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Beam handling with a Penning trap of a LINAC-based slow positron beam
Nuclear Instruments and Methods in Physics Research A 337 (1994) 246-252 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Beam handling with a Penning trap of a LINAC-based slow positron beam D. Segers *, J. Paridaens, M. Dorikens, L. Dorikens-Vanpraet
Universiteit Gent, Vakgroep Subatomaire en Stralingsfysica, Proeftuinstraat 86, B-9000 Gent, Belgium
(Received 6 September 1993) A Penning trap was constructed at the Ghent LINAC-based slow positron beam . It allows us to store the positrons without considerable loss m intensity and to create a quasi-continuous beam so that positron Doppler broadening measurements are possible. Starting from the stored positrons it is also possible to create a pulsed beam with pulse width of the order of 100 ns so that time-of-flight measurements are possible . 1. Introduction In a LINAC-based slow positron beam, the positrons are created during the electron pulse of the accelerator. The physical process involved is the bremsstrahlung- and pair creation process. The main advantage of such slow positron beams is that a huge number of positrons is available. LINACs are mostly pulsed and this results in a pulsed positron beam . The pulse length and the repetition frequency are dependent on the particular electron accelerator used . The pulsed character of the beam can be an advantage for some kinds of experiments; i.e. those experiments where there is a time correlation between the positron beam and the measured signal . Examples of such experiments are the positronium velocity spectroscopy measurements using the time of flight of positronium in vacuum [1]. In order to have an adequate time zero definition, electron LINAC pulses of the order of nanoseconds are needed . LINAC-based slow positron beams can be used for experiments in the atomic physics or condensed matter fields . In the condensed matter field there is an increasing interest for material physics with slow positrons. Especially depth profiling measurements with the Doppler broadening of the annihilaton radiation on layered structures reveal very interesting information . To perform such experiments one needs a positron beam with a constant intensity and which is continuous in time (i .e . a non-pulsed slow positron * Corresponding author .
beam). Such slow positron beam characteristics are needed because the resolution of the germanium detector is very much influenced by the absolute count rate and the count rate variations during the measurements. To perform slow positron depth profiling measurements with a LINAC-based slow positron beam the intensity of the pulsed slow positron beam has to be spread out in time in order to obtain a quasi-continuous beam . During the measurements the slow positron beam intensity has to be kept as constant as possible . Typical slow positron intensities at LINAC-based slow positron beams range from the order of a few 10 7 to a few 10 9 slow positrons per second . The count rate in measurements of the Doppler broadening of the annihilation radiation with a germanium detector is limited to about 10 ° counts per second, which means that for such measurements one can afford to lose some positron intensity during the process of quasi-continuous beam production . The quasi-continuous slow positron beam starting with a pulsed LINAC-based beam is created with the help of a Penning trap . This idea was proposed by Hulett et al . [2] and has been used by several groups [3-6] with varying degrees of success.
2. Experimental setup The Ghent slow positron beam [5,7,8] is pulsed with a frequency of 300 Hz . The pulse length is typically 3 ws . Under normal working conditions a typical slow positron intensity of 4 x 10 7 slow positrons per second
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V. All rights reserved SSDI0168-9002(93)E0917-H
247
D. Segers et al. /Nucl. Instr. and Meth. in Phys. Res. A 337 (1994) 246-252
is easily obtained . This results in 1.3 x 10 5 slow positrons per pulse. These positrons are magnetically transported from the electron LINAC to the experimental chamber by a 41 m long transport system . The general layout of the transport system is shown in ref. [8]. In a first attempt [5] to construct a quasi-continuous slow positron beam a Penning trap was installed before the end of the transport system . The idea is to trap the positrons during the positron pulse and gradually release them during the time interval (of 3.3 ms) between two successive positron pulses . The length of the Penning trap is determined by the length of the slow positron pulse and the transport energy of the positrons. In our first experiments [5] this resulted in a Penning trap with a length of 6 m. It was constructed by inserting three tubes (a long central one an two short reflecting tubes at the ends) in a 0.10 m inner diameter transport tube . The vacuum in the transport system was 10 -5 Pa and before and after the Penning trap a turbo-molecular pump was installed. The magnetic field was obtained by a solenoid (producing a field of 10 mT) wound immediately onto the transport tube . The Penning trap could not be sufficiently outgassed because baking-out of the transport tube at the Vane
15~
1 .5 cm collimator
CEO
position of the Penning trap was impossible . The transport tube was constructed out of one piece (6 m long), into which the tubes of the Penning trap were inserted . This caused the vacuum in the middle of the trap to be certainly not as good as the measured value (of 10 -5 Pa) on the vacuum meters just above the pumps at both ends of the trap . It was estimated that during operation the vacuum in the middle of the Penning trap was no better than 10 -4 Pa . Positrons with an energy of 30 eV travel a distance of over 10 km in the time interval between two accelerator pulses (3 .3 ms). The slow positron intensity can be described as :
where z is the distance travelled, N is the number of residual gas molecules per unit volume and o, is the total cross section for positron-gas scattering . A typical value for this cross section is 5 x 10 -20 m2. Using Eq . (1) we can calculate the relative intensity for two vacuum conditions . The results are summarised in Table 1, from which it is clear that a much better vacuum is needed in order to have a sufficiently high positron intensity after storage of about 3 ms . The
22 Helmholtz coils
1 .5 cm collimator
vane
480
z
helicoflex metal joints
ion pump turbo pump
70
turbo pump Helmholtz coil
vacuum tube
dc .curfent wires for oompeosahon of the earth magnetic field
Fig. 1 . Layout of the installation of the Penning trap . Dimensions are given in cm . A 4.8 m long vacuum tube put together with metallic helicoflex joint is evacuated with ion pumps. The magnetic transport field is generated with Helmholtz coils. Differential pumping is applied (see stainless steel collimator with a central hole of 0.015 m). Vanes can shut off the Penning trap . The earth's magnetic field is adequately compensated .
