Answer to Question #209 Submitted to "Ask the Experts"

Q

About the hand-held gamma probes used in surgery: Two kinds of detectors exist, semiconductor (CdTe) and a crystal (NaI). What are the differences between these two detectors and how do they work?

A

The NaI detector consists of an NaI crystal optically mounted to a photomultiplier tube (also called simply phototube). The crystal has the property that incident radiation interacting in the NaI produces sparks of light (scintillations, that's why this detector is also called a scintillation detector). These sparks of light are transmitted into the photomultiplier tube where the light produces electrons; the number of electrons is amplified (multiplied) in the phototube by a factor of a million or more. The signal out of the phototube is transformed, with the help of proper electronics, into a pulse that signifies the incidence of a photon; the pulse is recorded by various means, easily available. The CdTe is a semiconductor material; normally a semiconductor material acts like an insulator. When radiation (e.g., a photon) strikes it, electrons are freed from their electronic states in the atoms of CdTe. The electrons leave behind "holes," i.e., electronic states that are empty. If an electric field is applied, the electrons will move towards the positive voltage; it turns out that the "holes" behave like positive charges and move towards the negative voltage. The charge produced by an incident particle is thus collected with the help of the external voltage (i.e., the electric field), amplified, and recorded. Nicholas Tsoulfanidis, Professor University of Missouri - Rolla