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Biomedical subjects

T B Borak

Publications and source records attributed to T B Borak.

33 records · Page 2Linked to original sources

Use of a simulation model in occupational dose assessment at a high-level radioactive waste repository.

Occupational radiation dose assessment at a nuclear facility can benefit from a systems approach that recognizes the complex dynamics of the facility. A simulation model can be a useful tool for accomplishing this goal. This technique can quantify the uncertainty in estimates of occupational dose equivalent and illustrate the effects of system interactions. A simulation model of operations at a high-level radioactive waste repository together with a radiological dose assessment model were used to illustrate this technique. Annual individual and collective occupational dose equivalents were estimated in the truck cask receiving and handling stations at a repository. The uncertainty of these estimates is shown through the use of complementary cumulative distribution functions. The effects of changing parameters in cask handling operations are examined.

Computer Simulation↗

Randomization of grab-sampling strategies for estimating the annual exposure of U miners to Rn daughters.

Periodic grab sampling in combination with time-of-occupancy surveys has been the accepted procedure for estimating the annual exposure of underground U miners to Rn daughters. Temporal variations in the concentration of potential alpha energy in the mine generate uncertainties in this process. A system to randomize the selection of locations for measurement is described which can reduce uncertainties and eliminate systematic biases in the data. In general, a sample frequency of 50 measurements per year is sufficient to satisfy the criteria that the annual exposure be determined in working level months to within +/- 50% of the true value with a 95% level of confidence. Suggestions for implementing this randomization scheme are presented.

Air Pollutants↗

Verification of the generalized Kusnetz method for measuring potential alpha energy from Rn daughters.

The equations describing serial radioactive build-up and decay contain symmetries which provide flexibility for measuring potential alpha energy. Mathematical analysis indicates that sample and count times may be interchanged or combined to enhance measurement protocols. Experiments have verified that these mathematical properties can be used for routine measurements of working level from Rn daughters.

Alpha Particles↗

Radon-222 and 222Rn progeny concentrations measured in an energy-efficient house equipped with a heat exchanger.

Radon-222 and 222Rn progeny concentrations, barometric pressure and pressure differentials between inside and outside were measured continuously in the basement of a recently constructed energy-efficient house in metropolitan Denver, CO. Although the monitoring equipment was developed primarily for underground mines, it proved to be applicable for house monitoring. Results indicate that for tightly sealed houses, forced-flow transport does not significantly contribute to the 222Rn present even when the pressure within the house is less than the outside pressure by 0.8 Pa (.006 mm Hg). Calculations of 222Rn levels using diffusion as the primary transport mechanism are in agreement with observed data. The diffusion coefficient of 222Rn in the walls and floor surrounding the basement is higher than values previously reported. Ventilation by means of a heat exchanger reduces the 222Rn levels in accordance with measured air exchange rates, regardless of the pressure differential between inside and outside.

Air Pollutants↗

A comparison of alpha spectroscopy and gross alpha techniques for determining working level in uranium mines and houses.

We have completed an evaluation of two methods that estimate potential alpha energy of radon daughters suspended in air. One method approximates Working Level from a single measurement of gross alpha activity using a generalization of procedures originally described by Kusnetz and Rollé. The second is a linear combination of the count rates from RaA and RaC' determined from a single measurement using alpha spectroscopy as originally suggested by Schiager . These methods were compared in terms of precision and accuracy for a range of radon-daughter concentrations. Even though alpha spectroscopy provides information on two independent variables, it does not always improve the accuracy and precision of the estimate of Working Level in uranium mine atmospheres and houses.

Air Pollutants↗

Neutron quality parameters versus energy below 4 MeV from microdosimetric calculations.

Charged-particle production by neutrons and the resulting energy-deposition spectra in micron-sized spheres of tissue of varying diameters were calculated from thermal energies to 4 MeV. These data were used to obtain dose-average values of several quality-indicating parameters as functions of neutron energy and of tissue sphere diameter. The contrast among the parameters is shown and discussed. Applications are made to two neutron spectra, one a fission spectrum in air and the other a moderated spectrum at the center of an irradiated cube of water.

Energy Transfer↗

Calculations of charged-particle recoils, slowing-down spectra, LET and event-size distributions for fast neutrons and comparisons with measurements.

A rapid system has been developed for computing charged-particle distributions generated in tissue by any neutron spectra less than 4 MeV. Oxygen and carbon recoils are derived form R-matrix theory, and hydrogen recoils are obtained from cross-section evaluation. Application to two quite different fission-neutron spectra demonstrates the flexibility of this method for providing spectral details of the different types of charged-particle recoils. Comparisons are made between calculations and measurements of event-size distributions for a sphere of tissue 1 micrometer in diameter irradiated by these two neutron spectra. LET distributions have been calculated from computed charged-particle recoils and also derived from measurements using the conventional approximation that all charged particles traverse the chamber. The limitations of the approximation for these neutron spectra are discussed.

Carbon↗

Effects of intraoperative hyperthermia on peripheral nerves: neurological and electrophysiological studies.

The tolerance of peripheral nerves to heat may limit the heat dose which can be applied to tumours. This may be particularly important in intraoperative hyperthermia (IOHT) for pelvic and retroperitoneal tumours. Furthermore the effects of hyperthermia alone must be known before its effects can be assessed in combination with irradiation. In this study injury to sciatic nerves was evaluated in 30 beagle dogs for 1 year following IOHT. IOHT was performed using a water circulating hyperthermia device with multichannel thermometry system. Neurological and electrophysiological examinations were done before, during and after IOHT treatment. Electrophysiological examinations showed a significant decrease in sciatic nerve conduction velocity and potential amplitude immediately after 60 min of heating for all temperatures. The greatest decrease in conduction velocity was observed for a temperature of 45 degrees C. Full recovery of nerve conduction velocity was observed 3 weeks following hyperthermia for all dogs except for those exposed to 45 degrees C. Neurological findings correlated with electrophysiological results. All five dogs which had nerve exposed to 45 degrees C for 60 min had severe neurological changes, with recovery taking place between 3 and 11 months after treatment. Based on these results it appears that temperatures to the peripheral nerve exceeding 44 degrees C for 1 h are likely to cause significant, but not necessarily permanent, nerve injury.

Action Potentials↗