu in the latter case.

ff

ective nuclear interaction within the time-dependent Hartree-Fock

framework to assess the e

ff

ect of inclusion of the tensor terms of the Skyrme interaction on the fusion window

of the

16

O?

16

O reaction. We find that the lower fusion threshold, around the barrier, is quite insensitive to

these details of the force, but the higher threshold, above which the nuclei pass through each other, changes by

several MeV between di

ff

erent tensor parametrisations. The results suggest that eventually fusion properties

may become part of the evaluation or fitting process for e

ff

ective nuclear interactions

^{-}Decaying states in 194Re: Shape evolution in neutron-rich osmium isotopes, Physical Review C - Nuclear Physics 85 (3)

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 ³ softness for 184?192W and an oblate minimum for 194W.

The mass distributions for giant dipole resonances in S-32 and Sn-132 decaying through particle emission and for deep-inelastic collisions between O-16 nuclei have been investigated by implementing the Balian-Vénéroni variational technique based upon a three-dimensional time-dependent Hartree-Fock code with realistic Skyrme interactions. The mass distributions obtained have been sown to be significantly larger than standard TDHF results.

Recent experimental data on the low-lying states in W-190 show a change in the E(4(1)(+))/E(2(1)(+)) behavior compared to less neutron-rich neigbors. Self-consistent axially-deformed Hartree-Fock calculations, using a separable monopole interaction, of nuclei in the vicinity of W-190 are performed to systematically examine the evolution of ground state quadrupole deformations. It is found that the neutron number N=116 causes a coexistence of oblate and prolate shapes, with a weak dependence on proton number, thereby hindering the development of these isotones as well-deformed rotors.

A previously unreported isomer has been identified in Mo-99 at an excitation energy of E-x = 3010 keV, decaying with a half-life of T-1/2 = 8(2) ns. The nucleus of interest was produced following fusion-fission reactions between a thick Al-27 target frame and a Hf-178 beam at a laboratory energy of 1150 MeV. This isomeric state is interpreted as an energetically favored, maximally aligned configuration of nu h (11/2) circle times pi(g (9/2))(2).

^{16}O+

^{16}O

^{32}S are extracted based on both the time-dependent Hartree-Fock and the time-dependent density matrix methods. The latter goes beyond the mean-field approximation by taking into account the effect of two-body correlations, but in practical simulations of fusion reactions we find that the total energy is not conserved. We analyze this problem and propose a solution that allows for a clear quantification of dissipative effects in the dynamics. Compared to mean-field simulations, friction coefficients in the density-matrix approach are enhanced by about 20 %. An energy-dependence of the dissipative mechanism is also demonstrated, indicating that two-body collisions are more efficient at generating friction at low incident energies.