EP0858674A1

EP0858674A1 - Time-of-flight mass spectrometer with position-sensitive detection

Info

Publication number: EP0858674A1
Application number: EP96937307A
Authority: EP
Inventors: Uwe Becker; Franz Heiser
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 1995-11-03
Filing date: 1996-10-31
Publication date: 1998-08-19
Anticipated expiration: 2016-10-31
Also published as: DE19541089A1; WO1997017718A1; US6031227A; DE59607015D1; EP0858674B1

Abstract

The disclosure relates to a time-of-flight mass spectrometer with position-sensitive detector (20) for determining the energy and momentum of photo-dissociated ions. The detector (20-24) has one or more electron multipliers (21, 22) and an anode matrix (23) located behind the electron multipliers and used for determining the point of impact of the ions. The time-of-flight mass spectrometer has a device (30, 33) for determining the time of flight of the ions.

Description

Time-of-flight mass spectrometer with position sensitive

Detection

The invention relates to a time-of-flight mass spectrometer with a position-sensitive detector, which has at least one electron multiplier and an anode arrangement for detecting the electrons released in the electron multiplier.

To understand the processes involved in light-induced ionization and subsequent dissociation of molecular systems, it is necessary to identify the reaction products and to determine their energy and momentum. Photoionized molecules usually decay into electrons and ions, which have to be detected simultaneously. The energy and momentum of these photodissociated fragments can be determined, for example, by means of a time-of-flight mass spectrometer with position-sensitive detector, with which the time of flight and the point of impact can be determined by the fragments. The position-sensitive detectors known in the prior art have various disadvantages. Detectors based on resistive anodes are relatively widespread, but generally have very long dead times, which limit the time resolution of the system. Other detectors consist of a matrix of crossed anodes, which are interconnected in rows and columns, as described, for example, in the publication by JHD Eland in Meas. Be. Technol. 5, 1501-1504 (1994). With these detectors too, the dead times are high due to the use of delay lines for indirect position determination from signal transit time measurements and digital circuits for time measurement. It is therefore an object of the present invention to provide a flight time mass spectrometer with a position sensitive detector with improved time and location resolution.

This object is achieved by the subject matter of patent claim 1. Advantageous embodiments result from the subclaims.

The detector according to the invention consists of at least one electron multiplier and an anode matrix arranged behind each electron multiplier, in which anodes are connected to one another in rows or columns and each row or column is connected to an output connection. The anodes detect the electrons released in the electron multiplier in a position-sensitive manner. The time measurement is carried out in a device independent of the anode matrix. This device can be, for example, an analog circuit with which a time resolution of approximately 100 ps can be achieved. This represents an improvement in the time resolution by more than a factor of 10 compared to the prior art.

The invention is explained in more detail below with reference to the figures.

Show it:

1 shows a schematic illustration of a first embodiment of the invention;

2 shows a schematic diagram to explain the processes taking place during the detection;

3 shows a schematic representation of a further embodiment of the invention; Fig. 4 is a block diagram of the embodiment of Fig. 3; and

5 shows an example of an anode pattern of an anode matrix.

The basic mode of operation of the detector according to the invention is shown schematically and by way of example in FIG. 1. In an interaction zone 12 of a time-of-flight mass spectrometer 10, approximately monochromatic synchrotron radiation SR strikes the molecules of a molecular beam MB. The synchrotron radiation can, for example, be emitted by a synchrotron storage ring and guided through a wavelength-selective element such as a grating or crystal monochromator 1. As a result of the interaction of the synchrotron radiation with the molecules, these are ionized and, if appropriate, decay into individual ions and electrons, of which the ions are to be detected in a spatially and time-resolved manner by a detector 20 arranged at the end of a drift tube 11.

2, the measuring principle is shown schematically on the basis of the decay of a CO molecule. Based on the measured positions and times of the ions striking the detector 20, their energy and momentum can be determined shortly after the fragmentation. The detector 20 consists of two multi-channel plates 21, 22 arranged one behind the other and the anode matrix 23 which is arranged at such a distance from the second multi-channel plate 22 that an electron cloud triggered in the multi-channel plates hits at least one anode. Instead of multi-channel plates, other electron multipliers can also be used. In the present case, the anode matrix consists of 900 flat anodes, which are regularly arranged in 30 rows and 30 columns. Overall, the anode matrix thus consists of 1800 partial anodes, two of which each form a flat anode. An example of the geometric shape of the partial anodes is explained below with reference to FIG. 5. The anodes are arranged and dimensioned such that multi-channel plates or so-called microsphere plates with a diameter of 40 mm can be used. The anodes are at a potential 2800 V higher than the front side of the first multi-channel plate 21, which is at -4000 V with respect to ground. This voltage focuses the electron cloud in such a way that it hits at least one anode (two partial anodes). The partial anodes are connected to each other along each row (X) or each column (Y) and each row and each column is connected to an output connection, so that a total of 60 wires have to be led out of the vacuum chamber. An anode is struck at each event and a signal is generated on an X and a Y line. These signals are first fed to a position decoder 24 for determining the position. The position decoder 24 responds when two adjacent partial anodes have been hit by the electron wave at the same time.

