- Email: [email protected]
D atoms extracted from a storage cell with a Lamb-shift polarimeter
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337 www.elsevier.com/locate/nima
Nuclear polarization measurement of H/D atoms extracted from a storage cell with a Lamb-shift polarimeter R. Engelsa,, R. Emmerichb, K. Grigorievc, J. Leyb, M. Mikirtytchiantsc, R. Rathmanna, J. Sarkadia, H. Paetz gen. Schieckb, H. Seyfartha, G. Tenckhoffb, T. Ullricha, A. Vassilievc a
Institut fu¨r Kernphysik, FZ Ju¨lich, 52425 Ju¨lich, Germany Institut fu¨r Kernphysik, Universita¨t zu Ko¨ln, Zu¨lpicher Str. 77, 50937 Ko¨ln, Germany c St. Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia
b
Available online 11 September 2004
Abstract The occupation numbers of the individual hyperfine substates in a beam of hydrogen or deuterium atoms are measured with a Lamb-shift polarimeter. Therefore, the nuclear polarization of an atomic beam can be determined with high precision of about 0:5% within 20 s. The measurements are carried out with the atomic beam source for the internal polarized storage-cell gas target of ANKE (I 7 1016 atoms/s). The polarization of a slow (500–2000 eV) ion beam with currents higher than 1 nA can be measured as well. When the intensity of the atomic beam is reduced by a factor of 100, the Lamb-shift polarimeter still provides a precise measurement with an error of about 2% within 30 s. Thus, it is thought possible to determine the nuclear polarization of the atomic gas inside the storage cell, when a sample of these atoms effuse into the Lamb-shift polarimeter through a sample tube. First, results obtained with a simple Teflon cell, a new ionizer with an internal getter pump, a newly designed Wien filter and a larger photomultiplier are presented. Planned studies to optimize the polarization measurement are discussed as well. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.Pj; 29.25.Lg Keywords: Lamb-shift polarimeter; Polarized target
1. Introduction
Corresponding author.
E-mail address: [email protected] (R. Engels).
Since two years the Lamb-shift polarimeter (LSP) [1] is routinely used to measure the nuclear polarization of hydrogen and deuterium beams
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.08.094
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337
3.0 . 10 6 2.5 . 10 6 2.0 . 10 6 1.5 . 10 6 1.0 . 10 6 0.5 . 10 6
52.5
55.0
57.5
60.0
Magnetic field in the spinfilter [mT]
62.5
the spin filter, the polarization of the atomic beam (or of a slow ion beam of 500–2000 eV) can be determined. With an overall efficiency of the polarimeter of 1010 ; a beam of 1016 atoms/s produces some 106 photons/s in the photomultiplier (Fig. 1). A statistical error of about 0:002 is obtained within 30 s. The polarization PLy ; resulting from the Lymana spectrum, is lower than the real polarization pz of the atomic beam. A number of corrections are necessary, which are discussed in Ref. [1]. The knowledge of the correction factors determines the absolute accuracy which can be obtained with the LSP.
