Properties of isomers measured by high resolution laser spectroscopy

Jon BillowesBirmingham – Manchester – Jyväskylä

laser/IGISOL collaboration

1. Nuclear charge radii –measurement and interpretation

2. The 8- isomers in 176Yb, 178Hf

3. Isomers and ground states of yttrium

4. Measurements by laser resonance ionization

Properties of isomers measured by high resolution laser spectroscopy

Isotope Shift and Hyperfine Structure

Isotope shift of atomic transition

176Hf 178Hf

Analysis yields the change in nuclear mean square charge radius

Nuclear size, static and dynamic deformations

Hyperfine structure of atomic transition

177Hf

Nuclear spin I

Magnetic moment µ

Quadrupole moment Qs

ppm shift

(Isotope shift found using centroids of hyperfine multiplet)

+40 kV

Ion beam cooler

Light collection region(Laser resonance fluorescence)

Reduces energy-spread of ion beam (<1 eV)

Trap and accumulates ions – typically for 300 ms

Releases ions in a 15 µs bunch

The laser/IGISOL facility at Jyväskylä

Sensitivity gains using the RFQ ion-cooler

8000 ions/sec

5.3 hours

2000 ions/sec

48 minutes

Photons from laser-excitation of radioactive 88ZrLaser frequency

200

100

30

0

BEFORE

AFTER

(Photon-ion coincidence method)

(bunched beam method)

Isotope Shift

Isotope shift of atomic transition

176Hf 178Hf

Analysis yields the change in nuclear mean square charge radius

Nuclear size, static and dynamic deformations

ppm shift

Transition energy difference between isotopes A and A’

+ mass shift (zero for isomers)

Deformed nucleus Spherical nucleus

Mean square charge radii

δ<r2 >

(fm

2 )Volume

Deformation

Pairing effects?

Odd-even staggering

Radii of multi-quasiparticle

isomers178Hf 16+

The K=8 isomers in 178Hf and 176Yb

Structure of 2-neutron 8- state in 176Yb

Structure of 2-qp 8- state in 178Hf is

60%(2ν)+40%(2π)

The 176Yb 8- isomer

Production: (d,pn) at 13 MeV, 5.5 µAFlux: 200 isomers/sec (total flux at A=176: 8,400 ions/sec)

176, 176mYb laser resonance spectrum

Yb+

λ= 328.9 nm

δ<r2>176,176mYb fm2

-0.0224(1)

The 178Hf 8- isomer

λ= 301.3 nm

Hf+

Production: 176Yb ( α, 2n) at 25 MeV, 5μA

Isomer beam: 3,000 per second

8- δ<r2>178,178m1 = -0.0384(1) fm2

16+ δ<r2>178,178m2 = -0.0873(20)fm2 (Boos et al.)

16+

8-

23/2-

Negative isomer shifts in multi-quasiparticle isomers

8-

Calculation by Bordeaux Group (Pilletet al, Nucl. Phys. A 697 (2002) 141.)

Without pairing effects, Isomer shift = +0.051 fm2

Including pairing Isomer shift = -0.033 fm2

(Experiment: -0.087(2) fm2)<r2> for proton states

36.097 fm2

28.888 fm2

84 86 88 90 92 94 96 98 100 102 104A Zr

18.4

18.8

19.2

19.6

20.0

20.4

20.8

fm2

<β2>=0.4

<β2>=0.2

<β2>=0.3

<β2>=0.1

<β2>=0.0

Charge radii of Zr isotopes

40Zr

mea

n sq

uare

cha

rge

radi

us

Deformed nucleus

Spherical nucleus

Spherical droplet modelMyers & Schmidt, Nucl. Phys. A410 (1983) 61.

