PubMed Health⌕ Search

Biomedical subjects

T B Borak

Publications and source records attributed to T B Borak.

At least 19 recordsLinked to original sources
PubMed1998

Wall effects observed in tissue-equivalent proportional counters from 1.05 GeV/nucleon iron-56 particles.

Tissue-equivalent proportional counters (TEPCs) have been used to measure energy deposition in simulated volumes of tissue ranging in diameter from 0.1 to 10 microm. There has been some concern that the wall used to define the volume of interest could influence energy deposition within the sensitive volume because it has a density significantly greater than that of the cavity gas. These effects become important for high-velocity heavy ions. Measurements of energy deposition were made for 1 GeV/nucleon iron particles in a TEPC simulating a 1-microm-diameter sphere of tissue. The TEPC was nested within a particle spectrometer that provided identification and flight path of individual particles. Energy deposition was studied as a function of pathlength through the TEPC. Approximately 30% of the energy transfer along trajectories through the center of the detector escapes the sensitive volume. The response of the TEPC, for trajectories through the detector, is always larger than calculations for energy loss in a homogeneous medium. This enhancement is greatest for trajectories near the cavity/wall interface. An integration of the response indicates that charged-particle equilibrium is essentially achieved for a wall thickness of 2.54 mm. However, estimates of the linear energy transfer for the incident particles are influenced by these wall effects.

Gases↗
PubMed1995

Alpha pulse height distributions with ZnS(Ag) scintillator.

A flask coated with ZnS(Ag) scintillator is one of the most accurate detectors available for measuring 222Rn. To maintain this accuracy, the counting system consisting of a photomultiplier tube and associated electronics must be checked on a regular basis. A combination of an alpha source and a ZnS(Ag) scintillator is commonly used for these purposes. This paper compares the pulse height distributions of 4 alpha sources with the pulse height distribution from a 100 cm3 scintillation flask containing 222Rn. The source that most closely reproduced the distribution from an actual 222Rn sample in a 100 cm3 scintillation flask consisted of a sealed flask, of the same type, which contains a small piece of uranium-ore.

Alpha Particles↗
PubMed1995

In-situ determination of the diffusion coefficient of 222Rn in concrete.

Measurements of the effective diffusion coefficient, De, of 222Rn in concrete are important for accurate determination of transport mechanisms and computer modeling of radon entry into structures. A method for in-situ determination of De as well as the emanation fraction, F, is described. It is based on continuous measurement of the radon flux from an intact slab and the concentrations on both sides. A non-linear regression algorithm was used to fit these data to the steady state solution to Fick's law. The regression output includes estimates of both De and F. The method has the advantage over typical laboratory procedures since it measures an undisturbed surface area of concrete where the samples exhibit moisture, temperature, and loading conditions that are experienced in a real structure. Effective diffusion coefficients ranged from 1.1 x 10(-8) to 4.4 x 10(-8) m2 s-1 for measurements of concrete floor slabs in two structures. The measured emanation fraction of radon in concrete in one structure was 0.2 +/- 0.02. A sensitivity analysis indicated that the values of De and F estimated by the regression procedure are strongly dependent on the thickness of the slab. The porosity of the concrete had little affect on the regression results.

Air Pollutants, Radioactive↗
PubMed1995

A method for computing the decision level for samples containing radioactivity in the presence of background.

It is often necessary to make a decision whether or not a sample contains radioactivity in excess of background. The most common procedure is a deterministic approach that compares the result from each sample against a critical level, Lc. Originally Lc was derived from the case where each sample is paired with a blank or control that is known not to contain excess radioactivity. However, most analytic laboratories do not process a blank with each and every sample. Another approach computes Lc using a collection of measurements to form a "well defined" background. This paper presents a method for determining a decision level, L delta, that includes the uncertainties in both the mean and variance of background and how these combine to form uncertainties in estimating the tails of the background distribution. The values of L delta are greater than Lc for a given Type I error. It can be applied whenever the true distribution of background is either normal or log normal. The process is not restricted to counting statistics and is valid for identifying excess contamination of any type providing that unknown samples are processed identically to background samples.

Biological Assay↗
PubMed1994

Intraoperative radiation (IORT) injury to sciatic nerve in a large animal model.

