RE: Micro-R Meter

["Edwards, Richard W" <Richard.Edwards@PSS.Boeing.com>]

Since you haven't mentioned how you plan to use the instrument, there's
not much I can say beyond giving you some very general guidance.

1)  Many micro-R meters are actually NaI detectors coupled to a pulse
counter and displaying out in units of microR/hr. In general, I find
these to be very energy dependent in the ranges that I work (<0.02 to
0.32 MeV). If you are working with a source that emits a known and fixed
photon spectrum (such as a single radionuclide), these aren't bad. You
may have some trouble getting them calibrated correctly.

2)  Pressurized ion chambers work quite well within their design limits.
They have similar problems in that the increased wall density needed to
contain the pressurized gas raises the lower energy threshold higher
than I typically need. Also, I am not aware of one that can measure down
in the grass of 3 to 30 micro-R/hr. 

3)  Tissue equivalent plastic scintillators, such as the Bicron
micro-rem (which happens to be the only off-the-shelf version I know of)
work fairly well. I've recently (i.e., in the past two hours) seen some
indications that the low energy response is a little more variable than
I thought. I'm seeing plus or minus 25% in side by side measurements of
an Am-241 photon source when taken with supposedly identical units.
Those same units are giving identical responses to Cs-137 photons at the
same approximate dose rate. Still, I can conditionally recommend them.

4)  There are a number of manufacturer's that have dose integrating ion
chambers. I have found these to be very useful in those situations where
I can clearly define the time the instrument is exposed (either by
checking the time the instrument is on or that the radiation is being
generated).


Rick Edwards, Analyst
The Boeing Company
richard.w.edwards@boeing.com



As always, any opinions are mine and may or may not reflect the opinions
of my employer, coworkers or spouse.