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21 November 2009

Answer to Question #2880 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

I have a question about an IPM7 personnel monitor. When using argon 90%, methane 10% mix in a proportional counter, how important is quality of the gas (moisture and specification)? How much effect would there be if the gas is slightly out of specifications, say 93%:7%? Can IPM7 be run using from 5% to 10% methane? What difference does this make to the proportional counting, i.e., higher or lower voltage required?

I couldn't find the IPM7 through an online search, but I assume you've got an early model Nuclear Enterprise Ltd. (NE) "Installed Personnel Monitor." My search did show NE's IMP9. Glenn Knoll's text Radiation Detection and Measurement, in chapter 6 on proportional counters, gives an excellent review of this subject. In particular, the effects of gas impurities in the "gas multiplication" process are discussed in detail, i.e., from the initiating radiation interaction, production of secondary electrons, and an "avalanche" pulse producing a count. Most flow-through proportional counters use P-10 gas (i.e., 90% argon, 10% methane) However, lower methane quench gas concentrations will work. Knoll's Table 6-1 in chapter 6 shows the characteristics of P-5 gas (i.e., 95% argon, 5% methane). I'd follow the manufacturer's procedure for IPM setup using the gas in question, vary the high voltage with a check source in a fixed position, and plot the count rate vs. high voltage. You want to set your operating high voltage toward the middle of the flat portion of the "plateau."

David J. Allard, CHP

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