D. Segers et al. /Nucl. lnstr and Meth.
248
Table 1 Relative positron intensity as a function of the vacuum for positron travelling a distance of 10 km in the Penning trap Vacuum (Pa) 10 -4 10 -6
I/h
5 .8 x 10 -6 0.88
results of Table 1 are in agreement with the experimental results presented in ref. [5], where it was seen that the positron intensity vanished after a storage time of 1 ms . In order to obtain better beam characteristics a new version of the Penning trap was built. Special care was paid to the vacuum conditions . A schematic drawing of the new Penning trap is given in Fig. 1 . In order not to construct a too long Penning trap, the transport energy of the slow positrons was chosen at 25 eV . The Penning trap was constructed with six metal tubes each 0.8 m long (total length of the trap is 4.8 m) and an inner diameter of 0.2 m. The tubes were put together with aluminium helicoflex joints (HN 200) . The vacuum in the Penning trap is created with two (one at each side of the trap) 150 I/s Perkin-Elmer ion pumps. The Penning trap is connected to the transport system and differential pumping is used to ensure a good vacuum in the trap . Therefore a stainless steel collimator with a central hole of 0.015 m is inserted into the tubes of the transport system (see Fig . 1) at
Lit
Phys. Res. A 337 (1994) 246-252
the beginning and erid of the Penning trap . A valve is installed before and after the Penning trap and collimator setup so that the Penning trap can be completely shut off. Just before and after the Penning trap a supplementary turbo-molecular pump is installed. The magnetic guiding field of 10 mT in the Penning trap is generated with 22 Helmholtz coils (diameter 0.45 m) . (Two supplementary coils are used to create the field over the T-pieces where the ion pumps are installed.) Heating ribbon is wound onto the vacuum tubes so that the whole Penning trap can be baked out up to temperatures of 450°C. After a 24 h bake-out at 450°C and after cooling to room temperature a total pressure of 10' Pa is measured . Since the positrons have to travel more than 10 km during the time between two positron pulses, special care is taken to compensate for the earth's magnetic field . With the help of five parallel conductors placed in two horizontal and two vertical planes (sec Fig. 1) a magnetic field is created in the horizontal and vertical directions . These components can be adjusted independent of each other so that the influence of the earth's magnetic field can be perfectly cancelled. The generation of the electric reflecting potentials is kept very simple, with flat circular grids. During the positron pulse the entrance grid is kept at 0 V potential, while the exit grid is at around 25 V. After the Penning trap is loaded with positrons the potential of the entrance grid is raised well above (to 55 V) the transport potential of the positrons. By applying a
w 0 0 U
4, O w. N Q
channel number (1 channel = 3 ps) Fig. 2 . The Penning trap is loaded with positrons during the 3 Ws long positron pulse. The positrons are stored for different confinement times (1 = 0 .3 ms, 2 = 0 .6 ms, 3 = 0 .9 ms, 4 = 1 .2 ms, 5 =1 .5 ms and 6 = 1 .8 ms) and are then released during a time interval of 0 .27 ms . No great loss of positron intensity is seen after a confinement time of 1 .8 ms . A multi-scaling measuring technique is used and each channel on the x-axis equals 3 ws .
