A. J. Waker

An accelerator based system for in vivo neutron activation analysis measurements of manganese in human hand bones

Manganese (Mn) is an essential nutrient for growth and development. Unfortunately, overexposure can lead to neurological damage, which is manifested as a movement disorder marked by tremors. Preclinical symptoms have been found in populations occupationally exposed to the element, and it is suggested that in late stages of the disorder, removing the Mn exposure will not prevent symptoms from progressing. Hence, it is desirable to have a means of monitoring Mn body burden. In vivo neutron activation analysis (IVNAA) is a technique which allows the concentration of some elements to be determined within sites of the body without invasive procedures. Data in the literature suggests that the Mn concentration in bone is greater than other tissues, and that it may be a long term storage site following exposure. Therefore, using the McMaster KN-accelerator to produce neutrons through the 7Li(p,n)7Be reaction, the feasibility of IVNAA for measuring Mn levels in the human hand bone was investigated. Mn is activated through the 55Mn(n,gamma)56Mn reaction, and the 847 keV gamma rays emitted when 56Mn decays are measured outside the body using NaI(Tl) detectors. An optimal incident proton energy of 2.00 MeV was determined from indium foil and microdosimetry measurements. Hand phantom data suggest a minimum detectable limit of approximately 1.8 ppm could be achieved with a reasonably low dose of 50 mSv to the hand (normal manganese levels in the human hand are approximately 1 ppm). It is recommended the technique be developed further to make human in vivo measurements.

Miniature tissue‐equivalent proportional counters for BNCT and BNCEFNT dosimetry

A dual miniature tissue-equivalent proportional counter (TEPC) system has been developed to facilitate microdosimetry for Boron Neutron Capture Therapy (BNCT). This system has been designed specifically to allow the analysis of the single event charged particle spectrum in phantom in high intensity BNCT beams and to provide this microdosimetric information with excellent spatial resolution. Paired A-150 and 10B-loaded A-150 TEPCs with 12.3 mm3 collecting volumes have been constructed. These TEPCs allow more accurate neutron dosimetry than current techniques, offer a direct measure of the boron neutron capture dose, and provide a framework for predicting the biological effectiveness of the absorbed dose. Design aspects and characterization of these detectors are reviewed, along with an exposition of the advantages of microdosimetry using these detectors over conventional dosimetry methods. In addition, the utility of this technique for boron neutron capture enhancement of fast neutron therapy (BNCEFNT) is discussed.

Development of gas microstrip detectors for digital X-ray imaging and radiation dosimetry

Our recent work in the application of gas microstrip detector (GMD) technology to the fields of digital X-ray imaging and radiation dosimetry is described. The GMD can measure the position and the energy of individual photons at the high counting rates encountered in X-ray imaging. GMD based imaging systems have high detective quantum efficiency and permit improvement of image quality and contrast using display windowing and measured energy information. Results are presented on the performance of a prototype GMD imaging system operated with a xenon/methane 90/10 gas mixture at 1 atmosphere. Results are also presented on the performance of a GMD filled with tissue equivalent gases for applications in the field of radiation dosimetry in mixed neutron and /spl gamma/ fields. The results show that the GMD can be used for dosimetric discrimination between different types of radiation in mixed field environments.

Design of a multi-element TEPC for neutron monitoring.

Tissue-equivalent proportional counters (TEPCs) have long been considered suitable candidate instruments for more accurate neutron monitors in nuclear power plants. It has also been recognised that the production of truly light-weight devices based on TEPCs requires further effort directed towards increasing their sensitivity and decreasing their physical size. This paper deals with the construction of a multi-element TEPC (METEPC) designed to have the sensitivity of a 12.7-cm (5-in.) diameter spherical TEPC, but with approximately one-tenth of its physical size. Construction of the METEPC is achieved by machining 61 elongated cylindrical cavities in a single block of A150 TE plastic. Comparative measurements carried out in neutron fields with mean energies ranging from 34 to 354 keV demonstrate that the METEPC constructed does match the sensitivity of a 5-in. spherical TEPC and that microdosimetric lineal energy spectra measured with both detectors have the same features and show the same changes with neutron radiation quality.

