A Precision Lamb-shift Polarimeter for the
Polarized Gas Target at ANKE/COSY
R. Engels , R. Emmerich† , J. Ley† , M. Mikirtytchiants , H. Paetz gen.
Schieck†, F. Rathmann , H. Seyfarth and A. Vassiliev
Institut für Kernphysik, Forschungszentrum Jülich, Leo-Brandt-Str., 52425 Jülich, Germany
†
Institut für Kernphysik, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
High Energy Physics Dept., St. Petersburg Nucl. Phys. Inst., 188350 Gatchina, Russia
Abstract. The Lamb-shift polarimeter [1], described here, allows the polarization measurement
of a beam of hydrogen (deuterium) atoms, or of a slow proton (deuteron) beam (500 - 2000 eV)
with an absolute precision better than 1% within a few s. For a hydrogen beam of 3 10 16 atoms/s,
3 106 photons/s are registered in a photomultiplier. The signal-to-background ratio in the Lyman–α
spectrum is excellent, thus beam intensities of one to two orders of magnitude less would still be
sufficient to carry out a precise measurement. Therefore, it will be possible to extract a few percent
of the atoms from a storage cell to measure the polarization in the cell. In order to obtain absolute
nuclear polarization values the polarization derived from the Lyman-α spectrum has to be corrected
by a number of correction factors calculated with high precision from the behaviour of different
components of the polarimeter.
INTRODUCTION
To measure the polarization of a hydrogen or deuterium beam the atoms will be ionized
in an electron-impact ionizer [3],[4] with an efficiency of about 10 3 . The ionization
volume is located in a strong magnetic field ( 160 mT) to decouple the nuclear and the
electron spins. At the same time 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 Lamb-shift polarimeter (LSP) by an electrostatic deflector. The
necessary precession of the polarization back onto the beamline direction is archieved
by a Wien filter. This Wien filter also acts as a mass filter and therefore only protons
(deuterons) arrive in the cesium cell. By charge exchange with the cesium metastable
atoms in the 2S state are produced. To define the polarization of the metastable hydrogen
(deuterium) a strong magnetic field ( 55 mT) in the cesium cell is required. The spin
filter [5] transmits only metastable atoms in different 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 then quenched
by an electric field (Stark effect). The emitted Lyman-α photons are registered by a photomultiplier sensitive to photons with a wavelength of 121 nm. The number of photons
counted in one hyperfine state is proportional to the number of atoms with the same
nuclear spin in the primary beam. From the ratio of the photons, which are produced at
53.5 mT and 60.5 mT in the spin filter, the polarization of the atomic beam or a slow
ion beam of 500-2000 eV is calculated. With an overall efficiency of the polarimeter of
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
897
Photons/sec in the photomultiplier
3 .10
2.5 .10
6
6
6
2 .10
6
1.5 .10
6
1 .10
6
0.5 .10
52.5
55.0
57.5
60.0
mag. field in the spinfilter [mT]
62.5
FIGURE 1. The Lyman-α spectrum of a polarized hydrogen beam in the hyperfine state 1. The
polarization PLy 0780 0002 was measured in less than 30 s.
10
10
a beam of 1016 atoms/s will produce some 10 6 photons/s in the photomultiplier
(Fig. 1). Thus a statistical error of ∆PLy 0003 is obtained within 30 s.
CORRECTION FACTORS
The polarization PLy resulting from the Lyman-α spectrum is lower than the real polarization pz of the atomic beam. Some corrections are necessary. The knowledge of the
correction factors determines the absolute accuracy which can be obtained with the LSP.
1. The ionizer can produce protons from residual gas molecules like H 2 O, H2 , Cn Hn or
H2 after the recombination of atoms in the ionizer. With the atomic beam entering
the ionizer an H
2 ion current of the same order of magnitude as for the H ions
will be produced. So up to 10% of the proton beam is made from unpolarized
particles and has to be subtracted in the Lyman-α spectrum. This will increase the
polarization value by up to 10%.
2. While the polarization of hydrogen atoms in hyperfine substates 2 and 4 depends on
the magnetic field, it is constant for the pure substates 1 and 3 (Fig. 2). Therefore,
it is necessary for the calculation of the polarization in arbitrary magnetic fields
B to know the occupation numbers of the atoms in these substates for B ∞. By
varying the magnetic field in the ionizer it is possible to determine the correction to
the polarization. For example at 18 A in the solenoid coil, the magnetic field is 133
mT and only 935% 01% of pz will be measured. With this procedure it is even
possible to identify the single hyperfine substates or some mixtures of these states
in the beam. This resulting correction factor of about 7% is not needed for the pure
hyperfine states 1 and 3.
