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Properties of a fast timing detector using an electron multiplier
Nuclear Instruments and Methods in Physics Research
538
A300 (1991) 538-541
North-Holland
Properties of a fast timing detector using an electron multiplier Keiko Yuasa-Nakagawa and Sang-Moo Lee
Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Takahide Nakagawa and Isao Tanihata
RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-01, Japan
Received
5
June
1990
and m revised form
21
September 1990
A new tinung detector for the time-of-flight method has been invented and successfully tested with heavy-ion beams. The detector consists of a thin foil and an electron multiplier . The best time resolution of 260 ps is obtained for "I Br . The time resolution is also studied in relation to the number of secondary electrons emitted from the foil .
1. Introduction
2. Description of the detector
Time-of-flight telescopes were used for the analysis of fission fragment mass distributions and for mass identification of heavy-ion reaction products up to A = 50-60. The development of fast timing detectors and improved electronics allow time resolutions in the subnanosecond region for all particles with A >_ 4 over a wide energy range. The most important development in this method is the design of a suitable time pickup detector . Because minimizations of energy and angle stragglings are required, in particular for large masses and low energy particles, thin time pickup detectors are essential. Normally a secondary electron emission detector serves fast timing signals . It consists of a thin foil which emits secondary electrons and a multiplier, e.g . a microchannel plate [1], an open photomultiplier [2] etc., which amplifies the electrons and supplies a timing signal . We have constructed a simple detector which consists of a thin foil and an electron multiplier. A test of the detector with a-particles and fission fragments was already reported [3]. One of the main characteristics of this detector is to detect secondary electrons directly by the first dynode of the electron multiplier ; therefore it is very compact . It does not need such a clean high vacuum as for channel plate detectors; furthermore it is cheap and easy to operate. Here we report the more extensive test measurements with several different par35 81 ticles (160, Cl and Br). 2 shows the description of the detector . Section Section 3 gives the experiment and the results. 0168-9002/91/$03 .50 © 1991 -
The mechanical construction of the detector system is illustrated m fig. 1 . The particles pass through a thin foil and secondary electrons are emitted from the foil . A foil was made by evaporation of Au (20 Fig/cm2) on a formvar foil of 10 ltg/cm2. The Au-foil provides a better yield of secondary electrons [4] than the AI-foil, which we have used in the test experiment [3] with a-particles from a 252Cf-source. The secondary electrons are accelerated by the potential difference between the foil and the grid (V,cc = VG - VF ; V, and VF , the voltage applied to the grid and the foil, respectively). The grid is made of a 97% transparent electroformed mesh . The secondary electrons are detected directly by
Elsevier Science Publishers B.V . (North-Holland)
Fig. 1. The description of the detector .
K Yaasa-Nakagawa et al / A fast timing detector
the first dynode of the electron multiplier . The multiplied electrons are collected by the anode as an output signal . The distance between the foil and grid is 2 mm and that between the grid and first dynode is 50 mm. The electron multiplier (Hamamatsu 82362) is a proximity mesh type which has 23-stage Cu-BeO mesh dynodes. It has a uniform sensitivity for incident position and high pulse linearity. Furthermore, it provides high immunity to magnetic fields and is quite compact (length - 6 cm) compared to other types of electron multipliers . An electron is multiplied up to 10 6 in this electron multiplier . It can be operated at a pressure of about 10 -4 Torr, or lower. The silicon surface barrier detector (SBD, 100 mm2, 150 l.Lm thick) for the stop signal and the energy information is placed at the end of the flight path (10 cm). For keeping a good timing property, we have chosen a SBD having a small capacitance (C - 70 pF) and a low resistivity (p - 1300 2 cm) [5]. We have supplied an overbias which is twice as high as a normal one. The SBD was placed parallel to the foil and grid to avoid a difference in the flight path between the foil and the SBD. Timing signals from the electron multiplier were amplified by a fast amplifier (IV62, developed by the Hahn-Meitner Institut, Berlin) and led into a constant fraction discriminator (CFD, Tennerec 454) which furnishes a timing signal independent of the initial pulse height and of the rise time . The signals from the SBD were amplified by a fast preamplifier (IV48, HMI, Berlin) and the timing of the signals was obtained by another CFD. Then the time difference between these two signals was deterrmned by a time-to-amplitude converter (Ortec 467) . 