Any effort to observe the CEνNS process will share many common background concerns with rare-event searches such as attempts to observe neutrinoless double-beta decay and dark-matter interactions with nuclei. Owing to the significant interest and investment (both capital and intellectual) in rare-event searches, an
understanding of the nature of these backgrounds and techniques for mitigating their effects is well established in the literature [110, 132]. Additionally, due to the proximity to the SNS neutron-production target which is an intense source of both high-energy neutrons and moderate-energy neutrinos, there are some unique background concerns that must be considered.
5.2.1: Steady-state ambient and cosmogenic backgrounds
Though this category of backgrounds represents a wide variety of sources, the common feature is that they show no correlation with the SNS facility operation or the beam. This category encompasses radioactivity from decay or decay-related sources external to the detector; radioactivity from the same sourceswithin the detector; cosmic ray interactions within the detector; daughter particles resulting from cosmic ray interactions within the closest layers of shielding; and daughter particles from cosmic ray interactions with external materials or outer shielding layers.
These backgrounds are generally common to those of concern to other sensitive experiments and many of the same mitigation techniques can be employed [110, 132]. A high-efficiency muon-veto system, consisting of 5-cm thick plastic scintillator panels, located inside of the outer-most shielding layer but surrounding the high-Z shielding, allows for the identification and rejection of events which are possibly contaminated by µ-induced neutron production in the experiment shielding [17]. The passive components of the shielding structure, which is discussed in greater detail in Sec. 5.3, work to greatly reduce the ambient backgrounds through a combination of hydrogenous materials (efficient at moderating energetic neutrons and attenuating the total transmitted neutron flux) and lead, a high-Z material which is effective in moderating the flux of γ-rays from external sources.
5.2.2: Prompt neutrons from SNS
The SNS is described as “.. a one-of-a-kind research facility that provides the most intense pulsed neutron beams in the world..” [91], and while the ancillary neutrino output makes a CEνNS measurement realizable, the neutrons produced by the SNS could undermine the viability of any such attempt. Neutron backgrounds are of particular concern for a CEνNS measurement as energy depositions from both CEνNS and elastic neutron scattering are in the form of recoiling nuclei. This concern is shared with WIMP dark-matter searches [116], noting again the common observable in CEνNS interactions and WIMP-nucleus scattering (§1.6, §2.2).
Prompt neutrons incident on the CsI[Na] detector will have traveled out of the liquid-mercury spallation production target, through any close-proximity shielding around the target, and through ∼19 meters of a
combination of structural materials (e.g., concrete, rebar) and “backfill”, itself a combination of dirt, gravel, or rocks. At the site of the CEνNS-search, flux of low-energy neutrons from the spallation target will be strongly attenuated by the considerable amount of intervening material. However, energetic neutrons which do approach neutrino alley may interact with material near the experiment, either in the shielding, walls, or floor, for instance, and ultimately reach the CsI[Na] installation as lower energy neutrons. Prompt-neutron transport is difficult to model a priori with high accuracy.As an approximate model of the spectrum of downscattered neutrons, a power-law energy spectrum is adopted, as suggested by the spectral data from various other neutron-background measurements conducted by the COHERENT Collaboration [17], and determination of parameters for this model is discussed in Sec. 5.5.
Prompt, SNS-beam neutrons produce a background which is fundamentally very similar to ambient or cosmogenic neutron backgrounds. Though the involved energies may differ, the hydrogenous shielding elements (§5.3) should be effective to reduce the flux of prompt neutrons witnessed by the CsI[Na] detector. Since the flux is not reduced by shielding to entirely negligible levels (§5.5), a prompt-neutron feature is included in spectral models used to analyze the collected CEνNS data (§6.2.2).
5.2.3: SNS-operation-related backgrounds
Located within the area of the SNS in which the COHERENT suite of experiments are located (§2.7) is a large pipe that is a part of the SNS building and target exhaust system. This “hot off-gas” (HOG) pipe contains the flow of numerous radioisotopes produced in either the target or other systems; target gases are sent through numerous purification systems to remove mercury, noble gases, and tritium prior to exhaust through the HOG, while other systems are not subject to the same kind of prefiltering [158, 210]. With the target off-gas contaminants mitigated, much of the activity in the HOG is likely from the water cooling system [28, 158], which suggests the HOG contents feature common oxygen activation products such as11C, 13N, and15O, all of which decay viaβ+emission, yielding, ultimately, 511-keVγrays. Though some 511-keV
γflux is expected from ambient sources, the intensity of this additional source has modest variability in time and is confined to a single, extended geometry. Dosimeters situated along neutrino alley, as a part of the radiation safety mission at SNS, observe meaningfulγ-radiation dose increases during periods of SNS beam operation, and this is attributed to the HOG.
High-Zshielding meant to address genericγ-ray background sources is a standard component of sensitive- experiment shielding configurations [110, 116, 132]. As the 511-keV γ-rays from the HOG are of modest energy, the contribution from this source to collected spectra in the CEνNS search is easily mitigated and does not require special consideration or unique shielding requirements. The muon-veto systems for experiments in
neutrino alley could be sensitive to HOGγ-rays, however, and some consideration must be given when setting thresholds so that veto-system efficiency isn’t meaningfully compromised while maintaining a manageable trigger rate during SNS operation.
5.2.4: Neutrino-induced neutrons
As the most intense pulsed neutrino source in the world [99], the SNS environment is subject to a somewhat unique potential source of background events: neutrino-induced neutron production on materials near detection elements. Neutrino-induced neutrons, or NINs, were considered as a possible background during the LSND experiment [155] and have been offered as a possible explanation for the annual-modulation observed by the DAMA/LIBRA dark matter experiment [84]. Though the NIN-based explanation of the DAMA/LIBRA phenomenon was quickly and soundly refuted [33, 46], the original suggestion nevertheless highlights the need to consider this source of background for very-sensitive experiments, especially in the presence of a neutrino flux more significant than that from the sun.