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Construction and timing system of the EPOS beam system
Applied Surface Science 255 (2008) 42–45
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Construction and timing system of the EPOS beam system M. Jungmann a,*, R. Krause-Rehberg a, A. Mu¨ller a, A. Krille a, G. Brauer b a b
Fachbereich Physik, Martin-Luther-Universita¨t Halle-Wittenberg, D-06099 Halle, Germany Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
The Forschungszentrum Dresden-Rossendorf provides an intense pulsed 40 MeV electron beam with high brilliance and low emittance (ELBE). The pulse has a length of 1–10 ps and a repetition time of 77 ns, or in slow mode 616 ns. The EPOS system (ELBE Positron Source) generates by pair production on a tungsten converter and a tungsten moderator an intense pulsed beam of mono-energetic positrons. To transport the positrons to the laboratory (12 m) we constructed a magnetic beam guidance system with a longitudinal magnetic field of 75 G. In the laboratory outside the cave, the positron beam is chopped and bunched according to the time structure, because the very sharp bunch structure of the electron pulses is broadened for the positron beam due to transport and moderation. ß 2008 Elsevier B.V. All rights reserved.
Available online 15 May 2008 PACS: 29.27.Eg 29.25.Rm 07.77.Ka 78.70.Bj, 29.20.Mb Keyword: Positron beam
1. Introduction In the last years, the investigation of crystal lattice defects and open-volume cavities of nanometres scale has become more important. With the EPOS system it is possible to determine materials with different methods, positron lifetime spectroscopy, age–momentum correlation and Doppler broadening spectroscopy can be used [2]. Another advantage of the EPOS system is the fact that the measurements are very fast. With the multidetector system we expect a detection rate of approximately 6.2 105 counts/s. Due to the fact that we have the alternative of two repetition-times, fast and slow lifetimes can be measured. We will look at some parts of the beam in more detail. 2. Einzel lens At the beginning of the positron lens (see Fig. 1), a mesh grid made of stainless steel with an open area of 89% is located. The first tube has a length of 170 mm, the others 50 mm each. The length of the tubes is 10 mm. The moderator has a potential of
* Corresponding author. Tel.: +49 0345 5525571; fax: +49 0345 5527158. E-mail address: [email protected] (M. Jungmann). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.176
+2000 V. The mesh grid of the first and the third part of the Einzel lens have earth potential. Thus, the positrons obtain the transport energy. On the second electrode, 675 V are applied. So the focal length is 1.1 m. On this point, the magnetic field is beginning. On the focus, the diameter of the positron beam is 0.8 mm. Due to the gyration on the magnetic field, expansion of the positron beam is avoided. Because the converter chamber and the tubes material is aluminium, the whole construction is build at a flange of stainless steel, so that electrical feedthrough’s can be used. The second tube is mounted on the electric connection, so further electric isolation is not necessary. The first electrode is made of a thin cylinder with a wall thickness of 0.025 mm. The slight wall thickness is essential because the heat input should be not so high by the electron beam. 3. Chopper As the very sharp bunch structure is broadened on the way to the laboratory we use a chopper which is described in [1]. The aim is that the positron beam has a time slot of 2 ns with a repetition time of 77 ns. Before the beam arrives at the chopper, an aperture limits the diameter of the positron beam. The main component consists of an aperture, which is used in optical instruments.
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
43
Fig. 1. Exploded view of the Einzel lens, and a SEM view of the mesh grid.
In z-direction, a magnetic field of 75 G is applied. If between two plates an electric field is applied in x-direction, the positrons obtain a transversal energy in that direction. Therefore, the radius of gyration is larger and the positrons cannot pass the plates of the chopper. The pairs of electrodes with an antipodal electric field are spaced at intervals of one length of gyration (126.3 mm), so that the transversal energy is compensated when the positrons drop out of the chopper. A draft of the chopper is shown in Fig. 2. The plates of the chopper and the rotary feedthrough are installed on a double-sided bored flange. By means of a thread rod with a left-hand and right-hand thread, the aperture can be changed. In a range of 2 ns the chopper will
open the way for the positrons because the voltage is lower than 100 V. Otherwise (Fig. 3), the positrons will be more deflected and annihilate in the aperture material. As signal for the electrodes we use a damped oscillation with a frequency of 130 MHz, superposed with a direct voltage (Fig. 4). The oscillation will be stimulated at regular intervals with the signal from the ELBE system. 4. Construction and magnetic guidance field The floor of the concrete channel between the cave 111b and the positron laboratory is 2.2 m lower than the electron beam axis
Fig. 2. Draft of the chopper.
