# Development of a transportable neutron spectrometer for operational dosimetry within the DONEUT project

Marc Labalme, Sébastien Bozec, Yohann Brelet, G. de Bouteiller, Alban

Hemmer, E. Moncanis, Antoine Roussel, Claude-Alexandre Simonetti,

Jean-Lionel Trolet

► To cite this version:

Marc Labalme, Sébastien Bozec, Yohann Brelet, G. de Bouteiller, Alban Hemmer, et al. Development of a transportable neutron spectrometer for operational dosimetry within the DONEUT project. 9th International Conference on Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA 2025), Jun 2025, Valencia, Spain. pp.10011, 10.1051/epj-conf/202533810011. hal-05359307

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# Development of a transportable neutron spectrometer for operational dosimetry within the DONEUT project

M. Labalme¹*, S. Bozec², Y. Brelet¹, G. de Bouteiller¹,², A. Hemmer¹,², E. Moncanis¹,², A. Roussel¹,², C.A. Simonetti¹,², J.L. Trolet²

¹LPC Caen, Normandie Univ, ENSICAEN, UNICAEN, CNRS/IN2P3, Caen, France

²Marine Nationale, EAMEA, Cherbourg-en-Cotentin, France

(*) labalme@lpccaen.in2p3.fr

**Abstract**—Accurate neutron dose rate estimation demands precise measurement of both kinetic energy and neutron flux. This study introduces a multi-instrumented transportable neutron spectrometer/dose rate meter, developed as part of the DONEUT project. The prototype, optimized for 0–20 MeV neutron dose rates, is based on a polyethylene moderator with thermal neutron detectors at different depths to reconstruct energy spectra via unfolding algorithms. The initial DONEUT V1 design consists of a 25 cm diameter polyethylene cylinder with 32 embedded ZnS:LiF scintillators, coupled to a SiPM matrix via light guides and wavelength-shifting fibers. GEANT4 Monte-Carlo simulations guided detector configuration and design optimization. Unfolding methods (MAXED and GRAVEL) were applied to various moderator volumes and neutron energy spectra, including Am/Be and ²⁵²Cf sources. This led to a “dome” prototype, DONEUT V1+, which improves vertical angles response uniformity and reduces weight. A sixth detection depth was added to enhance deconvolution robustness, particularly in the epithermal range, while the number of detectors was reduced from 32 to 24. A custom Python implementation of the GRAVEL algorithm was developed, replacing traditional codes, increasing flexibility and processing speed for energy spectra reconstruction. Experimental tests with moderated and unmoderated Am/Be sources were performed, benchmarked against NNS measurements, demonstrated dose equivalent rate (H*(10)) accuracy within 15%, even at low dose rates (~1 µSv/h) over a 10-minute measurement. The prototype reliably assesses dose rates from 1 to 100 µSv/h. A new version, DONEUTV2, insensitive to gamma background, is under development to extend the range to several mSv/h

**Keywords** —neutron, dose rate meter, unfolding algorithm, ZnS: LiF.

## I. INTRODUCTION

Accurate measurement of neutron dose equivalent rates is a critical requirement in nuclear safety and radiation protection, particularly in demanding environments such as nuclear-powered submarines. These operational settings necessitate compact, reliable, and fast-response instruments capable of characterizing neutron fields with high precision. The

DONEUT (DOsimétrie NEUTrons) project was initiated to address this need, aiming to develop a transportable neutron spectrometer able to provide real-time or near-real-time spectral and dose rate information.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Pictures of the three version of developed prototypes, DONEUT V1 (2021) in the left, DONEUT V1+ (2024) in the center and DONEUT V2 (2025) in the right.</figcaption></figure>

The project, launched in 2019, conducted at the Laboratoire de Physique Corpusculaire de Caen (LPC) in partnership with the French Navy's Nuclear Physics Laboratory (LPN) at EAMEA, has evolved through several design iterations of spectrometer prototypes, as presented in Fig. 1. The primary objective was to create a system capable of measuring neutron fluence and energy distributions across a broad energy range from thermal energies up to 20 MeV, and from these, derive ambient and individual dose equivalents (H*(10) and Hp(10)) thanks to unfolding algorithms. It consists on designing a transportable neutron detector prototype able to measure dose equivalents in compliance with a stringent set of specifications derived from both civilian and military nuclear requirements:

- Ambient dose equivalent detection thresholds ranging from 1 µSv/h to 10 mSv/h;

- Maximum measurement time of 10 minutes;

- Neutron energy spectrum coverage from 0.025 eV to 20 MeV;

- Dose equivalent calculation accuracy of 20% or better;

- Gamma/neutron discrimination to ensure environmental insensitivity;
- Isotropic response for omnidirectional source measurements;

- Portability: < 15 kg and < 60 cm in diameter.

