236 Chapter 4. Liquid Filled Detectors
as therecombination coefficient. Certainly for a parallel plate geometry the electric
field intensity is constant at each point and therefore independent ofx. For cylin-
drical geometry it generally suffices to use the radial component of the electric field
intensity in the above relation.
The recombination coefficient in the above equation is determined experimentally.
Unfortunately not much data is available for liquids that could potentially be used
in radiation detectors. Also, one finds differences in reported values. Nevertheless,
one can get an approximate idea of the recombination by using the reported values
in the above equation (see example below).
Example:
A parallel plate liquid xenon filled ionization chamber is exposed to a flux
of γ-rays, which is producing 5× 103 charge pairs per second midway
between the two electrodes. Compute the number of electrons that survive
the recombination if the applied electric field is 1 kV/cm. Assume the
recombination coefficient to be 100V/cm.Solution:
Since it is a parallel plate chamber, we can assume that the electric field inten-
sity is constant throughout its active volume. We can then simply substitute
the given values in equation 4.3.2 to get the desired result.N =
N 0
1+K/E(x)=5 × 103
1 + 100/ 1000
=4. 5 × 103 charge pairs4.5.B Parasitic Electron Capture and Trapping
One of the major problems associated with using a liquid as the ionizing medium
is the capture of electrons by the impurity molecules. The reader should note that
the capture process is separate from the recombination effect we just studied. The
recombination process involves the fall of an electron back into the conduction band
of the liquid while in the capture process the electron gets captured by an impurity
molecule. Another difference is that the recombination process generally occurs near
the site of the charge pair production. On the other hand the capture process does
not occur with this preference.
The impurity in a liquid filled detector can be of two types. One is the parasitic
capture and trapping impurity and the other is the so called reversible attachment
impurity. Most of the signal deteriorating effects in a detector are caused by the
impurity of the first kind, in which once the electron gets captured, it does not
get re-emitted and hence is said to have been trapped. The process of electron
attachment to a trapping impurity molecule (XY) can be written as
e+(XY)→(XY)−∗. (4.5.1)