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R. N. Mohapatra
reactions in the early part of the twentieth century, Bethe and Critchfield’s
work made a major advance towards understanding the vast energy supply at
the solar core. Yet it was not clear how to test this theory. It came only after
solar neutrinos were discovered by Ray Davis and in subsequent experiments
at Super-Kamiokande and SNO. They clearly pointed to the correctness of
the nuclear reaction picture for solar energy, since according to this picture,
neutrinos emitted in the nuclear fusion process are responsible for energy
generation in the Sun. However, they only detected the higher energy tail of the
neutrinos emitted from the Sun, which is only a small fraction of the neutrinos
emitted and a small percentage of the fusion reaction responsible for solar
luminosity. By themselves, they cannot therefore confirm the detailed fusion
model of the solar energy production. The most dominant energy source in
the Sun is the hydrogen–hydrogen fusion part of the nuclear reaction, which
produces more than 90% of the solar energy and produces only very low energy
neutrinos. There is thus a one-to-one correspondence between the low energy
neutrinos and the solar luminosity. To prove this fusion picture in more detail,
one needs to measure the low energy part of the neutrino spectrum in as much
detail as possible.
There are four classes of nuclear fusion reactions going on in the Sun
constantly: they are known as (see Fig. 13.6) pp, Be, B, and pep reactions
with neutrinos coming out with different energies from them. As noted, the
pp reaction is the most dominant source with the characteristic energy of the
neutrinos coming from them being in the 0.1–0.4 MeV range. The Ray Davis
and Super-K and SNO are only sensitive to the B (Boron) neutrinos, which go
from few MeV up to 10 MeV and thus are not sensitive to the most dominant
part of the solar energy generation reaction.
This important gap was filled by the gallium detector experiments Gallex
(Italy) and SAGE (Russia) and by one of the more recent additions to this
group, the Borexino experiment in Gran Sasso (Italy) that uses a liquid
scintillator for detection. The way the low energy neutrino detection confirms
the solar fusion model is as follows: in the hydrogen fusion reaction, there are
two neutrinos emitted, together with 26 MeV of energy as photons for each
two neutrino. The energy emitted comes out as solar luminosity, which has
been measured fairly accurately. Thus if the emitted number of neutrinos is
measured also accurately, that will confirm the solar fusion model. The Gallex,
SAGE, and Borexino experiments have done this measurement and have
confirmed the solar fusion model. This provides a complete understanding
of the longstanding problem of energy generation by the Sun (Fig. 13.6).
Another important aspect of understanding the Sun involves knowledge of
what elements are in it and how much of each element is there. We know
R. N. Mohapatra
reactions in the early part of the twentieth century, Bethe and Critchfield’s
work made a major advance towards understanding the vast energy supply at
the solar core. Yet it was not clear how to test this theory. It came only after
solar neutrinos were discovered by Ray Davis and in subsequent experiments
at Super-Kamiokande and SNO. They clearly pointed to the correctness of
the nuclear reaction picture for solar energy, since according to this picture,
neutrinos emitted in the nuclear fusion process are responsible for energy
generation in the Sun. However, they only detected the higher energy tail of the
neutrinos emitted from the Sun, which is only a small fraction of the neutrinos
emitted and a small percentage of the fusion reaction responsible for solar
luminosity. By themselves, they cannot therefore confirm the detailed fusion
model of the solar energy production. The most dominant energy source in
the Sun is the hydrogen–hydrogen fusion part of the nuclear reaction, which
produces more than 90% of the solar energy and produces only very low energy
neutrinos. There is thus a one-to-one correspondence between the low energy
neutrinos and the solar luminosity. To prove this fusion picture in more detail,
one needs to measure the low energy part of the neutrino spectrum in as much
detail as possible.
There are four classes of nuclear fusion reactions going on in the Sun
constantly: they are known as (see Fig. 13.6) pp, Be, B, and pep reactions
with neutrinos coming out with different energies from them. As noted, the
pp reaction is the most dominant source with the characteristic energy of the
neutrinos coming from them being in the 0.1–0.4 MeV range. The Ray Davis
and Super-K and SNO are only sensitive to the B (Boron) neutrinos, which go
from few MeV up to 10 MeV and thus are not sensitive to the most dominant
part of the solar energy generation reaction.
This important gap was filled by the gallium detector experiments Gallex
(Italy) and SAGE (Russia) and by one of the more recent additions to this
group, the Borexino experiment in Gran Sasso (Italy) that uses a liquid
scintillator for detection. The way the low energy neutrino detection confirms
the solar fusion model is as follows: in the hydrogen fusion reaction, there are
two neutrinos emitted, together with 26 MeV of energy as photons for each
two neutrino. The energy emitted comes out as solar luminosity, which has
been measured fairly accurately. Thus if the emitted number of neutrinos is
measured also accurately, that will confirm the solar fusion model. The Gallex,
SAGE, and Borexino experiments have done this measurement and have
confirmed the solar fusion model. This provides a complete understanding
of the longstanding problem of energy generation by the Sun (Fig. 13.6).
Another important aspect of understanding the Sun involves knowledge of
what elements are in it and how much of each element is there. We know