High detector counts rates, whether caused by high isotope concentrations or high background contributions, decrease confidence in AMS measurements through the requirement for significant deadtime corrections. Numerous chemical and spectrometric methods are available to suppress AMS counting backgrounds, but high ion rates are sometimes unavoidable from AMS samples, especially for biomedical and nuclear forensic applications. Ion source intensities continue to rise for even natural AMS isotopes such as 14C, increasing requirements for accurate and precise deadtime corrections. Shaping amplifiers typically have pulse processing widths of a few microseconds, limiting ultimate system performance, but the digital conversion of peak heights from these amplifiers is the most common source of system deadtime. We mated a very fast NIM-based ADC to our CAMAC-based data acquisition system to obtain stored digitized peak heights in the memory units before the shaped pulses return to baseline. A random pulser and 2 µs full-width-at-base shaping amplifier were used to evaluate the deadtime response of the ADC/memory module system; the results obtained showed that the 10% deadtime level was reached at a pulse rate of 40 kHz. With these high count rate units, precise deadtime corrections can be derived from well-known fixed pulse widths rather than from ADC conversion efficiencies, and deadtime corrections for high count rate high precision 14C measurements are greatly reduced in significance.
This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract W–7405–Eng–48.
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See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)