Creation of polarized ultracold neutrons and observation of Ramsey resonance for electric dipole moment measurement

Size: px

Start display at page:

Download "Creation of polarized ultracold neutrons and observation of Ramsey resonance for electric dipole moment measurement"

Lindsay Rose
5 years ago
Views:

1 Hyperfine Interact (2013) 220:89 93 DOI /s Creation of polarized ultracold neutrons and observation of Ramsey resonance for electric dipole moment measurement K. Matsuta Y. Masuda K. Hatanaka S. C. Jeong S. Kawasaki R. Matsumiya M. Mihara Y. Watanabe D. Nishimura Y. Morita K. Asahi T. Adachi J. Martin A. Konaka A. Miller C. Bidinosti T. Dawson L. Lee C. Davis D. Ramsay W.van Oers E. Korkmaz L. Buchman Published online: 22 March 2013 Springer Science+Business Media Dordrecht 2013 Abstract Polarized UCNs have been created by selecting only one spin state passing through a magnetized Fe foil. Typical degree of polarization was about 90 %. The polarization relaxation time in the prototype Ramsey cell was T 1 = s. Clear Proceedings of the 4th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2012), Beijing, China, September K. Matsuta (B) M. Mihara Y. Morita Department of Physics, Osaka University, Toyonaka, Osaka , Japan matsuta@vg.phys.sci.osaka-u.ac.jp Y. Masuda S. C. Jeong S. Kawasaki Y. Watanabe T. Adachi High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki , Japan K. Hatanaka R. Matsumiya RCNP, Osaka University, Ibaraki, Osaka , Japan D. Nishimura Department of Physics, Tokyo University of Science, Noda, Chiba , Japan K. Asahi Tokyo Institute of Technology, Meguro-ku, Tokyo , Japan J. Martin C. Bidinosti T. Dawson Department of Physics, University of Winnipeg, Winnipeg, MB, Canada R3B 2E9 A. Konaka A. Miller L. Lee C. Davis D. Ramsay L. Buchman TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2A3 W. van Oers Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T2N2 E. Korkmaz Department of Physics, University of Northern British Columbia, Prince George, BC, Canada V2N4Z9

2 90 K. Matsuta et al. Ramsey resonance spectra have been observed for two precession time settings, t c = 100 ms and 30 s. The transverse relaxation time T 2 was about 50 s. Keywords Polarized UCN n-edm Ramsey resonance 1 Introduction UCNs (ultracold neutrons) are very low energy neutrons, which are reflected by material surfaces, so that they can be stored in a material bottle. UCNs are very useful for various researches including fundamental physics. One of the great interests which can be studied using UCN, is the neutron EDM (electric dipole moment) [1], which breaks time reversal symmetry and is a clue to understand baryogenesis in the universe. The standard model of particle physics predicts far smaller baryon asymmetries than observed. To explain larger EDMs, if observed, one needs new physics, such as supersymmetric theory (SUSY). The last EDM measurement at ILL, Grenoble, provides the upper limit of ecm[2]. We are planning to measure the neutron EDM in the region of ecm,using our intense UCN source. In the present study, Ramsey resonance spectra have been observed for the preparation of the future precision measurement of the EDM. 2 Experimental Production of UCNs with the prototype KEK-RCNP UCN source UCNs were produced using our prototype UCN source installed in Research Center for Nuclear Physics (RCNP), Osaka Univ. Spallation neutrons were generated bombarding a Pb target with a 1 μa proton beam at 400 MeV. Neutrons were cooled down by a roomtemperature and a 10K-D 2 O moderators, and were down-scattered by the phonons in 0.8 K superfluid He-II to become UCN. Thus obtained UCNs were transported by a UCN guide to the vertical direction up about 1 m, bent to the horizontal direction, and then transported to the experimental port with the critical energy E c of 90 nev. For the performance test of the UCN source, the beam was turned on for 100 s, and UCNs were observed for 500 s after the beam was off, at the exit of the source with a 3 He detector. The UCN counting rate reached 10 k counts/s. The saturated UCN density estimated for the best conditions reaches 26 UCN/cm 3 at E c = 90 nev [3], which is comparable with the 60 MW UCN source in ILL, Grenoble. Creation of polarization The UCNs were then polarized utilizing a strong magnetic potential, passing through a 10-μm thick magnetized Fe foil, then guided to the Ramsey cell, and stored in it. The cell was basically a flat cylinder with a domeshaped ceiling made of Silica glass with a Cu-Be floor coated with diamond like carbon (DLC), the size of which is 210 mm in diameter and about 160 mm high (5.5 l). The cell was placed under a weak magnetic field B 0 of typically 2 μt, generated by a spherical coil, in a double Permalloy magnetic shield. After some elapsed time, the UCNs were extracted from the cell and went through the same magnetized Fe foil which worked also as an analyzer of the UCN spin states, and were then detected with the 3 He detector. Because of the loss at the wall collision, number of UCNs were gradually decreased. Typical UCN storage life-time