Peripheral nerve appears to be a dose-limiting normal tissue in the clinical application of intraoperative radiation therapy (IORT). To assess IORT injury to peripheral nerve, three groups of five beagle dogs received doses of 12, 20 or 28 Gy to the surgically exposed and isolated right sciatic nerve in the mid-femoral region using 6 MeV electrons. The left sciatic nerve of each dog served as its own control. As a surgical control five dogs received surgical exposure of the nerve only. Monthly neurologic exams, electromyogram and nerve conduction studies were performed following treatment for 12 months. After that dogs were euthanatized and histologic studies of nerves were done to define the degree of axon and myelin loss as well as presence of fibrosis and vascular lesions for different doses of IORT. Results showed that the threshold dose most likely related to expression of severe radiation damage to the nerve in this model is between 20 and 25 Gy. Radiation injury to peripheral nerve appears to be the result of direct radiation effects on Schwann cells and nerve vasculature and secondary effects resulting from damage to regional muscle and vasculature. A theoretical mechanism of radiation injury to peripheral nerve is proposed.

Animals↗
PubMed1994

Partitioning of 222Rn entry into a structure surrounded by soil.

This paper describes the entry rate of 222Rn into a basement structure surrounded by a sandy clay loam soil. The highest indoor radon concentrations occurred when the rate of entry was lowest. Data from in-situ measurements were used to identify the entry pathways and also the origins of the radon during periods when the entry rate was low. Results indicated that 25% of the radon entered through the floor-wall joint and 75% through the floor and walls. About 30% of the radon originated in the concrete. Diffusion was the primary transport mechanism. However, radon entry through the floor-wall joint was a combination of diffusion and a convective flow between the subslab region and the interior of the structure.

Air Pollutants, Radioactive↗
PubMed1994

A method for computing random chord length distributions in geometrical objects.

A method is described that uses a Monte Carlo approach for computing the distribution of random chord lengths in objects traversed by rays originating uniformly in space (mu-randomness). The resulting distributions converge identically to the analytical solutions for a sphere and satisfy the Cauchy relationship for mean chord lengths in circular cylinders. The method can easily be applied to geometrical shapes that are not convex such as the region between nested cylinders to simulate the sensitive volume of a detector. Comparisons with other computational methods are presented.

Monte Carlo Method↗
PubMed1993

Characterization of 222Rn entry into a basement structure surrounded by low-permeability soil.

An experimental facility has been developed to monitor the entry rate and concentration of 222Rn in a basement-type structure surrounded by soil having a permeability on the order of 10(-11) m2. A data acquisition system recorded environmental conditions outside and inside the structures, plus basement air exchange rate information, every 15 min. Indoor 222Rn concentrations ranged from 400 to 1,400 Bq m-3. The observed 222Rn entry rate is highly variable and has two primary components; a constant input rate caused by diffusion of 222Rn through the concrete walls and floor, and a variable rate that depends upon indoor-soil pressure differentials of only a few pascals. Pressure differentials are dependent upon wind speed and wind direction. Stack effect was not significant. During a 2-wk period, with relatively calm winds, diffusion through the concrete walls and floor plus the floor-wall joint accounted for more than 80% of the total 222Rn entry.

Air Pollutants, Radioactive↗
PubMed1991

Determination of time-varying 222Rn concentrations using flow-through scintillation flasks.

This paper describes a method to obtain time-varying 222Rn concentrations from the series of gross alpha measurements recorded with a flow-through scintillation detector. The continuous input of 222Rn into the detector is treated as a series of independent pulses. The response of the detector to a single pulse containing 222Rn can be measured in the laboratory and is used to determine a normalized detector response function (NDRF). The NDRF contains information on the combined effects of detector design, operating parameters, and the presence of 222Rn progeny. The estimation of individual plate-out or counting efficiencies is not needed, as they are appropriately included in the NDRF. The equation describing the response function can be inverted to yield the actual 222Rn concentration in terms of detector output. The technique was applied to actual indoor monitoring data and the results presented.

Air Pollutants, Radioactive↗
PubMed1989

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↗
PubMed1986

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↗
PubMed1985

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↗
PubMed1985

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↗
PubMed1984

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↗
PubMed1983

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↗