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Article SiLiF Neutron Counters to Monitor Nuclear Materials in the MICADO Project

Luigi Cosentino 1, Quentin Ducasse 2 , Martina Giuffrida 1,3, Sergio Lo Meo 4, Fabio Longhitano 1 , Carmelo Marchetta 1, Antonio Massara 1, Alfio Pappalardo 1,†, Giuseppe Passaro 1, Salvatore Russo 1 and Paolo Finocchiaro 1,3,*

1 INFN Laboratori Nazionali del Sud, 95123 Catania, Italy; [email protected] (L.C.); [email protected] (M.G.); [email protected] (F.L.); [email protected] (C.M.); [email protected] (A.M.); alfi[email protected] (A.P.); [email protected] (G.P.); [email protected] (S.R.) 2 CEA, DES, IRESNE, Nuclear Measurement Laboratory, Cadarache, F-13108 Saint-Paul-lez-Durance, France; [email protected] 3 Centro Siciliano di Fisica Nucleare e Struttura della Materia, 95123 Catania, Italy 4 ENEA Centro Ricerche, 40129 Bologna, Italy; [email protected] * Correspondence: fi[email protected] † Current address: Extreme Light Infrastructure Nuclear Physics, 077125 Magurele, Romania.

Abstract: In the framework of the MICADO (Measurement and Instrumentation for Cleaning And Decommissioning Operations) European Union (EU) project, aimed at the full digitization of low- and intermediate-level radioactive waste management, a set of 32 solid state thermal neutron detectors



 named SiLiF has been built and characterized. MICADO encompasses a complete active and passive characterization of the radwaste drums with neutrons and gamma rays, followed by a longer-term Citation: Cosentino, L.; Ducasse, Q.; monitoring phase. The SiLiF detectors are suitable for the monitoring of nuclear materials and Giuffrida, M.; Lo Meo, S.; Longhitano, F.; Marchetta, C.; Massara, A.; can be used around radioactive waste drums possibly containing small quantities of actinides, as Pappalardo, A.; Passaro, G.; Russo, S.; well as around spent fuel casks in interim storage or during transportation. Suitable polyethylene et al. SiLiF Neutron Counters to moderators can be exploited to better shape the detector response to the expected neutron spectrum, Monitor Nuclear Materials in the according to Monte Carlo simulations that were performed. These detectors were extensively tested MICADO Project. Sensors 2021, 21, with an AmBe neutron source, and the results show a quite uniform and reproducible behavior. 2630. https://doi.org/10.3390/ s21082630 Keywords: neutron detectors; radwaste management; radwaste monitoring

Academic Editor: Kelum Gamage

Received: 19 March 2021 1. Introduction Accepted: 6 April 2021 Published: 8 April 2021 In the framework of the MICADO (Measurement and Instrumentation for Cleaning And Decommissioning Operations) European Union (EU) project [1], whose goal is to

Publisher’s Note: MDPI stays neutral establish new, fully digital assessment and management of low- and intermediate-level with regard to jurisdictional claims in radioactive waste, we have produced and characterized a full set of radiation counters published maps and institutional affil- suitable for monitoring gamma rays and neutrons. In particular the goal of the Work iations. Package 7 is to set up a granular radwaste monitoring system to be used during the final MICADO demonstration [2]. Indeed, the practice so far has been to store the radwaste drums following their initial characterization, the only monitoring consisting in an overall set of few ambient detectors and periodic manual checks done by operators. The proposed system for online real-time monitoring consists of an array of many radiation sensors to be Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. deployed all around a number of radioactive waste drums, in order to collect counting-rate This article is an open access article data in real time and to make them available to a software platform under development distributed under the terms and named DigiWaste. In order to be suitable for mass deployment these detectors have to be conditions of the Creative Commons compact, reasonably inexpensive, robust, easy-to-use and reliable. Attribution (CC BY) license (https:// Continuous radiological monitoring of radwaste has to be based on the measurement creativecommons.org/licenses/by/ of gamma and neutron radiation, since these are penetrating types of radiation easily 4.0/). detectable out of the drums [3]. This is the reason why the focus in MICADO was placed

Sensors 2021, 21, 2630. https://doi.org/10.3390/s21082630 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 2630 2 of 22

on the development of detectors for gamma rays and neutrons. The overall system must be modular, so that one can easily modify number and placement of the sensors around the drums, and it has to be scalable in order to make it possible to tailor it to small-, medium- and large-scale storage configurations without conceptual limitations [4–6]. In this paper we focus on the neutron detection, and describe the SiLiF thermal neutron counters that we developed. They are based on a solid state silicon detector coupled to a thin layer of a neutron converter material, which upon capture of a low energy neutron produces an alpha particle and a triton that can be detected in the silicon [6–18]. As compared to the well known thermal neutron detectors, based on 3He whose price has been increasing and whose availability has been decreasing in the last decades, the SiLiF technology allows to achieve comparable performances at about one order of magnitude lower prices [7]. A set of SiLiF detectors was built and characterized by means of an intense AmBe neutron source whose flux was simulated and measured in the relevant positions. As a final exercise, using the characterization results, we made an estimate of the detection sensitivity with respect to the possible presence of small amounts of 240Pu inside a waste drum.

2. Materials and Methods The mainstream technology for thermal neutron detection so far has been based on 3He, that is an artificially produced gas with a 5330 b cross section for the following reaction.

n + 3He → 3H (0.191 MeV) + p (0.573 MeV) (1)

The main source of 3He is the nuclear weapons program of USA and Russia, as a byproduct of the 3H (tritium) supply which undergoes a nuclear decay into 3He with a 12.3 years half life. The shortage of 3He in the last decades, along with its price increase, has led to a wide research for alternative neutron detection technologies [7]. The operating principle of the SiLiF neutron detector, as a viable alternative to 3He tubes [7], is straightforward: following a thermal neutron capture by 6Li, the 7Li compound nucleus decays into an alpha particle (4He) and a triton (3H), emitted back to back with high energy according to the reaction

n + 6Li→ 3H (2.73 MeV) + 4He (2.05 MeV) (2)

Semiconductor detectors, e.g., silicon diodes, can be used in combination with a neutron reactive film, usually made of 6Li or 10B and called neutron converter. Such a film converts thermal neutrons into charged particles which are then detected by the silicon diode. Indeed 6Li and 10B have a quite high cross section at thermal neutron energy, respectively 940 and 3840 b, which decreases with the inverse of the neutron velocity. In such a scheme fast neutrons can also be detected by surrounding the detector with a suitable moderator box, typically made from polyethylene, which slows neutrons down to thermal energy. The use of 6Li was preferred to 10B because, following a neutron’s capture, it has a unique decay channel with no gamma rays emitted and a higher available kinetic energy, and it decays into lighter particles that are easier to detect [8–13]. A valid neutron count is registered whenever the signal produced by a SiLiF detector, which is proportional to the energy deposited into the diode, is higher than a predefined threshold. With this technique, moderately inexpensive solid-state neutron detectors featuring good gamma/neutron discrimination can be built. Since 6Li is chemically very reactive we decided to employ 6LiF, a stable salt enriched at 95% in 6Li, that can be deposited by evaporation onto a suitable substrate. Instead of the direct deposition onto the silicon diode we chose to use independent detector and converter, which allows a better modularity and reconfigurability [14]. SiLiF detectors in different configurations (single, sandwich, double sandwich) with various converter thicknesses have already been tested in laboratory with moderated AmBe sources, and also tested with neutron beams and with spent fuel. The collected data were Sensors 2021, 21, 2630 3 of 22

compared with Monte Carlo simulation results [15] and with efficiency calibration data taken with a certified neutron source at a metrology institute [16], showing a perfect mutual agreement. These detectors can be successfully employed for long-term online monitoring of neutron radiation, and could represent a viable ingredient for both safety and security in a nuclear waste storage facility. The most convenient counting threshold can be suitably chosen according to the desired gamma/neutron discrimination and to the type of waste (i.e., neutron and gamma spectra expected) to be monitored. Such an alternative to 3He tubes has several advantages: • 6LiF is much cheaper and more easily available than 3He; • any solid state detector can in principle be used to detect the secondary particles; • it is operated at low voltage, typically 30–50 V as opposed to ≈1000 V of a gas detector; • the semiconductor and the neutron converter can be replaced independently in case of damage; • a double-sided silicon diode can be used to double the neutron detection efficiency [17,18]. In the framework of MICADO we had to build 32 SiLiF detectors, to characterize them, and to check their reliability and repeatability versus appropriate simulations. As for the detection behavior we made use of GEANT4 [19], widely used in the nuclear and particle physics community. For the neutron source simulation we opted for the Monte Carlo code Fluka [20], widely used and better suited for the simulation of fluxes for radiation protection applications, and finally the MCNP [21] code was employed for the simulation of a radwaste drum.

