Polarized 11Li beam at TRIUMF and its application for spectroscopic study of the daughter nucleus 11Be

1. Physics motivation new delayed decay spectroscopy using polarized nucleus

2. Polarizer for RNB at TRIUMF ISAC polarization by collinear optical pumping special cares to achieve high polarization 8Li: 80%, 9Li: 56%, 11LI: 55%,

20Na: 57%, 21Na: 56%, 26Na: 55%, 27Na: 51%, 28Na: 45%

3. Experimental results with polarized 11Li beam 　　　　 spin-parity assignments of levels in 11Be

4. Summary

T. Shimoda1, Y. Hirayama1,2, H. Izumi1, H. Hatakeyama3, K.P. Jackson4, C.D.P. Levy4, H. Miyatake2, M. Yagi1, H. Yano1

1: Osaka Univ., 2: KEK. 3: Univ. of Tokyo, 4:TRIUMF

CONTENTS

β-delayed decay spectroscopy

polarized

assignment

a new method of β-delayed decay spectroscopy

β-decay from a spin polarized nucleus

-decay angular distribution

A takes very different values depending on the final state spin.

A: asymmetry parameter of allowed -decayP: polarization of the parent nucleus

～ 0

11Li → 11Be

Measurement of β-decay asymmetry

R-detectorL-detector

polarization

: β-ray counts: β-ray counts

when spin orientation is reversed

free frominstrumentalasymmetry

Excited States of 11Be

F. Ajzenberg-Selove, Nucl. Phys. A506 (1990) 1.

■ only a few spin-parity assignments

prevents comparison between experiment and theory level by level

■ low level density at high energy region

11Be

10Ben

S1n=504 keV

T1/2 = 8.5 ms

Most of the states decay by neutron emission.

Isotope Separator / ACcelerator

radioactive nuclear beams produced in target fragmentation induced by a 500 MeV 100A proton beam

TRIUMF ISAC

commissioned in Aug. 2001

Alkali RI beam from ISOL A+1 beam at 10 – 60 keV

neutralizer charge exchange in a Na vapor jet

A+1 + Na → A0 + Na+1 : 90% efficiency

optical pumping for fast neutral beam in collinear geometry two laser beams to pump the two ground-state hyperfine levels longitudinal polarization

re-ionizer collision with a cold He gas target (12K) A0 → A+1 : 66% efficiency

transversely nuclear-polarized ion beam

bend

1.9 m

Shimoda11Li decay spectroscopy

MinamisonoANa moments,-decay symmetry

Kiefl 8Li -NMRcondensed matter physics

neutralizerre-ionizer

unpolarized 11Li+1

30.48 keV

polarized 11Li+1

B→10Gauss

beam velocity tuning

C.D.P. Levy et al.Nucl. Instr. and Meth.B204 (2003) 689

TRIUMF ISACPolarized Beam Line

pumping within 2.6s

D1673 nm

905 MHz

laser freq.

pumping the two ground-state hyperfine levels in order to achieve high polarization

Electro-Optic Modulator (EOM)

Only 1/3 laser power is used for each optical pumping.

driven at the hyperfine splitting frequency

>> laser line width ～ 1 MHz

energy (Doppler) broadening of the neutralized beam

multiple collisions with Na atoms in the neutralizer

6.3 eV

EOM-219 MHz

EOM-328 MHz

two EOMs in series

broadening the laser line width

laser beam

8Li

P ～ 20%

P ～ 70%

AP (coin. with 0.32 MeV ) = － 0.43 ± 0.005

known

A = -1

Polarization measurement

= 0.80 ( 0 opening angle of the -detector)

= 0.98 (= 12.3 ms, T1 = 570 ms in Pt

-1.0 (3/2- → 1/2- )

11Li: P = 0.55 ± 0.007

8Li

simulation by rate equation

E903 at TRIUMF

Li-glass scintillator: Δn = 0.92% x 2, n =2.1%@15 keV, n = 1.3%@80 keV En 1 keV≧ Flight Length: 130 mm 　　　　　　　　　　　　　　　　　　　　　　

Ge detector: HPGe, 50 and 60 %, Δ= 3.2x10 @3 MeV

plastic scintillator: Δn = 1.8% x 6, n = 19%@2 MeV, En ≧500 keV Flight Length: 1.5 m

-ray telescope: Δ = 14.7% x 2, 　 = 90%

-3

- n, - n-- , coincidence

30.48 keV

En = 1 keV – 9 MeV

Detector Setup Detector Setup 11Ligs

11Be*+10Be* + n

10Begs +

Y. Hirayama et al., Phys. Lett. B611 (2005) 239

neutron TOF spectrum and coincident -decay asymmetry

high energy neutrons

11Ligs 11Be*+

10Be* + n

New Level and Decay Schemes of 11Be New Level and Decay Schemes of 11Be

Ex, I

log ft

spectroscopic factor

11Li → 11Be

11Be→ 10Be + n

determined

■

■

■

decay path■

F. Ajzenberg-Selove, Nucl. Phys. A506 (1990) 1.

Y. Hirayama et al., Phys. Lett. B611 (2005) 239

Summary

● Highly polarized (50 – 80%) radioactive nuclear beams of alkali ions (Li, Na) have been successfully produced at the collinear optical pumping system of TRIUMF ISAC.

