Ion traps, atomic masses and astrophysics. Outline. Positive ray parabolas. British beginnings

Transcription

1 Ion traps, atomic masses and astrophysics Kumar S. Department of Physics and Astronomy University of Manitoba Outline Some history Atomic masses Ion traps rp-process nucleosysnthesis Physics Day Slide 1 Physics Day Slide 2 British beginnings Positive ray parabolas J.J. Thomson (1913) Positive ray parabola apparatus Coterminous electric and magnetic fields Screen Gas discharge ion-source Physics Day Slide 3 Physics Day Slide 4 1

2 The 1 st mass spectrometer Meanwhile back in Chicago F.W. Aston (1919) Photographic plate A.J. Dempster (1918) Monoenergetic ion source Direction focusing; no energy focusing Electric field Magnetic field Physics Day Slide 5 Physics Day Slide 6 Canadian Family Tree Evolution of precision Atomic mass determinations group at the University of Manitoba Physics Day Slide 7 Physics Day Slide 8 2

3 Information from atomic masses Binding energies of nucleons ( BE..) = Z ( M M ) N M M( Z, N) ( B. E.) nuclear p e n elect Energy released in nuclear reactions and decays Q = ( M M ) ( M M ) T B P E (B.E.) nuclear ~ 10 MeV/nucleon Differences in B.E. ~ 1MeV Need to know this to 0.1-1% (1-10 kev) or better in some cases. Need a precision of 10 ppb or better At the Univ. of Manitoba Two mass spectrometers were built: Manitoba II: Betsy Manitoba I: Big Ed Physics Day Slide 9 Physics Day Slide 10 Measurements made The lure of unstable nuclides Z N Physics Day Slide 11 Physics Day Slide 12 3

4 Penning Traps Overview of the CPT apparatus at ANL To study unstable nuclides we need: Precision Accuracy Sensitivity Ion traps can provide all of these. transfer tunable line degrader gas cell RFQ ion guide isobar separator Enge spectrograph Penning trap triplet RFQ ion trap velocity filter laser ion source target chamber Physics Day Slide 13 ATLAS Physics beam Day Slide 14 The anatomy of a Penning trap How a Penning Trap works-1 B Constant axial magnetic field particle orbits in horizontal plane Shapes of the electrodes Correction electrodes ω = c free to escape axially qb m Carefully chosen materials Physics Day Slide 15 Physics Day Slide 16 4

5 How a Penning Trap works-2 B Add an axial harmonic electric field to confine particles axial oscillations: ev ω z = 2 md Radial motion split into two components by electric field: ω : reduced cyclotron freq. ω - : magnetron frequency Physics Day Slide 17 Where: and ω = ω ω ω c z ω = ω ω c How a Penning Trap works-3 Ion motion in the radial plane: vx = ρ ω sin( ω t) ρ ω sin( ω t) vy = ρ ω cos( ω t) ρ ω cos( ω t) P = qv E ower absorbed by ion in electric field: ρ ρ - Physics Day Slide 18 How a Penning Trap works-4 How a Penning Trap works-7 For a dipole field: Resonances at ω D = ω and ω - - Recall: ω = c ω c depends only on: qb m For a quadrupole field: Resonances at ω Q = ω ω - = ω c - - the mass the magnetic field not on the electric fields Can use ω c to make accurate and precise mass measurements Physics Day Slide 19 Physics Day Slide 20 5

