Quenching factor for low-energy nuclear recoils in a plastic scintillator

Plastic scintillators are widely used in industry, medicine, and scientific research, including nuclear and particle physics. Although one of their most common applications is in neutron detection, experimental data on their response to low-energy nuclear recoils are scarce. Here, the relative scintillation efficiency for neutron-induced nuclear recoils in a polystyrene-based plastic scintillator (UPS-923A) is presented, exploring recoil energies between 125 and 850 keV. Monte Carlo simulations, incorporating light collection efficiency and energy resolution effects, are used to generate neutron scattering spectra which are matched to observed distributions of scintillation signals to parameterize the energy-dependent quenching factor. At energies above 300 keV the dependence is reasonably described using the semiempirical formulation of Birks and a $kB$ factor of ($0.014\pm 0.002$) g MeV${}^{-1}$ cm${}^{-2}$ has been determined. Below that energy, the measured quenching factor falls more steeply than predicted by the Birks formalism.

Figure 1

Energy spectrum acquired from exposure to a ${}^{137}$Cs $\gamma$-ray source. The data acquired with the CAEN acquisition (solid black spectrum) with a threshold of 5 photoelectrons is shown in comparison to the simulation data (red dashed spectrum).

Figure 2

Energy depositions in scintillator from neutron-induced nuclear recoils coming from an Am-Be source (red hatched spectrum—referring to the scale on the right). The $y$ axis on the left refers to the neutron flux from the source (black dashed spectrum) and the differential neutron spectrum when entering the scintillator bar (black solid spectrum).

Figure 3

Energy depositions in scintillator from neutron-induced nuclear recoils coming from a ${}^{252}$Cf source (red hatched spectrum—referring to the right-hand scale). The left-hand scale refers to the original neutron spectrum (black dashed spectrum) in comparison with the differential rate of neutrons entering the scintillator bar (black solid spectrum).

Figure 4

Background-corrected energy spectrum originating from irradiation with an Am-Be source (gray shaded area) in comparison with simulations using the quenching factor $Q_i$ as a constant parameter for the whole energy range. The best agreement with the real data is met by the curve featuring $Q_i=0.1$ (blue solid spectrum). The peak at $\sim$90 phe is the 2.2 MeV radiative capture $\gamma$ ray from hydrogen. The inset shows the impact of different constant quenching factors at low photoelectron values. A marked discrepancy between simulation and data suggests that an energy-dependent quenching factor may provide a better physical description for low recoil energies.

Figure 5

Background-corrected energy spectrum originating from irradiation with a ${}^{252}$Cf source (gray shaded area) in comparison with simulations using the quenching factor $Q_i$ as a constant parameter for the whole energy range and a close up of the very low-energy part of the spectrum as an inset at the top right.

Figure 6

Nuclear recoil quenching factor in polystyrene plastic scintillator UPS-923A as a function of recoil-energy deposition extracted by mean of ${\chi }^2$ minimization from comparison of simulations to data from a ${}^{252}$Cf (solid green) and an Am-Be (dashed blue) exposure, respectively. The hatched areas represent the 68$\%$ confidence limit bands (forward slashes for ${}^{252}$Cf, backslashes for Am-Be).

Figure 7

Simulations using energy-dependent value for $Q_i\left(E\right)$ in comparison with background-corrected data acquired with the CAEN analog-to-digital converter (black hatched spectrum) from irradiation of the scintillator with a ${}^{252}$Cf and Am-Be source (inset), respectively. The best fit using ${\chi }^2$ minimization is shown by the red solid histogram (squares). For comparison, the blue dot-dash spectrum (circles) shows the use of a constant quenching factor (from best fit to data).

Figure 8

Fraction of nuclear recoil energy depositions coming from carbon nuclei relative to the proton recoil contributions in the plastic scintillator averaged from exposures to both Am-Be and ${}^{252}$Cf neutron sources.

Figure 9

Fit of semiempirical calculations of Birks for combined proton and carbon stopping powers from varying the $kB$ factor (solid red) to the measured quenching factors (black with hatched error band) above 300 keV nuclear recoil energy yields: $kB=0.0135$ g MeV${}^{-1}$ cm${}^{-2}$. Additionally, curves assuming scattering off protons or off carbon nuclei only are also shown. Below this energy a clear divergence of measurement from the Birks description can be observed.