Development of a low-energy monoenergetic neutron source for applications in low-dose radiobiological and radiochemical research

The McMaster University 3 MV KN Van de Graff accelerator facility primarily dedicated to in vivo neutron activation measurements has been used to produce moderate dose rates of monoenergetic fast neutrons of energy ranging from 150 to 600 keV with a small energy spread of about 25 keV (1sigma width of Gaussian) by bombarding thin lithium targets with 2.00-2.40 MeV protons. The calculated dose rate of the monoenergetic neutrons produced using thin lithium targets as functions of beam energy, target thickness, lab angle relative to beam direction, and the solid angle subtended by the sample with the target has also been reported.

Opportunities to improve the in vivo measurement of manganese in human hands

Manganese (Mn) is an element which is both essential for regulating neurological and skeletal functions in the human body and also toxic when humans are exposed to excessive levels. Its excessive inhalation as a result of exposure through industrial and environmental emissions can cause neurological damage, which may manifest as memory deficit, loss of motor control and reduction in the refinement of certain body motions. A number of clinical studies demonstrate that biological monitoring of Mn exposure using body fluids, particularly blood, plasma/serum and urine is of very limited use and reflect only the most recent exposure and rapidly return to within normal ranges. In this context, a non-invasive neutron activation technique has been developed at the McMaster University accelerator laboratory that could provide an alternative to measure manganese stored in the bones of exposed subjects. In a first pilot study we conducted recently on non-exposed human subjects to measure the ratio of Mn to Ca in hand bones, it was determined that the technique needed further development to improve the precision of the measurements. It could be achieved by improving the minimum detection limit (MDL) of the system from 2.1 microg Mn/g Ca to the reference value of 0.6 microg g(-1) Ca (range: 0.16-0.78 microg Mn/g Ca) for the non-exposed population. However, the developed procedure might still be a suitable means of screening patients and people exposed to excessive amounts of Mn, who could develop many-fold increased levels of Mn in bones as demonstrated through various animal studies. To improve the MDL of the technique to the expected levels of Mn in a reference population, the present study investigates further optimization of irradiation conditions, which includes the optimal selection of proton beam energy, beam current and irradiation time and the effect of upgrading the 4pi detection system. The maximum local dose equivalent that could be given to the hand as a result of irradiation was constrained to be less than 150 mSv as opposed to the previously imposed dose equivalent limit of 20 mSv. A maximum beam current, which could be delivered on the lithium target to produce neutrons, was restricted to 500 microA. The length of irradiation intervals larger than 10 min, was considered inconvenient and impractical to implement with Mn measurements in humans. To fulfil the requirements for developing a protocol for in vivo bone Mn measurements, a revised estimate of the dose equivalent has been presented here. Beam energy of 1.98 MeV was determined to be optimal to complete the irradiation procedure within 10 min using 500 microA beam current. The local dose equivalent given to hand was estimated as 118 mSv, which is lower by a factor of 1.5 compared to that of 2.00 MeV. The optimized beam parameters are expected to improve the currently obtained detection limit of 2.1 microg Mn/g Ca to 0.6 microg Mn/g Ca. Using this dose equivalent delivered to the central location of the hand, the average dose equivalent to the hand of 74 mSv and an effective dose of approximately 70 microSv will be accompanying the non-invasive, in vivo measurements of bone Mn, which is little over the chest radiograph examination dose.

Neutron fields inside containment of a CANDU{reg_sign} 600-PHWR power plant

Neutron fields in six locations inside containment of a CANDU600-PHWR power plant were characterized using Bonner-sphere spectrometry. Unfolded fluence spectra were used to predict and understand the behavior of a rem meter (a moderator-type dose equivalent survey instrument). The suitability of employing commonly-used sources such as 241Am-Be for calibrating the rem meter was investigated by calculational means. Results of these calculations suggest that employing a calibration field more representative of the power-plant fields would likely provide more accurate dose equivalents.

