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

Time-of-flight mass spectrometer with a position-sensitive detector (20) for determining energy and pulse of photodissociated ions, the detector (20-24) comprising one or a plurality of electron multipliers (21, 22) and an anode array (23) arranged behind the electron multipliers for determining the position of impringement of the ions, and the time-of-flight mass spectrometer including devices (30-33) for determining the time of flight of the ions.

The present invention relates to a time-of-flight mass spectrometer with a 
position-sensitive detector which comprises at least one electron 
multiplier and an anode array for detecting the electrons released in the 
electron multipliers. 
For understanding the processes that take place during light-induced 
ionization and subsequent dissociation of molecular systems, the reaction 
products must be identified and energy and pulse of said products must be 
determined. Photoionized molecules normally dissociate into electrons and 
ions which must be detected simultaneously. Energy and pulse of such 
photodissociated fragments can, for instance, be determined by a 
time-of-flight mass spectrometer, including a position-sensitive detector, 
with the aid of which the time of flight and the position of impingement 
of the fragments can be determined. The position-sensitive detectors which 
are known in the prior art have various disadvantages. Relatively frequent 
use is made of detectors based on resistive anodes which, however, have 
very long dead times as a rule, whereby the time resolution of the system 
is limited. Other detectors consist of a matrix of anodes that are crossed 
and interconnected in rows and columns, as are, for instance, illustrated 
in the publication by J. H. D. Eland in Meas. Sci. Technol. 5, 1501-1504 
(1994). In these detectors, too, the dead times are long because of the 
use of delay lines for an indirect position determination from signal 
travel time measurements and of digital circuits for the time measurement. 
It is therefore the object of the present invention to provide a 
time-of-flight mass spectrometer comprising a position-sensitive detector 
of improved time and position resolution. 
This object is achieved by the subject of patent claim 1. Advantageous 
embodiments follow from the subclaims. 
The detector of the invention consists of at least one electron multiplier 
and an anode matrix which is arranged behind each electron multiplier and 
in which anodes are respectively interconnected in lines or columns and 
each line and each column are connected to a respective output terminal. 
The electrons which are released in the electron multiplier are detected 
in position-sensitive fashion by the anodes. The time is measured in a 
device that is independent of the anode matrix. This device may, for 
instance, be an analog circuit with the aid of which a time resolution of 
about 100 ps can be achieved. This is an improvement in time resolution by 
more than a factor 10 in comparison with the prior art. 
The invention will now be explained in more detail with reference to the 
figures, of which:

FIG. 1 shows the basic function of the detector of the invention in a 
schematic and exemplary manner. In an interaction zone 12 of a 
time-of-flight mass spectrometer 10 an approximately monochromatic 
synchrotron radiation SR impinges on the molecules of a molecular beam MB. 
The synchrotron radiation may, for instance, 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, the latter are ionized and 
possibly dissociate into individual ions and electrons, of which the ions 
are to be detected in time- and position-resolved fashion by a detector 20 
which is arranged at the end of a drift tube 11. 
FIG. 2 schematically illustrates the measuring principle with reference to 
the dissociation of a CO molecule. On the basis of the positions and times 
measured for the ions that impinge on the detector 20, energy and pulse of 
said ions shortly after fragmentation can be determined. The detector 20 
consists of two sucessively arranged multichannel plates 21, 22 and of the 
anode matrix 23 which is arranged at such a distance from the second 
multichannel plate 22 that an electron cloud which has been released in 
the multichannel plates impinges on at least one anode. Instead of 
multichannel plates, it is possible to use other electron multipliers. In 
the present case the anode matrix consists of 900 planar anodes which are 
arranged in regular fashion in 30 lines and 30 columns. Hence, on the 
whole, the anode matrix consists of 1800 partial anodes, of which two 
partial anodes respectively form a planar anode. An example of the 
geometrical shape of the partial anodes will be explained further below 
with reference to FIG. 5. The anodes are arranged and dimensioned in such 
a manner that multichannel plates or so-called microsphere plates with a 
diameter of 40 mm can be used. The anodes are at a potential which is by 
2800 V higher than the front side of the first multichannel plate 21 which 
is at -4000 V relative to ground. This voltage focuses the electron cloud 
such that it impinges on at least one anode (two partial anodes). The 
partial anodes are respectively interconnected along each line (X) and 
each column (Y), and each line and each column are connected to an output 
terminal, so that a total of 60 wires have to be guided out of the vacuum 
chamber to the outside. Hence, upon each event, an anode is hit and a 
respective signal is produced at an X-line and a Y-line. These signals are 
first supplied to a position decoder 24 for detecting the position. The 
position decoder 24 will respond when two neighboring partial anodes have 
simultaneously been hit by the electron wave. 
