20.5 Ion Detectors

• the Faraday cup neutralizes ions to provide a high-precision measurement • the electron multiplier generates a pulse of current for each ion • the photon multiplier uses conversion dynodes to monitor both positive and negative ions • the two multipliers are capable of fast measurements, thus compatible with TOF or Fourier transform methods

20.5 : 1/4 Faraday Cup

slit Ions exiting the analyzer are slowed by Faraday cup a positive potential and impact the ion + grounded electrode. At the electrode beam + they are ______by electrons that RL travel through the load resistor. This ion suppressor causes a voltage across the load resistor that is measured by a high input impedance voltmeter.

The impacted electrode is at an angle so that any secondary ions that are created are ______by the grounded wall.

Of all ion detectors, the Faraday cup provides the most precise relationship between the number of ions exiting the analyzer and the electrical measurement. It is used in ______experiments such as isotope dilution. Because very small currents are measured, the Faraday cup cannot be used for experiments involving rapid scanning.

20.5 : 2/4 Electron Multiplier

ion beam An electron multiplier is constructed

just like a photomultiplier but without 16e- 4e- 3 - a ______. It has dynodes 10 e RL

constructed from a Cu/Be alloy, which dynodes is a good secondary electron emitter. +V

The dynode potentials are such that the first is grounded and each subsequent one is 100 V more positive. This makes the voltage measurement a little tricky because one end of the load resistor is connected to positive ______. On average each dynode multiplies the current by ×4. A typical electron multiplier will have 10 to 15 dynodes for electron gains of 106 to 108. When used with magnetic analyzers the ions have sufficient kinetic energy to eject the first electron. With other analyzers the ion has to be accelerated into the first dynode.

Since the output pulse has a statistical variation it is not as ______as the Faraday cup. However, electron multipliers are very fast, making them compatible with TOF analyzers.

20.5 : 3/4 Photon Multipliers

The detector has two ______dynodes, a phosphorescent screen, and an ordinary photomultiplier.

The dynodes convert either ______ions into electrons. The electrons are accelerated into the screen which emits light. The light is detected with a current to voltage converter.

The advantages of the device are the ability to handle both positive and negative ions, a ______, and long lifetime.

It is not as precise as a Faraday cup.

20.5 : 4/4