Re: Radon Screening

[Jeff Sawyer <74652.2361@CompuServe.COM>]

Franz, you wrote

> a reading of 500dpm/100cm2 for alphas and ten
time for betas seems to be rather high - especially for plate out of radon
progeny.

Most personnel contamination monitors try to cover as much body area as possible
using arrays of large detectors. The problem with most large area detectors used
in personnel contamination monitors is that they are typically larger (much)
than 100 cm^2. The Eberline PCM-1C (commonly found in  power plants) uses
detectors that are 500 cm^2. This means that you only need 100 dpm/cm^2 to cause
an alarm when the activity is uniformly distributed over the detector area.
Newer instruments such as the PCM-2 refered to by Riasp use slightly smaller
detectors (about 325 cm^2) which improves the situation a bit. Several
instruments of this type also use sum zones which are designed to detect a 100
cm^2 source positioned over two or more adjacent detectors. On the PCM-2 this
means that the area of the small sum zone is 650 cm^2 and even worse a sum zone
of 4 detectors has an area of 1,300 cm^2 resulting in an alarm for 500 dpm with
only 38.5 dpm/100 cm^2 distributed.

The problem is exacerbated (my only attempt at big words today) in facilities
that use polyester coveralls which are a great collector of radon daughters. It
is quite easy to get a uniform contamination from radon daughters on this type
of clothing that will alarm a PCM from any manufacturer. There is a lot of
experience at many power plants with people sitting on a bench waiting to decay
before they can exit the monitors. I've even lost a pair of pants at Lasalle
County Station due to radon and a tight flight schedule - but that's another
story...

The PCM-2 does have a novel radon compensation routine that looks at the ratio
of alpha to beta and subtracts counts based on a signature. It has been found to
be effective in power plants that are routinely monitoring for beta only.  The
algorithm is not effective in situations with alphas and betas present in the
contamination. (As I am sure everyone knows, radon daughter products are not
"contamination" in the regulatory sense of the word.)

Jeff Sawyer
Eberline Instruments
74652.2361@compuserve.com