[Halld-tagger] Photon flux calibration

[Alexander Somov]

Hi,

Here are some thoughts; most of them have already been
discussed.

It's important to check a PS acceptance sensitivity to
the beam profile at the converter (I guess/hope that the
effect is going to be relatively small).

Once the PS acceptance is determined we can use it for the
lumi determination and the tagger efficiency monitoring. The
PS acceptance can be calibrated in special runs.

One way to calibrate the PS acceptance is to use TAC counters
and the tagger energy. We can probably use a thin radiator and
large beam current; we don't care about tagging efficiency
at this point unless the beam profile remains the same. The
PS rate at small current is critical. If I remember correctly
(have to check), the PS coincidence rate is  about 40 kHz  for
the 10-3 converter for high-lumi runs ( 2.2 um current)
For 10^3 times smaller current the rate is going to be
about 40 Hz (we acquire ~24k events in 10 min of running,
ideally we have to bin events into a few energy bins).

Michael, regarding storing the PS/tagger info, we can have a
special trigger type and readout only the PS and Tagger crates
only. Reading out a few hits only is not an issue at 100 KHz rate
(these triggers can be prescaled as well).

Cheers,
        Sascha