# Omniscinti<sup>TM</sup>: simultaneous detection of alpha/beta/gamma/fast and thermal neutron with a triple-layered phoswich detector

Camille Frangville, Guillaume Bertrand, Frédérick Carrel, Clement Lynde, Victor Buridon, Matthieu Hamel

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Camille Frangville, Guillaume Bertrand, Frédérick Carrel, Clement Lynde, Victor Buridon, et al. "Omniscinti<sup>TM</sup>: simultaneous detection of alpha/beta/gamma/fast and thermal neutron with a triple-layered phoswich detector." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2024, 1068 (Novembre 2024), pp.169790. 10.1016/j.nima.2024.169790. cea-05360446

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# OmniscintiTM: simultaneous detection of alpha/beta/gamma/fast and thermal neutron with a triple-layered phoswich detector

Camille Frangville,\* Guillaume H. V. Bertrand, Frédérick Carrel, Clément Lynde, Victor Buridon, Matthieu Hamel\*

Université Paris-Saclay, CEA, List, F-91120 Palaiseau, France

camille.frangville@cea.fr and matthieu.hamel@cea.fr

ORCID numbers: 0000-0003-2527-2371 (CF); 0000-0003-2061-9241(GHVB); 0000-0003-0081-2757 (CL); 0000-0002-3499-3966 (MH).

## Abstract

The aim of our study is to develop a detector that can provide a counting response when exposed to any kind of radionuclide decaying by the emission of alpha, beta, fast neutrons (being thermalized by their environment or not) and/or gamma rays, accompanied with the categorization of such impinging radiation. To reach this, only a single sensor is being used, which is composed of a three-layered, fully organic phoswich (a morphological stacking of different plastic scintillators) showing various thicknesses and decay times. Fast, thermal neutrons and gamma-rays detection are obtained through a thick ⁶Li-doped scintillator. Beta and alpha detection are performed respectively with thin and ultrathin films manufactured with dedicated thickness and decay times that have been optimized thanks to simulation studies. A standard Pulse Shape Discrimination method based on the charge comparison method is used to separate alpha, beta, neutrons and gamma rays contributions. We present the preparation and characterization of various configurations: quintuple discrimination, beta detection and alpha spectrometry (performed on a cocktail of alpha emitters). These particular experimental results are discussed and expended in the frame of complex multi-sources scenario. This all-purpose concept is trademarked as Omniscinti™.

## Keywords

Scintillation; photoluminescent polymers; radioprotection; homeland security; nuclear instrumentation

## Introduction

The fast characterization of a radiologically unknown field can become a quite challenging task. Relevant examples can be found in first responders acting in homeland security or operators in decommissioning or dismantling nuclear plants. Without any background on the area to be probed, several detectors may be required. These detectors are usually specialized towards one or eventually two natures of disintegrations, e.g. Bonner spheres for neutron dosimetry or alpha/beta contaminameters.
In this context, a “universal” detector would be highly appreciated, especially for homeland security where both fast and global determinations of the threat are of primary importance. Here the word “universal” may be restricted to the two above-mentioned application domains, where there is no strict need to detect, for example, muons or heavy nuclei. Looking into the literature, there is, to the best of our knowledge, no one-piece detector aiming at detecting at the same time alpha, beta, neutrons (fast and/or thermal) and gamma rays. In other words, often one disintegration is missing in the most advanced detectors. In particular, detecting fast neutrons is still challenging, especially if the detector is exclusively composed of inorganic scintillators. The most advanced example is probably the four-channel microdevice composed of scintillating nanoparticles that was developed for the same purpose but failed to detect fast neutrons.¹ Another attempt of discriminating alpha emitters, fast neutron and gamma rays with a one-piece plastic scintillator (EJ-299-33) highlighted an intricate separation of alpha contributions from fast neutrons.² In this regards, discriminating all five contributions with a mono composition approach appear to be impossible, thus, additional technique must be combine to reach our goal.

A phoswich is the contraction term for “phosphor sandwich” and consists in stacking scintillators that present different photoluminescent features, thus providing a morphological discrimination towards particles with different penetration ranges.³ Thus, the phoswich strategy can become particularly useful in such kind of universal detection, with each layer being designated to one detection task, thus allowing for example alpha/beta discrimination or beta/gamma discrimination in mixed fields. Two-layer phoswich detectors are widely known in the field of radiation detection, in particular due to their optimized design for surfaces contamination survey.⁴ Thin and thick plastic scintillators architectures are used to discriminate beta from a strong gamma background. Another example is a thin inorganic scintillator (e.g. ZnS:Ag) on top of a thick plastic scintillator, this is commercially employed as an alpha/gamma discriminating agent. **Erreur ! Signet non défini.** The reversed configuration (thin plastic on thick inorganic) sparks also many interest for dual to triple discrimination. A particular example is a combination of a 50 µm to 1 mm plastic scintillator layered on top of a 15 cm $BaF_2$ prism. This set up provided multiple signatures in presence of different heavy nuclei cations from light to moderate masses.⁵ This study showed that it is possible to discriminate charged cations, protons, fast neutrons and gamma rays. This strategy using both Pulse Shape Discrimination (PSD) and $dE/dx$ discrimination paves the way to the separation of multiple disintegrations schemes but still the literature does not provide full separation of the elements we are aiming at. Three-layer, fully inorganic⁶ or an organic/inorganic blend⁷ phoswich detectors are known to discriminate alpha, beta and gamma events, but once again without any possibility to detect at the same time fast or thermal neutron or to sort beta from gamma rays.⁸ An older achievement of three layer organic/inorganic phoswich exhibited an alpha, beta and thermal neutrons discrimination.⁹ Relevant research has been performed by the group of Morishita et al. in this field, in particular with ultra-thin plastic scintillators for alpha or beta detection with good gamma background rejection thanks to the small thickness of the scintillator¹⁰,¹¹ or with PSD-capable organic monoliths for the same purpose.¹²

A focus on neutron detection associated to other particles is less straightforward. Some formulation of plastic scintillators are known to discriminate fast or thermal neutrons from gamma rays,¹³ but a phoswich form factor does not grant the detection of all features: $n_{fast}/n_{thermal}$/gamma discrimination,¹⁴,¹⁵,¹⁶ $\alpha/\beta/n_{thermal}/\gamma$,¹⁷ **Erreur ! Signet non défini**. $\alpha/\beta/n_{fast}/\gamma$ discrimination. **Erreur ! Signet non défini.** It is in this scientific and technological space that the Omniscinti™ scintillator finds a suitable and relevant place. We present herein our latest results with fully organic phoswiches. First, the rationale of such a multi-sensor will be exposed, followed by MCNP simulations that will settle the best configuration, in particular the thickness of the dedicated layers towards alpha and beta detection. Then the chemistry and the photophysics underneath its operation mode will be explained.
At last, examples with different layer configurations and various cocktails of radioactive sources will be presented as a case study for the potential of Omniscinti™. Each experiment will show their respective scatter plots comprising five regions of interest (α/β/n<sub>thermal</sub>/n<sub>fast</sub>/γ). A focus on beta and alpha detection efficiencies will also be extended on a variety of relevant emitters (¹⁴C, ³⁶Cl, ⁶⁰Co, ⁹⁰Sr/⁹⁰Y and ²³⁷Np, ²³⁹Pu, ²⁴¹Am, ²⁴⁴Cm).

Experimental

Materials and methods

Solvents were purchased from VWR or Carlo Erba. Polystyrene, styrene, methacrylic acid, 2-
methylbutyric acid, lithium hydroxide and fluorophores were purchased from Sigma-Aldrich. Styrene
and methacrylic acid were distilled under reduced pressure to eliminate inhibitors and impurities.
Other reagents were used without further purification. Lithium *alpha*-valerate was prepared according
to a previous published procedure and was freshly crystallized from acetone prior to the preparation
of the lithium-loaded plastic scintillator.¹⁸ Thin films were prepared from 280k polystyrene containing
the suitable fluorophore composition,¹⁹ the whole being dissolved in standard grade toluene. The
solution was cast on a glass plate using a MSK-AFA-H200A semi-automatic coater (MTI Corp.), thus
leading to thin films after solvent evaporation. When necessary, the optical connexion between the
layers was performed using EJ-550 optical grease (Eljen Technology).

Photoluminescence

Fluorescence spectra were recorded at 90° angle with a Horiba Jobin Yvon Fluoromax-4P device, monitored with FluorEssence software. The photoluminescence decay characteristics of the materials were investigated with the Time-Correlated Single Photon Counting (TCSPC) module available on the Fluoromax-4P spectrofluorometer, with an excitation provided by a laser diode operating at 274 nm, which allows exciting the full scintillator. Decay spectra were fitted using DAS6 software (Horiba Jobin Yvon), the quality of the fit was determined so that the χ² value was less than 1.30.

Scintillation experiments

Particle discrimination analyses were performed using the following experimental setup. A phoswich detector was coupled with EJ-550 optical grease to either a H11284MOD or a R6231 photomultiplier tubes (Hamamatsu, see text for high voltage values). The phoswich detectors were not covered by any diffusive or reflective layer. The following radioactive sources were used in this study: ¹³⁷Cs (662-keV gamma only, 472 kBq), ²⁵²Cf (261 kBq or 155 kBq), ¹⁴C ($E_β^{mean}$ = 49 keV, $E_β^{max}$ = 159 keV, 5.6 kBq), ⁶⁰Co (beta and gamma, $E_β^{mean}$ = 95 keV, $E_β^{max}$ = 317 keV, 3.27 kBq), ³⁶Cl ($E_β^{mean}$ = 316 keV, $E_β^{max}$ = 709 keV, 6.47 kBq), ⁹⁰Sr/⁹⁰Y ($E_β^{mean}$ = 196 keV, $E_β^{max}$ = 546 keV for ⁹⁰Sr, $E_β^{mean}$ = 927 keV, $E_β^{max}$ = 2279 keV for ⁹⁰Y, 5.04 kBq), ²³⁷Np ($E_α^{main}$ = 4788 keV, 73 Bq over 2π), ²³⁹Pu ($E_α^{main}$ = 5156 keV, 127 Bq or 3.4 kBq over 2π), ²⁴¹Am ($E_α^{main}$ = 5485 keV, 214 Bq over 2π), and ²⁴⁴Cm ($E_α^{main}$ = 5805 keV, 500 Bq or 1.24 kBq over 2π), and two mixed alpha sources **S1** [²³⁷Np (307 Bq) + ²⁴¹Am (393 Bq) + ²⁴⁴Cm (93 Bq)] and **S2** [²³⁷Np (365 Bq) + ²³⁹Pu (409 Bq) + ²⁴¹Am (355 Bq) + ²⁴⁴Cm (91 Bq)).²⁰ Most sources were calibrated in November 2020, except **S1** and **S2**. Alpha and beta sources were in contact with the upper layer of the phoswich detector. Gamma and neutron/gamma sources were located ca. 6 cm away from the upper layer of the same phoswich. The ²⁵²Cf neutron/gamma source was partially thermalized and shielded using 5 cm of HDPE. The anode signal fed either a CAEN DT5743 or a DT5730B digitizer. Scintillation pulses were then recorded using distributed CAEN softwares and post-processed with a homemade procedure.²¹
## Simulation parameters

MCNP6.2 studies²² were performed on two or three layers of plastic scintillators. For all configurations, the density of the detector was 1.023 and the diameter of the cylinder 60 mm. The source-to-detector distance was equal to 1 m for the ¹³⁷Cs gamma source (simulated dose rate 1 µSv/h) and 1 mm for alpha and beta sources (simulated activity: 40 Bq). A 1 keV threshold was applied.

