Effects of Energetic Protons

High-energy proton dose tests were made at the Harvard Cyclotron Laboratory where an engineering front illuminated CCID-17 was exposed to a well-characterized proton beam with 40 MeV peak energy. Exposures were made at room temperature in ambient air with a lead aperture that allowed irradiation of a controlled section of the image array. Irradiation occurred in incremental steps to a final total of 400 Rads. A minimum 48 hour period at ambient condition elapsed between exposure and all subsequent characterization. While there is evidence to suggest changes in post radiation behavior immediately following exposure with time scales of hours no changes were seen following the 2 day period - all damage features appear permanent and unaffected by subsequent temperature cycles.

As with the X-ray test the most noticeable result of proton beam exposure was the production of an area of enhanced dark current coincident with the exposed region. This dark current could not be measured for temperatures below -90 c. No flickering or hot pixels were detected at any temperature.

Proton damage is expected to produce traps whose presence can be seen by an increase in CTI, and indeed a gradual increase in CTI at a rate of 2 x 10^-5 per kRad was seen when tested with a spatially uniform 25 count/sec/output Fe-55 source. A clocking method was used to create an adjustable line of charge in the image array. At -120 c this sacrificial charge technique only influenced pixels in the next few rows suggesting a detrapping time of order 100 microseconds. At -90 c the CTI is four times greater and the utility of the charge injection method more pronounced although improvements of any performance measure such as CTI or resolution could never be increased beyond levels achieved by normal operation at -120 c. Additional proton irradiation of just the frame store area had no affect upon the measured CTI but did continue to degrade the gain and resolution of the device. This is a consequence of the way in which CTI is measured.

Figure  4.121 shows the observed variation of CTI with proton dose.
 
 

Figure 4.120: Dark current map showing circular area of exposure to 5.9 keV photons. Total dose was 900 rads (Si); this map was obtained with the device at -60C. At nominal operating temperature (-120C) no increase in dark current is measurable. Device quadrants 0-3 are from left to right; the serial register is toward the top of the picture.

 
  Figure 4.121: Charge transfer inefficiency (CTI) vs. accumulated ionizing dose of 40 MeV protons for a front-illuminated ACIS CCD. The curves labelled c0,c1, and c2 pertain to exposed areas of the device. The curve labelled c3 pertains to an area well-shielded from proton radiation.