Measurement of the neutron fields produced by a 62 MeV proton beam on a PMMA phantom using extended range Bonner sphere spectrometers

The experimental characterization of the neutron fields produced as parasitic effect in medical accelerators is assuming an increased importance for either the patient protection or the facility design aspects. Medical accelerators are diverse in terms of particle type (electrons or hadrons) and energy, but the radiation fields around them have in common (provided that a given threshold energy is reached) the presence of neutrons with energy span over several orders of magnitude. Due to the large variability of neutron energy, field or dosimetry measurements in these workplaces are very complex, and in general, cannot be performed with ready-to-use commercial instruments. In spite of its poor energy resolution, the Bonner Sphere Spectrometer (BSS) is the only instrument able to simultaneously determine all spectral components in such workplaces. The energy range of this instrument is limited to Eo 20 MeV if only polyethylene spheres are used, but can be extended to hundreds of MeV by including metal-loaded spheres (extended range BSS, indicated with ERBSS). With the aim of providing useful data to the scientific community involved in neutron measurements at hadron therapy facilities, an ERBSS experiment was carried out at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) of INFN—LNS (Laboratori Nazionali del Sud), where a proton beam routinely used for ophthalmic cancer treatments is available. The 62 MeV beam was directed towards a PMMA phantom, simulating the patient, and two neutron measurement points wer

Determination of the long-term equilibrium factor indoors by means of makrofol track-etched detectors

From the point of view of radioprotection, it is worthwhile to determine the long-term equilibrium factor indoors to better ascertain the dose which population is exposed to. The method used in our laboratory to obtain long-term equilibrium factors indoors is based on simultaneous measurements of 222Rn and 214Po in air by using a detector with two Makrofol-ED (polycarbonate) foils covered with aluminised mylar: one placed inside a diffusion chamber, and the other one in direct contact with air, allowing, respectively, the measurement of 222Rn and 214Po activity concentrations. The equilibrium factor is estimated from the ratio of these two concentrations. The detector has been calibrated at the radon chamber of the National Radiological Protection Board (NRPB), Chilton, UK. Sensitivity values of e 04( 0.8560.04) (tr Qcm22 ) /( kBq Qm23 Qh ) and e 44(3.060.2) (trQcm22 ) /( kBq Q m23 Qh ) have been obtained from the calibration for the measurement of 222Rn and 214Po, respectively. A radon activity concentration of 44 BqQm23 and a mean equilibrium factor of 0.42 have been obtained in a pilot application indoors

Radiological characterisation in view of nuclear reactor decommissioning: On-site benchmarking exercise of a biological shield

Nearly all decommissioning and dismantling (D&D) projects are steered by the characterisation of the plant being dismantled. This radiological characterisation is a complex process that is updated and modified during the course of the D&D. One of the tools for carrying out this characterisation is the performance of in-situ measurements. There is a wide variety of equipment and methodologies used to carry out on-site measurements, depending on the environment in which they are to be carried out and also on the specific objectives of the measurements and the financial and personnel resources available. The extent to which measurements carried out with different types of equipment or methodologies providing comparable results can be crucial in view of the D&D strategy development and the decision-making process. This paper concerns an on-site benchmarking exercise carried out at the activated biological shield of Belgian Reactor 3 (BR3). This activity allows comparison and validation of characterisation methodologies and different equipment used as well as future interpretation of final results in terms of uncertainties and sensitivities. This paper describes the measurements and results from the analysis of this exercise. Other aspects of this exercise will be reported in separate papers. This paper provides an overview of the on-site benchmarking exercise, outlines the participating organisations and the measurement equipment used for total gamma, dose rate and gamma spectrometry measurements and finally, results obtained and their interpretations are discussed for each type of measurement as a function of detector type. Regarding the dose measurements, results obtained by using a large variety of equipment are very consistent. In view of mapping the inner surface of the biological shield the most appropriate equipment tested might be the organic scintillator, the BGO or even the ionisation chamber. In addition, for mapping this surface, the most appropriate total gamma equipment tested might be the LaBr$_{3}$(Ce), the thick organic scintillator or the BGO. These measurements can only be used as a secondary parameter in a relative way. Results for the gamma spectrometry are very consistent for all the equipment used and the main parameters to be determined

Estimation of neutron-equivalent dose in organs of patients undergoing radiotherapy by the use of a novel online digital detector.

Neutron peripheral contamination in patients undergoing high-energy photon radiotherapy is considered as a risk factor for secondary cancer induction. Organ-specific neutron-equivalent dose estimation is therefore essential for a reasonable assessment of these associated risks. This work aimed to develop a method to estimate neutron-equivalent doses in multiple organs of radiotherapy patients. The method involved the convolution, at 16 reference points in an anthropomorphic phantom, of the normalized Monte Carlo neutron fluence energy spectra with the kerma and energy-dependent radiation weighting factor. This was then scaled with the total neutron fluence measured with passive detectors, at the same reference points, in order to obtain the equivalent doses in organs. The latter were correlated with the readings of a neutron digital detector located inside the treatment room during phantom irradiation. This digital detector, designed and developed by our group, integrates the thermal neutron fluence. The correlation model, applied to the digital detector readings during patient irradiation, enables the online estimation of neutron-equivalent doses in organs. The model takes into account the specific irradiation site, the field parameters (energy, field size, angle incidence, etc) and the installation (linac and bunker geometry). This method, which is suitable for routine clinical use, will help to systematically generate the dosimetric data essential for the improvement of current risk-estimation models

Experimental characterization of the neutron spectra generated by a high-energy clinical LINAC

