Question

Explain the nature of escape peaks in gamma spectroscopy with a HP Ge detector. How does their efficiency depend on the geometry of the detector? How it is possible to improve the energy resolution of the Ge detector electronically?

Answer 1

Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources.

When an incident gamma with sufficient energy enters the crystal it can create an electron-positron pair. When the positron annihilates, two gammas with equal energy at 0.511 MeV are produced which leave with an angular separation of 180°. For small detectors, it is very probable that both ?1 and ?2 will escape from the detector before they make any further interactions in the crystal. The energy thus absorbed would be E? – 1.02 MeV, that is double escape peak. As the detector size increases, the probability is greater than either ?1 or ?2 will make a photoelectric interaction within the crystal. If one of these gammas does make a photoelectric interaction, the energy of the event that is recorded in the detector is the Single-Escape Peak. For even larger detectors, the probability of photoelectric interactions is even greater when both ?1 and ?2 interact and the total energy of the gamma is absorbed in the crystal.

It is ideal for a detector like this to maximize volume in order to absorb as many gamma-rays as possible. In order to do this, one detector should have as coaxial geometry. There are electrodes connected to contact on the inside of the coaxial and the outside of the coaxial. A potential is applied across the coaxial and a potential is applied across the detector. Due to the geometry of the detector, electron-hole pairs have to travel different distances to get to the electrodes, depending on where they were created. Because this collection time is not constant, the pulse shape is also not constant as well.

Creating the electron-hole pairs is the mechanism by which the semiconductor detects radiation. When a charged particle passes through a semiconductor, electron-hole pairs are created along the path of the charged particle. These electrons allow for the conduction of electricity. In short, this conduction of electricity allows a pulse to be formed. The larger the energy of the incident particle, the more electron-hole pairs are formed, and thus a higher pulse is the result.

High purity germanium, or simply HPGe, detectors are semiconductor detector crystals that are manufactured from ultrapure germanium. The reason that high level of purity in the material is desired has to do with the depletion region. The depletion region is desired to be as large as possible. The depletion equation is

where e is the electronic charge, is the dielectric constant, V is the reverse bias voltage, and N is the net impurity concentration in the bulk semiconductor material. As it follows from the Eq. 6, the lower the impurity concentration, the higher the depletion depth is. Germanium is chosen for the reason that current manufacturing techniques allow for germanium to be refined such that the purity concentration is as low as 1010 atoms/cm^2 . At this level of impurity, a depletion depth of 10 mm can be obtained with a reverse bias voltage of less than 1000V.

So how we can improve energy resolution by inserting impurities(Li etc.) in Ge detector crystals.