Answer to Question #6543 Submitted to "Ask the Experts"

What is the best method for determining G-M (Geiger-Müller) counting efficiency, then converting to MDA (minimum detectable activity) for scans and for fixed measurements? Does the source manufacturer give the emission percentage of the isotope in question? I am confused by the high-probe efficiency claimed by G-M manufacturers for "4 π geometry." How does this relate to efficiency of detecting surface contamination? Specifically, how can one determine ³²P efficiency (without making up a short-lived source from material)? Finally, where can one acquire NIST (National Institute of Standards and Technology)-trace beta check sources that can be used to determine G-M counting efficiency of surface contamination?

Thank you for your questions related to the appropriate calibration of survey instrumentation used in decommissioning. Allow me to first cite a few very useful reference documents that address your various questions. The first is NCRP Report No. 112 "Calibration of survey instruments used in radiation protection for the assessment of ionizing radiation fields and radioactive surface contamination." Section 7 provides guidance on source selection—such as calibration energy requirements (e.g., specific radionuclides suitable for calibration), source strength, and geometry. Appendix D in particular may be of interest, "Examples of calibration of a thin window G-M detector for assessment of surface contamination."

Two other pertinent references that come to mind include NUREG-1507, "Minimum detectable concentrations with typical radiation survey instruments for various contaminant and field conditions." This NUREG covers the calculation of both static and scan MDC in detail. Section 4 of NUREG-1507 addresses instrument efficiencies. The other reference that I'll mention is Volume 2 of NUREG-1757, "Consolidated NMSS Decommissioning Guidance,'' which also provides guidance on instrument calibration in the context of decommissioning surveys.

Calibration source manufacturers should provide the two-π emission rate of the beta source. The two-π emission rate is then used to calculate the instrument efficiency. The instrument efficiency is then multiplied by the surface efficiency to yield a four-π total efficiency. Again, Section 4 in NUREG-1507 covers this method in good detail, as well as providing typical efficiencies for G-M, gas proportional, and other detector types. This calibration approach for surface contamination instrumentation is in accordance with ISO-7503, which has been adopted by the MultiAgency Radiation Survey and Site Investigation Manual (MARSSIM) and various other related guidance documents.

Sometimes it is not feasible to obtain a beta calibration source for a particular energy (e.g., ³²P with 1.71 MeV beta max). We have had success developing a multipoint beta calibration curve using several beta-emitting radionuclides that span the energy range from a low of 67 keV (beta endpoint) for ⁶³Ni to a high of 2.28 MeV for ⁹⁰Sr/⁹⁰Y. Once you fit an equation to the calibration data, you then have the ability of determining efficiency for a particular energy. Pertaining to calibration sources, manufacturers should adopt the guidance in International Organization for Standardization (ISO-8769), "Reference sources for the calibration of surface contamination monitors—Beta-emitters and alpha-emitters"; 1988. A list of vendors is provided on the DDSC (Decontamination and Decommissioning Science Consortium) Web site.

Eric W. Abelquist, CHP