Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Development of 3 He comagnetometer for 129 Xe EDM measurement
Y. Ichikawa,∗1,∗2 M. Chikamori,∗2 Y. Ohtomo,∗2 E. Hikota,∗2 Y. Sakamoto,∗2 T. Suzuki,∗2 T. Sato,∗2S. Kojima,∗2 H. Shirai,∗2 H. Miyatake,∗2 T. Nanao,∗2 K. Suzuki,∗2 T. Inoue,∗3 T. Furukawa,∗1,∗4A. Yoshimi,∗1,∗5 C. P. Bidinosti,∗6 T. Ino,∗7 H. Ueno,∗1 Y. Matsuo,∗1,∗8 T. Fukuyama,∗9 and K. Asahi∗1,∗2A permanent electric dipole moment (EDM) that directly violates time reversal symmetry attracts muchattention, because an unknown CP-violating phasewhich is necessary for understanding the present
matter-dominated Universe, is expected to be probedby EDM. The present study aimed to measure theEDM in a 129 Xe atom to the order of 10−28 ecm, whichis beyond the present upper limit1) . We employed anactive nuclear spin maser2,3) to sustain the spin precession of 129 Xe over a long duration. The active spinmaser operates in the following manner. The 129 Xe
spin is longitudinally polarized through spin exchangewith optically pumped Rb atoms. Precession of the129
Xe spin in an applied static field is detected optically by transversely repolarized Rb atoms. By referring to the precession signal thus obtained, a feedback
magnetic field is generated such that its direction iskept orthogonal to the transverse component of thespin. The feedback field thus prevents decay of thetransverse magnetization.
In an EDM measurement, magnetometry is essential
because a large systematic uncertainty in frequency
arises from long-term drifts in the external magnetic
field. A comagnetometer using 3 He was incorporated
into the nuclear spin maser system in order to cancelout the drifts4) . Because a 3 He comagnetometer canmeasure the field exerted on the 129 Xe precession, it isan in situ magnetometer.
The main difficulty in realizing the 3 He comagnetometer stems from the fact that the spin-exchange
rate between 3 He and Rb is lower than that between
129
Xe and Rb by several orders of magnitude. Because
there is little source of polarization, spin relaxation atthe surface of the cell and impurity in the gas criticallydegrade the polarization of 3 He. Therefore, a GE180
glass with low magnetic impurity and low gas leakage was employed to fabricate the cell. The cell has aspherical shape with a diameter of 20 mm, containing
1 Torr of 129 Xe, 470 Torr of 3 He, 100 Torr of buffer N2gases, and Rb vapor. We typically achieved 3% of thepolarization and over 50 hours of the longitudinal spin
relaxation time for 3 He at 100 ◦ C.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Cyclotron and Radioisotope Center, Tohoku University
Department of Physics, Tokyo Metropolitan University
Research Core for Extreme Quantum World, Okayama University
Department of Physics, The University of Winnipeg
Institute of Material Structure Science, KEK
Department of Physics, Hosei University
Research Center for Nuclear Physics, Osaka University

Permalloy

Solenoid coil
B0 : 30 mG

λ/4 plate
HeaterPhotodiodePumping laser

Spherical gas cell

Diameter : 20 mm
Material : GE180
Xe : 1 Torr
3He : 470 Torr
N2 : 100 Torr
129

Wavelength : 794.76 nm
Line width : 10 MHz
Output power : 1 W

Probe laser
Wavelength : 794.76 nm (tunable)
Line width : 10 MHz
Output power : 10 mWFig. 1. Experimental setup for active spin maser.

The experimental setup used to test the 3 He comagnetometer is shown in Fig. 1. The gas cell is placed ina solenoid coil which generates a static magnetic fieldB0 (∼ 30 mG), and is enclosed in a 4-layer magnetic
shield. A circularly polarized pumping laser is incident
on the cell parallel to B0 . A probe laser passes throughthe cell in a direction orthogonal to B0 and is detectedby a photodiode. The signal from the photodiode is divided into two, each being lock-in-amplified with 129 Xeor 3 He precession frequency, and the resulting two beatsignals are obtained and processed individually at thesame time to generate their feedback magnetic fields
through two separate coils.
We succeeded in operating the masers of 129 Xe and3He concurrently4) . The individual determination precisions of the average frequencies achieved in 106 seconds for both 129 Xe and 3 He are ∼100 nHz. However,the frequency shift due to contact interaction with polarized Rb atoms prevents the 3 He comagnetometerfrom realizing its full potential because the strengthsof the Rb-129 Xe and Rb-3 He contacts are different.
Therefore, we decided to employ a double-cell geometry in which the gas volume is divided into a section
for optical pumping and another for optical spin detection in order to suppress Rb polarization in the optical detection section and thus reduce the frequency
shift5) . Development of the 3 He comagnetometer withthe double-cell geometry is in progress.
References
1) M. A. Rosenberry et al.: Phys. Rev. Lett. 86, 22 (2001).
2) A. Yoshimi et al.: Phys. Lett. A 304, 13 (2002).
3) A. Yoshimi et al.: Phys. Lett. A 376, 1924 (2012).
4) Y. Ichikawa et al.: Eur. Phys. J. Web of Conf. 66, 05007(2014).
5) E. Hikota et al.: Eur. Phys. J. Web of Conf. 66, 05005(2014).

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