1.) Chamber:
Two approaches are possible for the Final GEM detector active
area. If the detector is designed to only measure the elastic
electron profile for electrons which are detected by the Quartz
paddle, then the active area would be 8 cm wide x 13 cm high
(theta). If the detector is designed to be placed anywhere
within the Region 1 keepout zone, then the active area should be
at least 12 cm wide x 24.5 high. Both will be in the shape of a
trapezoid. The latter design has the advantage of being placed
anywhere in the Region 1 keep out zone and avoids having
the GEM chamber material within the electron shadow of the
collimator. In other words, some won't worry about have an
electron bounce off the chamber walls and into the detector
acceptance, just about it bouncing off copper, kapton, and ArCo2.
Below are some of the plots used for the above design criteria.
As described previously, the GEM
detector active area was defined as a 23 cm high and 11 cm wide
trapezoidal shape. A CAD drawing made prior to the Qweak tracking
group workshop on 12/06 is shown below. This CAD drawing will
need to be updated to have 12 cm active width instead of 11.
The image below is a composite of the elastic electron shadow at
the limits of the Region 1 keepout zone.
As you can see above, a detector which starts at R=7cm and
extends out to 30 cm which is 12 cm wide will catch all of the
elastically scattered electrons which pass through the Region 1
keepout zone.
As shown below, only about 24 percent of the above electrons actually make it to
the Quartz bar (the acceptance defining collimator is after
Region 1).
If we only want to detect the elastic electrons which make it to
the quartz then an 8 cm x 13 cm active detector area would
suffice, see below.
Below is the present conceptual design for Region 1 tracking.
The VFAT cards and ionization chamber frames are being
designed to fit behind the collimator without extending
into adjacent octants. Even numbered annular strips will be read out
on one side and odd numbered strips on the opposite side in
order to have plenty of space through the smallest 4 cm gap
between octants. The strips are 5 cm wide by the time they
make it to the two innermost readout cards. The 3 voltage
lines for the GEM amplifiers and the line for the cathode
will be moved to go around the top where there is plenty of
space for ionization chamber infrastructure.
2.)Readout electronics
Tony returned from CERN with a VFAT readout board. It is 5cm x 4 cm
in size and outputs
digital yes/no hits for 128 GEM channels in
LVDS format. It has an I2C programmable interface. I have
ordered a USB to I2C adapter and plan on using a laptop USB port
to program the chip for readout. There are a few VME-FPGA
interface cards out there which may be suitable for our
application. The concept would be to ave a ribbon cable connect
carry control and data signals directly to a VME card for DAQ.
3.) rotator
The minimum inner radius for the detector rotation hardware should be 30 cm, based on the images used for the final GEM detector design. I believe LaTech can begin ordering a new ring after the specifications are agreed upon at the next Qweak collaboration meeting. At present the brackets for the ring start at R=24.5 cm and the unislides start at R=23.3 cm. These need to start beyond R=30 cm. The ring radius must increase by at least 7 cm.