## Fit methods

### For the determination of the Figure of Merit

A quintuple fit-model was performed on PSD profiles to show that the five components (α, β, γ, n<sub>fast</sub> and n<sub>thermal</sub>), discriminated by an Omniscinti technology, can be identified. Mathematical functions to implement are determined through the shape of tasks associated to each component on the Energy x FoM two-dimensional plot. According to the number of parameters used to fit the whole FoM spectrum, the previous observation is useful as well to fix fit parameters.

For the model, Gaussian shapes are selected as fit functions for β, γ, n<sub>fast</sub> and n<sub>thermal</sub> contributions. For α and γ, the selected function is the following:

$$f(FoM) = A \cdot \frac{\sigma}{\tau} \cdot \sqrt{\frac{\pi}{2}} \cdot e^{\left(\frac{0.5 \cdot \sigma^2}{\tau^2}\right) - \left(\frac{FoM-\mu}{\tau}\right)} \cdot \left(1 - \operatorname{erf}\left(\frac{1}{\sqrt{2}} \cdot \left(\frac{\sigma}{\tau} - \left(\frac{FoM-\mu}{\sigma}\right)\right)\right)\right) \quad [1]$$

This multi-fit model used Mirror Spectrum Method (MSM, see below) using the xxx library to take into account the left asymmetric shape of α and γ FoM spectra.

### For the unfolding of alpha spectra

A fit method (using least square algorithm) was applied to unfold the contribution of individual spectra associated to pure alpha sources in a combination of various alpha emitters. Two mixed alpha sources were available (see S1 and S2 in the Scintillation experiments section). The goal of this method was to estimate the proportion of each individual alpha source inside those mixed sources. At first, individual spectra associated to pure alpha components (²³⁷Np, ²³⁹Pu, ²⁴¹Am and ²⁴⁴Cm) were measured and fitted by a single function. This function was given by a convolution product between a Gaussian and an exponential, through the following formula:

$$f(E) = A \cdot \frac{\sigma}{\tau} \cdot \sqrt{\frac{\pi}{2}} \cdot e^{\left(\frac{0.5 \cdot \sigma^2}{\tau^2}\right) - \left(\frac{E-\mu}{\tau}\right)} \cdot \left(1 - \operatorname{erf}\left(\frac{1}{\sqrt{2}} \cdot \left(\frac{\sigma}{\tau} - \left(\frac{E-\mu}{\sigma}\right)\right)\right)\right) \quad [2]$$

where values of μ, σ and τ are associated to the nature of the pure α source.

Alpha spectra have a left asymmetric profile. So, it is required to pass through the Mirror Spectrum Method (MSM) to fit them using the function in eq. [2].

MSM consists of an abscissa variable change (here the energy E) by a new one, still linearly dependent on E. In our cases, the new abscissa variable built is E' = 1000-E. This allows us to invert the spectrum shape as a mirror. So, fitting using function eq. [1] is applied on mirror spectra. Original spectra and associated fits are recovered through the inverted variable change (from E' to E).

Full methodology of the alpha spectra unfolding will be developed in an oncoming publication.
# Results and discussion

## Rationale

The ambition of the Omniscinti™ detector is to be able to perform an efficient discrimination between alpha, beta, gamma rays, fast and thermal neutrons. Noteworthy, this includes separation of beta from gamma rays, and fast neutrons from thermal neutrons. To reach this goal, two strategies are combined within one detector. First is the extension of traditional phoswich detectors that allow the as-known one-dimensional dE/dx discrimination. However, here we did not restrain to two layers, hence three layers could be added consecutively with different thicknesses, hereafter named "ultra-thin", "thin" and "thick", according to their main morphological characteristic. As mentioned, triple-layered phoswiches have already been published, but mostly with inorganic scintillators. **Erreur ! Signet non défini.** So this threefold fully organic phoswich already constitutes an advanced material. This is made possible thanks to a newly developed plastic scintillator set of formulation, which gave us access to tunable decay time in the 2-85 ns range. **Erreur ! Signet non défini.** This represents a good trade-off in terms of time constant, therefore perfectly suited to be embedded into three or more phoswich layers. A basis sketch is provided in Figure 1, and Table 1 summarizes all the main and secondary interactions we are expecting in each stage of the phoswich. It is noteworthy that the "main interaction" highlighted in Table 1 relates what would happen in a perfect world and secondary or even tertiary interactions (e.g. crosstalk) may occur as well, which will lead to trade-offs in terms of material design as we will see hereafter.

<figure><img src="image_2.png" /><figcaption>Figure 1. Schematic representation of the phoswich detector. Not to scale.</figcaption></figure>

<table><caption>Table 1. Various interactions that occur in each layer of the phoswich detector.</caption><thead><tr><td>Layer</td><td>Thickness intervals (μm)</td><td>Decay time intervals (ns)</td><td>Main interaction</td><td>Minor interaction</td></tr></thead><tbody><tr><td>Ultra-thin</td><td>15-20</td><td>70-240</td><td>Alpha</td><td>Low-energy beta</td></tr><tr><td>Thin</td><td>10-300</td><td>40-55</td><td>Beta</td><td>Gamma</td></tr><tr><td>Thick</td><td>&gt; 5000</td><td>2-10</td><td>Gamma, fast neutrons, thermal neutrons</td><td>High-energy beta</td></tr></tbody></table>

The following characteristics must be considered for all scintillators: morphology, thickness of each layer, and particular photophysical parameters such as emission wavelength and scintillation decay
time. The experimental morphology in this study is a cylinder whose diameter is close to 5 cm so that it suits well with most common photomultiplier tubes (PMTs). Nevertheless, our chemical technology is not limited by the sensor's surface by far. It is noteworthy that simulations were performed with a diameter of 6 cm. The desired thickness of the three layers is provided by MCNP simulations (see section below). The maximum of emission wavelength has been recorded via photoluminescence spectroscopy and is close to 425 nm (depending on the chosen secondary fluorophore), once again to comply with standard PMTs. The Omniscinti™ sensor can also be prepared in green- or red-emitting versions, since time and wavelength characteristics can be decorrelated in plastic scintillation design (see ref. **Erreur ! Signet non défini.** for the example of hybrid scintillators). While the scintillation decay property is given in our study by the slower primary fluorophore, the emission wavelength is chosen thanks to the wavelength-shifter, namely the secondary fluorophore.

To pave the way for this quintuple detection, we have decided to use pulse shape discrimination (PSD) between each layer (and in the same thick scintillator for fast and thermal neutron/gamma PSD), and in particular the Charge Comparison Method (CCM) to sort the different nature of interactions within the phoswich.²³ Since this pulse shape analysis requires a large difference between the scintillation decay time of each layer so that the nature of the various deexcitations would be perfectly sorted (in other words, it is the basis of almost all phoswich detectors). We benefited from our in-lab technique which allows preparing plastic scintillators with all required photoluminescence decay time ranging from 2 to 85 ns. **Erreur ! Signet non défini.** In this case, the thick scintillator is the fastest, with a scintillation mean decay time lower than 5 or 10 ns. The middle, thin layer must therefore present a scintillation decay time of more than 40 ns, and the upper, ultra-thin scintillator foil must have a decay time of more than 70 ns. Commercial slow plastic scintillators such as EJ-240 (Eljen Technology) or BC-444 (Saint-Gobain Crystals and Detectors) can also fulfil this requirement, as well as our in-house methodology to prepare plastics with decay ranging from 2 to 85 ns. **Erreur ! Signet non défini.** Having all these materials within reach, the construction of the three-layer phoswich allowing quintuple discrimination can start. Our next and logic step to decide on the size of our layers is to turn ourselves toward numerical simulation.

## Simulation study

MCNP6.2 simulations**Erreur ! Signet non défini.** were performed to study the impact of each layer's thickness on the response of the phoswich detector. The aim of this study is to favour the main interaction towards the other minor interaction (see Table 1). Another key parameter is our will to detect beta emitters with the broadest possible energy range, with a special interest towards ¹⁴C and an upper limit possibly up to ⁹⁰Sr/⁹⁰Y. Carbon-14 is a weak beta emitter with an average value of 49 keV and a maximum energy of 159 keV.²⁴ The other limit is strontium-90 that disintegrates by beta minus emission (Eβ = 546 keV) to yttrium-90, the latter disintegrating as well by beta minus emission but with a much higher energy (Eβ = 2279 keV). This broad energy range implies trade-offs: with a too thin scintillator, then ⁹⁰Sr/⁹⁰Y counting efficiency would be lowered. With a too thick foil, the beta/gamma discrimination would be worsened because of the increased sensitivity to gamma rays with the volume of the material, leading to gamma overcounting in this layer (minor interaction from Table 1).

A relevant perspective of such sensor is the possibility to perform both alpha and beta spectroscopy, thus leading to possible identification of the radioisotope that is impinging the phoswich detector. In the case of alpha spectroscopy, plastic scintillators are already known as efficient sensors, but they usually feature a moderate energy resolution, typically 16% for an NE-102 plastic scintillator coupled to a photomultiplier tube and exposed to a 5.5-MeV alpha particle. **Erreur ! Signet non défini.** This said,
various alpha emitters with an energy difference lower to this percentage would afford a comparable pulse height spectrum, thus precluding alpha identification.

Thus, simulation of the interaction of various alpha, beta and gamma emitters was performed in the three-layered phoswich detector.

## Beta/gamma discrimination optimization

A double-layer phoswich was simulated, with a thin plastic scintillator with thinnest ranging from 10 µm to 300 µm (L1), covering a thick plastic scintillator of 6 mm (L2). The four beta-emitting sources of interest ($^{14}$C, $^{36}$Cl, $^{60}$Co, $^{90}$Sr/$^{90}$Y, see Scintillation experiments section) were simulated here. The results of the simulation provide the percentage of pulses in L1 only and the mean energy that is deposited in both L1 and L2 layers.