2. The new ionizer The dominating correction factor stems from the recombination of the atoms in the ionizer. From the H2 molecules, the ionizer will produce þ Hþ 2 and H ions. The protons cannot be separated from those produced from the beam atoms. However, the ratio of extracted unpolarized Hþ and Hþ 2 ions, produced from H2 ; is a constant value of Hþ =Hþ 2 ¼ 0:095 0:008; measured by varying the H2 pressure in the ionizer volume. With beam on, the current of Hþ 2 ions is measured with a Faraday cup positioned behind the Wien filter. Then the known Hþ =Hþ 2 ratio yields the contribution by the unpolarized protons in the ion beam. Usually 10% of the protons stem from the molecules. Photons/s in the photomultiplier [arb. units]
Signal of the photomultiplier [photons/s]
produced by an atomic beam source (ABS). For this purpose, the atoms are ionized in an electronimpact ionizer [2] with an efficiency of about 103 : The ionization volume is located in a strong magnetic field (200 mT) to decouple the nuclear and electron spins. In addition, the field increases the collision probability of the accelerated electrons and the beam particles. The vertical ion beam is deflected into the horizontal beamline of the LSP by an electrostatic deflector. The necessary precession of the polarization back onto the beamline direction is achieved by a Wien filter. The Wien filter also acts as a mass filter and therefore only protons (deuterons) reach the subsequent cesium cell. There, by charge exchange with the cesium atoms, metastable atoms in the 2S state are produced. To define the polarization of the metastable hydrogen (deuterium) atoms, a strong magnetic field (55 mT) in the cesium cell is required. The spin filter [3] transmits only metastable atoms in single hyperfine states, depending on the magnetic field. All other atoms are quenched into the ground state. In the vacuum chamber at the end of the polarimeter the residual metastable atoms are quenched by an electric field (Stark effect). The emitted Lyman-a photons are registered by a photomultiplier, sensitive to photons with a wavelength near 121 nm. The number of photons counted in one hyperfine state is proportional to the number of atoms with equal nuclear spin orientation in the primary beam. From the ratio of the photons, which are produced at 53.5 mT and at 60.5 mT in
335
160 140 120 100 80 60 40 20
55.5
56.5
57.5
58.5
59.5
Magnetic field in the spinfilter [mT]
Fig. 1. The Lyman-a spectrum of a polarized hydrogen (left) and deuterium (right) beam. The measured polarization was pz ¼ 0:89 0:01 for hydrogen and pz ¼ 0:01 0:01 and pzz ¼ 0:81 0:01 for deuterium.
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337
336
To decrease this unpolarized background in the proton beam from the ionizer, a non-evaporable getter pump [4] was implemented into the ionizer. Especially the pumping speed for H2 of this getter material is very high (2000 l/s). Compared to earlier values, a reduction of the extracted Hþ 2 current by a factor of 40 could be achieved with the continuous beam from the ABS. Ion-current time spectra, taken with use of a ABS-beam chopper, installed in front of the ionizer, demonstrate the remaining recombination by the increase of the Hþ 2 current after opening the chopper (Fig. 2). By the getter pump the correction factor concerning recombination could be reduced by one order of magnitude. At the same time, the measured asymmetry in the Lyman spectrum was increased. Earlier, the error of this correction factor dominated the total correction factor. After the reduction of the H2 partial pressure in the ionizer the total error of the polarization measurements drops to 0:5%. The reduction of the unpolarized component in the Hþ current, extracted from the ionizer, would also increase the proton (deuteron) beam polarization from a source fed by the ionizer. With the getter pump it is now possible to determine the amount of H2 molecules in the ABS beam. The ratio of the ionization cross-sections for both ions, Hþ and Hþ 2 ; is well known [5] and the 180
Ion current [nA]
160
H+
140 120 100 80 60
H +2 (* 10)
40 20 0
0
25
50
75
100
125
150
175
200
225
Time [ms] Fig. 2. The Hþ and Hþ 2 currents of ions extracted from the ionizer, separated by the Wien filter and measured with a Faraday cup positioned behind the Wien filter. The beam of the ABS was opened and closed by a rotating chopper in front of the ionizer.
different ion currents are measured with the Wien filter and the Faraday cup (Fig. 2). It was found, that 3:1% 0:2% of the particles in the beam from the ABS are H2 molecules, which confirms the expected value.
3. The new photomultiplier The proton beam from the ionizer is decreased by several orders, when instead of the full ABS beam only a fraction of the gas atoms, effusing from a storage cell, will reach the ionizer. Therefore, the efficiency of the complete LSP must be increased. For this purpose a new quenching chamber was designed, with a photo0 0 multiplier [6] of a diameter increased from 10 to 20 and positioned closer to the electric quenching fields. Together with a better electric lens at the end of the ionizer, the efficiency of the complete LSP could be increased by one order of magnitude. Now an ion current of 1 nA is sufficient to get a good Lyman spectrum in a reasonable time of 30 s.