3. Experiments and results The detector was tested using 160 (5 .3 MeV/u), 3501 (3 .4 MeV/u), and 81 Br (1 .5 MeV/u) beams from the 12UD tandem accelerator at the University of Tsukuba. Ions elastically scattered by a gold target (- 60 [Lg/cm2) were detected with the TOF telescope at 10 ° . The electron multiplier was located at a distance of 103.5 cm behind the Au target . For each measurement, we changed the acceleration voltage (Vacc) between the foil and grid and tested the performance of the time response . Fig. 2 shows a time spectrum obtained by using a 84.8 MeV 160 beam . An extra time delay of 6 ns was used for calibrating the TAC. A smaller peak was obtained after putting in the delay. Fig. 3 shows the obtained time resolution for the 160 beam as a function of Vacc . The time resolution becomes better with increasing Vacc . The time resolution is mainly determined by three different components : 1) the transit time spread of
53 9
w
F-1 F-
a a w x m Ez w > w w CD
a w w z 360 400 280 320 CHANNEL NUMBER spectrum with a 84 8 MeV 160 beam . V. _ - 3.5 kV, VF = -4 kV and VSBD = 90 V. 240
Fig. 2. A
electrons between the foil and first dynode (OtED), 2) the intrinsic time resolution of the electron multiplier (OtEM), and 3) the time resolution of the SBD ('tSBD) . Then we can write the total time resolution in the following equation : Ot 2 = OtFD + OtE M + A tSBD1 Let us estimate the At,, as follows. Assuming a uniform electric field between the foil and the gnd and equal foil-to-first-dynode electron path lengths, the time spread between a secondary electron emitted with zero initial velocity from the foil and that with initial velocity vo is approximately given by the equation [6] At - vo lm o/eVdcc , (2) t
1000
p, v
8O0
z 0 600 E0 .1 0 w 400
w H 200 E-
0
0 .2
0 .4
0 .6
0 .8
1 .0
ACCELERATION VOLTAGE (Vacc )(KV)
Fig. 3. The behavior of the time resolution for a 84 .8 MeV 160 beam as a function of the acceleration voltage (Vacc). The solid line shows [(k,(O)/ V_)2 +(460)2]1/2. (k j (0) = 50 .1).
Yuasa-Nakagawa et al. / A jast timing detector
Table 1 The best time resolution for each particle Particles 160
(5 .3 MeV/u) 35 C1(3 .4 MeV/u) 81 Br (1 .5 MeV/u)
here k 2 = 2800 and AtsBD = 200 ps, which we have determined by a least squares fit of the data . As a result, it is clear that we should operate this type of detector using a foil which emits a large enough number of secondary electrons and applying a high acceleration voltage (Vacc) to get a good total time resolution .
The best time resolution [Ps] 450±40 340±40 260±40
where 1 is the distance between the foil and the first dynode, and e and m o are the charge and mass of electron, respectively. From this equation At,, is proportional to 1/Vacs . Then At is written as At = ~( kl/Vacc)2 + t]1EM + A tSBD
(3)
We can obtain variables (kl and [At 2 + Ot2BD]1/2) from the At values of various Vacc by a least squares method . The k1 and (At 2eM + AtSBD)1/2 for the 16 0 beam are 50 .1 ps kV and 460 ps, respectively . The solid line in fig. 3 shows At =
. (50.1/ Vacc)2 + (460)2
It is clear that the best time resolution for each particle has been obtained in the region where the acceleration voltage is high enough (Vac, >_ 0.4 kV). The best resolutions thus obtained for different particles are shown in table 1 . Next we discuss the relation between the time resolution and the number of secondary electrons. Here we discuss the values shown in table 1, because these values are independent of At,, . Gatti and Svelto [7] have shown that the variance of fluctuations in the arrival time of the total current pulse at the anode of the electron multiplier is AtEM = [I + EÁ(1 - I/N) 2 , an/N,
where N is the average number of secondary electrons emitted by an ion and EÁ the relative variance of the multiplier gain A, i .e ., EÁ = aÁ/A 2 . ah is the variance of the transit time of the current pulse from a single electron . If the number of secondary electrons is large, in practice N >_ a few tens, eq . (4) becomes OtEM = (1 + EÁ)ahIN .