2.1. The Detector According to the study and results shown in [15–18] we decided to employ the double-sided silicon detector MSX09-300, a 3 cm × 3 cm diode 300 µm thick with a 0.3 µm metallization Al layer on each face, produced by Micron Semiconductor Ltd., Lancing, UK. We remark that in principle any silicon diode can be changed into a SiLiF by applying a suitable 6LiF converter, and that the resulting geometrical efficiency scales up with the sensitive area. A wider diode, e.g., 5 cm × 5 cm, would almost triplicate the geometrical efficiency, but sensibly increasing the price and decreasing the signal-to-noise ratio due to the larger capacitance. Therefore the chosen diode was a tradeoff between price and performance. The detector configuration, shown in Figure1, consists of the silicon diode sandwiched between two 6LiF converter layers deposited onto carbon fiber substrates. The evaporation of the converters was undertaken in three batches of 22 units, by means of a rotating support explicitly constructed for this purpose in order to guarantee the uniformity of the deposition (Figure2). We remark that we used carbon fiber substrates from two Sensors 2021, 21, 2630 different manufacturers, respectively 0.6 and 1 mm thick. We will show later4 of 23 that this has consequences.

(a) (b)

FigureFigure 1. ((aa)) The The SiLiF SiLiF detector detector arrangement arrangement as a as double a double sided sided silicon silicon diode diode sandwiched sandwiched between between two two 6LiF layers deposited on carbon fiber substrates; (b) a real detector assembled inside its alu- 6LiF layers deposited on carbon fiber substrates; (b) a real detector assembled inside its aluminum box. minum box.

Figure 2. Sketch of the rotating support explicitly constructed for the evaporation of the 6LiF con- verters in order to guarantee the uniformity of the deposition.

Following the results shown in Lo Meo et al. [15], the areal density chosen for the 6LiF layer was 4300 µg/cm2. For each evaporation session a few samples from the inner and outer rings were tested, 28 out of 66 in total, by comparing their weight before and after the deposition on a precision scale. This allowed us to calculate the amount of 6LiF effectively deposited. The results, summarized in Figure 3, indicate a slight systematic overdeposition of about 1% with respect to the nominal value, with a small dispersion of about 2% (standard deviation).

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(a) (b) Sensors 2021, 21, 2630 Figure 1. (a) The SiLiF detector arrangement as a double sided silicon diode sandwiched between4 of 22 two 6LiF layers deposited on carbon fiber substrates; (b) a real detector assembled inside its alu- minum box.

6 FigureFigure 2.2. FigureSketch 2.of Sketchthe rotating of the support rotating explicitly support constructed explicitly constructed for the evaporation for the evaporation of the LiF ofcon- the 6vertersLiF converters in order into orderguarantee to guarantee the uniformity the uniformity of the deposition. of the deposition.

FollowingFollowing thethe resultsresults shownshown inin LoLo MeoMeo etet al.al. [[15]15],, thethe arealareal densitydensity chosenchosen forfor thethe 66LiF layerlayer waswas 43004300 µµg/cmg/cm2.. For For each each evaporation session a fewfew samplessamples fromfrom thethe innerinner andand outerouter ringsrings werewere tested,tested, 2828 outout ofof 6666 inin total,total, byby comparingcomparing theirtheir weightweight beforebefore andand afterafter thethe depositiondeposition onon aa precisionprecision scale.scale. ThisThis allowedallowed usus toto calculatecalculate thethe amountamount ofof6 LiF6LiF effectivelyeffectively deposited.deposited. The results, summarizedsummarized inin FigureFigure3 ,3, indicate indicate a a slight slight systematic systematic Sensors 2021, 21, 2630 5 of 23 overdepositionoverdeposition ofof aboutabout 1%1% withwith respectrespect toto thethe nominalnominal value,value, withwith aa smallsmall dispersiondispersion ofof aboutabout 2%2% (standard(standard deviation).deviation).

FigureFigure 3.3. DistributionDistribution ofof thethe deviation deviation of of the the effective effective areal areal density density with with respect respect to to the the nominal nominal value

(invalue percent) (in percent) for 28 testedfor 28 samples.tested samples.

2.2.2.2. TheThe NeutronNeutron ModeratorModerator TheThe detection detection efficiency efficiency for for thermal thermal neutrons neutrons (i.e., (i.e., 0.0250.025 eV) eV) is is maximum, maximum, as as the the 6 4 6Li(n,t) 4He cross sectionsection isis 940 940 b b and and decreases decreases as as the the inverse inverse 1/v 1/v of of the the neutron neutron velocity. velocity. In orderIn order to increase to increase the detectionthe detection efficiency efficiency for faster for faster neutrons neutrons one makes one makes use of use a moderator, of a mod- i.e.,erator, a material i.e., a material with low with atomic low numberatomic number that can that efficiently can efficiently slow down slow the down neutrons the neu- via multipletrons via elastic multiple collisions. elasticThe collisions. typical materialThe typica usedl material for this purposeused for is this polyethylene purpose (Cis 2poly-H4), whichethylene is a(C very2H4), convenient which is a solidvery moderator,convenient richsolid in moderator, hydrogen andrich easilyin hydrogen machinable. and easily The choicemachinable. of the The moderator choice of shape the moderator and thickness shape is typicallyand thickness done is by typically means of done Monte by means Carlo simulationsof Monte Carlo of the simulations neutron transport, of the neutro and isn atransport, tradeoff betweenand is a sizetradeoff and moderationbetween size level and tomoderation be achieved. level Indeed, to be the achieved. thicker Indeed, the moderator the thicker the more the moderator efficient moderation the more efficient can be achieved,moderation especially can be forachieved, high-energy especially neutrons for high that- needenergy more neutrons collisions that to need be thermalized. more colli- Unfortunately,sions to be thermalized. if the lower Unfortunately energy neutrons, if undergothe lower too energy many neutrons collisions, undergo their chance too many to be collisions, their chance to be captured by hydrogen via the 1H(n,γ)2H becomes increas- ingly high. Hence the choice of a thick but not too thick moderator. For our purposes we concentrated on the optimization of the moderator for neutrons with energy ranging from thermal to a few MeV, which is the typical spectral range one can expect out of a radwaste drum possibly containing fissile nuclear material. A moderator of 4 cm thick- ness, shown in Figure 4a, gave the best results and was replicated in 32 units. As the waste drums to be monitored could in some cases be embedded in a neutron moderating polyethylene matrix, we foresaw the possibility of using the detectors in the half-moderator configuration (Figure 4b), to avoid an overmoderation that would in- crease the chance for neutrons to be lost because they were captured in hydrogen. The detector in its standard configuration, with 10 × 10 × 10 cm3 size, is shown in Figure 4c.

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captured by hydrogen via the 1H(n,γ)2H becomes increasingly high. Hence the choice of a thick but not too thick moderator. For our purposes we concentrated on the optimization of the moderator for neutrons with energy ranging from thermal to a few MeV, which is the typical spectral range one can expect out of a radwaste drum possibly containing fissile nuclear material. A moderator of 4 cm thickness, shown in Figure4a, gave the best results and was replicated in 32 units. As the waste drums to be monitored could in some cases be embedded in a neutron moderating polyethylene matrix, we foresaw the possibility of using the detectors in the half-moderator configuration (Figure4b), to avoid an overmoderation that would increase the chance for neutrons to be lost because they were Sensors 2021, 21, 2630 captured in hydrogen. The detector in its standard configuration, with 10 × 10 × 106 of cm 23 3

size, is shown in Figure4c.

(a) (b) (c)

FigureFigure 4. 4. (a()a )3D 3D sketch sketch of of the the SiLiF SiLiF detector detector (in (in yellow) yellow) and and its its arrangement arrangement within within the the moderator moderator (in (in grey); grey); (b ()b a) aSiLiF SiLiF hostedhosted in in a ahalf half moderator; moderator; (c ()c f)inal final assembling assembling of of a aSiLiF. SiLiF.

TheThe detector detector response response to tomonoenergetic monoenergetic neutrons neutrons at 13 at energies 13 energies (0.025 (0.025 eV, eV,0.1 0.1eV, eV,1 eV,1 eV,10 eV, 10 eV, 100 100 eV, eV, 1 keV, 1 keV, 10 10keV, keV, 100 100 keV, keV, 1 MeV, 1 MeV, 2.5 2.5 MeV, MeV, 5 5MeV, MeV, 7 7MeV, MeV, 10 10 MeV) MeV) was was simulatedsimulated in in the the full full and half half moderator moderator configurations, configurations, along along with with the the no-moderator no-moderator one, one,by meansby means of theof the GEANT4 GEANT4 code. code. The The resulting resulting detection detection efficiencies, efficiencies, plotted plotted in in Figure Figure5 , 5,clearly clearly indicateindicate thatthat thethe full moderator provides provides a a rather rather flat flat efficiency efficiency from from thermal thermal to to 1 1MeV. MeV. Conversely, Conversely, as as expected, expected, in in case case of of no no moderator moderator the the detector detector is is basically basically sensitive sensitive onlyonly to to very very slow slow neutrons. neutrons. The The half half moderator moderator configuration configuration is isa aconven convenientient solution solution whenwhen one one expects expects neutrons neutrons already already partially partially moderated. moderated. The The lower lower detection detection efficiency efficiency belowbelow 1 1eV eV observed observed for for the the full full moderator, moderator, as as compared compared to to the the other other configurations, configurations, is is duedue to toa relatively a relatively important important neutron neutron capture capture by the by hydrogen the hydrogen of the of moderator. the moderator. It is also It is clearalso that clear the that moderator the moderator has little has littleeffect effect on the on thedetection detection efficiency efficiency for for neutrons neutrons at atthe the highesthighest energy energy (i.e. (i.e.,, close close to to10 10 MeV). MeV). The The figure figure also also shows shows the the detection detection efficiency efficiency in a in 4 a × 4 ×square4 square array array configuration configuration that thatwill willbe desc be describedribed later. later.