As awaited, the optimum thickness of the L1 scintillator is highly dependent with the energy of the radionuclide one is aimed. Table 2 summarizes the results for various L1 thicknesses. As one can see, a trade-off between low gamma perturbation and high-energy beta counting is difficult to reach. Thus, we preferred to focus on the optimization of the count rate of the two lowest-energy beta emitters, namely $^{14}$C and $^{60}$Co. In other words, we decided to stand the thin layer at a thickness of 150 µm, meaning that > 99% of $^{14}$C pulses and > 86% of $^{60}$Co would be counted in this layer. At this thickness, a 1 µSv/h $^{137}$Cs gamma background would account for less than 3.5% of the total counting in this layer. It is also the first thickness where the energy deposition of the incident gamma ray becomes higher than the deposited energy of $^{14}$C (42.7 keV vs. 40.4 keV, respectively).

<table><caption>Table 2. MCNP6.2 simulation results for $\beta/\gamma$ discrimination optimization for a 1 keV threshold. A 1 µSv/h gamma count rate for $^{137}$Cs is also provided for each thickness.</caption><thead><tr><th>L1 thickness (µm)</th><th>Emitter</th><th>% pulses in L1 only</th><th>E<sub>mean</sub> in L1 (keV)</th><th>E<sub>mean</sub> in L2 (keV)</th></tr></thead><tbody><tr><td rowspan="5">10</td><td><sup>14</sup>C</td><td>53.68</td><td>16.1</td><td>53.6</td></tr><tr><td><sup>60</sup>Co</td><td>34.54</td><td>14.5</td><td>101.0</td></tr><tr><td><sup>36</sup>Cl</td><td>9.95</td><td>11.3</td><td>281.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>4.21</td><td>10.1</td><td>796.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>0.27</td><td>7.5</td><td>241.0</td></tr><tr><td rowspan="5">50</td><td><sup>14</sup>C</td><td>88.48</td><td>34.2</td><td>55.0</td></tr><tr><td><sup>60</sup>Co</td><td>63.59</td><td>39.9</td><td>111.0</td></tr><tr><td><sup>36</sup>Cl</td><td>23.56</td><td>39.8</td><td>293.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>10.53</td><td>38.3</td><td>825.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>0.97</td><td>21.0</td><td>241.0</td></tr><tr><td rowspan="5">100</td><td><sup>14</sup>C</td><td>97.38</td><td>39.4</td><td>50.3</td></tr><tr><td><sup>60</sup>Co</td><td>78.46</td><td>56.1</td><td>112.0</td></tr><tr><td><sup>36</sup>Cl</td><td>33.33</td><td>67.2</td><td>293.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>14.74</td><td>67.5</td><td>838.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>1.83</td><td>34.3</td><td>241.0</td></tr><tr><td rowspan="5">150</td><td><sup>14</sup>C</td><td>99.48</td><td>40.4</td><td>42.0</td></tr><tr><td><sup>60</sup>Co</td><td>86.67</td><td>65.4</td><td>112.0</td></tr><tr><td><sup>36</sup>Cl</td><td>40.10</td><td>89.8</td><td>289.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>18.85</td><td>93.4</td><td>844.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>2.74</td><td>42.7</td><td>242.0</td></tr><tr><td rowspan="3">200</td><td><sup>14</sup>C</td><td>100.0</td><td>40.6</td><td>24.3</td></tr><tr><td><sup>60</sup>Co</td><td>91.79</td><td>71.1</td><td>110.0</td></tr><tr><td><sup>36</sup>Cl</td><td>46.35</td><td>109.0</td><td>285.0</td></tr></tbody></table>
<table><tbody><tr><td></td><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>21.47</td><td>117.0</td><td>845.0</td></tr><tr><td></td><td><sup>137</sup>Cs (γ)</td><td>3.55</td><td>51.7</td><td>242.0</td></tr><tr><td rowspan="5">260</td><td><sup>14</sup>C</td><td>n.s.</td><td>-</td><td>-</td></tr><tr><td><sup>60</sup>Co</td><td>95.38</td><td>75.1</td><td>110.0</td></tr><tr><td><sup>36</sup>Cl</td><td>52.60</td><td>130.0</td><td>280.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>24.61</td><td>144.0</td><td>845.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>4.42</td><td>61.2</td><td>242.0</td></tr><tr><td rowspan="5">300</td><td><sup>14</sup>C</td><td>n.s.</td><td>-</td><td>-</td></tr><tr><td><sup>60</sup>Co</td><td>96.92</td><td>76.7</td><td>109.0</td></tr><tr><td><sup>36</sup>Cl</td><td>56.25</td><td>142.0</td><td>276.0</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>26.70</td><td>160.0</td><td>845.0</td></tr><tr><td><sup>137</sup>Cs (γ)</td><td>4.97</td><td>67.3</td><td>242.0</td></tr></tbody></table>

n.s. not simulated

## Alpha detection optimization

A three-layer organic phoswich was simulated, with an ultra-thin plastic scintillator with thickness ranging from 10 µm to 40 µm (L'1), atop of a thin plastic scintillator with thickness greater than or equal to L'1 ranging from 20 µm to 40 µm (L'2), covering a thick plastic scintillator of 6 mm (L'3). The thickness of L'2 is always higher than the thickness of L'1. Three alpha-emitting sources were simulated here ($^{239}$Pu, $^{241}$Am, $^{244}$Cm). The results of the simulation provide the percentage of pulses in L'1 only and the mean energy that is deposited in both L'1, L'2 and L'3 layers. Here the simulation is aimed at optimizing the alpha counting against the beta counting, so gamma rays are not taken into account. The results are provided in Table 3. More than a 'traditional' α/β discrimination based on their pulse height, we also benefit from their pulse shape difference. This will indeed strongly reduce any cross-talk between alpha and beta region of interest which will be discussed later. But still the thickness of L'1 must not be too elevated to restrain any beta secondary interaction (see Table 1). This is particularly true for example for $^{14}$C. For a thickness as low as 10 µm, almost 54% of pulses from $^{14}$C beta are released in this layer, but with a 16 keV energy deposition only, so without any possibility to be superposed with alpha peaks, even considering an apparent lower energy due to ionization quenching. This quenching factor, a distinctive feature of alpha interaction, will be discussed below in the last part of this paper. In other words, this triple layer is particularly useful to discriminate alpha from beta with moderate energy, e.g. $^{60}$Co, and crosstalk from lower energy beta are easily avoidable with a right choice of energy threshold.

<table><caption>Table 3. MCNP6.2 simulation results for α/β discrimination optimization for a 1 keV threshold for $^{239}$Pu, $^{241}$Am and $^{244}$Cm alpha emitters on a three-layer plastic scintillator. Thicknesses more than 40 µm were not simulated.</caption><thead><tr><th>L'1 thickness (µm)</th><th>L'2 thickness (µm)</th><th>Emitter</th><th>% pulses in L'1 only*</th><th>E<sub>mean</sub> in L'1 (MeV)</th><th>E<sub>mean</sub> in L'2 (MeV)</th><th>E<sub>mean</sub> in L'3 (MeV)</th><th>E<sub>interest</sub> (MeV)<sup>‡</sup></th></tr></thead><tbody><tr><td rowspan="3">4</td><td rowspan="3">20</td><td><sup>239</sup>Pu</td><td>52.72</td><td>1.25</td><td>3.37</td><td>1.49</td><td>4.27</td></tr><tr><td><sup>241</sup>Am</td><td>52.29</td><td>1.23</td><td>3.44</td><td>1.88</td><td>4.37</td></tr><tr><td><sup>244</sup>Cm</td><td>52.01</td><td>1.21</td><td>3.48</td><td>2.24</td><td>4.42</td></tr><tr><td rowspan="3">4</td><td rowspan="3">40</td><td><sup>239</sup>Pu</td><td>52.72</td><td>1.25</td><td>3.97</td><td>0.00</td><td>4.81</td></tr><tr><td><sup>241</sup>Am</td><td>52.29</td><td>1.23</td><td>4.32</td><td>0.00</td><td>5.17</td></tr><tr><td><sup>244</sup>Cm</td><td>52.01</td><td>1.21</td><td>4.65</td><td>0.12</td><td>5.50</td></tr><tr><td rowspan="2">12</td><td rowspan="2">20</td><td><sup>239</sup>Pu</td><td>59.15</td><td>2.86</td><td>2.73</td><td>0.58</td><td>4.75</td></tr><tr><td><sup>241</sup>Am</td><td>58.26</td><td>2.85</td><td>2.90</td><td>1.02</td><td>4.93</td></tr></tbody></table>
<table><tbody><tr><td rowspan="3">12</td><td rowspan="3">40</td><td><sup>244</sup>Cm</td><td>57.40</td><td>2.84</td><td>3.00</td><td>1.42</td><td>5.07</td></tr><tr><td><sup>239</sup>Pu</td><td>59.15</td><td>2.86</td><td>2.86</td><td>0.00</td><td>4.84</td></tr><tr><td><sup>241</sup>Am</td><td>58.26</td><td>2.85</td><td>3.23</td><td>0.00</td><td>5.16</td></tr><tr><td rowspan="3">16</td><td rowspan="3">20</td><td><sup>244</sup>Cm</td><td>57.40</td><td>2.84</td><td>3.57</td><td>0.00</td><td>5.49</td></tr><tr><td><sup>239</sup>Pu</td><td>63.19</td><td>3.44</td><td>2.37</td><td>0.14</td><td>4.82</td></tr><tr><td><sup>241</sup>Am</td><td>61.59</td><td>3.46</td><td>2.63</td><td>0.57</td><td>5.10</td></tr><tr><td rowspan="3">16</td><td rowspan="3">40</td><td><sup>244</sup>Cm</td><td>60.25</td><td>3.46</td><td>2.78</td><td>1.00</td><td>5.29</td></tr><tr><td><sup>239</sup>Pu</td><td>63.19</td><td>3.44</td><td>2.38</td><td>0.00</td><td>4.83</td></tr><tr><td><sup>241</sup>Am</td><td>61.59</td><td>3.46</td><td>2.75</td><td>0.00</td><td>5.18</td></tr><tr><td rowspan="3">20</td><td rowspan="3">20</td><td><sup>244</sup>Cm</td><td>60.25</td><td>3.46</td><td>3.10</td><td>0.00</td><td>5.51</td></tr><tr><td><sup>239</sup>Pu</td><td>67.83</td><td>3.91</td><td>1.93</td><td>0.00</td><td>4.83</td></tr><tr><td><sup>241</sup>Am</td><td>65.54</td><td>3.96</td><td>2.30</td><td>0.13</td><td>5.10</td></tr><tr><td rowspan="3">20</td><td rowspan="3">40</td><td><sup>244</sup>Cm</td><td>63.82</td><td>3.98</td><td>2.55</td><td>0.55</td><td>5.43</td></tr><tr><td><sup>239</sup>Pu</td><td>67.83</td><td>3.91</td><td>1.93</td><td>0.00</td><td>4.83</td></tr><tr><td><sup>241</sup>Am</td><td>65.54</td><td>3.96</td><td>2.31</td><td>0.00</td><td>5.17</td></tr><tr><td rowspan="3">40</td><td rowspan="3">40</td><td><sup>244</sup>Cm</td><td>63.82</td><td>3.98</td><td>2.67</td><td>0.00</td><td>5.49</td></tr><tr><td><sup>239</sup>Pu</td><td>100.00</td><td>4.83</td><td>0</td><td>0.00</td><td>4.83</td></tr><tr><td><sup>241</sup>Am</td><td>96.52</td><td>5.17</td><td>0.13</td><td>0.00</td><td>5.17</td></tr><tr><td></td><td></td><td><sup>244</sup>Cm</td><td>89.40</td><td>5.44</td><td>0.55</td><td>0.00</td><td>5.51</td></tr></tbody></table>

* calculated as the number of counts in L'1 divided by the sum of the number of counts in L'1 and L'2.

