Gamma Ray Tracking

When a gamma ray hits a Germanium detector it behaves in many ways. Sometimes it can pass through without interacting, but usually it deposits some or all of its energy in the detector. A gamma ray has mean free path which is proportional to its energy, so for a given energy the larger the detector, the more likely we are to collect all the gamma ray energy. Ideally we will have a photoelectric absorption where all the gamma ray energy is converted to charge carriers in a single location, producing a single charge cloud of electrons and holes. Unfortunately a more likely mechanism is a Compton scatter where the gamma ray deposits part of its energy to create a charge cloud proportional to the energy lost and then scatters at an angle determined by the incident energy and the lost energy. The gamma ray now does one of three things: leaves the detector (depending on scatter angle and the location of the first Compton scatter), goes on to lose all its remaining energy by photo-absorption somewhere in the detector or thirdly loses part of its energy in another Compton scatter where part of the remaining gamma ray energy is absorbed, releasing another charge cloud. Typically a gamma ray deposits energy in 3 or 4 places within a detector by 2 or 3 Compton scatters and a final photo-absorption. At each of these 3 or 4 interaction sites a charge cloud of electrons and holes is created. The charge carriers are transported towards either the cathode or the anode under the influence of the detector's bias voltage. Moving charge is a current and that current induces charge in the anode and the cathode which is proportional to the amount of charge and how fast it is travelling and where it is. The detectors are operated in saturation so that charge carrier velocity is constant although in some areas of closed end coaxial detectors the non-uniform fields make this assumption untrue. (Another mechanism for interactions is for some or all of the gamma ray's energy to be used for pair production which releases an electron and a positron. The positron soon annihilates to release a pair of back to back gamma-ray photons, each with an energy of 511keV. This mechanism only operates at incident gamma ray energies of 1022keV and above, the probability increasing with energy.)

The diagram shows an example of a gamma ray which undergoes 3 Compton scatters at points A, B and C, releasing charge proportional to its energy loss at each point, before losing all its remaining energy at point D by photo-absorption.

Gamma ray tracking is a technique which correlates and reconstructs the multiple interactions of a single gamma ray in a segmented Germanium (Ge) detector or in an array of such detectors. The technique relies on the fact that although points A, B, C and D in the diagram above can be ordered in 24 different ways, only 1 of these sequences will obey the Compton scattering formula relating the energy deposited at A, B and C to the angle between the scattered and the incoming gamma ray. The locations of A,B, C and D are determined based on the segmentation of the Ge detector and pulse shape analysis. The success of the track reconstruction depends on the quality of the positional information. This project aims to investigate the optimal segmentation patterns and number of segments and also the best methods of pulse shape analysis to extract the position.

Gamma ray tracking opens up two new areas of physics instrumentation: firstly improving the correction of Doppler broadening and secondly the design of 4p gamma ray detector arrays (Germanium shells) without the need for escape suppression shields. The construction of Ge shell arrays permits the construction of more efficient arrays than conventional arrays of Ge inside suppression shields. In principle all interactions are accepted whereas for conventional escape suppressed Ge arrays only 20% of hits are accepted. This is because Gamma rays that interact in more than one Ge detector are no longer rejected, but tracked between detectors, allowing the array to be treated as one large very efficient detector accepting all incident gamma rays instead of many less efficient independent detectors rejecting all gamma rays that scatter and escape the detector.

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