YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources

Abstract

Recent studies indicate the resonance detector (RD) technique as an interesting approach for neutron spectroscopy in the electron volt energy region. This work summarizes the results of a series of experiments where RD consisting of YAlO 3 (YAP) scintillators were used to detect scattered neutrons with energy in the range 1 -200 eV. The response of YAP scintillators to radiative capture ␥ emission from a 238 U analyzer foil was characterized in a series of experiments performed on the VESUVIO spectrometer at the ISIS pulsed neutron source. In these experiments a biparametric data acquisition allowed the simultaneous measurements of both neutron time-of-flight and ␥ pulse height (energy) spectra. The analysis of the ␥ pulse height and neutron time of flight spectra permitted to identify and distinguish the signal and background components. These measurements showed that a significant improvement in the signal-to-background ratio can be achieved by setting a lower level discrimination on the pulse height at about 600 keV equivalent photon energy. Present results strongly indicate YAP scintillators as the ideal candidate for neutron scattering studies with epithermal neutrons at both very low ͑Ͻ5°͒ and intermediate scattering angles.

The very low angle detector for high-energy inelastic neutron scattering on the VESUVIO spectrometer

“…As shown in Ref. [19], raising the threshold above this value does not provide significant variations.…”

“…The latter relies upon two main steps: In the first one, the scattered neutron beam impinges onto the analyzer foil which provides the energy analysis by means of ðn; gÞ resonance absorption at a given resonance energy E 1 ; in the second step, the prompt gamma-rays are detected to determine the total neutron time of flight. The broad gamma-ray spectrum from the analyzer allows for the use of energy discrimination thresholds that improve the signal to background ratio (S/B), still guaranteeing an acceptable counting efficiency [19].…”

“…This results in an almost isotropic contribution. Indeed, the main gamma background source at times longer than about 100 ms is the borated wax shielding [19]. Boron ( 10 B) is commonly used in neutron shielding installations.…”

“…8 have been acquired setting a lower level discrimination (LLD) threshold of about 600 keV, thus resulting in a complete electronic rejection of these photons. The use of an LLD was found to be essential in order to improve the signal to background ratio (S/B) in the RD configuration [19,33]. Since the energy resolution of YAP scintillator is about 20% at 500 keV [34], the LLD threshold level has to be chosen at about 600 keV or higher in order to reject completely the 10 B photons.…”

“…To this aim, the experimental concept of the Resonance Detector (RD), proposed in the late 70s [13,14], has been revisited and optimized for the VESUVIO beam line [15,16] at the ISIS spallation neutron source [17]. Different types of photon detectors have been successfully tested on VESUVIO in the RD configuration for DINS measurements, at scattering angles above 201 [18][19][20]. Their use at the very small scattering angles ðp5 Þ required for high o and low q was a more recent development and required structural modifications of the original spectrometer layout.…”