Background studies for the MINER Coherent Neutrino Scattering reactor experiment

doi:10.1016/j.nima.2017.02.024

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 853, 1 May 2017, Pages 53-60

https://doi.org/10.1016/j.nima.2017.02.024 Get rights and content

Abstract

The proposed Mitchell Institute Neutrino Experiment at Reactor (MINER) experiment at the Nuclear Science Center at Texas A&M University will search for coherent elastic neutrino-nucleus scattering within close proximity (about 2 m) of a 1 MW TRIGA nuclear reactor core using low threshold, cryogenic germanium and silicon detectors. Given the Standard Model cross section of the scattering process and the proposed experimental proximity to the reactor, as many as 5–20 events/kg/day are expected. We discuss the status of preliminary measurements to characterize the main backgrounds for the proposed experiment. Both in situ measurements at the experimental site and simulations using the MCNP and GEANT4 codes are described. A strategy for monitoring backgrounds during data taking is briefly discussed.

Introduction

The cross section for the coherent elastic scattering of neutrinos off of nuclei (CEνNS) [1] is a long-standing prediction of the Standard Model, but has yet to be measured experimentally in part due to the extremely low energy threshold needed for detection with typical high flux neutrino sources such as nuclear reactors. Improvements in semiconductor detector technologies [2] which utilize the Neganov-Luke phonon amplification method [3] have brought CEνNS detection within reach. The Mitchell Institute Neutrino Experiment at Reactor (MINER) experiment, currently under development at the Nuclear Science Center (NSC) at Texas A&M University, will leverage this detector technology to detect CEνNS and measure its cross section. If successful, the CEνNSinteractions can be used to probe new physics scenarios including a search for sterile neutrino oscillations, the neutrino magnetic moment, and other processes beyond the Standard Model [4], [5], [6], [7]. The experiment will utilize a megawatt-class TRIGA (Training, Research, Isotopes, General Atomics) pool reactor stocked with low-enriched (about 20%) ²³⁵U. This facility has the unique advantage of possessing a movable core and provides access to deploy detectors as close as about 1 m from the reactor, allowing for a varying distance from the neutrino source to the detector. At these short baselines, we expect to detect as many as 20 events/kg/day in the range of recoil energy between 10 and 1000 eV_nr. This estimate is obtained by integrating the Standard Model differential cross-section over the neutrino energy spectrum of a megawatt ²³⁵U reactor and integrating the nuclear recoil energy from the specified sensitivity threshold up to the kinematic cutoff, as described in [4], [7]. The rate is cut to approximately a third if the detection threshold is instead 100 eV_nr, and diminishes as r⁻² with increasing distance, e.g. to as many as 5 events/kg/day at 2 m.

An important aspect of the proposed experiment are the backgrounds induced by both the core and environmental sources. These backgrounds include gammas and neutrons from the reactor, muons and muon-induced neutrons from cosmic rays, and ambient gammas. The rate of such backgrounds must be comparable to or below the expected rate of the neutrino recoil signal. We take a rate of 100 events/kg/day in the range of recoil energy between 10 and 1000 eV_nr as the target level of acceptable background rate, corresponding to a signal to background ratio of about 0.05–0.2. It has been demonstrated in [8] that a signal rate of 10 over a background rate of 100 events/kg/day is discoverable at the $5 σ$ level after a few months of integrated run time using a binned profile likelihood test statistic with marginalization over the background and flux normalization and assuming 2% systematic uncertainty. Events outside of this 10–1000 eV_nr energy window are acceptable to a level of about 100 Hz total event rate, dictated by the sampling rate of the data acquisition system. These higher energy events can serve to normalize backgrounds in the lower energy signal region.

The paper is organized as follows. In Section 2, a brief description of the experimental location is given. Section 3 describes the modeling of the reactor core and experiment in the MCNP and GEANT4 framework. 4 Gamma background measurements, 5 Neutron background measurements, 6 Muon background measurements describe the in situ measurements of the gamma, neutron, and cosmic muon backgrounds respectively, including comparison to the simulation for the gamma and neutron backgrounds. Section 7 combines the simulation with the in situ measurements to estimate a background rate in the detectors given a preliminary shielding design. Finally, status and prospects are described in Section 8.