‡ calculated as $E_{\text{mean}}(L'1) + E_{\text{mean}}(L'2) \times \text{Probability of counts in L'2}$.

Table 3 shows that below 12 µm for the thickness of L'1, there is a non-optimized alpha detection. Indeed, the mean energy is higher in L'2 than in L'1. It then becomes interesting when thicknesses between 16 µm and 20 µm for L'1 are considered. The most prominent part of the counts is now attributed to L'1 and most of the output energy is also emanating from this layer. The 16/40 µm configuration shows that all alpha are stopped within these two layers. Therefore, an optimum of 16 µm was considered for the thinnest layer.

## Preparation of the various layers

### Ultra-thin layer

Having access to micrometer-range plastic scintillator thin films with a satisfying thickness homogeneity is not straightforward. This homogeneity is mandatory since it guarantees the homogeneity of the detection of particles with high stopping power. Preparation methods include solvent evaporation,²⁵ spin coating,²⁶,²⁷,²⁸,²⁹ spray drying,³⁰ screen printing and Doctor-blading.³¹ Each technique shows both advantages and drawbacks, the simplest being the solvent evaporation since it does not require any expensive device. Spin coating and spray coating seem limited to low thicknesses, typically below 40 µm, so they are not suitable for the preparation of the thin layer of our phoswich detector. Our preliminary tests were conducted with solvent evaporation, but the best thickness homogeneity was around 15% over 100 cm², which is too far from our desired goal of 10% or less. In addition, we have seen with simulation that a modification of the thickness (thus a heterogeneity) may have a huge impact on both alpha and beta detection efficiency. We therefore decided to choose Dr. Blading method as it turned out to be the most versatile and reproducible method, at least in our hands. Using a semi-automatic coater device, materials with controlled thickness showing a standard deviation around 3% over a 100 cm² surface were readily obtained, and thicknesses as low as 5 µm
were achievable. The method was fast, reliable, solvent-abstemious, used commercial products such as the 280k polystyrene and was easy to prepare.

Concerning the decay time, hence the nature of the scintillator, three different possibilities were studied. The upper limit of hybrid scintillators is close to 85 ns, **Erreur ! Signet non défini.** but one can also think of the slowest decaying plastics, whose decay extends up to 285 ns (such as the commercially available EJ-240 or BC-444 plastic scintillators). Our experiments based on the digitizer performances revealed that a $\Delta\tau$ (the decay time difference between two neighboring layers) of 30 ns was sufficient to efficiently sort the interactions using the charge comparison method, as we will see in the following section. Therefore, our target was an ultra-thin layer of 15 µm showing a minimum decay time of 70 ns or a long-decaying layer of 240 ns based on a proprietary formulation was also studied (see Table 4).

### Thin scintillator

The thin scintillator was prepared according to the same procedure as for the ultra-thin scintillator, except that the formulation and Doctor-blade set-point was different so that the thickness was close to 150 µm and an intermediate decay time close to 40 ns (Table 4).

### Thick scintillator

This layer was prepared according to our literature precedent. **Erreur ! Signet non défini.** The principle of the neutron/gamma discrimination has been extensively discussed **Erreur ! Signet non défini.** and lies in the fact that heavy particles show a higher energy deposition per volume,³² hence leading to the creation of triplet states. In other words, gamma rays produce secondary electrons in the scintillator which have little or no chance to produce triplet states. Fast neutrons produce scattered protons that are able to ionize the matter, thus leading to theoretically 75% of triplet states after recombination. The same process occurs for alpha and triton, which are released after thermal neutron capture with ⁶Li, but these two particles are heavier than the proton, hence they create more triplet state densities. Ultimately, if two triplet states are close enough to annihilate each other, one jumps to its singlet state by intersystem crossing and deexcites with the emission of delayed fluorescence. In such triple-discriminating plastic scintillator, the enhancement of the triplet-triplet annihilation is made possible thanks to large amounts of 2,5-diphenyloxazole (PPO), paving the way to fast neutron/gamma discrimination. Thermal neutrons are detected after 6-lithium absorption, giving rise to the typical signature of the emitted alpha.³³ Our thick scintillator is thus composed of 0.05 wt% of isotopically enriched lithium-6. The thick scintillator exhibit a typical Factor of Merit, (calculation detailed in ESI), of 0.7 for $E \in [130; 577]$ keVee between fast neutrons and gamma rays.

### Three layer phoswich preparation

The whole phoswich detector was generally prepared by assembling the different layers altogether using the minimum of optical grease, thus allowing us to substitute easily one layer by another so as to study its influence on the global detection performances. A picture of such scintillator is provided in Figure 2A. Table 4 gives the main characteristics of the two phoswiches that will be used as case studies in this work. The thick scintillator was the same for both phoswich **PO-1** and **PO-2**.

<table><caption>Table 4. Characteristics of the different phoswiches presented in this work.</caption><thead><tr><th rowspan="2">Phoswich</th><th colspan="2">Thick layer</th><th colspan="2">Thin layer</th><th colspan="2">Ultra-thin layer</th></tr><tr><th>Thickness (mm)</th><th>Decay time (ns)</th><th>Thickness (µm)</th><th>Decay time (ns)</th><th>Thickness (µm)</th><th>Decay time (ns)</th></tr></thead><tbody><tr><td>PO-1</td><td>6.6</td><td>3.3</td><td>139</td><td>39</td><td>22</td><td>239</td></tr><tr><td>PO-2</td><td>6.6</td><td>3.3</td><td>150</td><td>35</td><td>17</td><td>71</td></tr></tbody></table>
Instrumentation

Digitization and signal processing

Various digitizers designed for Pulse Shape Discrimination are now available on the market. We turned
our attention to two CAEN digitizers: one is the DT5730B which provides real-time PSD bidimensional
spectra that can also be processed off-line thanks to the list mode, thus giving access to the whole
histogram dataset. In other words, when the pulses are sorted, giving either lobes (fast neutrons, beta
and gamma rays) or ovals (thermal neutrons and alpha), a focus on a dedicated zone allows recovering
the desired spectrum against the others. However the short and long gates cannot be modified offline.
The other digitizer is a DT5743 whose pulse-processing properties are more advanced than the
previous device permitting post-processing as the optimization of short and long gates, **Erreur ! Signet
non défini**. but it does not allow visualizing the real-time construction of the scatter plots.

For both digitizers, we used an algorithm based on the charge comparison method to sort the pulses
into a scatter plot. Again, we used a photophysical approach to distinguish all physical phenomena.
Thus, the scintillator allowing the PSD between neutrons and gamma rays is the fastest; the
intermediate scintillator (for beta detection can be finely tuned between 2 and 85 ns and the ultra-
thin layer is slower than the intermediate scintillator. Figure 2B shows the example of three different
pulses recorded by Time-Correlated Single Photon Counting (with 274-nm UV excitation) with an
example of the positioning of the temporal gate that will allow the charge comparison method.

<figure><img src="image_3.png" /><figcaption>Figure 2. A. Picture of OmniscintiTM PO-2 under 368-nm UV lamp. B. TCSPC of the three individual layers that are representative of sample PO-2, with decay values of 3.3 ns (blue curve), 35 ns (red curve) and 71 ns (black curve). The green dotted line represents an example of the positioning of the short gate, here 60 ns after the top of the pulse (λ<sub>ex</sub> = 274 nm, λ<sub>obs</sub> = 425 nm).</figcaption></figure>

PSD temporal gate optimization

Since many possibilities are offered for the decay time of the thin and ultra-thin scintillator, correlated
with the positioning of the long gate, we benefited from recent advances in software-assisted
parameterization of the digitizer. **Erreur ! Signet non défini.** This was performed using off-line
processing with the CAEN DT5743 digitizer, and was performed with all α/β/n/γ emitters to apprehend
the best temporal parameters. The example we discuss in this sub-section has been performed with
phoswich **PO-1**, which shows the highest difference in terms of scintillation decay time, since the ultra-
thin layer was prepared with a very long decay time of 239 ns and the thick scintillator's decay is less than 3.3 ns. More precisely, the parameters of this Omniscinti™ were as follows: classical thick layer (i.e. triple-discriminating scintillator with 6.6 mm thickness), thin layer with thickness 139 µm and decay time 39 ns, ultra-thin layer with thickness 22 µm and decay time 239 ns. This phoswich was therefore exposed to a combination of ¹⁴C (β), ²³Pu (α) and thermalized ²⁵²Cf (n/γ) radioactive sources, and the temporal gates were modified offline from 15 to 60 ns for the short gate and from 300 to 1150 ns for the long gate. The peak-to-peak distance of the scatter plots for all temporal gates combination are summarized in Figure 3 (note that bidimensional spectra and corresponding results are detailed in Figure S1 and Table S1 of the Supplementary Information). Figure 3 thus allows a visual inspection of the temporal parameters and their overall influence. Thereby, it is seen that incorrect charge comparison parameters are able to degrade, even inhibit the discrimination. Obviously, the effect is more pronounced when the short gate is incorrectly set. For instance, the whole dynamic of the scatter plot is barely used when the short gate is 60 ns, due to the fact that the Q tail/Q total value is very low in all cases. The best trade-off was chosen where most of the difference values were superior to their mean value. Accordingly, the short gate and the long gate were set at 30 ns and 750 ns, respectively.