Figure 5. Simulated neutron detection efficiency in different moderator configurations (see the text for details).

2.3. The Neutron Source In order to test and characterize the detectors we made use of an intense AmBe source nominally emitting 2.2 × 106 neutrons/s installed in an experimental hall at the

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(a) (b) (c) Figure 4. (a) 3D sketch of the SiLiF detector (in yellow) and its arrangement within the moderator (in grey); (b) a SiLiF hosted in a half moderator; (c) final assembling of a SiLiF.

The detector response to monoenergetic neutrons at 13 energies (0.025 eV, 0.1 eV, 1 eV, 10 eV, 100 eV, 1 keV, 10 keV, 100 keV, 1 MeV, 2.5 MeV, 5 MeV, 7 MeV, 10 MeV) was simulated in the full and half moderator configurations, along with the no-moderator one, by means of the GEANT4 code. The resulting detection efficiencies, plotted in Figure 5, clearly indicate that the full moderator provides a rather flat efficiency from thermal to 1 MeV. Conversely, as expected, in case of no moderator the detector is basically sensitive only to very slow neutrons. The half moderator configuration is a convenient solution when one expects neutrons already partially moderated. The lower detection efficiency below 1 eV observed for the full moderator, as compared to the other configurations, is due to a relatively important neutron capture by the hydrogen of the moderator. It is also

Sensors 2021, 21, 2630 clear that the moderator has little effect on the detection efficiency for neutrons at6 ofthe 22 highest energy (i.e., close to 10 MeV). The figure also shows the detection efficiency in a 4 × 4 square array configuration that will be described later.

FigureFigure 5. 5. SimulatedSimulated neutron neutron detection detection efficiency efficiency in in different different moderator moderator configurations configurations (see (see the the text text forfor details). details).

2.3.2.3. The The Neutron Neutron Sou Sourcerce InIn order to to test test and and characterize characterize the the detectors detectors we madewe made use of use an of intense an intense AmBe AmBe source sourcenominally nominally emitting emitti 2.2 ng× 10 2.26 neutrons/s× 106 neutrons/s installed installed in an in experimental an experimental hall at hall the at INFN the Laboratori Nazionali del Sud (LNS). Because of radiation protection requirements the source and its moderator are enclosed in a 95 × 75 × 85 cm3 iron box (Figure6a). The source is surrounded by a first polyethylene case followed by 30 cm thick paraffin that slow down the high energy neutrons (up to 10 MeV) it emits. The outer 5 cm of the shielding are made from borated paraffin that stops the vast majority of the outgoing thermalized neutrons. Still a very small amount of fast neutrons can escape such a shield, as will be discussed later. We remark that this is a legacy source from the 60’s and the effective geometry of its moderator is only roughly known (i.e., the paraffin, whose actual density and uniformity are not exactly known). Due to its huge activity (≈34 GBq of 59 KeV gamma rays from 241Am), modifying the source arrangement was not allowed for radiation protection reasons, and only a restricted set of operations was possible. However, such a configuration, heavily disturbed by the surrounding material assembly, gives rise to multiple neutron scattering and to a huge abundance of gamma rays. This makes the source somehow resembling a radwaste drum and, therefore, constitutes a good exercise to test and characterize the detectors. The source setup was simulated with Fluka [20], thus producing a 3D map of the expected neutron flux inside and around the box, as shown in Figures7 and8. In addition, the expected neutron energy spectrum was simulated in three positions, namely front, inside and top highlighted with red boxes in the figures, where the SiLiF measurements were done. The three simulated spectra are plotted in Figure9 (the top flux in the figure refers to det05 in the 4 × 4 array of Figure6b, as explained later). The total neutron flux expected in the front position is around 310 n/cm2/s, in the inside position it is expected around 263 n/cm2/s. The maximum flux expected in the top position, that is heavily shielded, is around 1.9 n/cm2/s. Sensors 2021, 21, 2630 7 of 23

INFN Laboratori Nazionali del Sud (LNS). Because of radiation protection requirements the source and its moderator are enclosed in a 95 × 75 × 85 cm3 iron box (Figure 6a). The source is surrounded by a first polyethylene case followed by 30 cm thick paraffin that slow down the high energy neutrons (up to 10 MeV) it emits. The outer 5 cm of the shielding are made from borated paraffin that stops the vast majority of the outgoing thermalized neutrons. Still a very small amount of fast neutrons can escape such a shield, as will be discussed later. We remark that this is a legacy source from the 60’s and the effective geometry of its moderator is only roughly known (i.e., the paraffin, whose ac- tual density and uniformity are not exactly known). Due to its huge activity (≈34 GBq of 59 KeV gamma rays from 241Am), modifying the source arrangement was not allowed for radiation protection reasons, and only a restricted set of operations was possible. How- ever, such a configuration, heavily disturbed by the surrounding material assembly, Sensors 2021, 21, 2630 gives rise to multiple neutron scattering and to a huge abundance of gamma rays.7 This of 22 makes the source somehow resembling a radwaste drum and, therefore, constitutes a good exercise to test and characterize the detectors.

(a) (b)

Figure 6. (a) The AmBe neutron sourcesource boxbox withwith aa SiLiFSiLiF detectordetector duringduring aa measurementmeasurement in in the the front front position. position. ( b(b)) The The 4 4× × 4 Sensors 2021, 21, 2630 8 of 23 array of SiLiF detectors duringduring thethe measurementmeasurement inin thethe toptop position.position.

The source setup was simulated with Fluka [20], thus producing a 3D map of the expected neutron flux inside and around the box, as shown in Figures 7 and 8. In addi- tion, the expected neutron energy spectrum was simulated in three positions, namely front, inside and top highlighted with red boxes in the figures, where the SiLiF measure- ments were done. The three simulated spectra are plotted in Figure 9 (the top flux in the figure refers to det05 in the 4 × 4 array of Figure 6b, as explained later). The total neutron flux expected in the front position is around 310 n/cm2/s, in the in- side position it is expected around 263 n/cm2/s. The maximum flux expected in the top position, that is heavily shielded, is around 1.9 n/cm2/s.

(a) (b)

FigureFigure 7. 7. (a(a) )Drawing Drawing of of the the top top view view of of the the section section at at3030 cm cm height height of ofthe the neutron neutron source source box; box; the thesource source position position is in- is dicatedindicated by by a small a small dark dark circle, circle, the the red red squares squares indicate indicate the the frontfront andand insideinside positionspositions where where the the measurements measurements were were done; done; ((bb)) t thehe corresponding neutron flux flux obtained by means of a simulation with the Fluka code.

(a) (b) Figure 8. (a) Drawing of the 3D side view of the neutron source box, the red box indicates the top position; (b) the corre- sponding neutron flux obtained by means of a simulation with the Fluka code.

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(a) (b) SensorsFigure2021, 21 7., 2630(a) Drawing of the top view of the section at 30 cm height of the neutron source box; the source position is in-8 of 22 dicated by a small dark circle, the red squares indicate the front and inside positions where the measurements were done; (b) the corresponding neutron flux obtained by means of a simulation with the Fluka code.

(a) (b)

Sensors Figure2021, 21 8., 2630 (a) Drawing of the 3D side view of the neutron source box, the red box indicates the top position;top (b) the corre-9 of 23 Figure 8. (a) Drawing of the 3D side view of the neutron source box, the red box indicates the position; (b) the correspondingsponding neutron neutron flux obtained flux obtained by means by means of a simulation of a simulation with withthe Fluka the Fluka code. code.

FigureFigure 9. 9. NeutronNeutron flux flux simulated simulated in in the the three three measurement measurement positions positions with with Fluka, Fluka, and and with with MCNP MCNP 240 onon the the detectors detectors in in the the radwaste radwaste package package exercise exercise with with 240PuPu (see (see the the Discussion Discussion section). section). 3. Results 3. Results 3.1. Measurements in the Front Position 3.1. Measurements in the Front Position The first set of measurements that was performed was a preliminary check that the detectorsThe first were set operationalof measurements and behaved that was as performed expected. was Each a preliminary detector in turn,check insidethat the its detectorsmoderator, were was operational placed on a and small behaved cart and as positioned expected. in Each front detector of the source in turn, just inside out of itsthe moderator,box with the was open placed door on (Figure a small6a). cart The and detector positioned was biasedin front at of 50 the V, source in order just to operateout of the in boxfull with depletion the open regime door and (Figure be sensitive 6a). The ondetector both faces was biased as required. at 50 V, The in chargeorder to preamplifier operate in fullemployed depletion was regime an ORTEC and be 142B, sensitive which on has both a nominalfaces as required. gain of 25 The mV/MeV, charge preamplifier connected to employedan ORTEC was 672 an spectroscopy ORTEC 142B, amplifier which whosehas a nominal output wasgain sent of 25 to mV/MeV, an Amptek connected MCA8000A to anmultichannel ORTEC 672 analyzer.spectroscopy The resultingamplifier spectrum, whose output as expected, was sent reaches to an upAmptek to 2.73 M MeVCA8000A which multichannelis the maximum analyzer. kinetic energy The resulting of the tritons spectrum, when emitted as expected, perpendicularly reaches up from to 2.73the surface MeV whichof the is6LiF the layer.maximum The emission kinetic energy from deeper of the positionstritons when and/or emitted with perpendicularly different angles from gives therise surface to a lower of the energy 6LiF deposition layer. The in emission the silicon, from hence deeper a low positions energy tail and/or in the with spectrum. different The anglessame appliesgives rise to theto a alpha lower emission energy deposition whose maximum in the silicon, energy hence is 2.05 a MeV. low Theenergy superposition tail in the spectrum. The same applies to the alpha emission whose maximum energy is 2.05 MeV. The superposition of the triton and alpha deposited energy, plus the effect of the emis- sion from different depths/angles, produces a characteristic spectrum shape, as can be observed in Figure 10 that also proves that all the detectors were actually working properly. The detector naming scheme in the figure is related to the original numbering from the manufacturer. The “N” suffix refers to a different (new) batch of detectors which has a higher depletion voltage.