Characterization of miniature tissue-equivalent proportional counters for neutron radiotherapy applications

A miniature tissue-equivalent proportional counter (TEPC) system has been developed to facilitate microdosimetric measurements in high-flux mixed fields. Counters with collecting volumes of 12.3 and 2.65 mm3 have been constructed using various tissue-equivalent wall materials, including those loaded with 10B for evaluation of the effects of the boron neutron capture reaction. These counters provide a measure of both the absorbed dose and associated radiation quality, allowing an assessment of the utility and relative effectiveness of various neutron radiotherapy techniques such as boron neutron capture therapy (BNCT), boron neutron capture enhanced fast neutron therapy (BNCEFNT) and intensity modulated neutron radiotherapy (IMNRT). An evaluation of the physical parameters affecting the measured microdosimetric spectrum, the gas multiplication characteristics and the measurement of absorbed dose is presented. In addition, important aspects of the calibration and low energy extrapolation techniques for the microdosimetric spectrum are provided.

Neutron spectrometry and dosimetry study at two research nuclear reactors using Bonner sphere spectrometer (BSS), rotational spectrometer (ROSPEC) and cylindrical nested neutron spectrometer (NNS)

Neutron spectrometry and subsequent dosimetry measurements were undertaken at the McMaster Nuclear Reactor (MNR) and AECL Chalk River National Research Universal (NRU) Reactor. The instruments used were a Bonner sphere spectrometer (BSS), a cylindrical nested neutron spectrometer (NNS) and a commercially available rotational proton recoil spectrometer. The purposes of these measurements were to: (1) compare the results obtained by three different neutron measuring instruments and (2) quantify neutron fields of interest. The results showed vastly different neutron spectral shapes for the two different reactors. This is not surprising, considering the type of the reactors and the locations where the measurements were performed. MNR is a heavily shielded light water moderated reactor, while NRU is a heavy water moderated reactor. The measurements at MNR were taken at the base of the reactor pool, where a large amount of water and concrete shielding is present, while measurements at NRU were taken at the top of the reactor (TOR) plate, where there is only heavy water and steel between the reactor core and the measuring instrument. As a result, a large component of the thermal neutron fluence was measured at MNR, while a negligible amount of thermal neutrons was measured at NRU. The neutron ambient dose rates at NRU TOR were measured to be between 0.03 and 0.06 mSv h⁻¹, while at MNR, these values were between 0.07 and 2.8 mSv h⁻¹ inside the beam port and <0.2 mSv h⁻¹ between two operating beam ports. The conservative uncertainty of these values is 15 %. The conservative uncertainty of the measured integral neutron fluence is 5 %. It was also found that BSS over-responded slightly due to a non-calibrated response matrix.

Intercomparison of Monte Carlo radiation transport codes to model TEPC response in low-energy neutron and gamma-ray fields

Tissue-equivalent proportional counters (TEPC) can potentially be used as a portable and personal dosemeter in mixed neutron and gamma-ray fields, but what hinders this use is their typically large physical size. To formulate compact TEPC designs, the use of a Monte Carlo transport code is necessary to predict the performance of compact designs in these fields. To perform this modelling, three candidate codes were assessed: MCNPX 2.7.E, FLUKA 2011.2 and PHITS 2.24. In each code, benchmark simulations were performed involving the irradiation of a 5-in. TEPC with monoenergetic neutron fields and a 4-in. wall-less TEPC with monoenergetic gamma-ray fields. The frequency and dose mean lineal energies and dose distributions calculated from each code were compared with experimentally determined data. For the neutron benchmark simulations, PHITS produces data closest to the experimental values and for the gamma-ray benchmark simulations, FLUKA yields data closest to the experimentally determined quantities.