Improved modelling of the neutron source for neutron activation experiments Steven Lilley, R Pampin, L Packer Neutronics and Nuclear Data Group NPL Neutron Users Club November 2011 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority
Background to the work MCUNED Comparison with previous source Modelling effect of different beam parameters Location Beam Energy Beam width Future plans Conclusions Outline
Background
Background New programme of work to measure reaction cross sections by neutron activation at AWE ASP facility Supporting the experiment campaign is a modelling exercise to help predict what will be measurable, optimise the experiments and hopefully estimate uncertainties due to various effects Original MCNP model was used in conjunction with FISPACT to identify measurable reactions MCNP cannot model deuteron interactions Neutron source was predicted based on relativistic mechanics, each emission angle had a fixed energy. Point source rather than distributed source
MCUNED is a patch for MCNPX Allows modelling of light ions including low energy deuterons by sampling from cross section data MCUNED Also adds a variance reduction technique for generation of secondary to greatly speed up the calculations Created by Patrick Sauvan at UNED Spain JEFF meeting December 2010 MCUNED Applicability to Nuclear Data Benchmarking and accelerator radioprotection Studies. P. Sauvan, J.Sanz, F. Ogando, New capabilities for Monte Carlo simulation of deuteron transport and secondary products generation, Nuclear Instruments & Methods In Physics Research Section A, Vol 64, pp 323-330 (2010)
Comparison with previous source MCUNED predicts a much broader peak. Original source distribution has no neutrons created with an energy more than 15MeV Standard model beam parameters Beam radius 5mm Beam Energy 300Kev Beam Position Centre of target
Changing Beam Position Shifting the beam away from the central position reduces the number of higher energy neutrons reaching the foil Total number of neutrons in the foil reduces by approximately 1/3
Changing Beam Width Neutron flux at foil per source deuteron for different beam radii Effect on selected energy bins of the beam radii 6.40E-06 3.00E-06 N flux per source Deuteron n/cm 2/s 6.20E-06 6.00E-06 5.80E-06 5.60E-06 5.40E-06 N flux per source deuteron n/cm 2/s 2.50E-06 2.00E-06 1.50E-06 1.00E-06 5.00E-07 15.7 bin 14.9 bin 14.6 bin 5.20E-06 4mm 5mm 6mm 7mm 8mm 9mm 10mm 0.00E+00 4mm 5mm 6mm 7mm 8mm 9mm 10mm As expected as the radius increases fewer neutron per source deuteron pass through the foil However not all energy groups decrease. The peak seems to shift towards lower energies
Geometry explanation Foil 5cm 1cm 1cm Beam Beam Beam Target Basic set up Centred beam Off axis beam Wide beam
Changing Beam Energy 8.00E-06 N e u t r o n f l u x p e r s o u r c e d e u t e r o n a t f o i l n / c m 2 / s 7.00E-06 6.00E-06 5.00E-06 4.00E-06 3.00E-06 2.00E-06 1.00E-06 0.00E+00 0 50 100 150 200 250 300 350 400 450 Deuteron Energy (Kev) N f l u x p e r s o u r c e d e u t e r o n 2.50E-06 2.00E-06 1.50E-06 1.00E-06 5.00E-07 1.00E-10 9 10 11 12 13 14 15 16 17 18 Neuteron energy (Mev) 150Kev D 300Kev D As expected as the energy increases the neutron flux increases because of cross section Higher energies mean a broader peak shifted towards higher energy neutrons
Conclusions MCUNED has been applied to investigate the effects of varying different beam parameters Changing beam parameters not only changes the absolute number of neutrons reaching the target foil but also changes the spectrum shape. Different parameters change the spectrum seen by the foil in different ways mapping out a complex parameter space for each energy group The model is now more realistic than the previous point source model MCUNED needs more benchmarking and validation, but it s a great improvement over previous methods.
Future work Investigate the effects of the tritium distribution Investigate effects of deuterium build up in the target Longer term aim is to set up a collaboration with UNED and others to add extra benchmark/ validation to MCUNED Possibly including work at ASP or others? Apply MCUNED to other systems?
Acknowledgements MCUNED Patch kindly provided by P Sauvan UNED Spain. AWE ASP team for details of the ASP Accelerator and on going support to experimental program Questions?