Microchannel Plate Detector (MCP)
The RMS focal plane detector of choice is the microchannel plate plus thin foil detector . These detectors have better characteristics for our application than the PSAC detectors. They have better timing and can withstand much higher count rates extending to several hundred kHz with 100% efficiency for heavy ions. The image can be slightly distorted, particularly at the edges, but this can be corrected in software.
When a heavy ion passes through the thin foil, electrons are ejected from the surface of the foil. We use aluminzed mylar foil of 0.9 μ thickness (~128 μg/cm2 mounted at 45o or 60o with respect to the beam. For slow moving heavy ions, many electrons are ejected. These electrons are accelerated by placing a negative high voltage (-300 V) on the foil and a large positive high voltage (> 1600 V) on the MCP detector which lies parallel to the foil. Two MCP plates are used to provide amplification and the resulting electrons are deposited on a resistive anode. The deposited charge is detected from the 4 corners of the anode and a position is derived from the charge division. In addition, the detectors use magnetic imaging to constrain the electrons from the foil and improve the position resolution as well as to compress the image so that larger foils than the MCP can be used. Currently, the 8x10 cm2 detector observes an image about 15 cm in length.
Position information degrades at high count rates but should be maintained at rates exceeding 25 kHz. The timing signal for this detector has been successfully used at rates exceeding 150 kHz. Higher rates should be possible. At such rates, the detector should be operated in coincidence with other detectors for inclusion into the data acquisition.
The foil can be repositioned out of the beam without breaking vacuum. Two versions are used routinely at the RMS:
Fig. 1 - A photograph of the large MCP at the final focus. A calibration grid instead of a foil is in the beam path. Note the side and central collimators used to block portions of the focal plane. The effective area is 6 x 15 cm2.
Fig. 2 - A photograph of the small position sensitive MCP used at the achromatic focus, behind the target, and after the large MCP at the final focus. In the latter position, it is often used in proton radioactivity experiments where its closer positioning to the implantation point minimizes data loss due to scattering. The effective area is 6 x 6 cm2.
Fig. 3 - This MCP has a different design. It uses grids to make an electrostatic mirror which deflects the electrons to the MCP. The foil is typically 10 μg/cm2 to minimize energy loss and scattering. The detector may be used in front of the target station and act a beam counter for low intensity beams. Note that the beam passes through the grid and may result in unwanted activity deposited on these thin wires. The effective area is 2 cm in diameter.
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This file last modified Monday January 08, 2007