Development of laser spectroscopic method using superfluid helium for the study of low-yield nuclei

Transcription

1 Development of laser spectroscopic method using superfluid helium for the study of low-yield nuclei Meiji University, Dept. of Phys. / RIKEN Nishina Center Kei IMAMURA INPC 2016, Adelaide Australia, Sep 11-16

2 Laser spectroscopy of atoms using superfluid helium - Measurement of Nuclear spin I, moment μ I Powerful tool: Laser spectroscopy Problem: Low efficiency OROCHI (OpHcal RI-atom ObservaHon in Condensed Helium as Ion-catcher) Accelerator Ion beam Separator e Target He II Radio frequency or Microwave Atoms Zeeman structure Hyperfine structure Nuclei Nuclear spin Nuclear moment Laser

3 Advantage of superfluid helium High trapping efficiency Beam Stop ion & neutralize Laser Cryostat He II 133 Cs D1 atomic spectra in He II Intensity [arb. unit] Extremely low background (λ laser ) AbsorpHon 発光励起 λ laser λ LIF Emission (λ LIF ) Vacuum LIF: Laser Wavelength induced fluorescence [nm] λ:wavelength Ref : Y.Takahasi et al, Phys. Rev. Lett (1999) Suitable for studying low-yield exotic nuclei

4 Op9cal pumping and double resonance method I=1/2, J=1/2 m F = P F=1 1/2 F=0 σ + LIF σ + Spin polarizahon LIF 2 S 1/2 F=1 σ + F=1 F=1 F=0 m F = F=0 F=0 RF LIF MW/RF MW LIF intensity Expected spectra Resonance frequency Frequency

5 Result of experiment using RIPS at RIKEN and iden9fied difficul9es Successful observation of HFS/Zeeman resonance Beam intensity: , Mes. time: 40 min. HFS resonance of 87 Rb LIF intensity [ x 10 3 cps] Frequency [MHz] LIF intensity [cps] Zeeman resonance using Rb Improvement of S/N ra9o 2.Small resonance signal intensity 84 Rb beam Magnetic field [gauss] X. F. Yang et al., PRA 96, (2013) LIF intensity [cps] Imperfect atomic polarization 1. Laser stray light Magnetic field [gauss] Lower yield nuclei ( < 100 pps)

6 1.Laser stray light -How to reduce?- Change the system for the wavelength separa9on Cf: Previous florescence detec9on system Laser New one Laser Slit Interference filter Lens Lens Monochromator Photomul9plier tube Fiber Photomul9plier tube Reduc9on of noise due to laser stray light

7 1.Laser stray light -Performance evalua9on- Cryostat Rb Gas cell (with He buffer gas) Comparing the S/N ra9o Interference filter Fiber PMT PMT Ti:Sa laser Monochromator

8 Collisional transfer λ LIF :794nm (Observed) S 1/2 P 1/2 1.Laser stray light -Result of experiment- Laser wavelength: nm P 3/2 λ Laser 780 nm Previous system:s/n New system:s/n= Number of detected photons [cps] At least 10 Hmes improvement! Previous detec9on system LIF signal 10-2 Laser stray light 10-3 New detec9on system PMT dark count level Laser wavelength [nm]

9 MagneHc field 2.Small resonance signal intensity -Was MW power enough?- Rb Gas cell Helmholtz coils Scanning magne9c field Resonance SaturaHon MW power[w] MW antenna Maximum intensity?? Required MW power?? Appling MW (fixed frequency) Laser LIF intensity B=0 peak y B Resonance Peak height y R y 0 y R y B y 0 y 0

10 2.Small resonance signal intensity Result and discussion- Peak height Satura9on MW power : 5.5 W MW power last applied MW power [W] LIF intensity [cps] x Frequency [MHz] Increasing MW power Obtaining 15 9mes higher peak height Laser: ~ nm Laser power: 0.7~0.8 mw, φ=1 mm MW frequency:3.057 GHz Total improvement of S/N ra9on 10 (New detec9on system) 15 (increasing MW power) x = 150 9mes!

