The Long and the Short of it: Measuring picosecond half-lives… Paddy Regan Dept. of Physics, University of Surrey, Guildford, GU2 7XH, UK

Electromagnetic Transition Rates EM transition rates ( = 1 / decay mean lifetime,  ) are related to the multipole order of the transition by the expression T if ( L) = 1 /  = (ang.mom. fact) x (E  ) 2L+1 x B( L:I i ->I f ) Thus e.g, for E2 transitions, B(E2:I i ->I f ) = k. E  -5 x T if Thus, if B(E2) is big (lots of overlap of wavefunctions), T if 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)

For lifetimes, , in units of seconds and and E  in MeV, T(E1) =1/  = 1.59 x E  3 B(E1) T(E2) =1/  = 1.22 x 10 9 E  5 B(E2) T(E3) = 1/  = 5.70 x 10 2 E  7 B(E3) B(ML) in units of e 2 fm 2L i.e., transition rates get slower (i.e., longer lifetimes associated with) higher order multipole decays.

Annual Review of Nuclear Science (1968) 18 p

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

136 Xe+ 198 Pt Dobon et al., nano to microsecond isomer tagging ?

Identification of new ‘seniority’ isomer in 136 Ba, N=80 isotone. J.J. Valiente-Dobon, PHR, C.Wheldon et al., Phys. Rev. C69 (2004) T 1/2 =91(2) ns

Structure of 8 + final state changes from 134 Xe -> 136 Ba ? See Valiente-Dobon, PHR, Wheldon et al., PRC69 (2004) Isomer decay also depends on structure of final state N=80, ( h 11/2 ) isomers

Pair Truncated Shell Model Calculations (by Yoshinaga, Higashiyama et al. Saitama) predict yrast 8 + in 136 Ba no longer mostly ( h 11/2 ) -2 but rather, (  d 5/2 ) 2 (  g 7/2 ) 2

BaF 2 ‘fast timing’ data from H. Mach et al. Contribution to ENAM 2001 Allows an ordering of the gammas under isomer from their (~ps) lifetimes.

From Henryk Mach et al., 96 Pd.

Use (BaF 2,BaF 2 ) coincidences below isomers to get B(E2) values ( & order gamma-transitions) 96 Pd H. Mach et al.,

100 Pd

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. 80 Se beam (Z=34; N=46) + 24 Mg target (Z=12; N=12) to make 104 Pd* (Z=46; N=58). Boil off 4 neutrons to leave 100 Pd which decays by gamma to ground state. Whole process from creation to gs takes < 1ns.

100 Pd Z=46 4 proton holes in Z=50: N=54 4 neutron particles above N=50

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+ 2+ →0+2+ →0+ 6+ →4+6+ → Pd

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m tof ~ 4ps tof~ 9ps tof~ 12ps tof~ 15ps tof~ 30ps

E s = E o ( cos(41.5 o )) E s = E o * E s = E o ( cos(139.5 o )) E s = E o * EoEo E s (139.5 o )E s (41.5 o ) Time of flight = 16.8  m per ps

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 4+ →2+4+ →2+ 2+ →0+2+ →0+ 6+ →4+6+ →4+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 6+ →4+6+ →4+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 6+ →4+6+ →4+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 6+ →4+6+ →4+

24 Mg ( 80 Se, 4n) 100 Pd E B =268 MeV, v/c ~5.8c 75  m 150  m 200  m 250  m 500  m 6+ →4+6+ →4+

Some (cool) numbers? Power: From 2 ps transition of E  =800 keV P =  E /  t = (800 x 1.6 x J) / (2 x s) → 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 Mv 2 v = sqrt ( 280 x 1.6 x J x 2 / 80 x 1.66 x kg) v= 2.6x10 7 m/s = 8.6% speed of light. Recoil velocity (measured from E s = E o ( 1+ v / c COS (  )) ~ 5.6% of c v rec ~ 1.7x 10 7 ms -1 = 16.8  m per ps

Now over to Vassia to get the final (accurate numbers)….does the structure change (from ~vibrator to ~rotor) with spin ? To be continued…