the long and the short of it: measuring picosecond half-lives…

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 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)

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.

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

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

136Xe+198PtDobon et al.,

nano to microsecond isomer tagging ?

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

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

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

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.

From Henryk Mach et al., 96Pd.

Use (BaF2,BaF2) coincidences below isomers to get B(E2) values ( & order gamma-transitions)

96Pd H. Mach et al.,

100Pd

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

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

4+ →2+2+ →0+

6+ →4+

100Pd

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

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

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m4+ →2+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m4+ →2+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m4+ →2+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m4+ →2+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

4+ →2+2+ →0+

6+ →4+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

6+ →4+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

6+ →4+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

6+ →4+

24Mg (80Se , 4n) 100Pd EB=268 MeV, v/c ~5.8c

75m

150m

200m

250m

500m

6+ →4+

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

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

• To be continued…