Response of CsI[Na] to nuclear recoils: Impact on coherent elastic neutrino-nucleus scattering

A new measurement of the quenching factor for low-energy nuclear recoils in CsI[Na] is presented. Past measurements are revisited, identifying and correcting several systematic effects. The resulting global data are well described by a physics-based model for the generation of scintillation by ions in this material, in agreement with phenomenological considerations. The uncertainty in the new model is reduced by a factor of four with respect to an energy-independent quenching factor initially adopted as a compromise by the COHERENT Collaboration. A significantly improved agreement with Standard Model predictions for the first measurement of coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\nu \mathrm{NS}$) is generated. We emphasize the critical impact of the quenching factor on the search for new physics via $\mathrm{CE}\nu \mathrm{NS}$ experiments.

Figure 1

(a) Quenching factor suggested for CsI[Na] as part of the first $\mathrm{CE}\nu \mathrm{NS}$ observation [3]. An unphysical energy-independent behavior was adopted to accommodate the large dispersion in calibration data available at the time [5, 7, 14, 15], visible in the figure. The resulting $1-\sigma$ uncertainty is shown as a grayed band. (b) Present global data and physics-based QF model developed in Sec. 5 (dotted line). The inset expands the $\mathrm{CE}\nu \mathrm{NS}$ ROI for CsI[Na] at a spallation source. Horizontal error bars are removed for clarity.

Figure 2

Data quality in the new QF measurement for cesium and iodine recoils in CsI[Na], illustrated here for the 65° neutron scattering angle. Prompt coincidences between the CsI[Na] crystal under 2.24 MeV neutron irradiation and the Bicron 501A backing detector occur at $\sim 220\mathrm{ns}$ in the horizontal time scale [6]. The spilling of elastic scattering events to later times is due to a $\sim 600\mathrm{ns}$ CsI[Na] scintillation decay constant [4] and the limited light yield ($15.4\mathrm{PE}/\mathrm{keV}$) involved. Events from neutron capture, inelastic scattering, and afterglow comprised of up to a few PE, are also indicated.

Figure 3

Measured energy for NRs from neutron scattering. Labels state scattering angle, distance between CsI[Na] and backing detector, exposure time to the D-D generator, simulated mean NR energy $⟨E_{\mathrm{rec}}⟩$, and best-fit QF obtained from the analysis (see text). The decrease in event rate with larger angle is characteristic of forward-peaked elastic scatters.

Figure 4

Comparison of NR light yield obtained for the smallest scattering angle tested, and its best-fit simulated prediction. A dotted line shows the triggering efficiency of the calibration setup, derived as in [4, 6]. A correction based on this efficiency is applied to the data points shown, prior to comparison to simulations. A greyed band in the inset shows the $1-\sigma$ uncertainty in the best-fit QF value (see text).

Figure 5

Tests of UBA PMT saturation effect under ${}^{241}\mathrm{Am}$ irradiation of a CsI[Na] crystal. The operating bias used in four QF calibrations described in the text is indicated by labeled arrows. The top axis shows PMT gain, derived from manufacturer specifications. The low-bias range of these measurements was defined by the ability to clearly separate unamplified SPEs from PMT noise. A logistic function is used to fit the data (solid lines). Error bars merge the uncertainties from fits to SPE and ${}^{241}\mathrm{Am}$ charge distributions (see text).

Figure 6

COHERENT CsI[Na] $\mathrm{CE}\nu \mathrm{NS}$ signal (data points, [3]) projected on energy and time after SNS proton spill, compared to previous SM predictions (dotted histogram) and a revision using the new QF model presented in this work (solid histogram). An improved agreement with the SM is obtained, with much smaller uncertainty in signal prediction (see text).

Figure 7

$\mathrm{CE}\nu \mathrm{NS}$ signals on germanium at a spallation source. A vertical line shows the best contemporary threshold for 1 kg PPCs [53]. Red traces correspond to ${\mu }_{{\nu }_{\mu }}=5\times {10}^{-10}{\mu }_{\mathrm{Bohr}}$, beyond the present limit of ${\mu }_{{\nu }_{\mu }}<6.8\times {10}^{-10}{\mu }_{\mathrm{Bohr}}$ [54]. Blue lines assume zero neutrino magnetic moment. Solid lines correspond to a Lindhard QF model with free parameter $k=0.18$ (inset, [48]). Dotted lines add an adiabatic factor (Sec. 5, $E_0=0.5\mathrm{keV}$) to a $k=0.19$ Lindhard model. The degeneracy of possible interpretations for hypothetical data following the two overlapping lines is evident: new physics could be erroneously claimed with high confidence, or missed while blatantly present, depending on the QF model settled upon.