Presently there are four profile detectors mounted on the annulus. Two hodoscopes (hodoscope "U" or "scint.1" and hodoscope "D" or "scint.2") are located on the upstream side of the annulus at the opposite sides of the beam pipe. Each hodoscope consists of two scintillator planes (plane "A" upstream and plane "B" more downstream). Two Cerenkov profile detectors (there will be four of them in the future) are placed on the back side of the annulus (downstream side). All four (six) profile detectors can be moved remotely along the annulus radius. The annulus can be rotated around the beam axis (presently it has to be done manually). Behind the annulus there are two fixed lead glass detectors, the purpose of which is to measure, in combination with the scintillator hodoscopes, the radial distribution of pion rate.

Scintillator hodoscopes

Each scintillator plane consists of 15 fingers 1.5 cm wide (along annulus radius), 2.0 cm long (asimuthal direction), and 0.7 cm thick (beam direction). Fingers, arranged in two rows in each plane, are slightly overlapping. As the result the total length covered by the fingers is only 21.6 cm. We refer to fingers using numbers, with number 1 corresponding to the finger at the smallest radius.

To increase the granularity of the hodoscopes the two scintillators planes (planes "A" and "B" separated by 30.8 cm) are staggered in the radial direction by 0.75 cm. Thus the effective length of the hodoscopes, defined by overlap of planes "A" and "B", is 20.9 cm. See hodoscope outlay for details. This range is divided in 29 bins (bin width 0.72 cm) selected by requirement of coincidences between appropriate fingers in planes "A" and "B" (see the Table below - note the difference between using planes "A" and "B" in hodoscopes "U" and "D")

Bin numberRel. bin centerCoinc. fingers, Hod. "U" Coinc. fingers, Hod. "D"
1 0.36 cm 1A-1B 1A-1B
2 1.08 cm 2A-1B 1A-2B
3 1.80 cm 2A-2B 2A-2B
4 2.52 cm 3A-2B 2A-3B
5 3.24 cm 3A-3B 3A-3B
6 3.96 cm 4A-3B 3A-4B
... ... ... ...
29 20.52 cm 15A-15B 15A-15B

The smallest radius the active edge of the hodoscope can reach is 15.8 cm (the center of the first bin at R = 16.2 cm, the center of the last bin at R = 36.3 cm).

All 60 photomultipliers in the hodoscopes are powered by a CAEN HV power supply located in the electronics hut (Chassis 2, negative voltages). To reduce number of cables reaching the annulus photomultipliers are arranged in 16 groups (groups of 4 or 3 PM's), each group is connected to one HV channel. To find correspondence between fingers and HV channels refer to the "Cable Map for Scintillator and Cerenkov Detector" diagram prepared by Ross Hicks (channel 0: finger 1A hod. U, ..., channel 15: finger 15B hod. D). The fingers are gain-matched at -1200 V (1.5 MeV left in a finger by a minimum ionizing particle produces signal 300 mV). Unfortunately, it turned out that at this voltage some groups of 4 PM's draw current exceeding the HV power supply limit (3 mA). At the nominal voltage that we are using now (-1120 V) this signal will be 100 - 150 mV, still well above the discriminator threshold (as on Feb. 12 the thresholds are set at -80mV).

Cerenkov detectors

The sensitive part of Cerenkov profile detectors (a quartz block) is 0.5 cm wide (in radial direction), 1.4 cm long (asimuthal 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.). The smallest radius the center of the block can reach is 15.3 cm. The detectors can be moved towards larger radii by a distance of about 46 cm. They are placed at the opposite sides of the beam pipe at the same asimuthal angles as the scintillator hodoscopes. The detector behind the "U" hodoscope is named "Cerenkov 1", the detector behind the "D" hodoscope - "Cerenkov 3".

The HV's for Cerenkov detector PM's are taken from the same HV Chassis 2 that supplies the hodoscopes, however, from a board that delivers positive voltages (channels 48 - 51). The initial HV's are set to +1500 V.

Lead Glass detectors

Each of the two lead glass detectors is made from two SF6 lead glass slabs 22.9 cm (9") long (radial direction), 3.2 cm wide (asimuthal direction) and 6.3 cm thick (beam direction). Two slabs are arranged one behind the other, and separated by a lead filter 2.7 cm thick. In front of the upstream slab there is a lead preradiator 1.2 cm thick. Each slab is viewed by a separate photomultiplier. Both detectors are mounted on the opposite sides of the beam pipe on a stand tilted by an asimuthal angle of 16 deg clockwise when looking downstream of the beam. The lead glass covers radial range from 17.1 cm up to 40.0 cm. Front and back upper (lower) slabs are labeled "LG1"and "LG2" ("LG3" and "LG4"), respectively.

Initially the voltage on four PM's attached to the slabs is set to -1600 V for LG1 and LG3 and -1700 V for LG2 and LG4 (HV Chassis 2, channels 16 - 19)

Each signal from LG1 and LG3 is fanned out to 7 channels of a discriminator through attenuators: 0 dB, 6 dB, 14 dB, 20 dB, 26 dB, 34 dB and 40 dB (attenuations: 1, 2, 5, 10, 20, 50, and 100). Signals from LG2 and LG4 are split to 6 channels only (no attenuator 26 dB). The discriminator threshold is set to -30 mV. Details can be found in the Steve's logbook with electronics diagrams.


During the test run the annulus must be rotated manually. There are marks on its circumference at 15 deg intervals. The positive direction of the rotation is counterclockwise when looking downstream the beam. The angle 0 deg corresponds to the detector radial motion along the vertical line with the hodoscope "U" and Cerenkov 1 being above the beam pipe and the hodoscope "D" Cerenkov 3 being below. At angle -16 deg the hodoscopes and Cerenkov detectors move along radius at the asimuthal angle coinciding with the lead glass detector tilt (it happens as well at 164 deg, however, now the hodoscope "U" and Cerenkov 1 are below the beam pipe). The allowed range of annulus rotation is from -16 deg until +180 deg. The readout point is the upper crossing of the annulus circumference with the circumference of the right lower small wheel supporting the annulus and marked with red arrow (sorry, it sounds rather complicated, ask me if you need clarification). The annulus is locked by two small wooden blocks slipped underneath.

Motion control

The safe range of detector motion is secured by two limit switches set at the smallest and the largest radii detector can travel. The smallest radius is defined by requirement that a detector chassis will not enter the region R < 12.5 cm (the radius of the beam pipe is 12.0 cm). The large radius limit switch marks the position where detectors should be parked when not in use. There is an additional "home" switch used as a reference point. When detector approaches this point moving from larger radii, it stops. The position calibration of the detector is right only if the encoder counter shows at this point 0 counter (+/- 10). If it is not the case, it should be reset to 0. It is important that for this check the detector approaches "home" from larger radii (it accounts for a finite distance at which the switch is activated).

The motion is controlled from LabView panels set in the counting house. Before moving detectors check that the velocity, acceleration and deceleration of motion are set to 100.00 RPM or RPM/s (use the "panel for experts" for this purpose). If these numbers are higher torque used to move the detectors can be too large causing erratic motion of detectors.

Detector position calibration

Hod. "U" (scint.1), position of the center of the first bin:
R(cm) = 16.2 +(counts+18840)/4000

Hod. "D" (scint.2), position of the center of the first bin:
R(cm) = 16.2 +(counts+34671)/4000

Cer. 1, position of the center of the quartz:
R(cm) = 15.3 +(counts+37981)/4000

Cer. 3, position of the center of the quartz:
R(cm) = 15.3 -(counts-43483)/4000 (note different signs!)

Piotr Decowski
SLAC February 7, 2001