Isotope shift measurements of 11,9,7 Be +

Transcription

1 Eur. Phys. J. A, (9) DOI 1.11/epja/i Regular Article Experimental Physics THE EUROPEAN PHYSICAL JOURNAL A Isotope shift measurements of 11,9,7 Be + A. Takamine 1,M.Wada 1,,a,K.Okada 3, T. Nakamura 1, P. Schury 1,T.Sonoda 1, V. Lioubimov 1,9, H. Iimura, Y. Yamazaki 1,5, Y. Kanai 1, T.M. Kojima 1,A.Yoshida, T. Kubo, I. Katayama,S.Ohtani 7, H. Wollnik,and H.A. Schuessler 9 1 Atomic Physics Laboratory, RIKEN, -1 Hirosawa, Wako, Saitama , Japan Nishina Center for Accelerator Based Science, RIKEN, -1 Hirosawa, Wako, Saitama , Japan 3 Department of Physics, Sophia University, 7-1 Kioicho, Chiyoda, Tokyo 1-55, Japan Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki , Japan 5 Graduate School of Arts and Science, The University of Tokyo, Meguro, Tokyo 153-9, Japan Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 35-1, Japan 7 Institute for Laser Science (ILS), University of Electro-Communications, Chofugaoka, Chofu, Tokyo, 1-55, Japan II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany 9 Department of Physics, Texas A&M University, College Station, TX 773, USA Received: 9 January 9 / Revised: 9 May 9 Published online: 3 October 9 c Società Italiana di Fisica / Springer-Verlag 9 Communicated by C. Signorini Abstract. We have performed precision atomic spectroscopy of trapped radioactive Be isotopes aiming at studies of the charge and magnetization radii of these nuclei especially for a single-neutron halo nucleus 11 Be. Some experimental results and the status of the analysis are discussed. PACS. 1.1.Gv Nucleon distributions and halo features 1.1.Ft Charge distribution 31.3.Gs Hyperfine interactions and isotope effects 37.1.Ty Ion trapping 1 Introduction Neutron halo nuclei were discovered through interaction cross-section measurements at intermediate energies [1]. Their extraordinary large cross-sections are considered to be due to weakly bound neutrons extending as a halo around the nuclear core []. The discovery ignited various studies of such halo nuclei, both experimental and theoretical. In recent years, the nuclear charge radii of halo nuclei have been determined in a nuclear-model independent manner by precision optical spectroscopy of atoms. The charge radii of the two-neutron halo nucleus He and the four-neutron halo nucleus He were determined at Argonne National Laboratory in a magneto-optical trap [3, ]. The charge radius of the two-neutron halo nucleus 11 Li was determined by two-photon spectroscopy at TRIUMF by the ToPLis group from GSI [5]. For Be isotope, two experiments at RIKEN and ISOLDE are in progress []. We have developed an online ion trap for precision atomic spectroscopy where unstable Be ions can be stored for extended durations and laser-cooled down to a very low temperature [7 1]. This is an ideal condition to perform a mw@riken.go.jp Fig. 1. Sketch of the charge and magnetization radii of the neutron halo nucleus 11 Be. double-resonance spectroscopy to determine the absolute optical transition energies as well as the hyperfine splitting energies with high accuracies, which allows us to deduce not only the isotope shifts of optical transitions but also the isotope shifts of the hyperfine constant. From these two isotope shifts, we can determine the nuclear charge radii and the nuclear magnetization radii. It should be noted that 11 Be is considered to have a 1 Be core and a valence halo neutron. In the naive picture shown in fig. 1, the charge radius of 11 Be is represented by the core size while the magnetization radius is represented by the radius of the extended halo neutron. In this way, we can clearly justify whether the valence neutron is really distributed with a large radius by a reliable pure-optical probe.