4. First cell tests With these improvements of the LSP, it now should be possible to measure the polarization of hydrogen and deuterium atoms effusing from a storage cell into the ionizer. In this case, only a fraction of 104 of the ABS-beam intensity reaches the ionizer. With an ionization efficiency of about 103 ion beams of a few nanoampere are expected behind the ionizer. The current of unpolarized protons, produced from the residual gas, has a comparable value. In the first test with a setup of feeding, storage and sampling tube, produced by standard machining of solid Teflon no increase in the proton signal could be observed, when the chopper was open. Only the Hþ 2 -ion current increased by 250 pA. When the Teflon cell was coated by fomblin oil, the proton current increased by 250 pA (Fig. 3) and that of the Hþ 2 ions only by 200 pA. Thus, there was too much recombination on the surface of the Teflon cell in the first attempt.
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337 18 16
Ion current [nA]
14
Chopper open
12 10 8
+ ∆ H = 250 pA
6 4 2 0
0
5
10
15
20
25
Time [ms]
Fig. 3. The proton current in the Faraday cup, when the chopper between storage cell and ionizer was open and closed. The signal increased for open chopper by 250 pA.
The number of unpolarized protons stemming from the residual gas in the ionizer is still more than 20 times greater than the signal difference, when the chopper is opened and closed. Longer pumping may help to increase the signal-to-background ratio. At present, the ion-current difference is too small to produce an effect in the light signal after charge exchange into metastable hydrogen and quenching in front of the photomultiplier.
337
In addition to the polarization measurements at ANKE, the LSP is an important component of an ISTC project [7] to determine the polarization of hydrogen and deuterium molecules after recombination of polarized atoms. Another application of the ABS will be the planned measurement of the reactions ~ d þ~ d! 3 ~ ~ nþ He and d þ d ! p þ t; which are of interest in the context of ‘polarized fusion’. For this purpose the ABS beam will be used as a jet target. After passing the target section the deuterium atoms are ionized and the deuterons are accelerated up to 100 keV and guided back to cross the ABS beam in the target section. After deceleration to 2 keV the polarization of the ions can be measured with the LSP. Furthermore, application of the corrections will yield the polarization of the jet target.
Acknowledgements This work has been supported by Deutsche Forschungsgemeinschaft (project 436 Rus 113/ 430), the German Ministry of Education and Research (project Rus 99/686), and FZ Ju¨lich (FF&E contract 41445283).
5. Discussion and future plans
References
Before the LSP can be used to measure the polarization of the atoms in the polarized gas target of the ANKE experiment at the cooler synchrotron (COSY) in Forschungszentrum Ju¨lich, these problems have to be solved. The signal difference (with chopper open and closed) necessary for a sufficient signal-to-background ratio is at least a few nanoampere. A lock-in amplifier could help in improving the sensitivity of the measurement.
[1] R. Engels, R. Emmerich, J. Ley, M. Mikirtytchiants, F. Rathmann, H. Seyfarth, G. Tenckhoff, H. Paetz gen. Schieck, A. Vassiliev, Rev. Sci. Instr. 74 (11) (2003) 4607. [2] H.F. Glavish, Nucl. Instr. and Meth. 65 (1968) 1. [3] J.L. McKibben, G.P. Lawrence, G.G. Ohlsen, Phys. Rev. Lett. 20 (1968) 1180. [4] SAES Getters, 2002 Lainate, Milano, Italy. [5] Y.-K.- Kim, et al., Electron-impact cross section data base hhttp://physics.nist.gov/PhysRefData/Ionizationi; 2002. [6] Electron Tubes Lim., Bury Street, Ruislip, UK. [7] ISTC project no. 1861, PNPI Gatchina, IKP Ju¨lich, IKP University of Cologne, 2001.
Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337 www.elsevier.com/locate/nima
Nuclear polarization measurement of H/D atoms extracted from a storage cell with a Lamb-shift polarimeter R. Engelsa,, R. Emmerichb, K. Grigorievc, J. Leyb, M. Mikirtytchiantsc, R. Rathmanna, J. Sarkadia, H. Paetz gen. Schieckb, H. Seyfartha, G. Tenckhoffb, T. Ullricha, A. Vassilievc a
Institut fu¨r Kernphysik, FZ Ju¨lich, 52425 Ju¨lich, Germany Institut fu¨r Kernphysik, Universita¨t zu Ko¨ln, Zu¨lpicher Str. 77, 50937 Ko¨ln, Germany c St. Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia
b
Available online 11 September 2004
Abstract The occupation numbers of the individual hyperfine substates in a beam of hydrogen or deuterium atoms are measured with a Lamb-shift polarimeter. Therefore, the nuclear polarization of an atomic beam can be determined with high precision of about 0:5% within 20 s. The measurements are carried out with the atomic beam source for the internal polarized storage-cell gas target of ANKE (I 7 1016 atoms/s). The polarization of a slow (500–2000 eV) ion beam with currents higher than 1 nA can be measured as well. When the intensity of the atomic beam is reduced by a factor of 100, the Lamb-shift polarimeter still provides a precise measurement with an error of about 2% within 30 s. Thus, it is thought possible to determine the nuclear polarization of the atomic gas inside the storage cell, when a sample of these atoms effuse into the Lamb-shift polarimeter through a sample tube. First, results obtained with a simple Teflon cell, a new ionizer with an internal getter pump, a newly designed Wien filter and a larger photomultiplier are presented. Planned studies to optimize the polarization measurement are discussed as well. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.Pj; 29.25.Lg Keywords: Lamb-shift polarimeter; Polarized target
1. Introduction
Corresponding author.
E-mail address: [email protected] (R. Engels).
Since two years the Lamb-shift polarimeter (LSP) [1] is routinely used to measure the nuclear polarization of hydrogen and deuterium beams
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.08.094
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337
3.0 . 10 6 2.5 . 10 6 2.0 . 10 6 1.5 . 10 6 1.0 . 10 6 0.5 . 10 6
52.5
55.0
57.5
60.0
Magnetic field in the spinfilter [mT]
62.5
the spin filter, the polarization of the atomic beam (or of a slow ion beam of 500–2000 eV) can be determined. With an overall efficiency of the polarimeter of 1010 ; a beam of 1016 atoms/s produces some 106 photons/s in the photomultiplier (Fig. 1). A statistical error of about 0:002 is obtained within 30 s. The polarization PLy ; resulting from the Lymana spectrum, is lower than the real polarization pz of the atomic beam. A number of corrections are necessary, which are discussed in Ref. [1]. The knowledge of the correction factors determines the absolute accuracy which can be obtained with the LSP.
