For those who haven't yet decided whether to attend the annual Health Physics Society meeting next month in Pittsburgh, Pennsylvania, here is a glimpse of some of the talks slated for the special accelerator session on Tuesday morning, July 15. This promises to be another great session, so plan now to attend.
L. Scott Walker, T. Hill, F. Tovesson
Personnel from the Weapons Neutron Research (WNR) Center and Operational Health Physics Group (RP-1) at the LANSCE accelerator have begun developing a series of neutron fission threshold detectors for use in the high-energy region. Differential fission cross-section measurements for gold were carried out at the WNR using a standard parallel-plate ionization chamber. The preliminary results show that a high-energy neutron threshold detector may be possible. Future measurements may include tantalum, tungsten, platinum, iridium, osmium, bismuth, and lead. Theoretically, it is possible to measure threshold fission down to iron. These new materials, along with precise fission measurements taken at the WNR, will allow threshold-binning of the neutron spectra as the threshold of the fission events increases in energy. Development of the neutron threshold detector will require further fission detector development, such as thin-film targets and double-sided chambers or the use of a time projection chamber, to learn more about the actual neutron-induced physics. Both fission detector development programs are underway in collaboration with other institutions, such as IRMM in Geel, Belgium, and Lawrence Livermore National Laboratory, and will be available to the high-energy neutron threshold detector development program.
S. Baker, F. Moore, W. Munyon
A fission fragment ion source is installed for use at the ATLAS Accelerator located at Argonne National Laboratory. This source ultimately contains 37 GBq of 252Cf and generates a variety of different neutron-rich fission fragments for nuclear and astrophysics research. Apart from the fission ions of interest, other fission fragments are also released most notably noble and halogen gases such as isotopes of xenon, krypton, iodine, and bromine. These fission fragments are vented through the helium gas purge line in the air-effluent system for discharge into the environment. The purpose of this paper is to report on how much air-affluent radioactivity is produced a priori, discuss various air-affluent control mechanisms, describe the process in place for monitoring releases real-time, and estimate the dose consequences at the site boundary for routine releases. In the proposed system, fission gas effluent passes through small HEPA filters and activated charcoal cartridges before being exhausted through a large downstream HEPA filter. The exhaust system has a flow rate of 42 cubic meters per minute and the effluent is discharge from the stack located 15 meters above ground level. The amount of radioactivity discharged is determined by continuously measuring the activity concentration in the exhaust line using a flow-through ion chamber; effluent flow rates are measured using a mass flow meter. Characterization measurements during start-up verify the results of the calculations. Exposure rates at the site boundary and for the nearest off-site neighbor are determined using the USEPA CAP-88 computer program. The projected annual doses meet stringent regulatory requirements for DOE facilities.
D. Elder, J. F. Harmon, T. B. Borak
Skyshine is the term used for radiation that originates near the surface of the earth with an upward velocity and then is scattered back by the molecules in the atmosphere. Skyshine radiation is of concern because it can contribute dose to the public in areas beyond the boundary of the radiation production facility. A Varian Trilogy accelerator was recently commissioned at the Colorado State University Veterinary Teaching Hospital. The Trilogy is capable of producing 6 MV and 10 MV photons for radiation therapy. The gantry rotates about the isocenter and approximately 25% of the workload is directed toward the ceiling. The roof area above the radiation therapy suite is a restricted area due to high radiation levels, making skyshine a concern for the adjacent roof areas and on the ground in the vicinity of the facility. A Reuter-Stokes RSS-120 pressurized ion chamber, an 8-L spherical ionization chamber filled with 25 atmospheres of argon, was used to measure radiation levels at ground level and on the roof. Readings were taken of background radiation, leakage, and total radiation at each location for various gantry angles and field sizes. The data was analyzed to determine the skyshine component at each location for each beam condition.