From this equation, we can say that AtEM is proportional to 1/VN -- as has been found in many cases for photomultipliers, i.e . In fig. 4 the best time resolutions for three particles are plotted as a function of FN. We have used the values in ref. [4] and [81 for the number of secondary electrons N . The solid line shows the relation At =
2 (k2/r) + OtSBD ,
(7)
4. Conclusion For the TOF method, we have constructed a timing detector which consists of a thin foil and an electron multiplier. It has been tested with several beams : 16 0, 35 Cl and 81 Br . The best time resolution of 260 ps has been obtained for the 81 Br-beam (1 .5 MeV/u) . The intrinsic time resolution for the electron multiplier was 170 ps . The total time resolution of this detector system is described by the equation Ot 2 = At2D + At22rv1 + OtSBD
where Ot ID is the transit time spread between the foil and the first dynode and is proportional to 1/ Vac, ( Vacs: the acceleration voltage between the foil and the grid), AtEM is the intrinsic time resolution of the electron multiplier, and AtSBD is the contribution of the surface barrier detector to the total time resolution . When Vacc is high (Vacs >_ 0.4 kV), the contribution of At,, is small and the time resolution is mainly determined by AtEM and AtSBD . In addition At,M is proportional to 1/F ; here N is the number of secondary electrons emitted from the foil . From the present results, it is clear that this detector should be operated using a foil which emits a large number of secondary electrons, applying a high acceler-
,., 800 in
a.
v
z 600 C H
81 Br
n 04
w
w 200 I
15 20 0 5 10 NUMBER OF SECONDARY ELECTRONSvN Fig. 4. The total time resolution as a function of the number of secondary electrons (VN) .
K Yuasa-Nakagawa et al / A fast timing detector ation voltage between the foil and the first dynode of the electron multiplier and using a SBD which has a good time response to get a good total time resolution . References [I] T. Nakagawa and W. Bohne, Nucl . Instr. and Meth . A271 (1988) 523 ; R H. Kraus Jr ., B.J. Vieira, H. Wollink and J.M . Wouters, Nucl . Instr. and Meth . A264 (1988) 327; W. Strazecki, A.M . Steffanim, S. Lunardi and C. Signonm, Nucl . Instr. and Meth. 193 (1982) 499. [2] W.F. Schneider, B. Kohlmeyer and R. Bock, Nucl . Instr. and Meth . 87 (1970) 253;
[3] [4] [5] [6] [7]
[81
54 1
W.F . Schneider, B. Kohlmeyer, W Pfeffer, F Puehlhofer and R. Bock, Nucl . Instr. and Meth . 123 (1975) 93, T. Nakagawa, I. Tanihata and K. Yuasa-Nakagawa, Nucl . Instr. and Meth A290 (1990) 136 J. Girard and M. Bolore, Nucl . Instr and Meth . 140 (1977) 279. T. Nakagawa and K. Yuasa-Nakagawa, Jpn. J Appl . Phys . 28 (3) (1989) 38 . L .K Anderson and M. Didomemco Jr ., Advances in Microwaves, ed . L. Young (1970) vol. 5, p. 1. E Gatti and V. Svelto, Symp . Nuclear Instruments, ed. J.B . Birks, 1962, p. 35 ; F. de la Barre, Nucl . Instr . and Meth . 102 (1972) 77 . H.G . Clerc, H.J . Gehrhardt, W. Patsher, W. Schaefer and R. Bass, Nucl . Instr. and Meth . 113 (1973) 325.