2 37 The European Physical Journal A Prototype of SLOWRI Ring Cyclotron Production Target RIPS Energy Degrader Helium Gas Cell Carbon-OPIG Q-Mass Linear Paul Trap high energy RI beam ~1GeV slow RI beam ~ev UV laser 313nm Fig.. Overview of the experimental setup. Principle The finite mass and size of a nucleus influences the electron binding energies of an atom. The atomic level energy, E, is expressed as E = E μ M E + μ M K + F r c, (1) where E is the energy under the assumption that the nucleus is an infinitely heavy point charge, μ the reduced electron mass, M the nuclear mass, K = 3 i<j ( p i p j )/μ the mass polarization parameter, F the field shift constant, and rc the mean-square charge radius. In the case of Be, the field shift contribution is as small as 1 MHz, whereas the transition energy of the optical transition is about 1 9 MHz and the mass contribution is about 1 GHz. The charge radii can be determined only if the relative accuracies of the measurements are as good as 1 9 and the theoretical calculation for K and F are provided with sufficiently high accuracies. For singly charged Be + isotopes the calculations were recently performed [11]. The isotope shift of the magnetic hyperfine constant is known as the hyperfine anomaly or the Bohr-Weisskopf effect [1] which stems from the different distribution of the magnetization in a nucleus. The magnetic hyperfine constant A can be described by A = A pt (1 + ɛ) with the hyperfine constant for a point dipole nucleus A pt and the hyperfine anomaly ɛ. Since A pt is hard to obtain, we usually take the differential hyperfine anomaly Δ 1, by comparing the ratio of A to the nuclear g-factor: Δ 1, = A 1/g 1 A /g ɛ 1 ɛ, () which is approximately equivalent to the isotope shift of ɛ. Since the magnitude of ɛ is as small as 1 1 5, accurate measurements for both the hyperfine constants and the nuclear g-factors are required to deduce the hyperfine anomaly. For the neutron halo nucleus 11 Be +,anextraordinary large anomaly of ɛ 1 3 was theoretically predicted simply due to the large radius of the valence s 1/ -neutron [13]. 3 Experimental method In order to perform precision atomic spectroscopy experiments, preparations of slow ion beams or trapped ions are essential prerequisites. We have worked on the development of a universal slow radioactive ion beam facility Fluorescence Intensity [Counts / ms] Laser Cooled 7 Be + UV Laser Frequency - 957,37 [GHz] Fig. 3. Fluorescence intensity of laser cooled 7 Be + ions as a function of the cooling laser frequency scanning from lower to higher frequency. A transition from a broad peak to a sharp peak with a characteristic dip indicates a transition from an ion cloud to an ion crystal. The ion temperature was evaluated to be < 1 mk from the line width of the sharp peak. LIF Signal Detector Probe Laser Frequency g.s. cooling probe F=1 F= F= F=1 Fig.. Optical-optical double resonance in Be +. SLOWRI at RIKEN [7]. The present Be experiments were performed at the prototype SLOWRI (fig. ), where high-energy radioactive Be isotope beams at 1GeV were provided by the RIKEN projectile fragment separator RIPS [1] and thermalized in a He gas catcher cell. Then, the thermal ions were collected and extracted by an RF-carpet and further transported into a high-vacuum region by a mm long Carbon-OPIG [15]. After contaminant ions were removed by a quadrupole mass filter, Be ions were collected in a cryogenic linear Paul trap and cooled by He buffer gas collisions. In the trap the velocities of Be + ions were further reduced by laser cooling using a UV laser at 313 nm which is resonant to the S 1/ P 3/ transition. A typical cooling spectrum of 7 Be + is shown in fig. 3. Under such a condition, optical pumping into a maximum or a minimum magnetic sub- 3

3 A. Takamine et al.: Isotope shift measurements of 11,9,7 Be PIAS Position Analyser Iodine Saturated Spectroscopy Beam Expander Frequency Comb EOM POL ~5 m MBD- SHG POL Interference Filter Ion Trap PC Slow Be Ion Beam 99-1 Ring Dye Laser Verdi V-1 Beam Expander B Verdi V Ring Dye Laser w/ SHG Intra Cavity EOM POL ~5 m Trap Chamber Frequency Comb Fig. 5. Schematic layout of the laser setup and the trap chamber. level is achieved and it is ready to perform laser-microwave or optical-optical double-resonance spectroscopy. A laser-cooling spectrum such as shown in fig. 3 is insufficient to determine the resonant frequency with high accuracy because of its asymmetric peak profile in addition to power broadening