A Low Background Beta Detection System using a Time Projection Chamber

TL;DR

This work demonstrates a Micromegas-based Time Projection Chamber (TPC) with an anti-coincidence detector to enable low-background beta detection. A Geant4-based detector model with digitization is validated against $^{90}$Sr beta data, and TMVA Boosted Decision Trees are used to discriminate beta signals from backgrounds, achieving a background rate of $0.49$ $\mathrm{cpm/cm^2}$ while preserving $55\%$ of the beta events within a $7$ cm diameter region. The study identifies environmental gamma radiation and detector-material radioactivity (notably FR-4 field cage and anode PCBs) as major background sources and demonstrates substantial background reductions through shielding and material substitutions, with simulations predicting a potential background of $0.0012$ $\mathrm{cpm/cm^2}$ at the same beta efficiency. The work provides a practical pathway to portable, low-background beta measurements by combining detailed simulations with targeted hardware optimizations (flexible PCBs and oxygen-free copper) and shielding. These findings have implications for environmental monitoring and rare-event background reduction where beta measurements must be disentangled from ambient radiation.

Abstract

In this paper, we present a Time Projection Chamber (TPC) system for low-background beta radiation measurements. The system consists of a TPC with two-dimensional-strip readout Micromegas and an anti-coincidence detector with readout pads for cosmic ray veto. The detector system utilize an AGET-based waveform sampling system for data acquisition. The beta detection capability of the system was verified through experimental test using $^{90}$Sr beta source. Additionally, a dedicated simulation program based on Geant4 was developed to model the entire detection process, including responses to both the beta source and background radiation. Simulation results were compared with experimental data for both beta and background samples, showing good agreements. The simulation samples were utilized to optimize and train classification models for beta and background discrimination. By applying the selected model into test data, the system achieved a background rate of 0.49 $\rm cpm/cm^2$ while retaining more than 55% of $^{90}$Sr beta signals within a 7 cm diameter detection region. Further analysis revealed that approximately 70% of the background originates from environmental gamma radiation, while the remaining contribution mainly comes from intrinsic radioactivity of detector materials, particularly the FR-4 based field cage and readout plane. Based on the knowledge gained from the experiments and simulations, an optimization of the TPC system has been proposed, with simulation predicting a potential reduction of the background rate to 0.0012 $\rm cpm/cm^2$.

Figures (16)

• Figure 1: Schematic lay-out (left) and the mechanical drawing (right) of the TPC system.
• Figure 2: Arrangement of the TPC's readout strips (a) and the anti-coincidence detector's readout pads (b).
• Figure 3: Simulated voltage distribution within the central 120mm$\times$120mm region of the TPC. The voltages correspond to typical settings used during beta radiation detection. Due to the symmetry of the design, only one-eighth of the TPC is modeled.
• Figure 4: A photograph of the TPC system during beta background test.
• Figure 5: A beta event detected by the TPC. Top row: 2D projected tracks on XZ and YZ plane. The Z dimension of the track is denoted by the signal arrival time on each strip. Bottom row: waveform amplitudes on X and Y strips.