FCAL Backgrounds

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Detector Configuration

  • Geometry as of January 15, 2007
  • Double-Hit Resolution: 75 ns
  • Threshold: 2 MeV (30 MeV in repository)
  • Maximum Number of Hits: 500 (100 in repository)

Items for Discussion

  • Change threshold and maximum number of hits in repository.

The "maximum hits" being modified here is the maximum number of hits per block. Multiple hits in a single block that occur within a certain time interval (currently 75 ns) are merged into a single hit, so to exceed 100 hits in a block is essentially impossible within the time interval of the simulation. An error message appears in the output any time an event hit record is truncated because it hit the maximum hit limit, and this has not occurred in hundreds of millions of events of all kinds that I have simulated so far. So this change will have no effect. [rtj]

  • GlueX note under development to describe mechanism for background suppression in the FCAL. Timing information from the FADC will be key in removing background hits.
  • At what dose does radiation damage begin to become an issue?
    • This source cites this paper and notes a drop in response for TF1 lead glass of e after about a 2 krad dose.

The figure at the bottom of the page (below) shows the degradation in the response of a lead glass block versus run time in the Radphi experiment. The block was located next to the beam hole, with a distance 4 cm from the edge of the block to the beam center. The Radphi calorimeter was located 1 m downstream from the Radphi target, a small block of beryllium 0.08 radiation lengths thick. A square hole of 2 x 2 blocks was removed for the beam hole, around which the 8 blocks showed essentially the same degradation as the one shown. The step at the end was the result of boosting the HV on the phototube, and does not show a recovery due to treatment of any kind. From the point of view of electromagnetic backgrounds, the Radphi photon beam is matched to GlueX running at an intensity of 8 10^{7} tags/s, 80% of full-intensity running. The increased end-point energy in GlueX makes essentially no difference to the background, which is dominated by the low-energy bremsstrahlung component and completely misses the high-energy pairs which are kinematically restricted to lie very near the beam axis. The reduction in the low-energy bremsstrahlung that comes from collimating the coherent beam has already been taken into account in this comparison. These data show that the 1/e lifetime of a block located within the forward 2 degree cone in Radphi was about 800 hours. Mapping this onto the GlueX geometry involves competition between intensity reduction from increased distance (1/d^{2}) and growth from reduced angle (1/\theta ^{3}). Getting this factor right requires a full simulation down to thresholds well below 1 MeV, but the result should be within about a factor of two of the Radphi rate. The Radphi calorimeter was essentially the same detector running in a high-energy bremsstrahlung photon beam, so I consider this estimate to be very reliable at the precision level of a factor of 2, which is probably as precise as we need for planning purposes.[rtj]


The background rates in Hz for each block in the FCAL.
The total radiation dose per second in the FCAL. This calculation assumes uniform deposition of energy throughout the block. This is not actually correct since, for example, the upstream end of the block will receive a higher dose than the downstream end.
The gain factor versus integrated run time for a lead glass block located next to the beam hole in the Radphi experiment.