{DEMAND}: {A}n organic glass scintillator array for direct measurements of $(α,n)$ and $(p,n)$ reactions in inverse kinematics

Abstract

We have developed an array of up to 20 organic glass scintillator detectors, each consisting of a $3~\mathrm{cm}\times3~\mathrm{cm}\times3~\mathrm{cm}$ scintillator cube coupled to a photomultiplier tube. Sensitive to neutrons from a few hundred keV to several tens of MeV, the array was designed for use with recoil separators in inverse-kinematics measurements of astrophysical $(α,n)$ and $(p,n)$ reactions. It provides a prompt start signal for identifying recoil-neutron coincidences from the recoil time of flight. We characterize eight detectors used in an inverse-kinematics measurement of $^{22}\mathrm{Ne}(α,n)^{25}\mathrm{Mg}$, using source and in-beam data. From the widths of $γ$-ray Compton-edge peaks at 311, 341, 477, and 1062 keVee, we find an energy resolution described by $\mathrm{FWHM}(E)/E=\sqrt{A^2/E+B^2/E^2+C^2}$, with $A=0.00(7)$, $B=0.0618(54)$, and $C=0.135(5)$. The time-difference spectrum of nearest-neighbor $γ$-$γ$ coincidences from a $^{22}\mathrm{Na}$ source (250–600 keVee) gives a FWHM coincidence timing resolution of $910(40)$ ps, corresponding to a single-detector resolution of $640(30)$ ps. Mixed neutron-$γ$ fields from the in-beam reaction and a $^{241}\mathrm{AmBe}$ source show pulse-shape discrimination with an $n$-$γ$ figure of merit $>1$ down to about 100 keVee. Below 100 keVee, the total neutron yield can still be extracted using two-Gaussian fits to energy-gated pulse-shape discrimination spectra. Monte Carlo simulations of the present $^{22}\mathrm{Ne}(α,n)^{25}\mathrm{Mg}$ measurement give a neutron singles detection efficiency of $1.56(45)$, with uncertainty dominated by systematic effects in the simulation inputs.