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Th DPG Verhandlungen Powered By Docstoc
					                        Optical access to the lowest
                         nuclear transition in 229Th:

                            Bridging the gap between
                        laser physics and nuclear physics
                            P.G. Thirolf, LMU München

Outline:                                             229mTh       isomer
                                             3/2+
- Introduction                               [631]
                                                                   DE = 7.6(5) eV
- Properties of 229mTh
                                                                   M1 transition
- Perspectives: fundamental constants
                                                                   t ≈ 104 s
                 .
                laser cooling/excitation
                                             5/2+
- Experimental approach at the MLL                   229Th
                                             [633]            ground state
- Conclusion

                                                     DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            Optical nuclear excitations ?

                                   atomic physics         nuclear physics

  level spacing                    DEA ~ 1-10 eV          DEA ~ 1-100 keV

  dimensions                        lL ~ 10-4 – 10-5 cm     RA ~ (2-10).10-13 cm

  coupling strength                       M/DEA ≤ (10-15 -10-16) I1/2
  (matter-field)

 bridge between laser/nuclear physics:
    very low-energetic nuclear levels required

 229Th: low-energy excited state

               - 1976 (Kroger, Reich):                 < 100 eV
               - 1990 (Reich, Helmer):                 -1 ± 4 eV
               - 1994 (Reich, Helmer):                3.5 ± 1 eV
               - 2005 (Guimaraes-Filho, Helene):      5.5 ± 1 eV (re-analysis)
                                                           DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            229mTh:   nuclear level scheme




                                                                  ~ 2% population of
                                                                    229mTh in a decay




                                                    7.6(5) eV
       T1/2~ 3-5h


 Iexc=3/2+            T1/2=7340 a
                                                       0.0076
 [631]
                       Igs=5/2+
                       [633]
                                                        DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
               229mTh:           revision of excitation energy

 - use (cooled) X-ray microcalorimeter (NASA)
 - identify low-energy 3/2+  5/2+
   ground state transition energy from
   differences between ‘high-energy’ transitions


                                         E(229mTh)= DE29-DE42

                                          result:
                                          (3.5 ± 1.0) eV → (7.6 ± 0.5) eV = 163± 11 nm




                                                                     DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München   B.R. Beck et al., PRL 98 (2007) 142501
                            Unique properties of       229Th



         isotope with lowest excited state in nuclear physics

         very long-lived isomeric state (~ 3-5 hours)
         compared to equivalent atomic transitions (~ seconds)

            natural line width:               D  104 Hz
            extremely sharp transition:       DE / E  1020

         Perspectives:
          nuclear vs. atomic system:
            stable against external pertubations
            (small nuclear dimensions, shielding by electrons)

                 -   metrology (‘nuclear clock’)
                 -   time-dependence of fundamental constants (e.g. a)
                 -   qubit for quantum computing
                 -   coherent excitations (Mößbauer spectroscopy)
                                                          DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                    229mTh   and fundamental constants

 - time-dependence of natural constants ?              V.W. Flambaum, PRL 97 (2006) 092502

               temporal variation in transition energy provides enhanced
                                                                    .
               sensitivity by 105 – 106 for fine structure constant a/a
               and strong interaction parameter (mq/LQCD)


229mTh       transition:            a X q 
                                    5
                                 10 0.1                  with    X q  mq / L QCD
                                     a X 
                                           q 



                                    X q
energy shift:                           MeV
                                    Xq

         frequency shift:        3 1020 X q / X q Hz     V. Flambaum: arXiv:0801.1994v1
                                                                           [nucl-th] (2008)
         with transition width ~ 10-4 Hz:
        sensitivity to variation of Xq: ~10-24/yr (atomic clock limit: 10-14/yr)
                                                               DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                     Laser excitation of the isomer ?

closed 2-level system in electron shell:


                                                    a' ,'
                                                                                 1
            a ,      '
                                          2

                                   1                a' ,
                                                                       229mTh


            a ,
                            229Th(g.s.)        after nuclear transition:
                                                change of nuclear moments, spin
      a: nuclear, : electronic                 change of hyperfine splitting,
                                                 total angular momenta
                                                1 out of resonance (~GHz)
          double resonance method               drop in resonance fluorescence
          ‘electron shelving’
                                           Peik, Tamm, Eur. Phys. Lett. 61 (2003) 181
                                                         DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            229Th3+:          suitable level scheme

 lowest electronic levels of 229Th3+: Rn core + single valence electron
                                            6p6 7s 2S1/2
                                                              (metastable, ~1s)
                      6p6 6d 2D5/2
                                                          6p6 6d 2D3/2


                                     690 nm
                                              ~GHz

         6p6 5f 2F7/2


                                               6p6 5f 2F5/2

 closed 3-level L system and closed 2-level system available
  for laser cooling and fluorescence detection
- constant resonance fluorescence scattering with closed optical transition
- after nuclear excitation: out of resonance
        spectroscopy of laser-cooled                 229Th3+   (e.g. in linear Paul trap)
             (see also talk by M. Bussmann: A28.5 –Friday- )
                                                                      DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            Experimental Approach

 final goal: laser excitation of isomeric transition
      requires precise knowledge of transition energy
      present precision of transition energy: 6.7.10-2

 improvement of DE/E:

          233U      229(m)Th + a : in buffer gas cell




                                                          DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                               “Isomer generator”
                                buffer gas cell



    a detector


                                                  233U

                                RFQ:
 needle                         ion transport channel
                    229(m)Th


