Low-energy structure of exotic nuclei studied at the NSCL using - PowerPoint PPT Presentation

Low-energy structure of exotic nuclei studied at the NSCL using β -delayed γ ray and β -NMR spectroscopies P.F. Mantica Chemistry and NSCL Michigan State University East Lansing, MI 48824 mantica@msu.edu SQS04 February 19, 2004

Coupled Cyclotron Facility Layout • Existing apparatus: 4 π -Array (N2), 92-inch chamber (N3), S800 magnetic spectrograph (S3), Superball neutron multiplicity meter (S2), RPMS Wien Filter (S1); movable large-solid-angle detectors: Miniball, Neutron Walls, NaI array • Major new equipment: – segmented Ge-array for γ -ray Doppler shift correction – Si-strip-CsI array for high efficiency charged particle coincidence experiments – Superconducting “sweeper” magnet for n-coincidences at 0 degrees – Modular neutron array (MONA) for high-efficiency neutron detection

Spin Polarization via Fragmentation • Fragments collected off the central beam axis. • Polarization as large as 20% for 12 B fragments at wings of momentum distribution. • I n initial experiments no spin polarization detected at the peak of the momentum yield curve. • Provides a means for measuring ground state dipole moments of exotic nuclei. Asahi et al., Phys. Lett. B251, 488 (1990)

Factors I nfluencing Magnitude of Spin Polarization in Fragmentation • Magnitude of spin-polarization dependent on beam, target, and beam energy • I n general, non-zero polarization at the peak of the momentum yield distribution observed when medium-mass targets employed Okuno et al. , Phys. Lett. B335, 29 (1994)

Details of the Kinematical Model -k /p f x θ def Projectile-rest frame y θ L R = (X, Y , Z) Beam z x Θ Projectile Target k = (k , k , k ) x y z Target = − Θ + Θ l k R cos k R sin z y x When Θ = 0 When Θ ≠ 0 = l P z / L ( ) = − l k y R ≅ θ − θ k p z x 0 L def = = < = P 0 at p p P 0 at p p 0 0

Nucleon Pick-up Reactions = + p p p 18 O (E = 80 MeV/ nucleon) F PF t 0.985 outgoing projectile target part nucleon < p/A> F /< p/A> beam 0.980 From momentum 0.975 conservation, the data to the Al left are consistent with the Ta 0.970 nucleon picked up with the 0.965 Fermi momentum 230 MeV/c oriented along the direction of 0.960 the projectile motion 19O 18N 17C ⎡ ⎤ p 1 p = + F P ⎢ ⎥ A p PF Fermi ⎣ ⎦ A A A F F P Souliotis et al. , Phys. Rev. C46, 1383 (1992) Pfaff et al. , Phys. Rev. C51, 1348 (1995)

Spin Polarization via Nucleon Pickup = + p p p F PF t outgoing projectile target part nucleon At the peak of the momentum distribution, < p F > = p 0 , < p PF > = p beam , and < p t > = p Fermi spin polarization is positive As the momentum of the outgoing particle decreases, the momentum of the nucleon picked up in the target should increase (since the projectile momentum is constant) spin polarization should increase D. Groh, Ph.D. Thesis (2002)

Dipole Magnet for Nuclear Moment Measurements • A small dipole magnet will be located in the S1 vault for nuclear moment measurements. – magnet gap = 10 cm – capability for catcher cooling – B max = 5000 Gauss – improved PMT performance at – optional vacuum chamber high B fields

37 K Spin Polarization 150 MeV/ A 36 Ar on Be target Reaction: 36 Ar + p → 37 K 37 K fragments implanted into a KBr crystal T 1/ 2 ( 37 K) = 1.23 s Q β + EC ( 37 K) = 6.1 MeV Polarization monitored by pulsed magnetic field method Maximum polarization observed when separator tuned just off the peak production of 37 K

Potential I mpact Expt. 02001: Ground state magnetic moment of 57 Cu T.J. Mertzimekis et al.

Monopole Shift π f 7/ 2 - ν f 5/ 2 Proton-neutron Lowering of 5/ 2 - due to 40 strong π f 7/ 2 - ν f 5/ 2 interaction is strongest p 1/ 2 f 5/ 2 monopole interaction when the orbitals they p 3/ 2 28 28 occupy strongly overlap. f 7/ 2 f 7/ 2 This overlap is maximum 20 d 3/ 2 when l n ~ l p . The s 1/ 2 attractive nature of the neutrons protons monopole interaction may lead to a re-arrangement of the single-particle N= 29 isotones 34 orbitals. ∑ ~ ε = ε + 2 j j V j j v ν π ν π j j M j ν ν π 32 j π Prisciandaro et al. , PLB 510, 17 (2001)

β -Delayed γ -Ray Spectroscopy • Beta decay lifetime of parent β – A p X Decay energy of parent • n Beta decay branching from • parent to daughter Low-energy level structure • of daughter γ 28 126 82 A 20 p Y + 50 1 n - 1 [MeV] Focus on the decay of odd-odd nuclei, which will selectively populate low- energy states of even- even daughter

NSCL Beta Counting System and Calorimeter Permits the correlation of fragment implants and subsequent beta decays on an event-by-event basis I mplant detector: 1 each MSL type BB1-1000 4 cm x 4 cm active area 1 mm thick 40 1-mm strips in x and y Calorimeter: 6 each MSL type W 5 cm active area 1 mm thick 16 strips in one dimension Prisciandaro et al. , NI MA 505, 140 (2003)

