13 Neutrinos Oscillate and Hence They Weigh
95
wonderful book describing the history of physics and research on the Sun and
the neutrinos was written by Bahcall in 1989 [16]. Many things needed to be
included in getting this calculation accurate. It took several decades to get the
final number right, all done by Bahcall and his collaborators. They invented a
unit called SNU (solar neutrino unit) and expressed their result in SNUs. It is
given by the neutrino flux producing 10
−36 neutrino captures per target atom
per second. Typically, every second about 10 billion neutrinos pass through
every centimeter square of area on the Earth. The detailed numbers, of course,
depend on the energy of the arriving neutrino. To detect a single solar neutrino
in the detector, one needs to have a lot of target atoms—typically as large a
number present in a ton of the detector material.
13.4 Ray Davis and Catching of Solar Neutrinos
A key question towards the end of the nineteenth century was, “Where does
the Sun get its energy that comes to us as light every day?” This question
occupied great minds of the early twentieth century. In 1911, Arthur Eddington
wrote a long article on stars for Encyclopedia Britannica. In it, he calculated
the energy emitted due to gravitational effect in the Sun and found it to be
totally inadequate to explain observed solar luminosity. That then left a new
puzzle in physics. Recall, continuous electron spectrum in beta decays was
already another puzzle at the time. A clue came in 1920 when Francis Aston
showed that the mass of the helium atom was slightly less than the masses of
four protons. Physicists knew that solar luminosity had something to do with
protons at the core of the Sun. Aston’s observation led Arthur Eddington to
suspect that when four protons fuse together along with two electrons to form
the helium nucleus, they would have some missing mass, and that missing
mass is emitted as energy by Einstein’s mass energy relation E = mc
2 . But
nobody knew how to calculate the fusion rates in those days to make a reliable
prediction of solar luminosity.
The Sun is a massive hot star, which has several parts to its structure: the
core, the radiative zone, and the convective zone. The core is extremely hot
and dense. The radiative zone is the one through which energy generated in
the core is transmitted by photons. The convective zone comes after radiative
zone and contains gas at relatively low temperature and pressure. The neutrinos
are coming from the core where nuclear reactions are taking place. The
temperature of the solar core is about fifteen million degrees and it slowly
tapers down as one moves outward along the radius. The density of the gas
at the core is about 160 g per cubic centimeter. The challenge was, given this
95
wonderful book describing the history of physics and research on the Sun and
the neutrinos was written by Bahcall in 1989 [16]. Many things needed to be
included in getting this calculation accurate. It took several decades to get the
final number right, all done by Bahcall and his collaborators. They invented a
unit called SNU (solar neutrino unit) and expressed their result in SNUs. It is
given by the neutrino flux producing 10
−36 neutrino captures per target atom
per second. Typically, every second about 10 billion neutrinos pass through
every centimeter square of area on the Earth. The detailed numbers, of course,
depend on the energy of the arriving neutrino. To detect a single solar neutrino
in the detector, one needs to have a lot of target atoms—typically as large a
number present in a ton of the detector material.
13.4 Ray Davis and Catching of Solar Neutrinos
A key question towards the end of the nineteenth century was, “Where does
the Sun get its energy that comes to us as light every day?” This question
occupied great minds of the early twentieth century. In 1911, Arthur Eddington
wrote a long article on stars for Encyclopedia Britannica. In it, he calculated
the energy emitted due to gravitational effect in the Sun and found it to be
totally inadequate to explain observed solar luminosity. That then left a new
puzzle in physics. Recall, continuous electron spectrum in beta decays was
already another puzzle at the time. A clue came in 1920 when Francis Aston
showed that the mass of the helium atom was slightly less than the masses of
four protons. Physicists knew that solar luminosity had something to do with
protons at the core of the Sun. Aston’s observation led Arthur Eddington to
suspect that when four protons fuse together along with two electrons to form
the helium nucleus, they would have some missing mass, and that missing
mass is emitted as energy by Einstein’s mass energy relation E = mc
2 . But
nobody knew how to calculate the fusion rates in those days to make a reliable
prediction of solar luminosity.
The Sun is a massive hot star, which has several parts to its structure: the
core, the radiative zone, and the convective zone. The core is extremely hot
and dense. The radiative zone is the one through which energy generated in
the core is transmitted by photons. The convective zone comes after radiative
zone and contains gas at relatively low temperature and pressure. The neutrinos
are coming from the core where nuclear reactions are taking place. The
temperature of the solar core is about fifteen million degrees and it slowly
tapers down as one moves outward along the radius. The density of the gas
at the core is about 160 g per cubic centimeter. The challenge was, given this