RE: Ion chamber versus GM

[don.parry@cis.state.mi.us]

I know this comes a little late in the discussion, but I wanted to add one 
caution when using the 450 P.  Our program has many years of experience 
using these meters, (we currently have 20 of them) and I perform the semi 
annual calibrations on the instruments.  While I agree that they are 
durable, light, fast responding instruments  capable of measuring a wide 
range of intensities, we have had an occasional problem with chambers that 
lose their pressure.  This can result in a large under response of the 
instrument.  During our January calibration, I had one instrument that read 
only 72 mR/hr in a 238 mR/hr field.  The meter tested fine 6 months earlier 
and the user had no indication that the ion chamber had depressurized.  We 
have experienced at least 3 of these failures over the years.  More common 
is the slow leaking of the chambers over time.  I almost always need to 
adjust calibration factors on each unit every 6 months to keep the device 
reading to within 5% of true.  Occasionally, some meters have had to be 
sent back to Victoreen to be repressurized before they can be adjusted to 
within 10% of true.  (Last year I sent 6 of our 20).  So while I think they 
are a good instrument, I would encourage users to test them frequently 
against a known radiation field, especially if you are relying on their 
accuracy in a high dose rate field.

Donald E. Parry
Radiation Safety Section
MI  Dept of Consumer & Industry Services
Voice: 517-241-1989; Fax: 517-241-1981
E-mail: don.parry@cis.state.mi.us
Web site: www.cis.state.mi.us/bhs/divisions/hfs/rss/


Donald E. Parry
Radiation Safety Section
MI  Dept of Consumer & Industry Services
Voice: 517-241-1989; Fax: 517-241-1981
E-mail: don.parry@cis.state.mi.us
Web site: www.cis.state.mi.us/bhs/divisions/hfs/rss/