Answer to Question #1146 Submitted to "Ask the Experts"

Q

Radiation technicians stated they could repair light leaks on alpha probes with black electrical tape until up to 10% of the mylar was covered. There is no procedural allowance afforded for such repairs in the national standards. (1) Have you ever seen such field repairs done on a routine basis? I can't ever recall seeing such "repairs" before on in-service instruments. (2) What is your opinion on the technical adequacy? As a matter of principle I don't much care for it, but I can acknowledge that given the correction factors already applied to hand-held field instruments, up to 10% of the surface area might be reasonable.

A

The aluminized mylar on ZnS(Ag) alpha scintillator probes is to prevent ambient light from causing photomultiplier tube pulses. This will cause an increase in background, which is normally very low for this type probe due to excellent pulse height discrimination possible (that is, for a 100 cm² probe, background is on the order of 1 to 2 counts per minute). However, as noted, this mylar is very easily torn, thus your technicians have used black electrical tape to stop the light leak. I've seen such repairs on these probes in Department of Energy facilities. And although not ideal, I'd agree that up to 10% area might be reasonable, given the calibration and correction factor may have a ±15% error already. Nonetheless, it may be helpful to test the entire surface area of one or two of the probes with a well-collimated alpha source to determine if the complete area responds uniformly. Also, if the radiation technicians are decreasing the probe area with tape, one could also adjust the correction factor up proportionately with the decreased area. There would have to be an estimate of the tape area to probe area for such an adjustment. A large area alpha source would also be helpful to sort all this out. Lastly, as a practical matter, the level of accuracy desired also depends on what is being surveyed (for example, U compared with Pu) and why (for example, personnel, facility screening survey, or free release of an item, etc.). David J. Allard, CHP