GB2039140A

GB2039140A - An ion detecting device

Info

Publication number: GB2039140A
Application number: GB7941775A
Authority: GB
Original assignee: Kratos Ltd
Current assignee: Kratos Ltd
Priority date: 1978-12-20
Filing date: 1979-12-04
Publication date: 1980-07-30

Abstract

An ion-detecting device comprises an apertured entrance plate (10), a scintillator or phosphor (13), and a dynode assembly (11). Potentials are applied such that positive/ negative ions received by the dynode assembly effect electron emission therefrom causing scintillator or phosphor light output pulses which are detected by photomultiplier (16). Utilization in a mass spectrometer is disclosed. The dynode assembly may be withdrawn from the detector for operation in another mode. <IMAGE>

Description

SPECIFICATION Improvements in and relating to ion detecting means This invention relates to ion-detecting devices, in particular for use in mass spectometers but also suitable for use in a wide range of ion detecting applications.

An ion detector, as used on a mass spectrometer of similar device, is a device for measuring ion intensity. Various types of detectors are known, ranging from simple, low sensitivity devices, to complex constructions containing inherent amplification and other properties, these being located in the vacuum space of a mass spectrometer for example. The simplest detector is a flat plate in which, when the plate is struck by a stream of ions, a flow of charge takes place constituting a detectable current. However, this device has poor sensitivity. More sensitive devices respond to single ions. Probably the best known of these is the electron multiplier, consisting of either a single continuous dynode, or else discrete dynodes. Incident ions are attracted to the device by means of a suitable potential applied to its entrance. Each impinging ion causes a number of electrons to be released.

These are attracted along the device by a progressively increasing positive potential, causing a cascade of secondary electrons at each collision with the dynode surface. The "packet" of electrons due to each single ion is collected at the end of the dynode chain on an appropriate surface. Subsequent amplification and smoothing or counting of the output pulses allows the intensity of the ion beam to be measured. In a normal mode of operation, the electron multiplier is used to measure positive ion beam intensities. The entrance to the multiplier is held at a high negative potential (typically 3kV) and the end of the multiplier at earth. In this way, amplification may proceed at earth potential, simplifying electronic design. When used to measure negative ion beams, some modification may be required.Although negative ion beams of higher energy than the potential applied to the entrance of the multiplier will still enter the device, beams of lower energy will be repelled. To counter this one of two methods may be used. Either the ion entrance plate is connected to earth, which presents great difficulty and often instability in subsequent amplification which must float at high positive potential, or else a collision dynode may be used. This collision dynode is placed in front of the entrance to the multiplier and converts the negative ions to positive ions by charge exchange or other processes. The positive ions then enter the electron multiplier and detection proceeds as before.

Systems containing electron multipliers are sensitive to damage due to vacuum accidents or to other processes which lead to a deterioration in gain (the number of electrons reaching the end of the device for one ion entering it).

A known type of ion-detecting device (sometimes referred to as a Daly metastable detector) has an entrance plate through which an ion beam can pass, and a scintillator disposed adjacent the entrance plate. This device, which possesses high gain and stability over long periods, is however only effective for positive ions. In operation, by the application of appropriate potentials, the positive ions which have passed through the entrance plate are decellerated and repelled, to strike the back of the entrance plate with high energy (typically 1 OkV or more). Electrons are liberated and accelerate to the scintillator.The light pulses caused by the impinging electrons are detected by a photomultiplier mounted outside the vacuum system, and subsequent amplification of the signal from the anode of the photomultiplier, which is at earth potential, yields a measurement of the ion beam intensity. This system is not as sensitive to a vacuum accident as a conventional electron multiplier, and since the photomultiplier is within its own vacuum envelope, high stability of gain may be achieved over long periods.

A requirement exists for an ion-detecting device which possesses the advantage of the above-mentioned detector of utilising a scintillator and photomultiplier, but without the inherent disadvantage of being effective only for positive ions.

According to the invention, an ion-detecting device is provided comprising an entrance member having therein an entrance aperture or slit through which an ion bears can pass, a dynode assembly located in the path of the ion beam on the downstream side of the entrance member, the dynode assembly consisting of one or more stages of gain, a scintillator or phosphor, and means for applying such potentials to the dynode assembly and to the scintillator or phosphor that when when a beam of ions is received by the dynode assembly, an electon beam is produced at the outlet of the dynode assembly and accelerated to the scintillator or phosphor.

