Physics and Engineering of Radiation Detection

6.5. Photodetectors 395

I 2

I 1

I 2 I 1

Time

(a)

Number of Pulses

Incident Photon

Emitted Photoelectron

(b)

Pulse Height

>

Output Signal

Figure 6.5.22: (a) Analog mode of PMT op-

eration. If the incident photon intensity is

higher than the resolving power of the read-

out electronics, a pulse-overlapped (or time

integrated) signal is obtained. In this mode

the counting of individual pulses is not pos-

sible. (b) Pulse height spectrum of a typical

PMT at two different incident photon inten-

sities.

this electron multiplies into a total charge on anode of

Qanode =(1. 6 × 10 −^19 )(2× 106 )

=3. 2 × 10 −^13 C.

A pulse of widthtw=16ns, will then produce a peak current of

Ipeakanode =

Qanode

tw

=

3. 2 × 10 −^13

16 × 10 −^9

=20μA.

The corresponding voltage across the amplifier’s input having impedance

Rload= 100 Ω is then given by

Vout = Ianodepeak ×Rload

=(20× 10 −^6 )(100)

=2. 0 mV.

This shows that the typical voltage output of a PMT working in digital mode

is quite small and a good signal-to-noise ratio would require very sensitive and

stable readout electronics.