The success was due to (i) pumping of the two ground-state hyperfine levels and (ii) matching of the laser line width to the Doppler broadened absorption line of the beam.

● The highly polarized beam is a very powerful tool to explore the excited states of unstable nuclei by applying the new method of -delayed decay spectroscopy.

The -decay asymmetry parameter is useful (i) to assign the peaks of the decaying particles and (ii) to assign the spin-parity of the daughter states.

Ring dye laserCoherent 899-21Dye: DCM SPECIAL/LC 6501

9W

673 nm cwcircular polarizedfor 11Li

frequency referenceto actively stabilizethe ring dye laser

8mm 12mmLaser system

8Li: 80%, 9Li: 56%, 11LI: 55%, 20Na: 57%, 21Na: 56%, 26Na: 55%, 27Na: 51%, 28Na: 45%

Achieved polarization Achieved polarization

Pumping for 11Be+ beam is in progress.

Doppler-shift tuning

deceleration bias (Na vapor cell) tuning to adjust ion beam velocity so as to meet the Doppler shift

absorption line

scanning velocity

11Li

D1 transition frequency/wave number for Li atoms at rest

14903.30 cm-1 33.6 GHz

14904.41 cm-1

Doppler shift

1486.17 cm-1

30.48 keV 11Li

Na vapor cell bias tuning

1 eV → 17.8 MHz

R.E. Azuma et al., Phys. Rev. Lett. 43(1979)1652

3He ionization chamber

thermal neutron

Low energy neutrons

spurious due to resonance in detector

16 keV72 keV

high energy neutrons

6Li-doped scintillators

-decay asymmetry En = 72±5 keV = 25±15 keV= 3/2-

-detector = L+, L-, R+, R-laser helicity

En = 16±1 keV = 9±3 keV= 5/2-

Low energy neutrons

12±2.6 %

5.1±1.5 %

AP (peak B) = － 0.377 ± 0.009(solid angle) =0.80(spin relaxation) =0.98

P = +0.48 ± 0.017

coincidence

Doppler broadening due to neutron recoil 100±50 fs

β-decay

n-decay

γ-decay

γ-rays γ-rays

mixed but one is dominant

n coincidence

hidden peaks !

spin-parity assignment

High Energy NeutronsHigh Energy Neutrons

β-n-γ-coincidenceβ-n-γ-coincidenceexistence previouslyclaimed by Aoi et al.

neutron energy

neutron detection efficiency curvelevel width

→ neutron peak profiles

✓

check the calculatedneutron peak profiles

Γ=216±55 keV

Γ=243±55 keV

β-n-γcoincidence

5

7

✓

reproduction of spectra based on observed decays and assigned spins-parities

reproduction of spectra based on observed decays and assigned spins-parities

✓ detector time resolution✓

spin-parity✓ → asymmetry spectrum

further included neutron decays

from known levelsfrom assumed levels

reproduction of spectra by including unknown neutron decays from known levels and assumed levels

reproduction of spectra by including unknown neutron decays from known levels and assumed levels

New Level and Decay Schemes of 11Be New Level and Decay Schemes of 11Be

Ex, I

log ft

spectroscopic factor

11Li → 11Be

11Be→10Be + n

determined

■

■

■

from known levels

from assumed levelspreviously unknown transitions

neutron spectroscopic factorneutron spectroscopic factor

11Be → 10Be + n

neutron penetrability

: partial decay width

channel radius

l–th order Hankel function of the first kind

overlapping between 11Be and 10Be

K = 1/2 －

K = 3/2 －

Anti-symmetrizedMolecular Dynamics

Anti-symmetrizedMolecular Dynamics

K =1/2+

Y. Kanada-En’yo and H. Horiuchi, Phys. ReV. C66 (2002) 024305

Y. Kanada-En’yo, H. Horiuchi, A. Dote, Nucl. Phys. A687 (2001) 146c

ab-initio fully microscopic theory

without any model assumptions such as mean field, clustering, ・・・

K = 1/2 －

K = 3/2 －

K =1/2+

■ 2α-cluster states (α+α+3n) rotational bands

■ single-cluster state (α+5n+2p)

single cluster state

3n: p-shell (0hω)

2n: sd-shell (2hω)

1n: sd-shell (1hω)

log-ft

Spectroscopic FactorSpectroscopic Factor assumed lowest possible LTransitions with the largest spectroscopic factor are shown.

?

?

new cluster states?