2. The new ionizer The dominating correction factor stems from the recombination of the atoms in the ionizer. From the H2 molecules, the ionizer will produce þ Hþ 2 and H ions. The protons cannot be separated from those produced from the beam atoms. However, the ratio of extracted unpolarized Hþ and Hþ 2 ions, produced from H2 ; is a constant value of Hþ =Hþ 2 ¼ 0:095 0:008; measured by varying the H2 pressure in the ionizer volume. With beam on, the current of Hþ 2 ions is measured with a Faraday cup positioned behind the Wien filter. Then the known Hþ =Hþ 2 ratio yields the contribution by the unpolarized protons in the ion beam. Usually 10% of the protons stem from the molecules. Photons/s in the photomultiplier [arb. units]
Signal of the photomultiplier [photons/s]
produced by an atomic beam source (ABS). For this purpose, the atoms are ionized in an electronimpact ionizer [2] with an efficiency of about 103 : The ionization volume is located in a strong magnetic field (200 mT) to decouple the nuclear and electron spins. In addition, the field increases the collision probability of the accelerated electrons and the beam particles. The vertical ion beam is deflected into the horizontal beamline of the LSP by an electrostatic deflector. The necessary precession of the polarization back onto the beamline direction is achieved by a Wien filter. The Wien filter also acts as a mass filter and therefore only protons (deuterons) reach the subsequent cesium cell. There, by charge exchange with the cesium atoms, metastable atoms in the 2S state are produced. To define the polarization of the metastable hydrogen (deuterium) atoms, a strong magnetic field (55 mT) in the cesium cell is required. The spin filter [3] transmits only metastable atoms in single hyperfine states, depending on the magnetic field. All other atoms are quenched into the ground state. In the vacuum chamber at the end of the polarimeter the residual metastable atoms are quenched by an electric field (Stark effect). The emitted Lyman-a photons are registered by a photomultiplier, sensitive to photons with a wavelength near 121 nm. The number of photons counted in one hyperfine state is proportional to the number of atoms with equal nuclear spin orientation in the primary beam. From the ratio of the photons, which are produced at 53.5 mT and at 60.5 mT in
335
160 140 120 100 80 60 40 20
55.5
56.5
57.5
58.5
59.5
Magnetic field in the spinfilter [mT]
Fig. 1. The Lyman-a spectrum of a polarized hydrogen (left) and deuterium (right) beam. The measured polarization was pz ¼ 0:89 0:01 for hydrogen and pz ¼ 0:01 0:01 and pzz ¼ 0:81 0:01 for deuterium.
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337
336
To decrease this unpolarized background in the proton beam from the ionizer, a non-evaporable getter pump [4] was implemented into the ionizer. Especially the pumping speed for H2 of this getter material is very high (2000 l/s). Compared to earlier values, a reduction of the extracted Hþ 2 current by a factor of 40 could be achieved with the continuous beam from the ABS. Ion-current time spectra, taken with use of a ABS-beam chopper, installed in front of the ionizer, demonstrate the remaining recombination by the increase of the Hþ 2 current after opening the chopper (Fig. 2). By the getter pump the correction factor concerning recombination could be reduced by one order of magnitude. At the same time, the measured asymmetry in the Lyman spectrum was increased. Earlier, the error of this correction factor dominated the total correction factor. After the reduction of the H2 partial pressure in the ionizer the total error of the polarization measurements drops to 0:5%. The reduction of the unpolarized component in the Hþ current, extracted from the ionizer, would also increase the proton (deuteron) beam polarization from a source fed by the ionizer. With the getter pump it is now possible to determine the amount of H2 molecules in the ABS beam. The ratio of the ionization cross-sections for both ions, Hþ and Hþ 2 ; is well known [5] and the 180
Ion current [nA]
160
H+
140 120 100 80 60
H +2 (* 10)
40 20 0
0
25
50
75
100
125
150
175
200
225
Time [ms] Fig. 2. The Hþ and Hþ 2 currents of ions extracted from the ionizer, separated by the Wien filter and measured with a Faraday cup positioned behind the Wien filter. The beam of the ABS was opened and closed by a rotating chopper in front of the ionizer.
different ion currents are measured with the Wien filter and the Faraday cup (Fig. 2). It was found, that 3:1% 0:2% of the particles in the beam from the ABS are H2 molecules, which confirms the expected value.
3. The new photomultiplier The proton beam from the ionizer is decreased by several orders, when instead of the full ABS beam only a fraction of the gas atoms, effusing from a storage cell, will reach the ionizer. Therefore, the efficiency of the complete LSP must be increased. For this purpose a new quenching chamber was designed, with a photo0 0 multiplier [6] of a diameter increased from 10 to 20 and positioned closer to the electric quenching fields. Together with a better electric lens at the end of the ionizer, the efficiency of the complete LSP could be increased by one order of magnitude. Now an ion current of 1 nA is sufficient to get a good Lyman spectrum in a reasonable time of 30 s.
