SiPM Radiation Hardness Tests
Although not discussed in the original requirements of the tagger microscope photo-detection, recent measurements have raised radiation hardness (specifically due to neutrons) as a concern for silicon photomultiplier (SiPM) readout. Radiation tests of Hamamatsu and SensL 3x3mm2 units have been started by Dr. Yi Qiang at Jefferson Lab. A similar test is necessary for the Photonique SiPM's currently intended for outfitting the tagger microscope.
To suggest appropriate dose the following considerations are offered:
- since a sensor undergoes annealing (as has already been reported in literature) concurrently with the irradiation, an equilibrium may exist between the two processes or, at the very least, the degradation is slowed.
- much higher dose rates are applied in radiation hardness tests since the time scale of these tests is much smaller than the lifetime of the equipment in the Hall D complex
For these reasons, attempting to match the radiation dose of some nominal lifetime of the equipment based on some nominal neutron background is not important. Instead, a proper measurement of the damage rate, based on the separate performance parameters: dark rate, photo-detection efficiency and gain, as well as the healing rate of these parameters will offer a model for estimating the lifetime of these devices in their intended role. Therefore, doses should be selected roughly based on how much it takes to demonstrate a change in these performance parameters.
- dose rate is quoted per mass irradiated
- the mass of the silicon medium in a SiPM scales with the area (width of the wafers, for our purposes, taken to be roughly the same between various SiPMs)
- dark rate (current) scales with the area of the wafer
it seems therefore that similar doses should be applied to the Photonique SiPM as have been found effective in measurements with Hamamatsu and SensL units.
Proposed Measurement Program
The following procedure uses as a unit of radiation dose the rem based on the neutron dosimeter used in the study by Y. Qiang et.al. referenced above.
- Irradiation with monitoring until the dark rate in the SiPM has increased by a factor 2 over the initial conditions, then allow the SiPM to self-anneal and monitor its recovery.
- Monitoring will consist of full measurements (see below) taken at several points during irradiation, and periodically during annealing.
- The initial period between measurements while irradiating should be 1 rem, to be adjusted after a trend is observed.
- The initial period between measurements while annealing should be one day, to be adjusted after a relaxation time is observed.
- Once the data up to factor-2 dark rate increase and subsequent annealing have been analyzed, proceed with a longer irradiation period until significant curvature is seen in the dark rate vs. dose curve. then repeat the annealing measurements to compare with the initial relaxation rates.
- Irradiate the unit for a much longer duration - until some saturation of the performance parameters is seen. This will help determine the offset in the model of the deterioration of performance parameters.
"Full measurement" above refers to the following:
- collection of an ensemble of pulse integrals in the dark to determine the dark rate and gain
- collection of an ensemble of pulse integrals with a pulser at low setting to determine gain, and photo-detection efficiency
Performing (2) at low enough pulser settings to count photons allows a check of the gain against the result in the dark count analysis. In the worst case, higher intensity light pulses may be used: the efficiency is then determined using the gain determined in (1).