Section snippets

Description of experimental site

The NSC reactor facility pool is surrounded by roughly 2 m of high density concrete (about 3.5 g/cm³ density) which acts as a shield to the high flux of neutron and gamma byproducts in the reactor. A cavity in this wall, dubbed the “Thermal Column”, was used in the past to facilitate close proximity to the reactor for material neutron irradiation. The cavity is located in the lower research area of the NSC and is in the same horizontal plane as the reactor core (see Fig. 1). The cavity has many

Reactor core model

Fission processes in the reactor produce large fluxes of both gammas and neutrons near the core. The energy spectrum and production rate of these backgrounds are predicted using a core model developed at the NSC, shown in Fig. 3, and applied in the MCNP [9] framework. The TRIGA reactor of the NSC features a 90 fuel element, low-enriched uranium core operating at a nominal power of 1 MW. The fuel burn-up of the relatively new core (installed in 2006) is modeled in a 15 axial layer configuration

Gamma background measurements

Background measurements have been conducted in the experimental cavity using a commercial High Purity Germanium (HPGe) detector shown in Fig. 6 (Canberra GC2020, approx. 0.5 kg). Due to the large volume of water between the detector and the reactor core, these measurements were dominated by gamma interactions. A commercially available shield was used to limit the rate registered by the detector, while maintaining a simple geometry for matching with simulations. The shield is cylindrical,

Neutron background measurements

Due to administrative and safety constraints preventing deployment of traditional detectors, the background neutron measurement was thus far restricted to measurements needed for validation of the computational models. The validation measurement was performed using a 6×6 in. copper foil (see Fig. 11) that was activated by neutrons in the experimental cavity. The copper acts as an absorber of thermal neutrons and can be used to verify the integrated thermal neutron flux by measuring the

Muon background measurements

Bolometric detectors with low thresholds are particularly vulnerable to large energy depositions from atmospheric muons. A typical solution for low rate experiments to this problem is to install such detectors deep underground, maximizing the overburden, and thus shielding of the detector. For detecting higher rate processes, such as neutrino interactions near a nuclear reactor, a higher muon rate can be tolerated. The experimental cavity proposed for this experiment provides some overburden in

Rate estimate with shielding

We then used the GEANT4 setup to estimate the backgrounds with a full preliminary shielding design, as shown in Fig. 5. We generated approximately 3×10⁹ gamma and 4×10⁹ neutron events with the core at the closest possible proximity to the experimental cavity (at the face of the graphite block shown in Fig. 1). The simulation included 4 germanium detectors and 4 silicon detectors, each represented as 100 mm diameter, 33 mm thickness cylinders, and backgrounds were assessed by determining the energy

Summary and future prospects

We have performed in situ measurements and detailed simulations of expected backgrounds for the proposed MINER experiment with the goal of detecting CEνNS. Simulations reproduce the measurements of thermal neutrons and gammas and were used to estimate the expected backgrounds with a full shielding designed to bring the backgrounds down to a level compatible with a measurement of the CEν NS signal in the MINER experiment. This simulation has shown that it is indeed possible to reduce both

Acknowledgements

The authors gratefully acknowledge the Mitchell Institute for Fundamental Physics and Astronomy for seed funding, as well as the Brazos HPC cluster at Texas A& M University (brazos.tamu.edu) and the Texas Advanced Computing Center (TACC) at the University of Texas at Austin (www.tacc.utexas.edufor providing resources that have contributed to the research results reported within this paper. We also gratefully acknowledge the TAMU Nuclear Science Center for facilitating the MINER experiment and

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Background studies for the MINER Coherent Neutrino Scattering reactor experiment

Abstract

Introduction

Section snippets

Description of experimental site

Reactor core model

Gamma background measurements

Neutron background measurements

Muon background measurements

Rate estimate with shielding

Summary and future prospects

Acknowledgements

Nucl. Instrum. Methods

Nucl. Instrum. Meth.

Nucl. Instrum. Methods

Coherent neutrino nucleus scattering as a probe of the weak neutral current

Phys. Rev.

Probing new physics with coherent neutrino scattering off nuclei

JHEP

Prospects for measuring coherent neutrino-nucleus elastic scattering at a stopped-pion neutrino source

Phys. Rev.