DETECTOR CONFIGURATION
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 number | Rel. bin center | Coinc. 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.
Annulus
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