<figure><img src="image_4.png" /><figcaption>Figure 3. Gaussian maximum distance peak to peak as a function of short and long PSD gates for PO-1; black box: Gaussian distance peak-to-peak illustration given as an example for short gate and the long gate respectively at 30 ns and 750 ns.</figcaption></figure>

Phoswich PO-2 was assessed using the DT5730B digitizer, which does not allow such semi-automatic optimization of the temporal parameters. However, manual optimization was set at 42 ns and 780 ns for the short gate and the long gate, respectively.
## Results of detection

### Mixture of radionuclides

The two phoswiches presented in Table 4 were tested against the same set of three radioactive sources, ¹⁴C, ²³⁹Pu and ²⁵²Cf surrounded by 5 cm HDPE, thereby exhibiting the five nuclear emissions of interest (alpha, beta, gamma rays, thermal and fast neutrons). The results are presented in Figure 4 and Figure 5 for PO-1 and PO-2, respectively. The comparison between all sources was not straightforward for the sake of clarity of the figures, as a highly active radioactive source may “hide” those with low activity. For both phoswiches, a powerful quintuple discrimination is clearly seen and four regions of interest corresponding to alpha, beta, neutrons and gamma can be drawn (see Table 5). Applying rectangular regions of interest allows therefore the access to the energy-calibrated pulse height spectra for each individual radionuclide (see Figure 4B and Figure 5B; calibration spectra are gathered in Figure S2 and S3 in SI).

<table><caption>Table 5. Example of rectangular regions of interest used to sort histograms according to the impinging ionizing radiation for phoswich PO-1 and PO-2. Digitizer were not the same for PO1 and PO2, with the use of DT5743 and DT5730B, respectively.</caption><thead><tr><th>Phoswich</th><th>Emitter</th><th>Abscissa (Q<sub>tot</sub>)</th><th>Abscissa (Energy (keVee))</th><th>Ordinate</th></tr></thead><tbody><tr><td rowspan="4">PO-1</td><td>Alpha</td><td>3.1-8.3</td><td>260-630</td><td>0.60-0.80</td></tr><tr><td>Beta</td><td>1.0-2.8</td><td>110-240</td><td>0.45-0.70</td></tr><tr><td>Fast and thermal neutrons</td><td>2.0-22</td><td>180-1620</td><td>0.27-0.42</td></tr><tr><td>Gamma</td><td>2.0-22</td><td>180-1620</td><td>0.10-0.26</td></tr><tr><td rowspan="4">PO-2</td><td>Alpha</td><td>5000-12000</td><td>250-600</td><td>0.40-0.62</td></tr><tr><td>Beta</td><td>0-3500</td><td>0-175</td><td>0.30-0.60</td></tr><tr><td>Fast and thermal neutrons</td><td>2300-35000</td><td>115-1750</td><td>0.19-0.33</td></tr><tr><td>Gamma</td><td>2300-35000</td><td>115-1750</td><td>0.00-0.1875</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Figure 4. A. Pulse shape discrimination spectra of OmniscintiTM PO-1. B. Energy histograms corresponding to each α, β, n and γ contributions from top to bottom according to the respective ROIs (see Table 5). C. (Qtail/Qtot) charge ratio histogram corresponding to the PSD spectrum taken in the range 130-577 keVee.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5. A. Pulse shape discrimination spectra of PO-2. B. Energy histograms corresponding to each α, β, n and γ contributions from top to bottom according to the respective ROIs. C. (Q tail /Q tot) charge ratio histogram corresponding to the PSD spectrum taken in the range 111-413 keVee.</figcaption></figure>

Calculating Figure of Merit values was not straightforward. The performances of our multi-fit model
were evaluated through a Kolmogorov test. Results of the fit done on the two phoswich detectors are
exposed in Figure 6 and Table 6.
<figure><img src="image_7.png" /><figcaption>Figure 6. Multi-fit method applied to FoM spectra for Omniscinti PO-1 (A) and Omniscinti PO-2 (B). 1: Zoom to show low-contributions components. 2: Cumulative function to check performances between multi fit and experimental data; Kolmogorov maximum difference is respectively 1.37% on for Omniscinti PO-1 and 1.40% for Omniscinti PO-2.</figcaption></figure>

<table><caption>Table 6. Multi-fit parameter values, state and associated errors.</caption><thead><tr><th colspan="5">PO-1</th></tr><tr><th>Component</th><th>Amplitude</th><th>Mean</th><th>Sigma</th><th>Tau</th></tr></thead><tbody><tr><td>γ</td><td>7.62 × 10<sup>-3</sup> (0.65%)</td><td>2.40 × 10<sup>-1</sup> (fixed)</td><td>2.70 × 10<sup>-2</sup> (fixed)</td><td>2.80 × 10<sup>-2</sup> (fixed)</td></tr><tr><td>n<sub>thermal</sub></td><td>7.10 × 10<sup>-4</sup> (7.04%)</td><td>2.88 × 10<sup>-1</sup> (fixed)</td><td>2.50 × 10<sup>-2</sup> (fixed)</td><td>-</td></tr><tr><td>n<sub>fast</sub></td><td>1.43 × 10<sup>-3</sup> (2.72%)</td><td>3.45 × 10<sup>-1</sup> (fixed)</td><td>3.92 × 10<sup>-2</sup> (fixed)</td><td>-</td></tr><tr><td>β</td><td>1.72 × 10<sup>-3</sup> (1.69%)</td><td>5.88 × 10<sup>-1</sup> (fixed)</td><td>6.40 × 10<sup>-2</sup> (fixed)</td><td>-</td></tr><tr><td>α</td><td>1.41 × 10<sup>-2</sup> (0.44%)</td><td>6.86 × 10<sup>-1</sup> (fixed)</td><td>1.71 × 10<sup>-2</sup> (fixed)</td><td>1.35 × 10<sup>-2</sup> (fixed)</td></tr><tr><th colspan="5">PO-2</th></tr><tr><th>Component</th><th>Amplitude</th><th>Mean</th><th>Sigma</th><th>Tau</th></tr><tr><td>γ</td><td>1.84 × 10<sup>-4</sup> (0.70%)</td><td>1.75 × 10<sup>-1</sup> (fixed)</td><td>4.35 × 10<sup>-2</sup> (fixed)</td><td>9.60 × 10<sup>-2</sup> (fixed)</td></tr><tr><td>n<sub>thermal</sub></td><td>1.79 × 10<sup>-5</sup> (9.21%)</td><td>2.30 × 10<sup>-1</sup> (fixed)</td><td>2.50 × 10<sup>-2</sup> (fixed)</td><td>-</td></tr><tr><td>n<sub>fast</sub></td><td>1.26 × 10<sup>-5</sup> (13.47%)</td><td>2.60 × 10<sup>-1</sup> (fixed)</td><td>2.50 × 10<sup>-2</sup> (fixed)</td><td>-</td></tr><tr><td>β</td><td>5.85 × 10<sup>-5</sup> (1.11%)</td><td>3.70 × 10<sup>-1</sup> (fixed)</td><td>1.00 × 10<sup>-1</sup> (fixed)</td><td>-</td></tr><tr><td>α</td><td>1.91 × 10<sup>-3</sup> (0.08%)</td><td>5.50 × 10<sup>-1</sup> (fixed)</td><td>2.09 × 10<sup>-2</sup> (fixed)</td><td>2.25 × 10<sup>-2</sup> (fixed)</td></tr></tbody></table>

Results show that the hypothesis concerning the choice of mathematical functions and parameters to fix is coherent. For both cases, Kolmogorov maximum difference is indeed below 1.5%. However, we show that this model is limited for low-contributions components as neutrons. This requires more performant analysis, using machine learning tools to improve the discrimination power before the fit step. Nevertheless, we succeeded in calculating the various figure of merit values (calculated according the common formula: distance of Gaussian maxima divided by the sum of full widths at half maximum in the range of 130-577 keVee for PO-1 and 111-413 keVee for PO-2). The numerical values of the two-by-two Factor of Merit (FoM) for the two phoswiches are gathered in Table 7. At a glance, one can see
that the alpha and beta contributions are better separated for phoswich **PO-1** than for phoswich **PO-2**. This was confirmed with the FoM values of FoM(γ/β) = 1.56 and FoM(γ/α) = 3.44 **for PO-1** and respectively FoM(γ/β) = 0.58 and FoM(γ/α) = 3.44 for **PO-2**. Overall FoM values from **PO-1** seem high enough to be further used as an unique probe for most of the case studies that may be found on radiological sites. On the other hand, the FoM discrimination factor finds its own limits in this complex case. For instance, FoM(β/α) seems higher for **PO-2** than **PO-1** (0.6 against 0.43) whereas the beta pulses of **PO-2** are more spread out than for **PO-1**, resulting in a Gaussian maximum shifted to the left. Further dedicated signal analysis will be developed to suit this new type of multi-layer scintillator.

<table><caption>Table 7. Factor of Merit values related to respective ($Q_{tail}/Q_{tot}$) charge ratio histogram from Figure 4C and Figure 5C for **PO-1** and **PO-2**, respectively.</caption><thead><tr><th></th><th>PO-1</th><th>PO-2</th></tr></thead><tbody><tr><td>FoM(γ / n<sub>fast</sub>)</td><td>0.64</td><td>0.34</td></tr><tr><td>FoM(γ / n<sub>thermal</sub>)</td><td>0.71</td><td>0.53</td></tr><tr><td>FoM(γ / β)</td><td>1.56</td><td>0.58</td></tr><tr><td>FoM(γ / α)</td><td>3.44</td><td>2.47</td></tr><tr><td>FoM(n<sub>thermal</sub> / β)</td><td>1.00</td><td>0.37</td></tr><tr><td>FoM(n<sub>thermal</sub> / α)</td><td>2.38</td><td>2.68</td></tr><tr><td>FoM(β / α)</td><td>0.43</td><td>0.6</td></tr></tbody></table>

## Detection of single radionuclides

### Beta emitters

As demonstrated in previous studies and our simulations, clear beta discrimination is paramount as its signature is surrounded by the others. The large range of beta energy considered here linked to its centered position of its dedicated layer in the phoswich makes crosstalk minimization a crucial point for such multi-layer scintillator. The same phoswich detectors **PO-1** and **PO-2** were thus exposed to beta sources showing various energies, from a few tens of keV ($^{14}$C) up to ≈2 MeV ($^{90}$Sr/$^{90}$Y). As was mentioned in the simulation study, the thickness of the thin layer controls the efficiency towards both beta counting efficiency and gamma background rejection. However, beta spectroscopy can also be achieved since usually the full energy of the impinging beta is being released in the material. Figure 7 represents the scatter plots for phoswich **PO-2** (scatter plots for phoswich **PO-1** are presented in Figure S4), when exposed to various beta emitters. As one can see, strong visual differences between A-D plots comply with a probable beta identification. The $^{14}$C detection gives rise to an almost vertically oriented, slightly tilted to the right plume, when more energetic $^{36}$Cl and $^{90}$Sr/$^{90}$Y are more penetrating and excite both thin and thick layers of the phoswich. Since $^{60}$Co is also a gamma emitter, a contribution of gamma-related scintillation is also visible in its scatter plot. Noteworthy is the lower plume at the $Q_{tail}/Q_{total}$ ordinate for $^{90}$Sr/$^{90}$Y which shows both $^{90}$Sr and $^{90}$Y populations, whatever the phoswich being used. Note that for both phoswich detectors, the $^{90}$Y spectrum extends out of the number of channels due to its high energy.
<figure><img src="image_8.png" /><figcaption>Figure 7. Pulse shape discrimination spectra of Omniscinti™ PO-2 against different beta emitters: (A) <sup>14</sup>C; (B) <sup>60</sup>Co; (C) <sup>36</sup>Cl; (D) <sup>90</sup>Sr/<sup>90</sup>Y. Acquisitions during 120 seconds.</figcaption></figure>

The beta detection efficiencies of phoswich PO-1 and PO-2 were performed with the same digitizer CAEN DT5730. The results are given in Table 8, and are calculated on the basis of the interactions that occur in the thin layer ROI only (see Table 5 for the zone definition). This efficiency is highly dependent with the zone refining and the thickness of the layer of interest, as well as operating configuration (ROI, thickness, etc). For phoswich PO-1, the efficiencies were measured with DT5743 and are very high for low-energy beta emitters <sup>14</sup>C and <sup>60</sup>Co, with values exceeding 34%. The efficiency then falls down when the energy of the beta increases, reaching 11% only for the high-energy <sup>90</sup>Y beta emitter. Beta counting efficiencies for phoswich PO-2 were measured with DT5730 and are also satisfying and fall within or even encompass standard devices used for such contamination detection.<sup>34</sup> The better counting efficiency for <sup>36</sup>Cl and <sup>90</sup>Sr/<sup>90</sup>Y sources for PO-2 could be explained by the fact that it is thicker than PO-1 (150 µm vs. 139 µm, see Table 4).