44
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
Fig. 3. Simulation of the positron trajectories with different transversal energies. The dark lines show the deflection in y-direction by a voltage of 100 V. In this direction, the chopper plates can be moved. The bright lines show the deflection in x-direction. The position of the aperture is drawn with the black vertical line.
and 0.8 m below the ground floor. The channel has a length of 3.2 m. Thus it is necessary to use a movable transport system and bent tubes (Fig. 5). An advantage is that the fast positrons cannot follow the magnetic field in the bent tubes. So we do not need an E B filter. By reason of the radiation in the cave, the first part of the beam guidance field and the beam dump is made of aluminium. In the positron laboratory, a connection for a 22Na sources is planned so that experiments can be done if the precious electron
beam time is not available. Chopper and buncher are behind this connection, so it is possible to do lifetime measurements with the 22 Na sources too. In order to ensure that the beam diameter is hardly heightened, we have simulated the magnetic guidance field with the self-written program ‘‘mfield’’ [3]. The deviation of the magnetic field is lower then 6 G, and the gradient is below 0.11 G mm 1 (Fig. 6).
Fig. 4. The required electronic signal to operate the chopper.
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
45
Fig. 5. Cutting of the beam system in the range of the wall between cave and laboratory.
References [1] Bauer-Kugelmann, Technische Weiterentwicklungen am gepulsten Positronenstrahlsystem PLEPS. Fakulta¨t fu¨r Luft-und Raumfahrttechnik, Institut fu¨r Nukleare Festko¨rperphysik. Mu¨nchen, Universita¨t der Bundeswehr Mu¨nchen (2000). [2] R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors, Springer-Verlag, Berlin, 1999, , ISBN: 3-540-64371-0. [3] V. Bondarenko, Source code available via the corresponding author, MLU HalleWittenberg, 2004.
Fig. 6. The magnetic guidance field, simulated by the program ‘‘mfield’’ [3].
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Construction and timing system of the EPOS beam system M. Jungmann a,*, R. Krause-Rehberg a, A. Mu¨ller a, A. Krille a, G. Brauer b a b
Fachbereich Physik, Martin-Luther-Universita¨t Halle-Wittenberg, D-06099 Halle, Germany Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
The Forschungszentrum Dresden-Rossendorf provides an intense pulsed 40 MeV electron beam with high brilliance and low emittance (ELBE). The pulse has a length of 1–10 ps and a repetition time of 77 ns, or in slow mode 616 ns. The EPOS system (ELBE Positron Source) generates by pair production on a tungsten converter and a tungsten moderator an intense pulsed beam of mono-energetic positrons. To transport the positrons to the laboratory (12 m) we constructed a magnetic beam guidance system with a longitudinal magnetic field of 75 G. In the laboratory outside the cave, the positron beam is chopped and bunched according to the time structure, because the very sharp bunch structure of the electron pulses is broadened for the positron beam due to transport and moderation. ß 2008 Elsevier B.V. All rights reserved.
Available online 15 May 2008 PACS: 29.27.Eg 29.25.Rm 07.77.Ka 78.70.Bj, 29.20.Mb Keyword: Positron beam
1. Introduction In the last years, the investigation of crystal lattice defects and open-volume cavities of nanometres scale has become more important. With the EPOS system it is possible to determine materials with different methods, positron lifetime spectroscopy, age–momentum correlation and Doppler broadening spectroscopy can be used [2]. Another advantage of the EPOS system is the fact that the measurements are very fast. With the multidetector system we expect a detection rate of approximately 6.2 105 counts/s. Due to the fact that we have the alternative of two repetition-times, fast and slow lifetimes can be measured. We will look at some parts of the beam in more detail. 2. Einzel lens At the beginning of the positron lens (see Fig. 1), a mesh grid made of stainless steel with an open area of 89% is located. The first tube has a length of 170 mm, the others 50 mm each. The length of the tubes is 10 mm. The moderator has a potential of
* Corresponding author. Tel.: +49 0345 5525571; fax: +49 0345 5527158. E-mail address: [email protected] (M. Jungmann). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.176
+2000 V. The mesh grid of the first and the third part of the Einzel lens have earth potential. Thus, the positrons obtain the transport energy. On the second electrode, 675 V are applied. So the focal length is 1.1 m. On this point, the magnetic field is beginning. On the focus, the diameter of the positron beam is 0.8 mm. Due to the gyration on the magnetic field, expansion of the positron beam is avoided. Because the converter chamber and the tubes material is aluminium, the whole construction is build at a flange of stainless steel, so that electrical feedthrough’s can be used. The second tube is mounted on the electric connection, so further electric isolation is not necessary. The first electrode is made of a thin cylinder with a wall thickness of 0.025 mm. The slight wall thickness is essential because the heat input should be not so high by the electron beam. 3. Chopper As the very sharp bunch structure is broadened on the way to the laboratory we use a chopper which is described in [1]. The aim is that the positron beam has a time slot of 2 ns with a repetition time of 77 ns. Before the beam arrives at the chopper, an aperture limits the diameter of the positron beam. The main component consists of an aperture, which is used in optical instruments.
