6.5. Photodetectors 417
Problems
1.If the light yield of a scintillation material is 25,000 photons perMeV, estimate
its efficiency of producing green scintillation light.2.Estimate the number of luminous centers in aNaI:Tlscintillator if it emits
4 × 104 photons after absorbing 2MeVof energy. Assume its quantum efficiency
of luminous centers to be 0.96.3.Compute the efficiency of aCsI:Tlscintillator if its quantum efficiency of
luminous centers is 0.92 and the efficiency of luminous center activation is
0.85 (note that these two numbers have been arbitrarily assigned and therefore
should not be assumed to represent a real crystal).4.Calculate the minimum tapering angle of a fish-tail type light guide used in air
for a maximum angle of incidence of 75^0. Assume the refractive index of the
light guide to be 1.55.5.Assume that the light guide of previous exercise is immersed in light water.
Calculate the angle above which the total internal reflection inside the guide
will be guaranteed.6.You want to design a photomultiplier tube that could detect photons having
wavelength up to 520nm. Determine the work function of the photocathode
that would be suitable for the PMT.7.A PMT having a gain of 10^5 is exposed to a photon beam of intensity 1. 5 ×
105 s−^1 at a wavelength of 470nm. Compute the maximum anode current and
the voltage at the PMT output assuming that the quantum efficiency of the
photocathode is 60%.8.A photomultiplier detection system having a bandwidth of 100Hzhas a dark
current of 200pA. Determine the relative change in the fluctuations of the
dark current if the system bandwidth is changed to 1kHz.9.In an effort to determine the shot noise due to all sources the output current
of a PMT is first measured in the dark and then by exposing it to a photon
flux. The first measurement yielded a current of 1.6nAwhile the second one
gave a current of 20.5nA. Determine the total shot noise due to all sources.10.Determine the minimum power that can be detected from a PMT having a
background current of 300pAand a dark current of 1.7nA.11.Determine the relative change in the Johnson noise for a PMT when it is cooled
down from room temperature (300K) to 250K.12.A PMT has 12 dynodes, the last 8 of which have individual gains of 5 each.
Assume that the first 4 dynodes produce the same number of secondary elec-
trons, what should be their individual gains if the overall gain of the PMT is
to remain between 10^7 and 10^8.13.A PMT having 10 dynodes with a gain of 6× 106 is used to detect 540nmpho-
tons. Assuming that the incident power is 100nW, compute the photelectric