Recoil Proton Telescopes and Parallel Plate Avalanche Counters for the ²³⁵U(n,f) cross section measurement relative to H(n,n)H between 10 and 450 MeV neutron energy

Abstract

With the aim of measuring the 235U(n,f) cross section at the n_TOF facility at CERN over a wide neutron energy range, a detection system consisting of two fission detectors and three detectors for neutron flux determination was realized. The neutron flux detectors are Recoil Proton Telescopes (RPTs), based on scintillators and solid state detectors, conceived to detect recoil protons from the neutron-proton elastic scattering reaction. This system, along with a fission chamber and an array of parallel plate avalanche counters for fission event detection, was installed for the measurement at the n_TOF facility in 2018, at CERN. An overview of the performances of two RPTs — especially developed for this measurement — and of the parallel plate avalanche counters are described in this article. In particular, the characterization in terms of detection efficiency by Monte Carlo simulations and response to neutron beam, the study of the background, dead time correction and characterization of the samples, are reported. The results of the present investigation show that the performances of these detectors are suitable for accurate measurements of fission reaction cross sections in the range from 10 to 450 MeV.

A detector system for `absolute' measurements of fission cross sections at n_TOF in the energy range below 200 MeV

“…An important aspect for the selection of the proton emission angle is the overlap of protons from reactions in carbon nuclei with the recoil proton. This is illustrated in figure 2 which shows the maximum proton energy from 12 C(n,p) 12 B and n-p scattering for various emission angles. Only for emission angles smaller than 15 degrees it is possible to avoid interference of the two proton energy distributions for neutron energies up to about 200 MeV.…”

“…high-energy experiment is reported in a second paper [12] and the results of the measurements will be reported in forthcoming publications.…”

“…The beam profile and its energy dependence were measured with the PPAC. The details are presented in [12], but to summarize briefly, the beam profile followed a bell-shaped 'flat top' function, with the flat top becoming larger with increasing energy, and a FWHM ranging from 1.6 cm to 1.7 cm. The data acquisition system (DAQ) at n_TOF is based on SP Devices ADQ412-DC and ADQ14-DC-4C digitizers, with 12-bit and 14-bit resolution and variable sampling rates, set to 1 GS/s for this measurement.…”

“…Neutron-induced reactions on the carbon nuclei in the PE targets of the RPTs will produce a background of protons and deuterons from 12 C(n,px) and 12 C(n,dx) reactions. Deuterons can be discriminated using the differential energy losses in the transmission detectors, i.e.…”

“…where 𝑟 U is the nominal radius of the deposits. The neutron beam profile 𝜙(𝑥, 𝑦) was measured with the PPAC detector [12], while the alignment of the samples with respect to the axis of the beam was carried out using Gafchromic films. The uncertainty on 𝑘 U was determined by varying the position of the neutron beam relative to the sample, and trying different parametrizations for 𝜙(𝑥, 𝑦) (as for example the parametrizations obtained for neutron energies below 30 MeV).…”