Manual For Front Chambers of the FPP

Mark K.Jones
College of William and Mary, Charles Perdrisat
College of William and Mary, Vina Punjabi Norfolk State University

Revised June 19th, 1996

Contents

List of Figures

List of Tables

Chapter1
Physical Description of Chambers

The front chambers of the focal plane polarimeter (FPP) are straw tube drift chambers. There are two chambers each consisting of three U planes and three V planes which have been attached to opposites sides of a 284.5x124.5 cm aluminum frame (1.8 cm thick). The U and V planes are perpendicular to each other and rotated 45^° from the X (dispersion) direction. Each V plane has 160 straw s. Each U plane has 176 straws. The straws are arranged into modules consisting of 3 planes of 16 straws with the middle layer offset by half the distance between straw centers. Table 1.1 gives the length of each module in the planes

Drawings of the each chamber plane are show in Fig. 1.1 and Fig. 1. The planes are numbered U1,2,3 and V1,2,3 on chamber 1 and U4,5,6 and V4,5,6 on chamber 2. The higher the number the farther away from the VDC the plane is. The chambers are placed in the hadron stack so that the V planes are closest to the VDCs. The number one straw in the plane is the one at the most negative U (V) position. This means that for the V planes straw one is near the bottom of the frame and for the U planes straw one is near the top of the frame. Each module was positioned on the frame by placing the endblocks against 0.125'' diameter dowel pins and then screws were tightened.

The straws are made of a layer of 10 micron aluminum foil and two layers of 10 mil mylar plus heat setting glue. The inner diameter of the straws is 0.522 cm. A brass ferrule is glued to the inside of the straw tube with silver epoxy. A delrin feedthrough is placed in the brass ferrule. The delrin feedthrough has a center hole for a wire to be strung through. The delrin feedthrough also has 3 holes for gas flow into the straw. The wire is LUMA wire which is gold-plated tungsten with a few percent rhenium and has a diameter of 1 mil. The wire is held in place by soldering it to a brass pin which fits snugly into the delrin's center hole while the wire was strung at a tension of 43 g. The brass pin was sawed with a center slot that was 4 mil wide and 30 mil deep that the wire lays in. A rubber gas cap is placed at each end of the straw. The gas cap has a center hole for the pin to pass through and the gas cap fits over the delrin feedthrough and is glued to the brass ferrule. The gas cap has an off-center hole for the input of gas.

In both Fig. 1 and Fig. 1.1 the distance between the center of the active area of the chamber and the center of the nearest dowel pin to first straw in the plane is shown. One can determine the distance in the u or v coordinate to the first straw and these are listed in Table 1.2. In addition the distance in Z (along the particle direction) relative to the middle of the first plane is given in Table 1.2.

The straws are arranged into 3 planes of 16 straws each by inserted the straws into aluminum endblocks. Most of the endblocks were manufacture at JINR, Dubna. The straws were initially glued together in stacks of 13, 12, 12, and 11 and then inserted into the endblocks. Gluing the straws together reduced the sagging of the individual straws. A drawing of an endblock is shown in Fig. 1. The horizontal spacing between the centers of the straws is 1.095 cm and the vertical spacing between rows is .95 cm. The vertical distance between the bottom of the endblock and the center of the first row in 1.43 cm. The endblocks were positioned on the chamber frame by pushing the endblocks against dowel pins and then tightening screws to hold the endblocks in place. The dowel pins were .125 inches in diameter. The chamber frame was also manufactured in JINR, Dubna. The chamber frame is 1.8 cm thick. The endblocks were placed on the chamber frame so that the second layer shifted to the most negative U or V position.

The active area of the chamber is 60.0 cm in the y-direction by 209.0 cm in the x-direction. The center of the active area in the y-direction is 56.0 cm from the HV side of the frame, while the center of the frame is 62.25 cm away. The rail, on which the chamber moves, is 144 cm long. The endpiece of the subframe of the chamber is 129.54 cm wide. To place the center of the active area at the center of the rail the HV edge of the frame should be 13.48 cm from the end of the rail. The readout side of the chamber would be 0.98 cm away from the nearest end of the rail.

Chapter2
Electronics

2.1  Rutgers Low Voltage Power Supply Boxes

The low voltage boxes for the readout boards were designed and built by Rutgers University. Each of the front chambers has two LV boxes associated with them. In the LV boxes there are two power supplies. One box contains +5V and -5V supplies and the other contains two -5V supplies. (For the rear chambers some LV boxes have three supplies). The readout cards need +5 and -5V with the -5V drawing 1.52A per card and the +5V drawing .52A at the maximum. In ordinary usage the boards should draw about 70% of the maximum values. In Table 2.1 list the properties of each power supply and in which section of the chamber they are used.

The power connectors that connects the thick cable to the LV boxes is the same for all LV boxes, but the LV boxes 9 and 10 have a different wiring than LV boxes 7 and 8. This makes it important to connect each the cable to the its correct LV box. Also it is important to have the correct polarity at the readout cards, since the wrong polarity can blow the power capacitors on the cards.

2.2  Voltage distribution at the chamber

2.3  Readout Boards

The threshold for the discriminator is read off a voltage divider on the readout board where the main voltage is set by a outside power supply. The divider lowers the voltage by a ratio 10/1540 and this is the threshold on the discriminator. There is a voltage supply for each chamber. The threshold voltage is split to go in parallel to four sections of the chamber: V plane modules 1-5, V plane modules 6-10, U plane modules 1-5, and U plane modules 6-11. For each section of the chamber the threshold is supplied in series to the modules. There is a fuse before each series of modules. The fuses are 250mA so the maximum that one should set the power supply is 0.25A*(1550/18) = 21.5V as determined by the fuse that goes to modules 6 through 11 on the U plane (in both chambers). With 63 cards connected to one power supply the total current draw will be 0.87 A at 21.5V.

The signals on the readout boards are multiplexed into groups of eight, so that there are two output signals per board. The output of the comparators is feed into shaping circuitry ( the MC10102P chip on the board). The output of the shaping circuitry is a logic pulse which has different width for each of the eight wires in the group. The width of the pulse is determined by a removable resistor which goes between ground and pin 9 on the MC10102P chip. In Table 2.2 the nominal values of the resistance are listed for each wire. The actual resistance used might be different because a capacitor at pin 10 is part of the pulse shaping circuit and had a tolerance of 5%. Sometimes the resistance had to be changed by ±5% from the nominal value to match the pulse width for that wire. To check the widths of all readout boards, they were tested with a pulser. A summary of the results is shown in Table 2.2.

2.4  High Voltage

The high voltage is supplied by Lecroy 1463 HV modules. The HV modules has 24 HV slave outputs which are controlled in groups of 8 by three masters. So only the masters can be set and reset. The individual channels can be tripped but all 8 in the group need to be lowered to reset the HV. The HV modules are in slots 7 and 8 in the Lecroy HV crate. In Table 2.4 is the mapping between the Lecroy 1463 HV output and the chamber section that it controls. Also listed are the nominal voltage to set that section of chamber and the nominal current one expects.

2.5  Level shifter boards