New decay and isomer physics studies with fast scintillation detection arrays at Coulomb Barrier separator focal planes Paddy Regan Department of Physics University of Surrey & National Physical Laboratory, 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.

Annual Review of Nuclear Science (1968) 18 p

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).

Brighton & Surrey groups purchased ” x 2” LaBr 3 detectors from St Gobain (Dec. 2012). Mounted into designed holders with Hamamatsu PMTs Jan 2013.

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

PMT Digitizer CFD Trigger Anode Dynode 4 GS/s (250 ps sampling interval) 10 bit resolution, 1 Volt input dynamic range, 4 GHz Bandwidth To PC for offline analysis Source Digital CFD for timing CR-RC, Trapezoidal filters for energy Acqiris digitizer model DC252HF. Tests by Mohammad Nakhostin (U. Surrey)

Sample energy spectrum, 137Cs

Time resolution vs pulse sampling rate Energy gate: keV Still preliminary! Analog result

Digital time spectra for two different energy gates The width and peak location of time spectra move with energy

Energy resolution vs sampling rate

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%)

An experimental example: 138 Ce 80

( 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

T.Alharbi et al., Phys. Rev. C87 (2013) 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 poster by Frank Browne). 8 detectors used with from Feb. – March 2013 for use in 235 U(n,f) experimental campaign.

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?

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)

Delayed Gammas Isomeric state PROMPT gammas Isomeric state Tagging Methods at Jyväskylä Trigger-less Data Acquisition System JUROGAM, RITU, GREAT and the TDR. D.M. Cullen, Physics with Large Arrays of Novel Scintillators, Dublin, Jan 2014

Delayed γ rays but in prompt coincidence with each other (ps lifetimes) Measure LaBr 3 – LaBr 3 coincidences with ~200ps intrinsic LaBr 3 timing resolution. Extract lifetimes down to ~5ps (centroid shift). Isomeric state (~100ns – several µs) ~ 100ps ~ 50ps ~ 5ps D.M. Cullen, Physics with Large Arrays of Novel Scintillators, Dublin, Jan 2014 Ground state Technique

Possible array configuration at focal plane of RITU spectrometer at JYFL

An experimental challenge….

1.N=74 K-Isomers e.g. 138 Gd / 140 Dy? to address the deformation of ground-state bands approaching the proton drip-line. Deformations used in hindrance calculations and also proton decay tunnelling calculations but not yet measured. 2.Deformation of states in neutron deficient Pb ( ) under known isomers (11 - isomers and K isomers). 3.States below the 14 + isomer in 94 Pd giving information on n-p pairing Some potential Physics Experiments at RITU: D.M. Cullen, Physics with Large Arrays of Novel Scintillators, Dublin, Jan Minimum half-life possible is ~ few 100 ns (flight time through RITU, depends on v/c) 2.Maximum half-life possible is ~ few ms (depending on tagging method and implant rate (beam current)).

Acknowledgements Zsolt Podolyák, Peter Mason, Thamer Alharbi, Christopher Townsley, Daniel Terver, Laila Gurgi, Mohammad Nakhostin (Surrey) Alison Bruce, Oliver Roberts, Frank Browne (Brighton) Dave Cullen et al. (Manchester) Nicu Marginean et al., (Bucharest) Funding for detectors and DAQ from STFC UK.