GAS-FILLED DETECTOR
Radiation Detectors
As detector voltage is increased, the electric field has more influence upon electrons produced.
Sufficient voltage causes a cascade effect that releases more electrons from the cathode. Forces
on the electron are greater, and its mean-free path between collisions is reduced at this threshold.
Calculating the change in the capacitor’s charge yields the height of the resulting pulse. Initial
capacitor charge (Q), with an applied voltage (V), and capacitance (C), is given by Equation 6-4.
(6-4)
Q
CV
A change of charge (DQ) is proportional to the change in voltage (DV) and equals the height of
the pulse, as given by Equation 6-5 or 6-6.
(6-5)
DQ
CDV
(6-6)
DV
DQ
C
The total number of electrons collected by the anode determines the change in the charge of the
capacitor (DQ). The change in charge is directly related to the total ionizing events which occur
in the gas. The ion pairs (n) initially formed by the incident radiation attain a great enough
velocity to cause secondary ionization of other atoms or molecules in the gas. The resultant
electrons cause further ionizations. This multiplication of electrons is termed gas amplification.
The gas amplification factor (A) designates the increase in ion pairs when the initial ion pairs
create additional ion pairs. Therefore, the height of the pulse is given by Equation 6-7.
(6-7)
DV
Ane
C
where
DV = pulse height (volts)
A = gas amplification factor
n
= initial ionizing events
e
= charge of the electron (1.602 x 10-19 coulombs)
C = detector capacitance (farads)
IC-06
Page 12
Rev. 0
