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3He Nuclear Spin Relaxation in Cesium Coated Cells at Room Temperature
B. Chéron, H. Gilles, J. Hamel, M. Leduc, O. Moreau, E. Noël
To cite this version:
B. Chéron, H. Gilles, J. Hamel, M. Leduc, O. Moreau, et al.. 3He Nuclear Spin Relaxation in Cesium Coated Cells at Room Temperature. Journal de Physique III, EDP Sciences, 1995, 5 (8), pp.1287-1295.
�10.1051/jp3:1995104�. �jpa-00249379�
Classification Physics Abstracts
76.60E 32.80B 73.60D
~He Nuclear Spin Relaxation in Cesium Coated Cells at Room
Temperature
B. ChAron(~,~), H. Gilles(~), J. Hamel(~), M. Leduc(3), O. Moreau(~) and E. NoAl(~)
(~) Laboratoire de Spectroscopie Atomique, I-S-M-R-A-, Bd M~~ Juin, 14050 Caen Cedex, France (~) Universit6 de Caen, UFR de Sciences, Esplanade de la paix, 14032 Caen Cedex, France (~) Laboratoire Kastler Brossel de l'E.N.S., 24 rue Lhomond, 75231 Paris Cedex 05, France
(Received 19 January 1995, accepted 16 May 1995)
Abstract. The relaxation time T of ~He gas, nuclearly polarized by optical pumping, has been significantly increased by internally coating the glass cells with cesium. Values ofTj of order 45 hours
were recorded at
room temperature, resulting from collisions of the atoms with the cell wall. The possibility of keeping nuclear polarization for several days opens up new possibilities in NMR ~He magnetometry. Empirical means for characterizing the cesium coatings are proposed.
1. Introduction
3He atom has
a nuclear spin 1
= 1/2 which can be oriented by optical pumping [1] for use in fundamental physics experiments and applications in realization of high sensitivity magne-
tometers [2, 3], gyroscopes using maser effect or targets for nuclear physics [4]. The recent commercialisation of a high power laser diode for helium optical pumping has renewed the interest of these experiments [5,6].
At mom temperature, in an homogeneous magnetic field and at low densities, the longi- tudinal nuclear orientation disappears due to the collisions between the atoms and the cell.
This relaxation follows an exponential law characterized by a time constant T (longitudinal
relaxation time). With glass cells, T values of about one hour are usually obtained. For a
given pressure of 3He and a given cell size, these values strongly depend on the glass nature
and on its treatement preceding the cell filling. T values close to ten hours have been ex-
ceptionally obtained in the case of aluminosilicate glasses [7]. On the other hand, very long 3He nuclear relaxation times at low temperature (4 K) have been measured with cryogenic coatings of solid hygrogen [8]. We present here the first results obtained with a cesium coating
at room temperature and at low 3He pressure of the order of a few Torr. The choice of helium coating was inspired by the long relaxation times measured at E.N.S. with cesium coated cells at low temperature [9], which were attributed to the very small adsorption energy (2.3 K) of 3He on a cesium film. Similar results have been obtained recently on high pressures cells at Mainz [10]. This article describes measurements of T in various situations and a study of the evolution of the cesium coated cells. Efforts have been made to vary the amount of cesium and to characterize the coatings obtained.
© Les Editions de Physique 1995
1288 JOURNAL DE PHYSIQUE III N°8
Table I. Qualitative estimation of the cesium amount introduced into each ceil and electrical characteristics identified for each cell.
Cell Visual aspect CD(PF) R(11) C(pF)
A w1tllout cesium
"reference cell clear 2
co 0
B very l1gllt trace after filling 2 2400 16
no trace stabilized 2 >106 0
C film after filling 2 4000 8.6
stabilized 2 2700 17
D a few droplets stabilized 2 1260 27
E big drop stabilized 2 1370 25
2. Experimental Apparatus
The procedure consists in first creating a longitudinal orientation of 3He nuclear spins by optical pumping in a homogeneous magnetic field Bo, and then measuring the evolution of this
orientation periodically.
In order to reduce the effect of the magnetic field inhomogeneities, the experiment is carried out outside the laboratory, in the earth field and in a small log cabin located as far as possible
from sources of magnetic perturbations. To demonstrate the role of cesium, two cells are
simultaneously tested, the first one is internally coated ("cesiated" cell) while the second one
(without cesium) acts as a reference cell. Several cells have been prepared with various amounts of cesium as followed. Five spherical (4 cm diameter) cells made of Pyrex 732-01 are connected
to a vacuum system. Prior to the final filling, they are carefully baked at 200 °C and submitted
to several H-F- discharges in helium. A first cell is filled with 3He to 3.5 Torr before sealing off. It is the "reference cell". Various amounts of cesium are then distilled inside the remaining
cells before the final filling with 3He at the same 3.5 Torr pressure. As it is not easy to control the amount of cesium introduced into these cells, they are characterized by their visual aspect
reported in Table I.
