Beta-gamma point source on the skin problem--activity estimation and dose analysis.

This paper presents the analytical approaches used and results obtained by the authors in the evaluation of the dose significance of maintaining a microscopic particle of beta-gamma emitting radionuclides on the skin for a finite time interval. The correlation of the response of a portable measuring instrument with radioactivity content of the particle is discussed along with the subsequent evaluation of dose to a limited skin area. Examples are given to show how two independent theoretical approaches were used to estimate the activity of 60Co from a single poor geometry measurement made with a thin window ionization chamber and how the results compared with empirical results from laboratory measurements on a prepared 60Co source. The application of Loevinger's point beta source dose distribution function and other dosimetric information is presented. A summary table and simplified equations to predict beta dose rates to 1 cm2 of tissue at a depth of 7 mg cm-2 for 3.7 X 10(4) Bq point isotropic sources of hypothetical beta emitters ranging in end point energies from 0.2 to 3 MeV and in atomic number from 10 to 90 are also included.

An alternative method for the release criteria and calculation of the total dose equivalent to another individual from a patient treated with a therapeutic dose of 131I.

The Nuclear Regulatory Commission's Regulatory Guide 8.39, "Release of Patients Administered Radioactive Materials," permits licensees to authorize the release of patients treated with 131I [(T 1/2)p = 8.04 d] if the total dose equivalent to another individual from the released individual is not likely to exceed 5 millisieverts (0.5 rem). This paper guides the reader through the derivation of an easy-to-use formula that demonstrates how to calculate the total dose equivalent, using patient-specific parameters that an individual may receive from a patient who has been administered a therapeutic dose of 131I, as compared to the NRC's Reg. Guide 8.39 (see Appendix A). Patient-specific parameters include the individual thyroidal uptake, the patient's physical dimensions, attenuation of radiation by body tissues, buildup of secondary photons, whole body and thyroid retention functions as well as the distance and time the patient is around others. This paper also demonstrates how to calculate an inpatient's discharge time, so that the patient will not expose other individuals above the desired total dose equivalent set by the Radiation Safety Officer (RSO).

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%.

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 University's 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.

Intake retention functions and their applications to bioassay and the estimation of internal radiation doses.

This paper describes a way of obtaining and gives applications of intake retention functions. These functions give the fraction of an intake of radioactive material expected to be present in a specified bioassay compartment at any time after a single acute exposure or after onset of a continuous exposure. The intake retention functions are derived from a multicompartmental model and a recursive catenary kinetics equation that completely describe the metabolism of radioelements from intake to excretion, accounting for the delay in uptake from compartments in the respiratory and gastrointestinal tracts and the recycling of radioelements between systemic compartments. This approach, which treats excretion as the 'last' compartment of all catenary metabolic pathways, avoids the use of convolution integrals and provides algebraic solutions that can be programmed on hand held calculators or personal computers. The estimation of intakes and internal radiation doses and the use of intake retention functions in the design of bioassay programs are discussed along with several examples.

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.

Theoretical response of a ZnS(Ag) scintillation detector to alpha-emitting sources and suggested applications.

The classic problem of alpha absorption is discussed in terms of the quantitative determination of the activity of "weightless" alpha sources and the specific alpha activity of extended sources accounting for absorption in the source medium and the window of a large area ZnS(Ag) scintillation detector. The relationship for the expected counting rate gamma of a monoenergetic source of active area A, specific alpha activity C, and thickness H that exceeds the effective mass density range Rs of the alpha particle in the source medium can be expressed by a quadratic equation in the window thickness x when this source is placed in direct contact with the window of the ZnS(Ag) detector. This expression also gives the expected counting rate of a finite detector of sensitive area A exposed to an infinite homogeneous source medium. Counting rates y obtained for a source separated from a ZnS(Ag) detector by different thicknesses x of window material can be used to estimate parameter values in the quadratic equation, y = a + bx + cx2. The experimental value determined for the coefficient b provides a direct estimation of the specific activity C. This coefficient, which depends on the ratio of the ranges in the source medium and detector window and not the ranges themselves, is essentially independent of the energy of the alpha particle. Although certain experimental precautions must be taken, this method for estimating the specific activity C is essentially an absolute method that does not require the use of standards, special calibrations, or complicated radiochemical procedures. Applications include the quantitative determination of Rn and progeny in air, water, and charcoal, and the measurement of the alpha activity in soil and on air filter samples.

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.

Spectral energy effects in ESR bone dosimetry: photons and electrons.

The spectral energy-dependence of the radiation-induced ESR signal has been studied in ovine cortical bone. Crushed bone samples were irradiated using photon beams with effective energies in the range from 0.06 to 6 MeV, and electron beams with mean energies in the range from 2 to 10 MeV. The photon and electron data were normalized to a dose to bone of 50 Gy and the results are reported as response relative to the ESR signal for photon irradiation at 1.25 MeV (60Co). The photon irradiation results show that the ESR response is greatest at low energies with a relative value of 1.2 at 0.06 MeV. The relative response decreases, as the energy increases, to approximately 0.85 in the region of 2 to 3 MeV. These variations in the relative ESR responses are significantly less than the ESR energy-dependent responses reported in the literature for human tooth enamel and synthetic hydroxyapatite. An explanation for this difference is offered. For electron beam irradiations, the ESR signal is fairly constant with energy, and approximately equal to that at a photon energy of 1.25 MeV. Implications of these results are discussed.

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.