A feature of the present invention is that time measurement is performed by 
devices which are independent of the anode matrix and the remaining 
position-determining means. FIG. 1 illustrates the principle of time 
measurement. 
A thin metal plate which is mounted on the back side of the second 
multichannel plate 22 (see FIG. 2), or a metal coating applied to a 
substrate, serves to measure the time. The electrons which have been 
released in the multichannel plates pass in substantially unhindered 
fashion through the metal plate while moving towards the anode. In this 
metal plate, however, a pulse is generated (hereinafter designated as ion 
signal IS) which is used for time measurement. The time of said pulse is 
considered to be representative of the time when the ions impinge on the 
first multichannel plate. The start time of the ion analysis is taken as a 
reference point. In the present example, this is the time of ionization by 
the synchrotron radiation pulse which enters into the interaction zone. 
The so-called bunch marker (BM) signal, a pulse derived from the 
high-frequency control of the synchrotron, is considered to be 
representative of said time. These two signals are supplied to a plurality 
of time-to-amplitude converters (TAC) whose output signal is converted 
into a digital signal in analog-digital converters (ADC), the number of 
which corresponds to that of the TACS, and is supplied to the data 
acquisition interface 25. The next BM signal represents the stop time of 
the ion analysis and is therefore supplied as a stop signal to the TACs, 
while the ion signals are supplied to the TACs as start signals. The TACs 
have, for instance, a dead time of 2.5 .mu.s after the start signal, while 
the synchrotron period is 200-1000 ns. 
Instead of the TAC-ADC combination, means can also be used that directly 
convert the time magnitudes into digital signals (so-called 
time-to-digital converters, TDC). 
FIG. 3 illustrates a further embodiment of the present invention, in which 
the electrons are additionally detected by an electron spectrometer 50. 
Identical reference numerals have here been used for components 
corresponding to those of FIG. 1. The electron spectrometer 50 contains a 
drift tube 51 and an electron detector 52 which, as illustrated, may have 
a structure similar to that of the ion detector, i.e. it can contain two 
multichannel or microsphere plates and an anode, with the anode of the 
electron detector being possibly also configured as a large-area anode. In 
this embodiment, the signal of the electron detector (hereinafter 
designated as electron signal ES) is used as a reference signal for the 
time measurement of the ion events, i.e., it is supplied as the start 
signal to the TACs. The electron signal and the BM signal are supplied to 
a TAC 4 (see FIG. 4) for measuring the time of flight of the electrons. 
The electron signal is supplied as the start signal and the BM signal as 
the stop signal to the TAC 4. Hence, the interval between the occurrence 
of the electron signal and the BM signal that follows the BM signal 
causing the electron signal is always measured. The analog signal of the 
TAC 4 is supplied to an ADC 4, and the output signal thereof is supplied 
to the data acquisition interface 25. 
FIG. 4 shows details of the time measuring device in greater detail. The 
circuit as shown refers to the embodiment of FIG. 3. The travel 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 travel time is determined in TAC 4 in that, as mentioned, the 
electron signal is supplied as the start signal and the BM signal of the 
successive cycle, which has been received from the synchrotron 70, as the 
stop signal. The ion signals which are produced at the metal plate on the 
back side of the second multichannel plate are supplied along a line to a 
high-speed switch (MUX) 31 whose task consists in subsequently 
distributing the signals over the TACs 0-3. Hence, in the illustrated 
example, an electron and four ions can be detected in a measurement cycle. 
