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RA3. Experimental Equipment - A Rotating Target at the RMS
(C. J. Gross for the Decay Spectroscopy group)

Recent software improvements to our digital data acquisition system has enabled us to use unique trigger requirements [1] in order to search for fast sequential decay events at the recoil mass spectrometer (RMS). These techniques have led to our discovery of superallowed alpha decay [2] in the 109Xe-105Te-101Sn decay chain. This new trigger requirement allows us to suppress uninteresting events while pursuing short (< 1 μs) half-life radioactivities. The suppression is so large, that we could increase our implantation rate by a factor of 3-5 and still maintain our detection capability and data throughput. Thus, we have recently developed a rotating target for use at the RMS.

Figure RA3-1: (left) The rotating target mounted in the target chamber of the RMS. A U. S. penny is shown for scale. (right) A 58Ni target that was irradiated with 30+ pnA of 58Ni. The photographs may be viewed separately by clcking on them.

Our design is unusual in that we have opted for a system completely enclosed in the vacuum chamber thereby eliminating mechanical (or magnetic fluid) feedthrough devices. After several iterations of motors (stepper motor, DC motors with various gear ratios) we have settled on a 24 VDC motor [3] with a gear ratio of 3.7:1. Although motors rated for vacuum use (low-vapor pressure lubricant) have been used, we have also tested non-vacuum-rated motors and both appear to work in the vacuum for several days. We selected DC motors due to their simplicity to control (a bench-top power supply is all that is needed) and their lack of producing excess heat (the stepper motor required copper cooling fins to radiate heat in order to stay below 50° C). There does not appear to be a similar build up of heat with the DC motors.

In addition, we do not use a wheel with multiple targets but rather a single target of roughly 14 mm diameter. The target moves in a 7-mm-diameter circle allowing plenty of space for our approximately 2-mm-diameter beam to avoid striking the frame and scattering primary beam into our detectors. Our system consists of:

  • One DC motor rotating at a maximum of 7000 RPM
  • Gear ratio of planetary gearheads of 3.7:1, 9.4:1, 14.1:1
  • Gear ratio of target rotator apparatus of 32/37
  • One 14-mm-diameter target Thus, by changing the planetary gearhead ratio, the target can rotate at the same speed but with a lower motor RPM. Data compiled after a recent 200 hour test with gearheads of 3.7:1 are shown in table RA3-1.

    Table RA3-1 - Voltage and rotation values for a DC motor with a 3.7:1 gearing ratio
    Volts Motor Revolutions (RPM) Target Revolutions (RPM) Target Velocity (mm/s)
    Nominal Measured1 Nominal Measured2 Nominal Measured
    10 2917 2385 911 745 334 273
    12 3500 2944 1094 920 401 337
    16 4667 4116 1458 1286 535 471
    1Calculated based on measured rotations.
    2Measured with a strobe light.

    At 12 V, we have demonstrated that we can run 3-5 times as much beam as with a stationary 300 μm/cm2 Ni or Fe target which corresponds to 50 pnA of Ni or Fe beams [4]. Under these conditions we are able to sustain a rate of approximately 5000 implants/s on a 40-strip-by-40-strip double-sided silicon strip detector.

    The longevity of the motors has shown variablity which has led us to use the smallest gear ratios so that the motor does not turn at maximum rate. For example, two motors running full out at 7000 RPM and 14.1:1 gear ratio survived for less than 48 hours. However, a similar motor, operating at 3000-4000 RPM and 3.7:1 gear ratio, survived more than 200 hours. The target speed was the same in both tests. At less than 300 USD per motor-planetary-gear-head assembly, our goal of using only 1 motor per 5-day experiment appears to have been satisfied.

    [1] R. Grzywacz et al., Nucl. Instrum. and Methods Phys. Res. B 261, 1103 (2007).
    [2] S. N. Liddick et al., Phys. Rev. Lett. 97, 082501 (2006); R. Grzywacz et al., HRIBF Newsletter, Feature Article #4, Winter 2006
    [3] Faulhaber motors series 2232-024SR - planetary gearheads series 20/1 with a 3.71 reduction ratio (P/N 2232P0035),
    [4] K. P. Rykaczewski et al, AIP Conference Proceedings 961, Proton Emitting Nuclei and Related Topics, edited by L. S. Ferreira and P. Arumugam, Lisbon Portugal 17-23 June, 2007 (AIP, Melville New York, 2007) p 12.

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