Novel interferometer for cold neutrons using a pair of etalons

ELSk’IER

Nuclear Physics A72 1 (2003) 48 lc-484~ www.elsevier.com/locateinpe

Novel interferometer

for cold neutrons using a pair of etalons

M. Kitaguchiab, H. Punahashi”, T. Nakuraa, M. Hino”, and H. M. Shimizub ‘Department of Physics, Kyoto University, Kyoto 606-8502, Japan bRIKEN; 2-l Hirosawa, Wako, Saitama 351-0198, Japan CResearchReactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan A new type of cold-neutron interferometer has been developed using a pair of etalons. We have observed interference fringes with the contrast of 60%. The air gap of the etalon is lOpm, which produces much larger separation between two paths in the interferometer. The present results have paved the way for the more high precision measurements and the new type of experiments. 1. INTRODUCTION Neutron interferometer is a sensitive detector for very small interaction. An interferometer consists of two paths. Neutron counts vary with the relative phase between the two paths. The relative phase is provided by gravitational, magnetic, and nuclear interaction. We can measure such interaction and search speculative interactions from the phase of interference fringes. When there is some energy difference AE between the two paths, the relative phase is written as

where m is neutron mass, X is neutron wavelength, and L is interaction path length. The large dimensional interferometer using long wavelength neutron has the advantage to increase the sensitivity to small interactions. By using perfect Si crystal interferometer, some remarkable experiments were performed[l]. But Si interferometer can not deal with a neutron which has a wavelength longer than twice its lattice constant. The interaction length is limited by size of the crystal. In addition, the dynamical diffraction phenomena require a rather advanced analysis. 2. INTERFEROMETER

USING

MULTILAYER

MIRRORS

Multilayer neutron mirror is one of the most useful devices in cold and very cold neutron optics. The multilayer of two materials which have different potentials is understood as a one-dimensional crystal, which reflects long wavelength neutrons according to Bragg’s law. The feasibility of interferometer using multilayer mirrors was demonstrated in 1995[2]. The cold neutron interferometer using multilayer mirrors contained two pair mirrors 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/SO375-9474(03)01104-7

M. Kitaguchi et al. /Nuclear Physics A721 (2003) 481c-484~

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mirr&

pair mirror

Beam Splitting Etalon

Figure 1. Pair mirror and Beam Splitting Etalon. A pair mirror consisted of the two parallel multilayer mirrors and an intermediate gap layer, by depositing on a substrate (Fig.1). The pair mirror divided incident neutrons into two parallel beams. After the reflection off the 2nd pair mirror, the two beams were recombined and the optical path-difference was compensated. In this case, we could use long wavelength neutrons and we could also take long interaction length as the distance between the two pair mirrors. Later, Neutron Spin Interferometer(NSI)[3] was developed. In NSI case, one of the two mirrors in a pair mirror was spin-selective, and the two paths corresponded to two spin eigenstates respectively. But these system had a weak point. The two paths were overlapped each other because of thin gap layer. The gap thickness was less than a few micrometers, due to vacuum evaporation method. Therefore, we could not insert any devices between the two beams, and moreover, they could not be sensitive to some interactions depending on the beam separation. It is worth while enlarging beam separation in order to broaden the applicability of interferometer using multilayer mirrors. 3. INTERFEROMETER

USING

A PAIR

OF ETALONS

By using etalons, the separation can be enlarged[4]. An etalon is an optical device, mainly used in laser optics, which has two precision parallel planes. When neutron mirrors are deposited on the two planes of an etalon, the etalon divides incident glancing neutrons into two parallel beams with large separation (Fig.1). We want to call this type of devices as Beam Splitting Etalon (BSE). The test experiment was performed using cold neutron beam line MINE2 at the JRR3M reactor in JAERI. The beam had a wavelength of 8.8A and a bandwidth of 2.4% in FWHM. Figure2 illustrates the experimental setup. Two BSEs set in NSI forms a geometrical optics equivalent to Jamin interferometer, which is the oldest type of interferometer for practical use in the history of optics. The space-length was 9.75pm. This was 10 times as large as the gap of the conventional pair mirror. We deposited eight bilayers of Permalloy45/Ge for spin-selective mirror, and eight bilayers of Ni/Ti for normal mirror in a diameter of 20mm. The Bragg angle of the mirrors was 1.05degree. The magnetic field of phase-shifter coil gave the relative phase between the two paths. We have observed clear interference fringes with the contrast of 60%. One period of the fringes corresponds to the energy difference between the two paths of 20peV. 4. DISCUSSION Here we discuss two applications of the present type of interferometer with large spatial separation between the two beams. The Aharonov-Cssher(AC) effect is electromagnetic

