PPACs

The essential elements of each PPAC comprise a thin film anode plus a cathode circuit board. A diagram of an individual PPAC assembly is shown in Figure 5.

  

Figure 5: Assembly of an individual PPAC. The asymmetrically segmented anode and the cathode board with the attached Lexan spacer are shown.

The anode, made of approximately 100 $\mu$g/cm² thick stretched polypropylene with an approximately 20 $\mu$g/cm² aluminum coating, is glued to a 3.2 mm (0.125 in) thick G10 frame. The aluminum coating is segmented into two electrically isolated sections to provide azimuthal angle segmentation, one segment is $9.3^{\circ}$ wide and the other $18.6^{\circ}$wide. The aluminum segments on the stretched polypropylene were made by vacuum deposition with the back of the anode foil cooled by 0.5 torr of helium. The vacuum deposition was performed by clamping the rear of the anode frame against an O-ring on a similar shaped chamber that is mounted inside the vacuum evaporator. Helium was passed through this chamber during the vacuum deposition. The helium cooling was essential to produce coatings with good electrical properties, anode foil coatings made not using helium cooling were found to have dead areas.

A 3.2 mm (0.125 in) thick Lexan spacer separates the anode from the cathode. The spacer is slotted along the sides so the detector gas can circulate to the active region of the PPAC, preventing degradation of the detector performance over time due to radiation damage of the gas. The azimuthal width of the spacer is 28$^{\circ }$, and 30$^{\circ }$ for the anode frame. The spacer has a smaller opening angle to prevent edge effects, where the aluminized polypropylene is glued to the anode frame, from causing an electrical discharge to the cathode. The azimuthal width of the active area of the PPACs (defined by the spacer) is constant, so the total geometric $\phi$ acceptance remains constant at 78% over the entire polar range of the full detector assembly.

The cathode board is a three layer circuit board, shown in Figure 6. The active side (front) of the cathode board, where the particles are detected, is segmented into 1$^{\circ }$ wide traces of constant polar angle $\theta$. Each of the traces is connected to one tap of a delay line which is mounted on the back side of the cathode board. The delay line has a delay of 1 ns per tap and is made of passive delay chips manufactured by Rhombus Industries. The traces on the active side of the board are connected to the delay line on the back of the cathode board by plated-thru holes which are located near the edge of the board. The third (middle) layer is a ground plane located between the delay line side and the active side of the cathode. The ground plane acts to capacitively decouple the traces of the delay line on one side of the board from the sensing traces on the active side of the board. The addition of the ground plane considerably improves the performance of the delay line.

  

Figure 6: Front and back of a cathode board.

The signals from the anode and cathode are carried to the amplifiers by a strip-line transmission line that has four 50 ohm impedance traces on a central plane shielded on either side by ground planes. The PPAC end of the transmission line is forked, allowing two of the traces to connect to the cathode board and two of the traces to the segmented anode. The anode traces on the transmission line also carry the bias voltage ($\sim$400 V) to the PPAC anode. Figure 7 shows a photograph of an anode, a cathode board, transmission line and amplifier board.

  

Figure 7: Photograph (from top down) of individual anode foil, active side of a cathode board with attached transmission line, delay-line side (back) of cathode board, a separate transmission line, and one amplifier board.