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of the triton and alpha deposited energy, plus the effect of the emission from different depths/angles, produces a characteristic spectrum shape, as can be observed in Figure 10 that also proves that all the detectors were actually working properly. The detector naming Sensors 2021, 21, 2630 scheme in the figure is related to the original numbering from the manufacturer. The10 “N” of 23

suffix refers to a different (new) batch of detectors which has a higher depletion voltage.

FigureFigure 10. 10.The The characteristic characteristic spectrum spectrum shape, shape, measured measured for for all all the the SiLiF SiLiF detectors detectors in the in frontthe front position, posi- tion, is due to the superposition of the triton (2.73 MeV) and alpha (2.05 MeV) contribution smeared is due to the superposition of the triton (2.73 MeV) and alpha (2.05 MeV) contribution smeared down down by the effect of the emission from different depths/angles in the 6LiF converter. The energy by the effect of the emission from different depths/angles in the 6LiF converter. The energy bin size bin size is 47 keV. is 47 keV.

3.2.3.2. Gamma/NeutronGamma/Neutron DiscriminationDiscrimination. OneOne ofof thethe mainmain requirements requirements forfor a a neutron neutron detector detector is is the the insensitivity insensitivity to to gamma gamma rays.rays. Actually,Actually, itit isis notnot possiblepossible toto obtainobtain suchsuch aa perfect perfect behavior,behavior, therefore therefore one one usually usually refersrefers toto the the gamma/neutron gamma/neutron discriminationdiscrimination e.g.,e.g., thethe probabilityprobability toto detectdetect aa gamma gamma ray ray andand misinterpretmisinterpretit it as as a a neutron. neutron. A Aγ γ/n/n countcount raterate ratioratio typicallytypically acceptedaccepted asas quitequite good,good, withwith a a detector detector in in a a mixed mixed flux flux of of neutrons neutrons and and 1.17 1.17 and and 1.33 1.33 MeV MeV gamma gamma rays rays from from60 60Co,Co, isis about about 10 10−−44 [[2222,,2323].]. ProfitingProfiting byby thethe independentindependent assemblyassembly ofof converterconverter andand diode diode we we werewere able able to to simply simply evaluate evaluateγ γ/n/n inin twotwo steps.steps. First,First, wewe measured a normal normal spectrum spectrum in in the the frontfront positionposition then, then, after after reversing reversing the thetwo two carbon carbon fiber fiber substrates substrates upside upside down, down, we we repeated repeated the measurement. measurement. Obviously, Obviously, whenwhen measuring measuring with with the the reversed reversed converters converters no no triton triton or or alpha alpha can can reach reach the the silicon silicon diode,diode, soso thatthat nono detecteddetected event can be related related to to the the 66Li(n,t)Li(n,t)4He4He reaction reaction and and this this is isto tobe beconsidered considered as asbackground, background, even even though though there there will will be some be some additional additional consideration consideration of it ofin it the in the following. following. The The background background measurement measurement was was then then subtracted subtracted from the the real real measurement,measurement, and and the the resulting resulting spectrum spectrum is reportedis report ined Figure in Figure 11 as 11pure as neutronpure neutronspectrum, spec- alongtrum,with along the with measured the measured background. background.

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FigureFigure 11.11. The background background-subtracted-subtracted neutron neutron spectrum spectrum as as compared compared to to the the background. background. Also Also highlightedhighlighted are are three three different different contributions contributions to to the the background background (see (see text). text).

ByBy observingobserving the the measured measured background background one one sees sees three three contributions contributions with with clearly clearly differentdifferent behavior behavior (we (we remark remark that that no no full full energy energy gamma gamma deposit deposit is is expected expected to to occur occur in in suchsuch a a thin thin silicon silicon detector): detector): • TheThe firstfirst one,one, aa decreasingdecreasing exponentialexponential extrapolatedextrapolated inin thethe figurefigure upup toto 11 MeV,MeV, isis mainlymainly duedue to to the the abundant abundant 2.2 2.2 MeV MeV gamma gamma rays rays produced produced by by the the neutron neutron capture capture onon hydrogen hydrogen in in the the moderator moderator box. box. We We name name it it here here “ gamma1“gamma1”” contribution. contribution. • TheThe second second one, one, a a decreasing decreasing exponential exponential extending extending up up to 1.5to 1.5 MeV MeV and and extrapolated extrapolated in thein the figure figure up toup 2 to MeV, 2 MeV, is due is todue the to gamma the gamma rays producedrays produced by the by AmBe the AmBe source source itself. Indeed,itself. Indeed, whenever whenever an alpha an particle alpha particle from the from americium the americium reacts with reacts the beryllium,with the beryl- this decayslium, thisby emitting decays one by emitting neutron and one one neutron gamma and ray, one with gamma the gamma ray, with energy the between gamma 3.4energy and 4.4between MeV. We 3.4 estimated and 4.4 MeV. that roughly We estimated the same that number roughly of such the gammasame number rays and of neutronssuch gamma hit the rays detector and neutrons in the front hit measurementthe detector in position. the front Part measurement of this contribution position. isPart also of due this to contribution the elastic scattering is also due of neutronsto the elastic on silicon, scattering with of cross neutrons section on of silicon, a few barn,with andcross maximum section of silicona few barn, recoil and kinetic maximum energy, silicon from recoil 10 MeV kinetic neutrons, energy, of from about 10 1.33MeV MeV. neutrons, We name of about it here 1.33 “gamma2 MeV. ”We contribution. name it here “gamma2” contribution. • TheThe thirdthird one,one, withwith anan almostalmost constantconstant linearlinear trendtrend towardtoward highhigh energyenergy (“(“HEHE”” con-con- tribution),tribution), is is due due to to the the28 28Si(n,p)Si(n,p)2828AlAl reactionreaction whosewhose thresholdthreshold isis aroundaround 66 MeV MeV and and whichwhich hashas several several resonances resonances andand cross cross section section around around 300 300 b. b. InIn thisthis case case both both the the protonproton and and the the recoiling recoiling28 28AlAl depositdeposit kinetickinetic energyenergy inin thethe detector.detector. TheThe GEANT4GEANT4 simulation of of the the SiLiF SiLiF response response to to 10 10 MeV MeV neutrons neutrons is shown is shown in Fig- in Figuresures 12 12 and and 13, 13 respectively, respectively,, with with and and without without the the moderator moderator surrounding the detector.detector. InIn both both figures figures one one can can clearly clearly see see several several structures structures above above 3 3MeV MeV corresponding corresponding to to 28 28 resonancesresonances inin thethe cross cross section section of of the the 28Si(n,p)Si(n,p)28AlAl reactionreaction forfor whichwhich oneone measuresmeasures the the 28 28 kinetickinetic energy energy of of the the Al28Al and and of of the the proton, proton, while while the the gamma gamma ray fromray from the theAl de-28Al excitationde-excitation goes goes undetected. undetected. This This confirms confirms that that 4 cm4 cm of of polyethylene polyethylene is is not not enough enough to to efficientlyefficiently thermalize thermalize 10 10 MeVMeV neutrons, neutrons, therefore therefore a a fraction fraction of of these these neutrons neutrons retain retain a a kinetickinetic energyenergy highhigh enoughenough forfor aa nuclear nuclear interactioninteraction withwith thethe silicon silicon nuclei. nuclei. However,However, 6 4 whenwhen thethe moderator moderator isis present present there there is is still still a a contribution contribution from from the the Li(n,t)6Li(n,t)He4He reaction,reaction, visiblevisible in in the the 1 1 to to 3 3 MeV MeV range range in in Figure Figure 12 12.. Moreover,Moreover, both both figures figures show show the the low low energy energy 28 28 contributioncontribution below below 1.33 1.33 MeV MeV due due to to the the recoil recoil of siliconof silicon in the in theSi(n,n) 28Si(n,n)Si28 elasticSi elastic scattering. scatter- ing.

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FigureFigure 12.12. GEANT4GEANT4GEANT4 simulationsimulation simulation ofof of thethe the SiLiFSiLiF SiLiF responseresponse response toto 1010 to MeVMeV 10 MeV neutronsneutrons neutrons withwith withthethe moderatormoderator the moderator sur-sur- roundingsurroundingrounding thethe the detectordetector detector (see(see (see text).text). text).

Figure 13. GEANT4 simulation of the SiLiF response to 10 MeV neutrons without the moderator Figure 13. GEANT4GEANT4 simulation simulation of of the the SiLiF SiLiF response response to to 10 10 MeV MeV neutrons neutrons without without the the moderator moderator surrounding the detector (see text). surrounding the detector (see text).

