- Email: [email protected]
Ion-detectors for TOF mass analyzers
L *H __ _-.
INNil
!!I!8 ELSEVIER
B
Beam Interactions with Materials 8 Atoms
Nuclear
Instruments
and Methods
in Physics
Ion-detectors M. Gonin
Research
B 136-138 (1998) 124&1247
for TOF mass analyzers
*, Y.H. Chen ‘, Th. Horvath,
Il. Ph~~.silitrli.sc~hes Insiitut dcv hirwsiriit
Giessen
Heiwich-Buff-
M. The&s, H. Wollnik R&q 16. 35302 Gicww.
Gertncrr~~~
Abstract
Ion detection with good signal-to-noise ratio is required in TOF mass spectrometry for measurements over a wide range of masses. To reach high mass resolving powers it is also necessary to have detectors with good timing characteristics. We show some of our concepts to reach a detection system with such properties. 0 1998 Elsevier Science B.V.
1. Introduction In time-of-flight mass spectrometers (TOF MS) for laboratory use, it is common practice to only use acceleration voltages < 6 kV which leads to ion energies of 6 keV for singly charged ions. For moderate MS dimensions (I 6 1.5 m) this leads to a TOF of about t = 15 ps. A line width of At < 2 ns (for A = 28 amu) can be resolved by modern high speed detection electronics. The mass resolving power R = m/Am of such a device would be R = t/2At > 3750. An example of such a measurement is shown in Fig. 1. For moderate masses (~50 amu) the mass line widths are caused nowadays to a large extent by the detector and the detection electronics. Hence it is useful to improve the detector characteristics to improve the resolution of the TOF MS.
Most ion detectors rely on an ion-electron conversion process followed by a multiplication of the secondary electrons (SE) produced. At the low velocities considered in this work, the SE yield is approximately proportional to the molecule mass (addition rule) and proportional to the projectile velocity above some offset velocity of zz 45 mm/ ps. Low and high mass ions (4 amu < m < 500 amu) therefore have low electron production yields (Fig. 2) which leads to poor pulse height distributions (PHD) and to low detection efficiencies. The quality of a PHD is often indicated by the pulse height ratio (PHR), the peak to valley ratio (PVR), and sometimes by the peak to noise ratio (PNR) as indicated in Fig. 3. In the following we discuss some concepts to increase the quality of the PHD, to increase the detection efficiency and to improve the detector timing.
2. Current to voltage amplifier *Corresponding author. Tel.: +49 641 99 33 148: fax: +49 641 99 33 239; e-mail: [email protected]. ’ On leave from Department of Nuclear Science. National Tsing-Hua University. Taiwan. ROC. 0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PflSOl68-583X(97)00823-9
Let us assume that an ion generates one secondary electron which is multiplied 10” times by some electron multiplier and produces a current pulse of
M. Conin et al. I Nucl. Instr. und Meth. irt PIIJX Rex B 136-138
(IWS) 1244-1247
1245
PHR = Pm / FWHM
!
N,’
30
28.006,
I 32
m/q
co* 27 996 ‘, n 0
28.00 _~
28.05
width At = 2 ns and size I = Q/At z 80 uA. If this pulse is transmitted directly to an oscilloscope as is indicated in Fig. 4(a), this current flows through two 50 R resistors which are necessary to prevent signal reflections in the transmission line. The total resistance of R = 25 fi converts the current into a voltage of U = RI E 2 mV. The PHD of such a configuration is shown in Fig. 4(a). By inserting a current to voltage amplifier directly behind the detector as shown in Fig. 4(b), it is possible to convert the current directly into a slightly amplified voltage signal with the amplifier also working as 3.0
5 c 00.0,
P u)
=..--%__ I
0
200
,
400 particle
,
,
600
800
1000
100
150
signal voltage (mV)
m/q
13.5 ns
Fig. 1. Example of the measured resolving power of a time-offlight mass analyzer. The time resolution of the detection electronics is 0.5 ns.
3 9
50
28.10
Fig. 3. Definition PHD.
of the parameters
to describe
the quality
of a
a driver for the 50 R transmission line. Because the amplifier is placed closely behind the detector. reflected pulses have very short propagation times and are damped very quickly. In this way it is possible to increase the final signal height and to shift the PHD out of the main noise region.
