Figure 4: A typical spectrum from a good VLPC pixel at T=6.5 K
and U=6.5 V.
Figure 5: Spectrum showing a failed (`latched') VLPC pixel;
T=6.5 K and U=6.5 V.
Figure 6: Spectrum showing IR smearing from a VLPC pixel at
T=6.5 K and U=6.5 V.
Figures 4,5 and 6 show the three basic types of spectra that were observed. Figure 4 shows a typical good spectrum. There is clean separation between the pedestal, first and second photoelectron peaks and often one sees structure for the third, fourth, or even fifth photoelectron peak. (Note that the light level was set to peak at 2 photoelectrons at this temperature and bias.) Figure 5 shows a typical bad channel. Such pixels were observed to have very noisy oscilloscope signals and drew ten times more current than pixels that operated properly. Interestingly, some of these pixels seemed to radiate light to adjacent pixels on the same chip, causing the photopeaks to smear. This high-current effect has also been observed in SSPM's, of which we characterized fifty in the same apparatus, and there it has been shown that the effect was due to latch-up of the devices. In the SSPM, one could restore proper operation by lowering the bias to 1 or 2 volts. Further testing of the VLPC's revealed that they did not exhibit this toggling effect, but were, in fact, locked into the high-curent state and could not be restored to normal operation. Diasabling these latched pixels, by disconnecting their bias line, restores proper operation of their neighbouring channels. Figure 6 shows a spectrum marred by an IR light leak. These signals show the proper number of photoelectrons, but there is little to no separation between the individual photoelectron peaks. Electrically noisy but non-latched pixels show a similar type of spectrum, as do neighbours of photon-radiating latched pixels. One should note that for applications like the D0 fiber tracker, where each signal typically contains 10 photoelectrons, single photoelectron separation is not important. Thus, such pixels with intrinsically bad single photon resolution are useable as long as their noise rate is not excessively high.