Health Physics

Measuring Workload With Paired Detectors Linear accelerator workloads for each available photon energy are important quantities to know for radiation safety considerations, and presented is a technique to measure the workload using paired detectors. The signals from the two detectors can give sufficient information to separate the signal contributions from 6 and 18 MV photon fields and, combined with a signal-per-monitor-unit calibration factor, yields the number of monitor units delivered for each energy. CR-39 NTD is a neutron detector chosen for its ability to discriminate between 6 MV and 18 MV radiation fields. TLD-100 is a detector responsive to both 6 MV and 18 MV fields. These appeared to be a good choice for a detector pair. This experiment had both failures and successes to report. The CR-39 NTD and TLD-100 were not a successful pairing. The CR-39 NTD signals saturated under this experiment’s exposure conditions. The TLD-100 had a combination of detector noise and detector sensitivity that made extracting the 6 MV signal from the total signal impractical, unless the total exposure was overwhelmingly 6 MV. Nevertheless, the TLD-100 proved to be excellent for determining workloads when it was exposed to a single energy with 1% accuracy and 3% precision. The theory and data analysis showed the importance of understanding the noise contributions for the more general problem of pairing any two detector types. This experiment indicated the TLD-100 could be an excellent detector choice if paired with a suitable second detector.

A Comparative Study of Radon Levels in Federal Buildings and Residential Homes in Canada Shortly after revision of the Canadian radon guideline from 800 to 200 Bq m−3, Health Canada established the Federal Building Testing Program in 2007 to demonstrate federal leadership in raising awareness about radon risk and the need for testing. By the end of 2017, more than 7,600 federal workplaces had been tested for radon. As is the case in all radon surveys, radon levels vary widely; federal building results ranged from below the detection limit to more than 2,500 Bq m−3 in a few rooms of a few buildings. Weighted by the population of federal public servants across Canada, the average radon distribution in federal workplaces has a geometric mean of 22.0 Bq m−3 with a geometric standard deviation of 2.3. The population-weighted arithmetic mean is 34.3 Bq m−3, significantly lower than the population-weighted average radon concentration of 72.9 Bq m−3 in residential homes across Canada. On average, 2% of federal workplaces have radon concentrations above 200 Bq m−3, which is also significantly lower than the 7% of residential homes that tested above 200 Bq m−3. This comparative study demonstrated clearly that radon education and awareness in Canada should focus more on residential testing and remediation actions to effectively reduce the burden of radon-induced lung cancer.

210Po in Pacific Salmon From the West Coast of Canada and Its Contribution to Dose by Ingestion In response to public concern in Canada regarding health impacts attributable to the Fukushima Daiichi nuclear accident, oceanic seawater samples from the north Pacific and Arctic oceans, coastal seawater samples from 16 locations along the British Columbia coastline, and seafood samples (salmon, steelhead trout, and shellfish) from British Columbia coastal waters were collected and analyzed. This paper reports radiological analysis results of Pacific salmon samples (Oncorhynchus species) obtained from summer 2013 to fall 2016. While radioactive cesium from the Fukushima disaster was not detectable in most salmon samples, naturally occurring 210Po was measured in almost all individual samples in varying activity concentrations, from below the detection limit of 0.2 Bq kg−1 fresh weight up to 4.7 Bq kg−1 fresh weight. The average 210Po concentration among 297 salmon samples was 0.73 Bq kg−1 fresh weight. The average ingested radiation dose per kilogram of salmon from 210Po is estimated to be 0.88 μSv, and the average dose from 137Cs is estimated to be 0.0026 μSv. The annual dose from ingested salmon would be only a fraction of the worldwide average annual effective dose from exposure to natural background radiation (2,400 μSv y−1) (UNSCEAR 2000). The measurement results showed clearly that radiation doses to people consuming fish (such as salmon) from the Canadian west coast pose no health concern.

Model of Steady-state Temperature Rise in Multilayer Tissues Due to Narrow-beam Millimeter-wave Radiofrequency Field Exposure The assessment of health effects due to localized exposures from radiofrequency fields is facilitated by characterizing the steady-state, surface temperature rise in tissue. A closed-form analytical model was developed that relates the steady-state, surface temperature rise in multilayer planar tissues as a function of the spatial-peak power density and beam dimensions of an incident millimeter wave. Model data was derived from finite-difference solutions of the Pennes bioheat transfer equation for both normal-incidence plane waves and for narrow, circularly symmetric beams with Gaussian intensity distribution on the surface. Monte Carlo techniques were employed by representing tissue layer thicknesses at different body sites as statistical distributions compiled from human data found in the literature. The finite-difference solutions were validated against analytical solutions of the bioheat equation for the plane wave case and against a narrow-beam solution performed using a commercial multiphysics simulation package. In both cases, agreement was within 1–2%. For a given frequency, the resulting analytical model has four input parameters, two of which are deterministic, describing the level of exposure (i.e., the spatial-peak power density and beam width). The remaining two are stochastic quantities, extracted from the Monte Carlo analyses. The analytical model is composed of relatively simple functions that can be programmed in a spreadsheet. Demonstration of the analytical model is provided in two examples: the calculation of spatial-peak power density vs. beam width that produces a predefined maximum steady-state surface temperature, and the performance evaluation of various proposed spatial-averaging areas for the incident power density.