and shift effects induced by the cooling laser light. To measure the optical transition energy, we used the optical-optical double-resonance (OODR) spectroscopy method [1]. In the laser-cooling process, ions are optically pumped between the states S 1/ (largest F ) and P 3/ and strong laser-induced fluorescence is observed. If the optical pumping is not perfect, some fraction of the population remains in the other states. Under such a condition, if we use a weak probe laser resonant to the S 1/ (smaller F )- P 3/ transition, a part of the population in the smaller F state is transferred to the larger F state. Then, an increase in the laser-induced fluorescence signal can be observed (fig. ). The cooling and probe lasers alternately irradiated the trapped ions, using electro-optic modulators and polarizers to avoid the power broadening and shift effects. In this way, we can obtain a symmetric Lorentzian profile for the S 1/ - P 3/ transition with a width close to the natural line width. To measure the hyperfine constant, we applied lasermicrowave double-resonance (LMDR) spectroscopy. In this case, a cooling laser and microwave radiation were used and the microwave frequency directly represents the hyperfine transition frequency. A detailed description of LMDR for Be + ions can be found in our previous papers [17,1]. To measure the nuclear g-factor, we used a similar method but at high magnetic field. Under such high magnetic field, the hyperfine splittings decouples, to some extent, from the nuclear spin part and the electron spin part. We measure both the nuclear spin flip transitions and the electron spin flip transitions simultaneously by laser-microwave-uhf triple-resonance spectroscopy [1]. In this way, we can determine the hyperfine constant as well as the nuclear g-factor in units of the electron s g-factor with high accuracies. Figure 5 shows a schematic layout of the ion trap chamber and the laser system. Two 313 nm UV laser radiations were oscillated by ring dye lasers and second harmonic generators. The laser frequencies were monitored by commercial wave meters (Advantest Q3), an iodine molecular vapor cell and a clockwork optical femtosecond frequency comb (Menlo Systems FC15) [19]. The UV laser radiations were introduced into the center of the trap with an angle of 1 respect to the trap axis to avoid interference to the beam injection path. A weak magnetic field (. mt) produced by Helmholz-type coils was applied parallel to the cooling laser radiation. Using an accurate shunt resistor and a software feedback system, the current of the coils was stabilized to a level of 1 ppm. Two other pairs of coils also compensated stray magnetic fields to the order of μt. The laser-induced fluorescence from trapped ions was collected through a lens and an interference filter and detected by a two-dimensional photon counting system (PIAS, Hamamatsu). Preliminary results and discussion The OODR spectra for 7,9,11 Be + ions obtained in the first preliminary measurements are shown in fig.. The absolute laser frequencies of the probe laser were obtained from the beat signals to the frequency comb. To avoid fluctuations and jitters of the laser frequency, the beat signal and the fluorescence signal were accumulated simultaneously with a short bin period of < 1 ms. Then the spectra were reconstructed as a function of the true probe laser frequency. In these particular measurements, the total bandwidth of the frequency filters for the beat signal was narrow, only a part of the Lorentzian profile was obtained. From these three spectra, the resonance frequencies were determined to be ν( 7 Be + ) = (1.) MHz, ν( 9 Be + ) = (1.1) MHz, and ν( 11 Be + ) = (.3) MHz. The absolute transition frequencies of the S 1/ - P 3/ transition can be determined by taking into account the hyperfine structures and the Zeeman shifts. However, a careful evaluation of the optical pumping scheme is required to identify the probed states. We continue the measurements in the succeeding beam time to clarify the probed state and to improve the statistics. It should also be noted that the present OODR scheme is not applicable to the 1 Be isotope, since 1 Be + does