3. Fast detector By optimizing the geometry and some capacities of the detector it is possible to decrease the signal width below 1 ns for different types of electron multipliers. With the well known MCP electron multipliers we achieved about 560 ps (see Fig. 5). For the novel MSP electron multipliers we achieved 640 ps. A MSP (multi-sphere plate [3.4]) consists of glass beads 20-100 urn in diameter, sintered to form a thin, porous plate. The functional principle of a MSP is similar to that of a MCP, but the electron multiplication process does not occur within distinct channels but on the surface of the glass beads. For both types of multipliers the small signal widths lead to higher currents and hence to better PHDs.
mass m (amu)
Fig. 2. SE yield as a function of projectile mass at constant projectile energy. E= 6 keV. The function is derived from measurements with clusters impinging on metal surfaces. The solid line indicates the situation when the SE yield is proportional to the projectile velocity. However. for velocities <60 mm/us (dashed line) this proportionality no longer holds. The solid line function is derived from measurements with V clusters [I]. The dashed line is derived from measurements with HZ0 clusters [2].
4. Isochronous ion-electron
conversion surface de-
tector
With a low work function ion-electron conversion surface (e.g. CsI, MgO) it is possible to increase the production yield of secondary
1246 Vacuum Chamber __-_-_-_
Vacuum Chamber (a) r-----SMA-Feedthrough
I I
TDC
1_--_-__I Detector
Fig. 4. Electronic
scheme of a detection
system without
(a) and with (b) a current
incoming
to voltage
amplifier.
ions
+
-mot0.0
1 10
2.0
a
30
40
tme (lx)
Fig. 5. Comparison of the signal width of a detector with a chevron MCP and a double thickness MSP electron multiplier. The MCP signal width is slightly shorter.
ion-electron conversion surface
Fig. 6. lsochronous high energy physics
ion-electron experiments
conversion [6].
detector
used in
detector
Fig. 7. SIMION simulation of a detector with an ionelectron conversion surface and electrostatic particle optics. The inconing ions are indicated as squares and the secondary electrons as circles. Note that if the initial ions reach the conversion surface simultaneously, all electrons move to the detector in 5.2 * 1 ns according to the simulation.
M. Gonin et al. I Nucl. Instr. and Meth. in Phys. Res. B 136-138 (1998) 124461247
electrons considerably [5]. This leads to an increased initial signal height and better PHDs. By using a homogeneous magnetic field superimposed on an orthogonal electric field it is possible to obtain an isochronous electron transport from the conversion surface to the detector entrance (Fig. 6). This results in very good timing characteristics of such a detector, e.g. a signal jitter of about 80 ps. To avoid the handling of magnetic fields we are currently designing an electrostatic version of a detector with an ionelectron conversion surface. A computer simulation of such a detector with the code SIMION [7] is shown in Fig. 7. By tilting and shaping the conversion surface it is possible to correct the TOF differences of the incoming ions caused by the field which deflects the electrons to the detector. The TOF differences of the secondary electrons are about 2 ns. Hence, with a careful choice of geometry we reached signal widths At < 2 ns.
1241
Acknowledgements
The authors would like to thank Alexander Dodonov and his team. This work was supported by the German Bundesministerium fur Forschung und Technologie (BMFT) and the Swiss National Foundation (SNF).
References [l] W.O. Hofer. Nucl. Instr. and Meth. 170 (1980) 275. [2] R.J. Beuhler, L. Friedmann. Nucl. Instr. and Meth. 170 ( 1980) 309. [3] AS. Tremsin. J.F. Pearson, J.E. Lees, G.W. Fraser, Nucl. Instr. and Meth. A 368 (1996) 719. [4] El-Mu1 Technologies LTD, http:/&-mul.co.iU. [5] H.L. Seifert, Nucl. lnstr. and Meth. A 292 (1990) 533. [6] J. TrGtscher, Nucl. Instr. and Meth. B 70 (1992) 455. [7] D.A. Dahl, SIMION 3D Version 6.0, 43rd ASMS Conference Proceedings, 1995, p. 717.
