Physics and Engineering of Radiation Detection

374 Chapter 6. Scintillation Detectors and Photodetectors

Example:

ANaIscintillator is bombarded by ionizing radiation that results in the

deposition of 1.5MeV of energy per second. The scintillation photons thus

produced are detected by a PMT. Determine the current at thefirstdynode

of this PMT. Assume that 15% of photons are lost before reaching the

photocathode, which itself has a quantum efficiency of 20%. NaIproduces

about 40,000 photons perMeV.

Solution:

The number of electrons produced by the photocathode per unit time can be

estimated from

Ne=(Ns)(η)(QE),

whereNsrepresents the number of scintillation photons produced per unit

time,ηis the efficiency of the process of transfer of photons to the photocath-

ode, andQEis the quantum efficiency of the photocathode. The number of

photons being produced per second inNaIis

Ns=(40,000)(1.5) = 60, 000 s−^1.

Hence the number of photoelectrons being produced per second is

Ne =(60,000)(0.15)(0.2)

=1, 800 s−^1.

Since each electron carries a unit electrical charge, the current at the first

dynode is given by

I =(1,800)

(

1. 6 × 10 −^19

)

Cs−^1

=2. 9 × 10 −^15 A. (6.5.6)

It is evident that it will be quite a difficult to measure this current of about

0.3fAand also have a good signal-to-noise ratio. The reader should note

that even when we started with a scintillator that had one of the highest light

yields, the current at the first dynode turned out to be extremely small. Of

course in this case the incident photon flux was also very low, which refers to

the operation of the detector in low radiation fields. In high radiation fields

the photoelectric current could be several tens of nanoseconds. In very low

radiation fields, obtaining a measurable requires that the signal multiplication

structure of the PMT has a high enough gain. for example a PMT having a

gain of 10^4 would amplify the 0.3fAsignal to about 3pA, which may still be

too low for reliable measurements.

Every dynode in the electron multiplication structure does not have the same
shape. for example the first dynode is specially designed to maximize the collection
of photoelectrons that are already low in numbers. The structure of other dynodes
depends on their particular locations. The focusing requirements for dynodes are
also very stringent to avoid unnecessary loss of electrons. These requirements include
not only proper designing of the dynodes but also their placement. Due to these