state of the tpc tests

[Claude Petitjean <claude.petitjean@psi.ch>]

Dear friends,

following Peters discussion with Alexei, I can update you
a little about the state of the tpc tests at PSI:

After a successful finish of formation + test of the tpc,
first in 10 bar, then in 5 bar N_2, we baked the Alu tank
at 120 C and then filled it on March 7 with 10 bar fresh,
ultra pure protium. The tpc was tuned up to 6.5 kV on the
cathodes and an electrical drift field of up to 2 kV/cm
with essentially no problems. It shows on every wire nice
signals from cosmic muons.

Still, there is one problem left, which we have to study
carefully: When bringing in charges with the beta source
through the window, the chamber starts after a hysteresis
period of 10 - 40 minutes to draw a current of about 1 - 3
microamps (depending on the cathode voltage 6 to 6.5 kV).
No extra pulses are visible on the osci and the chamber
shows still everywhere the cosmic ray signals.

To get rid of this current we have to reduce the cathode
voltage to about 4.4 kV. After that we can soon run it
up again with no current.

After careful measurements of the noise levels on each 
wire, Malte found a hot spot on an adode wire (Nr 8 related
with cathodes away from any edges of the frames).

Next week we will continue to study this problem. If we
can cure it, the chamber would be ready to go into the
muE4 area. But because it is'nt cured yet, I dont want
to fix any exact schedule right now.

With best regards

Claude

Peter Kammel wrote:
> 
> Dear friends,
> 
> We had a good discussion with Alexei. We agreed on the
> dates for the Russian visits to work on the ePC (two
> months after April 24).
> 
> As regards Genna Petrov, my preference for the ePC was that he
> comes in July and then can stay there for the run. Alexei
> will discuss with Claude whether Genna is needed also
> for the TPC tests.
> 
> I updated Alexei about the PSI TPC status (with the information
> I got from Claude a few days ago). Alexei was concerned
> that PNPI tests had seen similar rate dependent current
> development and had hoped that this only occurs due to
> contaminations. But now it is a clean chamber! Of course
> it is critical to find out if the current or noise is
> concentrated on some specific spots in the chamber (even
> if it is not large enough to cause observable signals).
> My understanding from Claude is that the path of the current
> is exactly what you are investigating.
> 
> Good luck and have a nice weekend
> 
> Peter