Fwd: ionization chambers, micro rem meters, etc.

[Ccja@aol.com]

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Forwarded message:
Subj:    Re: ionization chambers, micro rem meters, etc.
Date:    96-11-13 14:38:59 EST
From:    Ccja
To:      radsafe@romulus.ehs.uiuc.edu

The 450P is a very fine instrument, but I don't recommend it for your
application, due to it's lack of flexibility. It's best used for photonic
surveys, where the E ranges from about 50keV to maybe 3MeV (although I think
that it's been checked against the 6 or 7 MeV lines from N16, and found to be
well within 20%). The I125 xrays for instance are pretty far down on the
response curve. You can always correct, of course, but that gets problematic
at close to background levels. Then there's the problem of betas: you have
two chamber walls to go through, with a total density>400mg/sq.cm.

I recommend the basic Model 450. In Seattle, where the gamma background is
60-70 mrem/y, a typical instrument background is 0.05-0.25 mrem/h (it's quite
calibration-dependent). The 450 has a removable buildup cap (200mg/cm2), so
that you can easily scan for low E photons and betas. It's geometry makes it
easier to use in close quarters (eg, hoods) than it's cousin the 450B, which
has an integral, sliding buildup cap/beta shield. But, if you need very fast
response, as for surveys of xray machines, those won't cut it. The only meter
that comes readily to mind, that'll do that, too, is Keithley's Model
36100/105, whichever one has an integrative mode (never buy an ion-chambered
meter that won't integrate, if you do, you'll be sorry). But that's $2500.

Why don't you ask Victoreen if you can give them both a testdrive, with no
obligation? Most manufacturers are good about this.

Chris Alston