AsAs a a consequence consequence oneone cancan makemake thethe followingfollowing points:pointspoints::  • TheThe countscountscounts above aboveabove 2.73 2.732.73 MeV, MeV,MeV, i.e., i.e.i.e. the,, thetheHE HEHEcontribution, contribution,contribution, have havehave to be toto ascribed bebe ascribedascribed to neutrons, toto neu-neu- 6 eventrons,trons, though eveneven thoughthough not interacting notnot interactinginteracting with the withwith6LiF thethe converter, 6LiFLiF converter,converter, and have andand to be havehave considered toto bebe consid-consid- in the detectioneredered inin thethe efficiency detectiondetection evaluation. efficiencyefficiency evaluation.evaluation.  • FittingFitting thethethe twotwotwo decreasing decreasingdecreasing exponentials exponentialsexponentials of ofof Figure FigureFigure 11 1111 can cancan provide provideprovide a realisticaa realisticrealistic estimate estimateestimate of theofof thethe gamma gammagamma contribution contributioncontribution in theinin thethe measured measuredmeasured neutron neutronneutron counts. counts.counts. IndeedIndeed,Indeed,, wewe calculatedcalculatedcalculated the thethe ratio ratioratio of ofof the thethe integrals integralsintegrals of ofofgamma1 gamma1gamma1and andandgamma2 gamma2gamma2to to theto thethe integral integralintegral of oftheof the thepure purepure neutron neutronneutronSiLiF SiLiFSiLiF spectrum spectrum spectrum for different for for different different energy energy energy thresholds. thresholds. thresholds. The resulting The The resulting resulting plot, shown plot, plot, showninshown Figure inin 14 FigureFigure, indicates 14,14, indicatindicat the γ/neses ratiothethe γγ as/n/n a ratioratio function asas aa offunctionfunction the chosen ofof thethe energy chosenchosen threshold. energyenergy threshold.threshold. Therefore, Therefore,inTherefore, cases where inin casescases one where onlywhere expects oneone onlyonly lower expectsexpects energy lowerlower gamma energyenergy rays gammagamma (i.e., roughly raysrays (i.e.(i.e. below,, roughlyroughly 2.2 MeV) belowbelow a −6 −6 −10 −10 2.2threshold2.2 MeV)MeV) aa at thresholdthreshold 1 MeV provides atat 11 MeVMeVγ/n providesprovides≈ 10 γ,γ and/n/n ≈≈ a1010 threshold−6,, andand aa thresholdthreshold at 1.5 MeV atat γ 1.51.5/n MeVMeV≈ 10 γγ/n/n, ≈≈ that 1010−1 is0,, thatathat very isis a gooda veryvery discrimination. goodgood discrimination.discrimination.

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Figure 14. TheThe γ/nγ/n ratio ratio as as a afunction function of of the the chosen chosen energy energy threshold, threshold, with with respect respect to tothe the gamma1gamma1 andand gamma2gamma2 casescases (see(see text).text).

IfIf oneone expectsexpects muchmuch higher higher energy energy gamma gamma rays, rays, generally generally produced produced by by the the presence presence of alphaof alpha emitters emitters and andlight li elementsght elements like Li like or Be, Li e.g., or Be, what e.g. happens, what happenswith AmBe, with a threshold AmBe, a atthreshold 1.5 MeV at is 1.5 recommended MeV is recommended with γ/n ≈ with10− γ4./n We ≈ 10 recall−4. We that recall in the that real in datathe real the gamma2data the regiongamma2 will region also will include also a include contribution a contribution of silicon of recoil silicon events recoil produced events produced by neutrons. by neu- trons. 3.3. Intrinsic Detection Efficiency Measurement 3.3. IntrinsicIn order Detection to determine Efficiency the thermal Measurement neutron intrinsic detection efficiency of each SiLiF detectorIn order we needed to determine to know the the thermal number neutron of impinging intrinsic detection neutrons inefficiency a given of position, each SiLiF so thatdetector we could we needed calculate to itknow as the the ratio number of the of number impinging of neutrons neutrons detected in a given to the position, impinging so ones.that we This could was calculate achieved it by as makingthe ratio use of the of a number reference ofdetector neutrons which detected had to been the previouslyimpinging calibratedones. This at was a metrology achieved institute by making in a certifieduse of a thermalreference neutron detector field which [16]. Thehad referencebeen previ- de- 6 2 tectorously is calibrated a SiLiF as at well, a metrology with only a institute single layer in a of certifiedLiF converter thermal 1.6 neutronµm thick field (410 µ [16].g/cm The). Atreference first we detector measured is a theSiLiF counting as well,rate with in only the ainside singleposition layer of with 6LiF theconverter reference 1.6 µm detector, thick then(410 weµg/cm repeated2). At first the samewe measured measurement the counting with each rate SiLiF in the detector. inside Forposition this measurement with the ref- allerence the detectorsdetector, werethen usedwe repeated without the their same moderator, measurement so that with they each were SiLiF basically detector. sensitive For onlythis measurement to the thermal all neutrons the detectors inside were the big used moderator without their box. Themoderator, spectrum so obtainedthat they withwere the reference detector is shown in Figure 15. The colored area extends upwards from the basically sensitive only to the thermal neutrons inside the big moderator box. The spec- valley corresponding to the alpha endpoint, and contains 97% of the detected tritons. The trum obtained with the reference detector is shown in Figure 15. The colored area extends detection efficiency under these conditions is 0.41% ± 0.001% [16], which implies that upwards from the valley corresponding to the alpha endpoint, and contains 97% of the the impinging flux at the inside position was 107 ± 1.6 n/s/cm2 to be compared with an detected tritons. The detection efficiency under these conditions is 0.41% ± 0.001% [16], expected thermal flux of roughly 130 n/s/cm2 from the simulation. We remark that the which implies that the impinging flux at the inside position was 107 ± 1.6 n/s/cm2 to be alpha endpoint is slightly shifted to lower energy (see the red area in Figure 15) because of compared with an expected thermal flux of roughly 130 n/s/cm2 from the simulation. We the energy loss in the thin air and aluminum dead layers between the converter and the remark that the alpha endpoint is slightly shifted to lower energy (see the red area in silicon diode. Figure 15) because of the energy loss in the thin air and aluminum dead layers between the converter and the silicon diode.

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Figure 15. The deposited energy spectrum collected in the inside position with the reference detec- tor.Figure The colored 15. The areadeposited starts energyfrom the spectrumspectrum alpha endpoint, collectedcollected and inin thethe containsinside insideposition position97% of the with with detected the the reference reference tritons detector. (seedetec- text).Thetor. The colored colored area area starts starts from from the alphathe alpha endpoint, endpoint, and and contains contains 97% 97% of the of detectedthe detected tritons tritons (see text).(see text). TheThe typical typical spectrum spectrum measured withwith thethe SiLiF SiLiF detectors detectors under under test test at theat theinside insideposition po- sitionis shown Theis shown intypical Figure in spectrumFigure 16. The 16. coloredmeasured The colored area with was area the normalized was SiLiF normalized detectors to the impinging tounder the impinging test flux at the and fluxinside provides and po- providessitionthe detection is theshown detection efficiency in Figure efficiency for 16. the The 1.5 forcolored MeV the 1.5 energyarea MeV was threshold. energynormalized threshold. The to detection the Theimpinging detection efficiency flux effi- wasand ciencyprovidesevaluated was theevaluated for detection all the 32for detectors,efficiency all the 32 withfordetectors, the two 1.5 nominal withMeV two energy energy nominal threshold. thresholds energy The atthresholds 1 detection and 1.5 at MeV, effi- 1 andciencyand 1.5 the MeV,was results evaluated and are the summarized results for all are the summarized in 32 Figure detectors, 17. in withFigure two 17. nominal energy thresholds at 1 and 1.5 MeV, and the results are summarized in Figure 17.

FigureFigure 16. 16. TheThe typical typical deposited deposited energy energy spectrum spectrum collected collected in the in inside the inside positionposition with withthe SiLiF the SiLiF detectorsFiguredetectors 16. under underThe test.typical test. The Thedeposited colored colored area energy area represents represents spectrum the collected the number number inof thedetected of detectedinside neutronsposition neutrons withfor forthe the thechosen SiLiF chosen 1.51.5detectors MeV MeV energy energyunder threshold, test. threshold, The coloredand and was wasarea normalized normalizedrepresents to the to impingingnumber the impinging of fluxdetected to flux provide neutrons to provide the for detection thethe detectionchosen effi- ciency for the chosen threshold. 1.5efficiency MeV energy for the threshold, chosen threshold. and was normalized to the impinging flux to provide the detection effi- ciency for the chosen threshold.

