Multichannel Pulse Analysis Chapter 18 Multichannel Pulse Analysis GK Lec-No-30-34 Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. 1

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18-I. SINGLE-CHANNEL METHODS-GK-2 © 2014 John Wiley & Sons, Inc. All rights reserved.

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E. MCA Dead Time in Analog Pulse Processing -GK-39 The dead time of an analog system MCA is usually comprised of two components: the processing time of the ADC and the memory storage time. The first of these was discussed earlier and, for a Wilkinson-type ADC, is a variable time that is proportional to the channel number in which the pulse is stored. The processing time per channel is simply the period of the clock oscillator. For a clock frequency of 100 MHz, this time is 10 ns per channel. Once the pulse has been digitized, an additional fixed time is generally required to store the pulse in the proper location in the memory. In modern memories, this access time is normally short (on the order of 10 ns ), but for some low-power memories it can be significantly longer. Thus, the dead time of an MCA using an ADC of this type can then be written where  is the frequency of the clock oscillator, N is the channel number in which the pulse is stored, and B is the pulse storage time. The analyzer control circuits will hold the input gate closed for a period of time that equals this dead time. A dead time meter is often driven by the input gate to indicate the fraction of time the gate is closed, as a guide to the user. One normally tries to arrange experimental conditions so that the fractional dead time in any measurement does not exceed 30 or 40% to prevent possible spectrum distortions. The automatic live time operation of an MCA described earlier is usually quite satisfactory for making routine dead time corrections. Circumstances can arise, however, in which the built in live time correction is not accurate. When the fractional dead time is high, errors can enter because the clock pulses are not generally of the same shape and duration as signal pulses. One remedy15'16 is to use the pulser technique described on p. 664 to produce an artificial peak in the recorded spectrum. If introduced at the preamplifier, the artificial pulses undergo the same amplification and shaping stages as the signal pulses. The fraction that are recorded then can account for both the losses due to pile-up and the analyzer dead time. Several authors17•18 have reviewed the live time correction problem and suggested conditions under which the pulser method is not accurate. © 2014 John Wiley & Sons, Inc. All rights reserved.

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