MSU Segmented Germanium Array (SeGA) Nominal 75% Ge crystal with etching of outer area of the crystal in each detector into 8 segments along the crystal axis and 4 segments perpendicular to it, for a total of 32 segments and 12 SeGA detectors around the one central contact. beta counting system during Expt. 02004. Efficiency ~ 5% at 1 MeV Mueller et al. , NI MA 466, 492 (2001)

Systematic Variation of E(2 + ) N= 32 N= 32 isotones 3000 E(4 + ) 2500 N= 32 Energy (MeV) 2000 1500 1000 E(2 + ) N= 34 500 0 18 20 22 24 26 28 30 32 34 Proton Number

What about N= 34? Lowering of 5/ 2 - due to GXPF1 interaction: 40 strong π f 7/ 2 - ν f 5/ 2 p 1/ 2 N= 34 magic structure f 5/ 2 monopole interaction expected p 3/ 2 28 28 f 7/ 2 f 7/ 2 20 d 3/ 2 s 1/ 2 neutrons protons N= 29 isotones 34 32 Honma et al. , PRC 65, 061301 (2002) Prisciandaro et al. , PLB 510, 17 (2001)

56 Sc Observables 56 Sc production rate ~ 3/ minute T 1/ 2 = 38 ± 5 ms Counts Time (ms) Absolute Energy (keV) I ntensity (% ) 592 ± 1 7 ± 2 689 ± 1 19 ± 4 Since total implant rate in DSSD less 751 ± 1 9 ± 3 than 20/ s, expanded correlation to 1127 ± 1 48 ± 11 nearest neighbor pixels 1160 ± 1 21 ± 5

56 Sc Levels: Beta Decay Liddick et al. , PRL in press

Summary Beta-NMR spectroscopy at the NSCL • Spin polarization observed for proton pick-up reactions at fragmentation energies • Polarization ~ 8% at peak of momentum distribution Apply method to measurements of g( 35 K) and • g( 57 Cu) • New data for isoscalar spin expectation values of T= 3/ 2 nuclides Beta-delayed gamma ray spectroscopy at the NSCL • Access to low-energy states in exotic nuclei • Half-lives, absolute branching ratios, (total beta energies) Tracking the monopole shift of ν f 5/ 2 orbital with • filling of π f 7/ 2 Measurement of E(2 + ) in 56 Ti does not support • shell closure at N = 34 for Ti nuclides

Collaborators Beta Decay Measurements Near N = 32 B.A. Brown, A.D. Davies, B. Fornal, T. Glasmacher, D.E. Groh, M. Honma, M. Horoi, R.V.F. Janssens, S.N. Liddick, D.J. Morrissey, A.C. Morton, W.F. Mueller, T. Otsuka, J. Pavan, J.I . Prisciandaro, H. Schatz, A. Stolz, S.L. Tabor, B.E. Tomlin, and M. Wiedeking Spin Polarization of 37 K A.D. Davies, D.E. Groh, S.N. Liddick, T.J. Mertzimekis, W.F. Rogers, A. Stolz, A.E. Stuchbery, B.E. Tomlin

Decays of Odd-A, N= 33 Ti and Ca The beta decay properties of the odd-A nuclides in this region also suggests migration of the ν f 5/ 2 orbital T 1/ 2 = 90 ± 15 ms 53 Ca n Proposed ν f 5/ 2 → π f 7/ 2 spin-flip decay [1] 53 Sc However, the P n value for 53 Ca is (40 ± 10)% [2] 1. Sorlin et al., Nucl. Phys. A632, 205 (1998) 2. Langevin et al., Phys. Lett. 130B, 251 (1983) Mantica et al. , PRC 68, 044311 (2003).

Magnetic Moments and Mirror Nuclei I sospin, T , is a quantum number that arises from the identical treatment of protons and neutrons due to the charge independence of nuclear forces. The z -component of isospin, T z = ( N – Z )/ 2, is a measure of the neutron–proton asymmetry in the nucleus. I f isospin is a good quantum number ( ) ( ) µ = µ + µ ∑ ∑ i i 0 3 i i J J The summed moments of mirror nuclei, those nuclei that differ simply by exchange of protons and neutrons, can be directly related to the expectation value of the isoscalar magnetic moment. ( ) ( ) ( ) µ = + µ ∑ ∑ J , T , T 2 T 1 i z 0 T i J z

I soscalar Spin Expectation Values: T = 1/ 2,3/ 2 Mirror Partners

Spins of Odd-Odd V and Sc Nuclides Since N = 32 is a good subshell closure for Ca, Ti, and Cr, can examine the angular momentum coupling between proton and neutron spins for odd-odd nuclides in this region. 56 1+ 57 58 1+ ν p 1/ 2 ν p 1/ 2 V ν f 5/ 2 ν f 5/ 2 π (f 7/ 2 ) 3 32 32 ν p 3/ 2 ν p 3/ 2 (5/ 2)- 56 55 57 ν p 1/ 2 Ti ν f 5/ 2 π (f 7/ 2 ) 2 32 ν p 3/ 2 (3,4)+ (1)+ 54 55 56 ν f 5/ 2 ν f 5/ 2 Sc ν p 1/ 2 ν p 1/ 2 π (f 7/ 2 ) 1 32 32 ν p 3/ 2 ν p 3/ 2 33 34 35

Odd-Odd Decay Schemes (3,4) + Janssens, Broda, Mantica et al. , PLB546, 55 (2002) Mantica et al. , PRC 67, 014311 (2003).