A feature of the present invention is that the time measurement is carried out by devices which are independent of the anode matrix and the other devices for determining the position. In Fig. 1 the principle of time measurement is shown.

A thin metal plate attached to the back of the second multi-channel plate 22 (see FIG. 2) or a metal coating applied to a carrier is used for the time measurement. The electrons released in the multi-channel plates pass through the metal plate essentially unhindered on their way to the anode. In this, however, a pulse (hereinafter referred to as ion signal IS) is triggered, which is used for time measurement. The time of this pulse is considered to be representative of the time of the impact of the ions on the first multi-channel plate. The starting point of the ion analysis is taken as the reference point. In the present example, this is the time of ionization by the synchrotron radiation pulse entering the interaction zone. As representative of this point in time, called bunch marker (BM) signal, a pulse derived from the high-frequency control of the synchrotron. These two signals are fed to several time-to-amplitude converters (TAC, time-to-amplitude converter), the output signal of which is in analog-to-digital converters (ADC, analog-to-digital converter), the number of which is that of the TACs corresponds to a digital signal and the data acquisition interface 25 is supplied. The next BM signal represents the stop time of the ion analysis and is therefore fed to the TACs as a stop signal, while the ion signals are fed to the TACs as start signals. For example, the TACs have a dead time of 2.5 μs after the start signal, while the synchrotron period is 200-1000 ns.

Instead of the TAC-ADC combination, devices can also be used which convert time quantities directly into digital signals (so-called time-to-digital converters, TDC).

FIG. 3 shows a further embodiment of the present invention, in which the electrons are additionally detected by an electron spectrometer 50. For the rest, the same reference numerals have been used for the components corresponding to FIG. 1. The electron spectrometer 50 contains a drift tube 51 and an electron detector 52, which, as shown, can be constructed in a manner similar to that of the ion detector, ie it can contain two multi-channel or microsphere plates and an anode, the anode of the electron detector can also be carried out over a large area. In this embodiment, the signal of the electron detector (hereinafter referred to as electron signal ES) is used as a reference signal for the time measurement of the ion events, that is, it is supplied to the TACs as a start signal. For the measurement of the electron flight time, the electron signal and the BM signal are fed to a TAC 4 (see FIG. 4). The electron signal is given to the TAC 4 as a start signal and the BM signal as a stop signal. So it always becomes Time period measured from the occurrence of the electron signal to that BM signal that follows the BM signal causing the electron signal. The analog signal of the TAC 4 is fed to an ADC 4 and its output signal is fed to the data coordination interface 25.

4 details of the device for time measurement are shown in somewhat more detail. The circuit diagram shown relates to the embodiment according to FIG. 3. The transit times of the ions detected by the ion detector are measured in TACs 0-3. They receive the start signal from the anode of the electron detector 52. The electron transit time is determined in the TAC 4, in which, as mentioned, the electron signal is supplied as the start signal and the BM signal taken from the synchrotron 70 of the subsequent cycle is supplied as a stop signal. The ion signals which arise on the metal plate at the rear of the second multi-channel plate are fed to a fast switch (MUX) 31 on a line, which has the task of subsequently distributing the signals to TACs 0-3 . In the example given, one electron and four ions can therefore be detected in one measurement cycle. The number of TACs is of course chosen arbitrarily and can be expanded very easily. An ADC is assigned to each TAC, which has a time dispersion of 0.1 ns per channel, which can also be selected to be smaller with a suitably fast rise time of the ion signals and can be reduced to 30 ps.

Before they are fed to the MUX or the TACs, the ion and electron signals are amplified by a factor of 50 to 100 by an amplifier (not shown) and then converted into standard pulses in a discriminator 32 (CFD, Constant Fraction Discriminator). This ensures that for original pulses with a similar shape (rise time) but different levels, the time measurement in the TAC is started at comparable times. The exact mode of operation of this commercially available component is described in more detail in the literature, for example in the dissertation by B. Langer, TU Berlin, 1992. The MUX 31 is connected to the position decoder 24 and sends a gate trigger signal to it in order to enable the association of the measured time signals with the measured location signals.

Since the signals from the anode rows and columns have very short rise times and a very short pulse duration (rise time ₊ duration £ 5 ns, amplitude approx. 5 mV), they can no longer be read out directly with a computer. A pattern recognition system is therefore interposed as a fast unit, which loads the corresponding information of the position into a very fast memory when an electron cloud occurs. This memory can already be written with a new event position after 5 ns, so that the dead time for reading out depends only on the speed of the fast memory in this pattern recognition system. The pattern recognition system is part of the data acquisition interface 25 and also has the task of binary coding the position signals.

