Re: Ionization at 80 eV? (Yes)

[FRAMEP@ORAU.GOV]

Let me throw in my two cents. As I understand it: 

For any single detector, the system goes through the ionization region,
the proportional region, and the GM region in that order. 

The voltages at which these regions are reached depend on 

1. the detector design (I.e. electric field strength)
2. the type of gas
3. the gas pressure

GMs often operate at lower voltages than proportional counters because
their design/gas/pressure allows them to reach the GM region "sooner"
than proportional counters.

With a GM, the aim is to get to the GM plateau at as low a voltage as
feasible given the various constraints. This is done by increasing the
steepness of the slope in the proportional region. With a proportional
counter, you operate on the slope between the ionization and GM
regions, not on a plateau (we're talking about the six region curve of
pulse size vs voltage, not the characteristic curve of count rate vs
voltage)

With a proportional counter, it is desirable to have a gradual slope in the
proportional region. The more gradual the slope, the smaller the effect
fluctuations in voltage will have on pulse size. As the steepness of the
slope in the proportional region is made more gradual, the operating
voltages in the proportional and the GM regions are shifted to the right
i.e., higher voltages.

I'd hesitate to push the point that GMs use noble gases while proportional
detectors use polyatomic gases. GMs gases include polyatomic quench
gases and proportional gases can be pure argon or krypton. The widely
used P-10 gas is 90% argon. But, as I understand it, it is true that the
polyatomic gases in the proportional gas are responsible for the shift in
the shape of the curve.

Wade is right. Price's Nuclear Radiation Detection has a nice discussion
about this. An oldie but a goodie (the book, not Wade).

Paul Frame
Professional Training Programs
ORISE
framep@orau.gov