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Neutron is one of the most produced secondary particle by medical and industrial accelerators, but it remains difficult to detect due to its neutral charge. Neutrons also require a particular attention in radiation protection due to their long range and their high relative biological effectiveness (RBE) [1]. For instance, in hadron therapy, ion beams can generate multiple neutrons both in the beam line and in the patient body. Neutrons are responsible for the major part of the out-of-field dose received by the patient away from tumor [2]. In industries, X-ray photons produced by electron accelerator are increasingly used for various applications such as food sterilization or polymers cross-linking. During these processes, neutrons can be produced by photo-nuclear reactions in the accelerator component and the irradiated object [3]. These photo-neutrons can lead to the activation of irradiated materials. A precise knowledge of the produced neutron spectrum for a given medical or industrial application is thus important to improve the radiation protection of workers and population in these installations. Monte Carlo (MC) simulation and analytical codes for radiation-matter interactions are considered to be powerful and practical tools for this purpose. However, there is still today a lack of nuclear data neccessary to validate models of neutron production used in theses codes.
The reference method for neutron spectrometry is the Bonner Spheres System (BSS). This system allows to reconstruct the neutron spectrum over a wide energy range, from meV to GeV. However this detection system, which is very cumbersone, is hardly compatible with the constraints of most medical and industrial environments. Maximum neutron flux is also a problem for some installations that use very high current, because of possible saturation effects [4]. In order to facilitate the acquisition of nuclear data in medical or industrial installations (confined space, high flux), it seems necessary to have a dedicated neutron spectrometry system. The DeSIs research team (Dosimetry, SImulation, Instrumentation), from IPHC laboratory (Strasbourg, France), has been working for a few years on the conception of such a neutron detector [5]. It is a recoil proton telescope (RPT) designed to operate within 5 to 30 MeV energy range (Figure 1.a). The instrument is made of a thin polyethylen converter (typically 50 µm) to generate recoil protons, 3 pixelated CMOS sensors specially developed to measure high rate proton emission angle and a silicium diode to measure the proton energy. The detector is extremely compact (10x10x8 cm³) to be placed easily in all the irradiation facilities. Its low detection efficiency (10⁻⁵), combined with CMOS performances, allow it to work in high fluxes environment (up to 109 n/s/cm2). The neutron spectrum is also reconstructed in real time, without complex and time consuming deconvolution process.
The RPT performances have been evaluated both with protons (CyRCE accelerator, IPHC, France) for the CMOS trajectography, and with neutrons (AMANDE facility, IRSN, France) for the spectrum reconstruction (Figure 1.b). A dedicated analysis software, including multiple selection cuts, has been developed to correct raw data from various backgrounds (fake tracks, double counts, …). The next step is to test (end 2021) the detector in real conditions in a proton therapy center (Center Antoine Lacassagne - France) and in an industrial facility (Aérial – France). Instrumentation, performance evaluation and the first data measured at medical and industrial facilities will be presented in this work.
[1] A. Ottolenghi et al., Assessment of cancer risk from neutron exposure – The ANDANTE project. Radiation Measurements 57 (2013) 68-73
[2] M. Hultqvist and I. Gudowska, Secondary absorbed doses from light ion irradiation in
anthropomorphic phantoms representing an adult male and a 10 year old child, Phys. Med. Biol. 55 (2010) 6633–6653
[3] O. Grégoire et al., Radiological safety of food irradiation with high energy X rays: theoretical expectations and experimental evidence, Radiation Physics and Chemistry 67 (2003) 169–183
[4] D.J. Thomas, A.V. Alevra, Bonner sphere spectrometers - a critical review, Nuclear Instruments and Methods in Physics Research A 476 (2002) 12–20
[5] R. Combe, N. Arbor, S. Higueret, D. Husson, Experimental characterization of a fast, pixelated CMOS sensor and design of a Recoil-Proton Telescope for neutron spectrometry, Nuclear Instrument and Methods in Physics Research A 929 (2019)