The internal dosimetry program at Los Alamos National Laboratory uses applications of Bayesian statistics in the estimation of intake dates and quantities, and associated committed doses from inhalation of {sup 239}Pu. The construction of prior distributions, based on urine bioassay results from populations of plutonium workers, has reemphasized the fact that current radiochemical techniques using {alpha}-spectroscopy do not provide detection limits that meet DOE regulations (i.e., detection of intakes resulting in a committed effective dose of 1 m Sv in the year of intake). We are in the process of constructing a bioassay analysis regimen that uses thermal ionization mass spectrometry in conjunction with {alpha}-spectrometry to optimize detection of intakes in a cost effective manner. The mass spectrometry system at Los Alamos has a detection limit of 7 gBq L{sup -1} for {sup 239}Pu in urine compared to 2 n-Bq L{sup -1} from classical radiometric methods. Results are used to construct prior distributions for worker and non-worker populations. The improved sensitivity, combined with the decision making capabilities of the Bayesian framework, provides a powerful method for detecting intakes in both plutonium workers and non-occupational populations.

Internal dosimetry may be divided into tow main problems: (1) the forward (scientific) problem of determining biokinetics models that describe how radionuclides are taken into the body, distributed in body tissues, and excreted, and (2) the inverse (mathematical) problem: given the measured amounts in excreta and assuming a biokinetic model, to determine the times and amounts of intakes into the body. The inverse problem of internal dosimetry is, in fact, a generic problem studied in other fields (e.g., image reconstruction, spectral deconvulution, and model parameter fitting). We have developed a code for plutonium internal dosimetry using the maximum entropy method, a method for solving underdetermined inverse problems with a positivity constraint. Within the framework of Bayesian statistics, we believe the definitive approach is to examine the Bayesian posterior probability describing the probability of an intake scenario (X{sub i}) read ( ... ) as {open_quotes}the set of,{close_quotes} where X{sub i} denotes the intake amount that occurs on the with day. For plutonium, for a worker with a long employment history, this is a very high dimensional probability space, since there may be on the order of 10,000 days when intakes may have occurred. Within this high dimensional space, we calculate themore » mean intake scenario as where denotes the expectation value over the posterior probability distribution. Similarly, we calculate uncertainties and other relevant quantities, such as X{sup 2}, as expectation values over the posterior distribution. Thanks to a recent breakthrough in describing the mathematical structure of the intake process (a Poisson sum representation of intakes), we have developed the initial version of a Bayesian expectation-value algorithm for internal dosimetry reconstructions.« less

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.

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.