Of course, the number of TACs as selected is an arbitrary one and can very 
easily be increased. Each TAC has assigned thereto an ADC which has a time 
dispersion of 0.1 ns per channel that at a suitably fast rise time of the 
ion signals can also be chosen such that it is smaller and can be reduced 
down to 30 ps. Before being supplied to the MUX and the TACs, the ion and 
electron signals are amplified by an amplifier (not shown) by the factor 
50 to 100 and are then converted in a discriminator 32 (CFD, constant 
fraction discriminator) into standard pulses. This has the effect that 
time measurement in the TAC is started at comparable times for original 
pulses with a similar shape (rise time), but different amplitude. The 
exact operation of this commercially available component is described in 
more detail in the literature, for instance in the dissertation by B. 
Langer, TU Berlin (Technical University of Berlin), 1992. The MUX 31 is 
connected to the position decoder 24 and sends a gate trigger signal to 
said decoder to permit an assignment of the measured time signals to the 
measured position signals. 
Since the signals from the anode rows and columns have very short rise 
times and a very short pulse duration (rise time+duration.ltoreq.5 ns, 
amplitude about 5 mV), they can no longer be read out directly with a 
computer. Therefore, a pattern recognition system is interposed as a rapid 
unit which upon occurrence of an electron cloud loads the relevant 
information about the position into a very rapid memory. Already after 5 
ns, a new event position can be written into this memory again, so that 
the dead time for readout will only depend on the speed of the rapid 
memory in this pattern recognition system. The pattern recognition system 
forms part of the data acquisition interface 25 and has the additional 
task to code the position signals in binary form. 
The electron pulses have a width of about 2 ns and an amplitude of 20 mV. 
The dead time of the system according to FIGS. 3 and 4 after detection of 
an electron is defined by the width of the ion signals and the dead time 
of the MUX 31 and is 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. Therefore, the system makes no fundamental distinction as to the 
smallest possible interval between two ion events, as the width of the 
signals is of essential importance. In case the amplified signals from the 
anode lines are digitized by an ultrafast ADC, it is also possible to 
interpolate between different lines to obtain a position resolution in the 
range of 0.1 mm instead of 1.5 mm, as in the illustrated examples. 
FIG. 5 shows an example of a position-sensitive anode matrix (cutout) used 
according to the invention. The anode matrix is formed as a planar 
pattern, with each anode comprising two partial anodes (A, B shown in the 
right part of the figure on an enlarged scale). The partial anodes A and B 
correspond to the subpixels in x-direction and y-direction, respectively. 
The partial anodes A and B are mounted on an anode plate. The partial 
anodes A are each connected in rows by electrical connections (not shown) 
on the back side of the anode plate. The partial anodes B are each 
connected in columns by electrical connections on the front side of the 
anode plate. Each row and column of partial anodes have assigned thereto a 
respective output terminal which is connected to a fast preamplifier. A 
special advantage of the invention is that the assignment of output 
terminals to each line and each column permits a genuine determination of 
the position without any signal travel time measurements in delay lines. 
As a result, the speed of the position-sensitive detector of the invention 
is considerably increased. A further advantage is that a possible increase 
in the anode matrix area for adaptation to a specific measurement 
structure does not, for instance, lead to a slowing down of the measuring 
operations. Furthermore, when the matrix area is increased by a certain 
factor, the number of conduction lines will just be increased by the 
square root of said factor. 
The position-sensitive detector of the invention can be used in many ways 
in all fields of application where the occurrence of particles, in 
particular, is to be measured with a high resolution of time and position. 
The invention has been described above with reference to a mass 
spectrometer. The detector of the invention, however, can also be used in 
an electron spectrometer or in an analyzer for neutral particles.