Exposing subtle details of the nuclear force through lifetime measurements

The lifetime of a nuclear state is a fundamental observable which gives a direct probe of the nuclear force.

The DPUNS degrader and target foils installed at the University of Jyväskylä in central Finland. Full details of the plunger can be found in reference [1].

Classically, the lifetime of a nuclear state, T, is given by the quantum mechanical Fermi-Golden rule,

,

where H is the nuclear force Hamiltonian, ψi and ψf are the initial and final nuclear states involved in the process and ρ(E)dE is the density of final states.

For nuclei, the transition between the initial and final states usually takes place with the emission of an electromagnetic gamma-ray which conserves angular momentum, parity and energy. This reserach programme uses the lifetime measurement of these gamma-ray transitions to examine subtle details of the nuclear force in exotic regions far from stability where weak but important parts of the nuclear force are no longer masked. As an example, lifetime measurements in exotic nuclei give direct information on the so called "effective proton and neutron charges" which define the theoretical nuclear transition matrix elements. With STFC support over the last 4 years, we have developed a Differential Plunger for Unbound Nuclear States (DPUNS) [1] which has allowed progress in the understanding of excited state lifetimes in exotic nuclei beyond the proton drip line to be made. A campaign of lifetime measurements of excited-states built upon proton-emitting states was performed and a total of 27 publications were made on this research over this three-year period with 3-completed and 2-ongoing PhD theses.

Two recent highlights have been:

  1. The lifetime of the nuclear states directly defines the nuclear collectivitiy, and gives information on how this quantum mechanical ensemble of ~200-300 strongly interacting nucleons can act together to produce very large transition rates orders of magnitude greater than the single-particle values. As such, the determination of nuclear collectivity is extremely important around closed shells where single-particle transition rates must dominate. This is especially important for the predictions of new closed shells in heavy nuclei (where all theoretical models predictions do not currently agree) and also around the special doubly magic N=Z=50 closed shell. Our recent measurements on 109I and 112Te appear to show anomalies in the trend of decreasing collectivity as the doubly-magic closed shell is approached [2]. This may constitute evidence for an isospin dependence of the effective charges predicted by Bohr and Mottelson in 1975. We are further testing these ideas with an expansion of our programme in Orsay, Paris this year.
  2. As part of our work on 151Lu, a non-adiabatic theoretical code was developed to calculate proton- and γ-emission in non-rotational nuclei [3] instead of the usual adiabatic approach for rotational nuclei. This code allows a better description of proton emission from mildly deformed nuclei and showed that the proton emitting state in 151Lu was oblate. Not only was this the best evidence to date for proton emission from an oblate nucleus, it also settled a long-standing theoretical debate on the shape of this nucleus [3].      

We are currently expanding this programme (2014-2018) with STFC funding to be able to study more exotic nuclei with the development of a new triple-foil plunger to be able to use radioactive beam developments at GANIL (France), CERN, RIKEN (Japan) and FAIR (Germany). In addition, we are constructing a charge-plunger which will be used to radically improve the knowledge of the role of the spin-orbit interaction in the stabilisation of superheavy nuclei. The spin-orbit interaction strength has a profound influence on the structure of heavy nuclei, creating shell gaps that stabilise against fission. Current theoretical models for heavy and superheavy nuclei disagree on the size and location of the next proton shell gap after Z=82. Microscopic-macroscopic models predict Z=114 and mean-field approaches predict Z=120, 124 & 126. The main reason for these discrepancies is that the models rely on large extrapolations from regions where the interactions are experimentally verified. 

[1]  M. J. Taylor, D. M. Cullen, et al. Nuclear Instr. and Methods in Phys. Res. A707 (2013) 143.

[2]  M.G. Procter, D. M. Cullen, et al., Physics Letters B 704 (2011) 118.

[3]  M. G. Procter, D. M. Cullen, M. J. Taylor et al., Physics Letters B 725 (2013) 79.

[4]  M.J. Taylor, D.M. Cullen et al. Phys. Rev. C 86 (2012) 044310.

 

Research Staff

  • Dr D. M. Cullen
  • Dr M. J. Taylor
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