1. for collaboration “Energy plus transmutation”
NEMEA-4 Workshop October 16-18, 2007 Prague, Czech Republic
Systematic studies of neutrons produced in the Pb/U assembly irradiated by relativistic protons and deuterons.
Vladimír Wagner
Nuclear physics institute of CAS, Řež, Czech Republic, E_mail:
for collaboration “Energy plus transmutation”
(Russia, Belarus, Germany, Greece, Poland, Ukraine, Czech Republic …)
1. Introduction
2. Integral neutron production
2.1 Used method
2.2 Overview of lead target data
2.3 Pb/U assembly data
3. Spatial distribution of neutron field
4.1 Comparison between experiment and simulation
4.2 Possible sources of discrepancies
4. Conclusions and outlooks

2. Energy plus Transmutation (EPT) Setup
Set-up: Lead target: diameter 8.4 cm, length 48 cm
Natural uranium blanket: rods with Al cladding total weight kg
Shielding box: polyethylene with 1 mm Cd on the inside side
Our main objectives: Neutron distribution studies – radiation samples
Results:
Understanding of sources of experimental data uncertainties – set of simulations of our set-up using MCNPX code.
Set of proton experiments with different energies was completed and analyzed, first two deuteron experiments were done.
Systematic comparison of experimental data was done (integral neutron production and its spatial distribution) , dependencies on beam energy were analyzed, comparison with lead target results
Systematic comparison of experimental data with MCNPX simulations

3. Experiments Ed = 1.6 GeV = 0.8 GeV/nucleon Simulations
Proton systematic:
Ep = 0.7 GeV
Ep = 1.0 GeV
Ep = 1.5 GeV
Ep = 2.0 GeV
Deuteron systematic:
Ed = 2.52 GeV = 1.26 GeV/nucleon
Ed = 1.6 GeV = 0.8 GeV/nucleon
Beam integral: 0.6 – 3.4·1013 protons or deuterons, irradiations - hours
Spatial distribution of neutron field ( different threshold reactions)
Reactions with thresholds from 6 MeV up to 46 MeV
Simulations
MCNPX code – Bertini, CEM, Isabel cascade model, INCL4
Used versions: MCNPX 2.6.C

4. The homogenous field of neutrons with energy 1 eV – 0.1 MeV
Shielding box with polyethylene (the Cd layer is used for thermal neutrons absorption)
The homogenous field of neutrons with energy 1 eV – 0.1 MeV
is produced inside container
Moderation – many times scattered neutrons → direction information is loosed
Dependence mainly on integral number of neutrons escaping target blanket set-up
Reaction 197Au(n,γ)198Au – only by moderated neutrons from container
Container with polyethylene:
size 100106111 cm3 weight 950 kg
Cd layer at inner walls – 1 mm thickness
Example of simulated (MCNPX) neutron spectra inside
shielding container with set-up “Energy + transmutation”
(spectrum on the top of U blanket 11 cm from the front)

5. Gold foils - 198Au production inside polyethylene shielding
Small changes with position
near the center – the best situation
We use gold foils
Determination of integral number of produced neutrons:
Similar to water bath method in novel variant (K. van der Meer: NIM B217 (2004) 202)
(determination of ratio between experimental and simulated data for different foils)
Experimental integral neutron number = obtained ratio  simulated integral number of neutrons
EPT set-up inside (Ep = 1.5 GeV)
Simple lead target inside (Ep = GeV)

6. Neutron production on lead target – dependency on target sizes
(MCNPX simulations)
σTOT (p+Pb) ~ 1.5 b → L = 100 cm → 0.7 %
L = 50 cm E = 1 GeV
Saturation – for lower beam energy done by ionization stopping
- for higher energy done by loose of protons by nuclear reactions
Such experimental
dependencies
A. Letourneau et al:
NIM B170(2000)299
(Ep=0.4, 0.8, 1.2, 1.8, 2.5 GeV)
R = 7.5 cm
R = 5 cm

7. Systematization of experimental data for lead target
Overview of experimental lead target results K. van der Meer: NIM B217 (2004) 202
(main part of used lead targets have R ~ 5 cm)
Simulations (MCNPX 2.6.C) of integral neutron production on “usual” (R = 5cm, L = 100 cm) target and target with saturated neutron production
Using MCNPX calculation we recalculated experimental results on the same target size:
(correction are usually only a few percent, exception are only data of Vasilkov with very large target)

8. Dependency of integral neutron number on beam energy
Our simple lead target result
Beam energy: < 1 GeV good description using MCNPX
> 1 GeV overestimation using MCNPX
Simulation/Experiment: GeV – 1.01; 1.0 GeV – 1.13; 2.0 GeV – 1.15, 3.0 GeV

9. EPT set-up – lead plus uranium
Strong influence of neutron capture
For some diameter maximal number of escaping neutrons, for larger target decreasing number of escaping neutrons
Maximal number of escaped neutrons from target for R = 20 cm, L = 150 cm

10. EPT set-up – dependency of integral neutron number on beam energy
More or less good description of integral neutron production by MCNPX simulation
Clearly visible is saturation of number of neutrons per energy unit near 1 GeV proton energy (energy per nucleon)
Beam energy/nucleon
Beam energy per particle

11. High energy neutrons – threshold neutron reactions
197Au(n,4n)194Au ETHR=24.5 MeV
Normalized to this foils
We see clear dependence of MCNPX description quality on beam energy

12. Possible source of experiment simulation differences
Neutron energy spectra for different beam energy
(longitudinal distance , radial distance 3 cm)
Higher beam energy → bigger contribution of neutrons with energy 7 MeV – 60 MeV

13. Conclusions and outlooks
EPT set-up and JINR Dubna accelerators are nice tools for ADTT benchmark experiments
Higher energy (E > 0.5 MeV) neutron background is suppressed but low energy neutron background is produced by shielding container → study of low energy neutron production is possible only without shielding container
Low energy neutrons are produced by thermal and resonance region. Inside container homogenous neutron field is produced. It is possible to use it for integral number of produced neutron determination.
Our obtained systematic for EPT set-up is possible to compare with systematic obtained for simple lead target.
Spatial distribution of high energy neutrons is also described by simulation qualitatively quit well, but there are quantitative differences. We see clear dependence of description quality on beam energy.
Low energy deuteron experiments (see O. Svoboda talk) are consistent with our proton beam data. We are waiting for first higher energy (4 GeV) deuteron experiment next month.
Experiments collected nice set of data for systematic benchmark comparison

14. The Proposal of High-energy Neutron Cross-section Measurements at TSL in Uppsala
Significant voids in the cross-section libraries of (n,xn)-reactions in many materials.
For example gold:
only (n,2n)-reaction was measured in detail,
(n,4n) reaction was measured only for energies < 40 MeV. Other
(n,xn) reactions with x > 4 were not studied at all
We use activation foils from Au, Bi, In, and Ta
Neutron beam at TSL in Uppsala is quasi-monoenergetic in the MeV range (standard energies 11, 22, 47, 95, 143, 174 MeV).
The neutron flux density is up to 5 105cm-2.s-1. About the half of intensity is in the peak
with FWHM ~ 2-6 MeV
measurements of cross-sections of (n,xn)-reactions (with x up to 9)
Proposal was sent to EFNUDAT PAC for October meeting