CDC/FDC ASIC Discussion Meeting, September 3, 2008

This was an ad hoc meeting between Fernando Barbosa (FB), Simon Taylor (ST), and Yves Van Haarlem (YvH) to discuss the FDC/CDC ASIC results.

Minutes

• We discussed the problematic behavior of the shaper gain as a function of CDC wire high voltage (table 2 in YvH's report). The gain depends on the high voltage setting, which suggests that the shape of the pulses is changing as a function of high voltage in a significant manner. The measurements were taken with an Fe-55 source positioned at the far end of a straw with respect to the preamp; nevertheless, for the higher high voltage settings there was evidence for saturation of the preamp, which would certainly distort the signals seen by the shaper (after the cable delay). This might explain the behavior in the shaper gain trend at higher voltage but does not explain the linear decrease for lower voltages. The origin of this behavior needs to be understood. The gain was determined by computing ratios of the averages of an accumulation of pulses before and after the shaper. FB suggested computing this ratio on a pulse-by-pulse basis, making sure that the pulses before the shaper are consistent with the linear range of the ASIC (below 400 fC charge at the input of the ASIC for <5% linearity and below 270 fC for <1%). It was suggested that for the lower voltage settings the noise relative to the signal becomes significant, which could also distort the pulse shapes. The rate plots indicate that for a given threshold setting the efficiency at 1650 V is very small; YvH was not able to see signals until 1700 V. Already at 1800 V there is some evidence for saturation, so the useful voltage range is quite small (100-150 V). Since the goal of the whole exercise is to establish the ASIC gain for the CDC, we don't want the shaper board to lead to a distorted view of the gain of the system. It was suggested that the interposer board be modified to add capacitors before the input of the ASIC to reduce the effective gain before amplification, which would hopefully put the signals being amplified in the "good" region of the ASIC performance.

• Another mysterious thing about the CDC response to the Fe-55 source is the "double hump" behavior in the accumulated pulse height spectra (see figures 4 and 5, for example). Since the gas is mostly (87%) Argon, we expect to see a prominent peak corresponding to the full 5.9 keV x-ray energy and a second much smaller (15%) amplitude peak at about half this energy (the "escape peak"). Instead, the lower peak appears to correspond to too much energy and its amplitude relative to the 5.9 keV peak seems to be too high. ST wondered whether the lemo-T at the output of the shaper being used to split the signal between a trigger arm and a readout arm may somehow be introducing a distortion of the spectra, but it was thought this was unlikely as the observed spectra were confirmed by scope traces taken without the t-piece. Another possibility we considered is the positioning of the source relative to the wire; there was a suggestion that the x-rays from the Fe-55 source were interacting directly with the wire. YvH will move the source to see if this makes a difference.

• Studies of both the CDC and the FDC need to be completed in the very near future. The design of the new version of the ASIC on Mitch Newcomers's end is near completion (~80% level). After we provide the necessary feedback to Mitch, FB estimates that Mitch will need about a month to finalize things before submission to MOSIS. The next available MOSIS submission date that meets this condition is October 27. FB will generate a spec sheet for Mitch specifying the new demands for the low and high gain settings. This spec sheet needs to be submitted to Mitch within 2 weeks.

• The existing CDC prototype suffers from the problem of sagging of the straws. Progress is being made in constructing another (smaller) prototype for the CDC that, among other things, will address this problem. If the straw sags with respect to the wire such that the wire is no longer in the center of the straw, this will cause distortions of the electric field inside the straw and could cause differences in the response depending on where particles pass through the chamber. Could this explain the behavior of the pulse height spectra?

• FB suggested that both the CDC and the FDC prototypes be studied in a beam test. It was also suggested that the CDC pulses be simulated with GARFIELD/HEED for both muons and X-rays, which may illuminate the apparent factor of 6 difference in pulse heights between the Fe-55 source and the cosmic rays.