The ionization chamber is a multi-segmented device similar in
construction to a previous one built at Daresbury Laboratory in England.
The anode is split into three segments along the flight path of the recoil;
the first two, which should detect 66% of the recoils' total energy,
will measure the energy loss crucial to the determination of Z. In addition,
the first two segments allow the rejection of bad events caused by Rutherford
scattering in the gas. Further segmentation across the focal plane will
allow the detector to be run at higher overall counting rates. This is
because events entering the chamber simultaneously but in different segments
will be processed separately and will not be rejected as pile-up.
This has been a limiting factor in many of the Daresbury experiments due
to the poor beam rejection. Should such segmentation be unnecessary,
the various anode segments can be linked together with jumpers.
Fig. 1 - A photograph of the top of the ionization chamber. Anode segments are jumpered together such that the left half of the detector is electrically separated from the right half.
The 11-12 cm anode-cathode separation is comparable to that used in other ionization chambers and amplifier time constants of 2-3 μs are used. The anode and cathode planes are constructed of circuit board which allows them to be made cheaply and easily modified. The anode is shielded by a Frisch grid and a position sensitive wire grid. This grid can be used to reject events scattered at large angles in the PSAC or MCP and to provide horizontal position information in cases when the PSAC is not present. In practice, we often remove the PSAC or MCP to minimize energy spread and thus we depend on this position information for our mass gates.
Fig. 2 - An energy loss versus total energy detected in the ionization chamber using a 17F/17O beam stripped at the target. The RMS was tuned to accept charge state 9+ (left) and 8+ (right). Although the gas pressure was insufficient to fully stop the 43 MeV ions, the isotopes are well separated.
The chamber windows are of commercial mylar of 0.9 micron (0.125 mg/cm2) thickness. The windows are supported by gold plated tungsten wires of 50-micron thickness. These wires will be connected through resistor chains in order to provide smooth electric field gradients. This technique has been done with much success at Daresbury. The anticipated maximum gas load is 50-60 torr. Despite the large windows, the vacuum remains below 10**-6 Torr in the focal plane region. Adequate pumping is provided by two turbomolecular vacuum pumps and one cryopump. Although many gases may be used and in the past isobutane was used, we now prefer CF4. This gas is non-explosive and has better timing (faster) while maintaining adequate energy resolution. The gas is circulated through the chamber by our gas handling system. The chamber housing is mounted on a sled which can slide back away from the flange containing the entrance window. This allows easy handling if the window ruptures or if the chamber and components must be inspected or replaced.
This device was built by the University of Surrey and Daresbury Laboratory in England with additional financial help from Vanderbilt University.
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This file last modified Monday January 08, 2007