Optimization of the thickness of a ZnS/⁶LiF scintillator for a high-resolution detector installed on a focusing small-angle neutron scattering spectrometer (SANS-U)

This study involves the upgrade of a high-resolution position-sensitive detector (HR-PSD) installed on the small-angle neutron scattering spectrometer (SANS-U) at the Japan Atomic Energy Agency. By using both neutron lenses and the HR-PSD, the accessible low-Q limit can be extended to the order of 10⁻⁴ Å⁻¹ [Q is the magnitude of the scattering vector defined by Q = (4π/λ)sinθ, where λ and 2θ are the wavelength and the scattering angle, respectively]. The HR-PSD consists of a cross-wired position-sensitive photomultiplier tube (PSPMT) and a commercial ZnS/⁶LiF scintillator. To improve the experimental efficiency of focusing small-angle neutron scattering (FSANS) experiments, a high-performance ZnS/⁶LiF scintillator developed at the Japan Atomic Energy Agency has been utilized. For the PSPMT and data-acquisition system installed on SANS-U, the thickness of the ZnS/⁶LiF scintillator was optimized by measuring the thickness dependence of the pulse-height spectra. Under the experimental conditions of SANS-U, the optimum thickness of the ZnS/⁶LiF scintillator (ZnS:⁶LiF = 2:1) was determined to be 0.433 mm by measuring the total counts and peak positions of the pulse-height spectra. Installation of the optimized ZnS/⁶LiF scintillator improved detection efficiency by 1.39 times over that of a commercial scintillator at the same level of background counts and Q resolution in FSANS experiments.