4 37 The European Physical Journal A frequency [MHz] MHz frequency [MHz] MHz frequency [MHz] MHz Fig.. OODR spectra of S 1/ P 3/ transition for 7,9,11 Be +. counts/bin counts/bin (a) σ + polarization ν + = 17.7() MHz 95 FWHM 3(1) khz microwave frequency 1 (MHz) (b) σ polarization ν = 19.1(13) MHz FWHM 35(13) khz microwave frequency 1 (MHz) Fig. 7. Microwave resonance spectrum of 7 Be +. LIF Counts per ms Microwave Resonance of 11 Be + 7.3(1)MHz preliminary Microwave Frequency [khz] (- MHz) Fig.. Microwave resonance spectrum of 11 Be +. Table 1. Present status of the isotope shifts measurements for Be isotopes. The relative accuracies for the S-P transition frequencies, the magnetic hyperfine constant A, and the nuclear magnetic moments μ I taken from published data are listed. The status symbols stand for C: completed, D: data taking, A: analyzing, P: preparing. Isotope S-P A μ I 7 Be A C P 9 Be 1 1 A 1 1 C 3 1 C 1 Be 1 D 11 Be A A 5 1 P not have hyperfine splittings. We will also measure the S 1/ - P 1/ transition for all possible isotopes. The ground-state hyperfine splittings of 7 Be + has been measured by LMDR spectroscopy (fig. 7) and the magnetic hyperfine constant was determined to be A 7 = 7.77(3) MHz [1]. From the hyperfine constant, the nuclear magnetic moment of 7 Be was also deduced, within the uncertainty due to the hyperfine anomaly, to be μ I ( 7 Be) = 1.399(1)μ N. A similar experiment has been performed for 11 Be. Figure is a microwave resonance spectrum of 11 Be + ions. The analysis of the data is in progress and the result will be reported soon. Table 1 summarizes the present status of the isotope shift measurements for Be isotopes. Our previous measurements of the S-P transition frequencies under buffer-gas-cooled conditions [9] were not sufficient to deduce the charge radii. Present experiments will provide relative accuracies of better than 1 9 or absolute accuracies of sub-mhz which will allow us to determine the charge radii of Be isotopes. For the magnetization radii, measurements of the hyperfine constants have been completed for all odd Be isotopes. However, the nuclear magnetic moments should be measured more accurately. We deduced the magnetic moment of 7 Be, however, the value obtained from the hyperfine constant cannot be used for the hyperfine anomaly. The magnetic moment of 11 Be was measured by the β-nmr method at ISOLDE [], however, the relative accuracy of 5 1 is not sufficient to deduce the hyperfine anomaly. We have prepared a combined trap for measurements of the nuclear magnetic moments as well as the hyperfine constants with relative accuracies better than 1 [1].

5 A. Takamine et al.: Isotope shift measurements of 11,9,7 Be Summary A combination of trapping, laser cooling and doubleresonance spectroscopy is one of the most accurate method for atomic spectroscopy. We have shown the great potential of this method especially for the applicability to radioactive ions for the first time. Radioactive Be ions, including a neutron halo nucleus 11 Be, produced at relativistic energy were thermalized and cooled down to the μev range and various precision laser spectroscopy experiments are in progress. The universal slow RI-beam facility, SLOWRI, at RIKEN RIBF will expand such precision atomic spectroscopy to a wide variety of radioactive nuclei including refractory elements. The authors thankfully acknowledge the contribution of the crew of the RIKEN Nishina Center for Accelerator-Based Science for their contribution to our on-line experiments. This work was supported by the Grants-in-Aid for Scientific Research from the Japan Society for the Promotion Science, by the President s Special Grant of RIKEN, and by the Robert A. Welch Foundation under grant A15. References 1. I. Tanihata et al., Phys. Rev. Lett. 55, 7 (195).. P.G. Hansen, B. Jonson, Europhys. Lett., 9 (197). 3. L.-B. Wang et al., Phys. Rev. Lett. 93, 151 (197).. P. Mueller et al., Phys. Rev. Lett. 99, 551 (7). 5. R. Sanchez et al., Phys. Rev. Lett. 9, 33 ().. W. Nörtershäuser et al., this conference. 7. M. Wada, et al., Nucl. Instrum. Methods: Phys. Res. B, 57 (3).. A. Takamine, et al., Rev. Sci. Instrum. 7, 1353 (5). 9. T. Nakamura et al., Phys. Rev. A 7, 55 (). 1. K. Okada et al., Phys. Rev. Lett. 11, 15 (). 11. Z.-C. Yan et al., Phys. Rev. Lett. 1, 3 (). 1. A. Bohr, V.F. Weisskopf, Phys. Rev. 77, 9 (195). 13. T. Fujita, K. Ito, T. Suzuki, Phys. Rev. C 59, 1 (1999). 1. T. Kubo et al., Nucl. Instrum. Methods: Phys. Res. B 7, 39 (199). 15. A. Takamine et al., RIKEN Accel. Prog. Rep., 17 (7). 1. D.J. Wineland et al., Opt. Lett. 5, 5 (19). 17. K. Okada et al., J. Phys. Soc. Jpn. 7, 373 (199). 1. T. Nakamura et al., Opt. Commun. 5, 39 (). 19. T. Udem et al., Nature 1, 33 ().. W. Geithner et al., Phys. Rev. Lett. 3, 379 (1999).