Physics and Engineering of Radiation Detection

4.5. Sources of Error in Liquid Filled Ionizing Detectors 235

This can also introduce nonlinearity in the detector’s response with respect to vary-
ing radiation levels. However with the introduction of high electric field across the
electrodes, the recombination probability can be significantly decreased.
Most of the recombination occurs near the radiation track in the detector, where
the charge pairs are produced. The electrons in a liquid quickly thermalize near
their production point and in the absence of an electric field fall back to the valence
band under the influence of the Coulomb field. This type of recombination is gen-
erally known asgeminaterecombination and is the major source of error in liquid
filled detectors. The high rate of thermalization is due to the inelastic scattering of
electrons near their point of generation. In the absence of any external electric field,
the thermalization time could be as short as a fraction of a picosecond for molecular
liquids at room temperature. This is a serious problem since it can lead to the loss of
information and has the potential of introducing nonlinearity in detector’s response.
Liquids having such short recombination times are therefore not suitable for use as
detection media. Fortunately, liquefied noble gases have recombination times on the
order of a few hundred picoseconds, which make them suitable for use in radiation
detectors. Table 4.5.1 lists the electron thermalization times in some liquefied gases
commonly used in radiation detectors.

Table 4.5.1: Electron thermalization times in liquefied noble gases (26; 24; 3).

Liquid Temp erature (K) Thermalization Time (ns)

Helium-4 4.2 0. 4 × 10 −^3

Argon 85 0. 9 ± 0. 2

Krypton 117 4. 4 ± 0. 2

Xenon 163 6. 5 ± 0. 5

The geminate recombination just described is not the only type of recombination
that occurs in liquids. Apart from this localized recombination, there is also volume
recombination effect that can happen anywhere in the liquid volume. Fortunately the
probability of volume recombination is much lower than the geminate recombination
simply because of the low concentration of ions away from the particle track. Also,
under the influence of the applied electric field, the electrons move towards anode
and the ions move towards cathode, which does not leave many ions or recombination
centers for electrons to recombine with.
It was mentioned earlier that the number of electrons that survive the recombi-
nation can be calculated from (see equation 4.3.2)

N=

N 0

1+K/E(x)

,

whereN 0 is the number of electrons produced by the incident radiation,E(x)is
the electric field intensity at positionxin the detector, andKis a constant known