George E. Chabot

Proton dosimetry in bone using electron spin resonance

We have studied the ESR response of proton-irradiated (in vitro) bone. The ESR response as a function of proton (E = 105 MeV) dose to bone was linear from 0 to 50 Gy and similar to the photon (E = 6 MV) dose response. The ESR depth response (Bragg) curve was depressed as compared to a depth-response curve determined with a parallel plate ionization chamber (PPIC). There was a short-term ESR signal fade in the Bragg peak region, likely attributable to the organic component in bone. We are continuing to investigate these latter two effects.

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.

Environmental monitoring for a low-level radioactive waste management facility: incinerator operations.

An environmental monitoring program has been developed for Harvard University, Southborough campus, to assess the local environmental concentrations of radionuclides released in incinerator effluents. The campus is host to the Universitys low-level radioactive waste management facility, which consists of 6,000 drum capacity decay-storage buildings; a 250 drum capacity decay-storage freezer; and a controlled-air incinerator. Developmental considerations were based on the characteristics and use of the incinerator, which has a capacity of 8 tons per day and is operated 5% of the time for the volume reduction of Type 0 and Type 4 wastes contaminated with a variety of radionuclides used in biomedical research-some in microsphere form. Monitoring was established for air, leafy vegetation, leaf-litter, and surface soil media. Field sampling was optimized regarding location and time based on the action of atmospheric, terrestrial, and biotic transport mechanisms. Preliminary results indicate transient concentrations of 3H and 125I in vegetation directly exposed to the dispersing plume. Measurable particulate depositions have not been observed.

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.

ESR dosimetry of human cortical bone irradiated by a therapeutic proton beam

Dosimetry based on the electron spin resonance (ESR) technique was used to study human cortical bone irradiated by a therapeutic proton beam. Bone samples were irradiated with 160-MeV protons and, for comparison, 6-MV photons, Additional samples were held aside as controls. The ESR peak-to-peak heights (PPH) for various doses were studied. A Bragg curve was generated by irradiating bone samples at various depths and measuring the ESR response. A companion Bragg curve was also generated via parallel plate ion chamber (PPIC) measurements. The proton-to-photon dose-response was 0.806/spl plusmn/0.034 (+1 s.d.) in the Bragg plateau. This ratio is lower than previously reported and is likely attributable to an intermediate-length (year,) decay component. In the Bragg curve analysis, an iterative technique for sub-millimeter sample depth-correction was employed. The Bragg peak position and height of the ESR data differed from the PPIC data. The ESR Bragg peak shift was -0.6 mm. The ratio of the ESR-to-PPIC Bragg peak height was 0.804/spl plusmn/0.029 (+1 s.d.), for a normalization in the Bragg plateau region. In conclusion, the difference in the peak position and the peak value of the Bragg curve may, in part, be from detector size effects, as noted in the very interesting work of Bichsel (1995). A direct comparison, however, can not be made because of differences in the two studies. The role of differential proton stopping-power effects in the bone response is discussed.

Carbon cartridge standards for 125I and suggested applications.

Carbon cartridge standards were prepared to assess the activity of 125I incident on, and adsorbed in, cartridge samples during air sampling. Each cartridge standard consisted of an 125I-spiked filter paper at a known depth, ranging from 0 to 19 mm, embedded in approximately 34 g of 20-30 mesh activated carbon contained within a 6.35 cm diameter by 2.22 cm deep metal cartridge with screened openings. The total counting efficiency values range from 17.8 to 20.8% for cartridges counted at 3.2 mm from a thin-crystal NaI(Tl) detector. The standards were analyzed using a front/back counting technique, and fitting functions were developed relating the front/back net counts ratio and counting efficiency to the 125I depth of burial. A method for determining sample activity that accounts for exponential radioiodine loading in cartridge samples is compared to a less complicated technique that assumes all the radioiodine is located at an equivalent depth of burial that is based on the sample front/back net counts ratio. In addition, methods are presented for determining airborne 125I activity for constant and variable concentrations. Variable concentrations are assumed to occur in a fume hood duct by one or more bulk releases as a result of iodinations that are performed during a given sampling interval. The two methods are shown to have maximum relative deviations ranging from -16 to +16%.

Simplified Solutions for Activity Deposited on Moving Filter Media.

AbstractSimplified numerical solutions for particulate activity viewed on moving filter continuous air monitors are developed. The monitor configurations include both rectangular window (RW) and circular window (CW) types. The solutions are demonstrated first for a set of basic airborne radioactivity cases, for a series of concentration pulses, and for indicating the effects of step changes in reactor coolant system (RCS) leakage for a pressurized water reactor. The method is also compared to cases from the prior art. These simplified solutions have additional benefits: They are easily adaptable to multiple radionuclides, they will accommodate collection and detection efficiencies that vary in known ways across the collection area, and they also ease the solution programming.