INNil
!!I!8 ELSEVIER
B
Beam Interactions with Materials 8 Atoms
Nuclear
Instruments
and Methods
in Physics
Ion-detectors M. Gonin
Research
B 136-138 (1998) 124&1247
for TOF mass analyzers
*, Y.H. Chen ‘, Th. Horvath,
Il. Ph~~.silitrli.sc~hes Insiitut dcv hirwsiriit
Giessen
Heiwich-Buff-
M. The&s, H. Wollnik R&q 16. 35302 Gicww.
Gertncrr~~~
Abstract
Ion detection with good signal-to-noise ratio is required in TOF mass spectrometry for measurements over a wide range of masses. To reach high mass resolving powers it is also necessary to have detectors with good timing characteristics. We show some of our concepts to reach a detection system with such properties. 0 1998 Elsevier Science B.V.
1. Introduction In time-of-flight mass spectrometers (TOF MS) for laboratory use, it is common practice to only use acceleration voltages < 6 kV which leads to ion energies of 6 keV for singly charged ions. For moderate MS dimensions (I 6 1.5 m) this leads to a TOF of about t = 15 ps. A line width of At < 2 ns (for A = 28 amu) can be resolved by modern high speed detection electronics. The mass resolving power R = m/Am of such a device would be R = t/2At > 3750. An example of such a measurement is shown in Fig. 1. For moderate masses (~50 amu) the mass line widths are caused nowadays to a large extent by the detector and the detection electronics. Hence it is useful to improve the detector characteristics to improve the resolution of the TOF MS.
Most ion detectors rely on an ion-electron conversion process followed by a multiplication of the secondary electrons (SE) produced. At the low velocities considered in this work, the SE yield is approximately proportional to the molecule mass (addition rule) and proportional to the projectile velocity above some offset velocity of zz 45 mm/ ps. Low and high mass ions (4 amu < m < 500 amu) therefore have low electron production yields (Fig. 2) which leads to poor pulse height distributions (PHD) and to low detection efficiencies. The quality of a PHD is often indicated by the pulse height ratio (PHR), the peak to valley ratio (PVR), and sometimes by the peak to noise ratio (PNR) as indicated in Fig. 3. In the following we discuss some concepts to increase the quality of the PHD, to increase the detection efficiency and to improve the detector timing.
2. Current to voltage amplifier *Corresponding author. Tel.: +49 641 99 33 148: fax: +49 641 99 33 239; e-mail: [email protected]. ’ On leave from Department of Nuclear Science. National Tsing-Hua University. Taiwan. ROC. 0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PflSOl68-583X(97)00823-9
Let us assume that an ion generates one secondary electron which is multiplied 10” times by some electron multiplier and produces a current pulse of
M. Conin et al. I Nucl. Instr. und Meth. irt PIIJX Rex B 136-138
(IWS) 1244-1247
1245
PHR = Pm / FWHM
!
N,’
30
28.006,
I 32
m/q
co* 27 996 ‘, n 0
28.00 _~
28.05
width At = 2 ns and size I = Q/At z 80 uA. If this pulse is transmitted directly to an oscilloscope as is indicated in Fig. 4(a), this current flows through two 50 R resistors which are necessary to prevent signal reflections in the transmission line. The total resistance of R = 25 fi converts the current into a voltage of U = RI E 2 mV. The PHD of such a configuration is shown in Fig. 4(a). By inserting a current to voltage amplifier directly behind the detector as shown in Fig. 4(b), it is possible to convert the current directly into a slightly amplified voltage signal with the amplifier also working as 3.0
5 c 00.0,
P u)
=..--%__ I
0
200
,
400 particle
,
,
600
800
1000
100
150
signal voltage (mV)
m/q
13.5 ns
Fig. 1. Example of the measured resolving power of a time-offlight mass analyzer. The time resolution of the detection electronics is 0.5 ns.
3 9
50
28.10
Fig. 3. Definition PHD.
of the parameters
to describe
the quality
of a
a driver for the 50 R transmission line. Because the amplifier is placed closely behind the detector. reflected pulses have very short propagation times and are damped very quickly. In this way it is possible to increase the final signal height and to shift the PHD out of the main noise region.
