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: 4π-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 12B 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

y z x

Beam

k = (k , k , k )

x y z

R = (X, Y , Z)

Θ

Target

Projectile

• k /p

f x

θL θdef

Projectile-rest frame

Target

Θ + Θ − = sin cos R k R k

x y z

l

L P

z /

l =

When Θ = 0

R ky

z

− = l

p p at P = =

When Θ ≠ 0

( )

def L x

p k θ θ − ≅

p p at P = <

Nucleon Pick-up Reactions

Pfaff et al., Phys. Rev. C51, 1348 (1995) Souliotis et al., Phys. Rev. C46, 1383 (1992)

18O (E = 80 MeV/ nucleon)

t PF F

p p p + =

• utgoing

projectile part target nucleon

0.960 0.965 0.970 0.975 0.980 0.985 19O 18N 17C Al Ta

< p/A> F/< p/A> beam

⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

Fermi P P PF F F F

p A p A A A p 1

From momentum conservation, the data to the left are consistent with the nucleon picked up with the Fermi momentum 230 MeV/c

• riented along the direction of

the projectile motion

Spin Polarization via Nucleon Pickup

t PF F

p p p + =

• utgoing

projectile part target nucleon At the peak of the momentum distribution, < pF> = p0, < pPF> = pbeam, and < pt> = pFermi 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

– Bmax = 5000 Gauss

– improved PMT performance at

– optional vacuum chamber

high B fields

37K Spin Polarization

150 MeV/ A 36Ar on Be target Reaction:

36Ar + p → 37K 37K fragments implanted into

a KBr crystal T1/ 2 ( 37K) = 1.23 s Qβ+ EC ( 37K) = 6.1 MeV Polarization monitored by pulsed magnetic field method Maximum polarization

• bserved when separator

tuned just off the peak production of 37K

Potential I mpact

• Expt. 02001: Ground state magnetic moment of 57Cu

T.J. Mertzimekis et al.

Monopole Shift πf7/ 2-νf5/ 2

N= 29 isotones 32 34

20 28 28 40

f7/ 2

protons neutrons

f7/ 2 p3/ 2 f5/ 2 p1/ 2 d3/ 2 s1/ 2

Lowering of 5/ 2- due to strong πf7/ 2-νf5/ 2 monopole interaction

Proton-neutron interaction is strongest when the orbitals they

• ccupy strongly overlap.

This overlap is maximum when ln ~ lp. The attractive nature of the monopole interaction may lead to a re-arrangement

• f the single-particle
• rbitals.

2

~

π π ν ν

π ν π ν

ε ε

j j M j j

v j j V j j

∑

+ =

Prisciandaro et al., PLB 510, 17 (2001)

β-Delayed γ-Ray Spectroscopy

[MeV]

20 28 50 82 126

n A p X 1

• n

A 1 p Y +

β– γ

• Beta decay lifetime of parent
• Decay energy of parent
• Beta decay branching from

parent to daughter

• Low-energy level structure
• f daughter

Focus on the decay of

• dd-odd nuclei, which will

selectively populate low- energy states of even- even daughter

Permits the correlation of fragment implants and subsequent beta decays on an event-by-event basis

NSCL Beta Counting System and Calorimeter

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

• ne central contact.

12 SeGA detectors around the beta counting system during

• Expt. 02004. Efficiency ~ 5%

at 1 MeV

Mueller et al., NI MA 466, 492 (2001)

Systematic Variation of E(2+ )

500 1000 1500 2000 2500 3000 18 20 22 24 26 28 30 32 34

Proton Number Energy (MeV)

E(4+ ) E(2+ )

N= 32 isotones

N= 32 N= 32 N= 34

What about N= 34?

GXPF1 interaction: N= 34 magic structure expected N= 29 isotones 32 34

20 28 28 40

f7/ 2

protons neutrons

f7/ 2 p3/ 2 f5/ 2 p1/ 2 d3/ 2 s1/ 2

Lowering of 5/ 2- due to strong πf7/ 2-νf5/ 2 monopole interaction

Honma et al., PRC 65, 061301 (2002) Prisciandaro et al., PLB 510, 17 (2001)

56Sc Observables

56Sc production rate ~ 3/ minute

Energy (keV) Absolute I ntensity (% ) 592±1 7±2 689±1 19±4 751±1 9±3 1127±1 48±11 1160±1 21±5

Time (ms) Counts

T1/ 2 = 38±5 ms Since total implant rate in DSSD less than 20/ s, expanded correlation to nearest neighbor pixels

56Sc 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( 35K) and

g( 57Cu)

• 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 νf5/ 2 orbital with

filling of πf7/ 2

• Measurement of E(2+ ) in 56Ti 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 37K 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 νf5/ 2 orbital

53Sc

Mantica et al., PRC 68, 044311 (2003).

53Ca

T1/ 2 = 90±15 ms

Proposed νf5/ 2 → πf7/ 2 spin-flip decay [1]

However, the Pn value for

53Ca is (40±10)% [2]

• 1. Sorlin et al., Nucl. Phys. A632, 205 (1998)
• 2. Langevin et al., Phys. Lett. 130B, 251 (1983)

n

Magnetic Moments and Mirror Nuclei

I f isospin is a good quantum number 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.

( ) ( )

J i J i

i i ∑ + ∑ =

3

µ µ µ 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, Tz = (N – Z)/ 2, is a measure of the neutron–proton asymmetry in the nucleus.

( ) ( ) ( )

J i T z

i T T , T , J

z

∑ + = ∑ 1 2 µ µ

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

Spins of Odd-Odd V and Sc Nuclides

Sc Ti V 54 55 56 55 56 57 56 57 58 1+ 1+ (3,4)+ (1)+ π(f7/ 2) 1 π(f7/ 2) 2 π(f7/ 2) 3

32

νf5/ 2 νp3/ 2 νp1/ 2 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.

32

νf5/ 2 νp3/ 2 νp1/ 2

32

νp1/ 2 νp3/ 2 νf5/ 2

32

νp1/ 2 νp3/ 2 νf5/ 2 (5/ 2)-

32

νf5/ 2 νp3/ 2 νp1/ 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).