D. Segers et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 246-252
249
100 0
b II w
90 80
c >
70
.° aó w
50
k U ay U N i+ K N 1 .,
60 40 30 20 10 0
0
0 .2
0.4
0 .6
0 .8
1
1 .2
1 .4
1 .6
1 .8
2
confinement time in ms Fig. 3. The positron intensity as a function of the confinement time for two different setups, the Penning trap as described in ref. [51 (curve 1) and the actual setup (curve 2). Both curves are normalised to the same time zero value (which is set to 100%). linearly decreasing potential at the exit grid the positrons are released, producing a quasi-continuous positron beam. The results are discussed in section 3. For experiments in the atomic physics field (i .e . ionisation and excitation of noble gases) we need to do time of flight measurements and as a consequence very short positron pulses (order of nanoseconds) are needed . The Penning trap allows us to produce such very short positron pulses by pulsing the exit grid . The obtained beam characteristics are illustrated in section 3.
3. Beam characteristics 3 .1. Quasi-continuous beam
In order to see the loss of positron intensity, positrons were stored for different confinement times (between 0.3 and 1.8 ms in steps of 0.3 ms) and then released during a time interval of 0.27 ms . The positrons were collected onto a target and the annihilation radiation was measured with a germanium detector coupled to a multi-scaling setup that was started with the pre-
100000
10000 0 U 4-i O N .D
C
1000
100
0
100
200
300
400
500
600
700
800
900
channel number (I channel = 3 }ts) Fig. 4. Positron intensity as a function of time in the time interval during two electron LINAC pulses . The positron intensity is measured with a multi-scaling measuring technique and each channel on the x-axis equals 3 Ws.
D Segers et al. /Nucl. Instr. and Meth. i n Phys. Res . A 337 (1994) 246-252
250
. C 0 0 U W O w. U Q
300
400
500
Fig. 5. Positron annihilation energy spectrum for positrons implanted with an energy of 50 eV and 20 keV. Both spectra are normalised to the same area under the 511 keV photo-peak. In the 50 eV spectrum a higher contribution in the Compton region is measured . This is due to the annihilation of ortho-positromum formed at the surface when implanting low-energy positrons. pulse of the accelerator . The pulse for the channel advance was generated every 3 ws . The results are shown in Fig. 2. It is seen that not much loss in positron intensity occurs for confinement times of the order of 2 ms . We have to stress that in the former version of the Penning trap [5] the positron intensity had dropped to zero after a confinement time of 1 ms . In Fig. 3 we represent the number of released positrons as a function of the confinement time . These data are obtained by integrating the curves of Fig. 2. These results are compared with the data of the first version of the Penning trap (described in ref. [5]). Both curves are normalised to the same confinement time zero value. For that purpose an exponential was fitted to the data of the former Penning trap (curve 1 in Fig. 062
3) and a straight line was fitted to the data of the present setup (curve 2 in Fig. 3). By extrapolating the curves to t = 0 and normalising this to 100%, we can compare the characteristics of both setups . It is clearly seen that with the present version of the Penning trap only a 15% loss of positron intensity occurs in the 3 .3 ms confinement interval . The positron intensity as a function of time during the release in the time interval in between two electron LINAC pulses is illustrated in Fig. 4. The shape of the distribution can be changed by altering the slope of the potential drop on the exit cylinder . Under real working conditions it was noticed that the shape of the time distribution could change drastically with worsening of the vacuum in the Penning trap (by warming up of the 21
a
061
2
06 0.59
"
058
"
057
17
0.56 055
0
-5
0
5
10
15
high tension (kV)
20
-5
0
5 10 15 high tenstion (kV)
20
Fig. 6. (a) Doppler broadening S-parameter and (b) peak to Compton ratio in a non-treated Al foil as a function of the positron implantation energy .
D. Segers et al. /Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 246-252 10000
a
10000-
251
b
ó U 0 w.
r_
0
200
400 600 800 channel number
(1 channel =3
1000
10 1Olu"1 .IIIIIIIIIIIII 0
ps)
200
400 600 800 channel number (1 channel= 3 us)
1000
Fig. 7. Pulsed slow positron beam (pulse width about 100 ns). Positrons are first stored in the Penning trap . The exit cylinder is electronically pulsed (pulse width 50 ns). The time interval between two successive positron pulses can be adjusted (a = 50 Ws, b =150 Ws). The positron annihilation radiation is measured with a BaF2 detector coupled to a multi-scaling setup (started with the pre-pulse of the LINAC, channel advance every 3 ws). vacuum tubes due to the power dissipation in the Helmholtz coils) .
To check the feasibility of positron Doppler broad-
ening measurements with this quasi-continuous slow
positron beam a foil of non-treated pure aluminium
was mounted into the measuring chamber and the
Doppler broadening S-parameter together with the peak to Compton ratio (obtained from the complete energy spectrum from 0 to 600 keV) was measured as a
function of the negative high tension on the sample (i .e . as a function of the positron energy). A positron annihilation energy spectrum is shown in Fig. 5 for two positron energies (i .e . 50 and 20 keV) . The higher Compton contribution in the 50 eV spectrum is a result
of the annihilation of ortho-positronium formed at the surface. This is easily measured with the peak to Compton ratio. Experimental results are shown in Fig. 6.
