Alan L. Justus

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 instruments dead time; whereas in the wide-pulse case, the accelerator pulse width is significantly longer than the instruments dead time. Examples are provided that highlight the various concepts and limitations.

Personal dose equivalent conversion coefficients for neutron fluence over the energy range of 20–250 MeV

Monte Carlo simulations were performed to extend existing neutron personal dose equivalent fluence-to-dose conversion coefficients to an energy of 250 MeV. Presently, conversion coefficients, H(p,slab)(10,alpha)/Phi, are given by ICRP-74 and ICRU-57 for a range of angles of radiation incidence (alpha = 0, 15, 30, 45, 60 and 75 degrees ) in the energy range from thermal to 20 MeV. Standard practice has been to base operational dose quantity calculations <20 MeV on the kerma approximation, which assumes that charged particle secondaries are locally deposited, or at least that charged particle equilibrium exists within the tally cell volume. However, with increasing neutron energy the kerma approximation may no longer be valid for some energetic secondaries such as protons. The Los Alamos Monte Carlo radiation transport code MCNPX was used for all absorbed dose calculations. Transport models and collision-based energy deposition tallies were used for neutron energies >20 MeV. Both light and heavy ions (HIs) (carbon, nitrogen and oxygen recoil nuclei) were transported down to a lower energy limit (1 keV for light ions and 5 MeV for HIs). Track energy below the limit was assumed to be locally deposited. For neutron tracks <20 MeV, kerma factors were used to obtain absorbed dose. Results are presented for a discrete set of angles of incidence on an ICRU tissue slab phantom.

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.

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

AbstractThis 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 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.

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

AbstractEvaluation 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.

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

AbstractThis 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 137Cs 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.

AbstractThis 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.