<table><caption>Table 8. Detection of beta emitters with Omniscinti™. Efficiencies were calculated over 2π in the beta ROI.</caption><thead><tr><th>Phoswich</th><th>Radionuclide</th><th>Efficiency (%)</th></tr></thead><tbody><tr><td rowspan="4">PO-1</td><td><sup>14</sup>C</td><td>34.0</td></tr><tr><td><sup>60</sup>Co</td><td>38.9</td></tr><tr><td><sup>36</sup>Cl</td><td>23.5</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>11.2</td></tr><tr><td rowspan="4">PO-2</td><td><sup>14</sup>C</td><td>14.0</td></tr><tr><td><sup>60</sup>Co</td><td>16.9</td></tr><tr><td><sup>36</sup>Cl</td><td>32.2</td></tr><tr><td><sup>90</sup>Sr/<sup>90</sup>Y</td><td>26.1</td></tr></tbody></table>
Alpha emitters

Alpha detection was here performed by the "ultra-thin" topmost layer of phoswiches **PO-1** and **PO-2** that were exposed to four different alpha emitters, namely $^{237}$Np, $^{239}$Pu, $^{241}$Am and $^{244}$Cm (only $^{239}$Pu and $^{244}$Cm for phoswich **PO-2**). The energies of their most prominent alpha disintegrations are reported in Table 9.35

<table><caption>Table 9. Main alpha disintegrations for $^{237}$Np, $^{239}$Pu, $^{241}$Am and $^{244}$Cm and their probabilities in parenthesis. Values are in keV.</caption><thead><tr><th>Radionuclide</th><th>Main disintegration</th><th>2<sup>nd</sup> disintegration</th><th>Lower disintegration</th></tr></thead><tbody><tr><td><sup>237</sup>Np</td><td>4788 (47.5%)</td><td>4771 (23%)</td><td>4767 (10%)</td></tr><tr><td><sup>239</sup>Pu</td><td>5156 (70.8%)</td><td>5143 (17.1%)</td><td>5106 (11.9%)</td></tr><tr><td><sup>241</sup>Am</td><td>5485 (84.4%)</td><td>5442 (13.2%)</td><td>5388 (1.7%)</td></tr><tr><td><sup>244</sup>Cm</td><td>5805 (76.6%)</td><td>5763 (23.3%)</td><td>-</td></tr></tbody></table>

Figure 8, Figure S5 and S6 show the corresponding scatter plots and pulse-height spectra obtained with the Omniscinti™ **PO-1** and **PO-2**, respectively with DT5743 and DT5730. Table 10 summarizes the key results of alpha detection. We can first comment on the good linearity of the MeV over channel plot (Figure 8F) in the region [4.8; 5.8] MeV from these four alpha emitters, which highlights a good consistency of this type of organic multi-layer phoswiches to perform alpha spectroscopy. It is also observed that the output energy is highly reduced from the incident alpha emitter, as is usually the case due to strong quenching. Here the ratio between output and incident energy falls in the range 7.3-9.5%, which is usually the case for plastic scintillators.36 The energy resolutions of 27.5%, 19.4%, 17.1% and 14.5% for $^{237}$Np, $^{239}$Pu, $^{241}$Am and $^{244}$Cm, respectively, are calculated for phoswich **PO-1**. These values are close to the reported value of an NE-102 plastic scintillator (16.1% for $^{241}$Am energy) but slightly higher. **Erreur ! Signet non défini.** The main reason for this poorer resolution can be attributed to the lower light output of the slow thin film than NE-102. In addition, the absence of reflective or diffusing material increases the loss of photons that have to travel the full phoswich detector towards the photodetector. Another reason could be attributed to the low thickness of the thin film, since Morishita et al. noticed a degradation of the energy resolution – reaching 16.7% for $^{241}$Am when detected with for 55-µm thin films. **Erreur ! Signet non défini.** It is noteworthy that here our thin film is 22 µm only and still displays an energy resolution of 17.1% for $^{241}$Am. Interestingly the resolution degrades rather fast when the energy of the alpha emitter decreases. This feature is still unclear to our eyes; already explained optical issues might be the main driver here. The detection efficiency is moderate to good (between 17.3% and up to 85.4%) and can be explained by the loss of photons in the refractive index variations of the phoswich detector. In addition, 237Np is also a beta emitter, as can easily be seen on Figure 8A, with the beta which appear at the low energy part of the α spectrum.

Phoswich **PO-2** was only tested against $^{239}$Pu and $^{244}$Cm emitters (see Figure S6). The observed resolution for these radionuclides was of the same range as that of phoswich **PO-1**, but the detection efficiency was much better, reaching an interesting 85% for $^{244}$Cm against ca. 73.2% for the first material used in this study. However, the energy position of the full absorption peaks was usually located at lower values for phoswich **PO-2** (see the output/main column). This would indicate a lower light output for the thin film of phoswich **PO-2** compared with the one of phoswich **PO-1**.
<figure><img src="image_9.png" /><figcaption>Figure 8. A, B, C and D: scatter plots for $^{237}$Np, $^{239}$Pu, $^{241}$Am and $^{244}$Cm alpha emitters, respectively for phoswich PO-1. E: respective pulse height spectra for $Q_{tail}/Q_{tot}$ ∈ [0.6; 0.8], expressed in %Counts. F: Main disintegration energy over detected channel for phoswich PO-1.</figcaption></figure>

<table><caption>Table 10. Detection of alpha emitters with OmniscintiTM. Efficiencies are calculated over 2π in the range given by the alpha ROI (see Table 5).</caption><thead><tr><th>Sample</th><th>Radionuclide</th><th>Output (keVee)</th><th>Output (keVee)/Main Energy (keVee) (%)</th><th>Resolution (%)</th><th>Efficiency (%)</th></tr></thead><tbody><tr><td rowspan="4">PO-1</td><td><sup>237</sup>Np</td><td>395</td><td>8.2</td><td>27.5</td><td>21.2</td></tr><tr><td><sup>239</sup>Pu</td><td>457</td><td>8.9</td><td>19.4</td><td>17.3</td></tr><tr><td><sup>241</sup>Am</td><td>501</td><td>9.1</td><td>17.1</td><td>19.1</td></tr><tr><td><sup>244</sup>Cm</td><td>553</td><td>9.5</td><td>14.5</td><td>73.2</td></tr><tr><td rowspan="2">PO-2</td><td><sup>239</sup>Pu</td><td>377</td><td>7.3</td><td>17.6</td><td>46.3</td></tr><tr><td><sup>244</sup>Cm</td><td>428</td><td>7.3</td><td>16.2</td><td>85.4</td></tr></tbody></table>

To dig further into alpha spectroscopy, we benefitted from the access to mixed alpha sources prepared in the Henri Becquerel National Laboratory (LNHB). Hereafter only phoswich detector PO-1 was assessed. A first source, named S1, exhibiting three alpha components $^{237}$Np, $^{241}$Am, $^{244}$Cm with respective activities 307 Bq, 393 Bq, 93 Bq and a second source, named S2, exhibiting 4 alpha components $^{237}$Np, $^{239}$Pu, $^{241}$Am, $^{244}$Cm with respective activities of 365 Bq, 409 Bq, 355 Bq and 93 Bq. The following scatter plots and pulse height spectra from the 3 or 4 mixtures of alpha signatures (grey crosses) are gathered in Figure 9. Furthermore, we performed a deconvolution of the pulse height
spectra with single alpha signatures (broken lines) with the deconvoluted alphas fit model, using equation [2]. The following reconstruction spectra (black lines, corresponding to the weighted sum of each alpha emitter) superposes correctly to the pulse height spectra. Maximum Kolmogorov distance is estimated at 0.55% for 3-α emitters source and 0.79% for 4-α emitters one. The properties of mixing sources S1 and S2 are reported in Table 11 and the details of the multi-fit method applied, for those cases, are explained below. Thereby, Omniscinti™, as a multi-layer phoswich, additionally succeeds in unfolding a mix of three to four alpha emitters with close emission energies (in the region [4.8; 5.8] MeV). This is an unprecedented result in a fully organic phoswich detector.

