During March two additional Cerenkov detectors were added to the annulus. It increased number of diagnostic detectors to the final number of six: two hodoscopes (hodoscope "U" or "scint.1" and hodoscope "D" or "scint.2) and four Cerenkov profile detectors. Positions of all detectors as well as of the annulus are now controlled remotely from the counting house and are incorporated in the DAQ data stream.

Modifications of scintillator hodoscopes

Tungsten preradiators were installed in both downstream hodoscope planes (planes "UB" and "DB"). Placed in front of the scintillators they cover area of 2 cm * 23 cm (matching the scintillator finger azimuthal length of 2 cm) and are 1.5 cm thick (about 5 radiation lengths).

See the detailed drawing of the hodoscope box to learn about exact scintillator box dimensions, placement of scintillator fingers, etc. (dimensions are in mm). The tungsten preradiator is not shown in this drawing.

The arrangement of the four scintillator boxes can be seen in this figure . We refer to individual fingers using their numbers starting at 1 for the finger closest to the beam.

Modifications of Cerenkov detectors

The sensitive part of Cerenkov profile detectors (the quartz block) is 0.5 cm wide (in radial direction), 1.4 cm long (azimuthal direction) and 0.7 cm thick (beam direction) (actually it is a 2.0 cm * 0.5 cm * 0.5 cm quartz block tilted by 45 deg.). See details in the drawing (dimensions in inches). Two new Cerenkov detectors (Cer 2 and Cer 4) are equipped with tungsten preradiators 0.5 cm thick in the radial direction (matching thickness of the quartz blocks) and about 1.5 cm thick in the beam direction. The preradiators are attached directly in front of quartz blocks. The shape and dimensions in mm of the preradiator are shown in this figure and its placement in the tube relative to the quartz block and the conical mirror in the next figure. Cer 1 and Cer 3 (without preradiators) are evacuated (pressure about 0.6 Torr) to prevent production of Cerenkov light in air in the conical mirror volume because it could significantly contribute to the light produced in the quartz block (up to about 20%). Preradiators in the Cer 2 and Cer 4 enhance light production in quartz blocks so much that the air contribution is negligible (however, probably these detecors won't be suitable for high electron flux measurements because of the PM saturation).

Cer 1 detector is mounted at the same azimuthal angle as hodoscope "U", Cer 2 at angle 90 deg clockwise when looking downstream the beam, Cer 3 - 180 deg (same as hodoscope "D"), Cer 4 - 270 deg.

Annulus calibration

The position of the annulus center (determined by two crossing strings spanned along two different wheel diameter orientations) at different wheel angles had been surveyed using a scope. The results averaged over three sets of measurements (error bars represent standard deviations of mean) are shown in the following two plots: plot 1, and plot 2. In x as well as in y the deviation of the center from the anticipated beam axis does not exceed 0.5 mm.

The angle of the wheel had been calibrated in terms of the encoder counts:
Theta (deg) = 176.82 - 0.13125e-3 * counts
The edge of the HOME switch is at 176.82 deg. It means that when wheel approaches this angle from smaller angle values (wheel moves counter clockwise), at the HOME position the encoder should show value of 0 (+-100). If this is not the case, at this position the encoder should be reset to 0. At angle 0 deg hodoscope "U" (scint 1) moves along vertical radius above the beam pipe, and hodoscope "D" (scint 2) moves along vertical radius below the beam pipe. Angle increases when the annulus moves counter clockwise when looking at it downstream the beam.

Calibration of detector positions

Radial positions of the profile detectors were measured using a scope with detectors placed at the HOME positions. Radius was determined in respect to the fixed point in space coinciding with position of the center of the wheel rotated to angle 90 deg. Knowing that 4000 encoder counts correspond to 1 cm displacement in the linear drive, these measurements yield calibration relations. For the Cerenkov detectors they are (positions of the center of the quartz block):
Cer 1:
R (cm) = 24.81 + counts/4000
Cer 2:
R (cm) = 25.64 - counts/4000
Cer 3:
R (cm) = 26.12 - counts/4000
Cer 4:
R (cm) = 24.63 - counts/4000
It turned out, however, that the supports of quite heavy scintillator boxes are not strong enough to prevent the hodoscopes from gravitational sagging. To account for it, the HOME radial positions of all four boxes were measured at different angular position of the wheel (0, 45, 90, 135, and 180 deg). The results together with lines representing fitted analytical dependence are presented in the figures: hod. U and hod.D. In these figures R's describe positions of the small radius edge of the box. The R's of the small radius edge of the first fingers are described then by the following formulas:
Hodoscope plane UA:
R (cm) = 20.30 - 0.4756*cos(theta) + counts/4000
Hodoscope plane UB:
R (cm) = 20.49 - 0.2452*cos(theta) + counts/4000
Hodoscope plane DA:
R (cm) = 24.39 + 0.3497*cos(theta) + counts/4000
Hodoscope plane DB:
R (cm) = 23.46 + 0.1242*cos(theta) + counts/4000

Piotr Decowski
SLAC April, 2001