INTRODUCTION

The single channel analyzer (SCA) or discriminator is the instrument which measures the energy (i.e., amplitude) of an amplifier's output pulse, and properly prepares a standardized pulse for input into counting devices. Introducing an amplifier's output pulse directly into a counting device (that is, a counter or ratemeter) with no discrimination of the amplitude, would give uncertain results due to the low level noise content. If it is desired to measure the gross number of the particles detected, a discriminator, set to pass only those pulses with amplitude above a fixed threshold, will insure that only detected nuclear events are counted. The measurement signifies that the event exceeded a desired threshold, and a standardized output pulse is generated in a form that can be easily acted on by other instruments appropriate to the application. Referred to as logic signals, the NIM Standard includes two signals: a "SLOW" positive 8 V signal, and a "FAST" negative 16 mA current pulse. CANBERRA also provides TTL outputs on all instruments.

The single channel analyzer not only permits the discrimination of pulses below a certain amplitude, but those above an upper threshold as well, allowing the measurement of only those events occurring in a selectable energy window. For convenience, the upper threshold may be defined and set as a �E window value which rides above the discriminator threshold E setting. Further, E may be controlled from an external voltage level by an SCA Sweep to drive the window across a selected energy range while the resulting energy spectrum is recorded.

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