3 Creation of polarized ultracold neutrons and observation of Ramsey resonance 91 (time constant) in the cell was as long as 120 s, after proper alkali degreasing of the surface. Without treatment, the storage life-time was very short (25 s). To observe the neutron polarization, neutron spins are flipped by a spin flipper, right before entering Ramsey cell, and the UCN counts were compared with those without spin flipping. Ramsey resonance To observe the precession of neutron spins, two coherent π/2 RF pulses of typically 60 Hz separated by correlation (precession) time t c were applied, during the UCNs were stored in the cell. The two RF pulses rotates neutron spins by 180 in total, resulting in the inversion of the spins, when the RF is correctly in the same phase as the spin precession. For slightly different frequencies, the resulting direction of the neutron spins after precession varies according to the phase difference between RF and the neutron precession. So the effect of the Ramsey resonance was observed as an oscillating pattern in the UCN counts, reflecting this phase difference. 3 Results and discussion The degree of polarization P observed with the spin flipper on and off, is defined as a flipping asymmetry shown in the following equation. P = N of f N on, (1) N of f + N on where N of f and N on are the UCN counts with flipper off and on. Typical polarization of UCN as a function of time is as shown in Fig. 1. From the figure, the relaxation time is determined as T 1 = s, which is much longer than the present storage life-time. The absolute value of the observed polarization includes the effect of the flipping efficiency. Thus obtained Ramsey resonance spectra are shown in Fig. 2. SocalledRamsey fringe is basically an oscillating pattern like in the figure due to the phase difference between the spin precession and the RF pulse. The degree of polarization in the Ramsey resonance is defined by the visibility α as α = N max N min, (2) N max + N min where N max and N min are the UCN counts at the maximum and the minimum points. From the spectrum for t c = 100 ms, this visibility is shown to be about 90 %. The spectrum for t c = 30 s shows long transverse relaxation time T 2, of the order of 50 s, which is consistent with the T 2 value estimated from the field inhomogeneity of 4 nt accross the whole cell region. Future experiment We are now constructing a new horizontal UCN source to have better transport efficiency compared with the present vertical one. With a 10 times more intense beam, we plan to generate 250 UCN/cm 3 (polarized), with the new UCN source. For the magnetometry in the future EDM experiments, 129 Xe nuclear spins will be injected [4] at the same time with UCNs into a cylindrical EDM cell, where a

92 K. Matsuta et al. Fig. 1 Typical UCN polarization as a function of time in the Ramsey cell. B 0 =20μT Fig. 2 Typical Ramsey resonance spectra. Left: B 0 = 20μT, t c = 100 ms.

4 92 K. Matsuta et al. Fig. 1 Typical UCN polarization as a function of time in the Ramsey cell. B 0 =20μT Fig. 2 Typical Ramsey resonance spectra. Left: B 0 = 20μT, t c = 100 ms. Right: B 0 = 2μT, t c = 30 s. Insert is the closeup view near the center cylindrically symmetric magnetic field and an electric field are applied. Frequency shifts due to the GPE (geometric phase effect) arising from the particle motion( 129 Xe in this case) in a magnetic field gradient generate false EDM effects and could be the most important source of error. However, the motion of the 129 Xe atom is largely suppressed by interatomic collisions resulting in a short mean free path, and a small GPE. In addition, the field gradient is precisely controlled by means of the NMR frequency ratio between the neutron spins and the 129 Xe spins free from the Earth s rotation effect, because of the same sign of the g-factors for neutron and 129 Xe. The statistical error of EDM is expressed as, δd stat. = /{2αEt c N}, where E is the electric field and N is the number of UCNs. We will achieve 300 s of storage life-time, by proper vacuum baking and dueterizing of the surface of the cell. The polarization relaxation time T 2 will be improved to the value close to 1000 s, by improving field homogeneity. Then, we can expect the statistical error of e cm achieved in a day. Thus, we set our goal for the precision of EDM with the future experiments at RCNP to be in the e cm region.

5 Creation of polarized ultracold neutrons and observation of Ramsey resonance 93 References 1. Lamoreaux, S.K., Golub, R.: Experimental searches for the neutron electric dipole moment. J. Phys. G 36, (2009) 2. Baker, C.A., Doyle, D.D., Geltenbort, P., et al.: Improved experimental limit on the electrc dipole moment of the neutron. Phys. Rev. Lett. 97, (2006) and references therein 3. Masuda, Y., Hatanaka, K., Jeong, S.C., et al.: Spallation ultracold neutron source of superfluid helium below 1 K. Phys. Rev. Lett. 108, (2012) 4. Masuda, Y., Asahi, K., Hatanaka, K. et al.: Neutron electric dipole moment measurement with a buffer gas comagnetometer. Phys. Lett. A376, 1347 (2012)

Ultracold Neutrons at TRIUMF

Ultracold Neutrons at TRIUMF Jeff Martin The University of Winnipeg Outline: UCN Production and source Physics experiments at the UCN source: neutron EDM Materials science with UCN? with VCN? The future