Difference Distributions Applicable to Certain Health Physics Measurements.

This paper discusses calculational methods for the determination of the difference distributions associated with certain health physics measurements. These measurements include the check-source response counts relative to an initial reference count, the Albatross (i.e., HPI model 2080B) neutron tube counts relative to the gamma tube counts, and those that involve the use of the automatic background subtraction feature of portable health physics instrumentation. Examples are provided that illustrate the methods for a few specific measurements. For the comparison of a daily source count to its previously determined reference value, minimum counts for various scenarios were presented in order to reliably meet required tolerance limits of ±10% and ±20%. In either case, it was found beneficial that the initial reference readings be established using a counting interval of longer length than the daily interval. For the comparison of Albatross neutron counts to the gamma counts, it was seen that the relative error in the difference distribution was still related to that of the parent distribution. It was seen, therefore, that an effective way of reducing the gamma influence on the Albatross was to increase the counting interval used, hence yielding a significantly larger mean count per interval. For the automatic background subtraction feature, it was noted that net count values near 0 counts would almost always have the negative values of the difference distribution truncated to 0 counts by commercially available off-the-shelf instrumentation, whereas significant net count values would be displayed correctly but with a larger associated variance than the gross count itself. This paper therefore also provides a technical basis for the necessary source strength of a check source in order to meet daily limits, the gamma field limitations of the HPI 2080B Albatross, as well as the consequences of automatic background subtraction.

Multiple Facets of the Poisson Distribution Applicable to Health Physics Measurements.

This paper discusses several ways to view the standard Poisson distribution as applied to health physics scaler counts. The intent is not so much to present calculational details but rather to present some underlying concepts. Examples are provided that illustrate the various concepts for several types of specific measurements. The concepts presented involve purely Poisson-based contributions to variance; extra-Poisson variance is presently ignored, and any counting system would be presumed to function properly and without counting loss.

A Purely Poisson-based Approach to Estimating Audible Scan Survey Sensitivities.

Several attempts have been made over the years to estimate the sensitivity of audible scan surveys. Some estimates involved certain factors of the background count rate, while others involved calculations utilizing parameters pulled from the normal (Gaussian) distribution, regardless of background count rate. This paper discusses the results of a purely Poisson-based calculational method for the determination of appropriate decision levels and affiliated detection limits associated with health physics audible scan surveys. Comparisons are made to the other approaches and to empirical tests, where available. Although agreement with previous estimates can be had over limited ranges of background count rate, order-of-magnitude differences are possible outside of those ranges. The previous estimates are generally conservative at the higher count rates but quite nonconservative at the lowest count rates. Several real-life operational examples are provided that illustrate the approach.

Dynamic Radioactive Source for Evaluating and Demonstrating Time-dependent Performance of Continuous Air Monitors.

Evaluation of continuous air monitors in the presence of a plutonium aerosol is time intensive, expensive, and requires a specialized facility. The Radiation Protection Services Group at Los Alamos National Laboratory has designed a Dynamic Radioactive Source, intended to replace plutonium aerosol challenge testing. The Dynamic Radioactive Source is small enough to be inserted into the sampler filter chamber of a typical continuous air monitor. Time-dependent radioactivity is introduced from electroplated sources for real-time testing of a continuous air monitor where a mechanical wristwatch motor rotates a mask above an alpha-emitting electroplated disk source. The mask is attached to the watch's minute hand, and as it rotates, more of the underlying source is revealed. The measured alpha activity increases with time, simulating the arrival of airborne radioactive particulates at the air sampler inlet. The Dynamic Radioactive Source allows the temporal behavior of puff and chronic release conditions to be mimicked without the need for radioactive aerosols. The new system is configurable to different continuous air monitor designs and provides an in-house testing capability (benchtop compatible). It is a repeatable and reusable system and does not contaminate the tested air monitor. Test benefits include direct user control, realistic (plutonium) aerosol spectra, and iterative development of continuous air monitor alarm algorithms. Data obtained using the Dynamic Radioactive Source has been used to elucidate alarm algorithms and to compare the response time of two commercial continuous air monitors.

Techniques to alleviate nuisance alarms observed by PCMs following 222Rn-progeny deposition on clothing.

This paper presents technically-based techniques to deal with nuisance personnel contamination monitor (PCM) alarms. The techniques derive from the fundamental physical characteristics of radon progeny. Some PCM alarms, although valid alarms and not actually "false," could be due to nuisance naturally-occurring radionuclides (i.e., radon progeny). Based on certain observed characteristics of the radon progeny, several prompt techniques are discussed that could either remediate or at least mitigate the problem of nuisance alarms. Examples are provided which demonstrate the effective use of the techniques.

Derivation of a simple relationship between pulsed and steady-state dose limits due to general recombination within air-filled ionization chambers.

This paper discusses various calculational and measurement methods involved with general recombination losses of air-filled ionization chambers exposed to either steady-state or pulsed photon fields. Derived therefrom is a simple relationship between the pulsed and steady-state cases that allows the determination of expected pulsed-field losses from steady-state loss measurements within, for instance, typical Cs calibration fields. Several examples are provided to illustrate each calculational and measurement method, as well as the newly derived relationship.

Risk-based containment and air monitoring criteria for work with dispersible radioactive materials.

This paper presents readily understood, technically defensible, risk-based containment and air monitoring criteria, which are developed from fundamental physical principles. The key for the development of each criterion was the use of a calculational de minimis level, in this case chosen to be 100 mrem (or 40 DAC-h). Examples are provided that demonstrate the effective use of each criterion. Comparison to other often used criteria is provided.

