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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: p.regan@surrey.ac.uk. Electromagnetic Transition Rates. EM transition rates ( = 1 / decay mean lifetime, t ) are related to the

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## The Long and the Short of it: Measuring picosecond half-lives…

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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: p.regan@surrey.ac.uk**Electromagnetic Transition Rates**EM transition rates ( = 1 / decay mean lifetime, t) are related to the multipole order of the transition by the expression Tif (lL) = 1 / t = (ang.mom. fact) x (Eg )2L+1 x B(lL:Ii->If) Thus e.g, for E2 transitions, B(E2:Ii->If) = k. Eg-5 x Tif Thus, if B(E2) is big (lots of overlap of wavefunctions), Tifis big and t is small (i.e. fast = collective); If B(E2) is small tis big (i.e. slow = isomer) Also, slower for larger L values, (classically further away)**For lifetimes, t, in units of seconds and and Eg in MeV,**T(E1) =1/t = 1.59 x 1015 Eg3 B(E1) T(E2) =1/t = 1.22 x 109 Eg5 B(E2) T(E3) = 1/t = 5.70 x 102 Eg7 B(E3) B(ML) in units of e2fm2L i.e., transition rates get slower (i.e., longer lifetimes associated with) higher order multipole decays.**In-beam, electronic technique (g-g-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**nano to microsecond isomer tagging ?**136Xe+198Pt Dobon et al.,**Identification of new ‘seniority’ isomer in 136Ba, N=80**isotone. T1/2=91(2) ns J.J. Valiente-Dobon, PHR, C.Wheldon et al., Phys. Rev. C69 (2004) 024316**Structure of 8+ final state changes from 134Xe -> 136Ba ?**See Valiente-Dobon, PHR, Wheldon et al., PRC69 (2004) 024313 Isomer decay also depends on structure of final state N=80, (nh11/2)-210+ isomers**Pair Truncated Shell Model**Calculations (by Yoshinaga, Higashiyama et al. Saitama) predict yrast 8+ in 136Ba no longer mostly (nh11/2)-2 but rather, (pd5/2)2(pg7/2)2**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.**Use (BaF2,BaF2) coincidences below isomers to get B(E2)**values ( & order gamma-transitions) 96Pd H. Mach et al.,**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.**100Pd**• Z=46 4 proton holes in Z=50: • N=54 4 neutron particles above N=50**100Pd**2+→0+ 4+→2+ 6+→4+ 500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**500mm**tof~ 30ps 250mm tof~ 15ps 200mm tof~ 12ps 150mm tof~ 9ps 75mm tof ~ 4ps 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**Es = Eo (1 +0.058cos(139.5o))**Es = Eo * 0.9558 Es = Eo (1 +0.058cos(41.5o)) Es = Eo * 1.0434 Time of flight = 16.8 mm per ps**4+→2+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**4+→2+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**4+→2+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**4+→2+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**2+→0+**4+→2+ 6+→4+ 500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**6+→4+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**6+→4+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**6+→4+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c**6+→4+**500mm 250mm 200mm 150mm 75mm 24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

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