- Email: [email protected]
Bunched beam generation of polarized positrons in TOPPS-II
Applied Surface Science 149 Ž1999. 16–19
Bunched beam generation of polarized positrons in TOPPS-II M. Irako
a,)
, R. Hamatsu a , M. Hirose b, T. Hirose a , H. Iijima a , T. Kumita a , K. Matsuzawa a , N.N. Mondal a
a
b
Department of Physics, Tokyo Metropolitan UniÕersity, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan Laboratory for Quantum Equipment Technology, Sumitomo HeaÕy Industries, 2-1-1 Yato-Cho, Tanashi, Tokyo 188-0001, Japan
Abstract We have constructed a bunched beam generator of polarized slow positrons, namely TOPPS-II ŽTokyo Metropolitan University P olarized P ositron Beam S ystem. using the magnetic transportation. The positrons emitted from 100 mCi 22 Na are implanted on the tungsten moderator of 6 mm thickness. Reemitted positrons, accelerated to 200 eV and guided with 100 G magnetic field of solenoid coils, are chopped to be 40 ns every 200 ns and bunched into 2 ns time width. In this paper, we present the results of the overall performance of TOPPS-II. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 41.75.F,H; 29.27.H; 36.10.D; 78.70.B Keywords: Polarized beam; Positron beam; Bunched beam; Moderator; Polarimeter; Positronium
1. Introduction It is well known that positrons Že H. emitted from b decay of a radio-isotope is longitudinally polarized with helicity of Õrc, where Õ and c are velocities of eq and light, respectively. In order to achieve high quality polarized slow eq beams, we have established the method of measuring slow eq polarization w1x and developed a software program POEM and SPPG which can calculate eq depolarization in magnetic and electric fields as well as in materials. On the basis of these simulations, we have constructed a polarized slow eq beam generator TOPPS w2x in which a Wien filter for spin rotation can be installed. In this report, we describe performance of the modified system TOPPS-II. For attaining eq
bunched beams of higher intensity, we modified the original TOPPS by excluding the Wien filter where the magnetic transportation cannot be applied.
q
)
Corresponding author. Tel.: q81-426-77-1111 ext. 3288; Fax: q81-426-77-2483; E-mail: [email protected].
2. Experimental apparatus 2.1. Polarized slow-positron generator TOPPS-II A schematic diagram of TOPPS-II is shown in Fig. 1. A 100 mCi 22 Na source having the diameter of 3 mm is deposited on the Be backing to suppress depolarization due to back-scattering of eq. Positrons emitted from bq decay are injected on a polycrystalline tungsten ŽW. moderator of 6 mm thickness and 22 mm diameter. In TOPPS, we set up a cleaning chamber Žsee Fig. 1. where the W moderator is cleaned and annealed in the ultra high vacuum of 10y1 0 Torr by electron bombardment of 3 kV–50
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 8 3 - X
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
17
Fig. 1. Schematic illustration of TOPPS-II.
mA for about 1 min. Positive current of 25 pA was measured through the moderator. Slow positrons reemitted from the moderator are accelerated to 200 eV and transported in a distance of 34 cm under the magnetic field of 100 G and deflected by 1408 to avoid background radiation emitted from the radioisotope. Beam profiles was observed with the beam monitor installed just in front of the beam chopper. Thus we obtained positrons with intensity 1.6 = 10 4 cps and the beam diameter 7 mm as shown in Fig. 2.
eV, the work function is derived as 2.5 eV from maximum energy of 3.3 eV.
2.2. Energy distribution of e q beam We measured the energy distribution of the eq beam at the sample position Žsee Fig. 1. by the retarding method. The typical data is shown in Fig. 3, where the solid line represents eq current and the dashed curve is obtained by differentiating the current curve. The peak energy and FWHM of eq beam are thus evaluated as 2.3 eV and 1.5 eV, respectively. Since the eq beam is accelerated to be 0.8
Fig. 2. The image of eq beam on the fluorescence screen set up behind the micro channel plates.
