Using LaBr3 Gamma-ray Detectors for Precision Lifetime Measurements of Excited States in ‘Interesting’ Nuclei

Paddy ReganDepartment of PhysicsUniversity of Surrey, UKp.regan@surrey.ac.uk

How is measuring the lifetime

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 single-particle wavefunctions.

Brighton & Surrey groups purchased 31 1.5” x 2” LaBr3 detectors from St Gobain (Dec. 2012). Mounted into designed holders with Hamamatsu PMTs Jan 2013.

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).

• (2012) Bought 31 x LaBr3 1.5” x 2” crystals for array (expect 36 in total).

• 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 (runs to 2015).

T1/2 = 1.4ns

Tests with 152Eu source to measure lifetime of I=2+ 122 keV level in 152Sm.

137Cs source gives (initial) testenergy resolution of ~3.5% at 662 keV.Note presence of internal radioactivity in detector.PMT HV range ~1300 V

1436 keV EC(2+→ 0+ in 138Ba)789 keV + -

In 138Ce

Ba x-rays from 137Cs & EC from 138La decay

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%)

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

34P19

Scientific Motivation for ‘Fast-Timing’ Studies 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 multipole gamma-rays.

•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

20

1d5/2

2s1/2

1d3/2

1f7/2

20

1d5/2

2s1/2

1d3/2

1f7/2

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

•Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration with some small admixture of [1d3/2 x (1s1/2)-1]

•4- state based primarily on [2s1/2 x 1f7/2] configuration.

•E3 can proceed by f7/2 → s1/2 (j=l=3 transition).

•Admixtures in 2+ and 4- states allow mixed M2/E3 transition.

15 protons 19 neutrons 15 protons 19 neutrons

DESPEC LaBr3 Detectors ‘Test’ Experiment

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-

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}

P.J.Mason et al., Phys. Rev C85, 064303 (2012)

138Ce

(h11/2)-2 only

N=80 Isotones

0+

2+

4+

6+

8+

10+isomer

Primarily(d5/2)2

Primarily(g7/2)2

•N = 80 isotones above Z = 50 display 10+ seniority isomers from coupling of (h11/2)-2

•6+ level decays also usually ‘hindered’ e.g., in 136Ba, T1/2 = 3.1(1)ns.

•Thought to be due to change in configuration and seniority.

N=80 Isotones•Neighbouring N=80 nuclei, 138Ce and 140Nd expected to show

similar 6+ → 4+ hindrance.

•Competing transitions to negative parity states •E1 decays, forbidden in truncated shell model space.

Restricted basis SM calculations give reasonable comparison with experimental (near-yrast) states in 138Ce.

What about transition rates?

130Te(12C,4n)138Ce @56 MeV

138Ce80

138Ce80

S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

T1/2 = 140(11)ps

Using “delayed” HPGe gate

138Ce – Lifetime of the 11+ State

188W

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

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

(Not really a fusion-evapreaction, but populates medium spin states).

See e.g., Dracoulis et al., J. Phys. G23(1997) 1191-1202

110 111 112 113 114 115 N

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. PLB190 (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

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

•16 mg/cm2 186W target with thick Pb backing •186W(7Li,p)188W. 31-, 33-MeV beam (Coul. Barr. ~ 29 MeV)

•Estimated 0.1-1 mb•Strongest channels:187Re (1-p transfer)189Ir (fusion-evap)

•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.

188W 188W

188W

Other Recent Uses of Fatima Detectors

• 21 detectors went RIKEN for use in EURICA array from Nov. 2012 (see talk by GL).

• 8 detectors used with EXOGAM@ILL from Feb. – March 2013 for use in 235U(n,f) experimental campaign.

F.Browne, H. Watanabe, A. Bruce, T. Sumikama et al.,

140 keV 2+ →0+ in 104Zr

Demonstrates that beta-gamma time differences can be used in projectile fragmentation/fissionspectroscopy to measure, e.g., 2+ lifetimes in many, even-even, exotic nuclei, down to ~100 ps?

Compares with T1/2=2.0(1) ns measurement by Hwang et al., Phys. Rev. C73, 044316 (2006)

EXILL + FATIMAEXOGAM

+ FATIMA LaBr3 array at ILL Grenoble

Ge-Ge-LaBr3-LaBr3

quadruple coincs between prompt gammas from fissionfragments in 235U(n,f) reaction usingthermal neutrons.. Massive data set, underanalysis (led by Koln group)

Possible array configuration at focal plane of RITU spectrometer at JYFL

• Characterized LaBr3 detectors for fast-timing measurements in the 100 ps to few ns range.

• 34P M2 strength measured approaching the island of inversion for N~20.

• 188W show reduction in ground state collectivity compared to lighter W isotopes.

• Other measurements using these detectors at EXILL+FATIMA (2013) ; EURICA (2013); DESPEC-FATIMA@FAIR (~2017) ; decay spect at RITU (from Mid 2014).

Acknowledgements

• Zsolt Podolyák, Peter Mason, Thamer Alharbi, Christopher Townsley (Surrey)

• Alison Bruce, Oliver Roberts, Frank Browne (Brighton)

• Nicu Marginean et al., (Bucharest)

• Funding for detectors and DAQ from STFC UK.