Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

■ Self-Quenching Gas (absorb ultraviolet (UV) photons)
A self-quenching gas is one that can absorb ultraviolet (UV) photons without
becoming ionized. One way to use this characteristic is to introduce a small
amount of organic vapor, such as alcohol or ether, into the tube. The energy
from the ultraviolet photons is then dissipated by dissociating the gas
molecule. Such a tube is useful only as long as it has a sufficient number of
organic molecules to dissociate, generally about 10 8 counts.
To avoid the problem of limited lifetime, some tubes use halogens (chlorine or
bromine) as the quench gas. The halogen molecules also dissociate in the
quenching process but they are replenished by spontaneous recombination at
a later time. Halogen quench tubes have an infinite lifetime and are preferred
for extended applications.
Reaction products of the discharge often produce contamination of the gas or
deposition on the anode surface and generally limit the lifetimes of geigermüller
tubes.
Charlie Chong/ Fion Zhang