Physics and Engineering of Radiation Detection

370 Chapter 6. Scintillation Detectors and Photodetectors

Example:

A photocathode produces a current of 20 nAwhen exposed to light of

wavelength 510nm. Determine the photoelectric current if the wavelength

of light is changed to 475 nmsuch that the incident power remains the

same. Assume that the material has the same quantum efficiency at the two

wavelengths.

Solution:

Equation 6.5.4 for photoelectric current can be written as

Ipe =(QE)

ePγ

hc

λ

= Kλ,

whereλis the wavelength of the incident light and we have lumped together

all the constant terms into one parameterK. This constant can be eliminated

by writing the above equation for the two wavelengths and then dividing one

with the other. Hence the required current is

Ipe, 2 = Ipe, 1

λ 2

λ 1

=20

475

510

=18. 6 nA. (6.5.5)

Fig.6.5.2 shows a typical spectral response curve of a photocathode. Here the
quantum efficiency of the photocathode is plotted against wavelength of incident
light. The interesting thing to note here is that the curve has a plateau where the
variation in efficiency is not very large. As soon as one goes beyond this plateau on
either side, the efficiency decreases rapidly. Therefore to build a good scintillation
detector with a PMT, one should ensure that the spectrum of scintillation light has
a peak somewhere in the middle of this plateau. There are some materials that have
very short plateaus as well and using those in PMTs is generally not a good idea
unless the scintillation spectrum is also narrow and has clearly defined peak that
occurs at or near the peak of the spectral response curve.
As noted above the particular requirements of the system drive the choice of a
photocathode in a PMT. Since there are a host of scintillation materials available
having their unique scintillation spectra, therefore efficient detection of those pho-
tons with a PMT would require the use of photocathode materials with matching
spectral response characteristics. Then there are also applications where the photons
to be detected are not coming from a scintillator. That is, a PMT is a photodetec-
tor that can be used to detect photons no matter from where they are coming. for
example the use of PMTs for detecting Cherenkov photons has recently gotten a lot
of attention in highly sensitive large scale neutrino detectors. In essence, a PMT is a
versatile photodetector that can be used to detect photons in virtually any environ-
ment provided it is equipped with a photocathode having the appropriate spectral
response.