1 Chapter No. 9 Measurements and Detection of Radiation, Nicholas Tsolfanadis, 2010, McGRAW-HILL BOOK INTRODUCTION TO SPECTROSCOPY

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8 9.4 MEASUREMENT OF A DIFFERENTIAL SPECTRUM WITH A SCA  determination of the number of particles within a certain energy interval  E for several values of energy;  Counting the number of pulses within a certain interval  V, for several pulse heights.  Differential spectrum measurement can be counted using SCA operating in the differential mode with the lower threshold of the SCA is set at Vl (or El) and the window has a width  V (or  E), then only pulses with height between Vl and Vl +  V are recorded. All pulses outside this range are rejected.  To measure the pulse spectrum of Fig. 9.6, start by setting the lower threshold at Vl, where Vl > Vo, with a certain window  V (e.g.,  V = 0.1 V) and then keeps lowering the lower threshold of the SCA.  Table 9.2 shows the results of the measurement, where n(V)  V is the number of pulses with height between V and V +  V. Figure 9.8 shows these results. It is assumed that the width is  V = Vi – Vi-1, where Vi, are the successive settings of the lower threshold of the SCA. It is important to note that one never measures the value of n(V), but only the product n(V) AV.

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BRIEF DESCRIPTION OF A MULTICHANNEL ANALYZER (MCA) To measure an energy spectrum of a radioactive source means to record the pulse-height distribution produced by the particles emitted from the source, which is achieved with the use of an instrument called the multichannel analyzer (MCA). Multichannel analyzers are used in either of two different modes: the pulse-height analysis (PHA) mode or the multichannel scaling (MCS) mode. The MCS mode is used to count events as a function of time. The individual channels of the memory count all incoming pulses for a preset time width At. After time At, the counting operation is switched automatically to the next channel in the memory, thus producing in the end a time sequence of the radiation being detected. For example, if the radiation source is a short-lived isotope, the MCS mode will provide the exponential decay curve that can be used for the measurement of the half-life of this isotope. The MCS mode is also useful for Mossbauer experiments. The PHA mode is the traditional function of an MCA and is used to sort out incoming pulses according to their height and store the number of pulses of a particular height in a corresponding address of the MCA memory called the channel number. In the PHA mode, an MCA performs the function of a series of SCAs placed adjacent to one another. When only one SCA with width AE is used, the experimenter has to sweep the spectrum by moving the lower threshold of the SCA manually (see Sec. 9.4). On the other hand, if one had many SCAs, all counting simultaneously, the whole spectrum would be recorded simultaneously.