MEETING NOTES
Hadronic Rate Estimates for Hall D

author:
Richard Jones

contributors:
David Doughty
Randy Macleod
Curtis Meyer
Elton Smith
Elliot Wolin

June 9, 1998

An informal meeting was held at Jefferson Lab for the purpose of agreeing on what beam conditions should be used as the basis for the section on trigger and rates in the design report. The collaboration has decided to base the design on an initial goal of 10MHz of tagged hadronic events, with allowance for 100MHz after the experiment is underway. What the total hadronic background rate in the detector will be under these conditions depends on the radiator orientation, the configuration of the tagger and collimator, etc. I presented rough information on how these things depend on each other at the meeting, and promised to circulate the information later in a more final form. This note is the result. The essential results are shown in the two following tables. Numbers in red have been changed since the beamline section of the earlier version of the design report.

input parameter	value for this run
beam energy	12 GeV
radiator-collimator distance	80m
collimator diameter (half-angle)	3.4mm (0.5 m/E)
radiator thickness (diamond)	14.7µm
crystal mosaic spread	50µr r.m.s.
beam current	3µA
virtual spot size at collimator	500µm
beta at focus	25m

 

E of peak	N in peak	polarization max (f.w.h.m.)	tag efficiency max (f.w.h.m.)	power on collimator	power on target	total hadronic rate	tagged hadronic rate
8 GeV	185 M/s	0.54 (1140MeV)	0.55 (720MeV)	5.3 W	810 mW	385 K/s	26 K/s
9 GeV	100 M/s	0.41 (900MeV)	0.50 (600MeV)	4.7 W	690 mW	365 K/s	14 K/s
10 GeV	45 M/s	0.27 (600MeV)	0.45 (420MeV)	4.2 W	600 mW	350 K/s	6.3 K/s
11 GeV	15 M/s	0.11 (240MeV)	0.29 (300MeV)	3.8 W	540 mW	345 K/s	2.1 K/s

Note first of all that the total hadronic rate in the detector is insensitive to the orientation of the crystal, and simply reflects the beam current, radiator thickness, and collimator diameter. The number of tagged photons on target, however, is very sensitive to crystal orientation and varies by an order of magnitude as the peak is moved from 8 to 11 GeV. Simply specifying 10 or 10 tagged photons per second does not give an adequate basis for discussing the trigger, because trigger rates depend on the energy of the peak at which that rate is obtained. I suggest that we normalize around the configuration at 9 GeV where the beam current of 3 µA gives 10 in the tagged peak, or 300nA gives 10 per second on target.

The polarization and tagging efficiency both reach their maximum values at the upper edge of the coherent peak. The distance in energy below the peak at which the polarization or tagging efficiency has dropped to half its peak value is listed in the table as f.w.h.m. The way that I obtained my hadronic rate is as follows. First I digitized the total ,p cross section from the HERA Report by that title. It can be seen as a gif file here. I then used a digitizing program to read 50 points off the curve. I then connected the points with a smooth curve and saved it in a histogram. The histogram is available here as a EPS file where the cross section is shown in µb. I then generated coherent bremsstrahlung spectra for each of the peak energies shown in the table above. I then multiplied this function by the total ,p cross section above, times a target thickness factor for 30cm of liquid hydrogen (1.28 protons/b). The resulting total hadonic rate production spectrum can be seen in this EPS file for the 9 GeV point. The tagged portion of that rate, of course, is simply the diffractive ,p cross section times 1.28 times column 2.