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
43
Fig. 1. Exploded view of the Einzel lens, and a SEM view of the mesh grid.
In z-direction, a magnetic field of 75 G is applied. If between two plates an electric field is applied in x-direction, the positrons obtain a transversal energy in that direction. Therefore, the radius of gyration is larger and the positrons cannot pass the plates of the chopper. The pairs of electrodes with an antipodal electric field are spaced at intervals of one length of gyration (126.3 mm), so that the transversal energy is compensated when the positrons drop out of the chopper. A draft of the chopper is shown in Fig. 2. The plates of the chopper and the rotary feedthrough are installed on a double-sided bored flange. By means of a thread rod with a left-hand and right-hand thread, the aperture can be changed. In a range of 2 ns the chopper will
open the way for the positrons because the voltage is lower than 100 V. Otherwise (Fig. 3), the positrons will be more deflected and annihilate in the aperture material. As signal for the electrodes we use a damped oscillation with a frequency of 130 MHz, superposed with a direct voltage (Fig. 4). The oscillation will be stimulated at regular intervals with the signal from the ELBE system. 4. Construction and magnetic guidance field The floor of the concrete channel between the cave 111b and the positron laboratory is 2.2 m lower than the electron beam axis
Fig. 2. Draft of the chopper.
44
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
Fig. 3. Simulation of the positron trajectories with different transversal energies. The dark lines show the deflection in y-direction by a voltage of 100 V. In this direction, the chopper plates can be moved. The bright lines show the deflection in x-direction. The position of the aperture is drawn with the black vertical line.
and 0.8 m below the ground floor. The channel has a length of 3.2 m. Thus it is necessary to use a movable transport system and bent tubes (Fig. 5). An advantage is that the fast positrons cannot follow the magnetic field in the bent tubes. So we do not need an E B filter. By reason of the radiation in the cave, the first part of the beam guidance field and the beam dump is made of aluminium. In the positron laboratory, a connection for a 22Na sources is planned so that experiments can be done if the precious electron
beam time is not available. Chopper and buncher are behind this connection, so it is possible to do lifetime measurements with the 22 Na sources too. In order to ensure that the beam diameter is hardly heightened, we have simulated the magnetic guidance field with the self-written program ‘‘mfield’’ [3]. The deviation of the magnetic field is lower then 6 G, and the gradient is below 0.11 G mm 1 (Fig. 6).
Fig. 4. The required electronic signal to operate the chopper.
M. Jungmann et al. / Applied Surface Science 255 (2008) 42–45
45
Fig. 5. Cutting of the beam system in the range of the wall between cave and laboratory.
References [1] Bauer-Kugelmann, Technische Weiterentwicklungen am gepulsten Positronenstrahlsystem PLEPS. Fakulta¨t fu¨r Luft-und Raumfahrttechnik, Institut fu¨r Nukleare Festko¨rperphysik. Mu¨nchen, Universita¨t der Bundeswehr Mu¨nchen (2000). [2] R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors, Springer-Verlag, Berlin, 1999, , ISBN: 3-540-64371-0. [3] V. Bondarenko, Source code available via the corresponding author, MLU HalleWittenberg, 2004.
Fig. 6. The magnetic guidance field, simulated by the program ‘‘mfield’’ [3].