GB2266407A

GB2266407A - Charged particle analyser

Info

Publication number: GB2266407A
Application number: GB9208531A
Authority: GB
Inventors: Keith Birkinshaw
Original assignee: Aberystwyth University; University of Wales
Current assignee: Aberystwyth University; University of Wales
Priority date: 1992-04-21
Filing date: 1992-04-21
Publication date: 1993-10-27
Also published as: WO1993021542A1; GB9208531D0

Abstract

A detector array comprises an array of sensor elements (3) connected to respective counters, and means for supplying clock pulses to the counters simultaneously; each sensor element (3) is arranged to respond to a pulse of charged particles incident thereon to stop the counter to which it is connected, for example to give a measure of the time-of-flight of the incident particles. The detector array may be used in a particle detecting and analysing instrument.

Description

Particle Analysinq Instrument This invention relates to a particle analysing instrument and particularly but not solely to an ion detection instrument and to a detector array for use in such an instrument.

In accordance with this invention, there is provided a detector array which comprises an array of sensor elements connected to respective counters, and means for supplying clock pulses to the counters simultaneously, each sensor element being arranged to respond to a pulse of charged particles being incident thereon to stop the counter to which it is connected.

The array of sensor elements may be a linear array or a two dimensional array. Each sensor element may comprise an elongate metal strip forming a sensor electrode on the surface of a substrate with an associated sensor circuit. The counters, and sensor circuits connected between the sensor electrodes and their respective counters, may form an integrated circuit within the substrate. Each sensor electrode and its sensor circuit may be as described in W091/00612. Thus each detector preferably comprises a sensor electrode and a sensor circuit (forming a sensor element) and a counter.

The detector array may be used in a particle detection and analysing instrument. In this case preferably the particles (e.g. ions) are incident on a multiplier, arranged so that a particle incident at a particular point on the surface of the multiplier causes a burst of particles (e.g.

electrons) to be emitted from the opposite side of the multiplier and strike a sensor electrode at a corresponding position in the array.

In one form, the instrument comprises an energymomentum analyser, arranged to allow a pulse of particles to pass into a field which causes the particles to follow curved paths, the curvature for each particle being dependent upon its mass and velocity. The particles are arranged to strike a sensor array in accordance with the invention: the time-offlight of a particle is measured by the count held in the counter, at the instant this is stopped, associated with the sensor electrode which the particle strikes. This time of flight is proportional to the particle momentum. The location of the sensor electrode which the particle strikes determines the distance through which the particle has passed, and is related to the particle energy. From these momentum and energy measurements, the particle mass and velocity can be determined.

The instrument may be used as a mass spectrometer.

In a second form, the instrument comprises a conduit for a pulse or burst of particles to pass along and strike a multiplier. Each particle causes a burst of particles (e.g.

electrons) to be emitted from the opposite side of the multiplier, which strike the sensor electrode array (preferably a two-dimensional array in this case). The pulses or bursts of particles act to stop the counters of a proportion of the sensor elements: the counts held by the "stopped" counters indicate the times of flight of the particles incident on the respective sensor elements.

In a third form, the instrument is arranged to measure the time-of-flight and the angle of incidence of particles on the sensor array.

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic diagram of a first form of instrument using a detector array in accordance with this invention; FIGURE 2 is a schematic diagram of a second form of instrument using a detector array in accordance with this invention; and FIGURE 3 is a schematic diagram of a third form of instrument using a detector array in accordance with this invention.

The instrument shown in Figure 1 comprises in its simplest form, a pair of parallel plates 2 across which a fixed potential difference is applied, to provide a steady electric field. The uniformity of the field can be improved by adding guard rings between the plates 2. One of the plates is provided with an electrode arrangement forming a gate 1 for the entry of a pulse of ions travelling along a predetermined straight-line path lying at an acute angle (preferably 450) to the plates 2. The electric field between the parallel plates 2 acts on the ions to deflect them through curved paths, to impinge upon a multiplier placed in front of a linear array 3 of sensor electrodes. The curvature of the path which each ion follows depends upon its mass and velocity.

The sensor electrodes in the linear array 3 are connected to respective counters via respective sensor circuits. A short pulse is applied to the gate 1 to allow a corresponding pulse of ions to pass into the space between the pates 2: at a known time with respect to the latter pulse, the counters connected to the sensors are started, to count clock pulses. When a pulse of charge from the multiplier, in response to an ion impact on the multiplier, is incident on any of the sensor electrodes, its counter is stopped, and its count at that instant represents the time-of-flight, i.e. the time taken for the ion to travel from the gate 1 to the sensor.