11 Summary and outlook *Developing a nuclear laser spectroscopy technique OROCHI for the study of low-yield exohc nuclei *In the online experiment, we successfully observed HFS/Zeeman resonance spectra. However, we required the beam intensity of 10 4 pps at minimum. * Towards lower-yield nuclei (< 100pps) Developing the new florescence detechon system EvaluaHng MW power dependence of resonance signal intensity. *As a result of development, we expect S/N raho approximately 150 Hmes higher. *In Dec. 2016, we will perform a beam experiment to evaluate minimum beam intensity to observe the double resonance spectra using the system.

12 Collaborators Meiji University / RIKEN Nishina Center : Kei Imamura Osaka Univ.: Tomomi Fujita Hosei University: Tsuyoshi Egami, Taishi Nishizaka, Daiki Tominaga, Takafumi Kawaguchi, Wataru Kobayashi, Makoto Sanjo, Yukari Matsuo RIKEN Nishina Center: Aiko Takamine, Yuichi Ichikawa, Hideki Ueno Tokyo Metropolitan University : Takeshi Furukawa NIRS: Takashi Wakui Meiji Univ.: Yutaro Nakamura, Hitoshi Odashiima Thank you for your asenhon!!

13 Backup slide

14 Resonance intensity vs MW antenna posi9on Resonance intensity Distance between MW antenna and gas cell[cm]

15 Signal intensity of double resonance spectra 5 P 1/2 Γ F=3 α β ν e :MW resonance rate Γ :pumping rate α, β : Spontaneous emission 5 S 1/2 F=2 ν e LIF intensity = Γ (α + β) 3Γ + 2(α + β) + β Γ/ν e

16 Comparison of new and old fluorescence system Laser Cf: Previous florescence detec9on system Laser Lens Slit Interference filter Photomul9plier tube Lens Photon intensity [cps] Specifica9on of spectrometer 10-8 Fiber Spectrometer (Selec9ng wavelength of light) Selected wavelength by spectrometer [nm] Laser stray light can be decreased by 10-8

17 Atomic bubble model in superfluid helium ExcitaHon Need more energy blue shiyed abs. spectrum Different atom-he distance broadened spectra Deform Deform De-excitaHon

18 Op9cal pumping of atoms in superfluid helium F=4 In vacuum P 1/2 F=3 S 1/2 F=4 F=3 Wavelength [nm] In He Ⅱ Wavelength [nm]

19 2.Small resonance signal intensity -Experimental setup- Cryostat Rb Gas cell Rb Gas cell MW antenna Helmholtz coils Scanning magne9c field Laser Fiber PMT Ti:Sa laser Spectrometer

20 Hyperfine structure in magnehc field W(F, m F ) = - ΔW/2(2I+1) μ I m F B/I ± (ΔW/2) [1 + {4m F /(2I+1)} (1+ε)x +(1+ε) 2 x 2 ] ΔW:hyperfine spli ng x = g J μ B B/ΔW ε= g N μ N /g J μ B P 3/2 ^ 2 P 1 2 F=3 λ Laser : nm (Rb D1) ^ 2 S 1 2 ΔW λ LIF :780 nm (Rb D2) F=4 F=3 F=2 E + E - m F =3 m F =-3 m F =-2 m F =2

21 Frequency sii due to applied magne9c field Applied magnehc field: B B B σ+ pumping ν hfs ν + ν - σ- pumping σ- pumping σ+ pumping ν - ν +

22 Double resonance spectra in He II LIF Intensity [count/bin] x Cs Au Frequency [MHz] K. Imamura et al., Hyperfine Interact., 230, 73 (2014) Y. Matsuura, Master Thesis, Meiji University(2010)

23 Cryostat for online experiment Liquid helium bath Liquid nitrogen bath Needle valve Beam injec9on port Superfluid helium bath