The measurements were performed at a count rate of 8 X 10 3 counts per second and the measuring time
was only 300 s, resulting in an area in the 511 keV annihilation line of about 3 X 10 5 counts. The statistical error on the experimentally determined S-parameters was 0 .001 . Measurements were performed repeatedly and the obtained results were reproducible. This illustrates that it is possible to perform Doppler broadening measurements with a quasi-continuous LINACbased slow positron beam . During measurements on
layered structures [9] it was noticed that the major problem with Doppler broadening measurements with such a beam is the long-term stability of the positron
intensity. Manual control of the focusing optics at the moderator is necessary to ensure a constant positron intensity during the measurements . Experimental details will be published elsewhere [9].
3.2. Generation of a pulsed slow positron beam with nanosecond pulse width
Some atomic physics experiments require time-offlight measurements, so that slow positron pulses in the nanosecond region are needed . An electronic pulser was constructed so that the voltage on the exit cylinder could be pulsed (from an upper tension adjustable between 0 and 50 V - to be chosen higher than the
transport tension - to 0 V). The pulse width is determined by a fixed cable length . In the actual setup 50 ns wide voltage pulses are obtained. The time interval
between the pulses can be adjusted. In Fig. 7 we illustrate the pulsed character of the obtained beam for different time intervals between successive positron pulses . The annihilation radiation at a target was measured with a BaF2 detector . The shaping of the pulses
in the preamplifier and amplifier setup resulted in decay times of the order of 6 ws. The pulses of the measuring chain were recorded with a multi-scaling setup (start pulse = LINAC pre-pulse, channel advance every 3 lts). Due to the long shaping time, the actual form of the positron pulse could not be measured . It was estimated (taking into account the difference in flight time of the positrons from the exit of the Pen-
ning trap to the target) that the actual width of the positron pulse does not exceed 100 ns . Experimental results with the pulsed nanosecond beam will be presented elsewhere [10] .
4. Conclusion It is demonstrated that slow positrons from a LINAC-based slow positron beam can be stored in a
252
D. Segers et al. /Nucl. Instr. and Meth to Phys. Res. A 337 (1994) 246-252
Penning trap and gradually released during the time interval between two successive electron LINAC pulses . Not much loss of positron intensity is seen for confinement times of the order of 2 ms . With the thus obtained quasi-continuous slow positron beam it is possible to perform positron Doppler broadening measurements. The exit cylinder of the Penning trap can also be pulsed so that pulses with a width of 100 ns can be obtained very easily . This allows us to perform timeof-flight measurements with a LINAC-based slow positron beam where the electron LINAC pulses are in the microsecond region . Acknowledgements This research is part of the programme of the IIKW Brussels, Belgium (contract 4.0002.91) . Financial support is acknowledged . In W. Mondelaers and the technical staff of the laboratory are acknowledged for the operation of the LINAC. Ing. C. van den Bossche is thanked for the design of the electronic pulser. The staff of the mechanical workshop are acknowledged for the elaborate construction of the Penning trap .
References [1] R. Howell, IT Rosenberg and M.J . Fluss, Nucl . Instr. and Meth . 43 (1987) 247. [2] L.D . Hulett, Jr ., T.A . Lewis, R.G. Alsmiller, Jr ., R. Peelle, S. Pendyala, J.M . Dale and T.M. Rosseel, Nucl . Instr . and Meth. B 24/25 (1987) 905. [3] Y Ito, O. Sueoka, M. Hirose, M. Hasegawa, S. Takamura, T. Hyodo and Y. Tabata, Proc. 8th Int. Conf Positron Annihilation, eds. L. Dorikens-Vanpraet, M. Dorikens and D. Segers (World Scientific, 1989) p. 583. [4] F. Ebel, W. Faust, H. Schneider and 1. Tobehn, Nucl . Instr. and Meth. A 274 (1989) 1 . [5] J. Paridaens, D Segers, M. Dorikens and L. DorikensVanpraet, Nucl . Instr. and Meth . A 295 (1990) 39 . [61 T. Akahane, T. Chiba, N. Shiotam, S Tanigawa, T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki and T. Tomimasu, Appl. Phys . A 51 (1990) 146. [7] J. Paridaens, D. Segers, M. Dorikens and L. DorikensVanpraet, Nucl . Instr. and Meth . A 287 (1990) 359. [8] J. Paridaens, D. Segers, M. Dorikens and L. DorikensVanpraet, in : Positron Beams for Solids and Surfaces, AIP Conf. Proc. 218: London, Ontario, Canada, 1990, eds. P. Schultz, G.R . Massoumi and P.J . Simpson (American Institute of Physics, 1990) p. 259. [9] D. Segers et al .; to be published. [10] R. Hippler et a].; to be published.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Beam handling with a Penning trap of a LINAC-based slow positron beam D. Segers *, J. Paridaens, M. Dorikens, L. Dorikens-Vanpraet
Universiteit Gent, Vakgroep Subatomaire en Stralingsfysica, Proeftuinstraat 86, B-9000 Gent, Belgium
