Size of the pulse from your detector

The magnitude of the pulse from a radiation detector is given by:

Qsignal = Eabs e  G

where Eabs is the energy of the absorbed ionizing radiation, e is the ionization efficiency of the detection material, and G is the gain (if any) of the detector. If the detector is a scintillation detector, then the quantum efficiency of the photodetector must also be factored in.

As an example, suppose you are detecting 0.662 MeV gamma-rays using a thallium doped cesium iodide (CsI(Tl)) scintillator coupled to a photomultiplier tube (PMT). CsI(Tl) has a scintillation efficiency of 65,000 photons per MeV. You refer to the specifications of your PMT and discover that the quantum efficiency of the photocathode in your PMT is 0.15 (15%) at the wavelength of emission for CsI(Tl) scintillators (540 nm). Also, the specifications for the PMT indicate that the PMT gain is 105 when set at the particular operating voltage you are using. The signal magnitude is calculated to be the product of these factors: 6.4 x108 electrons. This result can be multiplied by the conversion factor of 1.6 x10-19 to convert the signal size to Coulombs, which is 1.0 x10-10 Coulombs (100 picoCoulombs).

Another example is the detection of 60 keV gamma-rays in a silicon p-i-n photodiode detector (direct detection with no scintillator). The ionization efficiency of silicon is one electronic charge per 3.6 eV (e =1 / 3.6eV). p-i-n photodiodes have only unity gain, so G=1. These factors combine to form a charge signal of Qsignal = 1.7 x104 electrons, or 2.7 femtoCoulombs.

Scintillation efficiencies for various scintillator materials, as well as ionization efficiencies for a number of semiconductor materials can be found in Knoll GN, Radiation Detection and Measurement, 3rd ed., John Wiley & Sons, Inc. (2000).