Electron multipliers are commonly used in pulse-counting mode to detect ions from magnetic sector mass spectrometers. The electronics used to amplify, detect and count pulses from the electron multipliers always have a characteristic time interval after the detection of a pulse, during which no other pulses can be counted. This characteristic time interval is known as the x201C;dead time.x201D; The dead time has the effect of reducing the measured count rate compared with the x201C;truex201D; count rate.
In order to measure count rates accurately over the entire dynamic range of a pulse counting detector, such as an electron multiplier, the dead time of the entire pulse counting system must be well known. Accurate count rate measurement forms the basis of isotopic ratio measurements as well as elemental abundance determinations.
The procedure described herein has been successfully used to determine the dead time of counting systems on SIMS instruments. The accurate determination of the dead time by this method has been a key component of precision isotopic ratio measurements made by SIMS.
1.1 This practice provides the Secondary Ion Mass Spectrometry (SIMS) analyst with a method for determining the dead time of the pulse-counting detection systems on the instrument. This practice also allows the analyst to determine whether the apparent dead time is independent of count rate.
1.2 This practice is applicable to most types of mass spectrometers that have pulse-counting detectors.
1.3 This practice does not describe methods for precise or accurate isotopic ratio measurements.
1.4 This practice does not describe methods for the proper operation of pulse counting systems and detectors for mass spectrometry.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.