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

[David Lawrence]

Hi,

  This looks important enough that it should be put into an easily accessible wiki page so we can get
to it quickly while on shift. 

-Dave

On Oct 16, 2014, at 10:59 PM, Richard Jones <richard.t.jones at uconn.edu> wrote:

> <commissioning-normfact.gif>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
> <commissioning-normfact.eps>_______________________________________________
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