From RHESSI Wiki
|1st Author:||Steven Christe|
|2nd Author:||Albert Shih|
|Published:||28 January 2006|
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At RHESSI's heart are its nine hyperpure germanium detectors. Space is not only a harsh environment for living organisms but also for radiation sensors (for mostly the same reasons). High-energy charged particles zip along at speeds close to the speed of light and can deposit a lot of energy into materials, causing radiation damage as described below. These particles can come from a variety of sources: from the Sun in the form of solar energetic particles (SEPs...as discussed in a ), from the radiation belts, or from the all-pervading cosmic rays. As previously mentioned RHESSI is only lightly shielded, which makes it more susceptible to radiation damage. RHESSI's orbit takes it through the South Atlantic Anomaly where particle fluxes are very high, not to mention particle events which frequently hit RHESSI. All of these energetic particles can interact with RHESSI's germanium detectors and damage them.
To refresh everyone's memory, energy deposited (say, by a photon) in a semiconductor creates electron-hole pairs. These charges are then collected at opposite contacts through the use of a large voltage applied across the active volume. High energy radiation can create large disordered regions (on the order of 100 angstroms in size) in the germanium crystal, and these regions tend to accumulate negative charge. As a result, these regions act as hole traps and prevent holes from reaching the cathode. If a hole is trapped, then only a fraction of the energy of the incident photon is collected. This leads to tailing and a decrease in energy resolution.
Another effect of radiation damage (if allowed to go on for a while) is a loss of sensitivity. As the detector becomes more damaged, it becomes difficult to fully deplete the detector, and thus there is a reduction of effective area as there is less active volume. Both of these effects can be seen in RHESSI (see below)
Figure 1. A depiction of the radiation-damaged lineshape in one of the RHESSI detectors for observing a monoenergetic 1-MeV photon in the years since RHESSI's launch.
Figure 2. A comparison between the detection of the background line at 1370 keV between launch and four years afterwards, in the rear segments of each of the nine detectors. Note the line broadening due to radiation damage, and the lack of appreciable active volume in many detectors. As an aside, detector 2 did not have a function rear segment at launch.
Fortunately, RHESSI is not lost! One can repair radiation damage through the procedure known as annealing. The detectors are heated (to approximately 100 degrees celsius) for a certain period of time (at least a week) and the damage can be fixed! This procedure increases the mobility of the germanium atoms sufficiently to break up the previously mentioned large disordered regions into smaller clusters that no longer trap holes. Unfortunately, there is additional concern with annealing the RHESSI detectors compared to typical germanium detectors. RHESSI's detectors are each split into two segments, and the contacts for these segments are created by doping the inner surface with lithium. The contacts are normally separated by a region without any doping, but annealing the detector can cause the lithium to diffuse to the point where the contacts touch. In that event, the two segments will fuse into one.
Test annealing is currently being done down here at SSL on a spare RHESSI detector to determine the how long we can anneal the detectors before they desegment. If the RHESSI detectors become single segments, then their sensitivity to gamma-rays (from solar or cosmic origins) becomes reduced, although probably still better than their present sensitivity. We hope to be able to anneal the RHESSI detectors soon and make them new and fresh again!
Biographical note: Both Steven Christe and Albert Shih are currently graduate students at U.C. Berkeley.