A low background-rate detector for ions in the 5 to 50 keV energy range to be used for radioisotope dating with a small cyclotron
Accelerator mass spectrometry in tandem Van de Graaff accelerators has proven successful for radioisotope dating small samples. We are developing a 20 cm diameter 30 to 40 keV cyclotron dedicated to high-sensitivity radioisotope dating, initially for /sup 14/C. At this energy, range and dE/dx methods of particle identification are impossible. Thus arises the difficult problem of reliably detecting 30 to 40 keV /sup 14/C at 10/sup -2/ counts/sec in the high background environment of the cyclotron, where lower energy ions, electrons, and photons bombard the detector at much higher rates. We have developed and tested an inexpensive, generally useful ion detector that allows dark-count rates below 10/sup -4/ counts/sec and excellent background suppression. With the cyclotron tuned near the /sup 13/CH background peak, to the frequency for /sup 14/C, the detector suppresses the background to 6 x 10/sup -4/ counts/sec. For each /sup 14/C ion the detectors grazing-incidence Al/sub 2/O/sub 3/ conversion dynode emits about 20 secondary electrons, which are independently multiplied in separate pores of a microchannel plate. The output signal is proportional to the number of secondary electrons, allowing pulse-height discrimination of background. We have successfully tested the detector with positive /sup 12/C, /sup 23/Na, /sup 39/K, /sup 41/K, /sup 85/Rb, /sup 87/Rb, and /sup 133/Cs at 5 to 40 keV, and with 36 keV negative /sup 12/C and /sup 13/CH. It should detect ions and neutrals of all species, at energies above 5 keV, with good efficiency and excellent background discrimination. Counting efficiency and background discrimination improve with higher ion energy. The detector can be operated at least up to 2 x 10/sup -7/ Torr and be repeatedly exposed to air. The maximum rate is 10/sup 6.4/ ions/sec in pulse counting mode and 10/sup 9.7/ ions/sec in current integrating mode.
- Research Organization:
- Univ. of California, Berkeley, CA (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6767535
- Report Number(s):
- LBL-17804; ON: DE87006171
- Resource Relation:
- Other Information: Thesis. Portions of this document are illegible in microfiche products
- Country of Publication:
- United States
- Language:
- English
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Low background-rate detector for 40-keV ions using a conversion dynode and a microchannel-plate electron multiplier to reject low-energy ions, electrons, and photons
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Related Subjects
ION DETECTION
DYNODES
CYCLOTRONS
DESIGN
ISOTOPE DATING
MICROCHANNEL ELECTRON MULTIPLIERS
PERFORMANCE
ACCELERATORS
AGE ESTIMATION
CHARGED PARTICLE DETECTION
CYCLIC ACCELERATORS
DETECTION
ELECTRON MULTIPLIERS
ELECTRON TUBES
RADIATION DETECTION
440101* - Radiation Instrumentation- General Detectors or Monitors & Radiometric Instruments