538
A300 (1991) 538-541
North-Holland
Properties of a fast timing detector using an electron multiplier Keiko Yuasa-Nakagawa and Sang-Moo Lee
Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
Takahide Nakagawa and Isao Tanihata
RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-01, Japan
Received
5
June
1990
and m revised form
21
September 1990
A new tinung detector for the time-of-flight method has been invented and successfully tested with heavy-ion beams. The detector consists of a thin foil and an electron multiplier . The best time resolution of 260 ps is obtained for "I Br . The time resolution is also studied in relation to the number of secondary electrons emitted from the foil .
1. Introduction
2. Description of the detector
Time-of-flight telescopes were used for the analysis of fission fragment mass distributions and for mass identification of heavy-ion reaction products up to A = 50-60. The development of fast timing detectors and improved electronics allow time resolutions in the subnanosecond region for all particles with A >_ 4 over a wide energy range. The most important development in this method is the design of a suitable time pickup detector . Because minimizations of energy and angle stragglings are required, in particular for large masses and low energy particles, thin time pickup detectors are essential. Normally a secondary electron emission detector serves fast timing signals . It consists of a thin foil which emits secondary electrons and a multiplier, e.g . a microchannel plate [1], an open photomultiplier [2] etc., which amplifies the electrons and supplies a timing signal . We have constructed a simple detector which consists of a thin foil and an electron multiplier. A test of the detector with a-particles and fission fragments was already reported [3]. One of the main characteristics of this detector is to detect secondary electrons directly by the first dynode of the electron multiplier ; therefore it is very compact . It does not need such a clean high vacuum as for channel plate detectors; furthermore it is cheap and easy to operate. Here we report the more extensive test measurements with several different par35 81 ticles (160, Cl and Br). 2 shows the description of the detector . Section Section 3 gives the experiment and the results. 0168-9002/91/$03 .50 © 1991 -
The mechanical construction of the detector system is illustrated m fig. 1 . The particles pass through a thin foil and secondary electrons are emitted from the foil . A foil was made by evaporation of Au (20 Fig/cm2) on a formvar foil of 10 ltg/cm2. The Au-foil provides a better yield of secondary electrons [4] than the AI-foil, which we have used in the test experiment [3] with a-particles from a 252Cf-source. The secondary electrons are accelerated by the potential difference between the foil and the grid (V,cc = VG - VF ; V, and VF , the voltage applied to the grid and the foil, respectively). The grid is made of a 97% transparent electroformed mesh . The secondary electrons are detected directly by
Elsevier Science Publishers B.V . (North-Holland)
Fig. 1. The description of the detector .
K Yaasa-Nakagawa et al / A fast timing detector
the first dynode of the electron multiplier . The multiplied electrons are collected by the anode as an output signal . The distance between the foil and grid is 2 mm and that between the grid and first dynode is 50 mm. The electron multiplier (Hamamatsu 82362) is a proximity mesh type which has 23-stage Cu-BeO mesh dynodes. It has a uniform sensitivity for incident position and high pulse linearity. Furthermore, it provides high immunity to magnetic fields and is quite compact (length - 6 cm) compared to other types of electron multipliers . An electron is multiplied up to 10 6 in this electron multiplier . It can be operated at a pressure of about 10 -4 Torr, or lower. The silicon surface barrier detector (SBD, 100 mm2, 150 l.Lm thick) for the stop signal and the energy information is placed at the end of the flight path (10 cm). For keeping a good timing property, we have chosen a SBD having a small capacitance (C - 70 pF) and a low resistivity (p - 1300 2 cm) [5]. We have supplied an overbias which is twice as high as a normal one. The SBD was placed parallel to the foil and grid to avoid a difference in the flight path between the foil and the SBD. Timing signals from the electron multiplier were amplified by a fast amplifier (IV62, developed by the Hahn-Meitner Institut, Berlin) and led into a constant fraction discriminator (CFD, Tennerec 454) which furnishes a timing signal independent of the initial pulse height and of the rise time . The signals from the SBD were amplified by a fast preamplifier (IV48, HMI, Berlin) and the timing of the signals was obtained by another CFD. Then the time difference between these two signals was deterrmned by a time-to-amplitude converter (Ortec 467) . 