4th June 2009 - 39AA04

Speaker:
Title:

Abstract:

Arnau Rios
'Static and dynamical description of correlated nuclear systems'
Green's functions techniques provide a consistent description of strongly correlated many-body systems. Two applications of these techniques to the nuclear domain will be discussed. On the one hand, the implementation of the ladder approximation in terms of Green's functions includes the effect of short-range and tensor correlations into the one-body properties of nuclear matter. The momentum distribution is particularly sensitive to correlations and will be used to quantify their importance in the nuclear medium. On the other hand, for time-dependent systems, the Kadanoff-Baym equations describe the time evolution of the Green's functions, including correlation and memory effects. We will sketch their implementation for nuclear reactions and discuss a first model calculation based on the mean-field dynamics of one-dimensional nuclear slabs.

28th May 2009 - 39BB02

Speaker:
Title:

Abstract:

James Broomfield
'Calculations of mass distributions using the Balian-Vénéroni variational approach'
Existing mean-field models, namely the Hartree-Fock (HF) and time-dependent Hartree- Fock (TDHF) approaches, can be used to determine the expectation values for one-body observables, such as fragment mass, in nuclear reactions and decays but are known to underestimate the fluctuations in these values [1,2]. This is due to their assumption that each nucleon moves independently in a mean-field generated by the interactions between the nucleons neglected important two-body correlations. Balian and Veneroni [3] considered the variational determination of expectation values and fluctuations and obtained an improved formula for these fluctuations which can be implemented using existing TDHF codes. This approach has previously been implemented in a small number of test cases but symmetries and simplified interactions were used due to computational limitations [4-6].

In this work we first review the Balian-Veneroni approach. We then present calculations of the mass distributions for the decay of giant resonances in 32 S, 40 Ca and 132 Sn and in deep-inelastic and fusion-evaporation reactions for 16 O+16 O and 40 Ca+40 Ca using a three-dimensional code with the full Skyrme interaction comparing with the previous calculations and/or experimental data as appropriate. We find that the Balian-Veneroni approach consistently produces fluctuations that exceed the TDHF values but that the numerical problems inherent in running prolonged TDHF calculations, particularly due to emitted nucleons being reflected back from the boundaries of our spatial box. We are consistently able to obtain converged results for giant resonance calculations but encounter difficulties for the deep-inelastic scattering reactions and are unable to obtain reliable results for the fusion- evaporation reactions. Our results differ from those obtained previously. We discuss the sources of these difficulties and discrepancies.

[1] C. H. Dasso, T. Døssing and H. C. Pauli, Zeit. für Phys. A 289, 395-398 (1979).
[2] P.-G. Reinhard, R. Y. Cusson and K. Goeke, Nucl. Phys. A 398, 141-188 (1983).
[3] R. Balian and M. Veneroni, Ann. Phys. 281, 65-142 (2000); Ann. Phys. 187, 29-78 (1988); Ann. Phys. 216, 351-430 (1992).
[4] T. Troudet and D. Vautherin, Phys. Rev. C 31(1), 278-279 (1985).
[5] P. Bonche and H. Flocard, Nucl. Phys. A 437, 189-207 (1985).
[6] J. B. Marston and S. E. Koonin, Phys. Rev. Lett. 54(11), 1139-1141 (1985).

2nd April 2009

Speakers:

Title:
Abstracts:

Nawras Al-Dahan, Nasser Alkhomashi, Elizabeth Cunningham, Ed Simpson, Emma Suckling, Abderrahmane Yekhelef
'IOP Nuclear Physics conference practice talks'
Nawras Al-Dahan
Nasser Alkhomashi
Elizabeth Cunningham
Ed Simpson
Emma Suckling
Abderrahmane Yakhelef

19th February 2009

Speaker:
Title:

Abstract:

David Garrity
'Development of X-ray imaging techniques to materials science challenges in the Naval Nuclear Propulsion Programme (NNPP) '
The Naval Pressurised Water Reactor (PWR) program run by the Royal Navy is the safest of its kind in the world with a zero-accident record. However, there are continued materials challenges illustrated by such events as those occurring aboard HMS Tireless in Gibraltar in 2000, where a serious unforeseen fault in the reactor pipe work forced her to port on emergency diesel power and placed her out of commission for nearly 12 months for inspection and repairs.

