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Measurements of the Nuclear Polarisation of Optically Pumped 3He Atoms Using a 4He Magnetometer
E. Noël, H. Gilles, J. Hamel, O. Moreau, B. Chéron
To cite this version:
E. Noël, H. Gilles, J. Hamel, O. Moreau, B. Chéron. Measurements of the Nuclear Polarisation of Optically Pumped 3He Atoms Using a 4He Magnetometer. Journal de Physique III, EDP Sciences, 1996, 6 (8), pp.1127-1132. �10.1051/jp3:1996172�. �jpa-00249510�
Measurement of the Nuclear Polarisation of Optically Pumped
~He Atoms Using a ~He Magnetometer
E. No61, H. Gilles (*), J. Hamel, O. Moreau and B. Ch6ron (**)
Laboratoire de spectroscopie atomique, ISMRA, 14050 Caen Cedex, France
(Received 16 November 1995, revised 9 February 1996, accepted 20 May 1996)
PACS.32,80.Bx Level crossing and optical pumping PACS.67.65.+z Spin-polarized hydrogen and helium
PACS.07.55.Ge Magnetometers for magnetic field measurements
Abstract. A ~He magnetometer is used to detect the static magnetic field produced by optically pumped ~He nuclei submitted to a RF discharge. The experiment is made in a magnetic
field whose value (40 ~IT) is close to the earth field value. From this measurement, we determine the ~He nuclear polarisation Pn. For comparison, simultaneous determination of Pn is made by measuring the degree of circular polarisation of one of the fluorescence lines emitted from the
discharge.
R4sum4. Nous utilisons un magn4tombtre h h41ium-4 pour d4tecter le champ magn4tique statique cr44 par les spins nucl4aires d'atomes d'h41ium-3 soumis h d4charge et orient4s par pompage optique. L'exp4rience est r4alis4e dans un champ magn4tique directeur voisin de 40 ~IT (valeur voisine du champ terrestre). I partir de cette mesure, nous d4terminons la polarisation
nucl4aire Pn du gaz. Simultan4ment, nous comparons ce r4sultat h celui obtenu en mesurant Pn
par le degr6 de polarisation circulaire d'une raie de fluorescence 6mise par la d6charge.
Polarised ~He atoms have applications in different fields of physics ranging from quantum phenomena in the gaseous and fluid phases at low temperature [ii, magnetometry [2], to
polarised targets for nuclear physics experiments [3,4]. These experiments require control and
knowledge of the ~He nuclear polarisation Pn.
We briefly recall the optical pumping technique used to polarise ~He nuclei. 3He (2~Si)
metastable atoms are produced by a weak R-F- discharge and polarised by optical pumping [5]. Optical pumping is achieved by using a circularly polarised laser tuned to the (2~Si 2~P) transition at 1.08 /tm [5]. The hyperfine interaction couples the electronic and nuclear
polarisation in the (2~Si) state. The polarisation is then transferred to the ground state
through metastability exchange collisions.
Several methods can be used for measuring Pn. Two of them rely on optical measurements.
The first one consists in analysing the absorption of the pumping beam through the cell [6]. The second one, developed at E-N-S. Paris [7-9], deduces Pn from the degree of circular polarisation P~ of one of the fluorescence lines emitted from the discharge. This method takes into account
(* Author for correspondence
(**) also Universit4 de Caen, UFR de Sciences, 14032 Caen Cedex, France
© Les Editions de Physique 1996
l128 JOURNAL DE PHYSIQUE III N°8
the conservation of the nuclear spin during collisions rising atoms from the ground state to an upper state. In this state, the hyperfine interaction couples the electronic and the nuclear
polarisation. Then the polarisation of the light emitted as the atoms decay reflects the nuclear
polarisation Pn. The (3~D2 2~Pi) helium line at 668 nm is well suited for this purpose.