Figure 1 shows the experimental set-up. The optical pumping beam is emitted by a laser diode (SDL 6702,H1) located in the laboratory and operating at 20 mW of optical power. Its
single frequency emission at 1
= 1.08 ~lm is tuned to the helium Cg line (23Si> F
= 1/2 23Po
transition) [11]. This beam, transmitted through a 50 m long optical fiber of 200 ~tm core
diameter (PCS200), is circularly polarized before entering the cell (S) located outdoors. The
optical power available at the cell is about 5 mW. The transmitted power is measured by
a photodetector in order to monitor the nuclear polarization. Two pairs of coils allow to
transversely tip the magnetic moments resulting from the optical pumping of 3He (tipping
coils T) and to detect the voltage induced by the spins precession (pickup coils PU) [12].
a) optical pu~nping sequence:
o
PO,J/4
LD
~)
,
~ ( ~
~
S'
~
b) Measurement sequence:
T p~
RF s
SP A
Fig. I. Experimental set-up. a) LD: laser diode at 1.08 pm, BS: beam splitter, S': auxiliary ~He cell for frequency tuning, F: optical fiber of 200 pm core diameter, PO: linear polarizer, 1/4: quater wave plate, Ll,L2: focusing and collimating lenses, O: discharge oscillator, Ph: photodetector, Bo: earth
magnetic field, S: ~He cell to be tested, I: transmitted light intensity measurement, b) RF: tipping oscillator, G: gate, T: tipping coils, PU: pickup coils, A: low noise amplifier, SP: signal processing.
2.I. MEASUREMENT OF THE 3HE NUCLEAR POLARIzATION. Starting from zero, when the
pumping beam is switched on, the nuclear polarization rate reaches an asymptotic value P after about one minute. Colegrove et al. [13] showed that P is related to the variation of the
absorption of the beam by the cell through the relation:
Au A~ P(7.5 5P + 1.5P~)
Ao (3 + P2)
where AD et Ap are respectively the initial and the asymptotic values of the absorption. This
1290 JOURNAL DE PHYSIQUE III N°8
~ ~fll3X
t
i mn
Fig. 2. Heterodyne signal between the amplified induced e-m-f- and a local oscillator.
relation is valid for optical pumping with the Cg line and at a pumping power below saturation of this transition. From this relation, we evaluate the polarization rate P. This measurement, repeated with the five cells, gives in each case a value of P close to 13% with 5 mW of optical
power. We can thus conclude that the presence of cesium in the cell does not affect the optical pumping efficiency of 3He. Actually collisions between helium and cesium atoms in the gas
phase are likely to be negligible due to the low cesium vapor pressure (10~~ Torr at 25 °C).
We also note that the values of P obtained with 5mW of laser power are in good agreement with those predicted by the model of reference [11].
2.2. MEASUREMENT OF THE NUCLEAR RELAXATION TIME. When the steady-state of the
longitudinal nuclear orientation is reached, the discharge and the laser are switched off. Helium atoms are then briefly submitted to a R.F. magnetic field which induces a transverse component of the nuclear magnetic moment (tipping pulse). This component then precesses freely around the earth magnetic field. An induced voltage V is measured across the pickup coils. Figure
2 shows a typical recording of this signal: clearly its amplitude evolution, in a time scale of about 1 minute, is not an exponential decay; the signal rises first to a maximum Vmax before it decays down to zero. The higher is the coupling between the pickup coils and the sample magnetization, the stronger the distorsion from an exponential decay. For instance when the
quality factor of the pickup coils circuit is significantly lowered or when the magnetization gets close to zero, the decay returns to an exponential behaviour characterized by a T2 transverse
relaxation time, at the expense of a loss in the signal to noise ratio. These facts clearly
indicate a tendancy of the atomic system to built up a maser oscillation [14]. The equation of the transverse aimantation evolution which gives the non exponential signal (Fig. 2) shows that the maximum amplitude Vmax of this signal is proportional to the transverse magnetization just after the tilting pulse. We thus decided to measure Vmax to monitor periodically the nuclear
polarization after each tilting pulse. One can note that this transient transverse magnetization decays in a much shorter time (of order 1 minute) than the longitudinal magnetization IT is
order several hours). However, this measurement procedure induces a small orientation loss.
