Tests of a solution-grown stilbene scintillator in mono-energetic neutron beams of 565 keV and 5 MeV

Abstract

The results of measurements performed with a solution-grown stilbene scintillator placed in reference mono-energetic neutron fields are presented. The $\varnothing$ 25 mm organic scintillator was positioned in 5 MeV and 565 keV neutron fields delivered by the AIFIRA facility at CENBG. The goal of the experiment was to assess the performance of the solution-grown stilbene crystal (n-$\mathrm{\gamma }$ discrimination, response, anisotropy, sensitivity) relative to that of a BC501A liquid scintillator of larger size. Neutron pulse height spectra after gamma discrimination are compared. The results show that the stilbene crystal not only has a better discrimination capability than the BC501A (35% higher FoM) at 5 MeV, but is also able to separate neutrons from gamma-rays at 565 keV and below, a range where the BC501A is inoperative. This study also confirms the anisotropy of the crystal response, as already observed by other groups at different energies.

Introduction

The development of detectors capable of measuring the continuous-energy neutron spectrum in the intermediate-to-fast range (5 keV to 15 MeV) is of interest in numerous applications. Accurate neutron spectrum measurements in the intermediate range (5 keV to $\sim$600 keV) in mixed neutron–gamma-rays are notoriously difficult. The present work is motivated by the development of a fast neutron spectrometer for the MASURCA critical facility [1], [2], but it is also relevant to applications such as steady-state and pulsed neutron beam characterization, prompt and delayed fission neutrons in nuclear physics, fission and fusion reactors, in-core and out-of-core (neutron leakage) spectra in low-flux critical experiments, transmission and shielding benchmarks, neutron-induced material irradiations, detector calibration, neutron radiography, nuclear safeguards, etc. Among the aforementioned, measurements of the prompt fission neutron spectrum away from the 1 MeV peak have received considerable attention in recent years, since such data have major repercussion on fission models and applications [3], [4], [5], [6], [7].

The specific needs are for detectors having a reasonably good energy resolution (<10%) so as to distinguish broad peaks and valleys in the spectra, a small size for good spatial resolution (less than 50 mm), sufficient sensitivity and high efficiency so as to operate in low flux levels, able to discriminate neutrons from gamma-rays, reliable without requiring frequent calibrations or complicated correction factors, easy to handle and operate. Such neutron detectors would nicely complement classical low-resolution techniques, which rely on small foil activation and ionization chamber measurements, in combination with calculations, to infer detailed neutron energy distributions.

Organic scintillators are good candidates for such a purpose, being easy to set up and use, and representing a good compromise between detection efficiency, energy resolution and $n-\gamma$ discrimination capability. Liquid scintillators, such as BC501A, are generally considered reference detectors for neutron spectrometry in mixed radiations fields [8], but alternatives exist, in particular solid scintillators based on a plastic matrix or on stilbene crystals [9], [10], [11]. Among the latter, melt-grown-type stilbene single crystals have been used for measuring the outgoing neutron and gamma fields in transmission experiments [12], and for in-core and out-of-core neutron spectrum characterization in low-flux research reactors [13], [14]. The possibility of employing a single type of detector to form a spectrometric system covering a wide energy domain is an attractive feature of stilbene. In [15], it is stated that a set of stilbene crystals (with sizes varying from 20 mm $\times$ 10 mm to 40 mm $\times$ 40 mm) could be used for covering the neutron energy range between 0.4 and 20 MeV, and in [12] a similar set is used together with a lead cover for the 0.1–16 MeV energy domain.

Fairly recently, a promising new type of stilbene crystals has been produced by a different technique: solution-grown stilbene [16], [17]. Thanks to a more homogeneous structure, these crystals are expected to deliver an even greater light output and higher PSD FoM (Pulse Shape Discrimination Figure of Merit: 30%–40%) than liquid and classical melt-grown stilbene. However, possible limitations could arise from the anisotropy of their response [18], [19], [20], [21].

In this paper, the results of tests performed with a $\varnothing$ 25 mm solution-grown stilbene detector at reference mono-energetic neutron fields are reported. The aim of these tests was to assess the properties of this novel crystal scintillator compared to those of a well-known BC501A liquid scintillator.