The measurement of the nuclear polarisation can also be made by controlling the induced
voltage across pick-up coils by the precession of the nuclear spins [lo, iii. Finally, Pn can be measured by detecting directly the magnetic field created by the oriented ~He nuclei, using
a magnetometer. Here we consider a spherical low pressure (a few Torrs) ~He pumped cell
(radius p
= 2 cm) homogeneously polarised. The magnetic field b produced outside the cell is the same as the field created by a dipole located at its centre and characterized by a magnetic
moment Mo given by:
Mo " ILNPn ii)
where /t is the ~He nuclear magnetic moment (/t =1.08 x 10~2~ JT~~), and N is the number of atoms in the ground state.
On the dipole axis, at a distance r from the centre, b is given by:
Such a measurement has been achieved in the past by Cohen-Tannoudji et at. [12], using a Rubidium magnetometer based on parametric resonances and working in a very low ambient magnetic field (below loo pT).
Recently, using a cesium magnetometer, the measurement of the polarisation of ~He nuclei
has been made at a much higher pression (lo atm) [13] than in our experiment, leading to a
magnetic moment Mo three order of magnitude higher.
In this paper the measurement ofPn is made in a magnetic field close to the earth magnetic field value (40 /tT), and uses a laser pumped ~He magnetometer. In order to control the magnetometry signals, simultaneous measurement of Pn is made by monitoring the polarisation P~ of the 668 nm line.
The nuclear polarisation Pn has been measured for two spherical pyrex cells with 2 cm radius and filled with ~He gas under 2 and 3.5 Torr pressure. Due to the high level of magnetic fluctuations in our laboratory, the ~He cell is placed inside a /t-metal magnetic shield providing
an attenuation factor of 5 x 10~~ for the earth magnetic field. A stable magnetic field Bo of the order of 40 /tT is created with the help of a pair of Helmholtz coils. The ~He atoms
are
excited in the 2~Si state by a R-F- discharge and are longitudinally pumped with the classical technique [5]. A laser diode (model SDL 6702-Hl is tuned on the hyperfine transition giving the highest degree of polarisation at the working ~He pressure: (2~Si,F
= 1/2 - 2~Po) for
pressure below 3 Torr and (2~Si, F
= 3/2 - 2~Po over [14]. The pumping light is alternatively right or left circularly polarised with a period of about five times the pumping time. The
resulting magnetic moment Mo is then oriented either in the same or the opposite direction of the magnetic field Bo. It results a modulation of the magnetic field b used to improve the
detectivity.
The magnetic field produced by oriented nuclei is measured with a laser diode pumped
~He magnetometer developed in our laboratory [15]. The magnetometer probe consists in a
cylindrical ~He cell (4 cm length and 4 cm diameter) centered on the 3He pumping beam and placed close to the spherical 3He cell whose center is at O (Fig. I). Due to the symmetry of the configuration, the magnetometer gives a field value B which can be expressed as:
B=Bo+amo (3)
x'
LD
p ~He- H +l~lH I+ill+(H(
~
~
Bo
~<
Fig. I. Geometry of the measurement of the magnetic field produced by oriented ~He nuclei. LD:
Laser Diode; x x': ~He cell axis; z z': ~He pumping beam axis.
3He
L L
c- c+
F
D D pA
)
fi LD
Fig. 2. Experimental set-up used for measuring the electronic polarisation of the 668 nm line. LD:
~He pumping laser diode, Q~ quaterwave plate, Si, S2S switches, pA: picoammeter (Keithley model
485), D: photodiode (Hamamatsu Gl120), F: optical filter (Wratten 92), C+, C-. right and left circular analysers, L: lens.
where Bo is the measured magnetic field value for a non-polarised 3He sample and a a ge- ometrical coefficient. a is experimentally determined according to the following procedure:
the 3He cell is replaced by a small circular coil (4.7 mm diameter) whose center is at O and with its axis parallel to Mo. From known values of the D-C- current in the coil, we deduce
a = 5.13 x 10~3 TA~lm~2. This value is in good agreement with a theoretical estimation based on the calculation of the mean magnetic field intensity over the ~He probe volume.