In order to descriminate between this loss due to the tipping process and the natural decay (Tj), a series of identical tippings is made during a time very much shorter than n, from which
we evaluate that the loss per tipping is 2Slo. Figure 3 shows the corrected time evolution of the
polarization of the cell E on a logarithmic scale.
nuclear polarization rate %
0 20 40 60 80 100
time in hours
Fig. 3. Measurement of Tj: nuclear polarization (logarithmic scale) versus time.
Cell A (the reference cell, without coating) has a T value of about one hour. Cells C, D and
E have a common value of about 45 hours (+5 hours). This measurement repeated several
months later with the same cells kept on the shelf gave the same T value. On the other hand, for cell B, T was about 45 hours just after the filling and decreased to 30 hours after several
discharges. It is interesting to note that the cesium coating is then no longer visible. It is
important to check that the measured values of T are due to collisions of the helium atoms with the walls and not due to other causes. The pressure is much too low for helium-helium
collisions in the gas phase to have an influence on T [15]. On the other hand the influence
of the transverse magnetic field inhomogeneities ABt has to be carefully investigated. Onp
estimates ABt/Ar of order 1 nT/cm from rough measurements with a gradientmeter in our
experimental conditions. The T value resulting from this perturbation is estimated in the
appendix using references [16] and [17]. It is found to be much too long to contribute to the measured Tj values. Therefore, we can conclude that wall collisions are the main relaxation process in the present experiments.
2.3. TIME EvoLuTioN OF THE CELLS. It is interesting to note that even a very small amount of cesiurn, as in cell B, has a large effect on the relaxation time T. In order to define this amount more precisely, a series of electrical tests has been undertaken. Two electrodes, painted in silver lake on the external surface of the cell, constitute the two terminals of an electrical dipole. Without cesium coating, the corresponding dipole is equivalent to a single capacitor Co (Fig. 4a). A conductive film of cesium [18], spread on the inner side of the cell, modifies this dipole which can be represented by a capacitor Co connected in parallel with a
capacitor C in series with a resistance R (Fig. 4b). To confirm this assumption, we measure its electrical impedance as a function of the excitation frequency. A spectrum analyzer (HP8560A)
with its tracking generator is used for this measurement (Fig. 4c). The frequency evolution of the analyzer input signal for cell A (without cesium) confirms the capacitive character (Co of this dipole (Fig. 5a). Figure 5b shows, for cell E, the experimental frequency evolution of the
analyzer signal (full line). A good agreement is obtained with the dashed curve representing
the calculated response using the model of Figure 4b with the values Co " 2 pF, R
= 1370 fl
1292 JOURNAL DE PHYSIQUE III N°8
~co
~ ~~ (a)
~
~OF-~~~b>
tested ceil
TG Al (C)
Fig. 4. Electrical model of the cells. a) Cell without cesium coating. b) Cesiated cell. c) Impedance
measurement of the electrical dipole associated with the cell. TG: tracking generator (Z~ = 50 Q), AI:
analyzer input (Z,
= 50 Q).
30 mV
(c) (b)
(a)
o
loo k~iz lo xfliz
Fig. 5. Frequency evolution of the analyzer input signal a) for the cell A (without cesium coating).
b) for the cesiated cell E. The dashed line c) represents the curve calculated with the components
(Co = 2 pF, R =1370 Q, C
= 25 pF).
Table II. Ignition discharges power.
Cell A Cell B Cell C Cell D Cell E
lgnJfion power 0.42 0.64 0 96 1,72 1,94
(W)
et C
= 25 pF. The values of Co, R and C identified for each cell are reported in Table I.
The measurements with cells B and C were done first just after the filling (no discharge has been switched on and then after 3 or 4 discharges during about 10 minutes. The results show an evolution of the electrical characteristics towards the stabilized values. The electrical stabilized characteristics of the cell B, which contains a very light trace of cesium, are the
same as for cell A. However, the effect of cesium on T remains considerable: T decreases only from 45 hours to 30 hours. We can think that there is not enough cesium to form a single layer and that cesium atoms fill the glass holes where helium atoms are trapped. For cell C, the electrical characteristics are slightly modified in the opposite direction and T remains
constant. The identification of cells D and E was done only after several discharges. Their electrical characteristics always remain constant even after a few other discharges. Another effect of the conductive coating of cesium is revealed by the increase of the electrical power required to ignite the discharge as shown in Table II: it can be interpreted as a screening effect.
As a consequence, the temperature of the cesiated cells submitted to discharge increases with the cesium amount.
3. Conclusion
Using cesium coating in Pyrex cell, the 3He nuclear relaxation time T increases from 1 to
45 hours with very good reproducibility. The results agree well with recent measurements
performed by the Mainz group with aluminosilicate glass cells coated with various metals [10].
Moreover, the presence of cesium does not affect the optical pumping efficiency of 3He at
room temperature. The only minor drawback is a small increase of the ignition power of the
discharge.
