First Identification of Gamma Transitions in 157Pm and its Excited States

J. RangerDr. J. H. HamiltonDr. A.V. Ramayya

Dr. J.K. HwangE.H. Wang

Spontaneous Fission● Large, unstable nuclei fission

into smaller fragments and neutrons

● Resulting primary fragments emit several neutrons and form several daughter nuclei

● Each nucleus formed has a partner

● These partner nuclei are formed in excited states and decay by emission of γ-rays to their ground states

γ-decay● The nucleus is a quantum system, where all protons

and neutrons are bound by a strong short-range force and have their own wavefunctions

● Like electrons in an orbit, nucleons, which also move in orbits, can become excited and lose energy in the form of a high-energy photon

● Sometimes, the entire nucleus can rotate and vibrate, which are called collective motions

γ-decay252Cf → 157Pm* + 95Rb*

252Cf → 157Pm* + 94Rb* + 1n252Cf → 157Pm* + 93Rb* + 2 1n252Cf → 157Pm* + 92Rb* + 3 1n252Cf → 157Pm* + 91Rb* + 4 1n252Cf → 157Pm* + 90Rb* + 5 1n

157Pm* → 157Pm + γ rays

Gamma Spectroscopy● Germanium with Compton suppression

detectors● Three main processes in detection:

photoelectric interaction, Compton scattering, and pair production

● Produce net voltage (signal)

Gammasphere● 101 Ge crystal detectors in spherical array● 252Cf placed into the center● 252Cf only undergoes SF 3% of the time● Decays into a plethora of isotopes, and all

subsequent γ-rays were recorded

Gamma Spectrum from 252Cf

Experiment● Use coincidence data to verify level schemes and

establish new level schemes● Two γ-rays are in coincidence if they occur at roughly

the same time (less than 10-12 s apart)● A gated gamma energy is a peak entered into the

computer to show all peaks in coincidence with it

Experiment● If we know one transition energy of an

isotope or its partner, we can gate on it to look for other coincident peaks

● We can use double (γ-γ-γ) or triple (γ-γ-γ-γ) gates to do this

Promethium-157● Vanderbilt team

previously established level scheme for 156Pm

● Mass-gated spectra produced from 238U→9Be <fission showed these transitions belong to 157Pm and not to 156Pm

Double gate on 103.6, 372.5 keV

Triple gate on 103.6, 372.5, 124.7 keV

π 5/2 [532] orbital

Conclusion● By studying nuclei over long isotopic chains,

one can establish the structural changes as the proton number or neutron number varies

Acknowledgements ● Vanderbilt University ● Furman University● NSF● Tsinghua University● GANIL● Dr. J. H. Hamilton● Dr. A.V. Ramayya

● Dr. J.K. Hwang● Enhong Wang● Carol Soren● Dr. Bill Brantley

Questions?