Subatomic Physics

17.7. Nuclei Far From the Valley of Stability 537
In heavy nuclei, the situation is as shown in Fig. 17.12: The symmetry energy is
larger than the energy at which the continuum begins, and the analog states lie in
the continuum. Nevertheless, in the absence of the Coulomb interaction, the analog
states would remain bound, as can be seen in the following. Decay by neutron
emission will lead to a neutron and a nucleus (Z +1,N − 2). The isospin of the
ground state and the low-lying excited states of the nuclide (Z +1,N − 2) is given
by I = 1
. Isospin conservation forbids the decay of the analog
2 (N − Z − 3) = I> − 3
2
state with I = I> into a state with I> − 3
2
and a neutron. In the absence of the
Coulomb interaction, the threshold for proton emission is so high that a decay of
the analog state by proton emission is not possible.
17.7 Nuclei Far From the Valley of Stability
As we explained in chapter 14 the nucleon-nucleon interaction has not been calculated
in an exact form and instead approximate approaches are used. Most of these
rely ultimately on comparisons to data for their validation. The data can not simply
be that coming from single nucleon-nucleon scattering experiments because the
correlations that take place in larger nuclei involve other degrees of freedom (like
the three-body force.) As a consequence the models that have been developped
have good predictive power close to the line of stability, where there is a wealth of
data available, but not for nuclei far from stability, where data is poor. Moreover,
it is believed that imperfections of the models close to stability could be resolved
from studies of nuclei far from stability, where some features would be exaggerated.
For that reason, in recent years interest has focused on studies of exotic nuclei,
particularly those far from the valley of stability. The nuclei with large or small
Z/N have binding energies close to zero, and often decay by weak interactions with
lifetimes larger than 1 ms; they play a role in nucleosynthesis (see Chapter 19.)
Nuclei far from stability can be produced by using short-lived beams. These
radioactive beams can be produced by fusion-evaporation reactions (primarily light
nuclei), by fission-fragmentation, or by spallation reactions on a primary target.
They can then be reaccelerated in a time short compared to their lifetimes in rare
isotope or radioactive isotope accelerators to study reactions (or their by-products)
on secondary targets.
The shell model we have discussed earlier applies to nuclei in or near the valley of
stability, but far from it, normal shell closures tend to disappear (e.g., N =20near
the neutron drip line) and energy levels become more uniformly spaced. Fig. 17.13
shows how the position of the shells changes from nuclei near stability to those far
from it. (14) Doubly magic nuclei will be different than those close to the valley of
stability. (15)
14D.F. Geesaman, C.K. Gelbke, R.V.F. Janssens, B.M. Sherrill, Annu. Rev. Nucl. Part. Sci.
56, 53 (2006).
15R. Schneider et al., Z. Phys. A348, 241 (1994) and A352, 351 (1995).