Precision Sub-Nanosecond Lifetime Measurements

of Excited States for Some 'Interesting' Nuclei

Paddy Regan

Department of PhysicsUniversity of Surrey,

& National Physical Laboratory,

Teddington, UKp.regan@surrey.ac.uk ; paddy.regan@npl.co.uk

Some Nuclear Structure ‘Big’ Science Questions?

– How do protons and neutrons interact with each other?• Can we write down a nuclear ‘force’ equation or approximation?

– Evolution of nuclear single-particle structure in nuclei.• Why do nuclear excitations change from ‘single particle’ to

‘collective’ ?

– Why do some nuclei exhibit significant deformation from sphericity?

• How do we measure and identify nuclear ‘deformation’ ?

Excitation energy (keV)

Ground stateConfiguration.Spin/parity I=0+ ;Ex = 0 keV

2+

0+

PHR, Physics World, Nov. 2011, p37

Some nuclear observables?

1) Masses and energy differences2) Energy levels3) Level spins and parities4) EM transition rates between states5) Magnetic properties (g-factors)6) Electric quadrupole moments?

Essence of nuclear structure physics……..

How do these change as functionsof N, Z, I, Ex ?

What are the most useful ‘signatures’ of nuclearstructural evolution?

Why measure gamma rays?

• Characteristic, EM ‘fingerprint’ energies associated with de-excitations from excited nuclear states.

• Can be used to identify the isotope and amount of given radionuclides present.

• Can also give direct insight into internal nuclear structure by ordering of excited energy levels and rates of their decay.

How is measuring excited state

lifetimes useful?

Transition probability (i.e., 1/mean lifetime as measured for state which decays by EM radiation)

(trivial) gamma-rayenergy dependence oftransition rate, goes as. E

2L+1 e.g., E5 for E2s

for example.

Nuclear structure information. The ‘reduced matrix element’ , B(L) tells us the overlapbetween the initial and final nuclear wavefunctions…..This contains the ‘Physics’.

Weisskopf ‘single particle’ estimates for lifetimes of excited nuclear states. Based on EM model transitions for protons in spherical orbits.

B(E2: 0+1 2+

1) 2+1 E20+

12

2+

0+

In the nuclear totational model, B(E2: I→I-2) gives Qo by:

Qo = INTRINSIC (TRANSITION) ELECTRIC QUADRUPOLE MOMENT.

This is intimately linked to the electrical charge (i.e. proton) distribution within the nucleus.

Non-zero Qo means some deviation from spherical symmetry and thus somequadrupole ‘deformation’.

T (E2) = transition probability = 1/ (secs); E = transition energy in MeV

Idea ?

Use LaBr3 (halide scintillation) detectors in coincidence to measure bothdiscrete gamma-ray energies from decays from excited nuclear states

and

use the (measured) time difference between successive members of a gamma decay cascade to give direct measurement of the lifetimes of (i.e. transition rates from) the intermediate states.

LaBr3(Ce) has good timing properties:

Timing Resolution FWHM of 130-150 ps with 60Co for a Ø1”x1” crystal.

Acceptable energy resolution ~ 3% FWHM at 662 keV.

Peak emission wavelength in blue/UV part of EM spectrum (380 nm), compatible with PMTs

Why LaBr3(Ce)

Expected, E1/2 dependence of FWHM on gamma-ray energy.

138La, T1/2=1.02x1011 yearsA.A.Sonzogni, NDS 98 (2003) 515

5+ 138La

1435.8138Ba82

2+

0+

ec (66%)

0+

2+

138Ce80

788.7

- (34%)

Natural abundanceof Lanthanum (Z=57):

99.91% 139La (stable) 0.09% 138La

138La (primordial NORM) gives rise to internal background activity within these detectors.

La chemical separation also can induce some (chemically similar) Ac into the detector material….specifically 227Ac (22 year half-life).(see talk later by Sean Collins on 223Ra decay).

A ‘high(ish) background’ instrument…

J. McIntyre et al., NIM A 652, 1, 2011, 201-204

Activity: ~0.7counts/sec./cm3 ~0.1 counts/sec/cm3

EC

β-decay

α

0-255 keV788-1000 keV 1.5-3 MeV

The ROSPHERE Gamma-ray Spectrometer array (at IFIN-HH Bucharest)

• 14 HPGe detectors (AC) are used to detect coincident γ rays:– 7x HPGe dets. @ 37o

– 4x HPGe dets. @ 64o

– 3x HPGe dets. @ 90o

• 11 LaBr3(Ce:5%) detectors– 7x ø2”x2” and 4x ø1.5”x2”

(Cylindrical) @ 37, 64 and 90o w.r.t. the beam axis.

Some physics examples…

Description of Doubly-Magic +1 Nuclei …….e.g. 209Bi

Assume inert (double magic) core and single, unpaired particle

Description of Doubly-Magic +1 Nuclei

Assume inert core and single, unpaired particle

Description of Doubly-Magic +1 Nuclei

Assume inert core and single, unpaired particle

Description of Doubly-Magic +1 Nuclei

Assume inert core and single, unpaired particle

Description of Doubly-Magic +1 Nuclei

Assume inert core and single, unpaired particle

Lifetimes in 209Bi

209Bi=208Pb+p

3-

0+h9/2

h9/2

The g.s of 209Bi

The 1609 keV, 13/2+ level in 209Bi can be a mixture of the ground state (0+ ) in 208Pb coupled to the i13/2 single valence proton and the 3- octupole vibration coupled to the h9/2 single proton ground state proton configuration.

Z= 82, N=126

O.J.Roberts, A.M.Bruce et al.,

Could also show some E3 vibrational mode?

