Here's the latest on how the drift chambers and utility systems are doing;
on recent progress in tracking and on work plans for Oct/Nov.
High voltage: the DCHV program has been working during the September run and
has been used by many of you. It has worked well, but we are planning several
improvements for the start of physics runs in December. The program's
monitoring speed will be increased by using CAEN's channel grouping feature.
The communications have to be made more robust also, for example, by adding
remote resets to the crate controller. We have also experienced more serious
failures of entire HV boards. We have ordered ten new spare boards (we had six),
and are talking to CAEN about these problems.
Low Voltage: we have added remote monitoring of the voltage and currents
from each of the supplies. Operation has been trouble-free.
Gas system: as I reported in September, the gas system has been operating
successfully during the September run but it HAS NOT YET been fully commissioned.
Issues to be reviewed include the overpressure safety sytem (active and passive),
the leak detection safety system (presence of air in return lines), the
calibration of the monitors (temp., press., O2, mixture ratio), the use of
in-line bubblers for Region 1 (they provide visual verification of flow, but
cause pressure spikes in the pump feedback system), evaluation of the leaks in
Region 2, labelling (esp.in the Hall), and the general question of the gas
mixture, including possible bubbling through alcohol. I will schedule a gas
system review for early November. Note also: Howard Fenker has been doing aging
studies on Ar/CO2 (90:10). He sees degradation of pulse heights of ~ 80%/C/cm
without alcohol and ~ 30%/C/cm with it.
ADB/TDC system: the system is largely operational. We are still plagued by
a few cable disconnects. We will add the 11th FASTBUS crate (and 31st ADB crate)
during the downtime in order to instrument the backward angle wires of Region 1.
Chambers: we discussed chamber repair yesterday at our installation meeting.
Region 1 has a number of HV disconnects at the patch panel. We are planning to
remove both the cryotarget and the mini-toroid in order to provide access to fix
these problems.
Region 2 (sectors 5 and 6) have serious gas leaks. We are planning to fix these
leaks during the January down-time.
Region 3 (sector 3) has two (or more) broken wires in addition to a LV
disconnection. We decided to pull the chamber to remove the wires and fix the
LV problem.
On-line monitoring: much progress has been made in establishing programs and
procedures to monitor chamber and tracking functionality on-line (including
educating shift takers).
September run results:
Noise studies- we measured the hit occupancy (dominated by noise) as a function
of discriminator threshold. The problem was compounded by the fact that we have
different kinds of "noise" - scattered hits, "hot spots" (either single wires or
electronic groupings such as 16 wires from one signal cable), "border" noise
grouped along the inner or outer radii of a chamber, and "blasts" of noise in a
single super-layer. We were also concerned that our studies would be biased by
changing beam conditions. To study these effects we did two things: 1) we
varied the threshold for two superlayers from each region, one which was quiet
and the other noisy, and 2) to account for possible changing beam conditions, we
would vary the threshold for one superlayer in a chamber and leave the other
superlayer unchanged.
The results showed that the nominal thresholds (30 mV for Regs. 1,2 and 45 mV
for Reg. 3) were 5 mV above the "knee" in the falling noise for the noisy
regions. The quiet regions seemed to be well above the noise threshold, showing
little rise even when the threshold was lowered by 10 mV. We left the
thresholds at their nominal values. We chose not to lower the thresholds for
the quiet sectors because the noise seems to move around from chamber to chamber
and also because of the inherent simplicity in running all sectors of one region
at a common threshold.
Efficiency Plateau- after setting the discriminator thresholds, we varied the
high voltage and measured the chambers' layer efficiencies in two ways - in one
we compared the number of times a single superlayer track segment had 4, 5 or
6 hits and inferred from this distribution the single layer efficiency, while
in the other a layer was excluded from track finding and fitting and it was then
determined whether the excluded layer had a hit consistent with the found track.
Both methods gave consistent results: the knee of the plateau was within about
50 V of the nominal voltages. We left the chambers at their nominal voltages:
1900, 2100 and 2250 V potential difference for Regs. 1,2 and 3. There are still
questions about the absolute value of the efficiency - in particular, Reg. 1
seems to have a "speckling" of inefficient wires lying along layers. We don't
know what causes this.
Luminosity limits- we studied three aspects of the limitations on drift chamber
performance imposed by increasing luminosity: loss of tracking efficiency due
to increased noise, decreased chamber lifetime caused by increased current, and
practical aspects of keeping the drift chambers operating at high luminosity.
We're still studying the efficiency as a function of luminosity. We found that
both the hit occupancy and the drift chamber current varied approximately
linearly with luminosity as expected. We've had problems, however, in keeping
the high voltage on. The problem does not appear to be a fundamental one of
too high of a current. The high voltage channels drawing the most current
(supplying the intermediate area of Regs. 1,2 and 3) draw about 6 - 7 microamps
at 2.5 * 10^33 luminosity. We thus expect them to draw 25 - 30 microamps at
10^34, which is fine for our 40 microamp per channel HV boards. However, we
experienced a lot of high voltage trips on our first attempts. We'll
try again tonight.
Tracking: lots of work by many people; our resolution is still about a half to
one millimeter. Nikolai and Liming have achieved much better resolution for
single superlayers and a selected data set, showing that the problem is getting
a uniform calibration for all the wires.
regards, Mac