The electronic polarisation of the 668 nm line emitted from the 3He discharge is measured with the help of the simple apparatus represented in Figure 2: it consists in separately monitor-
ing with two photodiodes (Hamamatsu Gl120) the a+ and a- component intensities (respec- tively I+ and I- of the emitted line. Due to their low intrinsec magnetism, the photodiodes
can be placed very close (a few centimetersl to the 3He cell without disturbing the magnetic
field homogeneity. Wavelength selection is achieved thanks to the spectral characteristic of the
l130 JOURNAL DE PHYSIQUE III N°8
o
a)
Fig. 3. Simultaneous records of: a) the difference (I+ I-) of the right and left components intensities of the 668
nm optical line, b) the measured magnetic field produced by oriented ~He nuclei.
The measured polarisation Pn is equal to 6%.
~~
Ctl
~
0 2
p (O/
n ~
Fig. 4. Magnetic field produced by the pumped ~He cell
versus Pn. Full line is fitted. Pn is
measured with the optical line at 668 nm, AB is measured with the magnetometer.
photodiodes and by adding optical filters (Wratten 92). A low noise picoammeter (Keithley
model 485) measures (I+), (I-) or (I+ I-) according to switches position (see Fig. 2). The polarisation degree of the line is given by:
P~ = j~ j~ (4)
+ +
The nuclear polarisation Pn is deduced from the electronic polarisation according to the calibration of Bigelow et at. [9]. As the two detected beam directions are not parallel with the
magnetic field direction Bo la
= 15°), a small calculated correction (2%) is applied to this calibration.
Figure 3 shows the variations of the magnetic field when the 3He atom spins are oriented either in the same or the opposite direction of the magnetic field Bo. Variations of (I+ I-
are simultaneously recorded. The measured polarisation Pn is equal to 6$io.
Df
o 5 io 15 2o
Pn (%) (fluorescence)
Fig. 5. Comparison between ~He nuclear polarisation measured with the optical line at 668 nm and with the ~He magnetometer. Full line is theoretical, (.); p
= 3.5 Torr, (A) p
= 2 Torr.
Figure 4 represents the variations AB of the magnetic field (AB
= B Bo) versus the
nuclear polarisation Pn measured using the fluorescence technique. The evaluation of AB is made after averaging on five periods of the modulation of the nuclear polarisation. The optical pumping power density is constant and the different values of the nuclear polarisation Pn are obtained by modifying the R-F- discharge power across the cell filled with 3.5 Torr of 3He.
From the slope of Figure 4 and relation (3), we deduce the magnetic moment of the 3He cell:
(3.9 + 0.4) x 10~~ A m~ when Pn
= I. This experimental value is compared to the theoretical
one obtained with ii) Mo
= 4.I x 10~~ A m2 when Pn = I. The main error is due to the
determination of the geometrical parameters.
Figure 5 represents the values of the nuclear polarisation deduced from the magnetometer experiment versus values deduced from fluorescent polarisation measurements. A good agree-
ment is observed under various conditions of 3He
pressure, discharge power and optical power density.
This paper presents an original application of a ~He earth magnetic field magnetometer de- veloped in our laboratory. An improvement of its sensitivity is in progress and would make
possible the study of the evolution of the nuclear polarisation of a 3He sample initially po- larised. A potential application would be the control of polarised targets for nuclear physics experiments. Thanks to the high sensitivity of our magnetometer, we have demonstrated the
possibility of a direct measurement of the magnetic field created by oriented 3He nuclei in such
a magnetic field value. Furthermore, such a measurement could be made in the earth magnetic field (outside a magnetic shield) by the use of a differential method (gradiometer) in order to
reject the field fluctuations.
l132 JOURNAL DE PHYSIQUE III N°8
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