In line with these specifications, several prototypes were developed and evaluated under laboratory conditions: DONEUT V1 between 2021 and 2023, DONEUT V1+ in 2024 and DONEUT V2 from 2025. A synthesis of the key findings is presented in the following sections.

## II. INSTRUMENT DESIGN AND OPTIMIZATION

The DONEUT spectrometer is based on the principle of a multi-instrumented moderator volume, similar in concept to Bonner spheres [1] but adapted for compactness and rapid measurement. The core of the system is a polyethylene (PE) moderator, 25 cm diameter cylinder or sphere, within which several thermal neutron detectors are embedded at various depths. This configuration allows the system to be sensitive to different neutron energy ranges in only one measurement, enabling neutron energy spectral reconstruction through unfolding techniques.

To optimize the spectrometer's geometry and detectors configuration, extensive simulations using the GEANT4 Monte Carlo toolkit were conducted. These simulations evaluated various moderator shapes and detector placements, using neutron sources such as Am/Be and $^{252}$Cf, both moderated and unmoderated. The unfolding of simulated detector responses was performed using the MAXED and GRAVEL algorithms from the UMG 3.3 software suite [2].

The first prototype, referred to as DONEUT V1 developed from 2019 [3], consisted of a 25 cm diameter PE cylinder housing 32 ZnS:LiF (0.3mm x 10mm x 20mm) scintillation detectors arranged in concentric layers at five different depths (Fig. 1.). Each detector is composed of two layers of ZnS:LiF glued to a 3 mm thick plexiglass light guide, coupled to a Kuraray Y11(200) wavelength-shifting fiber that transmitted scintillation light to a 64-channels silicon photomultiplier (SiPM) matrix. The readout electronics is based on the CAEN DT5202 module, which integrates two Citiroc-1A chips and provides 64 channels with temperature compensation. Simulation results exhibit good energy spectra reconstructions and dose rates evaluations in agreement with the specification, ±20%, regardless of the energy spectrum.

## III. EXPERIMENTAL VALIDATION

Experimental campaigns were carried out at LPC Caen and LPN EAMEA Cherbourg to validate the performance of the DONEUT prototypes. Measurements were conducted using Am/Be neutron sources, both bare and moderated with a 25 cm PE sphere, as well as $^{137}$Cs gamma sources to assess gamma sensitivity.

The first prototype, DONEUT V1, was extensively tested with Am/Be sources for neutron dose rates ranging from 1 µSv/h to 100 µSv/h. Results obtained for both energy spectra and dose rates evaluations showed good agreement between

DONEUT (using MAXED or GRAVEL unfolding algorithms) and the reference NNS spectrometer (Detec) [4], with deviations within ±20% for various source configurations and dose rates as illustrated by the example presented in Fig. 2. The agreement is of the order of 5%. These tests validated the concept. Nevertheless, this prototype showed limitations versus the vertical angle and a high sensitivity to the inner detectors.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Dose rate vs neutron energy measured at LPN Cherbourg by DONEUT V1, unfolded with GRAVEL and MAXED and compared to the NNS results used as reference. Neutron source: Am/Be moderated in a 25cm diameter polyethylene sphere. The dose rate integrated on the all-energy spectra are calculated.</figcaption></figure>

New simulations were performed in 2024 whom results led to the development of an improved prototype, DONEUT V1+, featuring a dome-shaped moderator. This new geometry improved vertical angle response uniformity and reduced the overall weight of the system. Additionally, the number of detectors was reduced from 32 to 24, and a sixth depth layer was introduced to improve sensitivity in the epithermal range. This new prototype was extensively tested with Am/Be sources, maintaining measurement accuracy while improving portability and robustness.

## IV. DEVELOPMENT OF A PYTHON BASED UNFOLDING SOFTWARE

One of the key innovations of the DONEUT project was the development of a custom Python-based implementation of the GRAVEL unfolding algorithm. This new tool, referred to as DSP (Déconvolution Sous Python), was inspired by the work of T. Doležal [5] and offers significant improvements in processing speed and flexibility compared to the UMG3.3 software. While UMG required up to 20 minutes for data processing, the DSP algorithm reduced this time to under 30 seconds.

