JEOL Probe Users Listserver

Moderator: Ellery Frahm, [log in to unmask],
Electron Microprobe Lab, University of Minnesota

Post a message: send your message to [log in to unmask]

Unsubscribe: send "SIGNOFF PROBEUSERS" to [log in to unmask]

On-line help and FAQ: http://probelab.geo.umn.edu/listserver.html



*

A very important part of the P-10 gas is the "10", which is methane. Methane
is a quench gas which needs to be replenished.

 

"Pure noble gases can be used for alpha counting at low voltages where the
multiplication factor is below 100. As a rule however, a quench gas is added
to prevent the proportional counter from acting like a geiger muller
detector. During the formation of an avalanche, some gas molecules/atoms are
excited rather than ionized. In other words, the energy absorbed by these
proportional gas atoms/molecules promotes electrons to higher energy levels
rather than frees them completely from the atoms/molecules. When the
electrons deexcite and return to their original energy levels, they emit
photons of visible light or UV. The problem with this is that these photons
can interact with the proportional gas and cause the avalanche to spread
along the anode. This can result in a non-linear relationship between the
energy deposited in the detector gas and the size of the resulting pulse.
These photons, particularly if they interact with the cathode wall, can also
lead to the production of spurious pulses. The solution is to add a small
amount of a polyatomic quench gas such as methane. The quench gas
preferentially absorbs the photons, but unlike the fill gas (e.g., argon),
it does so without becoming ionized." From
http://www.orau.org/ptp/collection/proportional%20counters/introprops.htm 

 

-joe

 

Joe Geller

Geller MicroAnalytical Laboratory, Inc.

426e Boston St., Topsfield, MA 01983

tel 978 887-7000, fax 978 887-6671

www.GellerMicro.com