SANDD: A directional antineutrino detector with segmented Li-doped pulse-shape-sensitive plastic scintillator
Introduction
Nuclear power reactors are the most intense man-made sources of antineutrinos. A 1-GW thermal (GWt) nuclear reactor emits antineutrinos per second isotropically. As the composition of the reactor core evolves due to the consumption of 235U and the production of 239Pu, the antineutrino flux and energy spectrum both evolve downwards [1]. This makes it possible in principle to monitor the ON/OFF status and the fuel evolution of the core over time. Antineutrinos have a very low interaction cross section, which makes their detection challenging. The same property allows them to pass through large amount of material unimpeded, which makes them unshieldable. The detection of antineutrinos presents opportunities for non-intrusive real-time monitoring of the operational status of nuclear reactors for safeguards, which falls within purview of the International Atomic Energy Agency [2], [3]. Additionally, short-baseline measurements of reactor antineutrinos could offer valuable insights into the nature of the neutrino [4], [5], [6], [7].
The concept of continuous non-intrusive real-time monitoring of a reactor core using antineutrinos has been demonstrated several times. The concept was first demonstrated in the 1980’s at the Rovno nuclear power plant in the Soviet Union [8], [9]. It was followed by experiments at the Bugey nuclear power plant in France using Li-loaded liquid scintillator [10], [11] and in the 2000’s at the San Onofre Nuclear Generating Station (SONGS) in the US [12] using a 0.64-ton Gd-doped liquid-scintillator detector deployed at an overburden of 20 meters water equivalent (m.w.e.). Recently, the Nucifer experiment, a 0.8-m Gd-doped liquid scintillator detector located 7 m away from the 70-MWth Osiris reactor, provided a short baseline measurement of the reactor neutrino flux at an overburden of 12 m.w.e. [13].
Antineutrinos undergo the inverse beta decay (IBD) reaction on protons in a hydrogen-rich target volume: producing an MeV-scale positron that slows down and annihilates with an electron, emitting two 0.511-MeV gamma rays. The neutron thermalizes and can be detected via capture on nuclei with high thermal neutron capture cross sections, usually added to the detection medium in small concentrations, such as 155Gd, 157Gd, and 6Li.
While the capabilities of antineutrino detectors for nuclear safeguards have been demonstrated, practicalities associated with deployment need to improve [2]. Most short-baseline detectors have been deployed below ground to suppress fast neutrons caused by the cosmic ray muons [1], [9]. Thick hydrogenous shielding is required around the target material to further reduce the fast neutron flux. This increases the overall size and complexity of the detector and limits the range of possible deployment locations. To solve this issue, contemporary near-field detectors employ segmented geometry and materials capable of pulse-shape discrimination to reject fast neutron induced backgrounds and to select the delayed coincidence events that are confined both in time (200 s) and space (1 m). This approach greatly suppresses the accidental coincidence and the muogenic backgrounds, allowing for aboveground deployment. For example, PROSPECT is an aboveground segmented 4-ton Li-doped pulse-shape-sensitive liquid-scintillator detector that has demonstrated real-time monitoring of the 85-MW High-Flux Isotope Reactor (HFIR) at a 7-m standoff [14].
All of the short-baseline detectors described above employed organic liquid scintillators. Some of these liquids can be chemically toxic, volatile, and flammable. Energy depositions in the container holding the liquid may also not contribute to light production and detection. Furthermore, materials compatibility with the liquid scintillator containment can be problematic, and support and retaining structures require careful engineering. To improve the safety characteristics and ease of deployment, several efforts to develop compact aboveground solid (plastic) antineutrino detectors are underway. Current and proposed near-field detectors such as SANDD [15], [16], PANDA [17], SoLid [18], [19], DANSS [20], [21], CHANDLER [22], VIDAAR [23], IMSRAN [24], and NuLat [25], [26], [27] are segmented and employ pulse-shape-sensitive solid materials. These detectors2 utilize heterogeneous detection medium: the main “target” scintillator is wrapped in neutron-capture material either containing Gadolinium (as in PANDA, DANSS, VIDAAR, and ISMRAN) or Li in the form of LiZnS:Ag (as in CHANDLER and SoLid).
Among the aforementioned detectors, SANDD (Fig. 1) is the only detector that uses pulse-shape-sensitive plastic homogeneously doped with Li [28] (0.1%wt). In contrast to neutron capture on 155Gd, 157Gd, or H, which produces one or more gamma rays that deposit their energies over a larger volume, neutron capture on Li produces an alpha and a triton, which deposit their energies over a short range. Hence, the use of Li combined with the fine segmentation enables additional discrimination for/against neutron captures via rod multiplicity, improving IBD coincidence identification and further suppressing the high rate of backgrounds near the surface. The fine lateral and azimuthal segmentation of the scintillator elements also permits good spatial resolution, a requirement for directional sensitivity. Antineutrino direction can be statistically reconstructed from the average displacement vector defined by the positron and neutron capture locations [29]. Detectors such as Goesgen [30], [31], Palo Verde [30], [32], CHOOZ [33], Double CHOOZ [34], [35], [36], and more recently PROSPECT [37] have theoretically and experimentally studied their directional sensitivity to antineutrino flux. So far, Double CHOOZ reported the lowest angular uncertainty of 43 from 100 detected IBD events [36].