DSP algorithm was first validated through GEANT4 simulations. An example of the results obtained for an unmoderated Am/Be source is presented in Fig. 3. Better agreements have been obtained for DONEUT V1+ and DSP compared to the previous prototype for both moderated and unmoderated neutron energy spectra.
<figure><img src="image_4.png" /><figcaption>Fig. 3. DSP unfolding algorithm (GRAVEL INCERTITUDE) results obtained with DONEUT V1+ in GEANT4 simulations with an unmoderated Am/Be source.</figcaption></figure>

The new prototype using DSP was then extensively tested in laboratory, and results were compared to the one obtained using the NNS. An example of results obtained with a moderated Am/Be source at LPN is presented in Fig. 4. DSP produced energy spectra and dose rate estimates that were consistent with those obtained using the commercial NNS spectrometer, while avoiding the thermal peak overestimation as it has been observed with UMG. A good agreement on dose rates is obtained, within ±10% The improved performance obtained thanks to the DSP algorithm was particularly evident in the epithermal energy range, where it provided more accurate spectral reconstructions.

<figure><img src="image_5.png" /><figcaption>Fig. 4. DSP unfolding algorithm (GRAVEL INCERTITUDE) results obtained with DONEUT V1+ in tests with a moderated Am/Be source and compared to results obtained with the NNS.</figcaption></figure>

This new algorithm is the one used since 2024 in simulations and measurements. The results demonstrated that the DONEUT V1 and V1+ prototypes could achieve dose rate accuracies within 15–20% over a wide range of neutron fluxes, from 1 µSv/h to 100 µSv/h. The system exhibited good angular response uniformity, with deviations less than ±10% up to 45° from the vertical axis. Furthermore, the spectrometer showed low sensitivity to gamma background, with a neutron-to-gamma dose equivalent ratio (H*(10)γ / H*(10)n) of approximately 2.

V. CONCLUSIONS AND PERSPECTIVES

The DONEUT project successfully delivered a compact,
accurate, and fast-response neutron spectrometer for

operational dosimetry in complex radiation environments
achieving a 20% accuracy on neutron dose rates compared to
the NNS reference.

Future developments will focus on further miniaturization,
integration of neural network-based unfolding algorithms, and
extensive field testing. The transition to DONEUT V2 in 2025
marked a significant leap in hardware design. The new
prototype featured a more compact and spherical PE
moderator weighing less than 10 kg. Detector modules were
redesigned to integrate the scintillator, the fiber, and one SiPM
into a sealed, modular unit, improving mechanical robustness
and ease of deployment. The readout electronics was upgraded
with new home-made front-end electronics and to a “trigger-
less” acquisition system based on the LPC-made FASTER
digitizer in QDC mode (125 MHz, 12 Bits) [6]. This system
supports long charge integration windows, which are better
suited to the slow light emission of ZnS scintillators than the
one proposed by the CAEN module. Preliminary tests of
individual detectors demonstrated the ability to operate at high
dose rates (>500 µSv/h) while maintaining good energy
resolution and a very good neutron/gamma discrimination
even for dose rates ratio H*(10)γ / H*(10)n ~50.

ACKNOWLEDGMENT

Authors thank the region Normandie for its financial
support via a RIN projet emergent.

REFERENCES

[1] T.W. Bonner, Richard L. Bramblett, Ronald I. Ewing. « A new type of neutron spectrometer». Nuclear Instruments and Methods 9.1 (1960), p. 1-12.

[2] M. Reginato, B. Wiegel, A. Zimbal and F. Langner, “UMG 3.3, Analysis of data measured with spectrometers using unfolding techniques,” Energy Agency of the OECD (NEA), 2004.

[3] C.A. Simonetti, M. Labalme, J.L. Trolet and P. Mary, «Deconvolution methods used for the development of a neutron spectrometer», EPJ Web of Conferences 253, 09003 (2021)

[4] J. Dubeau et al., “A neutron spectrometer using nested
moderators,” Radiation Protection Dosimetry, vol. 150, no. 2, pp.
217, 2012.

[5] T. Doležal, "Unfolding the AmBe Neutron Spectra using GRAVEL and MLEM," Department of Engineering Physics, Air Force Institute of Technology, 2021. https://github.com/tylerdolezal/Neutron-Unfolding

[6] https://faster.in2p3.fr/