Study on the measurement of high energy neutrons using Zirconium capped CR-39 (Zr-CR-39) combined detector - Monte Carlo simulation and experimental validation

Abstract

Neutron measurement, be it dose or fluence is a challenging field in high energy accelerators where the neutron energy varies from thermal to high-energy neutrons usually extending beyond 20 MeV. CR-39 SSNTD (Solid State Nuclear Track Detector) used in neutron measurements has near flat energy response in the range of 500 keV to 10 MeV. By capping the CR-39 with a suitable converter such as Zirconium (Zr), it's response can be enhanced in the higher neutron energy ranges (beyond 10 MeV). Zr has significant (n,2n) cross-section of ∼1 b in the neutron energy range of 10–20 MeV. Hence, when high energy neutrons impinge on Zr, neutrons of lower energy are produced by (n,2n) reactions, which create additional recoil proton tracks in CR-39. At higher neutron energies (>20 MeV) additional tracks can also be created due to other charged secondaries such as deuteron, triton, alphas and recoil carbon and oxygen. The enhancement in track density of CR-39 in presence of Zr as compared to that of bare CR-39 is termed as the Total Enhancement Factor (TEF). Monte Carlo simulations with FLUKA and GEANT4 codes are used to optimize the Zr converter thickness with 14 MeV and 20 MeV, and are also verified with measurements. At an optimum converter thickness of 1 mm, FLUKA and GEANT4 Monte Carlo codes were used to observe the enhancement in the response of the combined detector (Zr-CR-39) against the bare detector for different mono-energetic neutron beams in the range 14 MeV - 1 GeV. The enhancement obtained for Zr converter are also compared with Lead (Pb) converter. Experimental studies were carried out to validate the simulations by irradiating Zr-CR-39 to 14 MeV, 20 MeV and high energy neutrons spectra available at CERF facility. Measured TEF values are 1.27, 2.5 and 7.85 for 14 MeV, 20 MeV and CERF neutrons spectra respectively. Results of simulations and measurements are found to be in agreement within 20%.