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Radiopurity screening of materials for rare event searches by neutron activation at the TRIGA reactor of Pavia

G. Baccolo, A. Barresi, D. Chiesa, M. Nastasi, E. Previtali, M. Sisti

TL;DR

The paper presents a comprehensive NAA-based radiopurity screening workflow implemented at the Milano-Bicocca Radioactivity Laboratory for samples irradiated at the TRIGA reactor in Pavia. It combines careful sample preparation, reactor-neutron flux characterization with BaTMAN unfolding, Monte Carlo efficiency modeling, and γ-spectroscopy to derive contaminant masses with sensitivities down to the ppt/ppq level for $^{40}$K, $^{238}$U, and $^{232}$Th. A dedicated experiment assesses systematic uncertainties from moderation and flux gradients, highlighting biases and the need for accurate flux spectra in activation analyses. The work provides a robust benchmark for future radiopurity campaigns and demonstrates how detailed neutron-flux and efficiency characterizations improve the reliability of NAA results in rare-event experiments.

Abstract

In the framework of physics experiments searching for rare events, the selection of extremely radiopure materials is a challenging task, as the signal of interest is often hidden by instrumental background. Neutron activation is a powerful technique to measure trace contaminants with high sensitivity but, to be properly applied, it requires a good characterization of neutron irradiation and $γ$-spectroscopy facilities. This paper presents the state-of-the-art workflow adopted by the Radioactivity Laboratory of the University of Milano-Bicocca for radiopurity screening of materials by neutron activation performed at the TRIGA reactor in Pavia. The ultimate sensitivity of the described workflow, in the absence of interfering activation products and without the application of radiochemical and/or active background rejection techniques, is $<10^{-14}$ g/g for $^{40}$K, $<10^{-12}$ g/g for $^{238}$U and of the order of $10^{-12}$ g/g for $^{232}$Th contaminations. Further details are here provided to address systematic uncertainties related to neutron irradiation that may bias results. To this aim, a dedicated neutron activation campaign was performed and the data were analyzed exploiting a Monte Carlo simulation model of the reactor and applying an unfolding technique to obtain a comprehensive characterization of the neutron flux in typical irradiation configurations. The results of this work provide a valuable benchmark for the application of neutron activation in future radiopurity screening campaigns.

Radiopurity screening of materials for rare event searches by neutron activation at the TRIGA reactor of Pavia

TL;DR

The paper presents a comprehensive NAA-based radiopurity screening workflow implemented at the Milano-Bicocca Radioactivity Laboratory for samples irradiated at the TRIGA reactor in Pavia. It combines careful sample preparation, reactor-neutron flux characterization with BaTMAN unfolding, Monte Carlo efficiency modeling, and γ-spectroscopy to derive contaminant masses with sensitivities down to the ppt/ppq level for K, U, and Th. A dedicated experiment assesses systematic uncertainties from moderation and flux gradients, highlighting biases and the need for accurate flux spectra in activation analyses. The work provides a robust benchmark for future radiopurity campaigns and demonstrates how detailed neutron-flux and efficiency characterizations improve the reliability of NAA results in rare-event experiments.

Abstract

In the framework of physics experiments searching for rare events, the selection of extremely radiopure materials is a challenging task, as the signal of interest is often hidden by instrumental background. Neutron activation is a powerful technique to measure trace contaminants with high sensitivity but, to be properly applied, it requires a good characterization of neutron irradiation and -spectroscopy facilities. This paper presents the state-of-the-art workflow adopted by the Radioactivity Laboratory of the University of Milano-Bicocca for radiopurity screening of materials by neutron activation performed at the TRIGA reactor in Pavia. The ultimate sensitivity of the described workflow, in the absence of interfering activation products and without the application of radiochemical and/or active background rejection techniques, is g/g for K, g/g for U and of the order of g/g for Th contaminations. Further details are here provided to address systematic uncertainties related to neutron irradiation that may bias results. To this aim, a dedicated neutron activation campaign was performed and the data were analyzed exploiting a Monte Carlo simulation model of the reactor and applying an unfolding technique to obtain a comprehensive characterization of the neutron flux in typical irradiation configurations. The results of this work provide a valuable benchmark for the application of neutron activation in future radiopurity screening campaigns.

Paper Structure

This paper contains 15 sections, 18 equations, 8 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Experimental methodology
  3. NAA sensitivity
  4. Measurement and analysis workflow
  5. Sample preparation
  6. Irradiation at the TRIGA reactor in Pavia
  7. $\gamma$-spectroscopy measurements at Milano-Bicocca
  8. Data analysis
  9. Expected NAA sensitivity at Milano-Bicocca
  10. Investigation of systematic uncertainties
  11. Experiment description
  12. Activation rate results
  13. Neutron flux results
  14. Discussion
  15. Conclusions

Figures (8)

  • Figure 1: NAA detection limits achievable at the Radioactivity Laboratory of Milano-Bicocca (with the BeGE detector after 6 hours irradiation in the Lazy Susan facility of the TRIGA reactor of Pavia) for the concentration of the three natural contaminants in a generic plastic sample weighing 10 g, in the absence of interfering activation products. For a detailed description of the plots refer to the text. The uncertainties on neutron flux, cross sections and the other parameters entering in Eq. \ref{['Eq:Sensitivity_1']} are not represented here.
  • Figure 2: (a) Al-Co and Al-Au samples sandwiched between two acrylic cylinders (label no.1) and positioned above an empty vial (label no.2). (b) Stack of 8 couples of Al-Co and Al-Au samples positioned inside small polyethylene vials. (c) Scheme (not to scale) of the setup in the top left photo. (d) Scheme (not to scale) of the setup in the top right photo.
  • Figure 3: Vertical profiles of the Specific Saturation Activities of the radionuclides measured after irradiating Al-Au and Al-Co samples in the Lazy Susan facility (samples no.3 to no.10 in LS--25 channel). The results of $\gamma$-spectroscopy measurements performed with two HPGe detectors (GeGEM and GeSilena) are shown with different markers and colors.
  • Figure 4: Relative differences of the Specific Saturation Activities of $^{198}$Au, $^{60}$Co, and $^{24}$Na, calculated with respect to the ones measured in lowest irradiation position of the LS--25 channel.
  • Figure 5: (a) Neutron flux spectra, shown in lethargic scale and normalized to 1, obtained from the MCNP simulation model of the TRIGA Mark II reactor in Pavia at various distances from the center of the core along the z-axis in the Central Channel (CC), Rabbit (Rab), and Lazy Susan (LS) irradiation facilities, and in 3 annulus volumes in the reflector region between the core and the Lazy Susan (for a description of the TRIGA reactor geometry refer to AbsoluteFlux). All the other spectra (not shown here) belonging to the set of guess spectra used in the unfolding analysis are characterized by similar shapes, comprised between the most and least thermalized spectra shown here. (b) Spectra, normalized to 1, obtained at the end of the unfolding analysis of the 10 samples irradiated in different positions and conditions in the LS facility described in this paper.
  • ...and 3 more figures