3. Experiments and results The detector was tested using 160 (5 .3 MeV/u), 3501 (3 .4 MeV/u), and 81 Br (1 .5 MeV/u) beams from the 12UD tandem accelerator at the University of Tsukuba. Ions elastically scattered by a gold target (- 60 [Lg/cm2) were detected with the TOF telescope at 10 ° . The electron multiplier was located at a distance of 103.5 cm behind the Au target . For each measurement, we changed the acceleration voltage (Vacc) between the foil and grid and tested the performance of the time response . Fig. 2 shows a time spectrum obtained by using a 84.8 MeV 160 beam . An extra time delay of 6 ns was used for calibrating the TAC. A smaller peak was obtained after putting in the delay. Fig. 3 shows the obtained time resolution for the 160 beam as a function of Vacc . The time resolution becomes better with increasing Vacc . The time resolution is mainly determined by three different components : 1) the transit time spread of
53 9
w
F-1 F-
a a w x m Ez w > w w CD
a w w z 360 400 280 320 CHANNEL NUMBER spectrum with a 84 8 MeV 160 beam . V. _ - 3.5 kV, VF = -4 kV and VSBD = 90 V. 240
Fig. 2. A
electrons between the foil and first dynode (OtED), 2) the intrinsic time resolution of the electron multiplier (OtEM), and 3) the time resolution of the SBD ('tSBD) . Then we can write the total time resolution in the following equation : Ot 2 = OtFD + OtE M + A tSBD1 Let us estimate the At,, as follows. Assuming a uniform electric field between the foil and the gnd and equal foil-to-first-dynode electron path lengths, the time spread between a secondary electron emitted with zero initial velocity from the foil and that with initial velocity vo is approximately given by the equation [6] At - vo lm o/eVdcc , (2) t
1000
p, v
8O0
z 0 600 E0 .1 0 w 400
w H 200 E-
0
0 .2
0 .4
0 .6
0 .8
1 .0
ACCELERATION VOLTAGE (Vacc )(KV)
Fig. 3. The behavior of the time resolution for a 84 .8 MeV 160 beam as a function of the acceleration voltage (Vacc). The solid line shows [(k,(O)/ V_)2 +(460)2]1/2. (k j (0) = 50 .1).
Yuasa-Nakagawa et al. / A jast timing detector
Table 1 The best time resolution for each particle Particles 160
(5 .3 MeV/u) 35 C1(3 .4 MeV/u) 81 Br (1 .5 MeV/u)
here k 2 = 2800 and AtsBD = 200 ps, which we have determined by a least squares fit of the data . As a result, it is clear that we should operate this type of detector using a foil which emits a large enough number of secondary electrons and applying a high acceleration voltage (Vacc) to get a good total time resolution .
The best time resolution [Ps] 450±40 340±40 260±40
where 1 is the distance between the foil and the first dynode, and e and m o are the charge and mass of electron, respectively. From this equation At,, is proportional to 1/Vacs . Then At is written as At = ~( kl/Vacc)2 + t]1EM + A tSBD
(3)
We can obtain variables (kl and [At 2 + Ot2BD]1/2) from the At values of various Vacc by a least squares method . The k1 and (At 2eM + AtSBD)1/2 for the 16 0 beam are 50 .1 ps kV and 460 ps, respectively . The solid line in fig. 3 shows At =
. (50.1/ Vacc)2 + (460)2
It is clear that the best time resolution for each particle has been obtained in the region where the acceleration voltage is high enough (Vac, >_ 0.4 kV). The best resolutions thus obtained for different particles are shown in table 1 . Next we discuss the relation between the time resolution and the number of secondary electrons. Here we discuss the values shown in table 1, because these values are independent of At,, . Gatti and Svelto [7] have shown that the variance of fluctuations in the arrival time of the total current pulse at the anode of the electron multiplier is AtEM = [I + EÁ(1 - I/N) 2 , an/N,
where N is the average number of secondary electrons emitted by an ion and EÁ the relative variance of the multiplier gain A, i .e ., EÁ = aÁ/A 2 . ah is the variance of the transit time of the current pulse from a single electron . If the number of secondary electrons is large, in practice N >_ a few tens, eq . (4) becomes OtEM = (1 + EÁ)ahIN .