M. Kitaguchi et al. /Nuclear Physics A721 (2003) 48lc-484~

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0.1

483~

0.15

Phase-shifter-coil

0.2

0.25

0.3

current [Al

Figure 2. Experimental setup and interference fringes. and quantum mechanical dual of Aharonov-Bohm effect[5]. The previous measurement using Si crystal neutron interferometer reported the discrepancy between experimental results and theory. More precision measurement using neutron beam must be performed. By using atom beam interferometer some precision measurements were performed, but the two paths in atom interferometer were not separated. The topological nature of AC effect could not be investigated with atom interferometer. By using the present type of interferometer we can insert electrodes between the two paths and take large interaction length. High precision measurement of AC effect can be carried out. In gravitationally induced quantum interference experiment, the discrepancy of 0.8% between experiment and theory was reported using Si interferometer[g]. In the case of Si interferometer the dynamical diffraction process requires a rather advanced analysis. On the other hand, by using Bragg case of the reflection off the thin multilayer mirrors, the correction of dynamical diffraction is negligible. The discussion will become clear. Table1 shows comparison of interferometer using multilayer mirrors and Si interferometer. In respects of wavelength and interaction length, the interferometer using multilayer mirrors has remarkable gains. In Si crystal case the Bragg angle is large and the two paths are separated, therefore, Si interferometer has the advantage to arrange experimental setup. The Bragg angle of multilayer mirror is a few degree and the area enclosed by the two paths is still small, however, we can enlarge the path separation by using large-spacing etalons and achieve the same size of the area as Si case in the ongoing plan. The correction of the dynamical diffraction effect is much smaller than Si case. Table 1 Interferometer using multilayer mirrors vs perfect Si crystal interferometer Si crystal multilayer mirrors wavelength - 18 10.4 - 1ooA interaction length 5 1Ocm - lm Bragg angle - 1Odeg N ldeg size of enclosed area N 10cm2 - 0.1cm2 (present prototype) 2 10cm2 (ongoing plan) 2 lm2 (future plan) correction of dynamical diffraction inevitable negligible

gain 10 10 0.01 >l > 1000

484~

M. Kitaguchi et al. /Nuclear Physics A721 (2003) 481c-484~

energy difference

1000

10000

100000 0.1

le+06 1

,b

1 a+07 total neutron COUntS measuring time [day] when average counting rate is 5cps

Figure 3. The precision of phase shift measurement.

Figure3 shows the estimation of the precision of measurement, by computer simulation. We can measure the interaction of the order of femto-electron-volt by a measurement for several days at the MINE2 beam line in the case of lm interaction length.

5. SUMMARY We have observed clear interference fringes with the contrast of 60% using etalons with the gap of 9.75pm. This is the first success of neutron interferometer using four independent multilayer mirrors. The present results have demonstrated the feasibility of the development of a cold neutron interferometer with a large path separation. The enlargement of the path separation enables us to carry out high precision measurements and new types of experiment.

ACKNOWLEDGMENTS The authors gratefully acknowledge fruitful discussions with Prof. T. Ebisawa, Prof. S. Tssaki, and Prof. Y. Otake. This work was supported by the inter-university program for common use JAERI and KUR, and financially by a Grant-in-Aid for Scientific Research (No.12740149) of JSPS, the REIMEI Research Resources of JAERI, and Special Coordination Funds for Promoting Science and Technology of the Ministry of Education of Japanese Government. One of the authors (M.K.) was supported by the Junior Research Associates Program of RIKEN.

REFERENCES 1. 2. 3. 4. 5. 6.

H. Rauch and S. Werner, Neutron Interferometry , OUP, Oxford (2000). H. Funahashi et al., Phys. Rev. A54 649 (1996). T. Ebisawa et al., Phys. Rev. A57 4720 (1998). M. Kitaguchi et al., Proc. of 7th Int. Symp. Foundations of Quontum (ISQM-Tokyo’Ol) pp.269-272 (2002). A. Cimmino et al., Nucl. Inst. and Meth. A440 579 (2000). K. C. Littrell et al., Phys. Rev. A56 1767 (1997).

Mechanics