Dear Photomultiplier Doctor,
I have a 100 µJ laser which operates at 500 nm. How many photoelectrons will this produce from a typical photomultiplier?
yours sincerely
Cedric de Lange
Dear Cedric,
Far too many. We use the Einstein relationship: E = hc/λ, where h is the Planck constant and c is the speed of light, to calculate the energy of one photon at this wavelength.
E = 6.6 x 10-34 x 3 x 108 / (500 x 10-9 )
E = 4 x 10-19 J
100 µJ produces n photons where
n = 10-4 / ( 4 x 10-19 ) = 2.5 x 1014 photons
As the quantum efficiency of the popular photocathodes is around 10% at this wavelength we have 2.5 x 1013 photoelectrons. Note that in 1 - 10 MeV, high light level scintillator applications we seldom get more than about 104 to 105 so your light levels are really high. You need to estimate what proportion of the laser light hits the photomultiplier. If it's anywhere near unity, then please speak to us about a replacement photomultiplier.
Regards,
Photomultiplier Doctor.
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