106
6 Scintillation Counter
the total γ -rays spectrum to be counted (i.e., area under the spectrum covering area
from C to D of Fig. 6.4). For either of these conditions EHT, a suitable amplification
and suitable channels of a pulse height analyzer are to be selected.
6.10.2 β-particulate Radiations
Optimum EHT and optimum amplifications are to be found out to cover the full
β-spectrum (i.e., either C or D of Fig. 5.3) such that the entire spectrum lies within
0–100 V range of the pulse height analyzer.
6.10.3 α-particulate Radiations
Optimum EHT and the optimum amplification are to be found before counting αparticulate radiation at either the photopeak (i.e., at X in Fig. 5.3B) or entire area
of the photopeak (i.e., the area of spectrum between K and L Fig. 5.3A). It may be
noted that with α-counting, normally one does not get an X -ray or Compton scattering
radiation (as obtained with γ -rays). As a result, background counts can be eliminated
very easily with pulse height analyzer, because with pure α-emitters, pulses of αparticle would be much bigger than that pulses produced by the background radiations
(Fig. 5.11).
6.11 Optimum Conditions for Counting
In either of the cases mentioned above, EHT applied to the photomultiplier tube and
amplification factor are set, such that the entire spectrum is within 0–100 V of the
pulse height analyzer limit. No hard and fast rule stands to achieve this setting, it
comes only by practice. Usually, rate meter (which gives an average count rate) is
used instead of scaler. A particular EHT is applied (usually a lower value) and the
entire range of the pulse height is scanned by altering the pulse height gate from 0
to 100 V. Count rate in the rate meter is recorded to check the nature of the spectrum
of radiation (Fig. 6.5).
For example, if the entire spectrum is found to lie within 0–20 V range of the pulse
height analyzer, then EHT is further increased and the entire operation is repeated.
This shifts the spectrum to a higher voltage.
If the rate meter shows low count rate, then amplification factor is further
increased. However, if activity recorded by the rate meter does not show much variation in counts for the entire range of pulse height (i.e., 0–100 V) and the count
rate is very high, then the amplification factor is reduced. By this trial and error
method eventually one gets a suitable EHT and amplification value to get a spectrum
6 Scintillation Counter
the total γ -rays spectrum to be counted (i.e., area under the spectrum covering area
from C to D of Fig. 6.4). For either of these conditions EHT, a suitable amplification
and suitable channels of a pulse height analyzer are to be selected.
6.10.2 β-particulate Radiations
Optimum EHT and optimum amplifications are to be found out to cover the full
β-spectrum (i.e., either C or D of Fig. 5.3) such that the entire spectrum lies within
0–100 V range of the pulse height analyzer.
6.10.3 α-particulate Radiations
Optimum EHT and the optimum amplification are to be found before counting αparticulate radiation at either the photopeak (i.e., at X in Fig. 5.3B) or entire area
of the photopeak (i.e., the area of spectrum between K and L Fig. 5.3A). It may be
noted that with α-counting, normally one does not get an X -ray or Compton scattering
radiation (as obtained with γ -rays). As a result, background counts can be eliminated
very easily with pulse height analyzer, because with pure α-emitters, pulses of αparticle would be much bigger than that pulses produced by the background radiations
(Fig. 5.11).
6.11 Optimum Conditions for Counting
In either of the cases mentioned above, EHT applied to the photomultiplier tube and
amplification factor are set, such that the entire spectrum is within 0–100 V of the
pulse height analyzer limit. No hard and fast rule stands to achieve this setting, it
comes only by practice. Usually, rate meter (which gives an average count rate) is
used instead of scaler. A particular EHT is applied (usually a lower value) and the
entire range of the pulse height is scanned by altering the pulse height gate from 0
to 100 V. Count rate in the rate meter is recorded to check the nature of the spectrum
of radiation (Fig. 6.5).
For example, if the entire spectrum is found to lie within 0–20 V range of the pulse
height analyzer, then EHT is further increased and the entire operation is repeated.
This shifts the spectrum to a higher voltage.
If the rate meter shows low count rate, then amplification factor is further
increased. However, if activity recorded by the rate meter does not show much variation in counts for the entire range of pulse height (i.e., 0–100 V) and the count
rate is very high, then the amplification factor is reduced. By this trial and error
method eventually one gets a suitable EHT and amplification value to get a spectrum