The invention will now be particularly described by way of example, with reference to the accompanying drawing in which the single figure shows diagrammatically an ion detector in accordance with the invention.

As shown in the drawing, the ion-detecting device comprises an entrance plate 10 having an entrance in the form of an aperture or slit 1 Oa disposed in the path of ions entering the ion detecting means, and a dynode assembly 11 mounted downstream of the entrance aperture in the path of ions which have passed through the aperture 10a. The dynode assembly can conveniently comprise one, orprefera- bly two, stages of an electron multiplier, in particular a venetian blind multiplier.

The dynode assembly and entrance plate are connected to a biassing circuit 1 2 to receive potentials such that when ions pass through the entrance plate, they are received by the dynode assembly and produce an emission of electrons from the dynode assembly.

The ion-detecting device further comprises a scintillator 1 3 comprising a layer of scintillating material formed on a transparent base and coated with a thin layer of electically conductive material (typically aluminium) to which the electrons are accelerated and which is connected to the biassing circuit in such a way as to receive a high positive potential. As an alternative to a scintillator, a phosphor can be used whose construction is the same as that of the scintillator but with a layer of phosphor material in substitution for the layer of scintillating material. The scintillator or phosphor is formed on the end of light pipe 15, which provides the transparent base.The intensity of light emitted from the scintillator or phosphor, when electrons from the dynode assembly or from the entrance plate impinge thereon, is dependent on the intensity of the ion beam passing through the entrance plate.

The entrance plate 10, dynode assembly 11 and scintillator 1 3 are contained in an envelope 14 within which a vacuum is maintained.

The light pipe 15, sealed in the wall of the vacuum envelope 14, leads from the scintillator or phosphor 1 3 to the inlet of a photomultiplier 16 which is so connected that amplification proceeds at earth potential. The outlet terminal 1 6a of the photomultiplier being connected via a pre-amplifier 1 7 to a main amplifier 18 from which the amplified signal can be taken to a recorder or signal indicator (not shown). The amplified signal thus provides a measure of the incident ion intensity.

It will be apparent that, in operation, when ions pass through the entrance 1 0a, the dynode assembly 11 will be given a potential such as to enable it to attract the ions to itself.

This will be a positive potential if low energy negative ions are to be collected, or a negative potential if low energy positive ions are to be collected. If high energy ions of either polarity are to be collected, the dynode assembly can be held at earth potential.

If the dynode assembly consists of two dynodes, then the second (i.e. downstream) dynode will be positively charged with respect to the first dynode, typically by 250 volts. In this way, electrons which leave the first dynode are attracted to the second. This is true for any voltage applied to the first dynode, (i.e. positive, negative or earth).

The collection of ions by the dynode assembly will produce an emission of electrons to energise the scintillator, and the collection of electrons by the scintillator or phosphor will produce a light emission to energise the photomultiplier.

The device described above can thus be used to detect either positive or negative ions, and has approximately equal gain for ion beams of either polarity at any energy.

The dynode assembly can comprise assembly can comprise a single dynode, or a double dynode which provides one stage of electron multiplication.

If desired, the dynode assembly can be so constructed that it can be withdrawn from the detector to convert the detector for use in the mode of a Daly metastable detector.

Claims

1. An ion-detecting device comprising an entrance member having therein an entrance aperture or slit through which an ion beam can pass, a dynode assembly located in the path of the ion beam on the downstream side of the entrance member, the dynode assembly consisting of one or more stages of gain, a scintillator or phosphor, and means for applying such potentials to the dynode assembly and to the scintillator or phosphor that when a beam of ions is received by the dynode assembly, an electron beam is produced at the outlet of the dynode assembly and accelerated to the scintillator or phosphor.

2. An ion-detecting device according to claim 1 wherein the means for applying potentials to the dynode assembly are able to apply potentials to enable positive ions to be detected and potentials to enable negative ions to be detected.

3. An ion-detecting device according to claim 1 or claim 2 wherein said dynode assembly is movable out of the path of electrons flowing from the entrance member to the scintillator or phosphor.

4. An ion-detecting device according to any preceding claim wherein the scintillator or phosphor communicates with a photomultiplier via a light pipe.

5. An ion-detecting device according to claim 4 wherein the entrance member, the dynode assembly and the scintillator or phosphor are contained within an evacuated envelope, the light pipe passing through said envelope.

6. An ion-detecting device substantially as described herein with reference to the accompanying drawing.