898
1
Measured Polarization PLy
HFS 1
0.5
HFS 1+2
0
-0.5
HFS 2
-1
0
5
10
15
Current in the solenoid coil of the ionizer [A]
20
FIGURE 2. The measured polarization PLy of an atomic hydrogen beam in the hyperfine states 1, 2 or
1+2 as a function of the magnetic field in the ionizer.
3. With the Wien filter the polarization vector is rotated onto the horizontal beam line.
During this process 08% of the polarization will be lost for protons. For deuterons
this loss will increase up to 1%.
4. Like in the ionizer the magnetic field in the cesium cell influences the measured
polarization because the production of the α2 ( j 12; I 12) metastable
atoms is a function of the magnetic field. Due to the small critical field (6.34 mT)
of the metastable atoms, 994% of the polarization is conserved at 55 mT on the
beam axis.
5. The transmission of the metastable atoms through the spin filter depends on the
homogeneities of the magnetic field: at 53.5 mT the transmission is higher by
almost 1% than at 60.5 mT. Therefore the measured polarization of the hyperfine
states 1 and 4 will be lower by 02% and increased for the states 2 and 3.
6. The ionization efficiency of the ionizer is different for individual hyperfine states
[7], when the electrons in the ionization volume are polarized [6]. Together with
the unpolarized electrons from the hot filament electrons produced by ionization
of polarized atoms are captured in the potential trap. The contribution of these
electrons depends sensitively on the potentials and the magnetic field in the ionizer.
Therefore the changes in the ionization efficiency can be between 0 and 7%. The
resulting polarization-dependent correction factor varies usually between 0 and
07%.
SENSITIVITY
All measurements shown here have been made with a beam intensity up to the full
intensity of the ANKE-Jülich atomic beam source [2] (about 7 10 16 particles/s in two
spin states). The LSP is sensitive enough to measure the polarization of a proton beam
899
Measured polarization PLy
1
0.8
0.6
0.4
0.2
0
2
3
4
5
6
7
8
-8
9
10
Pressure in the deflector chamber [10 mbar]
FIGURE 3. Measured polarization PLy for a beam with atoms in the hyperfine state 1 as a function of
the pressure in the deflector chamber. The presuure decrease is due to the reduction of the incoming beam
intensity.
with strongly reduced intensity in a reasonable time. When the efficiency of the ionizer
is decreased by a factor of 100, the resulting 20 nA of polarized protons are sufficient to
get a good Lyman-α spectrum.
These ion beam intensities correspond to atomic beams of 6 10 14 atoms/s with the
ionizer working at an efficiency of 10 3 . However, it was observed that the measured
polarization PLy decreases (Fig. 3), when the beam flux is reduced and the pressure in the
deflector chamber drops. This is due to the constant beam-independent background in
the Lyman-α spectrum, which causes a reduction of the ratio of the number of protons
produced from the beam and from residual gas. The nuclear polarization p z can still be
determined by applying the proper correction factors.
DISCUSSION
The LSP ist an excellent relative polarimeter with a statistical error of 03% within 30 s.
Therefore it can be used for online tuning of the transition units in an atomic beam
source.
To derive the absolute polarization of the atomic beam with a high precision some
known corrections have to be applied. The correction factors increase the measured
polarization PLy of the hyperfine states 1 and 3 by about 11% and for the states 2 and
4 by about 19% with an error of 1%. These corrections are dominated by effects in
the ionizer. A new ionizer with a stronger magnetic field and a getter pump around the
ionization volume was designed and built to reduce these overall corrections to 5% at
an error of 05%. This ionizer will be tested before the end of 2002.
900
Photomultiplier signal [a.u.]
90
80
70
60
50
40
30
20
10
55.5
56.5
57.5
58.5
59.5
Magnetic field in the spinfilter [mT]
FIGURE 4. The Lyman-α spectrum of a polarized deuterium beam. (p z Ly 006; pzz Ly 109).
The LSP can be used for deuterium as well (Fig. 4). In this case the correction factors
are smaller due to the smaller critical field and the lower background of the residual gas
in the ionizer.
With the present LSP it will be possible to measure the polarization of the primary
atomic beam with intensities down to 1% of the present beam. It is planned to measure
the polarization of atoms in a storage cell at ANKE/COSY by extracting a small amount
of the atoms. First tests are under preparation.
ACKNOWLEDGMENTS
This work has been supported by the German Ministry of Education and Research
(BMBF) and FZ Jülich (FFE contract).
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