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The Long and the Short of it:Measuring picosecond half-lives…
Paddy Regan
Dept. of Physics, University of Surrey,
Guildford, GU2 7XH, UK
e-mail: [email protected]
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Electromagnetic Transition RatesEM transition rates ( = 1 / decay mean lifetime, ) are related to the multipole order of the transition by the expression
Tif (L) = 1 / = (ang.mom. fact) x (E)2L+1 x B(L:Ii->If)
Thus e.g, for E2 transitions, B(E2:Ii->If) = k. E-5 x Tif
Thus, if B(E2) is big (lots of overlap of wavefunctions), Tif is big and is small (i.e. fast = collective);
If B(E2) is small is big (i.e. slow = isomer)Also, slower for larger L values, (classically further away)
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For lifetimes, , in units of seconds and and E in MeV,
T(E1) =1/= 1.59 x 1015 E3 B(E1)
T(E2) =1/ = 1.22 x 109 E5 B(E2)
T(E3) = 1/ = 5.70 x 102 E7 B(E3)
B(ML) in units of e2fm2L
i.e., transition rates get slower (i.e., longer lifetimes associated with) higher order multipole decays.
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Annual Review of Nuclear Science (1968) 18 p265-290
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In-beam, electronic technique (t)
eg, PHR et al. Nucl. Phys. A586 (1995) p351
Fusion-evaporation reaction with pulsed beam (~1ns), separated by fixed period (~500ns). Using coincidence gamma-rays to see across isomer
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136Xe+198PtDobon et al.,
nano to microsecond isomer tagging ?
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Identification of new ‘seniority’ isomer in 136Ba, N=80 isotone.
J.J. Valiente-Dobon, PHR, C.Wheldon et al., Phys. Rev. C69 (2004) 024316
T1/2=91(2) ns
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Structure of 8+ final state changes from 134Xe -> 136Ba ?See Valiente-Dobon, PHR, Wheldon et al., PRC69 (2004) 024313
Isomer decayalso depends on structureof final state
N=80, (h11/2)-210+ isomers
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Pair Truncated Shell Model
Calculations (by Yoshinaga,Higashiyama et al. Saitama)predict yrast 8+ in 136Ba no longer mostly (h11/2)-2
but rather, (d5/2)2(g7/2)2
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BaF2 ‘fast timing’ data from H. Mach et al. Contribution to ENAM 2001
Allows an ordering of the gammas under isomer from their (~ps) lifetimes.
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From Henryk Mach et al., 96Pd.
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Use (BaF2,BaF2) coincidences below isomers to get B(E2) values ( & order gamma-transitions)
96Pd H. Mach et al.,
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100Pd
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Fusion evaporation…good way to populate nuclei for study…put into (high spin) excited states. Fuse beam and target nucleus and boil off excess energy via neutron evaporation. e.g. 80Se beam (Z=34; N=46) + 24Mg target (Z=12; N=12) to make 104Pd* (Z=46; N=58). Boil off 4 neutrons to leave 100Pd which decays by gamma to ground state. Whole process from creation to gs takes < 1ns.
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100Pd • Z=46 4 proton holes in Z=50:
• N=54 4 neutron particles above N=50
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
4+ →2+2+ →0+
6+ →4+
100Pd
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
tof ~ 4ps
tof~ 9ps
tof~ 12ps
tof~ 15ps
tof~ 30ps
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Es = Eo (1 +0.058cos(41.5o))Es = Eo * 1.0434
Es = Eo (1 +0.058cos(139.5o))Es = Eo * 0.9558
Eo Es (139.5o) Es (41.5o)
665 636 694
750 717 783
773 739 807
798 763 833
Time of flight = 16.8 m per ps
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m4+ →2+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m4+ →2+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m4+ →2+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m4+ →2+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
4+ →2+2+ →0+
6+ →4+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
6+ →4+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
6+ →4+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
6+ →4+
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24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c
75m
150m
200m
250m
500m
6+ →4+
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Some (cool) numbers?• Power: From 2 ps transition of E=800 keV
• P = E / t = (800 x 1.6 x 10-16 J) / (2 x 10 -12 s)→ 0.064 Watts per decay
Assume ~ 1000 per second measured = 64 W from per second…singles rate much larger (x 100 at least)
Beam Velocity: 280 MeV = ½ Mv2
v = sqrt ( 280 x 1.6 x 10-13 J x 2 / 80 x 1.66 x 10-27kg) v= 2.6x107 m/s = 8.6% speed of light.
Recoil velocity (measured from Es = Eo ( 1+ v/c COS ()) ~ 5.6% of c vrec ~ 1.7x 107ms-1 = 16.8 m per ps
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• Now over to Vassia to get the final (accurate numbers)….does the structure change (from ~vibrator to ~rotor) with spin ?
• To be continued…