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FigureFig 17.ureThe 17. intrinsic The intrinsic detection detection efficiency efficiency measured measured for for thethe 32 SiLiF detectors detectors in inthe the insideinside configuration,configuration, with withnominal nominal energy thresholds of 1 and 1.5 MeV. energy thresholds of 1 and 1.5 MeV. Figure 17. The intrinsic detection efficiency measured for the 32 SiLiF detectors in the inside configuration, with nominal energy thresholds of 1 and 1.5 MeV.We were surprised to discover that there were two distinct groups of detectors with differentWe were effic surprisediency: detectors to discover 02–11 (Figure that there17) have were a higher two thermal distinct neutron groups efficiency of detectors than with differenttheWe others. were efficiency: At surprised first we detectors tosupposed discover 02–11 that that this (Figurethere behavior were 17) wastwo have duedistinct a higherto a groupsdifferent thermal of batch detectors neutron of silicon with efficiency di- thandifferentodes, the buteffic others. thisiency: wasAt detectors excluded first we 02 –by supposed11 swapping (Figure 17) that the have converters this a higher behavior betweenthermal was neutrontwo due different to efficiency a different SiLi Fthan detec- batch of siliconthetors. others. diodes,We At were firstbut forced we this supposed to was conclude excluded that thisthat behaviorthe by difference swapping was due was the to in a converters thedifferent converter, batch between even of silicon though two di- differentthe SiLiFodes,amount but detectors. this of 6wasLiF Wewasexcluded werethe same. forcedby swapping to conclude the converters that the between difference two different was in theSiLi converter,F detec- even thoughtors. WeThen thewere amount we forced realized to of conclude 6thatLiF the was carbonthat the the same. fiber difference had come was from in the two converter, different evenmanufacturers, though the and amountdiscoveredThen of 6 weLiF that realizedwas the the secondsame. that the one carbon contains fiber a resin had with come an from unspecified two different amount manufacturers, of boron in it. and discoveredSuchThen an we amount that realized the of second thatboron the captures onecarbon contains fibera fraction had a resincome of the from with impinging two an unspecifieddifferent neutrons, manufacturers, amountthus reducing of and boron the in it. 6 Suchdiscoveredthermal an amount fluxthat effectivelythe of second boron reaching one captures contains the aLiF fractiona r esinconverter with of the anand unspecified impinging in the end reducesamount neutrons, theof borondetection thus reducingin it. effi- the Such an amount of boron captures a fraction of the impinging neutrons, thus reducing the thermalciency. flux To prove effectively this we reaching reported thein Figure6LiF converter18 the deposited and inenergy the end spectra reduces obtained the with detection thermalthe detectors flux effectively SiLiF03 reaching and SiLiF03N, the 6LiF belonging converter to and two in different the end siliconreduces batches. the detection The detectors effi- efficiency. To prove this we reported in Figure 18 the deposited energy spectra obtained with ciency.were To coupled prove withthis we6LiF reported converters in Figuredeposited 18 theonto deposited carbon fiber energy substrates spectra ofobtained the A and with SAT thethetypes detectors (coming SiLiF03 SiLiF03 from and different and SiLiF03N, SiLiF03N, manufacturers) belonging belonging to which two to different twoin a second different silicon run silicon batches.were swapped. batches. The detectors The detectors 6 werewere coupled with with 6LiFLiF converters converters deposited deposited onto ontocarbon carbon fiber substrates fiber substrates of the A ofand the SAT A and SAT typestypes ( (comingcoming from from different different manufacturers) manufacturers) which which in a second in a second run were run swapped. were swapped.

Figure 18. Deposited energy spectra obtained with the detectors SiLiF03 and SiLiF03N, belonging to two different silicon batches in the inside position. The detectors were coupled with 6LiF con- FigureFigure 18. DepositedDeposited energy energy spectra spectra obtained obtained with with the detectors the detectors SiLiF03 SiLiF03 and SiLiF03N, and SiLiF03N, belonging belonging to 6 to two different silicon batches in the inside position. The detectors were coupled with LiF con-6 two different silicon batches in the inside position. The detectors were coupled with LiF converters deposited onto carbon fiber substrates of the A and SAT types (coming from different manufacturers) which in a second run were swapped. Sensors 2021, 21, 2630 16 of 23 Sensors 2021, 21, 2630 16 of 23

Sensors 2021, 21, 2630 verters deposited onto carbon fiber substrates of the A and SAT types (coming from different15 of 22 verters deposited onto carbon fiber substrates of the A and SAT types (coming from different manufacturers) which in a second run were swapped. manufacturers) which in a second run were swapped. 3.4. Test with Fast Neutrons 3.4.3.4. TestTest with with Fast Fast Neutrons Neutrons A final test was performed using simultaneously 16 detectors aimed at measuring a A final test was performed using simultaneously 16 detectors aimed at measuring a low intensityA final test flux was of performed fast neutrons. using To simultaneously this purpose the 16 detectors detectors wereaimed arranged at measuring in a alow low intensity intensity flux flux of of fast fast neutrons. neutrons. To To this purpose the detectors detectors were were arranged arranged in in a a square 4 × 4 array in the top position upon the neutron source box. The efficiency of such a squaresquare 4 4× × 4 array in the top position upon thethe neutronneutron sourcesource box.box. TheThe efficiencyefficiency of of such such a a configuration, as resulting from a GEANT4 simulation, is flatter than in the single de- configuration,configuration, as as resulting resulting from from a GEANT4a GEANT4 simulation, simulation, is flatter is flatter than than in the in single the single detector de- tector configuration. This is due to an improvement of the moderation of fast neutrons configuration.tector configuration. This is dueThis to is an due improvement to an improvement of the moderation of the moderation of fast neutrons of fast due neutrons to the due to the contribution of neighboring moderators, as shown in Figure 5 for the detector contributiondue to the contribution of neighboring of neighboring moderators, moderators, as shown in as Figure shown5 for in theFigure detector 5 for inthe position detector in position 05. In Figure 19 we show the arrangement and numbering of the detectors, 05.in position In Figure 05. 19 In we Figure show 19 the we arrangement show the arrangement and numbering and ofnu thembering detectors, of the whereas detectors, in whereas in Figure 20 the behavior of the expected average detection efficiency between Figurewhereas 20 inthe Figure behavior 20 the of the behavior expected of the average expected detection average efficiency detection between efficiency thermal between and thermal and 2.5 MeV neutrons is reported. The efficiency modulation due to geometrical 2.5thermal MeV and neutrons 2.5 MeV is reported. neutrons is The reported. efficiency The modulation efficiency modulation due to geometrical due to geometrical effects is effects is clearly visible: the central detectors (05, 06, 09, 10) are the most efficient, the pe- clearlyeffects visible:is clearly the visible: central the detectors central detectors (05, 06, 09, (05, 10) 06, are 09, the 10) most are the efficient, most efficient, the peripheral the pe- ripheral ones are less efficient (01, 02, 04, 07, 08, 11, 13, 14), and the corner ones are the onesripheral are lessones efficient are less (01, efficient 02, 04, 07,(01, 08, 02, 11, 04, 13, 07, 14), 08, and 11, the13, corner14), and ones the are corner the least ones efficient are the least efficient (00, 03, 12, 15). (00,least 03, efficient 12, 15). (00, 03, 12, 15).

Figure 19. The 4 × 4 arrangement and numbering of the detector positions for the test with fast Figure 19. The 4 × 4 arrangement and numbering of the detector positions for the test with fast neutrons.Figure 19. The 4 × 4 arrangement and numbering of the detector positions for the test with fast neutrons. neutrons.

FigureFigure 20. 20. BehaviorBehavior of ofthe the simulated simulated average average neutron neutron detection detection efficiency efficiency between between thermal thermal and and2.5 Figure 20. Behavior of the simulated average neutron detection efficiency between thermal and 2.5 MeV2.5 MeV in the in top the position.top position. The efficiency The efficiency modulation modulation due to due geometrical to geometrical effects effects is clearly is clearly visible. visible. The MeV in the top position. The efficiency modulation due to geometrical effects is clearly visible. The errorThe errorbars indicate bars indicate only onlythe sta thetistical statistical uncertainty. uncertainty. error bars indicate only the statistical uncertainty. Moreover,Moreover, a a slight slight left left-to-right-to-right decrease decrease can can be be appreciated appreciated in in Figure Figure 20,20 ,due due to to the the Moreover, a slight left-to-right decrease can be appreciated in Figure 20, due to the slightlyslightly asymmetric asymmetric position position of of each each silicon silicon diode diode inside inside its its box box (Figure (Figure 11).). A comparison slightly asymmetric position of each silicon diode inside its box (Figure 1). A comparison betweenbetween a a corner corner and and a a central central detector detector efficiency efficiency at at var variousious energies energies is is shown shown as as an an between a corner and a central detector efficiency at various energies is shown as an exampleexample in in Figure Figure 2121.. In Figure 2222 wewe showshow the the simulated simulated detection detection efficiency efficiency as as a functiona func- example in Figure 21. In Figure 22 we show the simulated detection efficiency as a func- tionof the of impingingthe impinging neutron neutron energy energy for thefor 16the detectors 16 detectors in the in 4the× 4 × array 4 array arrangement. arrangement. tion of the impinging neutron energy for the 16 detectors in the 4 × 4 array arrangement.

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FigureFigure 21. 21. ComparisonComparison between between a a corner corner (det00) (det00) a andnd a acentral central (det05) (det05) detector detector simulated simulated efficiency. efficiency. Figure 21. Comparison between a corner (det00) and a central (det05) detector simulated efficiency. TheThe overall overall shapes shapes are are similar, similar, the the difference difference being being essentially essentially a scale a scale factor factor enhancement enhancement for for the the centralThe overall detector shapes due toare a similar,geometrical the difference effect of the being neighboring essentially moderators. a scale factor enhancement for the central detector due to a geometrical effect of the neighboring moderators. central detector due to a geometrical effect of the neighboring moderators.