- stopping range ~1 cm for a recoils
  in 50 mbar helium
- source in 60 mm distance from nozzle
- extraction times ~1 – 2 ms
- advantage: prompt decays already inside
  gas cell, only isomeric decays after collection
                                                         DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                                 Isomer Collection

  extraction of isomer through gas cell nozzle
  collection onto needle tip (diameter: ca. 40mm)
  ‘soft landing’ via suitable potentials
  optimize collection efficiency of isomers on needle tip (SIMION):


                                                                             RFQ
                            reflector
                                                                               ions from
 needle                                                                         gas cell

                  cap


                                                                              J. Neumayr
                                           aperture
                                                      DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                      Potential Quenching of Isomeric
                                 Transitions
 electron bridge processes:
    nuclear deexcitation via transfer of transition energy to atomic shell
     photon emission : Eg = DEIF – (ei – ef)
     fast decay of 229mTh instead ~104 s
                                                   g
                                                  n , en
  atomic shell/
  conductor
                              i        >                       f
                            ei = -Bi       IF
                                                           ef = -Bf
    nucleus
                              I        >           >           F
                                             F.F. Karpeshin et al., Hyperf. Interact. 162 (2005) 125

 reducing electron bridge probability:
   coating of needle with diamond-like carbon layer (DLC)
                                                                   DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                        Extraction test from gas cell:
                              Ra-Recoil Source
 extraction tests: 50 mbar helium
                    Si-Detector directly behind RFQ at 0o
  223Ra
              t1/2= 11.4 d
   a
                                             211Bi   215Po
  219Rn        t1/2= 4 s
  a
  215Po
              t1/2= 1.8 ms
  a
  211Pb       t1/2= 36 min
                                  extraction efficiency:     200 s accumulation
 211Bi
              t1/2= 2.2 min                                   7200 s measurement
  a                                              a              esource = 25%
                                 a       223Ra
 207Tl        t1/2= 4.8 min                                     edet = 50%
  
                                 219Rn                            eextr = 48%
 207Pb
                                             215Po
                                                       DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                                    233U:         sample composition

 g spectrum:

                            213Po
                  212Pb
                  217At




                                                                                      208Pb




                                                                                                   208Pb




                                                                                                               208Pb
                                                                  213Po




                                                                          209Pb
                                       208Pb




                                                                                  212Po
             229Th




            213Po
            208Pb

            229Th



                               209Pb
                               208Pb




                                               212Po




                                                          208Pb
                                                  213Po
                                                  212Po
                                                  208Pb




 background contamination: 232U (t1/2 = 69 a )                                      233U     (t1/2 = 1.6.105 a)
           232U/233U       ~ 2.10-5


                                                                          DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                                 a decay from needle

 - a detector moved beneath needle tip


               - gas cell ‘on’      217At
               - 3 hrs
  counts/ch.




                                             213Po

                            221Fr



                                                                                - gas cell ‘off’
                                                                                - 14.7 hrs


                                                       counts/ch.
                                            Ea [ch.]




                                                                                              Ea [ch.]
               collection of 229Th on needle works
                                                                    DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                  Search for VUV Fluorescence from
                               229mTh


 optical setup in detection chamber behind collection needle:




                   CaF2 lens




229Th                                       MCP/CCD                   M. Bussmann
(needle tip)                   VUV filter

    - collect ions from gas cell
    - focus UV light onto MCP/CCD (so far: 163 +/- 11 nm)
    - insert VUV filter with sharp absorption edge

                                                  DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                    MCP/CCD diagnostics at Gas Cell




CCD camera:
- 1600 x 1200 pixel, 14 bit
                                          MCP detector:       needle, 233U in
- cooled (t0= -20oC)                                                  gas cell
- low readout noise: 10 e- rms             - diameter 75 mm   lens
- dark current: -20oC: 0.01 e-/pixel.s     - chevron geometry
- exposure times: 5 ms – 49 days           - high gain: 108
- image integration in camera before readout
                                                   DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            Available VUV Filters

                                               163 +/- 11 nm
- customized UV filters
  with sharp absorption edges:                                                 filter 1


- cutoff wavelength can be shifted
  to lower values by changing the
  incidence angle



                                                                               filter 2

goal: determine wavelength
      to ~ 0.5 nm


                                                DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München
                            Conclusion
- 229mTh: enables optical access to nuclear transition

- 7.6(5) eV ground state transition (163±11 nm):
     connects metrology and fundamental physics
       - candidate for ultra-stable nuclear clock
       - sensitive to variations of fundamental constants

- mandatory: precise knowledge of transition energy

- technology available to address experimental search for VUV transition:
    233U isotopic material

    gas cell as ‘isomer generator’
    sensitive VUV detection equipment

- status of MLL measurements:
    233U material characterized with g spectroscopy

    extraction efficiency from gas cell optimized with Ra source (48%)
    collection on needle tip studied via a decays
                                                                      gas cell
    measurements started with low-noise MCP/CCD setup at Darmstadt, March 11, 2008
                                                  DPG Spring Meeting,
P.G. Thirolf, LMU München
                            Thanks to . . .


              M. Bussmann, D. Habs, H.-J. Maier,
              J. Neumayr, J. Schreiber, M. Sewtz,
              J. Szerypo




                                              DPG Spring Meeting, Darmstadt, March 11, 2008
P.G. Thirolf, LMU München

				
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