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A new time-of-flight mass spectrometer using channel multipliers as ion source and ion detector
NUCLEAR INSTRUMENTS AND METHODS 98 0972) 595-596; © NORTH-HOLLAND PUBLISHING CO. L E T T E R S TO T H E E D I T O R A NEW T I M E - O F - F L I G H T MASS S P E C T R O M E T E R U S I N G C H A N N E L M U L T I P L I E R S
AS I O N S O U R C E AND I O N D E T E C T O R R. D. A N D R E S E N and D . E .
PAGE
Space Science Department (ESLAB), European Space Research and Technology Centre, NoordwOk, The Netherlands Received 25 October 1971 A new type of ion source is described which is especially suitable for time-of-flight mass spectrometers. Such an instrument seems to have some advantages when compared with commercially
available residual gas analyzers. The fact that this new time-offlight spectrometer consumes low power makes it attractive for space flight instrumentation.
It has been demonstrated 1"3) that a channel multiplier (CM), activated by photoelectrons or beta electrons from a radioactive material, could be used to provide sufficient electron current to ionize neutral gas molecules, i.e. the device could be a useful "cold electron ion source". In our last paper 3) we stated that since the CM gives an electronic signal whenever ions are produced, it might be useful to consider this system as the basis of a time-of-flight residual gas analyzer. This note reports the first results using such an instrument. The experimental set-up is shown schematically in fig. 1. The essential components of this type of analyzer are (a) a curved CM, operating in the pulse-saturated mode and used as electron source for the production of ions, (b) a grid system for accelerating ions to uniform energy, (c) an ion drift tube region where the ions are separated according to their mass to charge ratio, and
(d) a curved CM, operating in the pulse-saturated mode, used as a sensitive fast-response ion detector. Suitable electronic circuitry is required for translating the time-dependent arrival of ions of different velocities into a time base that is related to mass numbers. It is possible to get a first overall view on the different kind of ions present using an oscilloscope. By triggering the time base of the scope with signals from the electron source CM the pulses from the ion detector CM can be observed. The time delay of the signals is proportional to the square root of the ion mass. A first experiment was built up using the CM Mullard B410 as electron source, the CM Bendix SEM 4219 as ion detector and a piece of stainless steel (1 cm diameter and 15 cm long) as ion drift tube. An electron t r a p - s e e fig. 1 - was provided and was biased to minimize backscattering of the electron beam. A 9°Sr source was used to trigger the electron source CM at a
electron source /(('~')")
( neg. 3.5 kV 90Sr point source
ionization region 'I~--
~l_J pos
:
( neg. Vs
\ ~ acceleration ~electron trap
neg. 35 kV neg.2.6 kV ~eg. VD
,f
f
/
~-field free drift tube
ion detector
region
tri99er
501l
oscilloscope Fig. I. The schematic diagram of the time-of-flight mass spectrometer using channel multipliers as ion source and ion detector.
595
596
R. D. A N D R E S E N A N D D. E. P A G E
repetition rate of 300 counts per second. Using an ion energy of typically 1 keV, the peak of H +, H~-, N +, O +, H 2 0 +, N ~ , O +, and CO~- could easily be experimentally resolved when analyzing the residual gas in a pressure range a r o u n d 10 -6 torr. It seems that the resolution obtainable with this kind of instrument might be comparable with the conventional time-offlight mass spectrometer4). The conclusion might be drawn that this new kind of electron source can be used for building up a simple and reliable time-of-flight residual gas analyzer which might also serve as a leak detector. Such an instrument would be attractive for space flight instrumentation
since it consumes extremely low power, the C M ' s are relatively rugged compared with filaments and they show practically no getter and outgassing effects.
References 1) g . D. Andresen and D. E. Page, Nucl. Instr. an.d Meth. 85 (1970) 141.
2) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 88 (1970) 99. 3) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 94 (1971) 429. 4) F. A. White, Mass spectrometry in science and technology (John Wiley & Sons, New York, 1968).
AS I O N S O U R C E AND I O N D E T E C T O R R. D. A N D R E S E N and D . E .
PAGE
Space Science Department (ESLAB), European Space Research and Technology Centre, NoordwOk, The Netherlands Received 25 October 1971 A new type of ion source is described which is especially suitable for time-of-flight mass spectrometers. Such an instrument seems to have some advantages when compared with commercially
available residual gas analyzers. The fact that this new time-offlight spectrometer consumes low power makes it attractive for space flight instrumentation.
It has been demonstrated 1"3) that a channel multiplier (CM), activated by photoelectrons or beta electrons from a radioactive material, could be used to provide sufficient electron current to ionize neutral gas molecules, i.e. the device could be a useful "cold electron ion source". In our last paper 3) we stated that since the CM gives an electronic signal whenever ions are produced, it might be useful to consider this system as the basis of a time-of-flight residual gas analyzer. This note reports the first results using such an instrument. The experimental set-up is shown schematically in fig. 1. The essential components of this type of analyzer are (a) a curved CM, operating in the pulse-saturated mode and used as electron source for the production of ions, (b) a grid system for accelerating ions to uniform energy, (c) an ion drift tube region where the ions are separated according to their mass to charge ratio, and
(d) a curved CM, operating in the pulse-saturated mode, used as a sensitive fast-response ion detector. Suitable electronic circuitry is required for translating the time-dependent arrival of ions of different velocities into a time base that is related to mass numbers. It is possible to get a first overall view on the different kind of ions present using an oscilloscope. By triggering the time base of the scope with signals from the electron source CM the pulses from the ion detector CM can be observed. The time delay of the signals is proportional to the square root of the ion mass. A first experiment was built up using the CM Mullard B410 as electron source, the CM Bendix SEM 4219 as ion detector and a piece of stainless steel (1 cm diameter and 15 cm long) as ion drift tube. An electron t r a p - s e e fig. 1 - was provided and was biased to minimize backscattering of the electron beam. A 9°Sr source was used to trigger the electron source CM at a
electron source /(('~')")
( neg. 3.5 kV 90Sr point source
ionization region 'I~--
~l_J pos
:
( neg. Vs
\ ~ acceleration ~electron trap
neg. 35 kV neg.2.6 kV ~eg. VD
,f
f
/
~-field free drift tube
ion detector
region
tri99er
501l
oscilloscope Fig. I. The schematic diagram of the time-of-flight mass spectrometer using channel multipliers as ion source and ion detector.
595
596
R. D. A N D R E S E N A N D D. E. P A G E
repetition rate of 300 counts per second. Using an ion energy of typically 1 keV, the peak of H +, H~-, N +, O +, H 2 0 +, N ~ , O +, and CO~- could easily be experimentally resolved when analyzing the residual gas in a pressure range a r o u n d 10 -6 torr. It seems that the resolution obtainable with this kind of instrument might be comparable with the conventional time-offlight mass spectrometer4). The conclusion might be drawn that this new kind of electron source can be used for building up a simple and reliable time-of-flight residual gas analyzer which might also serve as a leak detector. Such an instrument would be attractive for space flight instrumentation
since it consumes extremely low power, the C M ' s are relatively rugged compared with filaments and they show practically no getter and outgassing effects.
References 1) g . D. Andresen and D. E. Page, Nucl. Instr. an.d Meth. 85 (1970) 141.
2) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 88 (1970) 99. 3) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 94 (1971) 429. 4) F. A. White, Mass spectrometry in science and technology (John Wiley & Sons, New York, 1968).