(Received 6 September 1993) A Penning trap was constructed at the Ghent LINAC-based slow positron beam . It allows us to store the positrons without considerable loss m intensity and to create a quasi-continuous beam so that positron Doppler broadening measurements are possible. Starting from the stored positrons it is also possible to create a pulsed beam with pulse width of the order of 100 ns so that time-of-flight measurements are possible . 1. Introduction In a LINAC-based slow positron beam, the positrons are created during the electron pulse of the accelerator. The physical process involved is the bremsstrahlung- and pair creation process. The main advantage of such slow positron beams is that a huge number of positrons is available. LINACs are mostly pulsed and this results in a pulsed positron beam . The pulse length and the repetition frequency are dependent on the particular electron accelerator used . The pulsed character of the beam can be an advantage for some kinds of experiments; i.e. those experiments where there is a time correlation between the positron beam and the measured signal . Examples of such experiments are the positronium velocity spectroscopy measurements using the time of flight of positronium in vacuum [1]. In order to have an adequate time zero definition, electron LINAC pulses of the order of nanoseconds are needed . LINAC-based slow positron beams can be used for experiments in the atomic physics or condensed matter fields . In the condensed matter field there is an increasing interest for material physics with slow positrons. Especially depth profiling measurements with the Doppler broadening of the annihilaton radiation on layered structures reveal very interesting information . To perform such experiments one needs a positron beam with a constant intensity and which is continuous in time (i .e . a non-pulsed slow positron * Corresponding author .
beam). Such slow positron beam characteristics are needed because the resolution of the germanium detector is very much influenced by the absolute count rate and the count rate variations during the measurements. To perform slow positron depth profiling measurements with a LINAC-based slow positron beam the intensity of the pulsed slow positron beam has to be spread out in time in order to obtain a quasi-continuous beam . During the measurements the slow positron beam intensity has to be kept as constant as possible . Typical slow positron intensities at LINAC-based slow positron beams range from the order of a few 10 7 to a few 10 9 slow positrons per second . The count rate in measurements of the Doppler broadening of the annihilation radiation with a germanium detector is limited to about 10 ° counts per second, which means that for such measurements one can afford to lose some positron intensity during the process of quasi-continuous beam production . The quasi-continuous slow positron beam starting with a pulsed LINAC-based beam is created with the help of a Penning trap . This idea was proposed by Hulett et al . [2] and has been used by several groups [3-6] with varying degrees of success.
2. Experimental setup The Ghent slow positron beam [5,7,8] is pulsed with a frequency of 300 Hz . The pulse length is typically 3 ws . Under normal working conditions a typical slow positron intensity of 4 x 10 7 slow positrons per second
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V. All rights reserved SSDI0168-9002(93)E0917-H
247
D. Segers et al. /Nucl. Instr. and Meth. in Phys. Res. A 337 (1994) 246-252
is easily obtained . This results in 1.3 x 10 5 slow positrons per pulse. These positrons are magnetically transported from the electron LINAC to the experimental chamber by a 41 m long transport system . The general layout of the transport system is shown in ref. [8]. In a first attempt [5] to construct a quasi-continuous slow positron beam a Penning trap was installed before the end of the transport system . The idea is to trap the positrons during the positron pulse and gradually release them during the time interval (of 3.3 ms) between two successive positron pulses . The length of the Penning trap is determined by the length of the slow positron pulse and the transport energy of the positrons. In our first experiments [5] this resulted in a Penning trap with a length of 6 m. It was constructed by inserting three tubes (a long central one an two short reflecting tubes at the ends) in a 0.10 m inner diameter transport tube . The vacuum in the transport system was 10 -5 Pa and before and after the Penning trap a turbo-molecular pump was installed. The magnetic field was obtained by a solenoid (producing a field of 10 mT) wound immediately onto the transport tube . The Penning trap could not be sufficiently outgassed because baking-out of the transport tube at the Vane
15~
1 .5 cm collimator
CEO
position of the Penning trap was impossible . The transport tube was constructed out of one piece (6 m long), into which the tubes of the Penning trap were inserted . This caused the vacuum in the middle of the trap to be certainly not as good as the measured value (of 10 -5 Pa) on the vacuum meters just above the pumps at both ends of the trap . It was estimated that during operation the vacuum in the middle of the Penning trap was no better than 10 -4 Pa . Positrons with an energy of 30 eV travel a distance of over 10 km in the time interval between two accelerator pulses (3 .3 ms). The slow positron intensity can be described as :
where z is the distance travelled, N is the number of residual gas molecules per unit volume and o, is the total cross section for positron-gas scattering . A typical value for this cross section is 5 x 10 -20 m2. Using Eq . (1) we can calculate the relative intensity for two vacuum conditions . The results are summarised in Table 1, from which it is clear that a much better vacuum is needed in order to have a sufficiently high positron intensity after storage of about 3 ms . The
22 Helmholtz coils
1 .5 cm collimator
vane
480
z
helicoflex metal joints
ion pump turbo pump
70
turbo pump Helmholtz coil
vacuum tube
dc .curfent wires for oompeosahon of the earth magnetic field
Fig. 1 . Layout of the installation of the Penning trap . Dimensions are given in cm . A 4.8 m long vacuum tube put together with metallic helicoflex joint is evacuated with ion pumps. The magnetic transport field is generated with Helmholtz coils. Differential pumping is applied (see stainless steel collimator with a central hole of 0.015 m). Vanes can shut off the Penning trap . The earth's magnetic field is adequately compensated .
