Optical properties and pulse shape discrimination in siloxane-based scintillation detectors

T Marchi,F Pino,F Gramegna,M Degerlier,C L Fontana,G Collazuol,V Paterlini,M Cinausero,S M Carturan,E Zanazzi,S Moretto,A Quaranta,N Daldosso,D Fabris,M Vesco

doi:10.1038/s41598-019-45307-8

Abstract

The possibility to detect fast neutrons as a distinct signal from that one of γ-rays background is surely of great importance for several topics, spanning from homeland security to radiation monitoring in nuclear physics research plants. Nowadays, Helium-3 based detectors are extremely expensive, while the use of large volume liquid scintillators presents serious concerns related to spillage risks and waste disposal. A very attractive alternative is the use of commercially available solid scintillators, which exploits an aromatic polymer matrix entrapping very high loadings of primary dye, thereby enabling the use of pulse shape analysis (PSA) to discriminate between fast neutrons and γ-rays. In this work, we analyse in detail the optical features of a solid scintillator composed by polymethylphenylsiloxane (PMPS) as base polymer loaded with moderate amounts of 2,5-diphenyloxazole (PPO). Furthermore, fluorescence decay kinetics have been correlated to the observed pulse shape discrimination capabilities of this radiation and thermally resistant scintillator, whose performances have been discussed in terms of conformational features and excimers formation revealed by the optical analyses.

Highlights

ConceptsIn organic systems, the scintillation process is based on the energy transfer between aromatic groups of the either solid or liquid matrix and the dispersed dye molecules characterized by high quantum efficiency
We produced polymethylphenylsiloxane samples, added with low amounts of PPO as primary dye and a fixed amount of waveshifter, to obtain organic scintillators with the capability to discriminate between fast neutrons and γ-rays
This feature is related to the mechanism of triplet-triplet annihilation, which leads to delayed fluorescence and, in turn, to a longer decay time of the scintillation pulse in case of passage of particles with high ionization density, such as recoil protons from impinging neutrons

Summary

Introduction

ConceptsIn organic systems, the scintillation process is based on the energy transfer between aromatic groups of the either solid or liquid matrix and the dispersed dye molecules characterized by high quantum efficiency. Two nearby T1 molecules can undergo triplet-triplet annihilation (TTA) to give a ground S0 and an excited S1 singlet state that in turn decays with the emission of a delayed photon[17,18,19]. This delayed fluorescence has characteristic times of the order of tens of ns, with respect to the prompt emission, whose time scale is 1–2 ns, maintaining the same spectral response. The intensity of the component with longer lifetime increases with the concentration of triplet states, recoil protons give rise to scintillation pulses with a more intense long-living component with respect to Compton electrons, allowing their discrimination through pulse shape discrimination (PSD) techniques

Methods

Results

Discussion

Conclusion