2/92/13 1310 hi

2/910 h

M2

2/92/13 1310 hi

E3

209Bi

• 209Bi was formed in the proton transfer reaction channel at IFIN-HH Bucharest.

208Pb(7Li,α2nγ,)209Bi

• Target D 20 mg/cm2

• Beam D 4.5 pnA.

HPGe

LaBr3(Ce)

Analysis

Use discrete HpGe gated energies to create symmetrised 2-D and 3-D coincidence arrays such as LaBr3(E)-LaBr3(E)-T. Gate on gamma coincs to get T values.

• HPGe gates: To select the cascade• LaBr3(Ce:5%) gates: Above and below the

level of interest.

Analysis

STOPSTART

HPGe

LaBr3(Ce)

O.J.Roberts, A.M.Bruce et al.,

Analysis

Analysis

992, 1609

1133, 1609

1609, 1133

1609, 992Select gates and background

for subtraction.

Create E1-E2-T cube. Project out on T axis.

Background gate

O.J.Roberts, A.M.Bruce et al.,

2τ Half-life of the 13/2+ state in 209Bi established:

T1/2 = 110 (10) ps

34P19

‘Fast-Timing’ in 34P• 34P19 has I=4- state at E=2305

keV.

•Aim to measure a precision lifetime for 2305 keV state.

WHY?• A I=4-→ 2+ EM transition is allowed

to proceed by M2 or E3 multipolarity.

•M2 and E3 decays can proceed by f7/2 → d3/2 => M2 multipole f7/2 → s1/2 => E3 multipole

• Lifetime and mixing ratio information gives direct values of M2 and E3 transition strength

• Direct test of shell model wfs…

.’’’

Z=15 = N=19

34P19 (Simple) Nuclear Shell Model Configurations

20

1d5/2

2s1/2

1d3/2

1f7/2

20

1d5/2

2s1/2

1d3/2

1f7/2

I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2]

•Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration.

•M2 decay can proceed via f7/2 → d3/2 (j=l=2) transition.

15 protons 19 neutrons 15 protons 19 neutrons

34P19 (Simple) Nuclear Shell Model Configurations

20

1d5/2

2s1/2

1d3/2

1f7/2

20

1d5/2

2s1/2

1d3/2

1f7/2

I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2]

•Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration.

•M2 decay can go via f7/2 → d3/2 (j=l=2) transition.

15 protons 19 neutrons 15 protons 19 neutrons

34P19 (Simple) Nuclear Shell Model Configurations

20

1d5/2

2s1/2

1d3/2

1f7/2

I = 2+ [2s1/2 x (1d3/2)-1]

•Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration.

•M2 decay can go via f7/2 → d3/2 (j=l=2) transition.

M2 s.p. transition

Experiment details

18O(18O,pn)34P fusion-evaporation @36 MeV. 34P cross-section, ~ 5 – 10 mb

Target, 50mg/cm2 Ta218O enriched foil

18O. Beam from Bucharest Tandem (~20pnA).

Array 8 HPGe and 7 LaBr3(Ce) detectors

-3 (2”x2”) cylindrical-2 (1”x1.5”) conical-2 (1.5”x1.5”) cylindrical

4-

T1/2(4-) = 2.0(2) ns ; 4- → 2+ = M2 decay.Consistent with ‘pure’ f7/2 → d3/2 transition. Precision test of nuclear shell model at N=20

{429,1876} 4-

{429,1048}

188WMeasuring nuclear

(quadrupole) deformation.

Half-life of the yrast 2+ state in 188W

•Neutron-rich A ~ 190 nuclei, a long predicted prolate – oblate shape transition region. e.g. Bengtsson et al. Phys. Lett. B190 (1987) 1

•Unusual (energy) deviation at 190W compared to trend of other nuclides.

•Measurement of B(E2;2+ →0+) gives best measure of (evolution of) low-lying collectivity

2 neutrons more than heaviest stable Tungsten(Z=74) isotope (186W).

Populate 188W using 186W(7Li,ap)188W ‘incomplete fusion’ reaction.

110 111 112 113 114 115 N

•Sum of time differences between 143-keV (2+ → 0+) transition and any higher lying feeding transition.

T1/2 =0.87(12)

ns

Time difference between 143 keV2+→0+ and feeding transitions.

B(E2) gives value fordeformation for 188W of 2=0.18(1)

PES calculations based on a deformed Woods-Saxonpotential predict 2=0.19.

fragmentation/fission

~1GeV/u

fragmentseparator

350m

Facility for Antiproton and Ion Research (FAIR)

NUSTAR: SuperFRS and experiments on three (energy) branches…. > 800 collaborators

Low-energy branch / DESPEC

FATIMA for DESPEC

• FATIMA = FAst TIMing Array = A high efficiency, gamma-ray detection array for precision measurements of nuclear structure in the most exotic and rare nuclei.

• Specs.– Good energy resolution.– Good detection efficiency– Excellent timing qualities (~100 picoseconds).

• Purchased 31 x LaBr3 1.5” x 2” crystals for array (expect 36 in total).• Fully digital, time stamped DAQ in final array.

• Can use to measure lifetimes of excited nuclear states; provide precision tests of shell model theories of nuclear structure.

• UK contribution to DESPEC (Decay Spectroscopy) project within NUSTAR.

• Part of ~ £8M UK STFC NUSTAR project grant.

‘DESPEC’ Fast-Timing Array Design.

• GEANT4 Simulations for spectroscopy grade (small) 1.5”x2” LaBr3 detector crystals in compact geometry as part of the DESPEC project at FAIR.

DESPEC @ FAIR

Picture from 5. Apr 2013 (http://www.fair-center.de)

Gamma Spectroscopy:HISPEC / DESPEC