Count rate limitations for pulse-counting instrumentation in pulsed accelerator fields.

This paper discusses various concepts involved in the counting losses of pulse-counting health physics instrumentation when used within the pulsed radiation environments of typical accelerator fields in order to preestablish appropriate limitations in use. Discussed are the "narrow" pulse and the "wide" pulse cases, the special effect of neutron moderating assemblies, and the effect of pulse fine microstructure on the counting losses of the pulse-counting instrumentation. In the narrow-pulse case, the accelerator pulse width is less than or equal to the instrument's dead time; whereas in the wide-pulse case, the accelerator pulse width is significantly longer than the instrument's dead time. Examples are provided that highlight the various concepts and limitations.

How long to air sample? Practical considerations for short-term air sampling.

Transportable (e.g., "giraffe") air samplers can be used for both short-term "grab-sampling" situations involving a few minutes to several hours, as well as for long-term retrospective air sampling situations that typically last a week. It is the short-term coverage situation which begs the question: "how long a sampling time is adequate?" The basis for the answer (which depends mainly on the selected volumetric sampling rate and the counter sensitivity, as well as the concentration level to be measured) can be simply viewed as follows: that an adequate activity must be measureable on the air filter sample at the cessation of sampling. Equations for selection of sampling times are presented, as are example calculations for alpha counters in order to measure various target concentrations and exposures. The concepts presented here can be readily adapted to other counting systems as well as to different sampling flow rates.

Prompt retrospective air sample analysis--a comparison of gross-alpha, beta-to-alpha ratio, and alpha spectroscopy techniques.

The long-standing problem related to prompt analyses in continuous air sampling or monitoring has been the well-known interference of the radon- and thoron-progeny co-deposited on the filtration media with any potential suspect radionuclides. The solutions to this problem have been quite diverse, and have included, for example, simple gross-alpha screening, the use of beta-to-alpha ratios, and/or the use of alpha spectral analyses. In the context of week-long retrospective continuous air sampling, this paper will explain, in detail, the technical basis for the use of the simple gross-alpha screening, beta-to-alpha ratio, and alpha spectrometry techniques and demonstrate the efficacy (or lack thereof) of these methods with simple examples. Although the most sensitive analysis technique for week-long retrospective continuous air samples is no doubt a long-lived count performed typically after at least a four-day decay period, when necessary, certain prompt and semi-prompt techniques discussed here can approach a sensitivity that is within about an order of magnitude of the long-lived count.

A new integral-mode survey method for the determination of average surface contamination levels.

A new integral-mode survey method is described for the direct measurement of average surface contamination levels. It is a method made possible by the modern generation of integrating ratemeters. Experiments were conducted to show both the effects of sources at a distance from or off-center from a probe. Case trials were conducted that demonstrated the method. It allows essentially four surveys to be performed for the price of one.

Derivation of continuous air monitor equations for DAC and DAC-h.

Equations are derived that provide the numerical algorithms necessary for the calculations of both concentration (such as #DAC) and exposure (such as #DAC-h) within continuous air monitors (CAMs) employing collection media. Both calculations utilize measured counts over certain CAM counting intervals. The relationship to similar, although oft misinterpreted, equations given in International Organization for Standardization Standard 11929-5:2005 is detailed.

Probabilistic model evaluation of continuous air monitor response for meeting radiation protection goals.

Effective continuous air monitor (CAM) programs can eliminate or significantly reduce the amount of inhaled radioactive material following an accidental release. Numerous factors impact the levels of protection CAM programs provide to the workers during these releases. These factors range from those related to the capability of the CAM instrument (e.g., CAM alarm set point and length of counting intervals) to those related to CAM placement in the room relative to dispersion rates and patterns of the released material in a room. While the impact of many of these factors on alarm sensitivity has been investigated in isolation, there are no methods for holistic evaluations of CAM programs relative to radiation protection goals (RPGs) or the contribution of the factors, either individually or combined, toward limiting worker dose. In this study, worker exposure was predicted using CAM response models developed to evaluate protection levels for continuous and acute releases. Monte Carlo simulations of 10,000 releases were performed using various combinations of model parameter values, with associated uncertainty distributions, to assess the expected ability of a CAM program to meet RPGs, and, further, to assess the relative influence of each factor toward lowering worker exposure. Results showed that improvements to CAM instrument capability combined with better ventilation and CAM placement improve worker protection nonlinearly and that these improvements are critical to meet RPGs. The sensitivity analysis showed that ventilation-driven dilution had the greatest impact on exposure reduction with the selected counting interval for alarm decisions and the alarm set point as secondarily important.

A technical basis for the continued use of exposure as an acceptable operational quantity in radiation protection.

Within the tabulated values of the new [to U.S. Department of Energy (DOE)] radiation weighting factors, it can be seen that a doubling of the neutron factor occurs for the 0.1 to 2 MeV neutron energy range. Hence, with the effective replacement of the quality factor by these new radiation weighting factors (for the protection quantities), it has been widely understood that the new changes will most definitely impact neutron dosimetry. However, it is less well understood that the new changes could also affect photon (and beta) dosimetry, i.e., photon reference fields, instrument design, and instrument calibrations. This paper discusses the ramifications, and ultimately concludes that the use of exposure for workplace measurements complies with both current and amended DOE requirements.