3. Isomers and ground states of yttrium

b2b2

Qs=0

Problem: charge radius increase inconsistent with deformation derived from B(E2; 0+ →2+)

40Zr

Lack of Qs data to investigate problem further…. but wealth of ground states and isomers with I > ½ in neighbouring 39Y chain

I=0

I≠0 Prolate

J=0

J=1

Yttrium atomic structure

Hyperfine structure

I=0

Example of spectra for neutron-rich yttrium isotopes

(production by proton-induced fission of uranium)

Shift to lower laser frequency indicates increase

in the nuclear rms radius

Measured moment Qs is the projection of Q0 on quantization axis:

β 2=0.0

β 2=0.1

β 2=0.2

β 2=0.3

β 2=0.4

β 2=0.5

17.8

18.2

18.6

19.0

19.4

19.8

20.2

84 86 88 90 92 94 96 98 100 102 104Mass Number, A

<r2 >A /

fm2

Stable IsotopesRadioactivesIsomers

Yttrium charge radii Yttrium charge radii

N

Spherical

I=1

I=3

I=2

I=1

I=3

I=2

100Y

102Y

Mean square charge radius

(fm2)

Mean square radius from isotope shift data

Mean square radius predicted from spherical droplet model corrected for deformation

deduced from measured quadrupole moment

Neutron number

Spherical

Isotope shift data (ground states)

Droplet model + deformation from Qs

Isotope shift data (isomers)

Mean square charge radius

(fm2)

Charge radii of yttrium isotopes

p1/2

g9/2

Ground state and two isomers in 97Y

I=27/2

(The smaller nucleus is shifted to higher frequency)

9/2+ (πg9/2) Q0=-1.40(15) b δ<r2>97,97m1 = +0.122 fm2

27/2- (πg9/2)(νg7/2)(νh11/2) Q0 = -1.50(17) bδ<r2>97m1,97m2 = -0.098(1) fm2

Results for 97Y isomers

-Same effect in an oblate multi-quasiparticle state – the 3qp isomer is smaller than the 1qp isomer.

-Other isomers with same feature: 130Ba(8-), 202Pb(9-), 135Cs(19/2-)

-Appears to be a common feature for multi-quasiparticle states

The collinear-beams resonance ionization method5 µs 10 ns

50 Hz repetition rate lasers

Atom bunch

50 Hz delivery rate, synchronized with

laser pulse

All atoms from the ion source have a chance to be ionized

Resonance located by ion counting (not photon counting)

Doppler-broadening free

Ionization limit

308 nm

532 nm

Off-line tests with

27Al

4. Measurements by laser resonance ionization

Comparison with low-flux bunched beams

1.5 GHz

Photon counting (12 minutes)

Ion counting (4 minutes)

Sensitivity: 1 resonance ion per 30 atoms

within 1 µs time window

(compared with 1 photon per 50,000 atoms)

Ti:Sapphire lasers

Nd:YAG100W, 10 kHz

Pulsed dye lasersCopper vapour laser 45W, 10kHz

cw dye laser

cw pump laser

Isotope separatorIon guide

Fast Universal Resonant laser IOn Source FURIOS

Fast Universal Resonant laser IOn Source FURIOS

In-source laser spectroscopy (Doppler broadened)

LIST: Laser ion source trapMethod being developed by FURIOS collaboration at JYFL

Applications:

* Production of high isobaric purity isotope or isomer beams

* In-source laser spectroscopy

IGISOL

Pulsed laser beams

Manchester: J Billowes, P Campbell, A Nieminen, B Cheal, K Flanagan, M Avgoulea, B A Marsh, B W Tordoff

Jyväskylä: J Äystö, H. Penttilä, A Jokinen, J Huikari, S Rinta-Antila

Birmingham: G Tungate, D H Forest, M D Gardner, M Bissell

AcknowledgementsMatthew Gardner – Yttrium analysis

Bradley Cheal – Yttrium analysis

Mark Bissell – 178Hf(8-) analysis

Kieran Flanagan – 176Yb(8-) analysis

Present work: Tantalum gs & isomers

9- isomer (>1.2 x 1015 y)

180Ta

9-

1+ 0 (8h)75 keV

J=1

J=2J=1

264 GHz

Ta+ ion

296.

5nm

7/2+ ground state