OPPORTUNITIES IN NUCLEAR SCIENCE A Long-Range Plan for ...

ground state. This is referred to as fine structure in proton
decay and has been observed in a few cases, for
example, in 131 Eu (Z = 63), for which the data are shown
in the lower figure below.
p
(35/2 – )
(31/2 – )
(27/2 – )
(23/2 – )
(19/2 – )
(15/2 – )
(11/2 – )
(7/2 – )
141 Ho
(13/2 – )
(9/2 – )
p
(19/2 + )
(15/2 + )
(11/2 + )
(7/2 + )
(3/2 + )
(1/2 + )
Rotating proton emitters. The band structures seen on top of
the two proton-emitting states of 141 Ho are associated with the
rotation of this nucleus. Their presence confirms the sizable
deformation of 141 Ho that had been inferred originally from the
measured proton-decay half-lives.
The proton drip line and beyond. Properties of nuclei at or
near the proton drip line address a number of fundamental
questions. First, establishing the exact location of the drip
line represents a stringent test for mass models and constrains
the path of nucleosynthesis (rp-process). In addition,
because of the stability provided by the Coulomb
barrier, it is possible to study quasi-bound states in nuclei
that actually lie beyond those that would be bound by the
strong force alone. Such nuclei eventually decay, via quantum
tunneling through a three-dimensional barrier, by emitting
protons. A flurry of experimental activity studying this
decay mode (some of it summarized in “Beyond the Proton
Drip Line,” at left) has resulted in the complete delineation of
the drip line up to scandium (Z = 21) and, for odd-Z nuclei,
up to indium (Z = 49). A large number of proton emitters
have also been discovered between indium and bismuth (Z =
83), and we have achieved a quantitative understanding of
the properties of the decaying nuclei, such as their half-lives.
Deformation has also been found to affect proton decay significantly.
A new mode of nuclear decay, direct two-proton emission,
has been shown to occur in 18 Ne (see Figure 2.12).
This mode was predicted decades ago, but until recently,
experimental efforts had found only sequential emission
through an intermediate state, a mechanism energetically
forbidden in the case of 18 Ne. The characterization of the
Counts per 10 keV
90
70
50
30
131
63 Eu
3/2 +
t 1/2 = 18 ms
811
932
2 +
0 + 130
62 Sm
932
3.10 1/2 – 6.35 2 –
6.15 1 –
5.45 2 –
0.0 0 + 0.49 1/2 + 5.11 2 +
4.52
0.0 5/2 +
16 O + 2p 3.92
17 F + p
Other
states
18 Ne
0.0 0 +
18 Ne
Proton 1
Proton 2
10
811
Energy (keV)
Fine structure in europium. In 131 Eu proton decay has been
observed to both the ground state of 130 Sm and to the first
excited state of the deformed nucleus, as shown by the small
spectral peak observed at 811 keV.
Figure 2.12. A rare event. Two-proton decay, one of the most
exotic and elusive nuclear decay modes, has recently been
observed in 18 Ne. As seen from the energy relationship between
16 O, 17 F, and 18 Ne, emitting one proton after the other, first from
18 Ne to yield 17 F and then from 17 F to give 16 O, is forbidden for 18 Ne
states up to 6.4 MeV. The measured angular correlations between
emitted protons, along with their relative energy distributions, give
the first evidence of the two-proton decay, indicated by the long
red arrow, of an excited state of 18 Ne at 6.15 MeV.
37