5.6 Nature of Pulses Produced in Ionization Chamber
63
to sort out the individual pulses (of specific height) and count their numbers. If αparticles of a specific radioactive material are measured, then the height of square
pulses would be of the same height. If pulse height is calibrated in terms of MeV, then
a measure of pulse height gives the information regarding energy of the α-particles.
The number of such pulses recorded per unit time will give information about the
number of α-particles recorded by the instrument per unit time (Fig. 5.5B). If these
pulses are recorded on a strip chart recorder, a spectrum as shown in Fig. 5.3B will
be observed. On the other hand, if β-particles are recorded, they will create square
pulses of varied heights (Figs. 5.3D and 5.5A), and if instrument is connected to
the strip chart recorder, a spectrum like Fig. 5.3C will be observed. The instrument,
which can sort out these pulses according to their heights and keep an account of the
number of pulses per unit time of each pulse height, is called pulse height analyzer.
Cost of pulse height analyzer depends on the accuracy with which it can differentiate
one pulse height from the other. If we can scale the pulse height in the range of 0–
100, then the pulse height analyzer may divide this range into 100 channels (accuracy
becomes 1 volt) or more (to make the accuracy for differentiating the pulses by less
than one volt).
5.6.3 Relationship Between Energy of Radiation and Pulse
Height
Based on the concept of pulse height analyzer, as discussed in the previous section and
its application to current–voltage characteristic obtained with the ionization chamber
(Fig. 5.2), the following conclusions can be drawn:
• The pulse height produced in an ionization chamber would be very small, in the
order of micro to millivolts (up to B in Fig. 5.2). These pulses will have to be
amplified to a pulse height of 1 volt before they can be registered by a suitable
electronic circuit. Hence, a counter working in this region needs a sophisticated
amplifier. However, counters operating up to this region can differentiate pulses
produced by α- and β-particles.
• In the proportional counter, secondary ionization occurs; hence, pulse height initially formed due to primary ion-pairs are amplified to a large extent (region C
of Fig. 5.2) than in an ionization chamber (i.e., region B). Therefore, though a
proportional counter will also need an amplifier to amplify the pulses to 1 volt,
a relatively less sophisticated amplifier is needed as pulse height is normally of
millivolt height. However, counter working in this region will also be able to differentiate the pulses produced by α-particles and β-particles and pulses produced
by other undesirable processes or background radiation. These latter pulses are
called noise pulses.
• The pulses produced in Geiger region (region E) are amplified to such an extent
that they lose their originality, and all pulses produced due to α-particles or βparticles become identical in pulse height (in the order of 1 volt). Therefore, a
63
to sort out the individual pulses (of specific height) and count their numbers. If αparticles of a specific radioactive material are measured, then the height of square
pulses would be of the same height. If pulse height is calibrated in terms of MeV, then
a measure of pulse height gives the information regarding energy of the α-particles.
The number of such pulses recorded per unit time will give information about the
number of α-particles recorded by the instrument per unit time (Fig. 5.5B). If these
pulses are recorded on a strip chart recorder, a spectrum as shown in Fig. 5.3B will
be observed. On the other hand, if β-particles are recorded, they will create square
pulses of varied heights (Figs. 5.3D and 5.5A), and if instrument is connected to
the strip chart recorder, a spectrum like Fig. 5.3C will be observed. The instrument,
which can sort out these pulses according to their heights and keep an account of the
number of pulses per unit time of each pulse height, is called pulse height analyzer.
Cost of pulse height analyzer depends on the accuracy with which it can differentiate
one pulse height from the other. If we can scale the pulse height in the range of 0–
100, then the pulse height analyzer may divide this range into 100 channels (accuracy
becomes 1 volt) or more (to make the accuracy for differentiating the pulses by less
than one volt).
5.6.3 Relationship Between Energy of Radiation and Pulse
Height
Based on the concept of pulse height analyzer, as discussed in the previous section and
its application to current–voltage characteristic obtained with the ionization chamber
(Fig. 5.2), the following conclusions can be drawn:
• The pulse height produced in an ionization chamber would be very small, in the
order of micro to millivolts (up to B in Fig. 5.2). These pulses will have to be
amplified to a pulse height of 1 volt before they can be registered by a suitable
electronic circuit. Hence, a counter working in this region needs a sophisticated
amplifier. However, counters operating up to this region can differentiate pulses
produced by α- and β-particles.
• In the proportional counter, secondary ionization occurs; hence, pulse height initially formed due to primary ion-pairs are amplified to a large extent (region C
of Fig. 5.2) than in an ionization chamber (i.e., region B). Therefore, though a
proportional counter will also need an amplifier to amplify the pulses to 1 volt,
a relatively less sophisticated amplifier is needed as pulse height is normally of
millivolt height. However, counter working in this region will also be able to differentiate the pulses produced by α-particles and β-particles and pulses produced
by other undesirable processes or background radiation. These latter pulses are
called noise pulses.
• The pulses produced in Geiger region (region E) are amplified to such an extent
that they lose their originality, and all pulses produced due to α-particles or βparticles become identical in pulse height (in the order of 1 volt). Therefore, a