The electron pulses are about 2 ns wide and have an amplitude of 20 mV. The dead time of the system according to FIGS. 3 and 4 after detection of an electron is determined by the width of the ion signals and the dead time of the MUX 31 and results in less than 7 ns. In principle, the widths of the signals and the dead time of the MUX can each be reduced to less than 1 ns. The system therefore has no fundamental limit of the smallest possible interval between two ion events, since the width of the signals is essentially decisive. In the event that the amplified signals from the anode leads are digitized by an ultra-fast ADC, it is also possible to interpolate between different leads to a spatial resolution in the range 0.1 mm instead of 1.5 mm, as in the examples shown , to obtain. 5 shows an example of a position-sensitive anode matrix used according to the invention (detail). The anode matrix is formed as a flat pattern, each anode comprising two partial anodes (A, B shown enlarged in the right part of the figure). The partial anodes A and B each correspond to the sub-pixels in the x and y directions. The partial anodes A and B are attached to an anode plate. The partial anodes A are each connected in rows by electrical connections (not shown) on the back of the anode plate. The partial anodes B are connected in columns by electrical connections on the front of the anode plate. Each row or column of partial anodes is assigned an output connection, which is connected to a fast preamplifier. A particular advantage of the invention is that the assignment of output connections to each row and each column enables a true position determination without signal delay measurements in delay lines. This significantly increases the speed of the position-sensitive detector according to the invention. Another advantage is that any enlargement of the anode matrix area to adapt to a specific measurement setup does not lead to a slowdown of the measurement processes. Furthermore, when the matrix area is increased by a certain factor, the number of lines is increased only by the square root of this factor.

The position-sensitive detector according to the invention can be used in a variety of ways in all applications in which the occurrence of particles in particular is to be measured with a high time and location resolution. The invention has been described above with reference to a mass spectrometer. However, the detector according to the invention can also e.g. in an electron spectrometer or in an analyzer for neutral particles.

Claims

PATENT CLAIMS

1. time-of-flight mass spectrometer with position-sensitive detector (20-24), which has at least one electron multiplier (21, 22) and a regular anode arrangement (23) for detecting the electrons released in the electron multiplier, the anodes comprising row anodes and column anodes, the row anodes arranged in each row being electrically connected to one another and the column anodes arranged in each column being electrically connected to each other, characterized in that the anode arrangement has a number of first output connections corresponding to the number of its rows and one corresponding to the number of its columns A number of second output ports, the first and second output ports being connected to a position decoder (24) and the time-of-flight mass spectrometer including means (30-33) for determining the time of flight of charged particles.

2. Time-of-flight mass spectrometer according to claim 1, characterized in that the device (30-33) by a start signal representing a start time of a measurement

(BM, ES) and a stop signal (IS) caused by the impact of charged particles on the electron multiplier (21, 22) can be controlled.

3. Time-of-flight mass spectrometer according to claim 2, characterized in that the device (30-33) contains at least one time-to-digital converter (TDC) or at least one time-to-amplitude converter (30) which has two input connections for the supply of the Contains signals (BM, ES; IS) and an output connection connected to an analog-digital converter (33).

4. Time-of-flight mass spectrometer according to claim 3, characterized in that the device (30-33) contains at least two time-amplitude converters (30) and a switch (31) which has an input connection for the supply of the IS signal and one Has a large number of output connections connected to one input connection of the time-to-analog converter, and the switch (31) is designed to distribute subsequent IS signals of a measuring cycle to the output connections.

5. Time-of-flight mass spectrometer according to one of the preceding claims, characterized in that the electron multipliers (21, 22) and the anodes of the anode matrix (23) are so occupied with electrical potentials that the electron cloud triggered in the electron multiplier is at least one pair adjacent row and column anodes.

6. Time-of-flight mass spectrometer according to one of claims 2 to 5, characterized in that the starting time is given by the time of ionization of the molecules to be examined by a radiation pulse of electromagnetic Synchrotron radiation and the start signal (BM) derived from the high-frequency control of the synchrotron Signal is.

7. Time-of-flight mass spectrometer according to one of claims 1 to 4, characterized by an electron spectrometer (50) with an electron detector (52) for providing the start signal (ES).

8. Time-of-flight mass spectrometer according to claim 7, characterized by a time-to-analog converter (60) with two input connections for the independent supply of the signals

(BM; ES) and an output connection connected to an analog-digital converter (61) for the coincident recording of both the electron and the ion signals.

9. Time-of-flight mass spectrometer according to one of the preceding claims, characterized in that the electron multipliers (21, 22) are microsphere plates or multi-channel plates.