The production of unwanted neutrons by electron linear accelerators (LINACs) has attracted a special attention since the early 50s. The renewed interest in this topic during the last years is due mainly to the increased use of such machines in radiotherapy. Specially, in most of developing countries where many old teletherapy irradiators, based on 60Co and 137Cs radioactive sources, are being replaced with new LINAC units. The main objective of this work is to report the results of an experimental characterization of the neutron spectra generated by a high-energy clinical LINAC. Measurements were carried out, considering four irradiation configurations, by means of our recently developed passive Bonner sphere spectrometer (BSS) using pure gold activation foils as central detectors. This system offers the possibility to measure neutrons over a wide energy range (from thermal up to a few MeV) at pulsed, intense and complex mixed nγ fields. A two-step unfolding method that combines the NUBAY and MAXED codes was applied to derive the final neutron spectra as well as their associated integral quantities (in terms of total neutron fluence and ambient dose equivalent rates) and fluence-averaged energies. © 2010 Elsevier B.V. All rights reserved

Set-up of a passive Bonner sphere system for neutron spectrometry at mixed fields with predominant photon component based on activation detector

A passive Bonner sphere system (BSS), based on thermal neutron activation detectors, was developed to perform neutron spectrometry in pulsed and very intense (n-γ) fields with predominant photon component, as those produced by high energy (>10 MV) medical linear electron accelerators. In this paper, a description of the new system is presented together with an experimental characterisation of a portable Sodium Iodide (NaI) detector and a fixed high-purity Germanium one, both used to measure the induced γ-activity of the activated materials, respectively, in situ and in the laboratory. The choice of the activated materials is justified according to pre-established practical considerations and physical criteria. The response functions of the entire passive BSS were calculated using the MCNPX code. A preliminary experimental validation with a bare 252Cf source is given as well. © The Author 2007. Published by Oxford University Press. All rights reserved

Neutron spectrometry with a passive bonner sphere system around a medical LINAC and evaluation of the associated unfolding uncertainties

It is generally known that the use of high-energy electron linear accelerators (LINACs) in radiotherapy medical treatments may generate secondary neutrons, mainly via photonuclear (γ,n) giant dipole resonance reactions of incident photons with all the heavy materials present inside the gantry and along the beam line. A detailed knowledge (i.e., fluence energy distribution) of this parasite radiation, which is approximately isotropic and not confined within the primary LINAC beam field, would be of great interest to estimate the associated radiological risk for the patient and the working staff. It has been shown, in this study, that our recently developed passive Bonner sphere system, using pure gold activation foils as central detectors, is well adapted to measure neutron spectra at pulsed and intense mixed n-γ fields with high-energy photon component. This system was used to characterize the neutron field around a new generation medical electron LINAC. Two measurement positions (isocenter and maze entrance) inside the treatment room of this facility, with the machine operating in Bremsstrahlung photon mode, were chosen. The obtained specific 198Au saturation activities were processed by means of the NUBAY unfolding code, which performs a Bayesian estimation of a parameterized spectrum, to derive the final neutron spectra. Another unfolding method (MAXED), based on the maximum entropy principle and which may depend to some extent on considered the initial guess or default spectrum, was also applied to check the robustness of the NUBAY solutions as well as to carry out a sensitivity analysis to confirm their stability and to corroborate the associated uncertainties on their output results. Also presented are the obtained integral quantities, in terms of neutron fluence and ambient dose equivalent rates normalized to the primary LINAC photon dose, together with an estimation of their associated uncertainties due to the unfolding code used. © 2009 IEEE

3-D localization of radioactive hotspots via portable gamma cameras

International audienceA portable stereo gamma camera has been built, based on two hybrid pixelated detectors, each of them comprising a 1 mm thick CdTe substrate bump-bonded to the Timepix readout chip, a pixelated CMOS ASIC consisting of 256 × 256 pixels with 55 µm pitch.The technique described in this paper allows the automatic estimation of the distances between the prototype and the located radioactive sources by using triangulation based on epipolar geometry. The 3-D Cartesian coordinates of the sources are provided even when they are hidden behind an occluding material, such as a wall, or inside a barrel. Furthermore, the combination of the prototype with a depth sensor enables a wide range of applications, including the 3-D volumetric reconstruction of the scene under study as well as the capability of autonomously determining if an object is occluding the located radioactive sources.We discuss here the main features of the whole methodology developed and we also present the experimental results obtained at laboratory scale

A versatile calibration procedure for portable coded aperture gamma cameras and RGB-D sensors

International audienceThe present paper proposes a versatile procedure for the geometrical calibration of coded aperture gamma cameras and RGB-D depth sensors, using only one radioactive point source and a simple experimental set-up. Calibration data is then used for accurately aligning radiation images retrieved by means of the -camera with the respective depth images computed with the RGB-D sensor. The system resulting from such a combination is thus able to retrieve, automatically, the distance of radioactive hotspots by means of pixel-wise mapping between gamma and depth images. This procedure is of great interest for a wide number of applications, ranging from precise automatic estimation of the shape and distance of radioactive objects to Augmented Reality systems. Incidentally, the corresponding results validated the choice of a perspective design model for a coded aperture -camera

La dosimétrie et spectrométrie neutroniques : la contribution de l’université autonome de Barcelone

Depuis la création de l’université autonome de Barcelone, notre groupe a mené une recherche à la fois théorique et expérimentale sur les rayonnements cosmiques et leur détection au moyen de détecteurs passifs. La connaissance acquise sur le processus d’enregistrement des détecteurs solides de traces et les caractéristiques de ces détecteurs ont permis au groupe d’étendre sa recherche vers d’autres applications et d’autres types de détecteurs. Nous présentons nos travaux principaux de recherche sur ce thème en prenant en compte les résultats les plus significatifs dans cette étude