III.B.2. 65 Inch Scattering Chamber

The 65" Scattering Chamber is located on the 19deg. beam line. The major components are manufactured from stainless steel, which together with the use of an oil-free pumping system makes for an essentially hydrocarbon-free vacuum. The pumping system consists of a VARIAN Megasorb roughing stand (carbon vane plus liquid-N2 sorption pumps) and two cryo pumps (one 1000 l/sec and one 1500 l/sec). If the chamber has been open to air only briefly, the typical pump-downtimes are: ~30 minutes for roughing, ~30 additional minutes of cryo pumping to get into the 10-6 Torr range. The pump-down times are longer (perhaps by a factor of ~2) if the chamber has been open for an extended period, as during set-up.

A picture of the chamber with a gas detector and general mounts for Si detectors is shown in Figure III.2. The chamber has the following major components:

  1. Four (4) independent single-spoke gear rings for angular detector motion, mounted on an internal reference plate. General-purpose detector support stands are available that bring all gear-ring levels to the same height. Specific detector mounts for silicon surface-barrier detectors of various sizes, that can be positioned on precise radial rails, are available for general use. Relevant dimensions are given in Figures III.3 and III.4. The gear rings are directly driven by external motors and read by directly coupled angular encoders. Positioning accuracy and reproducibility is to within 0.01deg..

  2. A rotatable target wheel with 8 target positions for ANL standard- size target frames (1" x 1" and 1/16" thick). Other target sizes may be adapted with special fixtures that need to be built. A port is provided in the chamber lid above the target wheel that allows target transfer under vacuum from ANL (or similarly adapted) transfer units.

  3. An external collimator box with 4 combinations of collimator and anti-scattering collimator (maximum opening 3/4") at approximately 6 feet distance from the target position (see Fig. 3).

  4. An internal collimator system allowing easy exchange of collimators when the chamber is open during set-up. The collimator system is removable, offering the possibility for alternative internal beam collimation using collimators mounted on one of the gear rings.

  5. A Faraday cup with electron-suppression assembly, about 6 feet from the target position.

  6. Various feedthroughs; 50 standard (individually-insulated, 50X) BNC feedthroughs; 14 high-voltage feedthroughs (nine with SHV connectors, five with MHV connector on the outside and single- pin adapter on the inside, and one 50 kV connector); 9 gas feedthroughs (3 with 3/8" and 6 with 1/4" Polyflo connectors).

  7. One gas-handling system with electronic control for operation of a heavy-ion gas detector. The beam optics provides for a focus at the target with the following values for an ion beam of 4 mm mrad total transverse phase space: a full width of 1.3 mm, a full height of 1.6 mm, and horizontal and vertical divergences of 6 and 10 mrad respectively. These are typical values to be expected for light beams (12C, 160, etc.). For a Ni beam the values are typically larger by a factor of 2.

Also available is a collapsible time-of-flight arm for time-of-flight measurements with long flight paths (~1.5 meter), by mounting the target holder on one of the gear rings at the chamber entrance and making use of the full diameter of the chamber. The available angular range is 0deg. < [theta]lab < 50deg. where the flight path changes from ~150 cm to ~100 cm. The existing target holder houses 12 standard 1" x 1" target frames and the options for transferring targets under vacuum exists. Two "gridless" channel plate detectors utilizing thin (~20 ug/cm2) carbon foils are used in experiments utilizing the long time-of-flight arm.

A third target position (ladder with up to 10 positions) is available in a beam pipe cross located outside the chamber near the entrance. The horizontal ports in the beam pipe cross allows placement of [gamma]-ray detectors(NaI, Ge(Li) or Si(Li)) close to the target. In this configuration collimation slits and a charge-resetting foil can be inserted into the beam immediately after the target together with an electrostatic deflector, providing for a beam separator at 0deg.

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