6 rp-process measurements Observed elemental abundances cannot be reproduced by only considering nuclear reactions in quiescent stars. Need to consider some explosive processes as well: X-ray bursts rp-process (involves proton rich nuclides) Supernovae r-process (involves neutron rich nuclides) Mass measurements along the rpprocess From: Physics Day Slide 21 Time of flight (μs) Frequency applied Hz Time of flight (μs) Physics Day Slide 22 Frequency applied Hz rp-process path rp-process path 66 Se 65 As 67 Se 66 As 68 Se 67 As 68 As (p,γ) (γ,p) β 66 Se 65 As 67 Se 68 Se 66 As 67 As 68 As (p,γ) (γ,p) β Waiting-point 64 Ge 65 Ge Neutron stars: 1.4 M o, 10 km radius Waiting-point 64 Ge 65 Ge In equilibrium: (p,γ) (γ,p) Process stalls until β-decay 63 Ga 64 Ga 63 Ga 64 Ga waiting-point nuclide Z 62 Zn 9 NNovember accretion rate: 10-8 /10-10 M o /yr Normal star Physics Day Slide 23 Z 62 Zn 9 NNovember To determine equilibrium, need Q p Physics Day Slide 24 6

7 Waiting-point 66 Se 65 As 64 Ge rp-process path 67 Se 66 As 65 Ge 68 Se 67 As 68 As (p,γ) (γ,p) β Q e kt time scale A( Q) Dominated by β-decay Effective lifetime of waiting-point nuclides λ =λ λ λ exp{ Q kt} effective t1/2, eff ( 68 Se) (s) β p p p Effect of proton capture 63 Ga 64 Ga isotope production A( Q) e Q kt Z 62 Zn 9 NNovember energy production A( Q) Q e Q kt Physics Day Slide Qp ( 68 Se) (MeV) Precision required ~ kt ~ 100 kev (~ 1.5/10 6 ) Physics Day Slide 26 G. Audi and A.H. Wapstra, Nucl. Phys. A595, 409 (1995). Endpoint of the rp-process Refractory elements where little mass information is known Waiting-point nuclides rp-process measurements completed Effective lifetime of the waiting-point nuclide 68 Se t1/2, eff ( 68 Se) (s) Q p ( 68 Se) (MeV) CPT - AME SPEG - AME CSS2 - AME Pfaff et al. CSS AME FMA -AME CPT - HF CPT: J.A. Clark et al., Phys. Rev. Lett. 92, (2004). Pfaff: R. Pfaff et al., Phys. Rev. C 53, 1753 (1996). Physics Day Slide 27 AME: G. Audi et al., Nucl. Phys. A729, 337 (2003). SPEG: G.F. Lima et al., Phys. Rev. C 65, (2002). CSS2: A.S. Lalleman et al., Hyperfine Interact. 132, 315 (2001). CSS2 2003: D. Lunney, private communication. Physics Day Slide 28 FMA: A. Wöhr et al., Nucl. Phys. A742, 349 (2004). HF: B. A. Brown et al., Phys. Rev. C 65, (2002). 7

8 Effective lifetime of the waiting-point nuclide 64 Ge t1/2, eff ( 64 Ge) (s) Q p ( 64 Ge) (MeV) CPT - AME CPT - FRDM CPT - HF SPEG - AME Conclusions A Penning trap mass spectrometer is a powerful tool for the study of exotic nuclei. Can make measurements that shed light on: Astrophysics Tests of fundamental symmetries Nuclear structure Others AME: G. Audi et al., Nucl. Phys. A729, 337 (2003). Physics Day FRDM: P. Möller et al., At. Data Nucl. Data Tables Slide 59, (1995). HF: B. A. Brown et al., Phys. Rev. C 65, (2002). SPEG: G.F. Lima et al., Phys. Rev. C 65, (2002). Physics Day Slide 30 CPT Collaboration R.C. Barber, J.A. Clark, J. Fallis, H., K.S., Y. Wang Argonne National Laboratory B. Blank, J.P. Greene, J. Guest, A.A. Hecht, A.F. Levand, B. Lundgren, G. Savard, N. Scielzo, D. Seweryniak, I. Tanihata, W. Trimble, A.C. Villari, B.J. Zabransky F. Buchinger, J.E. Crawford, S. Gulick, J.K.P. Lee, G. Li J.C. Hardy G.D. Sprouse Physics Day Slide 31 8