When a gamma ray arrives in a semiconductor detector it usually deposits its energy in several different places, often scattering out of the detector leaving only part of its energy. The traditional way to deal with this partial energy collection was to surround the detector with an escape suppression shield so that incomplete energy measurement was detected and the partial measurement thrown away. This approach is very inefficient, typically throwing away 4 out of every 5 incoming 1MeV gamma rays.
A much better approach is to detect every gamma ray interaction point in the detector and in neighbouring detectors so that the whole path of a gamma ray can be tracked and used to measure not just the energy, but also the direction from which the original gamma ray came. This approach is known as gamma ray tracking (GRT) and it has led to big changes in nuclear physics instrumentation over the last 10 years.
GRT relies on 2 technologies - segmentation of detector contacts and digital signal processing. It works by subdividing the detector physically by segmenting the contacts and then subdividing each segment again electronically by digital signal processing. Each time the gamma ray interacts in the detector to release charge carriers, the interaction point can be detected along with the amount of energy deposited. Interactions follow certain well known rules which relate the scattering angle between points to deposited energy (the Compton Scattering formula and the Klein-Nishina formula). GRT reconstructs gamma ray paths by looking for groups of coincident interaction points which obey the rules for scattering.
Gamma ray spectroscopy in projects like AGATA benefits hugely from GRT because a 4-pi sphere can be instrumented with highly sensitive gamma-ray detectors without the need to waste space surrounding them with escape suppression shields. With GRT many more of the incident gamma rays are utilised because it is no longer necessary to throw away 80% of them due to incomplete energy deposition. The ability to find where the gamma-ray comes from also permits Doppler correction of measured energies from fast moving nuclei.
In the 1990's EPSRC awarded a grant to Daresbury with universities of Liverpool and Surrey to develop underlying technologies needed for GRT (segmented detectors and digital signal processing). Information about that work can be found on the original GRT project web pages (link opens in a new window).
The elements which make a GRT system are the same as those needed for a Compton camera which has several applications described in the Knowledge Exchange pages and also on the PORGAMRAYS page.
For more information about GRT work in the NPG please contact us.