It can be shown that the time-of-flight is proportional to the momentum (mass m times Velocity v) of the ion. It can further be shown that the distance of the sensor from the entry gate 1 is proportional to the energy (0.5mv2). Thus, from a measurement of the time-of-flight and a knowledge of the distance of each sensor of the array from the gate 1, both the mass and velocity of the different ions can be calculated.

From this a mass spectrum and a velocity spectrum of the incoming ions can de derived.

The linear array of sensor elements and their counters and associated circuits are formed together as an integrated circuit. A digital processing circuit is provided to read the counts held in the counters and process the data.

The instrument shown in Figure 2 comprises an elongate tube 10 which in use is evacuated. At one end the tube 10 is provided with a pulsed electron gun 12 to generate a pulse of electrons which impinges on a target to release ions. These ions are accelerated by a repelling pulse applied to an electrode 14, and travel the length of the tube 12 to impinge upon a micro channel plate electron multiplier 18. The tube 10 should be field free and a grid 16 is used to prevent electric field penetration from external sources. On the output side of the multiplier 18, there is a two-dimensional array 20 of sensor elements on an integrated circuit, each sensor being connected to a clock pulse counter. A digital processing circuit 24 may be provided to read the contents of the counters and process the data.

In this arrangement each counter will be stopped when its associated sensor receives a burst of electrons, in response to an ion striking the electron multiplier 18 at a corresponding location. The count held in the counter at this instant represents the time of flight of the ion, the counters being started at a known time with respect to the pulse applied to the electron gun.

The ions passing along the tube diverge slightly and impinge on the electron multiplier 18 over an area. The sensor array 20 extends over central part of this area, and covers typically 4mm2: typically there may be 256 sensors in the array. The times of flight which are recorded on the different counters provides a time-of-flight spectrum. If the repeller pulse ends before the first ions leave the acceleration region, the ions will all have the same momentum: if the repeller voltage is instead continuous, the ions would have the same energy.

Figure 3 shows an instrument having a linear array of sensor electrodes 30, a micro channel plate multiplier 32 and stopping potential grids 34. The array 30 will record the spatial positions of the ions. If the ions originate from a point source as indicated by T in Figure 3, the position of the detected ion will give the angle of emergence from the source.

As an example of the operation, a target T is bombarded to liberate ions which travel at different angles towards the multiplier. A short pulse is applied to a gate electrode 36 to allow a corresponding pulse of ions to pass to the multiplier. A predetermined potential is applied to the grids 34, such that only ions of greater than a predetermined energy will pass to the multiplier 32. The sensor electrodes in the linear array 30 are connected via respective sensor circuits to respective counters: when an ion strikes the multiplier 32, the sensor electrode at the corresponding position along the array receives a pulse of electrons so as to stop its counter, giving a measure of the time of flight of the ion from the gate 36. The time of flight gives the velocity of the ion, and the position of the respective sensor electrode along the linear array gives the angle of incidence. By measuring the velocityangle profile over a range of stopping potentials, the energyvelocity-angle profile of the ions is found; from the ion energies and velocities, the masses of the ions can be calculated. The data logging and processing circuit for the instrument is shown at 38.

Claims

1) A detector array which comprises an array of sensor elements connected to respective counters, and means for supplying clock pulses to the counters simultaneously, each sensor element being arranged to respond to a pulse of charged particles being incident thereon to stop the counter to which it is connected.

2) A detector array as claimed in claim 1, comprising a particle multiplier positioned in front of said array of sensor elements.

3) A detector array as claimed in claim 1 or 2, in which each sensor element comprises a sensor electrode tor receive the incident charged particles, and a sensor circuit, the sensor circuit being integrated with the respective counter.

4) A detector array substantially as herein described with reference to the accompanying drawings.

5) A particle detection and analysing instrument array as claimed in any preceding claim.

6) An instrument as claimed in claim 5, comprising means to allow a pulse of particles into a force field which causes the particles to follow curved paths dependent upon the mass and velocity of the individual particles, and to strike said sensor array, the counters of the array being started simultaneously so that their counts when stopped represent the time-of-flight of respective incident particles.

7) An instrument as claimed in claim 5, comprising a conduit for passage of a pulse of particles, the sensor array being positioned for incidence of the particles thereon, arranged so that the particles stop the counters of a proportion of the sensor elements and the counts of those counters when stopped represent the times of flight of the particles incident on the respective sensor elements.

8) An instrument as claimed in claim 5, arranged to measure the time-of-flight and angle of incidence of particles on the sensor array.

9) A particle detecting and analysing instrument substantially as herein described with reference to Figure 1, 2 or 3 of the accompanying drawings.