Liquid scintillator response to proton recoils in the 10--100 keV range

Liquid scintillator response to proton recoils in the 10–100 keV range

C. Awe, P. S. Barbeau, J. I. Collar, S. Hedges, and L. Li

Phys. Rev. C 98, 045802 – Published 5 October 2018

Abstract

We study the response of EJ-301 liquid scintillator to monochromatic $244.6 \pm 8.4$ keV neutrons, targeting the 10–100 keV proton recoil energy interval. Limited experimental information exists for proton light yield in this range, for this or any other organic scintillator. Our results confirm the adequacy of a modified Birks' model, common to all organic scintillator formulations, predicting a marked increase in quenching factor as proton energy approaches the few keV regime. The relevance of this behavior within the context of searches for low-mass particle dark matter is mentioned.

Received 9 May 2018

DOI:https://doi.org/10.1103/PhysRevC.98.045802

Physics Subject Headings (PhySH)

Neutron physics Nuclear astrophysics Nucleus-neutrino interactions

Radiation detectors

Nuclear Physics

Authors & Affiliations

C. Awe¹, P. S. Barbeau¹, J. I. Collar^2,*, S. Hedges¹, and L. Li¹

¹Department of Physics, and Triangle Universities Nuclear Laboratory, Duke University, Durham, North Carolina 27708, USA
²Enrico Fermi Institute, Kavli Institute for Cosmological Physics, and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA

^*Corresponding author: collar@uchicago.edu

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Vol. 98, Iss. 4 — October 2018

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Images

Figure 1
Experimental arrangement, with all dimensions to scale, derived from the MCNP-PoliMi simulation geometry. The scattering angle $θ$ and distance $L$ between the ${}^{6}{Li} I [Eu]$ backing detector and EJ-301 are indicated. A cross section of the small EJ-301 liquid scintillator cell is also shown.
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Figure 2
TOF for gammas (left peaks) and neutrons (right peaks) produced in the lithium foil, arriving at the plastic scintillator placed in the forward ( $θ = 0^{\circ}$ ) direction. TOF is relative to a logic signal in phase with POT pulses. Two distances $d$ between foil and scintillator were tested. For each, neutron velocity $v$ can be extracted from the expression $d / v - d / c = Δ t$ , where $c$ is the speed of light and $Δ t$ is the $n − γ$ difference in TOF. Both measurements agree within 1%, yielding an average neutron kinetic energy of $244.6 \pm 8.4$ keV.
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Figure 3
Signals in EJ-301, time-referenced to the onset of signals in the backing detector. The horizontal coordinate shows the onset of EJ-301 light emission, and the vertical its light yield integrated over 80 ns. As in Fig. 2, a time difference of $\approx 90$ ns between the indicated $n$ and $γ$ populations matches the expected 245 keV neutron TOF over the distance $L = 60$ cm between detectors. The $γ$ offset with respect to $t = 0$ is due to a rise time to reach an analysis threshold for ${}^{6}{Li} I [Eu]$ signals. Variations by $\approx 20$ ns in the position of $n$ signals were observed for different values of $θ$ , as expected from neutron energy losses in EJ-301 (Table 1).
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Figure 4
Background-subtracted residual spectra from proton recoils in EJ-301. The electron-equivalent energy scale, defined via ${}^{241}{Am}$ gamma calibration, is 2.1 PE/keV. Solid histograms correspond to simulated spectra for the best-fit QF values found. The sensitivity of these fits is illustrated by additional dashed greyed histograms at $θ = 15^{\circ}$ , $23^{\circ}$ , and $30 . 2^{\circ}$ . Those correspond to QF = 11.3%, 11.4%, 7.2%, whereas the best-fit values are 12.3%, 10.4%, 8.2%, respectively.
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Figure 5
Comparison of experimental and simulated proton recoil rates in EJ-301. Experimental rates are normalized to the indicated reference proton current delivered to the lithiated foil, using charge, number of events, and exposure times in Table 1. Simulated rates are for the best-fit neutron flux at the position of the EJ-301, stated in the label.
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Figure 6
Low-energy quenching factors for proton recoils in organic scintillators [10, 11, 29, 30, 31], including present results. A dotted black line represents the modified Lindhard model proposed in [11], and adopted by the COHERENT Collaboration for neutron background studies in [15]. Proton recoil light yields from [11] are converted here to a quenching factor via a 3 eV mean photon energy for NE-110 scintillation [10] and a 9.2 photon/keV scintillation light yield for ERs in this material [32].
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