Uncertainty of the thyroid dose conversion factor for inhalation intakes of 131I and its parametric uncertainty.

Inhalation exposures of 131I may occur in the physical form of a gas as well as a particulate. The physical characteristics pertaining to these different types of releases influence the intake and subsequent dose to an exposed individual. The thyroid dose received is influenced by the route through which 131I enters the body and its subsequent clearance, absorption and movement throughout the body. The radioactive iodine taken up in the gas-exchange tissues is cleared to other tissues or absorbed into the bloodstream of the individual and transferred to other organs. Iodine in the circulatory system is then taken up by the thyroid gland with resulting dose to that tissue. The magnitude of and uncertainty in the thyroid dose is important to the assessment of individuals exposed to airborne releases of radioiodine. Age- and gender-specific modelling parameters have resulted in significant differences between gas uptake, particulate deposition and inhalation dose conversion factors for each age and gender group. Inhalation dose conversion factors and their inherent uncertainty are markedly affected by the type of iodine intake. These differences are expected due to the modelling of particulate deposition versus uptake of gas in the respiratory tract. Inhalation dose estimates via iodine gases are very similar and separate classifications may not be necessarily based on this assessment.

Analysis of an internal kinetic model for free and bound tritium.

Internal dosimetry models that currently drive regulatory compliance decisions assume that tritium retention kinetic behavior can be modeled by a single exponential function. This is contrary to the results of a number of modeling techniques, which indicate that while elemental tritium (HT) and tritiated water (HTO) are the most commonly released forms of tritium, organically-bound tritium (OBT) doses can be quite significant. In this paper, a unified two-compartment model of the retention kinetics of HTO and OBT is examined for the purpose of investigating the importance of metabolic routes not considered in the ICRP one- and two-exponent models; namely the transfer of tritium from the HTO compartment to the OBT compartment and vice versa. In particular, the effect of intake ratio is investigated, and a detailed analysis of dosimetric implications is performed. For typical combined intakes of HTO and OBT, the number of disintegrations from the two tritium forms can be roughly equal. This result, when combined with the suggested greater biological effectiveness of OBT, indicates effective doses will be greater than those obtained from a single exponential model. The results of this study corroborate previous findings using the two-compartment model for the cases of HTO-only and or OBT-only intakes and compare well with data taken from studies on animals and human subjects.

A modified ICRP 66 iodine gas uptake model and its parametric uncertainty.

Intakes via inhalation may occur from radionuclides released in the form of a gas. The chemical characteristics pertaining to the release influence the intake and subsequent dose to an exposed individual. Gases are taken up or absorbed in the entire respiratory tract and the associated uptake mechanisms are quite different from deposition of particulates. Gaseous iodine can exist in various chemical forms, e.g., elemental iodine, inorganic, and organic iodine compounds. These different chemical species play an integral role in the gaseous uptake o f iodine in t he respiratory tract. Gas uptake in the various regions of the respiratory tract results in the intake of iodinated material into the body. The radioactive iodine taken up in the gas-exchange tissues is absorbed into the bloodstream of an individual and subsequently transferred to other organs. Iodine in the circulatory system can then be taken up by the thyroid gland, with resulting dose to the thyroid. The magnitude and uncertainty in regional gas uptake is important in the assessment of individuals exposed to airborne releases of radioiodine. The current ICRP 66 model is rudimentary and estimates regional gas uptake based on solubility and reactivity of the different radionuclides entering the respiratory tract. The modified model proposed here employs methodology and a mathematical structure to determine estimates of fractional gas uptake rather than defaulting to literature values, as in the current ICRP model. Model parameters have been assigned input distributions and estimates of uncertainty have been determined. A sensitivity analysis of these parameters has been performed to demonstrate the importance of each of these parameters. The sensitivity analysis ranks the model-input parameters by their importance to estimates of regional gas uptake. The model developed herein may be used for improved estimation of gas uptake in the respiratory tract and subsequent dose estimates from the different chemical forms of radioiodine.

A comparison between default EGS4 and EGS4 with bound Compton cross sections when scattering occurs in bone and fat.

