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Figure 3. Mirror Energy Differences (MED), i.e. differences in
excitation energy for analogue states in mirror nuclei, for the
heaviest known mass triplet, A=54 (red line), extracted from
experiments employing the EUROBALL and more recently the
RISING spectrometers. To illustrate the cross- conjugate symmetry,
within the shell, MEDs of mass A=42 are shown as well (green
line). MEDs are originated by isospin breaking interactions in the
nucleus. Theoretical description (blue line), with large-scale SM
calculations, comprises the single particle, radial and multipole
(alignment) Coulomb contributions, as well as a sizeable additional
isospin breaking contribution not understood hitherto (black lines).
Consequently, pairs of nuclei with mirroring values
of N and Z (across the N=Z line) are expected to have
excitations differing only by the Coulomb contributions
(mirror energy). Precise measurements of mirror energies
thus provide a stringent test of the effective nuclear
interaction. Indeed, existing studies (Figure 3) have suggested
modifications to the standard nuclear interaction
used in shell- model calculations. Therefore, there is a
need to measure mirror pairs with large isospin quantum
numbers to maximise the difference in the Coulomb
contributions. These studies will only be possible using
the next generation RIB facilities. The challenge is to
measure mirror energies near or even beyond the proton
drip-line and to investigate if shape changes and shape
coexistence could play a role in the differences between
isobaric multiplets in medium-mass nuclei. Therefore,
instrumentation combining gamma-ray tracking and
light charged-particle detector arrays is of paramount
importance.
Limits of existence in proton-rich nuclei
and 100 Sn
One of the most relevant questions in nuclear physics
concerns the limits of existence of nuclei, in terms of the
number of protons and neutrons. Experimental as well
as theoretical efforts are continuously made to map the
drip-line. Presently the one-proton drip-line is known up
to mass A~180, whereas the two-proton drip-line is only
known up to A=54.
One- and two-proton emitters are spectacular examples
of open quantum systems, which allow the study
of coupling between bound nuclear states and the continuum.
Furthermore, proton emitters also provide a
wealth of spectroscopic information, testing the validity
of nuclear models beyond the drip line. The dynamics
of the two-proton radioactivity provides an insight
into the pairing interaction and Josephson tunnelling in
nuclear matter. After the recent observation of the ground
state two-proton decay of 45 Fe, the understanding of the
physics involved requires more detailed data on the twoproton
decay of 48 Ni and 54 Zn as well as investigations of
heavier candidates, 59 Ge, 63 Se, and 67 Kr. In addition to
the total decay energy and the partial half-life, the energy
sharing between the two protons and in particular the
angle between the protons should be measured.
Prompt particle decay in competition with gamma
decay from excited or isomeric states is a rare phenomenon.
It was identified in the A~60 region and explained
in some cases, as a consequence of an abrupt shape
transition in nuclei with a limited Coulomb barrier. While
the prompt proton decay has been known for more than
a decade, recently the prompt emission of alpha particles
was identified for 58 Ni. The nuclear structure information
extracted from the analysis of this process is far from
being understood and requires experimental as well as
theoretical efforts.
The most relevant nucleus lying at the proton dripline
is the N=Z=50 nucleus 100 Sn. It is expected that
any collective phenomena present in this nucleus will
be reinforced by the coherent contribution of protons
and neutrons. The information regarding the structure
of this nucleus is very scarce. More detailed data will
provide an insight into what extent shell gaps, collectivity
or low-lying vibrational states are preserved.
The studies of charged particle decay from isomeric
or ground states will require efficient arrays of high resolution
charged- particle detectors with discrimination
capabilities, used in conjunction with large gamma-ray
detector arrays. The investigation of rare two-proton
decay requires pictures of individual decay events
obtained by employing novel imaging time-projectionchambers
(Figure 4).
Studies of the prompt particle decay can be extended
to heavy nuclei by employing the recoil-decay tagging
method. The investigation of delayed one- and twoproton
emitters is still limited by the flight time through
the separator. One ambitious experimental goal is to fill
the gap between the prompt and the several hundred
ns delayed emission.
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