The focus of this paper is a characterization of the full 9-liter SANDD (Fig. 2). It builds on earlier work [15], which characterized a prototype of the central module with shorter pulse-shape-sensitive rods and no Li doping. The aim of the previous work was to test a data acquisition system capable of recording key performance metrics such as pulse-shape-parameter (PSP), energy (), z-position resolution and particle ID via segment multiplicity (). The relevant performance metrics in this work are similar, but also include PSP of the neutron capture on Li. Fig. 1 shows a rendering of the full sized 9-liter SANDD. The full 9-liter SANDD described here employs the same readout for the central module, and a dual-end readout of scintillator bars with 1” photomultiplier tubes (PMTs) for the outer modules.
The central module of SANDD consists of an 8 × 8 array of Li-doped pulse-shape-sensitive plastic rods (5.4 mm 5.4 mm 40.64 cm) coupled to a pair of 64-pixel SiPM arrays (Fig. 2(a)). The central module of SANDD is surrounded by a layer of 2.54 cm 2.54 cm 40.64 cm Li-doped pulse-shape-sensitive plastic bars, which is further surrounded by a layer of 5.08 cm 2.54 cm 40.64 cm Li-doped pulse-shape-sensitive plastic bars. Each bar is coupled to a pair of 1” PMTs (Fig. 2b). The performance characteristics of the central module, one of the middle layer bars, and one of the outer layer bars were measured and compared separately to a GEANT4 simulation [38], [39]. Henceforth we refer to each of these components as component 1, component 2, and component 3, respectively, as described in Table 1.
The three different components of SANDD are shown in Fig. 2 and parameterized in Table 1. Analysis cuts were established to retain the majority of IBD events while removing a large fraction of background events. As a result, antineutrino detection efficiency and uncertainty in the direction of the reactor antineutrino flux are predicted by a tuned simulation.
Section snippets
Design of SANDD
Details of the composition and the fabrication of the Li-doped pulse-shape-sensitive plastic scintillator can be found in Ref. [40]. Multiple solid scintillator sections were prepared over an 8 months period. Some sections were machined for the 64-rod inner module, while the others were machined into the outer layer bars. The rods were then optically coupled to a pair of SiPM arrays (SensL J-60035 series, 50.44 mm 50.44 mm [41]) using BC-630 silicone optical grease, as shown in Fig. 2(a).
Characterization of the SANDD components and validation of the simulation framework
The metrics that impact the performance of SANDD include energy resolution, PSP, light attenuation length (which impacts energy resolution), position resolution and rod multiplicity. A detailed description of the data analysis methods can be found in Ref. [15]. Here, only the pertinent aspects of those methods are highlighted.
Waveforms saved to disk were analyzed as follows: First, the baseline was calculated by averaging the first 70 pre-trigger samples (280 ns) and 30 pre-trigger samples
The simulated performance of SANDD
In this section, the selection of positron and neutron capture candidates and the rejection of common backgrounds in the full SANDD, consisting of a full assembly of components 1, 2 and 3 (Fig. 1), is discussed. Neutron capture and positron candidates were selected by establishing cuts on the energy, rod multiplicity, and PSP. Coincidence time window was then optimized to select positron–neutron pairs and to estimate the antineutrino detection efficiency. Lastly, directional capability of SANDD
Conclusion
SANDD is a directional antineutrino detector that could potentially operate aboveground. The fine segmentation and the pulse-shape discrimination capability of SANDD allows the prompt-delayed coincidence to be discriminated against the cosmic background. Components of SANDD were characterized and a detailed Monte Carlo simulation code was developed and tuned to investigate the performance of SANDD. Analysis cuts were developed to strike a balance between the antineutrino detection efficiency
CRediT authorship contribution statement
F. Sutanto: Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. T.M. Classen: Software, Validation, Data curation. S.A. Dazeley: Conceptualization, Methodology, Software, Validation, Resources, Data curation, Writing - review & editing, Supervision, Project administration, Funding acquisition. M.J. Duvall: Software. I. Jovanovic: Funding acquisition, Writing - review & editing. V.A. Li: Investigation, Data curation, Writing
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to acknowledge Natalia Zaitseva of LLNL for helpful discussions regarding scintillator development. We thank Jingke Xu, along with Ultralytics and Struck engineers for their support and useful discussions. We thank Marc Bergevin for useful discussions on the Geant4 optical photon simulation. The research of F.S. was performed under the appointment to the Lawrence Livermore Graduate Scholar Program Fellowship and Rackham Merit Fellowship. The work of I.J. and F.S. was partially
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