18
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
Fig. 4. A schematic cross-section of the beam chopper and buncher.
given in Eq. Ž1.. The intensity of the bunched beam is 100 eqrs. Fig. 3. The energy distribution curve Žsolid line. and d NrdV Ždashed line. plot of polarized eq beam accelerated to 0.8 eV.
2.3. Bunched positron beam For many experiments using polarized eq, we have to distinguish orth-positronium ŽoPs. and parapositronium ŽpPs. by measuring their lifetime. Thus, we insert the chopper and buncher to attain the bunched eq beam. The chopper extracts every 200 ns the eq beam with the time width of 40 ns which the buncher compressed to 1 ns. For velocity modulation of the eq beam, the ideal pulse potential V Ž t . applied to the acceleration gap is V Ž t . s y Ž mL2r2e t 2 . q E0
2.4. Gamma-ray detector Annihilation g-rays are measured with a detector consisting of 10 NaI scintillation counters being arranged around the target material for positronium generation. Hence we can separate oPs and pPs by measuring each g-ray as well as its lifetime.
Ž 1. q
with parameters defined as follows, m: e mass, e: eq charge, L s 1.8 m: distance between the gap and the focusing point, E0 : initial eq energy, t: time for eq to travel the distance L w3,4x. However, in this experiment we tentatively utilized the pulse generator to provide the sine wave to examine technical details on our bunching system. In order to generate efficiently electric field at the gap, we insert a ferromagnetic core FINEMET having large inductance, i.e., 0.68 = 10y6 H inside the toroidal cavity as shown in Fig. 4. The time spectrum for chopped and bunched eq beams are shown in Fig. 5. The sharp peak with the width of 1 ns emerges on the relatively wide distribution, which is attributed to deviation of the applied pulse from the ideal pulse
Fig. 5. The time spectrum of chopped and bunched eq beam.
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
3. Results and discussion We have constructed the polarized eq beam generator TOPPS-II in which the buncher is installed to generate bunched eq beams with the time width of 2 ns. However, this beam should be improved in two respects namely improvement of the time structure of the bunched beam and the buncher efficiency which is suppressed in the present system to low value, i.e., 60% due to wide energy spread of eq beam of 200 eV. The improvement of the time structure is achieved by introducing a device which can generate pulses closer to the ideal pulse described in Eq. Ž1. as well as increasing the inductance of the ferromagnetic core leading to the better reproduction of an applied pulse at the gap. Since the high transportation energy, i.e., 200 eV, causes wide energy spread around 15 eV at the curved section giving rise to deterioration of the buncher efficiency, we are now attempting to transport eq beam at low energy of a few eV to the chopper and then to accelerate up to 200 eV in front of the buncher. Thus, the energy spread can be reduced down to the intrinsic energy spread of a few eV as shown in Fig. 3. After completion of these improvements, we will measure the magnitude of eq polarization using the
19
polarimeter w1x and then utilize TOPPS-II to investigate spin dependent properties of magnetic materials.
Acknowledgements This research was partially supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Sports and Culture of Japan and the Proposal-Based Advanced Industrial Technology R & D Program from New Energy and Industrial Development Organization ŽNEDO. of Japan.
References w1x T. Kumita, M. Chiba, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, M. Irako, N. Kawasaki, Y. Kurihara, T. Matsumoto, H. Nakabushi, T. Omori, Y. Takeuchi, M. Washio, J. Yang, Appl. Surf. Sci. 116 Ž1997. 1. w2x M. Irako, M. Chiba, M. Fukushima, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, T. Kumita, N.C. Mazumudar, M. Washio, Mater. Sci. Forum 255–257 Ž1997. 750. w3x M. Hirose, T. Nakajyo, M. Washio, Mater. Sci. Forum 255– 257 Ž1997. 674. w4x M. Hirose, T. Nakajyo, M. Washio, Appl. Surf. Sci. 116 Ž1997. 63.