D. Segers et al. /Nucl. lnstr and Meth.
248
Table 1 Relative positron intensity as a function of the vacuum for positron travelling a distance of 10 km in the Penning trap Vacuum (Pa) 10 -4 10 -6
I/h
5 .8 x 10 -6 0.88
results of Table 1 are in agreement with the experimental results presented in ref. [5], where it was seen that the positron intensity vanished after a storage time of 1 ms . In order to obtain better beam characteristics a new version of the Penning trap was built. Special care was paid to the vacuum conditions . A schematic drawing of the new Penning trap is given in Fig. 1 . In order not to construct a too long Penning trap, the transport energy of the slow positrons was chosen at 25 eV . The Penning trap was constructed with six metal tubes each 0.8 m long (total length of the trap is 4.8 m) and an inner diameter of 0.2 m. The tubes were put together with aluminium helicoflex joints (HN 200) . The vacuum in the Penning trap is created with two (one at each side of the trap) 150 I/s Perkin-Elmer ion pumps. The Penning trap is connected to the transport system and differential pumping is used to ensure a good vacuum in the trap . Therefore a stainless steel collimator with a central hole of 0.015 m is inserted into the tubes of the transport system (see Fig . 1) at
Lit
Phys. Res. A 337 (1994) 246-252
the beginning and erid of the Penning trap . A valve is installed before and after the Penning trap and collimator setup so that the Penning trap can be completely shut off. Just before and after the Penning trap a supplementary turbo-molecular pump is installed. The magnetic guiding field of 10 mT in the Penning trap is generated with 22 Helmholtz coils (diameter 0.45 m) . (Two supplementary coils are used to create the field over the T-pieces where the ion pumps are installed.) Heating ribbon is wound onto the vacuum tubes so that the whole Penning trap can be baked out up to temperatures of 450°C. After a 24 h bake-out at 450°C and after cooling to room temperature a total pressure of 10' Pa is measured . Since the positrons have to travel more than 10 km during the time between two positron pulses, special care is taken to compensate for the earth's magnetic field . With the help of five parallel conductors placed in two horizontal and two vertical planes (sec Fig. 1) a magnetic field is created in the horizontal and vertical directions . These components can be adjusted independent of each other so that the influence of the earth's magnetic field can be perfectly cancelled. The generation of the electric reflecting potentials is kept very simple, with flat circular grids. During the positron pulse the entrance grid is kept at 0 V potential, while the exit grid is at around 25 V. After the Penning trap is loaded with positrons the potential of the entrance grid is raised well above (to 55 V) the transport potential of the positrons. By applying a
w 0 0 U
4, O w. N Q
channel number (1 channel = 3 ps) Fig. 2 . The Penning trap is loaded with positrons during the 3 Ws long positron pulse. The positrons are stored for different confinement times (1 = 0 .3 ms, 2 = 0 .6 ms, 3 = 0 .9 ms, 4 = 1 .2 ms, 5 =1 .5 ms and 6 = 1 .8 ms) and are then released during a time interval of 0 .27 ms . No great loss of positron intensity is seen after a confinement time of 1 .8 ms . A multi-scaling measuring technique is used and each channel on the x-axis equals 3 ws .
