Re: Re[2]: Ionization at 80 eV? (Yes)

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

I sent a much longer reply directly, but for a given detector the
operating voltage is a function of gas and mode.  Proportional
voltage is lower than geiger, but normal proportional gases
operate at higher voltages than geiger gases.

Geiger gases tend to be mostly noble gases which give high gas gains
at relatively low voltages, but suffer from the emission of photons
when cations neutralize at the cathode.  These photons are what propagate
the pulse and make the pulse both wide ~ msec and energy insensitive.

Proportional gases tend to be partially to mostly polyatomic gases
such as methane or isobutane.  Polyatomics reduce the emission of
cathodic photons which are a result of the difference in ionization
potential of the gas and the cathode, by opening vibrational channels
for the gas to disipate the energy.  Gases like methane do not have
as much gas gain at a given voltage and detectors must be driven
to higher voltages to get satisfactory pulses.

Real fun starts when you use a "magic" gas mixture.  A small amount
of very strong electron scavenger such as CF3Br (Freon 13B1) mixed
with a noble gas allows a detector to be driven to very high gas
gains without cathodic photon propagation.  You get pulses that
are similar in width to proportional pulses ~ microsec, but have
pulse heights through 50 ohms of half a volt or more!  Because
the 13B1 is such a strong scavenger, the sensitive volume only
extends a couple of mm from the anode.  Useful in low background and
position sensitive work.

Dale
dale@radpro.uchicago.edu