Using LaBr 3 Gamma-ray Detectors for Precision Lifetime Measurements of Excited States in ‘Interesting’ Nuclei Paddy Regan Department of Physics University of Surrey, 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-ray energy dependence of transition rate, goes as. E  2L+1 e.g., E  5 for E2s for example. Nuclear structure information. The ‘reduced matrix element’, B( L) tells us the overlap between the initial and final nuclear single-particle wavefunctions.

Brighton & Surrey groups purchased ” x 2” LaBr 3 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 LaBr 3 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).

T 1/2 = 1.4ns Tests with 152 Eu source to measure lifetime of I  = keV level in 152 Sm.

137 Cs source gives (initial) test energy 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 138 Ba) 789 keV +  - In 138 Ce Ba x-rays from 137 Cs & EC from 138 La decay

138 La, T 1/2 =1.02x10 11 years A.A.Sonzogni, NDS 98 (2003) La Ba ec (66%) Ce  - (34%)

A ‘high(ish) background’ instrument… J. McIntyre et al., NIM A 652, 1, 2011, Activity: ~0.7counts/sec./cm 3 ~0.1 counts/sec/cm 3 EC β-decay α keV keV MeV

34 P 19

Scientific Motivation for ‘Fast-Timing’ Studies in 34 P 34 P 19 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  f 7/2 → d 3/2 => M2 multipole  f 7/2 → s 1/2 => 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

34 P 19 (Simple) Nuclear Shell Model Configurations 20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  I  = 2 + [  2s 1/2 x ( 1d 3/2 ) -1 ]I  = 4 - [  2s 1/2 x 1f 7/2 ] Theoretical predictions suggest 2 + state based primarily on [  2s 1/2 x ( 1d 3/2 ) -1 ] configuration and 4 - state based primarily on [  2s 1/2 x 1f 7/2 ] configuration. M2 decay can proceed via f 7/2 → d 3/2 (  j=  l=2) transition. 15 protons 19 neutrons

34 P 19 (Simple) Nuclear Shell Model Configurations 20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  I  = 2 + [  2s 1/2 x ( 1d 3/2 ) -1 ]I  = 4 - [  2s 1/2 x 1f 7/2 ] Theoretical predictions suggest 2 + state based primarily on [  2s 1/2 x ( 1d 3/2 ) -1 ] configuration and 4 - state based primarily on [  2s 1/2 x 1f 7/2 ] configuration. M2 decay can go via f 7/2 → d 3/2 (  j=  l=2) transition. 15 protons 19 neutrons

34 P 19 (Simple) Nuclear Shell Model Configurations 20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  I  = 2 + [  2s 1/2 x ( 1d 3/2 ) -1 ] Theoretical predictions suggest 2 + state based primarily on [  2s 1/2 x ( 1d 3/2 ) -1 ] configuration and 4 - state based primarily on [  2s 1/2 x 1f 7/2 ] configuration. M2 decay can go via f 7/2 → d 3/2 (  j=  l=2) transition. M2 s.p. transition

20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  20 1d 5/2 2s 1/2 1d 3/2 1f 7/2  I  = 2 + [  1d 3/2 x ( 2s 1/2 ) -1 ]I  = 4 - [  2s 1/2 x 1f 7/2 ] Theoretical predictions suggest 2 + state based primarily on [  2s 1/2 x ( 1d 3/2 ) -1 ] configuration with some small admixture of [  1d 3/2 x ( 1s 1/2 ) -1 ] 4 - state based primarily on [  2s 1/2 x 1f 7/2 ] configuration. E3 can proceed by f 7/2 → s 1/2 (  j=  l=3 transition). Admixtures in 2 + and 4 - states allow mixed M2/E3 transition. 15 protons 19 neutrons

DESPEC LaBr 3 Detectors ‘Test’ Experiment 18 O( 18 O,pn) 34 P MeV. 34 P cross-section,  ~ 5 – 10 mb Target, 50mg/cm 2 Ta 2 18 O enriched foil 18 O. Beam from Bucharest Tandem (~20pnA). Array 8 HPGe and 7 LaBr 3 (Ce) detectors -3 (2”x2”) cylindrical -2 (1”x1.5”) conical -2 (1.5”x1.5”) cylindrical

4-4-

4-4-

T 1/2 (4 - ) = 2.0(2) ns ; 4 - → 2 + = M2 decay. Consistent with ‘pure’  f 7/2 →  d 3/2 transition. Precision test of nuclear shell model at N=20 {429,1876} 4-4- {429,1048}

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

138 Ce

( h 11/2 ) -2 only N=80 Isotones isomer Primarily (  d 5/2 ) 2 Primarily (  g 7/2 ) 2 N = 80 isotones above Z = 50 display 10 + seniority isomers from coupling of ( h 11/2 ) level decays also usually ‘hindered’ e.g., in 136 Ba,T 1/2 = 3.1(1)ns. Thought to be due to change in configuration and seniority.

N=80 Isotones Neighbouring N=80 nuclei, 138 Ce and 140 Nd 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 138 Ce. What about transition rates?

130 Te( 12 C,4n) 138 MeV

138 Ce 80

S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999) T 1/2 = 140(11)ps Using “delayed” HPGe gate 138 Ce – Lifetime of the 11 + State

188 W

2 neutrons more than heaviest stable Tungsten (Z=74) isotope ( 186 W). Populate 188 W using 186 W( 7 Li,ap) 188 W ‘incomplete fusion’ reaction. (Not really a fusion-evap reaction, but populates medium spin states). See e.g., Dracoulis et al., J. Phys. G23 (1997) N

Half-life of the yrast 2 + state in 188 W 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 190 W 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 188 W 16 mg/cm W target with thick Pb backing 186 W( 7 Li,  p) 188 W. 31-, 33-MeV beam (Coul. Barr. ~ 29 MeV) Estimated   mb Strongest channels: 187 Re (1-p transfer) 189 Ir (fusion-evap)

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

T 1/2 =0.87(12) ns Time difference between 143 keV 2 + → 0 + and feeding transitions.

188 W

Other Recent Uses of Fatima Detectors 21 detectors went RIKEN for use in EURICA array from Nov (see talk by GL). 8 detectors used with from Feb. – March 2013 for use in 235 U(n,f) experimental campaign.

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

140 keV 2 + → 0 + in 104 Zr Demonstrates that beta-gamma time differences can be used in projectile fragmentation/fission spectroscopy to measure, e.g., 2+ lifetimes in many, even-even, exotic nuclei, down to ~100 ps?

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

EXILL + FATIMA EXOGAM + FATIMA LaBr 3 array at ILL Grenoble Ge-Ge-LaBr 3 -LaBr 3 quadruple coincs between prompt gammas from fission fragments in 235 U(n,f) reaction using thermal neutrons.. Massive data set, under analysis (led by Koln group)

Possible array configuration at focal plane of RITU spectrometer at JYFL

Characterized LaBr 3 detectors for fast-timing measurements in the 100 ps to few ns range. 34 P M2 strength measured approaching the island of inversion for N~ W show reduction in ground state collectivity compared to lighter W isotopes. Other measurements using these detectors at EXILL+FATIMA (2013) ; EURICA (2013); (~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.