From this equation, we can say that AtEM is proportional to 1/VN -- as has been found in many cases for photomultipliers, i.e . In fig. 4 the best time resolutions for three particles are plotted as a function of FN. We have used the values in ref. [4] and [81 for the number of secondary electrons N . The solid line shows the relation At =
2 (k2/r) + OtSBD ,
(7)
4. Conclusion For the TOF method, we have constructed a timing detector which consists of a thin foil and an electron multiplier. It has been tested with several beams : 16 0, 35 Cl and 81 Br . The best time resolution of 260 ps has been obtained for the 81 Br-beam (1 .5 MeV/u) . The intrinsic time resolution for the electron multiplier was 170 ps . The total time resolution of this detector system is described by the equation Ot 2 = At2D + At22rv1 + OtSBD
where Ot ID is the transit time spread between the foil and the first dynode and is proportional to 1/ Vac, ( Vacs: the acceleration voltage between the foil and the grid), AtEM is the intrinsic time resolution of the electron multiplier, and AtSBD is the contribution of the surface barrier detector to the total time resolution . When Vacc is high (Vacs >_ 0.4 kV), the contribution of At,, is small and the time resolution is mainly determined by AtEM and AtSBD . In addition At,M is proportional to 1/F ; here N is the number of secondary electrons emitted from the foil . From the present results, it is clear that this detector should be operated using a foil which emits a large number of secondary electrons, applying a high acceler-
,., 800 in
a.
v
z 600 C H
81 Br
n 04
w
w 200 I
15 20 0 5 10 NUMBER OF SECONDARY ELECTRONSvN Fig. 4. The total time resolution as a function of the number of secondary electrons (VN) .
K Yuasa-Nakagawa et al / A fast timing detector ation voltage between the foil and the first dynode of the electron multiplier and using a SBD which has a good time response to get a good total time resolution . References [I] T. Nakagawa and W. Bohne, Nucl . Instr. and Meth . A271 (1988) 523 ; R H. Kraus Jr ., B.J. Vieira, H. Wollink and J.M . Wouters, Nucl . Instr. and Meth . A264 (1988) 327; W. Strazecki, A.M . Steffanim, S. Lunardi and C. Signonm, Nucl . Instr. and Meth. 193 (1982) 499. [2] W.F. Schneider, B. Kohlmeyer and R. Bock, Nucl . Instr. and Meth . 87 (1970) 253;
[3] [4] [5] [6] [7]
[81
54 1
W.F . Schneider, B. Kohlmeyer, W Pfeffer, F Puehlhofer and R. Bock, Nucl . Instr. and Meth . 123 (1975) 93, T. Nakagawa, I. Tanihata and K. Yuasa-Nakagawa, Nucl . Instr. and Meth A290 (1990) 136 J. Girard and M. Bolore, Nucl . Instr and Meth . 140 (1977) 279. T. Nakagawa and K. Yuasa-Nakagawa, Jpn. J Appl . Phys . 28 (3) (1989) 38 . L .K Anderson and M. Didomemco Jr ., Advances in Microwaves, ed . L. Young (1970) vol. 5, p. 1. E Gatti and V. Svelto, Symp . Nuclear Instruments, ed. J.B . Birks, 1962, p. 35 ; F. de la Barre, Nucl . Instr . and Meth . 102 (1972) 77 . H.G . Clerc, H.J . Gehrhardt, W. Patsher, W. Schaefer and R. Bass, Nucl . Instr. and Meth . 113 (1973) 325.