During the FSU workshop, I offered to produce an estimate of the electromagnetic backgrounds in the Hall D detector. I thought this would be a good way I could contribute because I can build on what I have already done in a similar study for the Radphi detector. Here I outline what I had in mind to do, and let's discuss how that should be modified to best address the background issues for the CDR. Then I will produce the Monte Carlo data and post it in the form of an ntuple, with documentation, from which various groups can extract numbers. As a starting point, I pose the following questions.

1. As a function of z-position inside the solenoid, what is the flux profile of (a) electrons and positrons, and (b) gamma rays vs distance from the beam axis?
2. What is the energy spectrum of this background, as a function of position?
3. What are the major sources of the background?

Although answers to these questions come out roughly in that order, you have to know something about the answer to question 3 in order to get started. From our experience in Radphi, and I guess from common sense, I suggest that the background is almost entirely electromagnetic in origin, that comes from two sources of more or less equal importance: beam halo and target conversions. The Radphi study concentrated on beam halo. In this study we have to consider both sources. I propose the following simple simulation to get things started.

1. GEANT v3.21 simulation with hadronic interactions turned off, that is, with electromagnetic showers as the only photon interactions.
2. Photon beamline consisting of radiator, helium pipe, collimation and sweeping system just upstream of the target, followed by a few meters of air and a 15cm liquid hydrogen target. How thick should the windows on the target be, in radiation lengths?
3. Uniform solenoidal magnetic field present. How many KG?
4. Sample the e+e- and gamma flux at a set of maybe 5 discrete z-planes: upstream end of the target, center of target, downstream end of the target, 1m downstream of target, and exit from solenoid.
5. Some guess for the amount of material close to the target and just downstream, including mounting hardware and cables as much as these can be estimated at the present time. This does not need to be a detailed geometrical description with discrete parts; just a discrete set of planes or cylinders of dummy material with approximately equivalent thickness in radiation length is all that is required. Can someone supply something for starters?

Something simple like this I could complete in two-three days, provided with the inputs listed above. Anything reasonable would suffice for a start, provided that it gets documented. I am willing to do the Monte Carlo. Would you or someone else in the detector group be willing to provide me these inputs in a form that could be posted for documentation purposes, i.e. reasonably self-contained?

Material

1. The standard target I believe we have been discussing for Hall D is 30 cm long.
2. Target Windows: Probably a good starting point are materials used in the Hall B  cryogenic target  (2x170 microns of mylar for photon beam targets and  120 microns of kapton for electron beam target).
3. Uniform solenoidal field
4. For the purpose of triggering it is becoming increasingly clear that we will need a fast detector surrounding the target. We have talked about the possibility of scintillating fibers, for example. In CLAS, we use our "start counter," which is 3 mm of plastic scintillator. Using this technology, thinner scintillator sheets are not practical. The use of round scintillating fibers will require at least two layers for efficiency leading to ~4 mm of material. Additional tracking layers will add even more material. To start simulations, I propose we put in the equivalent of 3 mm of scintillator (0.3 g/cm2) in a  can-type arrangement as suggested by Paul Smith.
5. To all of this we should add some estimate of material required for tracking detectors which are being simulated by Curtis and others.

What/Where to Simulate

1. Obtaining integrated rates should be made easy.
2. Your proposal for various z positions and radial positions from the beam line is probably the best because our detectors will have cylindrical shapes. Another variable of interest is the scattering angle (or angle from center of the target). But since the target is so long, that the angle may not be very useful right around the target.