Kenneth W. Skrable

Dosimetric model for the gastrointestinal tract.

A dosimetric model is proposed for the gastrointestinal tract based upon the physiological model of EVE (1966a). A general equation describing the kinetics of linear first order phenomena is applied to obtain the burden of radionuclides or disintegrations in the contents of the various segments of the GI tract. The model gives equations for the calculation of the “dosimetric” average dose equivalent and dose equivalent rate to an entire segment as well as instantaneous values at any location within a given segment as applicable to single or continuous uptakes of parent and daughter radionuclides. Allowance is made for both the absorption of radionuclides as well as mass from the contents of all segments; although, this is not always considered significant. The model permits the determination of the quantities of a radionuclide absorbed into the blood, the ratio of daughter to parent disintegrations and the maximum to average dose equivalent rate in a particular segment of the GI tract. Numerical examples are given for various intakes. Maximum permissible daily ingestion rates of fictitious single soluble and insoluble radionuclides with an effective energy term of unity in all segments are given over a large range of half-lives and are compared to values calculated on the basis of current ICRP recommendations. It is proposed that ratios of the maximum to dosimetric average dose equivalent rate be used to define a distribution factor to take into account relatively high dose rates at particular locations within a segment of the GI tract. All equations have been derived using the more fundamental units of atoms or disintegrations and disintegration rates rather than the more popular pCi and pCi-day units. The former units are more fundamentally related to dosimetric quantities of interest.

Determining the specific alpha activity of thick sources using a large-area zinc sulfide detector

A simple method using a large-area zinc sulfide detector to determine the total specific alpha activity of thick sources is presented. A previous paper shows how the linear absorption properties of weightless alpha sources can be applied to thick sources placed in direct contact with a varying thickness of window material. A quadratic relationship between the detector response and absorber thickness was derived for sources whose thickness exceeds the range of the alpha particle. The coefficient of the linear term in the quadratic expression is used to calculate the total specific alpha activity of a source in contact with the window of the detector. This relationship is tested by obtaining alpha absorption data from solid sources of known specific alpha activity, fitting the data to the theoretical relationship and comparing the results to the known activities.

Blood-organ transfer kinetics.

Abstract Exact and approximate kinetics equations relating to the transfer and elimination of radionuclides from the blood and various organs in the body are presented. Although they are limited to simple first order kinetics, instantaneous uniform mixing in the blood and all organ pools, and the behavior of a single metabolic species, they are not limited by the number of transfer organs. In addition, the approach used here may he extended to other less limiting cases (e.g. various metabolites or daughters, slug flow, etc.). These expressions may be used to estimate the instantaneous activity or the total number of disintegrations of a radionuclide in the blood or various organs of reference in the body, hence, also the respective dose rates and doses. The exact kinetics equations may be used to relate measurements of radionuclides in excreta to burdens in the body. The approximate expressions greatly simplify the mathematics and yet provide sufficiently accurate results with a maximum deviation of about 23% from exact mathematical expressions over most time intervals of interest. They do give better results for exposure intervals long compared to the effective mean lives of the radionuclide in the various organs of reference, and they yield the exact steady state expressions. Fortunately, this condition is often satisfied for the relatively long standard exposure interval of 50 years that is applied to occupational exposure. In addition, the steady state expressions may be used along with metabolic data of the distribution of elements in the body, diet and excreta to estimate values of the rate constants used in both the exact and approximate expressions. A comparison of the exact and approximate expressions is given for the uranium metabolic model of Wrenn et al. (Wr78), and a comparison is made with current ICRP models (ICRP68a).

Basic applications of the chi-square statistic using counting data.

The chi-square statistic has many scientific applications, including the evaluation of variance in counting data and the proper functioning of a radiation counting system. This paper provides a discussion of the fundamental aspects of the chi-square test using counting data. Practical applications of the chi-square statistic are discussed, including the estimation of extra-Poisson variance and dead time for a counting system. The consequences of passing or failing the chi-square test are discussed regarding the proper estimator for the population variance of the counting data. Example scenarios are used to provide insight into the applications of the chi-square statistic and the interpretation of values obtained in hypothesis testing.

Reevaluation of the committed dose equivalent from 232Th and its radioactive progeny

Multicompartmental models were used in ICRP Publication 30 to describe the metabolism of radioactive elements and their retention in specific organs and tissues. Despite their use of more complicated and sophisticated metabolic models than those used in its earlier ICRP Publication 2 in 1959, the ICRP assumed that the radioactive progeny of 232Th that are produced in the body metabolize like their parents in its Publication 30. This assumption was made for mathematical simplicity and out of necessity when organs and tissues named for the parent are not included in the model of the progeny. This simplifying assumption can lead to overestimates of doses to tissues, especially the critical cells on bone surfaces. More realistic metabolic models and parameter values for 232Th and its radioactive progeny have been developed to estimate the total committed dose equivalent from 232Th and all of its radioactive progeny per unit intake of 232Th only. It is believed that this research has led to (1) more realistic estimates of doses from 232Th and its radioactive progeny over any applicable period of time after an intake, (2) more appropriate derived limits, and (3) metabolic models that can be used in the design of bioassay programs.

The issues concerning the use of an annual as opposed to a committed dose limit for internal radiation protection

The scientific, technical, practical, and ethical considerations that relate to the use of an annual as opposed to a committed dose limitation system for internal radiation protection are evaluated and presented. The concerns about problems associated with the more recent ICRP committed dose recommendations that have been expressed by persons who are currently operating under an annual dose limitation system are reviewed and discussed in terms of the radiation protection programme elements that are required for an effective ALARA programme. The authors include in this and a follow-up article a comparison of how these alternative dose limitation systems affect the economic and professional livelihood of radiation workers and the requirements that they impose upon employers. Finally, they recommend the use of an ICRP based committed dose limitation system that provides protection of workers over an entire occupational lifetime without undue impact on their livelihood and without undue requirements for employers.

A simple method for assessing exposure to internal emitters

One of the most challenging aspects of regulatory compliance can be demonstrating compliance with internal dosimetry requirements. For long-lived alpha-emitting radionuclides in particular, the sensitivity and accuracy of bioassay analysis and whole body counting may not allow for adequate assessment of intakes. Simple and effective measures can be used to control the workplace for the internal hazards associated with long-lived radioactive material using methods that measure directly the air to which workers are exposed. This paper provides an easy assessment tool that uses direct measurement of the specific activity of dusts in breathing zone air to evaluate internal exposures. Using this method, sensitive assessments can be made to determine if intakes are likely to have occurred and, if so, at what magnitude. It is not a substitute for confirmatory bioassay or whole body counting but a simple method to evaluate expectations for internal exposures.