Geant4 simulation of a Proton Recoil Telescope for the measurement of the 235U(n,f) cross section up to 1 GeV at n_TOF

L. Damone1,3, R. Baccomi5, N. Colonna3, L. Cosentino6, P. Finocchiaro6, S. Lo Meo8, A. Manna2,7, C. Massimi2,7, P. Milazzo5, L. Rinaldi5, G. Vannini2,7, A. Ventura2,7

(The n_TOF Collaboration (www.cern.ch/ntof))1 Department of Physics M. Merlin, Università degli studi di Bari “Aldo Moro “, 2 Department of Physics and Astronomy, Alma mater studiorum Università di Bologna, 3 INFN Bari, Italy, 4 INFN Bologna, Italy, 5 INFN

Trieste, Italy, 6 INFN - Laboratori Nazionali del Sud, Catania, Italy, 7 INFN Bologna, Italy 8 ENEA, Bologna, Italy

CONCLUSIONSThe measurement is challenging (especially close to 1 GeV) but

feasible. Simulations indicate that at high energies a large background due to interactions of neutrons with the carbon in the polyethilenetarget is present. However, this can be measured and subtracted.

A prototype has been built and tested at n_TOF. A first measurementshould take place in 2017.

Scientific motivation

The 235U(n,f) cross section is one of the most important cross-sectionstandards used as reference in many fields. In particular, it is commonlyused to measure the neutron flux in reactors, and in various neutronfacilities worldwide, including n_TOF.The 235U(n,f) cross section is a standard at thermal neutron energy (25meV) and between 0.15 MeV and 200 MeV.

Despite the increasingimportance of the high-energy region, atpresent no data existon the 235U(n,f)reaction above 200MeV, and one has torely on highly uncertaintheoretical estimates.

Figure 1: The 235U(n, f) cross section above 200 MeV predicted byvarious libraries and theoretical calculations.

n_TOF at CERN is the onlyneutron facility in the worldwhere this measurement canbe performed, thanks to a veryhigh flux and a neutronspectrum that extends uo to 1GeV.

Figure 2: Neutron flux in EAR1 and EAR2 at the n_TOF facility atCERN.

The Proton Recoil Telescope

nH+C

p

The setup consists of a targetmade of hydrogenousmaterial (polyethilene) and aProton Recoil Telescope, todetect and identify therecoiling protons from the n–p reaction.

1. Neutrons scattered from the targetcan undergo a n-p reaction in thedetector;

1. neutron interaction with Carbon in thetarget produces protons and otherlight charged particles.

n n

p

n

np

n

GEANT4 Simulations of the Proton Recoil Telescope

Several telescope configurations have beensimulated. The chosen setup for thesimulations consists of four Silicondetectors, and four fast plasticscintillators to identify particles :

For a well defined solid angle, a trapezoidalshape has to be used for the scintillators

Proton

Deuterontriton

n

Figure 3: Schematic drawing of the PRT, as simulated inGEANT4. The telescope is mounted at an angle of 20 degreesrelative to the neutron beam direction.

Figure 4: E-∆E spectrum for ìneutron of E=250 MeV impinging on the Poliethylene radiator.

The chosen configuration is able to stopprotons only up to 150 MeV.Nevertheless, higher energy protonsfrom the n-p reaction can still beidentified and separated from thebackground caused by neutronreactions with carbon in thepolyethylene target.

According to GEANT4 simulations, with aselection in the E-∆E plot the backgroundrelated to neutrons scattered in thescintillators can be reduced to a fewpercent only

.Figure 5: Signal/background ratio as a function of neutronenergy with and without cuts on the E-∆E plots, for neutronsinteractions in the scintillators.

Figure 6: Background percentage of neutrons interaction with thecarbon in the polyethylene radiator.

We studied with Geant4 simulations the best configuration forthis measurement.

CH2 radiator

CH2 radiator

Fast plastic scintillators

A measurement of the 235U(n,f) cross section above 200 MeV is “urgently”needed, according to a pressing request from IAEA.

The 235U(n,f) cross section will be measured relative to the elasticneutron–proton scattering (n–p reaction), the best known and generallyaccepted primary reference at high energy.

The main issue in this measurement is the background. There are twopossible sources of background, that affect the measurement.

The test run at n_TOF

Neutron-induced reactions with Carbonin the polyethilene target is a problem tobe reckoned with. Even with the E-∆Eselection, the background at highenergy accounts for 60% of the events.However, this background componentcan be measured with a Carbon target,and subtracted.

A prototype of the Proton Recoil Telescope has beenbuilt by the INFN team and mounted in EAR1 atn_TOF.

Data have been taken for aweek. The analysis is inprogress, but the preliminaryresults are encouraging.

Si Det. Si Det. Si Det. Si Det. Pl. Scint. Pl. Scint. Pl. Scint. Pl. Scint.

300 µm 300 µm 300 µm 200 µm 0.5 cm 3 cm 6 cm 6 cm

Polyethilenetarget Si detectors