3. Fast detector By optimizing the geometry and some capacities of the detector it is possible to decrease the signal width below 1 ns for different types of electron multipliers. With the well known MCP electron multipliers we achieved about 560 ps (see Fig. 5). For the novel MSP electron multipliers we achieved 640 ps. A MSP (multi-sphere plate [3.4]) consists of glass beads 20-100 urn in diameter, sintered to form a thin, porous plate. The functional principle of a MSP is similar to that of a MCP, but the electron multiplication process does not occur within distinct channels but on the surface of the glass beads. For both types of multipliers the small signal widths lead to higher currents and hence to better PHDs.
mass m (amu)
Fig. 2. SE yield as a function of projectile mass at constant projectile energy. E= 6 keV. The function is derived from measurements with clusters impinging on metal surfaces. The solid line indicates the situation when the SE yield is proportional to the projectile velocity. However. for velocities <60 mm/us (dashed line) this proportionality no longer holds. The solid line function is derived from measurements with V clusters [I]. The dashed line is derived from measurements with HZ0 clusters [2].
4. Isochronous ion-electron
conversion surface de-
tector
With a low work function ion-electron conversion surface (e.g. CsI, MgO) it is possible to increase the production yield of secondary
1246 Vacuum Chamber __-_-_-_
Vacuum Chamber (a) r-----SMA-Feedthrough
I I
TDC
1_--_-__I Detector
Fig. 4. Electronic
scheme of a detection
system without
(a) and with (b) a current
incoming
to voltage
amplifier.
ions
+
-mot0.0
1 10
2.0
a
30
40
tme (lx)
Fig. 5. Comparison of the signal width of a detector with a chevron MCP and a double thickness MSP electron multiplier. The MCP signal width is slightly shorter.
ion-electron conversion surface
Fig. 6. lsochronous high energy physics
ion-electron experiments
conversion [6].
detector
used in
detector
Fig. 7. SIMION simulation of a detector with an ionelectron conversion surface and electrostatic particle optics. The inconing ions are indicated as squares and the secondary electrons as circles. Note that if the initial ions reach the conversion surface simultaneously, all electrons move to the detector in 5.2 * 1 ns according to the simulation.
M. Gonin et al. I Nucl. Instr. and Meth. in Phys. Res. B 136-138 (1998) 124461247
electrons considerably [5]. This leads to an increased initial signal height and better PHDs. By using a homogeneous magnetic field superimposed on an orthogonal electric field it is possible to obtain an isochronous electron transport from the conversion surface to the detector entrance (Fig. 6). This results in very good timing characteristics of such a detector, e.g. a signal jitter of about 80 ps. To avoid the handling of magnetic fields we are currently designing an electrostatic version of a detector with an ionelectron conversion surface. A computer simulation of such a detector with the code SIMION [7] is shown in Fig. 7. By tilting and shaping the conversion surface it is possible to correct the TOF differences of the incoming ions caused by the field which deflects the electrons to the detector. The TOF differences of the secondary electrons are about 2 ns. Hence, with a careful choice of geometry we reached signal widths At < 2 ns.
1241
Acknowledgements
The authors would like to thank Alexander Dodonov and his team. This work was supported by the German Bundesministerium fur Forschung und Technologie (BMFT) and the Swiss National Foundation (SNF).
References [l] W.O. Hofer. Nucl. Instr. and Meth. 170 (1980) 275. [2] R.J. Beuhler, L. Friedmann. Nucl. Instr. and Meth. 170 ( 1980) 309. [3] AS. Tremsin. J.F. Pearson, J.E. Lees, G.W. Fraser, Nucl. Instr. and Meth. A 368 (1996) 719. [4] El-Mu1 Technologies LTD, http:/&-mul.co.iU. [5] H.L. Seifert, Nucl. lnstr. and Meth. A 292 (1990) 533. [6] J. TrGtscher, Nucl. Instr. and Meth. B 70 (1992) 455. [7] D.A. Dahl, SIMION 3D Version 6.0, 43rd ASMS Conference Proceedings, 1995, p. 717.