Proposal to Unstack the FMS: July 2013-October 2014
We propose to unstack the lead glass in the FMS in order to repair approximately 200 PMT bases
connected to the large cells and approximately 10 small cells. At the same time, the platform would be
altered to move the FMS towards the IR by about 50 cm and make changes to the pumping system to
accommodate a hadron calorimeter behind the current lead glass stack. The unstacking would be
completed in the summer of 2013 and the FMS would remain out of the 2014 run. It would be
restacked at the end of the summer of 2014 in time for run 2015.
1) Work would begin July 2013 by dismantling the FMS enclosures and documenting the current
state of the cells and cabling. Dismantling of the enclosure would take 1 week, followed by an
initial unstacking of ~20-30 cells to assess the scope of repair work more fully.
2) Complete unstacking and stowing of the FMS will take approximately 2 months. The actual
unstacking should be performed by senior personnel, while the handling of the dismounted
pieces will be performed by a team of students from PSU.
3) Stowing of the lead glass which does not require repair must be made in a building with climate
control and access to the outdoors (see task 5, below). We have rejected the STAR IR and
storage building nearby as being unsuitable or unsafe. One consideration is the weight of the
lead glass (~5-10 tons). Currently, Bob Soja is inquiring about a suitable storage area.
4) Stowing of the Lead glass to be repaired must be a place with access to an electronics
laboratory.
5) Using a lab setup in the STAR-BNL electronics area and PSU, we would more fully test individual
channels for performance. It is thought that unacceptable gain variations of more than +/-20%
seen in the large cells are due to failures of three zener diodes on the last 3 dynodes of the PMT
tube base. Current estimates of the repair time for each PMT base is about 2-3 hours. Repair
time for the small cells is estimated at several weeks, mostly due to the time it takes to diagnose
and repair the tube bases. Thus, the repair time for all cells is estimated to be 3-4 months, with
several people devoted to the effort.
6) We will pursue an effort to expose the lead glass to strong sunlight to “bleach out” the f-center
damage incurred by several years of beam exposure. Lab tests will be required to quantify the
effect on the absorption length, possibly at several wavelengths, as a function of light dosage.
We will pursue a method to expose the glass to sunlight without fully unwrapping them, eg, we
will carefully cut the tape which holds the single layer of mylar in place, fold back the flaps,
expose the cell to sunlight, then re-tape the mylar. According to an publication from Fermilab
Exp835, the exposure time should be from 11-24 hours for highly irradiated Lead glass. The
recovery is not perfect, with a loss of transmission on the order of -10% in the UV portion of the
spectrum.
7) Personnel available for this repair work are S.Trentalange, S. Heppelmann and 2 graduate
students and ~5 undergraduates from Penn State University.
8) We will attempt to fix ~20 of the small cells. These missing channels are mainly due to
malfunctioning tube bases. We can attempt to repair these bases or substitute new ones. I
estimate the cost at ~$50/base x 20 bases = $1000.
9) Costs involved are ~1000 zener diodes, plus other discrete components to repair broken PMT
bases. Dremmel tools and extra soldering stations may be required. Estimated cost ~$500
10) A major cost may be mu metal tape to reinstall the magnetic shielding on the FMS enclosure.
We will attempt to make some measurements with a portable hall probe and also investigate
the use of ferrous-based tape as a substitute. Estimated cost $1000.
Total cost $2500.