Radiography can reveal crack growth and is already used for monitoring materials post- manufacture for inspection of defects. However, it does not provide any information about the underlying material structure, for example residual stresses or phase transformations that can compromise the macroscopic strength of a component. X-ray diffraction provides detailed information on the crystalline structure and therefore a probe for monitoring such changes to the material.

Generally in the past it has been limited to surface measurements with low energy X-rays, where X-rays are coherently scattered off the top few layers of atoms, providing little information on bulk engineering components. However, using higher energy (up to several hundred keV) X-rays it can probe the structure in the bulk of relatively thick samples [1] [2], with the constraint that the coherent scattering component of the total attenuation cross section becomes weaker with increasing energy. This requires the use of small diffraction angles and tight collimation of the primary beam in order to extract information about the crystalline lattice spacings of the sample.

The work presented here focuses on the development of 2D and 3D bench-top Energy Dispersive X-ray Diffraction (EDXRD) imaging systems which have been initially used to measure the martensite transformation in austenitic stainless steel, an important heat treatment process for hardening steels that has its roots in the manufacture of samurai swords. The formation of martensite within the steel can be correlated to the distribution of plastic strains which have been modelled with a Finite Element (FE) code. Upon quenching the steel, martensite forms along these areas of high plastic strain, which can be correlated to distinct changes in the X-ray diffraction spectrum obtained, in this way providing a coarse map of changes to the material. Development of the higher precision 3D system is aimed at increasing the resolution of mapping, and providing the capability to correlate changes with depth.

These systems prove both complementary to high-energy synchrotrons due to their low cost and relative ease of setting up, and necessary for any attempt at in-situ measurements. In parallel to the development of the 3D system, a prototype pipe work scanning system is under development, using up to 200 keV X-rays with a more rugged and compact EDXRD system, for proof-of-concept that a pipe work system could be implemented in-situ.

[1] D.J. Garrity, P.M. Jenneson, R.D. Luggar and S.M. Vincent, Nucl. Instr. and Meth B 251 (2006) 197
[2] A. Steuwer, J.R. Santisteban, M. Turski, P.J. Withers and T. Bunslaps, J. Appl. Cryst. 37, (2004) 883-889

6th February 2009

Speaker:
Title:

Abstract:

Alexis Diaz-Torres
'Dissipative quantum dynamics in low­energy nuclear collisions: effects of decoherence within a new coupled­channels approach'
Coupled­channels approaches have been very successful in explaining several collision observables. However, problems remain. Foremost is the inability to describe elastic scattering and fusion measurements simultaneously and, related, the more recent failure to describe in a physically consistent way the below­barrier quantum tunnelling and above­barrier fusion yields [1].

These problems may be caused by the neglect of important physical processes (e.g., deep­inelastic) which cannot be treated within (standard) coupled­channels models. Measurements have shown that deep­inelastic processes occur even at sub­barrier incident energies, in competition with the process of quantum tunnelling, and thus fusion. The understanding of this complex interplay, at near­ and below barrier energies, requires a dynamical model which can describe coupling assisted tunnelling with dissipation. Neither existing models of fusion nor of deep­inelastic scattering can address both energy dissipation and quantum tunnelling.

In my talk, I will introduce a novel coupled­channels density matrix approach [2] that overcomes these difficulties. We exploit the Lindblad axiomatic approach for open quantum systems. The coupled­channels description is formulated with Lindblad's equation for a reduced density matrix. It describes the dynamical evolution of the reduced system (comprising the relative motion of the nuclei plus selected, intrinsic collective excitations) that irreversibly interacts with an environment.

The development provides a significant step towards an improved theoretical understanding of low­energy collision dynamics, as model calculations exhibit both quantum decoherence and energy dissipation. These cannot be treated within standard coupled­channels approaches. Effects of decoherence and dissipation on collision dynamics can be manifested at distances outside the fusion barrier radius, resulting in suppression of the quantum tunnelling probability, and thus fusion.

[1] M. Dasgupta et al., Phys. Rev. Lett. 99, 192701 (2007), and references therein.
[2] A. Diaz­Torres et al., Phys. Rev. C 78 (2008) 064604.