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           Collimator working group

A draft version of the primary collimator technical report is finished, and is being edited.

           Beamline Design

There are several goals for the design of the beamline. One of these goals is to allow a range of electrons useful to the downstream luminosity monitors to make it downstream without interacting upstream of the lumis. A second goal is to stop those electrons outside of the lumi range with one or two tungsten plugs at the locations of the cleanup collimators (in order to help contain the showers). From the point of view of the lumis, the 2 plug scenario is better because it allows for a larger range of accepted electrons.

Unfortunately, the preliminary beamline designs have yielded unacceptably high photon rates at the Region IIa chamber, and so far we have only considered electrons up to 3 degrees. The rates were calculated for the GEM region, the Region IIa chamber and the cerenkov bar. The GEMs and Region IIa chamber rates are calculated with argon attenuation, and the cerenkov bar rates are for photons which deposit greater than .5 MeV.

For the photons from eps, Richard Jones 1/theta^3 generator was used, and the isotropic generator was used for the mollers. The shower photons are from the primary beam. The geometry of the one and two plug cases is shown in Figures 1a and b respectively.

Figure 1a - Geometry of 1 plug case with tracks (scale x:y is 10:1).	Figure 1b - Geometry of 2 plug case with tracks (scale x:y is 10:1).

Figures 2a and b show the number of electrons per radiation length of lead and the photon attenuation in argon parameterization respectively. Figure 2a is shown as justification for needing at least 20 radiation lengths of lead. Actually, only the number of electrons is shown, and even with 20 radiation lengths you are pretty much back where you started, so we deduce that to contain the whole shower, perhaps even more radiation lengths are needed! Figure 2b shows the piecewise linear parameterization of the attenuation used to calculate the GEMs and Region IIa rates.

Figure 2a - Number of electrons vs. radiation lengths of lead.	Figure 2b - Plot showing piecewise linear argon attenuation parameterization.

Figures 3a and b show the total rates for the one and two plug cases for no shielding, Pb shielding, and W shielding. For some cases the rates go up when shielding is added. The rates at Region II are intolerably high, so we are investigating how to reduce the rates there. The first step is to figure out where the "hot spots" as seen by region II are.

Figure 3a - Log plot of rate vs. detector for 1 plug case, with no, Pb and W shielding.	Figure 3b - Log plot of rate vs. detector for 2 plug case, with no, Pb and W shielding.

Figure 4a shows a tomographic plot of the origin of photons from mollers as seen by Region II. This plot and histograms of vertz_o misled us into thinking that the bulk of the rate was coming thru the acceptance of the upstream cleanup collimator from the target, and then from the plug. However, we believe, and Figure 4b seems to confirm, that those photons which have an origin in the target in fact are being scattered in the first plug and/or the shielding into the Region II acceptance.

Figure 4a - Tomographic plot of origin of 0-3 degree photons hitting Region IIa chamber.	Figure 4b - Radial plot of photons at exit of upstream cleanup collimator.

           Drift Chamber Update

The machine shop has started making the next Region II chamber.