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 NEMEA-4 Workshop October 16-18, 2007 Prague, Czech Republic

1)Understanding of sources of experimental data uncertainties – set of simulations of our set-up using MCNPX code. 2)Set of proton experiments with different energies was completed and analyzed, first two deuteron experiments were done. 3)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 4)Systematic comparison of experimental data with MCNPX simulations Our main objectives: Neutron distribution studies – radiation samples 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 Energy plus Transmutation (EPT) Setup Results:

Proton systematic: E p = 0.7 GeV E p = 1.0 GeV E p = 1.5 GeV E p = 2.0 GeV Deuteron systematic: E d = 2.52 GeV = 1.26 GeV/nucleon E d = 1.6 GeV = 0.8 GeV/nucleon Experiments Beam integral: 0.6 – 3.4·10 13 protons or deuterons, irradiations - hours Reactions with thresholds from 6 MeV up to 46 MeV Spatial distribution of neutron field ( different threshold reactions) Simulations MCNPX code – Bertini, CEM, Isabel cascade model, INCL4 Used versions: MCNPX 2.6.C

The homogenous field of neutrons with energy 1 eV – 0.1 MeV is produced inside container 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) Container with polyethylene: size 100  106  111 cm 3 weight 950 kg Cd layer at inner walls – 1 mm thickness Reaction 197 Au(n,γ) 198 Au – only by moderated neutrons from container Dependence mainly on integral number of neutrons escaping target blanket set-up Moderation – many times scattered neutrons → direction information is loosed Shielding box with polyethylene (the Cd layer is used for thermal neutrons absorption)

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

Neutron production on lead target – dependency on target sizes R = 5 cm 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 (E p =0.4, 0.8, 1.2, 1.8, 2.5 GeV) R = 7.5 cm (MCNPX simulations) σ TOT (p+Pb) ~ 1.5 b → L = 100 cm → 0.7 %

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)

Dependency of integral neutron number on beam energy Beam energy: < 1 GeV good description using MCNPX > 1 GeV overestimation using MCNPX Simulation/Experiment: 0.5 GeV – 1.01; 1.0 GeV – 1.13; 2.0 GeV – 1.15, 3.0 GeV Our simple lead target result

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

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

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

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 Possible source of experiment simulation differences

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

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). measurements of cross-sections of (n,xn)-reactions (with x up to 9) The neutron flux density is up to cm -2.s -1. About the half of intensity is in the peak with FWHM ~ 2-6 MeV Proposal was sent to EFNUDAT PAC for October meeting