Figure 22. The simulated detection efficiency as a function of the impinging neutron energy for the 16FigureFigure detectors 22. 22.The Thein the simulatedsimulated 4 × 4 array detectiondetection configuration efficiencyefficiency in as theas a atop function function position. of of the the impinging impinging neutron neutron energy energy for for the the 16 detectors in the 4 × 4 array configuration in the top position. 16 detectors in the 4 × 4 array configuration in the top position. The simulated neutron flux in the top position has a main contribution between 100 keV andTheThe 5simulated simulatedMeV (Figure neutron 9), due flux flux to thein in the thepresence toptop positionposition of 5 cm has hasborated a main a main paraffin contribution contribution in the betweenouter between layer 100 of100keV the keV andsourceand 5 MeV 5box MeV (Figure which (Figure captures9), 9due), due to most the to thepresence of presencethe low of- 5energy of cm 5 cmborated neutrons. borated paraffin paraffinIts total in the invalue outer the outerof layer 1.9 n/s/cmlayerof the of2 sourceresulted the source box in boxwhichreasonable which captures captures agreement most most of with the of thelowa previous low-energy-energy directneutrons. neutrons. measurement Its Itstotal total value done value of by of1.9 means1.9n/s/cm n/s/cm of2 resulteda portable2 resulted in neu reasonable intron reasonable spectrometer agreement agreement Radeye with with a PX/WENDI aprevious previous direct direct[24], whichmeasurement measurement had provided donedone by bya valuemeansmeans of of of2.09 aa portableportable n/s/cm2 neutron.neu By troncombining spectrometerspectrometer the simulated RadeyeRadeye PX/WENDIPX/WENDIflux with the [2 [24 4simulated],], which which had haddetector provided provided effi- a ciencyavalue value weof of 2.09 estimated2.09 n/s/cm n/s/cm 2 the. By2. expected Bycombining combining counting the thesimulated simulated rate. Table flux flux with1 summarizes with the thesimulated simulated the comparisondetector detector effi- betweenefficiencyciency we measured we estimated estimated and the thesimul expected expectedated counting counting counting rates rate. rate. in Table Table the front11 summarizes summarizes and inside the thepositions. comparison comparison The neutronbetweenbetween counts measured measured measured and and simulated simulwith theated 4 counting counting× 4 array, rates ratescorrected in in the the forfront front theand simulatedandinside inside positions.efficiency positions. and The The forneutronneutron the intrinsic counts counts measureddetection measured effi with withciency the the 4 (4Figure× × 44 array,array, 17), corrected correctedprovided for forthe the thetotal simulated simulated neutron efficiency fluxefficiency for each and and detector.forfor the the intrinsic intrinsic In Figure detection detection 23 we compare efficiencyefficiency the (Figure ((a)Figure simulated 17 17),), provided provided and (b) theevaluated the total total neutron neutron fluxes, fluxand flux then for for each eachwe detector. In Figure 23 we compare the (a) simulated and (b) evaluated fluxes, and then we

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detector. In Figure 23 we compare the (a) simulated and (b) evaluated fluxes, and then we comparecompare the the (c) (c) simulated simulated and and (d) (d) measured measured counting counting rates. rates. Unfortunately, Unfortunately, due to due a broken to a brokenchannel channel in the electronics, in the electronics, the detector the indetector position in det10position did det10 not provide did not useful provide data. useful data. Table 1. Comparison of the measured neutron counting rates to the expectation from simulations. Table 1. Comparison of the measured neutron counting rates to the expectation from simulations. Measured Counts/s Simulated Counts/s Measured Counts/s Simulated Counts/s front 31.32 ± 0.16 ≈32 front 31.32 ± 0.16 ≈32 inside 39.78 ± 0.18 ≈54 inside 39.78 ± 0.18 ≈54

neutrons/cm2/s counts/s

(a) (b) (c) (d)

Figure 23. (a) Total neutron flux simulated in the top position in correspondence of the 4 × 4 SiLiF array; (b) The measured Figure 23. (a) Total neutron flux simulated in the top position in correspondence of the 4 × 4 SiLiF array; (b) The measured total neutron fluxflux withwith thethe 44 ×× 44 array. array. ( c) The simulatedsim ulated neutron neutron counting counting rates. rates. (d) The(d) measuredThe measured neutron neutron counting counting rates. rates. 4. Discussion 4. Discussion In the previous sections we have shown that the SiLiF detectors can fruitfully be em- In the previous sections we have shown that the SiLiF detectors can fruitfully be ployed to detect neutrons, in a rather wide energy range and with a very good gamma rejec- employedtion, in particular to detect considering neutrons, in that a rather typical wide commercial energy range neutron and detectors with a very claim goodγ/n gamma≈ 10−5 rejection,with respect in particular to the 137 Csconsidering gamma rays that of typical 662 keV. commercial This value neutron is to be detectors compared claim with γ/n the ≈ −5 137 10γ/n with≈ 10 respect−6 of SiLiF to the with Cs respect gamma to 2.2 rays MeV of 662 gamma keV. raysThis andvalue the is threshold to be compared at 1 MeV, with or −6 theeven γ/n better ≈ 10 with of SiLiF the γwith/n ≈ respect10−10 toif 2.2 using MeV a 1.5gamma MeV rays threshold. and the We threshold have also at 1 seen MeV, that or −10 evenSiLiF better detectors with can the provideγ/n ≈ 10 realistic if using measurements a 1.5 MeV threshold. of the total We have or thermal also seen neutron that SiLiF flux, detectorsrespectively, can when provide used realistic with or measurements without moderator. of the The total comparisons or thermal reported neutron in flux, Table re-1 spectivelyand Figure, when23 have used to bewith considered or without as beingmoderator. in good The agreement. comparisons Indeed, reported due toin theTable only 1 androughly Figure known 23 have features to be ofconsidered the source as moderator being in good box, weagreement. would have Indeed, been du satisfiede to the by only an roughlyagreement known between features measurements of the source and moderator simulations box, within we would one order have ofbeen magnitude. satisfied Quiteby an agreementsurprisingly, between the agreement measurements came out much and simulations better, thus suggesting within one that order the assumptions of magnitude. on Quitethe source surprisingly, geometry the were agreement realistic. came out much better, thus suggesting that the as- sumptionsThe agreement on the source between geometry the fluxwere measured realistic. with the SiLiF detectors and with the portableThe neutronagreement spectrometer between the in flux the top measuredposition with is quite the SiLiFreasonable detectors as well. and withIt is also the portableinteresting neutron to note spectrometer that the measured in the flux top (Figure position 23 b)is quite suggests reasonable a spatially as widerwell. fluxIt is thanalso interestingindicated by to the note simulation that the (Figure measured 23a). flux (Figure 23b) suggests a spatially wider flux than Theseindicated results by the prove simulation that SiLiF (Figure detectors 23a). can successfully be employed in the MICADO projectThese in order results to demonstrate prove that SiLiF the reliability detectors of can neutron successfully monitoring be employed in a medium-/long- in the MI- CADOterm storage project of in radioactive order to demonstrate waste drums. the An reliability interesting of neutron exercise monitoring has been done in a in medi- such uma framework,-/long-term tostorage investigate of radioactive the possibility waste drums. of detecting An interesting small quantities exercise ofhas actinides been done in inlow/intermediate such a framework, level to waste investigate drums. the A possiblepossibility radioactive of detecting waste small package quantities (RWP) of assem- acti- nidesbly was in considered, low/intermediate enclosed level in a standard waste drums. 220 l drum A possible (86 cm height,radioactive 57 cm w diameter)aste package with (RWP)a steel/polyethylene assembly was considered, matrix (67/33% enclosed in mass), in a standard in order 220 to estimate l drum (86 the cm SiLiF height, sensitivity. 57 cm diameter)The drum, withsketched a steel/polyethylene in Figure 24 along matrix with four (67/33% SiLiF detectors in mass), around in order it, was to estimate supposed the to SiLiFcontain sensitivity. 100 g of 240 ThePu, drum, which sketched is an isotope in Figure typically 24 along present with whenever four SiLiF there detectors is plutonium. around it,We was calculated supposed the to rate contain of prompt-fission 100 g of 240Pu, neutrons which is produced an isotope by typically such an amountpresent whenever according there is plutonium. We calculated the rate of prompt-fission neutrons produced by such

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toan the amount data in according Table2. Then to the we data made in aTable rough 2. estimate Then we of made the total a r expectedough estimate rate of of neutrons the total impingingexpected rate on theof neutrons four silicon impinging detectors, on considering the four silicon a uniform detectors, output considering flux from a the uniform cylin- deroutput surface, flux assuming from the acylinder 10% neutron surface, absorption assuming inside a 10% the neutron drum, and absorption fully thermalized inside the 2 6 neutronsdrum, and reaching fully thermalized the 9 cm LiF neutrons converter. reaching In Table the3 9we cm listed2 6LiF the converter. relevant In numbers Table 3 for we thelisted calculation, the relevant and numbers the resulting for the neutron calculation, counting and rate the in resulting the set of neutron four detectors counting is rate about in 4.5thecounts/s, set of four i.e., detectors about 1.1 is about counts/s 4.5 counts/s, in each detector. i.e., about 1.1 counts/s in each detector.

(a) (b)

FigureFigure 24.24. SketchSketch of of the the simulated simulated radwaste radwaste package package along along with with four four SiLiF SiLiF detectors detectors around around it. (a it.) (Topa) Top view; view; (b) ( bp)erspective perspective view. view.

