Position Sensitive Avalanche Counter (PSAC)

Position Sensitive Avalanche Counter (PSAC)
Located at the focal plane of the recoil mass spectrometer, this detector provides mass identification of the spacially separated recoil products. Although still available, our standard detector is now a large area microchannel plate (MCP) detector.

The PSAC has a 36x10 cm2 active area and consists of four or five wire planes constructed from 0.32 cm. FR4 printed circuit board. The boards are stacked as closely as possible in order to provide a more uniform electric field and to minimize the gas volume. Approximately -200 volts are placed on the first and last wire planes and the middle wire plane (anode) voltage adjusted to provide adequate signal gain. This voltage is typically greater than +300 V (+400 V for alpha particles). The second and fourth wire planes are operated at ground potential with each wire leading to a delay line which converts position to time differences. This arrangement provides a uniform electric field across both position planes.

All wires are 20 micron gold--plated tungsten separated by 1 mm with every two wires tied together. Isobutane gas is constantly circulated through the chamber at a pressure of approximately 3 torr. Commercially available mylar (0.9 μ) is used for the windows and supported with wires of 50 micron diameter. The transmission of such an arrangement is approximately 94%.

Fig. 1 - A photograph of the PSAC.

Spacial sensitivity is achieved through time delays of the X and Y signals. As an ion passes through the chamber, charge is induced on the X and Y wire planes. Each wire is electrically connected to the others through a passive delay line (2 ns delay per wire). The signals are taken from both ends of the active area and the time difference between the signals is related to the position of the transversing ion. The large size of the X plane, requires that the active area be separated into 2 halves. In addition to the X and Y signals, signals from the anode is typically used to gate any additional detectors firing in coincidence and classify the event as a recoil event.

Fig. 2 - A photograph of the focal plane area of the RMS. The PSAC or MCP is positioned in the left half of the chamber. The right half is the ionization chamber. The ion chamber can be removed and other detectors installed. In this photograph, the large MCP is installed.

The PSAC is housed in a chamber which is bolted into the larger focal plane chamber. The chamber has room for 16 vacuum feedthroughs using LEMO connectors and two gas feedthroughs. The gas intake has a perforated hose attached in order to ensure that the gas is spread evenly throughout the chamber. The gas will be free flowing and will not be recirculated. Two 300 l/s turbomolecular vacuum pumps are positioned directly under the PSAC in order to maintain good vacuum. The PSAC is a transmission detector so that an Ionization Chamber, Double-sided Silicon Strip Detector, or Moving Tape Collector may also be used to detect the mass separated ions.

Collimators may be placed in front of the PSAC in order to block unwanted ions from reaching the focal plane. This detector can cover the entire focal plane if the RMS is operated in diverging mode. In most experiments, this detector has been replaced by the large area MCP due to count rate limitations. In this gas counter, the well focussed mass-separated ions (1000+ ions per second) would ultimately cause sparks which would deposit a coating on the wires including the position planes. This coating reduces the gain of the signals and results in "holes" in the spectra. The wires can be cleaned with alcohol and performance recovered..

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