Effects of 90Sr on Tree Swallow Nestlings Near Groundwater Contaminant Plumes Discharge of groundwater contaminant plumes has created elevated concentrations of 90Sr in some aquatic sediments at Chalk River Laboratories. Tree swallows (Tachycenita bicolor) feed and supply their nestlings almost exclusively with airborne insects that developed as larvae in aquatic sediments. To monitor the uptake and test for potential detriment due to 90Sr in a terrestrial animal, we measured the gross beta concentrations in the bone of 12-d-old tree swallow nestlings in areas having sediments with elevated levels of gross beta (90Sr and 90Y) and in several control areas where sediment gross beta was primarily due to naturally occurring 40K. Nesting behavior and reproductive success of the tree swallows were similar regardless of the gross beta concentrations in sediments near their nest boxes. Radiation can damage DNA and cause micronuclei to form in cells, so we examined the frequency of micronuclei in erythrocytes of nestlings. The formation of micronuclei in the erythrocytes of the nestlings was also similar wherever nestlings were analyzed. The results revealed no significant increases even near sediments with the highest gross beta levels. At Perch Lake, where Chalk River Laboratories has a large area of 90Sr-contaminated sediments, the bones of 12-d-old nestlings contained gross beta concentrations as high as 29 Bq g−1. This would produce a skeletal dose rate of 9 μGy h−1, which is one-fourth of the threshold dose rate of 40 μGy h−1, above which detriment could occur. Failing to find any indication of detriment in the field study, we irradiated wild eggs in the lab and returned them to their nest for natural incubation, hatching, and feeding by the parents. There was an increase in formation of micronuclei following a dose of 3.2 Gy, and the other results were consistent with existing literature.

Radon Off-gassing From Military Artifacts Military historical artifacts found in museum displays and storage locations were analyzed for their 226Ra and 222Rn progeny activities to determine the fraction of 222Rn lost to the environment. Gamma-ray spectroscopy using high-purity germanium detectors was used to determine 226Ra activity and infer 222Rn activity based on 214Pb and 214Bi. Analyses were conducted without affecting the structural integrity of the artifacts. 226Ra was measured directly after correction for solid angle and finite sample-detector distance. Although 222Rn can be similarly analyzed, the collection in charcoal of 222Rn off-gassed from the artifact after the establishment of secular equilibrium was preferable. 222Rn off-gassing rates vary greatly between the six devices studied, with a maximum off-gassing rate of 1,850 ± 50 Bq h−1. Large variations in off-gassing rate were also observed between an additional 30 nominally identical dials, with a mean and standard deviation of 7.7 ± 7.1 Bq h−1. The work is not predictive of airborne 222Rn activity within museums, where building size and ventilation are significant and unique to each location. However, the significant off-gassing rates and their large variation suggest that 222Rn activities may be elevated in enclosed locations, such as aircraft cockpits and storage facilities.

Monitoring and Dose Assessment for Children Following a Radiation Emergency—Part II: Calibration Factors for Thyroid Monitoring Past radiological and nuclear accidents have demonstrated that monitoring a large number of children following a radiological and nuclear emergency can be challenging, in accommodating their needs as well as adapting monitoring protocols and applying age-specific biokinetics to account for various ages and body sizes. This paper presents the derived calibration factors for thyroid monitoring of children of all ages recommended by the International Commission on Radiological Protection using four selected detectors at given times following a short-term (acute) intake of 131I by inhalation. These calibration factors were derived by Monte Carlo simulations using the models of various detectors and pediatric voxel phantoms. A collection of lookup tables is presented in this paper which may be directly used as a quick reference by emergency response personnel or technical experts performing thyroid monitoring and assessment without doing time-consuming calculations.

Observation of Ground-level Enhancement Across Canada’s Fixed Point Surveillance Network During the 20 January 2005 Solar Event This paper presents the count rate enhancement observed across Canada’s Fixed Point Surveillance network during the solar event on 20 January 2005 and explores the feasibility and value of applying the Fixed Point Surveillance network’s long-term and continuous observations for space weather monitoring. The count rate, recorded in the high-energy channel of RS250 sodium iodide detectors, reflects the detector’s response to muonic and electromagnetic components of the cosmic ray shower. During the event peak time, simultaneous count rate increases have been observed across many Fixed Point Surveillance network stations at enhancements varying from 10% to 18%, 12- to 15-fold less than relative increases in neutron detector observations.

Optimization of a Neutron Long Counter Design by Monte Carlo Simulation In a search to optimize neutron long counter design for overall efficiency and flat energy response, Monte Carlo simulations were carried out for a variety of detector design parameters using the Monte Carlo N-Particle Extended code. Based on the standard long counter design by McTaggart, moderator diameter, moderator back length, and longitudinal hole diameter were sequentially varied, and the sensitivity of each parameter to the long counter response was systematically analyzed. For each design, simulations were done in the neutron energy range of 1 keV to 10 MeV. From the simulation results, it turned out that out of the three moderator parameters, the moderator diameter is most sensitive for optimizing the long counter response. As the last design parameter, the effect of the central slow-neutron counter was investigated, which showed a significant difference in the response. The investigation of each design parameter gave clear insight on its effect on the long counter response and enabled one to determine the optimum condition.

Streamlining Regulatory Activities Within Radiation Therapy Departments Using QATrack+ Radiation therapy departments are faced with the challenge of tracking numerous quality control tests as well as monitoring service events affecting radiation therapy treatment units. Service events, in particular, pose a challenge since the clinic must be able to provide evidence to the regulatory body that both the service work and any required follow-up tests were recorded and authorized by the appropriate staff. This article presents an integrated approach to tracking quality control tests and service event logs using QATrack+. The newly developed version of this quality assurance software integrates quality control tracking with the service event log, allowing a direct link between a service event and any initiating routine tests or follow-up tests that are performed. This improves the ability of a licensee to ensure compliance with regulations and permits a simple platform from which to access all machine equipment tests and service events. Furthermore, this improves the ability of a department to assess the service record of equipment and to identify trends in failure modes.