Pulse Processing Chapter No. 17 Radiation Detection and Measurements, Glenn T. Knoll, Third edition (2000), John Willey . 1

I  Linear and Logical Pulse II  Instruments Standard Ch 17 GK I  Linear and Logical Pulse II  Instruments Standard III  Application Specific Integrated Circuits IV  Summary of Pulse Processing Units V  Components common to many applications. VI Pulse Counting Systems VII Pulse Height Analysis VIII Digital Pulse Processing IX  System Involving Pulse Timing a) Time Pick off methods b) Measurements of Timing Properties  c) Modular Instruments for Timing Measurements X Pulse shape discrimination 3

C. Scalers or Counters-1 As the final step in a counting system, the logic pulses must be accumulated and their number recorded over a fixed period of time. The device used for this purpose may be a simple digital register that is incremented by one count each time a logic pulse is presented to its input. such devices are sometimes called scalers as a historic anomaly that dates from the time when digital registers of reasonable size were not widely available. Then it was common to use a scaling circuit to divide the input pulse repetition rate by a fixed factor such as 100 or 1000 so that the rate would be low enough to be directly recorded by an electromechanical register. These systems have been replaced by all-electronic digital registers. The scaling function persists only in the sense that an overflow output pulse is often provided when the maximum content of the register is exceeded (usually no less than HP or 106 counts). We henceforth refer to such units as counters· because that term more adequately describes the actual function. Counters are commonly operated in one of two modes: preset time and preset count.

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F. Deadtime in Counting Systems An important consideration in many counting applications is the loss of events due to the dead time of the system. For some detectors (notably the G-M tUbe) the detector mechanism itself limits the minimum interval between events for which two distinct pulses can be counted. More often, however, the detector will be capable of producing pulses that are separated by a time that is less than the dead time inherent in the operation of an electronic component in the signal chain, and therefore it will be this component that determines the system dead time. In the simple counting systems shown in Fig. 17.7, this limiting component is usually the integral discriminator or the SCA Although there are many exceptions, the dead time of a discriminator or SCA is typically related to the width of the linear pulse presented to its input and is characteristically a microsecond or two larger than this width. The validity of the corrections for dead time losses discussed in Chapter 4 depends oUlt the assumption that the dead time is constant for all events. The inherent dead time of elec.."; tronic units can sometimes vary with the amplitude or shape of the input pulse, so that steptt to set the dead time artificially are warranted in some critical applications. In this approach,‘ the dead time is standardized by an element such as a linear gate, which is held closed for a: fixed period of time following each pulse. This time is chosen to be larger than the dead time of any component in the system, so that accurate corrections can be made, even under con~ ditions in which wide variations in pulse amplitude are encountered. The dead time of this element may also be measured conveniently by direct observation on an oscilloscope.

10.8 COINCIDENCE-ANTICOINCIDENCE MEASUREMENTS In radiation measurements it is desirable or necessary to discard the pulses due to certain types of radiation and accept only the pulses from a single type of particle or from a particle or particles coming from a specific direction. Here are two examples of such measurements: 1. Detection of pair-production events. When pair production occurs, two 0.511-MeV gammas are emitted back-to-back. To insure that only annihilation photon are counted, two detectors are placed 180" apart, and only events that register simultaneously (coincident events) in both detectors are recorded. 2. Detection of internal conversion electrons. Radioisotopes emitting internal conversion (IC) electrons also emit gammas and X-rays. The use of a single detector to count electrons will record not only IC electrons but also Compton electrons produced in the detector by the gammas. To eliminate the Compton electrons, one can utilize the X-rays that are emitted simultaneously with the IC electrons. Thus, a second detector is added for X-rays and the counting system is required to record only events that are coincident in these two detectors. This technique excludes the detection of Compton electrons.

For example, consider a coincidence measurement involving two detectors (Fig. 10.42). The detector signals are fed into a coincidence unit, which then is used to gate the corresponding ADCs. The amplified detector pulses that are coincident are thus digitized by the ADCs, and the information is stored in the memory of the system. Any event that reaches the memory is defined like a point in a two-dimensional space. For example, if a pulse from ADC1 has the value 65 (i.e., ADC channel 65) and one from ADC2 has the value 18, the event is registered as 6518 (assuming 100 channels are available for each parameter). The measured data may be stored in the computer, for subsequent analysis, and may also be displayed on the screen of the monitor for an immediate preliminary assessment of the results. The results of a dual-parameter system such as that shown in Fig. 10.42.