[Halld-cal] Photon detection efficiency in the gap region

[Beni Zihlmann]

Hi All,
the network to the outside world is still down but I do not want to wait
any longer to distribute some results on the study of photon detection
efficiency in the gap region between FCAL and BCAL.

Attached is a first version of the report outlining what was done and
conclusions. As soon as the portal and doc-DB is again reachable
it will be posted there.

The main conclusions are as follows:
1) In the current geometry the gap is about 1.5 degree wide and the
     efficiency basically drops to zero around 11 degree. This is the region
     where trajectories from the target have to pass through the BCAL 
readout.

2) The effect of the last FDC package frame can be clearly seen because
      trajectories from the target that otherwise have free path to the FCAL
      completely missing the BCAL AND its readout will path right through
      the thickest part of the last FDC frame. Reducing the efficiency 
by about
      30%.

3) Because of point 2) a geometry was made where the FDC most downstream
     package was moved upstream by 20cm such that trajectories from the 
target
     passing through the FDC frame will also pass through the BCAL 
readout section.
     As the loss in the BCAL readout is already substantial additional 
loss in the FDC
     frame is not essential. As a result the efficiency gap width around 
11 degree was
     reduced from about 1.5 degree to 1 degree.

4) In addition the effect of running the FDC cables downstream was also 
investigated,
     however in less detail. It was found that for the nominal geometry 
this will increase
     the gap width only slightly but deepens the efficiency gap caused 
by the last FDC
     frame  by another 20% absolute( from 60% to 40%).

5) There are two more drops in efficiency around 12 and 16 degree 
labeled as "dip"
      regions. These are caused by the FDC frames 3 and 2 respectively. 
The exact
      location in angle depends on the vertex at the target and the 
depth of the dips
      greatly depend on the photon energy.

6) There is an additional energy dependence in all this and generally 
can be viewed as:
     the lower the energy the lower the efficiency. In addition there is 
a general substantial
     drop in efficiency for energies below 0.5GeV.

sorry for the long mail.
cheers,
Beni


-------------- next part --------------
A non-text attachment was scrubbed...
Name: photon_detection_efficiency.pdf
Type: application/x-itunes-itlp
Size: 633653 bytes
Desc: not available
Url : https://mailman.jlab.org/pipermail/halld-cal/attachments/20110712/f9158d3e/attachment-0001.bin