AA Monte Monte Carlo Carlo simulation simulation of ofthis this setup setup was was done done by means by means of the of MCNP the MCNP code, which code, producedwhich produced the impinging the impinging flux onto flux the onto detectors the detectors as shown as shown in Figure in 9Figure. By combining 9. By com- thisbining flux this with flux the with simulated the simulated detector detector efficiency efficiency we obtained we obtained 4.9 counts/s 4.9 counts/s to be expected to be ex- inpected the set in of the four set detectors of four detectors (i.e., ≈1.2 (i.e. counts/s/detector),, ≈1.2 counts/s/detector), very close very to close the to value the fromvalue thefrom rough the rough calculation calculation above. above. The sameThe same simulation simulation also producedalso produced the spectrumthe spectrum of the of gammathe gamma rays rays impinging impinging on on the the four four SiLiF SiLiF detectors, detectors, which which is is shown shown in in Figure Figure 25 25 along along withwith thethe cumulativecumulative one.one. TheThe mostmost prominentprominent peak,peak, asas expected,expected, isis aroundaround 2.22.2 MeVMeV duedue toto thethe H(n,H(n,γγ)D)D reactionreaction inin polyethylene.polyethylene. SimpleSimple integrationsintegrations ofof thisthis spectrumspectrum showshow thatthat thethe expectedexpected raterate ofof gammagamma raysrays betweenbetween 11 andand 2.252.25 MeVMeV onon thethe fourfour detectorsdetectors isis aboutabout 3030 counts/s,counts/s, and that above 2.252.25 MeVMeV oneone expectsexpects aa raterate ofof about about 14 14 counts/s, counts/s, whichwhich isis wellwell discriminateddiscriminated fromfrom neutronsneutrons byby thethe SiLiF.SiLiF.

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FigureFigure 25. 25. TheThe simulated simulated spectrum spectrum of of the the gamma gamma rays rays impinging impinging on on the the four four SiLiF SiLiF detectors detectors (green (green line)line) and and the the corresponding corresponding cumulative cumulative distribution distribution (red (red dotted dotted line). line).

TheThe expected expected neutron neutron counting counting rate rate indicates indicates that that in in a a medium medium-term-term monitoring a a setset of of four four SiLiF SiLiF plac placeded around around a a drum drum should should in in principle principle be be able able to to detect detect and and monitor monitor the presence of even a lower quantity of 240Pu. As also observed in the top measurements, the presence of even a lower quantity of 240Pu. As also observed in the top measurements, the SiLiF detectors can reliably measure at very low neutron counting rates, because of the SiLiF detectors can reliably measure at very low neutron counting rates, because of their high neutron selection threshold which cuts off any electronic noise. We remark their high neutron selection threshold which cuts off any electronic noise. We remark that that in runs of 1–2 h with no radioactive source the SiLiF detectors did not register any in runs of 1–2 h with no radioactive source the SiLiF detectors did not register any neu- neutron count. The only disturbing signals could come from a very intense high-energy tron count. The only disturbing signals could come from a very intense high-energy gamma ray flux several orders of magnitude higher than the neutron flux, as shown in gamma ray flux several orders of magnitude higher than the neutron flux, as shown in Section 3.2 and Figure 14. This implies that, under the assumption of a similar flux of Section 3.2. and Figure 14. This implies that, under the assumption of a similar flux of neutrons and high-energy gamma rays, the sensitivity to neutrons is at least of the order neutrons and high-energy gamma rays, the sensitivity to neutrons is at least of the order of 10−3–10−4. For instance, being the expectation about 1 count/s on each detector, the of 10−3–10−4. For instance, being the expectation about 1 count/s on each detector, the dif- difference between 90 g and 100 g of 240Pu would be seen at one standard deviation in just 240 ferencetwo minutes, between whereas 90 g and 10 g100g of 240 ofPu Pu would would produce be seen≈100 at one total standard counts indeviation one detector in just in 240 twoless minutes, than 20 min. whereas 10 g of Pu would produce ≈100 total counts in one detector in less thanIn the 20 MICADO minutes. perspective the drums will be first characterized with several methods basedIn on the active MICADO and passive perspective neutron the and drums gamma will ray be measurements, first characterized then the with monitoring several methodsdetectors based will beon used active to and check passive the longer-term neutron and stability gamma of ray their measurements, radiological behavior.then the monitoringPossible counting detectors asymmetries will be used between to check the the four longer detectors-term would stability signal of their an asymmetry radiological in behavior.the fissile Possibledistribution, counting or, should asymmetries this occur between afterwards, the a four change detectors in the internalwould signal structure an asymmetryof the radwaste in the package. fissile distribution, Obviously, foror, ashould more precisethis occur evaluation afterwards, of the a sensitivitychange in onethe internalshould takestructure into account of the radwaste also the background package. Obviously, from the neighboringfor a more precise drums. evaluation of the sensitivityOne last pointone should concerns take the into possible account radiation also the damage background of silicon from in the a radiation neighboring field. drums.The radiation hardness claimed by the manufacturer of the silicon is up to 1013 n/cm2. It isOne known last that point the concerns main damage the possible comes radiation from fast damage neutrons of whichsilicon canin a dislocate radiation silicon field. Theatoms radiation thus creating hardness defects claimed in the by semiconductorthe manufacturer lattice, of the with silicon a realistic is up to energy 1013 n/cm threshold2. It is knownfor dislocation that the inmain silicon damage of 34 comes eV [25 from], roughly fast neutrons corresponding which can to the dislocate deposited silicon energy atoms of thusa recoil creating from adefects neutron in withthe semiconductor average energy lattice, of about with 400 a eV. realistic However, energy even threshold considering for dislocationthe total neutron in silicon flux of on 34 the eV detectors, [25], roughly that corresponding is about 11 n/cm to 2the/s fromdeposited the simulation, energy of a a recoillife longer from thana neutron many with hundreds average of yearsenergy is toof beabout foreseen 400 eV. under However, these conditions. even considering Indeed, theas observedtotal neutron in reference flux on the [26 ],detectors, similar detectors that is about were 11 exposed n/cm2/s tofrom a fast the neutron simulation, fluence a life of longer3 × 10 10than/cm m2 anywithout hundred appreciables of years changes is to be in foreseen their characteristics. under these conditions. Indeed, as observed in reference [26], similar detectors were exposed to a fast neutron fluence of 3 × 1010/cm2 without appreciable changes in their characteristics.

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Table 2. Calculation of the rate of fission neutrons produced by 100 g of 240Pu (the relevant nuclear data were taken from [27]).

Fissioning Species 240Pu T1/2 [y] 6561 T1/2 [s] 2.07 × 1011 decay constant τ [y] 9466 decay constant τ [s] 2.99 × 1011 decay rate [1/s] 3.35 × 10−12 fission Branching Ratio 5.70 × 10−8 fission rate [1/s] 1.91 × 10−19 atomic mass [amu] 240.05 N atoms/gram 2.51 × 1021 sample mass [g] 100 N atoms in sample 2.51 × 1023 fission rate in sample [1/s] 4.79 × 104 /fission 2.16 neutron rate from sample [1/s] 1.03 × 105

Table 3. Rough estimate of the total expected neutron rate on the four silicon detectors, considering a uniform output flux from the radioactive waste package (RWP) cylinder surface, a 10% neutron absorption inside the drum and fully thermalized neutrons reaching the 6LiF converter.

Radwaste Package Matrix Inox/CH2 67/33% RWP radius [cm] 28.5 RWP extended radius [cm] 38.5 RWP height [cm] 86 RWP extended side area [cm2] 20,804 RWP top + bottom area [cm2] 9313 RWP total exit area 30,117 SiLiF active area [cm2] 9 number of SiLiF units 4 rough geometrical efficiency 1.20 × 10−3 thermal neutron detection efficiency 4% total neutron counting efficiency 4.8 × 10−5 n absorption factor (guess) 0.1 counts/s in 4 SiLiF from sample 4.5

5. Conclusions The described tests, measurements and simulations have allowed us to prove the suitability of the SiLiF technology for neutron detection. The set of 32 detectors we built proved to have a uniform behavior, apart from an unforeseen systematic shift of the detec- tion efficiency due to the converter substrate, which was investigated and fully understood. We can conclude that SiLiF is quite a promising candidate for future applications for low and intermediate level radioactive waste monitoring, and is going to be tested soon in a real environment within the MICADO project.

Author Contributions: Conceptualization and supervision, P.F.; detector design, P.F., F.L., A.P., L.C.; detector construction, F.L., A.P. and G.P.; chemical depositions, A.M. and C.M.; test and measure- ments, F.L., A.P., M.G., L.C. and P.F.; simulations, S.L.M., S.R. and Q.D.; manuscript preparation, P.F. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded within the framework of European Union’s Horizon 2020 research and innovation program under grant agreement No 847641, project MICADO (Measurement and Instrumentation for Cleaning and Decommissioning Operations). This work was also supported by the Centro Siciliano di Fisica Nucleare e di Struttura della Materia [Grant 02/2019]. Institutional Review Board Statement: Not applicable. Sensors 2021, 21, 2630 21 of 22

Informed Consent Statement: Not applicable. Data Availability Statement: The data are in the document. Acknowledgments: We are grateful to Marco Ripani for the constant encouragement and for the initial support within the INFN-Energy program that allowed us to start the MICADO project. Conflicts of Interest: The authors declare no conflict of interest.

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