D. Segers et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 246-252
249
100 0
b II w
90 80
c >
70
.° aó w
50
k U ay U N i+ K N 1 .,
60 40 30 20 10 0
0
0 .2
0.4
0 .6
0 .8
1
1 .2
1 .4
1 .6
1 .8
2
confinement time in ms Fig. 3. The positron intensity as a function of the confinement time for two different setups, the Penning trap as described in ref. [51 (curve 1) and the actual setup (curve 2). Both curves are normalised to the same time zero value (which is set to 100%). linearly decreasing potential at the exit grid the positrons are released, producing a quasi-continuous positron beam. The results are discussed in section 3. For experiments in the atomic physics field (i .e . ionisation and excitation of noble gases) we need to do time of flight measurements and as a consequence very short positron pulses (order of nanoseconds) are needed . The Penning trap allows us to produce such very short positron pulses by pulsing the exit grid . The obtained beam characteristics are illustrated in section 3.
3. Beam characteristics 3 .1. Quasi-continuous beam
In order to see the loss of positron intensity, positrons were stored for different confinement times (between 0.3 and 1.8 ms in steps of 0.3 ms) and then released during a time interval of 0.27 ms . The positrons were collected onto a target and the annihilation radiation was measured with a germanium detector coupled to a multi-scaling setup that was started with the pre-
100000
10000 0 U 4-i O N .D
C
1000
100
0
100
200
300
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500
600
700
800
900
channel number (I channel = 3 }ts) Fig. 4. Positron intensity as a function of time in the time interval during two electron LINAC pulses . The positron intensity is measured with a multi-scaling measuring technique and each channel on the x-axis equals 3 Ws.
D Segers et al. /Nucl. Instr. and Meth. i n Phys. Res . A 337 (1994) 246-252
250
. C 0 0 U W O w. U Q
300
400
500
Fig. 5. Positron annihilation energy spectrum for positrons implanted with an energy of 50 eV and 20 keV. Both spectra are normalised to the same area under the 511 keV photo-peak. In the 50 eV spectrum a higher contribution in the Compton region is measured . This is due to the annihilation of ortho-positromum formed at the surface when implanting low-energy positrons. pulse of the accelerator . The pulse for the channel advance was generated every 3 ws . The results are shown in Fig. 2. It is seen that not much loss in positron intensity occurs for confinement times of the order of 2 ms . We have to stress that in the former version of the Penning trap [5] the positron intensity had dropped to zero after a confinement time of 1 ms . In Fig. 3 we represent the number of released positrons as a function of the confinement time . These data are obtained by integrating the curves of Fig. 2. These results are compared with the data of the first version of the Penning trap (described in ref. [5]). Both curves are normalised to the same confinement time zero value. For that purpose an exponential was fitted to the data of the former Penning trap (curve 1 in Fig. 062
3) and a straight line was fitted to the data of the present setup (curve 2 in Fig. 3). By extrapolating the curves to t = 0 and normalising this to 100%, we can compare the characteristics of both setups . It is clearly seen that with the present version of the Penning trap only a 15% loss of positron intensity occurs in the 3 .3 ms confinement interval . The positron intensity as a function of time during the release in the time interval in between two electron LINAC pulses is illustrated in Fig. 4. The shape of the distribution can be changed by altering the slope of the potential drop on the exit cylinder . Under real working conditions it was noticed that the shape of the time distribution could change drastically with worsening of the vacuum in the Penning trap (by warming up of the 21
a
061
2
06 0.59
"
058
"
057
17
0.56 055
0
-5
0
5
10
15
high tension (kV)
20
-5
0
5 10 15 high tenstion (kV)
20
Fig. 6. (a) Doppler broadening S-parameter and (b) peak to Compton ratio in a non-treated Al foil as a function of the positron implantation energy .
D. Segers et al. /Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 246-252 10000
a
10000-
251
b
ó U 0 w.
r_
0
200
400 600 800 channel number
(1 channel =3
1000
10 1Olu"1 .IIIIIIIIIIIII 0
ps)
200
400 600 800 channel number (1 channel= 3 us)
1000
Fig. 7. Pulsed slow positron beam (pulse width about 100 ns). Positrons are first stored in the Penning trap . The exit cylinder is electronically pulsed (pulse width 50 ns). The time interval between two successive positron pulses can be adjusted (a = 50 Ws, b =150 Ws). The positron annihilation radiation is measured with a BaF2 detector coupled to a multi-scaling setup (started with the pre-pulse of the LINAC, channel advance every 3 ws). vacuum tubes due to the power dissipation in the Helmholtz coils) .
To check the feasibility of positron Doppler broad-
ening measurements with this quasi-continuous slow
positron beam a foil of non-treated pure aluminium
was mounted into the measuring chamber and the
Doppler broadening S-parameter together with the peak to Compton ratio (obtained from the complete energy spectrum from 0 to 600 keV) was measured as a
function of the negative high tension on the sample (i .e . as a function of the positron energy). A positron annihilation energy spectrum is shown in Fig. 5 for two positron energies (i .e . 50 and 20 keV) . The higher Compton contribution in the 50 eV spectrum is a result
of the annihilation of ortho-positronium formed at the surface. This is easily measured with the peak to Compton ratio. Experimental results are shown in Fig. 6.
