Hall was open until around 17:00 to allow snakes to be filled and to look
at raw signals from wire chambers.
Wire target which has been giving some problems in the past few days was
fixed last night again but so far has not been tested.
Analysis of previous day's running show that the wire chambers are working
reasonably well (efficient and good signal to background) when first turned
on. After several hours with beam however the signal to background ratio
gets worse for the inner chamber and later for the middle chamber and
eventually for the outer chamber. Different thresholds or operating
voltages do not help significantly though lower operating voltages have
better S/B but are inefficient. Supposition is that problem is due to
space charge or field emission causing noise on the wires. Radiation from
the beam, injection, or higher density of tracks increases the problem and
thus affects nearer chambers first.
This is similar to the problem seen with helium:ethane gas mixture but does
not develop as quickly. With the helium:ethane mixture improvement was
seen by changing the resistor chain in the electronics boxes and by
increasing the ratio of ethane in the gas mixture.
To investigate the current situation further the resistors in boxes 0, 2,
4, and 6 of the left sector have been changed again. Specifically the
resistors to ground removed causing all signal to go directly to amp/disc
card. This means these boxes can be run at lower gas gains for efficient
operation and possibly reduce the ionization from the avalanches. As a
second test 1/16" lead foil was placed over the entrance window of the
right sector to shield the chamber from X-rays, Moellers, etc. and reduce
the incident radiation to the right sector.
Plan for this evening is to determine the operating voltages for boxes L0,
L2, L4, and L6 and then run the wire chamber for as long as possible to see
if and where the space charging problem appears. If neither of these
scenarios yield fruitful information we will increase the iso-butane ratio
to 70:30 which will also allow the chambers to be operated at a lower
voltage.
More information than you wanted right ! To bad, nothing else to due while
we are waiting for the beam.
Cheers,
Douglas
26-415 M.I.T. Tel: +1 (617) 258-7199
77 Massachusetts Avenue Fax: +1 (617) 258-5440
Cambridge, MA 02139, USA E-mail: hasell@mit.edu
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