<figure><img src="image_10.png" /><figcaption>Figure 9. A, C: scatter plots for $^{237}$Np-$^{241}$Am-$^{244}$Cm and $^{237}$Np-$^{239}$Pu-$^{241}$Am-$^{244}$Cm, respectively as mixed-sources; B, D: corresponding normalized pulse height spectra for $Q_{tail}/Q_{tot} \in [0.6; 0.8]$, and associated components deconvolution. C, F: cumulative functions used to apply Kolmogorov test.</figcaption></figure>

<table><caption>Table 11. Multi-components alpha sources percentages activities and results of least square component deconvolution.</caption><thead><tr><th>Alpha sources</th><th></th><th><sup>237</sup>Np</th><th><sup>239</sup>Pu</th><th><sup>241</sup>Am</th><th><sup>244</sup>Cm</th></tr></thead><tbody><tr><td rowspan="5">S1</td><td>Source activity (Bq)</td><td>306</td><td>n.a.</td><td>399</td><td>94</td></tr><tr><td>Alpha activity (%)</td><td>38.3</td><td>n.a.</td><td>49.9</td><td>11.8</td></tr><tr><td>Unfolded activity by least square method (%)</td><td>32.2</td><td>n.a.</td><td>58.4</td><td>9.3</td></tr><tr><td>Unfolded activity by Gauss' pivot method (%)</td><td>30.9</td><td>n.a.</td><td>59.8</td><td>9.4</td></tr><tr><td>Relative error between both methods (%)</td><td>4.6</td><td>n.a.</td><td>2.3</td><td>0.7</td></tr><tr><td rowspan="3">S2</td><td>Source activity (Bq)</td><td>370</td><td>410</td><td>355</td><td>97</td></tr><tr><td>Alpha activity (%)</td><td>30.01</td><td>33.33</td><td>28.79</td><td>7.87</td></tr><tr><td>Unfolded activity by least square method (%)</td><td>22.2</td><td>41.8</td><td>30.5</td><td>5.5</td></tr></tbody></table>

n.a.: non applicable

As mentioned, individual spectra associated to pure α components: $^{237}$Np, $^{239}$Pu, $^{241}$Am and $^{244}$Cm are measured and fitted using function [2] (cf. plot E, Figure 8). The fit of individual components allows to get function parameters associated to each pure α component (A, μ, σ and τ). Associated results are given in Table 12.
<table><caption>Table 12. Fit parameters values associated to each pure α source.</caption><thead><tr><th>Source</th><th>Amplitude (A)</th><th>Mean (μ)</th><th>Sigma (σ)</th><th>Tau (τ)</th></tr></thead><tbody><tr><td><sup>237</sup>Np</td><td>5.04 × 10<sup>-2</sup> (0.99%)</td><td>444.68 (0.05%)</td><td>30.88 (0.92%)</td><td>60.59 (1.06%)</td></tr><tr><td><sup>239</sup>Pu</td><td>6.49 × 10<sup>-2</sup> (0.31%)</td><td>481.36 (0.01%)</td><td>24.93 (0.29%)</td><td>43.09 (0.37%)</td></tr><tr><td><sup>241</sup>Am</td><td>6.07 × 10<sup>-2</sup> (0.52%)</td><td>522.23 (0.04%)</td><td>28.93 (0.48%)</td><td>33.50 (0.93%)</td></tr><tr><td><sup>244</sup>Cm</td><td>5.97 × 10<sup>-2</sup> (0.23%)</td><td>578.82 (0.02%)</td><td>26.33 (0.22%)</td><td>28.90 (0.45%)</td></tr></tbody></table>

This method provides a precise and consistent mathematical function taking into account the individual α spectra shapes. Associated errors on each parameter are below 1.1%.

In this way, parameters got from the fit of individual α source can be used to build a multi-fit model to extract and quantify the proportion of each α component inside S1 and S2. This multi-fit model is composed by the 3 (S1) or 4 (S2) mathematical functions [2]. Each of those are associated to individual α spectra embedded inside the considered mixing source.

For S1 and for $^{241}$Am and $^{244}$Cm, parameters σ and τ are fixed to values extracted from their respective individual fit spectrum. μ is set as free parameter to take into account baseline shift which can occur between individual source and mixing source measurements. Concerning $^{237}$Np, all parameters are set as free. In addition to the baseline shift, this is due to the non-negligible β contamination which appears at the low energy part of its α spectrum. Multi-fit performances are evaluated through a Kolmogorov test (Figure 9C) and parameters values are summarized in Table 13.

<table><caption>Table 13. Multi-fit parameters values for each α emitters of S1 and the associated measured proportions.</caption><thead><tr><th>Component</th><th>Amplitude</th><th>Mean</th><th>Sigma</th><th>Tau</th><th>p<sup>meas</sup></th></tr></thead><tbody><tr><td><sup>237</sup>Np</td><td>1.75e-2 (0.6%)</td><td>412.80 (0.04%)</td><td>26.85 (0.62%)</td><td>24.43 (1.40%)</td><td>32.3%</td></tr><tr><td><sup>241</sup>Am</td><td>2.93e-2 (0.27%)</td><td>526.44 (0.03%)</td><td>28.93 (fixed)</td><td>33.50 (fixed)</td><td>58.4%</td></tr><tr><td><sup>244</sup>Cm</td><td>5.1e-3 (1.85%)</td><td>580.77 (0.03%)</td><td>26.33 (fixed)</td><td>28.90 (fixed)</td><td>9.3%</td></tr></tbody></table>

The consistency of the measured proportions given by multi-fit method on S1, is checked through a Gauss' Pivot (GP) method. This is based on the average conservation of 3 variables used in the analysis. Those are PSD, $Q_{tot}$ and $Q_{tail}$. Despite the capability of this method to extract the measured proportions from analytical computation, this has limited performances. The first reason is due to the variables selection. It is better that these ones are fully independent in order to keep the same sensitivity degree between those. The second one is directly correlated to the average conservation laws based on the mean of the distribution of PSD, $Q_{tot}$ and $Q_{tail}$ for individual sources measurements and mixing one. This requires that baseline shift is negligible to get more performant results. According to these two limitations, GP was done to provide an order of magnitude of expected measured proportions.

The 3 equations injected inside GP and in accordance to the average conservation laws are the following ones:

$$P_{1}^{\text{meas}} \cdot Q_{\text{tail}_1} + P_{2}^{\text{meas}} \cdot Q_{\text{tail}_2} + P_{3}^{\text{meas}} \cdot Q_{\text{tail}_3} = Q_{\text{tail mix}}$$

$$P_{1}^{\text{meas}} \cdot Q_{\text{tot}_1} + P_{2}^{\text{meas}} \cdot Q_{\text{tot}_2} + P_{3}^{\text{meas}} \cdot Q_{\text{tot}_3} = Q_{\text{tot mix}}$$

$$P_{1}^{\text{meas}} \cdot P^{\text{SD}}_{1} + P_{2}^{\text{meas}} \cdot P^{\text{SD}}_{2} + P_{3}^{\text{meas}} \cdot P^{\text{SD}}_{3} = P^{\text{SD}}_{\text{mix}} \quad [3]$$
where index 1 refers to $^{237}$Np, 2 to $^{241}$Am and 3 to $^{244}$Cm.

Through GP method, measured proportions can be deduced by:

$$ P_1^{\text{meas}} = \frac{\begin{vmatrix} Q_{\text{tail}_2} & Q_{\text{tail}_3} & Q_{\text{tail}_\text{mix}} \\ Q_{\text{tot}_2} & Q_{\text{tot}_3} & Q_{\text{tot}_\text{mix}} \\ P_{\text{SD}_2} & P_{\text{SD}_3} & P_{\text{SD}_\text{mix}} \end{vmatrix}}{\begin{vmatrix} Q_{\text{tail}_1} & Q_{\text{tail}_2} & Q_{\text{tail}_3} \\ Q_{\text{tot}_1} & Q_{\text{tot}_2} & Q_{\text{tot}_3} \\ P_{\text{SD}_1} & P_{\text{SD}_2} & P_{\text{SD}_3} \end{vmatrix}}, P_2^{\text{meas}} = \frac{\begin{vmatrix} Q_{\text{tail}_1} & Q_{\text{tail}_3} & Q_{\text{tail}_\text{mix}} \\ Q_{\text{tot}_1} & Q_{\text{tot}_3} & Q_{\text{tot}_\text{mix}} \\ P_{\text{SD}_1} & P_{\text{SD}_3} & P_{\text{SD}_\text{mix}} \end{vmatrix}}{\begin{vmatrix} Q_{\text{tail}_1} & Q_{\text{tail}_2} & Q_{\text{tail}_3} \\ Q_{\text{tot}_1} & Q_{\text{tot}_2} & Q_{\text{tot}_3} \\ P_{\text{SD}_1} & P_{\text{SD}_2} & P_{\text{SD}_3} \end{vmatrix}}, P_3^{\text{meas}} = \frac{\begin{vmatrix} Q_{\text{tail}_1} & Q_{\text{tail}_2} & Q_{\text{tail}_\text{mix}} \\ Q_{\text{tot}_1} & Q_{\text{tot}_2} & Q_{\text{tot}_\text{mix}} \\ P_{\text{SD}_1} & P_{\text{SD}_2} & P_{\text{SD}_\text{mix}} \end{vmatrix}}{\begin{vmatrix} Q_{\text{tail}_1} & Q_{\text{tail}_2} & Q_{\text{tail}_3} \\ Q_{\text{tot}_1} & Q_{\text{tot}_2} & Q_{\text{tot}_3} \\ P_{\text{SD}_1} & P_{\text{SD}_2} & P_{\text{SD}_3} \end{vmatrix}} \quad [4] $$

Results from GP and comparison with multi-fit method are reported in Table 14:

<table><caption>Table 14. Proportion measured from GP method and comparison with multi-fit.</caption><thead><tr><th>Component</th><th>P<sub>i</sub><sup>meas</sup> (%) Multi-fit</th><th>P<sub>i</sub><sup>meas</sup> (%) GP</th><th>Relative error with multi-fit (%)</th></tr></thead><tbody><tr><td><sup>237</sup>Np</td><td>32.3%</td><td>30.86%</td><td>4.63%</td></tr><tr><td><sup>241</sup>Am</td><td>58.4%</td><td>59.78%</td><td>2.27%</td></tr><tr><td><sup>244</sup>Cm</td><td>9.3%</td><td>9.35%</td><td>0.74%</td></tr></tbody></table>

Despite their respective limitations, both methods are pretty consistent between those and show an agreement below than 5%. This proves that multi-fit method is performant and parameters state imposed (free or fixed) are justified.

For S2 and for ($^{237}$Np, $^{241}$Am and $^{244}$Cm) components, ($\sigma$ and $\tau$) parameters are fixed to the values obtained for each of those through 3-$\alpha$ emitters source multi-fit model. Concerning $^{239}$Pu, same parameters are fixed to ones got from its individual fit spectrum. For same reasons (baseline shift during measurements), $\mu$ parameter is let as free. As for previous model, performances of this are evaluated through Kolmogorov test (Figure 9F). Parameters values are summarized in Table 15.