Computer simulation packages are important tools in understanding how radiation interacts with matter. EGS4 is a photon/electron Monte Carlo transport program that is employed in the health/medical physics field. Due to its high energy roots, the default version of EGS4 treats all electrons as unbound and therefore uses the Klein-Nishina cross section formula to determine Compton scattering angle distributions and the probability of Compton scattering through the branching ratio. Researchers have created improvements to EGS4 that account for the bound Compton cross section as well as other scattering properties. Numerical experiments were performed on both the default code and modified EGS4 to examine output differences in low Z materials such as fat and bone. Four incident photon energies were considered. At higher energies (500 keV and 1 MeV) the default and modified EGS4 codes produced results within 2sigma of one another. At 50 and 100 keV differences in scattering angle distribution and branching ratio values were found. In addition, the number of photoelectric absorptions and Compton scatters were also different at these energies.

An experimental determination of FIDLER scanning efficiency at specific speeds.

As part of mass remediation efforts across the country some radiation detection systems are now being used in conjunction with data logging and positioning system technology. These systems can be used in the scanning mode, simultaneously recording both count rate and position. Following data analysis, hot spots can be identified and remediation efforts for that particular area can commence. This technique has been used for nearly a decade and has had success in accelerating preliminary remediation work while also reducing potential clean up costs. However, little work has been completed on how the sensitivity of these detection systems are affected when used with this technology because while the intrinsic efficiency of the detector is constant, scanning efficiency can vary depending on data sampling time and scanning speed. To better understand scanning efficiency for a detector attached to such a system, a device was developed which moved soil at a constant speed underneath a Field Instrument for Detecting Low Energy Radiation (FIDLER). Count rate was measured every 2 s as a 241Am source passed under the detector at speeds ranging from approximately 10 cm s(-1) to 100 cm s(-1). A surface source and a buried source were both examined. Experimental detection efficiency was calculated and compared to Monte Carlo generated results. For the surface source, the efficiency dropped to a value of approximately 1% at 100 cm s(-1). At the same speed, the buried source had a detection efficiency of 0.1%, primarily due to attenuation of the low energy photon in the soil. It was also noted that the response time of the meter affected the scanning efficiency. With a response time set at 1 s, higher average efficiencies were recorded but with a large standard deviation from the mean. Higher response time setting had the effect of reducing the variability of the reading but also reducing efficiency.

Peer versus authority as decision maker: are the demographics of the perceiver related to judgements of fairness?

In a role-playing study, 264 university students evaluated the fairness of procedures and outcomes used to punish a student for plagiarizing from a web site. Individuals reading the scenario gave the highest ratings of justice when the decision maker was a group of nonpeers (faculty), compared to when the decision maker was an individual or peer (student). Sex and self reported guilt of individuals reading the scenario also influenced their ratings of justice.

Materialism and credit card use by college students.

Much has been written in the popular press on credit card use and spending patterns of American college students. The proliferation of credit cards and their ease of acquisition ensure that students today have more opportunities for making more credit purchases than any other generation of college students. Little is known about the relationship between students' attitudes towards materialism and their use of credit cards. A study was conducted at three college campuses in the northeastern part of the United States where a total of 1,022 students were surveyed. Students' attitudes toward use of credit and their credit card balances were evaluated relative to their scores on Richins and Dawson's Materialism Scale (1992). Our findings suggest no significant difference between those individuals scoring high versus low on the Materialism Scale in terms of the number of credit cards owned and the average balance owed. Individuals high on materialism, however, significantly differed in terms of their uses for credit cards and their general attitude toward their use.

The new VARSKIN 4 photon skin dosimetry model.

A new photon skin dosimetry model, described here, was developed as the basis for the enhanced VARSKIN 4 thin tissue dosimetry code. The model employs a point-kernel method that accounts for charged particle build-up, photon attenuation and off-axis scatter. Early comparisons of the new model against Monte Carlo particle transport simulations show that VARSKIN 4 is highly accurate for very small sources on the skin surface, although accuracy at shallow depths is compromised for radiation sources that are on clothing or otherwise elevated from the skin surface. Comparison results are provided for a one-dimensional point source, a two-dimensional disc source and three-dimensional sphere, cylinder and slab sources. For very small source dimensions and sources in contact with the skin, comparisons reveal that the model is highly predictive. With larger source dimensions, air gaps or the addition of clothing between the source and skin; however, VARSKIN 4 yields over-predictions of dose by as much as a factor of 2 to 3. These cursory Monte Carlo comparisons confirm that significant accuracy improvements beyond the previous version were achieved for all geometries. Improvements were obtained while retaining the VARSKIN characteristic user convenience and rapid performance.