LB Thesis DAQ Writeup

The class of detectors used in the photon activation analysis of selenium were high purity germanium detectors. The specific detector used was the GEM40P4 detector from Ortec. This specific detector is cooled with liquid nitrogen to prevent electrons from easily jumping into the conduction band. If the detector is not cooled, then electrons can cross the band gap more easily due to thermal excitations which results in a high level of noise in the detector that will wash out the signal of interest. Once the detector is cold enough, a high voltage can be applied within the detector. For the GEM40P4 detector, the high voltage is set at 4.6kV in a positive bias. After the ionizing radiation passes through the detector, there are 3 main interactions that are of concern. The first interaction is the photoelectric effect. This means that incoming radiation has enough energy to fully liberate an electron creating an electron hole pair. Once the electron hole pair are created within the detector, the electric field created by the applied high voltage will drift the electrons to one terminal and the holes to another which creates a pulse. It should be noted that the size of the pulse is proportional to the energy of the incident photon. This interaction is typically dominant at lower energies.

The second type of gamma-electron interaction we will concern ourselves with is Compton Scattering. This is essentially an elastic collision involving a photon and an electron. As the photon scatters off of the electron, it loses some energy. This basically means that there are photons that can rattle around within the detector. This rattling creates a continuum of energies within the detector. An example is shown below.