The measurements were performed at a count rate of 8 X 10 3 counts per second and the measuring time
was only 300 s, resulting in an area in the 511 keV annihilation line of about 3 X 10 5 counts. The statistical error on the experimentally determined S-parameters was 0 .001 . Measurements were performed repeatedly and the obtained results were reproducible. This illustrates that it is possible to perform Doppler broadening measurements with a quasi-continuous LINACbased slow positron beam . During measurements on
layered structures [9] it was noticed that the major problem with Doppler broadening measurements with such a beam is the long-term stability of the positron
intensity. Manual control of the focusing optics at the moderator is necessary to ensure a constant positron intensity during the measurements . Experimental details will be published elsewhere [9].
3.2. Generation of a pulsed slow positron beam with nanosecond pulse width
Some atomic physics experiments require time-offlight measurements, so that slow positron pulses in the nanosecond region are needed . An electronic pulser was constructed so that the voltage on the exit cylinder could be pulsed (from an upper tension adjustable between 0 and 50 V - to be chosen higher than the
transport tension - to 0 V). The pulse width is determined by a fixed cable length . In the actual setup 50 ns wide voltage pulses are obtained. The time interval
between the pulses can be adjusted. In Fig. 7 we illustrate the pulsed character of the obtained beam for different time intervals between successive positron pulses . The annihilation radiation at a target was measured with a BaF2 detector . The shaping of the pulses
in the preamplifier and amplifier setup resulted in decay times of the order of 6 ws. The pulses of the measuring chain were recorded with a multi-scaling setup (start pulse = LINAC pre-pulse, channel advance every 3 lts). Due to the long shaping time, the actual form of the positron pulse could not be measured . It was estimated (taking into account the difference in flight time of the positrons from the exit of the Pen-
ning trap to the target) that the actual width of the positron pulse does not exceed 100 ns . Experimental results with the pulsed nanosecond beam will be presented elsewhere [10] .
4. Conclusion It is demonstrated that slow positrons from a LINAC-based slow positron beam can be stored in a
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D. Segers et al. /Nucl. Instr. and Meth to Phys. Res. A 337 (1994) 246-252
Penning trap and gradually released during the time interval between two successive electron LINAC pulses . Not much loss of positron intensity is seen for confinement times of the order of 2 ms . With the thus obtained quasi-continuous slow positron beam it is possible to perform positron Doppler broadening measurements. The exit cylinder of the Penning trap can also be pulsed so that pulses with a width of 100 ns can be obtained very easily . This allows us to perform timeof-flight measurements with a LINAC-based slow positron beam where the electron LINAC pulses are in the microsecond region . Acknowledgements This research is part of the programme of the IIKW Brussels, Belgium (contract 4.0002.91) . Financial support is acknowledged . In W. Mondelaers and the technical staff of the laboratory are acknowledged for the operation of the LINAC. Ing. C. van den Bossche is thanked for the design of the electronic pulser. The staff of the mechanical workshop are acknowledged for the elaborate construction of the Penning trap .
References [1] R. Howell, IT Rosenberg and M.J . Fluss, Nucl . Instr. and Meth . 43 (1987) 247. [2] L.D . Hulett, Jr ., T.A . Lewis, R.G. Alsmiller, Jr ., R. Peelle, S. Pendyala, J.M . Dale and T.M. Rosseel, Nucl . Instr . and Meth. B 24/25 (1987) 905. [3] Y Ito, O. Sueoka, M. Hirose, M. Hasegawa, S. Takamura, T. Hyodo and Y. Tabata, Proc. 8th Int. Conf Positron Annihilation, eds. L. Dorikens-Vanpraet, M. Dorikens and D. Segers (World Scientific, 1989) p. 583. [4] F. Ebel, W. Faust, H. Schneider and 1. Tobehn, Nucl . Instr. and Meth. A 274 (1989) 1 . [5] J. Paridaens, D Segers, M. Dorikens and L. DorikensVanpraet, Nucl . Instr. and Meth . A 295 (1990) 39 . [61 T. Akahane, T. Chiba, N. Shiotam, S Tanigawa, T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki and T. Tomimasu, Appl. Phys . A 51 (1990) 146. [7] J. Paridaens, D. Segers, M. Dorikens and L. DorikensVanpraet, Nucl . Instr. and Meth . A 287 (1990) 359. [8] J. Paridaens, D. Segers, M. Dorikens and L. DorikensVanpraet, in : Positron Beams for Solids and Surfaces, AIP Conf. Proc. 218: London, Ontario, Canada, 1990, eds. P. Schultz, G.R . Massoumi and P.J . Simpson (American Institute of Physics, 1990) p. 259. [9] D. Segers et al .; to be published. [10] R. Hippler et a].; to be published.