Secondary neutron spectrum from 250-MeV passively scattered proton therapy: measurement with an extended-range Bonner sphere system.

PURPOSE: Secondary neutrons are an unavoidable consequence of proton therapy. While the neutron dose is low compared to the primary proton dose, its presence and contribution to the patient dose is nonetheless important. The most detailed information on neutrons includes an evaluation of the neutron spectrum. However, the vast majority of the literature that has reported secondary neutron spectra in proton therapy is based on computational methods rather than measurements. This is largely due to the inherent limitations in the majority of neutron detectors, which are either not suitable for spectral measurements or have limited response at energies greater than 20 MeV. Therefore, the primary objective of the present study was to measure a secondary neutron spectrum from a proton therapy beam using a spectrometer that is sensitive to neutron energies over the entire neutron energy spectrum. METHODS: The authors measured the secondary neutron spectrum from a 250-MeV passively scattered proton beam in air at a distance of 100 cm laterally from isocenter using an extended-range Bonner sphere (ERBS) measurement system. Ambient dose equivalent H*(10) was calculated using measured fluence and fluence-to-ambient dose equivalent conversion coefficients. RESULTS: The neutron fluence spectrum had a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate energy continuum between the thermal and evaporation peaks. The H*(10) was dominated by the neutrons in the evaporation peak because of both their high abundance and the large quality conversion coefficients in that energy interval. The H*(10) 100 cm laterally from isocenter was 1.6 mSv per proton Gy (to isocenter). Approximately 35% of the dose equivalent was from neutrons with energies ≥20 MeV. CONCLUSIONS: The authors measured a neutron spectrum for external neutrons generated by a 250-MeV proton beam using an ERBS measurement system that was sensitive to neutrons over the entire energy range being measured, i.e., thermal to 250 MeV. The authors used the neutron fluence spectrum to demonstrate experimentally the contribution of neutrons with different energies to the total dose equivalent and in particular the contribution of high-energy neutrons (≥20 MeV). These are valuable reference data that can be directly compared with Monte Carlo and experimental data in the literature.

Using handheld plastic scintillator detectors to triage individuals exposed to a radiological dispersal device.

After a radiological dispersal device (RDD) event, people could become internally contaminated by inhaling dispersed radioactive particles. A rapid method to screen individuals who are internally contaminated is desirable. Such initial screening can help in prompt identification of those who are highly contaminated and in prioritising individuals for further and more definitive evaluation such as laboratory testing. The use of handheld plastic scintillators to rapidly screen those exposed to an RDD with gamma-emitting radionuclides was investigated in this study. The Monte Carlo N-Particle transport code was used to model two commercially available plastic scintillation detectors in conjunction with anthropomorphic phantom models to determine the detector response to inhaled radionuclides. Biokinetic models were used to simulate an inhaled radionuclide and its progression through the anthropomorphic phantoms up to 30 d after intake. The objective of the study was to see if internal contamination levels equivalent to 250 mSv committed effective dose equivalent could be detected using these instruments. Five radionuclides were examined: (60)Co, (137)Cs, (192)Ir, (131)I and (241)Am. The results demonstrate that all of the radionuclides except (241)Am could be detected when placing either one of the two plastic scintillator detector systems on the posterior right torso of the contaminated individuals.

Calibration of a Bonner sphere extension (BSE) for high-energy neutron spectrometry.

In a recent work, we constructed modular multisphere system which expands upon the design of an existing, commercially available Bonner sphere system by adding concentric shells of copper, tungsten, or lead. Our modular multisphere system is referred to as the Bonner Sphere Extension (BSE). The BSE was tested in a high energy neutron beam (thermal to 800 MeV) at Los Alamos Neutron Science Center and provided improvement in the measurement of the neutron spectrum in the energy regions above 20 MeV when compared to the standard BSS (Burgett, 2008 and Howell et al., 2009).However, when the initial test of the system was carried-out at LANSCE, the BSE had not yet been calibrated. Therefore the objective of the present study was to perform calibration measurements. These calibration measurements were carried out using monoenergetic neutron ISO 8529-1 reference beams at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. The following monoenergetic reference beams were used for these experiments: 14.8 MeV, 1.2 MeV, 565 keV, and 144 keV. Response functions for the BSE were calculated using the Monte Carlo N-Particle Code, eXtended (MCNPX). The percent difference between the measured and calculated responses was calculated for each sphere and energy. The difference between measured and calculated responses for individual spheres ranged between 7.9 % and 16.7 % and the arithmetic mean for all spheres was (10.9 ± 1.8) %. These sphere specific correction factors will be applied for all future measurements carried-out with the BSE.