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Ultra-trace analysis of U and Th in organic liquid scintillators with high sensitivity

A. Barresi, D. Chiesa, D. Merli, M. Nastasi, S. Nisi, E. Previtali, M. Sisti, M. Borghesi, A. Cammi, C. Coletta, G. Ferrante, L. Loi, G. Andronico, V. Antonelli, D. Basilico, M. Beretta, A. Bergnoli, A. Brigatti, R. Brugnera, R. Bruno, A. Budano, B. Caccianiga, V. Cerrone, R. Caruso, C. Clementi, L. V. D'Auria, S. Dusini, A. Fabbri, G. Felici, A. Garfagnini, M. G. Giammarchi, N. Giudice, A. Gavrikov, M. Grassi, N. Guardone, F. Houria, C. Landini, L. Lastrucci, I. Lippi, P. Lombardi, F. Mantovani, S. M. Mari, A. Martini, L. Miramonti, M. Montuschi, D. Orestano, F. Ortica, A. Paoloni, L. Pelicci, E. Percalli, F. Petrucci, G. Ranucci, A. C. Re, B. Ricci, A. Romani, P. Saggese, A. Serafini, C. Sirignano, L. Stanco, E. Stanescu Farilla, V. Strati, M. D. C Torri, C. Tuve', C. Venettacci, G. Verde, L. Votano

TL;DR

The paper presents an integrated ultra-trace screening method for natural uranium and thorium in organic liquid scintillators, combining neutron activation analysis with radiochemical preprocessing and beta/gamma coincidence detection. By sequentially applying liquid-liquid extraction, pre-irradiation UTEVA extraction chromatography, neutron irradiation, post-irradiation TEVA extraction chromatography, and GeSparK beta/gamma spectroscopy, the approach achieves some of the world’s best sensitivities for $^{238}$U and $^{232}$Th in LAB-based scintillators, namely $0.65\times 10^{-15}$ g/g and $1.9\times 10^{-15}$ g/g, respectively. The study provides detailed laboratory cleaning protocols, recovery efficiencies (U: $86\%\pm12\%$, Th: $43\%\pm10\%$), and blank measurements to quantify and mitigate contamination sources, particularly resin-derived backgrounds. The results enable robust radiopurity validation for JUNO-like detectors and outline practical paths to further improvements, including larger sample masses, alternative resins, and higher-efficiency detectors. Overall, this work advances the capacity to screen detector-media radiopurity at the $10^{-15}$ g/g level, with significant implications for next-generation rare-event experiments.

Abstract

Rare event searches demand extremely low background levels, necessitating ever-advancing screening techniques to enhance sensitivity. Liquid scintillators are highly attractive as detector media due to their inherent radiopurity and scalability in mass. In this work, we present a screening procedure to measure ultra-trace concentrations of natural contaminants -- $^{238}$U and $^{232}$Th -- with sensitivities at the \qty{E-15}{g/g} level. Our method combines neutron activation analysis with radiochemical techniques, followed by \bg\ coincidence spectroscopy to minimize interference backgrounds. This approach achieves sensitivities of \qty{0.65E-15}{g/g} for $^{238}$U and \qty{2.3E-15}{g/g} for $^{232}$Th, among the best reported worldwide. Potential pathways for further sensitivity improvements are outlined in the conclusions.

Ultra-trace analysis of U and Th in organic liquid scintillators with high sensitivity

TL;DR

The paper presents an integrated ultra-trace screening method for natural uranium and thorium in organic liquid scintillators, combining neutron activation analysis with radiochemical preprocessing and beta/gamma coincidence detection. By sequentially applying liquid-liquid extraction, pre-irradiation UTEVA extraction chromatography, neutron irradiation, post-irradiation TEVA extraction chromatography, and GeSparK beta/gamma spectroscopy, the approach achieves some of the world’s best sensitivities for U and Th in LAB-based scintillators, namely g/g and g/g, respectively. The study provides detailed laboratory cleaning protocols, recovery efficiencies (U: , Th: ), and blank measurements to quantify and mitigate contamination sources, particularly resin-derived backgrounds. The results enable robust radiopurity validation for JUNO-like detectors and outline practical paths to further improvements, including larger sample masses, alternative resins, and higher-efficiency detectors. Overall, this work advances the capacity to screen detector-media radiopurity at the g/g level, with significant implications for next-generation rare-event experiments.

Abstract

Rare event searches demand extremely low background levels, necessitating ever-advancing screening techniques to enhance sensitivity. Liquid scintillators are highly attractive as detector media due to their inherent radiopurity and scalability in mass. In this work, we present a screening procedure to measure ultra-trace concentrations of natural contaminants -- U and Th -- with sensitivities at the \qty{E-15}{g/g} level. Our method combines neutron activation analysis with radiochemical techniques, followed by \bg\ coincidence spectroscopy to minimize interference backgrounds. This approach achieves sensitivities of \qty{0.65E-15}{g/g} for U and \qty{2.3E-15}{g/g} for Th, among the best reported worldwide. Potential pathways for further sensitivity improvements are outlined in the conclusions.

Paper Structure

This paper contains 16 sections, 5 equations, 9 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Neutron activation analysis sensitivity
  3. Measurement Strategy
  4. Labware cleaning protocol
  5. Radiochemical procedures
  6. Liquid-liquid extraction
  7. Extraction chromatography
  8. Pre-irradiation EC
  9. Post-irradiation EC
  10. Neutron irradiation facility
  11. The $\beta$/$\gamma$ system GeSparK
  12. Recovery efficiency determination
  13. Measurement of blank samples
  14. Conclusions
  15. Experimental sensitivity calculation
  16. ...and 1 more sections

Figures (9)

  • Figure 1: Block diagram of the measurement procedure.
  • Figure 2: PFA vials with screw caps.
  • Figure 3: Vacuum system used for the extraction chromatography process. This system is necessary to guarantee a precise and stable flow rate of the solutions through the columns.
  • Figure 4: Results of the total efficiency of the complete measurement procedure for uranium and thorium The coloured regions are the $1\sigma$ bands around the means.
  • Figure 5: Time distribution of $^{239}\textrm{Pu}$ delayed events in two blank samples with one washing step (\ref{['fig:DTPlot1']}) and two washing steps (\ref{['fig:DTPlot2']}). The plots also show the PDFs obtained from the JAGS fit (see \ref{['S:np-239']}), where the dashed blue line represents the background component and the solid red line represents the total distribution. The observed exponential decay is consistent with the half-life of the $^{239}$Np metastable state. The volume of the extracting solution, the irradiation time, and the measurement duration are identical for both samples.
  • ...and 4 more figures