4. First cell tests With these improvements of the LSP, it now should be possible to measure the polarization of hydrogen and deuterium atoms effusing from a storage cell into the ionizer. In this case, only a fraction of 104 of the ABS-beam intensity reaches the ionizer. With an ionization efficiency of about 103 ion beams of a few nanoampere are expected behind the ionizer. The current of unpolarized protons, produced from the residual gas, has a comparable value. In the first test with a setup of feeding, storage and sampling tube, produced by standard machining of solid Teflon no increase in the proton signal could be observed, when the chopper was open. Only the Hþ 2 -ion current increased by 250 pA. When the Teflon cell was coated by fomblin oil, the proton current increased by 250 pA (Fig. 3) and that of the Hþ 2 ions only by 200 pA. Thus, there was too much recombination on the surface of the Teflon cell in the first attempt.
ARTICLE IN PRESS R. Engels et al. / Nuclear Instruments and Methods in Physics Research A 536 (2005) 334–337 18 16
Ion current [nA]
14
Chopper open
12 10 8
+ ∆ H = 250 pA
6 4 2 0
0
5
10
15
20
25
Time [ms]
Fig. 3. The proton current in the Faraday cup, when the chopper between storage cell and ionizer was open and closed. The signal increased for open chopper by 250 pA.
The number of unpolarized protons stemming from the residual gas in the ionizer is still more than 20 times greater than the signal difference, when the chopper is opened and closed. Longer pumping may help to increase the signal-to-background ratio. At present, the ion-current difference is too small to produce an effect in the light signal after charge exchange into metastable hydrogen and quenching in front of the photomultiplier.
337
In addition to the polarization measurements at ANKE, the LSP is an important component of an ISTC project [7] to determine the polarization of hydrogen and deuterium molecules after recombination of polarized atoms. Another application of the ABS will be the planned measurement of the reactions ~ d þ~ d! 3 ~ ~ nþ He and d þ d ! p þ t; which are of interest in the context of ‘polarized fusion’. For this purpose the ABS beam will be used as a jet target. After passing the target section the deuterium atoms are ionized and the deuterons are accelerated up to 100 keV and guided back to cross the ABS beam in the target section. After deceleration to 2 keV the polarization of the ions can be measured with the LSP. Furthermore, application of the corrections will yield the polarization of the jet target.
Acknowledgements This work has been supported by Deutsche Forschungsgemeinschaft (project 436 Rus 113/ 430), the German Ministry of Education and Research (project Rus 99/686), and FZ Ju¨lich (FF&E contract 41445283).
5. Discussion and future plans
References
Before the LSP can be used to measure the polarization of the atoms in the polarized gas target of the ANKE experiment at the cooler synchrotron (COSY) in Forschungszentrum Ju¨lich, these problems have to be solved. The signal difference (with chopper open and closed) necessary for a sufficient signal-to-background ratio is at least a few nanoampere. A lock-in amplifier could help in improving the sensitivity of the measurement.
[1] R. Engels, R. Emmerich, J. Ley, M. Mikirtytchiants, F. Rathmann, H. Seyfarth, G. Tenckhoff, H. Paetz gen. Schieck, A. Vassiliev, Rev. Sci. Instr. 74 (11) (2003) 4607. [2] H.F. Glavish, Nucl. Instr. and Meth. 65 (1968) 1. [3] J.L. McKibben, G.P. Lawrence, G.G. Ohlsen, Phys. Rev. Lett. 20 (1968) 1180. [4] SAES Getters, 2002 Lainate, Milano, Italy. [5] Y.-K.- Kim, et al., Electron-impact cross section data base hhttp://physics.nist.gov/PhysRefData/Ionizationi; 2002. [6] Electron Tubes Lim., Bury Street, Ruislip, UK. [7] ISTC project no. 1861, PNPI Gatchina, IKP Ju¨lich, IKP University of Cologne, 2001.