Physics and Engineering of Radiation Detection

3.5. Proportional Counters 189

used noble gas in proportional counters is argon mainly because of its lower cost as
compared to other inert gases.

B.2 Quenching............................

The avalanche multiplication in a proportional counter is a very localized pro-
cess. However it has a byproduct that has the ability to cause additional localized
avalanches. This byproduct is a photon with the wavelength in and around the
ultraviolet region of the spectrum, which is produced during the avalanche process.
The exact mechanism of the emission of these ultraviolet photons is the de-excitation
of the gas molecules. For example, for argon we have

γ+Ar → e+Ar+∗ - Ionization (3.5.10)

Ar+∗ → Ar++γuv - De-excitation. (3.5.11)

Hereγin the first reaction above symbolically representsanyionizing radiation and
γuvin the de-excitation process represents the ultraviolet photon emitted by argon.
The minimum energy of these ultraviolet photons in argon is 11.6eV. This energy is
unfortunately higher than the ionization potential of the metals commonly used in
proportional counters. Hence when an ultraviolet photon strikes the cathode wall it
may knock off an electron from the metal. If this electron enters the gas it gets accel-
erated between collisions with the gas molecules due to the high electric field inside
the chamber. Eventually it may approach the anode and cause another avalanche.
This process of secondary avalanche is graphically depicted in Fig.3.5.5. An obvious
way to solve this problem is to add an agent in the gas that has high absorption
coefficient for the photons in the ultraviolet region. Polyatomic gases (such asCH 4 )
fulfill this criteria since they have a number of closely spaced vibrational and ro-
tational energy levels. Also after absorption of the photon a polyatomic molecule
generally dissociates, which can be regarded as a radiationless process. The pro-
cess of decreasing the probability of secondary discharges is calledquenchingand an
agent used for this purpose is calledquencher. The advantage of using a quencher
in a proportional counter is evident from Fig.3.5.5.

+

γuv

Q

Q

+

γuv

Primary

Avalanche

Secondary

Avalanche QQuencher Charge Transfer Non−radiative Energy Loss

Without Quencher With Quencher

e

e

Anode

Cathode

Cathode

Incident

Radiation

Incident

Radiation

Anode

Cathode

Cathode

Figure 3.5.5: Positive effects of adding a quenching agent in a proportional counter.

Another effect that introduces nonlinearity in the response of a proportional
counter is the emission of an electron during the process of ion-electron recombina-
tion near the cathode. We saw earlier that due to the slow movement of ions, a space