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Using LaBr3 Gamma-ray Detectors for Precision Lifetime Measurements of Excited States in ‘Interesting’ Nuclei
Paddy ReganDepartment of PhysicsUniversity of Surrey, [email protected]
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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.
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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.
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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).
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T1/2 = 1.4ns
Tests with 152Eu source to measure lifetime of I=2+ 122 keV level in 152Sm.
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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
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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%)
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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
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34P19
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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
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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
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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
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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
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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
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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
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4-
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4-
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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}
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P.J.Mason et al., Phys. Rev C85, 064303 (2012)
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138Ce
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(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.
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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.
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Restricted basis SM calculations give reasonable comparison with experimental (near-yrast) states in 138Ce.
What about transition rates?
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130Te(12C,4n)138Ce @56 MeV
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138Ce80
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138Ce80
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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
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188W
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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
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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
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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)
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•Sum of time differences between 143-keV (2->0) transition and any higher lying feeding transition.
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T1/2 =0.87(12)
ns
Time difference between 143 keV2+→0+ and feeding transitions.
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188W 188W
188W
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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.
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F.Browne, H. Watanabe, A. Bruce, T. Sumikama et al.,
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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?
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Compares with T1/2=2.0(1) ns measurement by Hwang et al., Phys. Rev. C73, 044316 (2006)
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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)
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Possible array configuration at focal plane of RITU spectrometer at JYFL
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• 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).
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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.