<table><caption>Table 15. Multi-fit parameters values for each $\alpha$ emitters of S2 and the associated measured proportions.</caption><thead><tr><th>Component</th><th>Amplitude</th><th>Mean</th><th>Sigma</th><th>Tau</th><th>p<sup>meas</sup></th></tr></thead><tbody><tr><td><sup>237</sup>Np</td><td>1.18 × 10<sup>-2</sup> (0.87%)</td><td>432.75(0.04%)</td><td>26.85 (fixed)</td><td>24.43 (fixed)</td><td>22.2%</td></tr><tr><td><sup>239</sup>Pu</td><td>2.39 × 10<sup>-2</sup> (0.50%)</td><td>490.21 (0.07%)</td><td>24.93 (fixed)</td><td>43.09 (fixed)</td><td>41.8%</td></tr><tr><td><sup>241</sup>Am</td><td>1.49 × 10<sup>-2</sup> (0.58%)</td><td>550.74 (0.14%)</td><td>28.93 (fixed)</td><td>33.50 (fixed)</td><td>30.5%</td></tr><tr><td><sup>244</sup>Cm</td><td>2.99 × 10<sup>-3</sup> (5.01%)</td><td>604.18 (0.34%)</td><td>26.33 (fixed)</td><td>28.90 (fixed)</td><td>5.5%</td></tr></tbody></table>

## Conclusions

We present herein a universal scintillating detector supported by three different strategies: morphological (scintillator stacking), photophysical (fast neutron/gamma discrimination, hybrid scintillators) and chemical (thermal neutron absorber owing to $^{6}$Li doping). For the first time up to five natures of deexcitation are sorted within one single sensor. This fully-organic phoswich is thus able to separate gamma rays, fast neutrons, thermal neutrons, beta and alpha contributions with good to excellent Figure of Merit values (from 0.43 up to 3.44). Its performances were further confirmed with the detection beta emitters ranging from $^{14}$C up to $^{90}$Sr/$^{90}$Y with efficiencies of 34.0% and 11.2%, respectively, and to detect alpha emitters from $^{237}$Np to $^{244}$Cm with a resolution as good as 14.5% and an efficiency up to 73.2%. We furthermore accessed to alpha spectroscopy in the region [4.8; 5.8] MeV
and were able to separate and categorize successfully mixtures of 3 or 4 alpha emitters with standard
deviation of 26-28 keVee (given by σ, equation 2). Some of these key parameters may appear lower
than other detectors that are more specialized, but the purpose of this work was to provide a universal
detector, so-called Omniscinti™. Future work will consist on an optimization of both detectors and
signal processing, in particular in using more sophisticated deconvolution or sorting algorithms. Then
this new kind of phoswich detector will be implemented into a prototype of device and tested in real
field conditions with first responders.

Acknowledgements

The "Institut Carnot" at CEA List is acknowledged for the funding of the Capsud project. Sylvie Pierre and the "Laboratoire de Métrologie de l'Activité" from the Henri Becquerel National Laboratory are acknowledged for granting the access to mixed alpha sources.

Authors contributions

CF: Conceptualization, investigation, writing – original draft, writing – review and editing.

GHVB: Supervision, project administration, funding acquisition, writing – review and editing.

FC: Formal analysis, data curation, software, writing – review and editing.
CL: Data curation, software, writing – review and editing.
VB: Formal analysis, data curation, software, validation, writing – review and editing.
MH: Conceptualization, investigation, writing – original draft, writing – review and editing.

Conflict of interest

The Omniscinti phoswich detector is patented by some of the Authors.³⁷

References

1 Pellegrin, S. M.; Whitney, C.; Wilson, C. G. *J. Microelectromech. Syst.* **2010**, *19*, 1207-1214.
https://doi.org/10.1109/JMEMS.2010.2067435

2 Pagano, E. V.; Chatterjee, M. B.; De Filippo, E.; Russotto, P.; Auditore, L.; Cardella, G.; Geraci, E.; Gnoffo, B.; Guazzoni, C.; Lanzalone, G.; De Luca, S.; Maiolino, C.; Martorana, N. S.; Pagano, A.; Papa, M.; Parsani, T.; Pirrone, S.; Politi, G.; Porto, F.; Quattrocchi, L.; Rizzo, F.; Trifirò, A.; Trimarchi, M. *Nucl. Instr. Methods A* **2018**, *889*, 83-88. https://doi.org/10.1016/j.nima.2018.02.010

3 Min, S.; Seo, B.; Roh, C.; Hong, S.; Cheong, J. *Sensors* **2021**, *21*, 4047.
https://doi.org/10.3390/s21124047

4 https://www.nuviatech-instruments.com/product/hand-held-contamination-monitor
(accessed Feb. 4, 2022)

5 Futami, Y.; Mizota, T.; Pu, Y. H.; Honjo, Y.; Yuasa-Nakagawa, K.; Toyokawa, H.; Lee, S. M.; Furutaka, K.; Murakami, T.; Kasagi, J.; Yoshida, K.; Nakagawa, T. *Nucl. Instr. Methods A* **1993**, *326*, 513-516.
https://doi.org/10.1016/0168-9002(93)90853-A

6 White, T. L.; Miller, W. H. *Nucl. Instr. Methods A* **1999**, *422*, 144-147.
https://doi.org/10.1016/S0168-9002(98)01090-0
7 Yamamoto, S.; Ishibashi, H. *Nucl. Instr. Methods A* **2015**, 785, 129-134.
https://doi.org/10.1016/j.nima.2015.02.062

8 Yasuda, K.; Usuda, S.; Gunji, H. *IEEE Trans. Nucl. Sci.* **2001**, 48, 1162-1164.

9 Usuda, S. *Nucl. Instr. Methods A* **1995**, 356, 334-338. https://doi.org/10.1016/0168-9002(94)01212-1

10 Morishita, Y.; Yamamoto, S.; Izaki, K.; Kaneko, J. H.; Toui, K.; Tsubota, Y.; Higuchi, M. *Nucl. Instr. Methods A* **2014**, 764, 383-386. http://dx.doi.org/10.1016/j.nima.2014.07.046

11 Morishita, Y.; Hoshi, K.; Torii, T. *Nucl. Instr. Methods A* **2020**, 966, 163795.
https://doi.org/10.1016/j.nima.2020.163795

12 Morishita, Y.; Di Fulvio, A.; Clarke, S. D.; Kearfott, K. J.; Pozzi, S. A. *Nucl. Instr. Methods A* **2019**, 935, 207-213. https://doi.org/10.1016/j.nima.2019.04.024

13 Bertrand, G. H. V.; Hamel, M.; Normand, S.; Sguerra, F. *Nucl. Instr. Methods A* **2015**, 776, 114-128. http://dx.doi.org/10.1016/j.nima.2014.12.024

14 Hartman, J.; Barzilov, A. *EPJ Web Conf.* **2017**, 153, 07012.
https://doi.org/10.1051/epjconf/201715307012

15 Cester, D.; Lunardon, M.; Moretto, S.; Nebbia, G.; Pino, F.; Sajo-Bohus, L.; Stevanato, L.; Bonesso, I.; Turato, F. *Nucl. Instr. Methods A* **2016**, 830, 191-196. https://doi.org/10.1016/j.nima.2016.05.079

16 Hartman, J.; Barzilov, A. *Trans. Am. Nucl. Soc.* 2016, 114, 958-960.
https://www.ans.org/pubs/transactions/article-38731/

17 Usuda, S. *Nucl. Instr. Methods A* **1995**, 356, 334-338. https://doi.org/10.1016/0168-9002(94)01212-1

18 Frangville, C.; Hamel, M.; Montbarbon, E.; Bertrand, G. H. V.; Grabowski, A.; Lynde, C. *Mater. Chem. Front.* **2019**, 3, 1626-1631. https://doi.org/10.1039/c9qm00153k

19 Hamel, M.; Soumaré, M.; Burešová, H.; Bertrand, G. H. V. *Dyes Pigm.* **2019**, 165, 112-116.
https://doi.org/10.1016/j.dyepig.2019.02.007

20 Bé, M.-M.; Chisté, V.; Dulieu, C.; Kellett, M. A.; Mougeot, X.; Arinc, A.; Chechev, V. P.; Kuzmenko, N. K.; Kibédi, T. Luca, A.; Nichols, A. L. *Monographie BIPM-5 : Table of Radionucléides*. Bureau International des Poids et Mesures. 2016. ISBN 978-92-822-2264-5.

21 Lynde, C.; Montbarbon, E.; Hamel, M.; Grabowski, A.; Frangville, C.; Bertrand, G. H. V.; Galli, G.; Carrel, F.; Schoepff, V.; El Bitard, Z. *IEEE Trans. Nucl. Sci.* **2020**, 67, 679-687.
https://doi.org/10.1109/TNS.2020.2966886

22 Werner, C. J.; Bull, J. S.; Solomon, C. J.; Brown, F. B.; Mckinney, G. W.; Rising, M. E.; Dixon, D. A.; Martz, R. L.; Hughes, H. G.; Cox, L. J.; Zukaitis, A. J.; Armstrong, J. C.; Forster, R. A.; Casswell, L. *MCNP Version 6.2 Release Notes*, http://doi.org/10.2172/1419730

23 Brooks, F. D. *Nucl. Instr. Methods* **1959**, 4, 151-163. https://doi.org/10.1016/0029-554X(59)90067-9

24 http://www.lnhb.fr/nuclides/C-14_tables.pdf

25 Merín, R.; Tarancón, A.; Mitev, K.; Georgiev, S.; Dutsov, C.; Bagán, H.; García, J. F. J. *Radioanal. Nucl. Chem.* **2019**, 319, 135-145. https://doi.org/10.1007/s10967-018-6341-z

26 Norbeck, E.; Dubbs, T. P.; Sobotka, L. G. *Nucl. Instr. Methods A* **1987**, 262, 546-547.
https://doi.org/10.1016/0168-9002(87)90901-6

27 Weathers, L. W.; Tsang, M. B. *Nucl. Instr. Methods A* **1996**, 381, 567-568.
https://doi.org/10.1016/S0168-9002(96)00779-6

28 Inami, W.; Fukuta, M.; Masuda, Y.; Nawa, Y.; Ono, A.; Lin, S.; Kawata, Y.; Terakawa, S. *Opt. Rev.* **2015**, 22, 354-358. https://doi.org/10.1007/s10043-015-0043-3

29 Chakraborty, S.; Harris, K.; Huang, M. *AIP Adv.* **2016**, 6, 125113.
https://doi.org/10.1063/1.4972989
30 Thomson, J. European Patent Application EP0576090.

31 Lee, K. W.; Seo, B. K.; Han, M. J. *J. Nucl. Sci. Technol.* **2010**, *47*, 84-89.
https://doi.org/10.1080/18811248.2010.9711930

32 Horrocks, D. L. *Rev. Sci. Instrum.* **1963**, *34*, 1035-1040. https://doi.org/10.1063/1.1718650

33 Dumazert, J.; Frangville, C. In *Plastic Scintillators: chemistry and applications*; Hamel, M., Ed.; Springer Nature: Cham, 2021, p 91-137. https://doi.org/10.1007/978-3-030-73488-6_3

34 Kang, H.; Min, S.; Seo, B.; Roh, C.; Hong, S.; Cheong, J. H. *Chemosensors* **2020**, *8*, 106.
https://doi.org/10.3390/chemosensors8040106

35 http://www.lnhb.fr/nuclides/

36 Tretyak, V. I. *Astropart. Phys.* **2010**, *33*, 40-53.
https://doi.org/10.1016/j.astropartphys.2009.11.002

37 Frangville, C.; Hamel, M.; Bertrand, G. French Patent Application FR2106228, 2021.