Physics and Engineering of Radiation Detection

394 Chapter 6. Scintillation Detectors and Photodetectors

anode at all. It is therefore very important that, specially for measurement of low
photon intensities, a PMT with a high electron collection efficiency is chosen.

Emitted Photoelectron

Incident Photon

Time

(a)

Number of Pulses

Pulse Height

(b)

Figure 6.5.21: (a) Digital mode of PMT

operation. The figure shows generation of

pulses when the incident photon rate is low.

(b) Pulse height spectrum of a typical PMT.

X-axis is the pulse heights in a convenient

bin size and y-axis is the number of pulses

with heights lying within the corresponding

bin.

As shown in Fig.6.5.21, although each pulse is generated by a single photon, still
the height of the output pulse is not always the same. The major factor contributing
to this variation is the stochastic nature of the electron multiplication process. Since
the number of secondary electrons from a dynode depends on the incident electrons,
therefore even a small variation in the electron population at early stages can po-
tentially cause large variations in the charge collected by the anode. The fluctuation
in the number of electrons can be due to two factors, one is the inherent statistical
fluctuation in secondary electron emission process and the other is the collection
efficiency of the dynode.
If the count rate is so high that the associated electronics can not differentiate
between subsequent pules, the PMT output is a pulse-overlapped or time integrated
signal (see Fig.6.5.22). In such a situation the counting of individual pulses is not
possible and the output is an average current having typical shot noise characteris-
tics. This is called the analog mode of PMT operation.

Example:

Estimate the peak output current and voltage of the output pulse of a PMT

working in digital mode. Assume the tube gain and the output pulse width

are 10^6 and 16nsrespectively. Take the input impedance of the amplifier to

be 100 Ω.

Solution:

Since the PMT is operating in the photon counting mode therefore the output

pulse is generated by a single photoelectron incident on the first dynode. With

the tube gain of 2× 106 and the elementary electronic charge of 1. 6 × 10 −^19 C,