The RHESSI detectors are
diodes, operated in a unique segmented manner (please see our
for more detailed information).
All such detectors are sensitive to radiation damage by high-energy
particles in space.
In RHESSI's low-Earth orbit, the particles are those of the
Van Allen belts,
and the dosage can be readily predicted.
Thus when RHESSI was launched we were aware that the detectors would have
a finite useful lifetime.
Gradually, as damage builds up, the detector resolution and effective volume
Annealing - accomplished by heating the detector to a relatively high
temperature (say 100 C) and allowing it to soak at this elevated temperature
(say for a week) can restore functionality.
The RHESSI team had always planned this but became more confident as the
other Ge detector array, that on the gamma-ray observatory
benefited from routine annealing operations.
Cross-section of one of RHESSI's cylindrical Ge detectors, showing the
general structure of the electric fields in the detector volume.
The Sun would be at the top, and the front segment is that part of the
volume above the dashed line, the rear segment below.
Separate anodes (contacts made in the hollow central region) separately
collect the charge from photon interactions the two segments.
(above) shows the geometry of the RHESSI detectors, along with
the pattern of the electric field within the Ge volume that sweeps out the
charge left by a photon interaction.
Radiation damage to the crystal traps moving charges that are part of
the signal from a gamma-ray detection, resulting in a partial loss of
signal and poorer energy resolution.
It also creates permanent charges
throughout the detector volume, which can distort the imposed electric
field and, when the damage is particularly severe, create dead
volumes within the crystal.
The 2008 RHESSI anneal
After much preparation, the RHESSI team annealed the detectors in
November for the first time, almost six years after launch.
By this time the radiation damage had built up to such a degree that
normal operation, especially for gamma-rays, was rapidly becoming
The operation was worrisome, since the segmentation (see Figure 1) might
have been destroyed by the anneal.
detectors are not segmented and do not have this extra risk
That was the reason to wait so long for the first anneal, and to keep the
temperature up only for a limited period of time (one week at 90 C).
The result is shown in the figure below - a success roughly as expected,
but not without surprises.
The anneal did not restore the full resolution of the detectors, but that is
of minimal importance.
The main thing is that the sensitivity (volume and efficiency) have returned.
Ressponse of the RHESSI rear segments at the 511-keV line of positron
annihilation (a background feature).
Green was the situation before the anneal, and red the situation afterwards.
The black (initial)
and blue (mid-2006) behavior shows that the anneal fully restored the
detectors' effective volume if not the original energy resolution.
The annealing also restored the front segments to good performance, as shown
in Figure 3.
Seven individual detectors are shown as separate curves for three solar
flares observed in 2005 (left), then just before the anneal (middle), and at
present after the anneal (right).
Note the disorder and disagreement among the detectors just before (the
middle panel) and the good agreement afterwards.
Spectra of three solar flares observed during normal operation (left),
just before the anneal (center), and after the anneal (right).
These spectra cover the soft X-ray range with a peak at about 7 keV.
The radiation damage destroyed the agreement among the 9 detectors (central
panel) making analysis very difficult, but the anneal has restored the
With this successful annealing operation, RHESSI is ready for the energetic
flares expected in
, which is just beginning (see our
, in particular its Figure 3).
We can do it again if need be, and in fact expect to do so as radiation
damage again gradually builds up.
David Smith is a RHESSI team member at UC Santa Cruz.