Experimental Studies of Spatial Distributions of Neutrons Produced by Set-ups with Thick Lead Target Irradiated by Relativistic Protons

Vladimír Wagner

Nuclear physics institute of CAS, 250 68 Řež, Czech Republic, E_mail: wagner@ujf.cas.cz

for collaboration “Energy plus transmutation”

(Russia, Belarus, Germany, Greece, Poland, Ukraine, Czech Republic …)

1. Introduction

2. Main tasks 2.1 Measurement of neutron field 2.2 Studies of transmutation 2.3 Benchmark studies 2.4 Simulation codes

3. Experiments 3.1 Different experimental set-ups 3.2 Experimental methods 3.3 Experimental results

4. Comparison between experiment and simulation 4.1 Changes of neutron spectra 4.2 Spatial distributions of neutrons

5. Conclusions and outlooks

XVII INTERNATIONAL BALDIN SEMINAR ON HIGH ENERGY PHYSICS PROBLEMS

Main tasks

1) Measurement of neutron field around and inside simple and more complicated set-ups irradiated by relativistic protons:

a) simple thick targets

b) target with moderator around

c) target with uranium blanket

2) Study of the transmutation of different radioactive samples (from radioactive waste) by neutrons with different energy from different set-ups

3) Comparison of obtained collection of data for systematic set of proton energies with different model simulation - benchmark

Used simulation codes 1) LAHET + MCNP

LAHET {Los Alamos High Energy Transport} - spallation reactions, transport of particles and high energy neutrons

MCNP {Monte Carlo Code for Neutron and Photon Transport} – low energy neutron ( En < 20 MeV) transport calculation

2) MCNPX {Monte Carlo N-Particle Transport Code} – LAHET and MCNP, for neutrons up to 150 MeV libraries are used

Used versions: LAHET2.7 a MCNP4A

1) Calculation of neutron (proton) field by LAHET+MCNP or MCNPX

Two steps of calculations:

2) Calculation of produced nuclei numbers using neutron cross sections from evaluated libraries, experimental data ( En < 20 MeV (150 MeV)) < or LAHET calculations

Used versions: MCNPX 2.3.0.

Experiments

Used accelerators (JINR Dubna):

1) Synchrohasotron (VBLHE) – advantage – wide spectrum of possible energies Ep = 500 MeV až 7 GeV, 1012 – 1013 protons per hours

2) Nuclotron (VBLHE) – advantage – wide spectrum of possible energies

Ep = 500 MeV až 5 GeV, strong focusing, 1012 – 1013 protons per hours

3) Phasotron (DLNP) – proton energy 660 MeV, advantage: high beam

intensity I = 1 μA (1015 – 1016 protons per minutes) → short irradiation time, possibility to measure very short radioisotopes

Used set-ups

1) Simple thick targets (Pb, W):Tungstem target: diameter 2cm, length 60cm Ep = 1.5 GeVLead target: diameter 9.6 cm, length 50cm Ep= 0.66, 0.885, 1.3, 1.5 and 2.5 GeV

3) Complex set-up (Energy plus transmutation):

2) Thick target with moderator (paraffin)(GAMMA-2): Lead target: length 20 cm, around paraffine moderator

Lead target: diameter 8.4 cm, length 48 cmNatural uranium blanket: rods with Al cladding total weight 206.4 kg Ep = 0.7, 1.0, 1.5 and 2.0 GeV

Shielding box with polyethylene (the Cd layer is used for thermal neutrons absorption)

1) Advantages – better “dosimetric” situation, shielding from scattered “higher” energy neutrons (En > 0.5 MeV)

2) Disadvantage – 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 plus transmutation”(spectrum on the top of U blanket 11 cm from the front)

Container with polyethylene:

size 100106111 cm3 weight 950 kg Cd layer at inner walls – 1 mm thickness

Activation and radiochemical method Determination of neutron field by activation detectors:used foils: Al, Au, Bi, Co, Cu and La sampleAdvantage: small size, simple

Examples of threshold reactions: 197Au(n,2n)196Au, 197Au(n,4n)194Au,27Al(n,α)24Na,

209Bi(n,4n)206Bi, 209Bi(n,5n)205Bi, 209Bi(n,6n)204Bi 209Bi(n,7n)203Bi

Gamma activity is measured by HPGe detectors:

Determination of transmutation by radiochemical method:measured samples: I, Ra, Pu

Changes of spectra along the target

Ratio between production rates near to the front of the target and on the end of targetas function of reaction threshold energy

Neutron energy spectra for different positions x along the target

Parts of the neutron spectrum producing given isotope The same for radial distribution

asymmetricalproduction

High energyneutrons:

(example – experiment with Energy plus transmutation set-up, Ep = 1.5 GeV)

Comparison between experiment and simulation Low energy neutrons:

Set-up “Energy plus transmutation”with shielding container

Set-up “GAMMA-2” (lead target plus paraffin) without shielding containerCross section of 139La(n,γ)140La

(example – experiments with Ep = 1.5 GeV)

Position 11 cm from the front of the target

High energy neutrons (En > ~ 1 MeV):

Simple lead target – spatial distribution of neutrons

Influence of protons – necessity to know beam geometryand sizes ( diameter ~ 4 cm)

Example: experiment with Ep = 885 MeV

Good agreement, difference starts only

from position 40 cm

Looks that simulation under predicts shower development

Comparison of experiment and simulation – set-up “Energy and transmutation:

Radial distribution of 194Au productionat Au foil (distance from target front 11.8 cm)

Longitudinal distribution of production of 194Au (radial distance 3 cm)

Example – experiment with proton energy 1.5 GeV

Necessity to describe also influence of protons which partly interact with our foils.

Qualitative agreement but quantitative differences

Experimental decreasing of neutron intensity is slower for radial distribution

same point

Conclusions and outlooks

• 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 and it is good agreement between experimental and simulated form of spatial distributions along the set-up

• Spatial distribution of high energy neutrons is also described by simulation qualitatively quit well, but there are quantitative differences

• There are signs about under prediction of shower developed inside lead target

• Experiments collected nice set of data for systematic benchmark comparison

• It is necessary to analyze these data as soon as possible