Bunched beam generation of polarized positrons in TOPPS-II M. Irako
a,)
, R. Hamatsu a , M. Hirose b, T. Hirose a , H. Iijima a , T. Kumita a , K. Matsuzawa a , N.N. Mondal a
a
b
Department of Physics, Tokyo Metropolitan UniÕersity, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan Laboratory for Quantum Equipment Technology, Sumitomo HeaÕy Industries, 2-1-1 Yato-Cho, Tanashi, Tokyo 188-0001, Japan
Abstract We have constructed a bunched beam generator of polarized slow positrons, namely TOPPS-II ŽTokyo Metropolitan University P olarized P ositron Beam S ystem. using the magnetic transportation. The positrons emitted from 100 mCi 22 Na are implanted on the tungsten moderator of 6 mm thickness. Reemitted positrons, accelerated to 200 eV and guided with 100 G magnetic field of solenoid coils, are chopped to be 40 ns every 200 ns and bunched into 2 ns time width. In this paper, we present the results of the overall performance of TOPPS-II. q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 41.75.F,H; 29.27.H; 36.10.D; 78.70.B Keywords: Polarized beam; Positron beam; Bunched beam; Moderator; Polarimeter; Positronium
1. Introduction It is well known that positrons Že H. emitted from b decay of a radio-isotope is longitudinally polarized with helicity of Õrc, where Õ and c are velocities of eq and light, respectively. In order to achieve high quality polarized slow eq beams, we have established the method of measuring slow eq polarization w1x and developed a software program POEM and SPPG which can calculate eq depolarization in magnetic and electric fields as well as in materials. On the basis of these simulations, we have constructed a polarized slow eq beam generator TOPPS w2x in which a Wien filter for spin rotation can be installed. In this report, we describe performance of the modified system TOPPS-II. For attaining eq
bunched beams of higher intensity, we modified the original TOPPS by excluding the Wien filter where the magnetic transportation cannot be applied.
q
)
Corresponding author. Tel.: q81-426-77-1111 ext. 3288; Fax: q81-426-77-2483; E-mail: [email protected].
2. Experimental apparatus 2.1. Polarized slow-positron generator TOPPS-II A schematic diagram of TOPPS-II is shown in Fig. 1. A 100 mCi 22 Na source having the diameter of 3 mm is deposited on the Be backing to suppress depolarization due to back-scattering of eq. Positrons emitted from bq decay are injected on a polycrystalline tungsten ŽW. moderator of 6 mm thickness and 22 mm diameter. In TOPPS, we set up a cleaning chamber Žsee Fig. 1. where the W moderator is cleaned and annealed in the ultra high vacuum of 10y1 0 Torr by electron bombardment of 3 kV–50
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 8 3 - X
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
17
Fig. 1. Schematic illustration of TOPPS-II.
mA for about 1 min. Positive current of 25 pA was measured through the moderator. Slow positrons reemitted from the moderator are accelerated to 200 eV and transported in a distance of 34 cm under the magnetic field of 100 G and deflected by 1408 to avoid background radiation emitted from the radioisotope. Beam profiles was observed with the beam monitor installed just in front of the beam chopper. Thus we obtained positrons with intensity 1.6 = 10 4 cps and the beam diameter 7 mm as shown in Fig. 2.
eV, the work function is derived as 2.5 eV from maximum energy of 3.3 eV.
2.2. Energy distribution of e q beam We measured the energy distribution of the eq beam at the sample position Žsee Fig. 1. by the retarding method. The typical data is shown in Fig. 3, where the solid line represents eq current and the dashed curve is obtained by differentiating the current curve. The peak energy and FWHM of eq beam are thus evaluated as 2.3 eV and 1.5 eV, respectively. Since the eq beam is accelerated to be 0.8
Fig. 2. The image of eq beam on the fluorescence screen set up behind the micro channel plates.
