Resonant antineutrino induced electron capture with low energy bound-beta beams
R.G.C. Oldeman
0
M. Meloni
0
B. Saitta
0
0
Dipartimento di fisica,
Universit degli Studi di Cagliari and INFN
, Sezione di Cagliari, Cagliari,
Italy
Antineutrino induced electron capture is a resonant process that can have a large cross-section for beams of monochromatic antineutrinos. We calculate the crosssection of this process and investigate an experimental setup where monochromatic antineutrinos are produced from the bound-beta decay of fully ionized radioactive atoms in a storage ring. If the energy between the source and the target is well matched, the cross-sections can be significantly larger than the cross-sections of commonly used nonresonant processes. The rate that can be achieved at a small distance between the source and two targets of 103 kg is up to one interaction per 8.3 1018 decaying atoms. For a source-target distance corresponding to the first atmospheric neutrino oscillation maximum, the largest rate is one interaction per 3.2 1021 decaying atoms, provided that extremely stringent monochromaticity conditions (107 or better) are achieved in future ion beams.
1 Introduction
In ordinary electron capture, an electron from one of the s
orbitals is absorbed by the nucleus, and a neutrino is emitted.
If this process is energetically forbidden, it can be induced
by an incoming antineutrino, a process called antineutrino
induced electron capture ( EC) [1]. The cross-section is
resonant, and with a monochromatic source of antineutrinos of
the right energy, large cross-sections can be obtained. The
case of the 3H 3He system is of particular interest since
recoilless emission and absorption can result in a Mossbauer
effect for neutrinos [25]. Here we study the process for a
large number of target nuclei, and propose to use
boundbeta decays of fully ionized radioactive atoms as a source
of monochromatic antineutrinos. Throughout the paper, we
consider only nuclear allowed transitions, where the parity
e + ZA+1Y ZAX,
4 2/4
Q2 (Q Qt )2 + 2/4
with an electron vacancy in an s shell with a quantum
number of energy n is an electric dipole transition from an
electron in a (n + 1)p shell. We approximate this width as:
where m(ZAX) refers to the mass of the neutral atom [9]. The
antineutrino energy must be slightly higher than the Q value,
since a small fraction of the neutrino energy is used for the
kinetic energy of the final state:
All stable atoms, with halflife t1/2 > 1016 s, and a peak
cross-section peak 5 1042 cm2 for antineutrino induced
electron capture from the 1s orbital are listed in Table 1.
In Fig. 1 the peak cross-section for some elements are
compared to the antineutrino-proton cross-section and the
antineutrino-electron cross-section.
Cross-sections for electron capture from higher orbitals
are calculated in the same way. While b decreases as n3,
the radiative width decreases as n4, and the peak
crosssection scales linearly with n.
We find that the peak cross-section for antineutrino
induced electron capture can be several orders of
magnitude larger than the commonly used processes of
scattering on protons or electrons. For example, four targets (3He,
69Ga, 106Cd, and 112Sn) have peak cross-sections in
excess of 1041 cm2 for sub-MeV antineutrinos, while the
antineutrino-electron cross-section is less than 1044 cm2 at
those energies. However, the cross-section rapidly decreases
outside a narrow energy window, and to exploit these large
cross-sections requires a monochromatic source of
antineutrinos.
3 Bound-beta beams (3) (4) (5)
A possible source of monochromatic antineutrinos are
radioactive nuclei that undergo bound-beta decay, a process
confirmed experimentally in 1992 [10]. We consider fully
where Btot is the total binding energy of all electrons of the
atom [11]. Bn,I is the binding energy of a single electron in
orbital n in an otherwise fully ionized atom:
1We use the values from http://www.jlab.org/~gwyn/ebindene.html
compiled from [68].
Bn,I = me 1
Table 1 List of stable (t1/2 1016 s) elements with a peak
crosssection for antineutrino induced electron capture from the 1s orbital
of 5 1042 cm2 or larger
Natural
abundance
ionized atoms, where the bound-beta decay fraction is
largest due to the two available vacancies in the 1s orbital.
In general, the Q value of the bound-beta decay will not
exactly match the Q value at the target, even if the
reactions involve the same nuclei, because of nuclear recoil and
differences in electron binding energies.
By accelerating the ionized atoms and storing them in
a storage ring with straight sections, the Q value of any
antineutrino source through bound-beta decay, Qs , can be
matched to the Q value in the macroscopic target at rest,
Qt , even for different reactions.
For bound-beta decay of a fully ionized atom we find:
Table 2 List of nuclei with a bound-beta decay fraction of 0.20
or more
Fig. 1 Illustration of the peak cross-sections of antineutrino induced
electron capture from the 1s orbital for some representati (...truncated)