[Halld-offline] MC beam sim rates normalization for commissioning

[Richard Jones]

[image: Inline image 1]Sean,

I have matched the beam simulation spectrum to the calculated
bremsstrahlung spectral rate to compute the beam sim normalization factor
as shown above, assuming:

   - 100nA beam
   - 10 micron Aluminum radiator (1.12e-4 rad.len.)
   - 5mm collimator 75m from radiator

You can rescale this with radiator thickness * beam current for other
settings.

See the attached plot. The red curve is the computed rate incident on the
face of the primary collimator in units of /GeV/s under these conditions.
The histogram is a simulation of 1 million beam events with a special
ntuple configured to measure the spectrum at the primary collimator face,
converted from counts/bin to counts/GeV and scaled up by a factor 975 so
that it matches the theoretical rate curve.

The normalization factor is that each beam sim event subtends 1.025 ns.

Please be careful in how you use this, because in beam sim mode not every
event is written out to hddm. Only events with hits in the detector
actually get written to hddm, and many of the beam photons get stopped in
the primary collimator. So when you normalize you need to use the number of
triggers you actually simulated in the simulation (or the eventNo of the
last event) and not the number of event records that happen to appear in
the hddm output stream.

-Richard Jones
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