18
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
Fig. 4. A schematic cross-section of the beam chopper and buncher.
given in Eq. Ž1.. The intensity of the bunched beam is 100 eqrs. Fig. 3. The energy distribution curve Žsolid line. and d NrdV Ždashed line. plot of polarized eq beam accelerated to 0.8 eV.
2.3. Bunched positron beam For many experiments using polarized eq, we have to distinguish orth-positronium ŽoPs. and parapositronium ŽpPs. by measuring their lifetime. Thus, we insert the chopper and buncher to attain the bunched eq beam. The chopper extracts every 200 ns the eq beam with the time width of 40 ns which the buncher compressed to 1 ns. For velocity modulation of the eq beam, the ideal pulse potential V Ž t . applied to the acceleration gap is V Ž t . s y Ž mL2r2e t 2 . q E0
2.4. Gamma-ray detector Annihilation g-rays are measured with a detector consisting of 10 NaI scintillation counters being arranged around the target material for positronium generation. Hence we can separate oPs and pPs by measuring each g-ray as well as its lifetime.
Ž 1. q
with parameters defined as follows, m: e mass, e: eq charge, L s 1.8 m: distance between the gap and the focusing point, E0 : initial eq energy, t: time for eq to travel the distance L w3,4x. However, in this experiment we tentatively utilized the pulse generator to provide the sine wave to examine technical details on our bunching system. In order to generate efficiently electric field at the gap, we insert a ferromagnetic core FINEMET having large inductance, i.e., 0.68 = 10y6 H inside the toroidal cavity as shown in Fig. 4. The time spectrum for chopped and bunched eq beams are shown in Fig. 5. The sharp peak with the width of 1 ns emerges on the relatively wide distribution, which is attributed to deviation of the applied pulse from the ideal pulse
Fig. 5. The time spectrum of chopped and bunched eq beam.
M. Irako et al.r Applied Surface Science 149 (1999) 16–19
3. Results and discussion We have constructed the polarized eq beam generator TOPPS-II in which the buncher is installed to generate bunched eq beams with the time width of 2 ns. However, this beam should be improved in two respects namely improvement of the time structure of the bunched beam and the buncher efficiency which is suppressed in the present system to low value, i.e., 60% due to wide energy spread of eq beam of 200 eV. The improvement of the time structure is achieved by introducing a device which can generate pulses closer to the ideal pulse described in Eq. Ž1. as well as increasing the inductance of the ferromagnetic core leading to the better reproduction of an applied pulse at the gap. Since the high transportation energy, i.e., 200 eV, causes wide energy spread around 15 eV at the curved section giving rise to deterioration of the buncher efficiency, we are now attempting to transport eq beam at low energy of a few eV to the chopper and then to accelerate up to 200 eV in front of the buncher. Thus, the energy spread can be reduced down to the intrinsic energy spread of a few eV as shown in Fig. 3. After completion of these improvements, we will measure the magnitude of eq polarization using the
19
polarimeter w1x and then utilize TOPPS-II to investigate spin dependent properties of magnetic materials.
Acknowledgements This research was partially supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Sports and Culture of Japan and the Proposal-Based Advanced Industrial Technology R & D Program from New Energy and Industrial Development Organization ŽNEDO. of Japan.
References w1x T. Kumita, M. Chiba, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, M. Irako, N. Kawasaki, Y. Kurihara, T. Matsumoto, H. Nakabushi, T. Omori, Y. Takeuchi, M. Washio, J. Yang, Appl. Surf. Sci. 116 Ž1997. 1. w2x M. Irako, M. Chiba, M. Fukushima, R. Hamatsu, M. Hirose, T. Hirose, H. Iijima, T. Kumita, N.C. Mazumudar, M. Washio, Mater. Sci. Forum 255–257 Ž1997. 750. w3x M. Hirose, T. Nakajyo, M. Washio, Mater. Sci. Forum 255– 257 Ž1997. 674. w4x M. Hirose, T. Nakajyo, M. Washio, Appl. Surf. Sci. 116 Ž1997. 63.