TO: Distribution
FROM: B. Mecking
SUBJECT: Hall B Upgrade Meeting Notes
DATE: 12-March-1998
MINUTES OF THE 6-MARCH-1998 HALL B UPGRADE MEETING
Present: W. Brooks, V. Burkert, D. Cords, B. Mecking, Jim Mueller,
and S. Stepanyan
This was the first in a series of weekly meetings to discuss upgrade plans for
the Hall B equipment especially in view of the JLab Summer Workshop (June 15-
19, 1998) which will be devoted to a discussion of accelerator and equipment
upgrades. Minutes will be distributed to keep everybody informed, and to
encourage the participation of outside user groups in the development of the
upgrade plans.
Basic assumptions:
- the energy of the accelerator will increase. The JLab management has
presented a long range plan to the Department of Energy that will
result in reaching 12 GeV by the year 2006.
- independent of other detector developments (e.g. Hall D) there will be
a continuing need for the type of capabilities that are available at
CLAS:
o high luminosity operation in an electron beam environment
o good resolution charged particle detection with good particle I.D.
o capability to operate polarized targets in the center of the
detector
- the upgrade program should aim at improving the performance of CLAS in
the present energy range, and make the detector capable of coping with
the detection problems at higher beam energies.
The following specific topics were discussed:
1. Luminosity Increase for CLAS
Design luminosity for electron beam operation in CLAS is 10**34/cm**2/sec,
mainly limited by drift chamber occupancy. Commissioning results indicate
that it will be possible to reach this luminosity. An increase (by factors
of a few) would allow to complete the presently approved program faster
(very important in view of the backlog), and to get access to rare
processes. Important ingredients are
- A better understanding of what particles or processes contribute to the
occupancy of the detector elements. This can be studied with Monte
Carlo simulations with the parameters tuned to reproduce the
experimental results.
- Based on Monte Carlo study one may be able to come up with a better
shielding arrangement for Moller electrons and the associated
secondaries. A little bit of lead at the right place may go a long way
in reducing background.
- Faster drift chamber gas would help reducing the occupancy in the drift
chambers.
- Higher luminosity will require a corresponding improvement in trigger
and/or data acquisition capability.
2. Complete coverage for charged particles
Angular coverage for complete magnetic analysis is limited by the torus
coils. In addition, the Region I mechanical structure and the mini-torus
also make use of the shadow region IN FRONT of the coils. Charge particle
heading in the direction of a coil are presently not detected at all.
Ideally, one would like to determine the directions of the all charged
particles. This information could be used in a kinematical fitting process
to identify the reaction. At the very minimum the information could serve
to veto events with incomplete determination of the final state.
Reasonable goals for coverage are 3 - 130 degrees in Theta, and full
coverage in Phi. While one can think of solutions for photon beams
(silicon, scintillating fibers, gas detectors, ...), the harder problem is
to make such a detector work in an electron scattering environment.
Without magnetic shielding, the detector will have to cope with roughly
10**9 charged particles/sec!. An efficient magnetic shielding arrangement
seems to be a must. Two ideas were kicked around, both providing
longitudinal magnetic field to contain the transverse motion of Moller
electrons
- a thin, small diameter superconducting solenoid surrounding the target
and the beam line
- a (most likely) superconducting Helmholtz-type coil arrangement, with
the coils located upstream and downstream of the target.
What needs to be studied is the necessary field levels and shielding
arrangements to make these solutions work.
3. Cover the dead areas for photons
Same problem as for charge particles. Reasonable goals for coverage are 3
- 45 degrees in Theta (to match the calorimeters). The detector will have
to be very compact since it will have to live right in front of the torus
coils. Ideally, the detector should also give some information on charged
particles, like energy deposition, range, etc. Possible solutions are
- lead scintillating fiber detector with good energy resolution, position
resolution, and longitudinal segmentation
- crystals with photodiode readout
One important problem that needs to be studied is the interplay of this
detector with the calorimeter in the back (energy sharing, detection
efficiency at the edges, etc.).
4. Kaon/pion Separation at high momentum
Presently done by combining momentum and time-of-flight. Will likely
require aerogel Cerenkov counters. Interesting question: could that
detector be put inside (in front of the coils) also?
5. Electron Identification above 2.7 GeV
Presently done by combining energy deposition in the calorimeter (can also
make use of longitudinal and transverse deposition pattern) and Cerenkov
counter information. Interesting question: how important is it to detect
electrons with p>2.7 GeV/c (most of the interest may be in reactions with
high energy transfer)?
Other issues not discussed:
- upgrade to the tagged photon system
- virtual photon tagging.
Next upgrade meeting:
Friday, 13-March-1998 9:30 - 10.50 a.m. (max)
Trailer City Room 84
To encourage a broader participation I will donate my last box of
(fattening) Samoa girl scout cookies.
Distribution: CLAS Collaboration