New decay and isomer physics studies with fast scintillation detection arrays at Coulomb Barrier separator focal planes

Paddy ReganDepartment of Physics

University of Surrey&

National Physical Laboratory, 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.

Annual Review of Nuclear Science (1968) 18 p265-290

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).

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.

T1/2 = 1.4ns

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

PMTPMT

Digitizer

CFD

Trigger

AnodeDynode

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 timingCR-RC, Trapezoidal filters for energy

Acqiris digitizer model DC252HF.Tests by Mohammad Nakhostin (U. Surrey)

0 200 400 600 800 10000

100

200

300

400

500

600

700

Co

unts

Channel Number

Sample energy spectrum, 137Cs

0.0 0.5 1.0 1.5 2.0 2.5 3.0

250

300

Tim

e re

solu

tion

(ps)

Sampling interval (ns)

Time resolution vs pulse sampling rate

Energy gate: 1100-1400 keV

Still preliminary!

Analog result

100 150 200 250 300 350 4000

10

20

30

40

50

60

C

ount

s

Time (channel)

1100-1400 keV 100-400 keV

Digital time spectra for two different energy gates

The width and peak location of time spectra move with energy

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.53.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

Ener

gy re

solu

tion

(%)

Sampling interval (ns)

B

Energy resolution vs sampling rate

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%)

An experimental example:138Ce80

(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

T.Alharbi et al., Phys. Rev. C87 (2013) 014323

138Ce80

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 poster by Frank Browne).

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

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?

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)

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 LaBr3 – LaBr3 coincidences with ~200ps intrinsic LaBr3 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. 138Gd / 140Dy? 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 (188-194) under known isomers (11- isomers and K isomers).

3.States below the 14+ isomer in 94Pd giving information on n-p pairing.

4....

Some potential Physics Experiments at RITU:

D.M. Cullen, Physics with Large Arrays of Novel Scintillators, Dublin, Jan 2014

1. 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.