Validating current-mode operation of the Nested Neutron Spectrometer under high neutron fluence-rates in radiation therapy using a novel passive system with gold foils
Image credit: Felix MathewAbstract
Objective: Nested Neutron SpectrometersTM (NNS, Detec Inc., Gatineau, QC) are commercially available Russian-doll type neutron spectrometers that use He-3 counters for thermal neutron detection [1]. These spectrometers are calibrated by the vendor at the Ionizing Radiation Standards Laboratory of the National Research Council of Canada (IRS-NRC) using a reference Am-Be neutron source. During the calibration, akin to the method of Hagiwara et al. 2011 [2], a conversion factor is determined to convert the accumulated charge measured in current-mode operation to the number of neutrons that interacted with the sensitive volume of the He-3 detector. This calibration factor is provided to end-users, typically for determining neutron spectra at a particle accelerator or nuclear power plant environments where neutron fluence-rates are high. However, the accuracy of the conversion factor obtained using the Am-Be source (low neutron fluence-rate) is questionable when the neutron fluence-rates are high and beyond the range of calibration. A direct validation in a high fluence-rate environment is also impossible as the pulse-mode (an operation mode in which the number of neutron pulses is measured directly) fails due to pulse pile-up. Therefore, in this study, we sought to indirectly validate the current-mode of the NNS in a high fluence-rate environment using passive gold-foil neutron detectors. Materials and methods: We developed a novel passive-NNS by replacing the He-3 detector with a gold-foil disc (8 mm radius, 0.1 mm thick and 19.3 g cm-3 density) placed between two cylindrical inserts. The gold activation foil was positioned horizontally at the geometric center of the moderators of the NNS. Unlike the original active-NNS, the passive version does not suffer from pulse pile-up when used under high neutron fluence-rates. Hence, we used both the active and passive NNS to determine and compare the neutron spectra generated by the 15 MV beam of a Varian TrueBeam STx linac at the location of measurement (1 m away from the isocenter along the treatment-couch axis at the isocenter height) in a radiotherapy bunker of the McGill University Health Centre (MUHC). The spectral measurement procedure for the active NNS is well established [3] . The electrometer connected to the He-3 detector of the NNS measures the total charge accumulated in the He-3 detector during the irradiation. Using the vendor-provided response functions, calibration factor and an input guess spectrum the raw data are unfolded into a neutron fluence-rate spectrum using the modified MLEM-STOP algorithm [4]. The passive NNS on the other hand requires more elaborate pre-processing steps to obtain unfoldable raw data. Gold-foils get activated via the neutron capture reaction in a neutron field. The activated gold isotope (Au-198) radiates a characteristic photon of energy 412 keV with a half-life of 2.7 days. With a suitable gamma-ray spectrometer, the characteristic photon can be identified and the saturation activity of the foil can be determined. In our study, we used a High Purity Germanium (HPGe) detector at the SLOWPOKE neutron activation analysis laboratory of Polytechnique Montreal. The response functions of the passive NNS that were needed to unfold the foil saturation activity data―to obtain the neutron fluence-rate spectra―were generated through Monte Carlo simulations of the spectrometer in Geant4 (version 10.4. patch-2). The passive NNS with gold-foils was modelled accurately and its neutron interactions were simulated using the QGSP_BIC_HP physics model in Geant4. The response, defined as the ratio of the number of neutron capture reactions to the neutron fluence, was thus obtained and used to unfold the foil saturation activity data using the modified MLEM-STOP algorithm as for the active NNS. Results: The response functions of the passive system with gold foil were obtained from the Monte Carlo simulations in Geant4 for different moderator shell configurations. The neutron fluence-rate spectra at the location of measurement during the irradiation of the 15 MV beam in the radiotherapy bunker using both the active and passive NNS were also obtained. The histograms plotted show the spectra obtained with the average of three repeated measurements with both spectrometers and the shaded region is the standard uncertainty of each neutron spectrum. The spectra agreed reasonably well within uncertainties. Conclusion: In this study, a passive NNS with gold-foils was successfully developed and a functional workflow to use the spectrometer for neutron spectral measurements was established. The neutron fluence-rate spectrum at a location of interest was determined using the active NNS in its current-mode and with the passive gold-foil-NNS and compared. The spectra agreed reasonably well within uncertainties validating the accuracy of the use of the active NNS with He-3 detector in its current-mode under high neutron fluence-rate environments such as in radiotherapy. Relevance to CIRMS: This work was done by the first author as part of his master’s thesis. Through the validation of the current-mode of the active NNS, we brought the calibration from the standards lab to the end-user at high neutron fluence-rates. It is now plausible for any user to confidently perform neutron spectral measurements using this spectrometer under high neutron fluence-rates. Hence this research is relevant to the CIRMS mission. The first author, now a Ph.D. candidate, is continuing the use of this spectrometer to gain more insight into neutron-induced carcinogenic effects consistent with the mission of his research group. References: 1. Dubeau J, Hakmana Witharana SS, Atanackovic J, Yonkeu A, Archambault JP. A neutron spectrometer using nested moderators. Radiat Prot Dosimetry. 2012;150: 217–222. 2. Hagiwara M, Sanami T, Iwamoto Y, Arakawa H, Shigyo N, Mokhov N, et al. Shielding Experiments at High Energy Accelerators of Fermilab (III): Neutron Spectrum Measurements in Intense Pulsed Neutron Fields of The 120-GeV Proton Facility Using A Current Bonner Sphere Technique. Progress in Nuclear Science and Technology. 2011. pp. 52–56. doi: 10.15669/pnst.1.52 3. Maglieri R, Licea A, Evans M, Seuntjens J, Kildea J. Measuring neutron spectra in radiotherapy using the nested neutron spectrometer. Medical Physics. 2015. pp. 6162–6169. doi: 10.1118/1.4931963 4. Montgomery L, Landry A, Al Makdessi G, Mathew F, Kildea J. A novel MLEM stopping criterion for unfolding neutron fluence spectra in radiation therapy. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 2020. p. 163400. doi: 10.1016/j.nima.2020.163400
Felix Mathew
PhD Student
Logan Montgomery
PhD Student