We have clearly demonstrated that a small cesium droplet is largely sufficient to obtain the permanent increase of T. It is interesting to note the great efficiency of an invisible cesium trace: even though we no longer detect its presence by electrical tests, T increases to about 30 hours.
The application of cesium coated cell is very interesting for 3He magnetometry [19] 3He magnetometer operates sequencially: after an optical pumping cycle, the magnetic field value is deduced from the measurement of the free precession frequency. The increase of Tj extends
the operating time of the magnetometer. It is possible to imagine a magnetometer probe initially optically pomped and then freely working away during several days. The mean required
electrical power is then very low: several watts are needed during a few minutes for the optical pumping sequence and several milliwatts for the long measurement period.
Acknowledgments
It is a pleasure to thank B. Rocher for making and filling the cells.
1294 JOURNAL DE PHYSIQUE III N°8
Appendix A
We calculate the relaxation time of the longitudinal magnetization induced by the gradients
of transverse magnetic field ABt. We use the formalism of reference [16] where the movement of each atom is studied in the frame of the diffusion equation.
If one only takes into account the lowest diffusion mode (n
= I = 1), one obtains for a
spherical cell of radius R (from formula [B.34] in reference [17] ):
I _~jj )2~~ ~ j~2 I
T ~~~ Qii 16~ w(Tii
with
~jj~~~)2 ~~3 ~~"t)~hr
Iii and Qii are constants given in reference [17], fo
= Iwo/2~) is the frequency of the longitudinal magnetic field ED and Tii is the diffusion time for the fundamental mode.
j~2
For a sphere, rii = o.231-p where D is the diffusion coefficient (D
= 1440 cm~ s~~ at
D
1 Torr for 3He at 300 K [17]), p the pressure in Torr. One thus obtain:
= 386 ~~~~~~
in s~~
T ED
~
P Note that this value is independent of R. For ABt/Ar
= 1 nT/cm, Bo " o.5 x 10~~T (earth field), p
= 3.5 Torr, one gets T = 6250 hours.
References
[ii Walters G-K-, Colegrove F-D- and Schearer L.D., Phys. Rev. Lett. 8 (1962) 439.
[2] Leduc M. and Hamel J., Rev. Scien. Tech. Del., France 4 (1989) 31.
[3] European patent application, n°84305946.0, "Apparatus for measuring the magnitude of
a magnetic field", McGregor, Douglas D., Eds.
[4] Workshop on polarized ~He beams and targets (Princeton 1984) AIP conference proceedings 131
(1985); Chupp T-E-, Holt R-J- et MiIner R-G-, Annu. Rev. Nucl. Part. Sm. 45 (1994) 373.
[5] Bohler C-L- and Marton B-I-, Opt. Lett. 19 (1994) 1346.
[6] Ch6ron B, Gilles H., Hamel J., Moreau O. and No61E., Opt. Comm. 115 (1995) 71.
[7] Fitzsimmons W-A-, Tankersley L-L and Walters G-K-, Phys. Rev. 179 (1969) 156.
[8] Barb6 R., Lalo6 F. and Brossel J., Phys. Rev. Lett. 34 (1975) 1488;
LefAvre-Seguin V., Nacher P-J-, Brossel J., Hardy W-N- and Laloi F., J. Phys. France 46 (1985)
l145.
[9] Tastevin G., J. Low Temp. Phys. 89 (1992) 669.
[10] Heil W., Humblot H., Otten E., Schafer M., Surkau R. and Leduc M., Phys. Lett. A., to be
published.
[iii Nacher P-J- and Leduc M., J. Phys France 46 (1985) 2057.
[12] Abragam, the principles of nuclear magnetism, Oxford Science Publications (1961).
[13] Colegrove F.D., Schearer L.D. and Walters G-K-, Phys. Rev. 132 (1963) 2561.
[14] Richards M-G-, Cowan B-P-, Secca M-F- and Machin K., J. Phys. B 21 (1988) 665.
[15] Newbury N-R- et al. Phys. Rev. A48 (1993) 4411.
[16] LefAvre-Seguin V., Nacher P-J- and Lalod F., J. Phys. France 43 (1982) 737.
[17] Barb6 R., Leduc M. and Laloi F., J. Phys. France 35 (1974) 699 and 935.
[18] Jacquier P., ThAse de Doctorat d'Etat (Paris VI, 1991).
[19] Leduc M., Heil W., Humblot H., Otten E., Schafer M., Surkau R., Hamel J., Ch6ron B.
and Moreau O., R6cipient pour contenir un fluide notamment de l'h61ium 3, Brevet Brevatome, n°9410251.
