Getting under the skin of the solar enigma

Last month, the Borexino experiment with a large detector located deep underground at the Gran Sasso laboratory in Italy reported spectral observations of so-called pp neutrinos, demonstrating that 99 percent of the power emitted by the Sun is generated by the proton-proton nuclear fusion reactions at the solar core.

It was just 75 years ago, when Hans Bethe, Von Weizsacker and others, in 1939, showed by calculation of nuclear reaction rates that the fusion of hydrogen to helium via the pp cycle releases the required energy to power the Sun’s enormous radiation. Kelvin and Helmholtz suggested that the Sun releases energy by slow gravitational contraction, but even if it had contracted from infinite size to its present radius, the energy would have lasted hardly thirty million years, too short by geological time scales. 

Laws of energy
Sun is predominantly composed of lighter elements like hydrogen and it became clear that nuclear reactions converting hydrogen to helium at the hot solar core could provide the required energy. Nuclear reactions release million times more energy than chemical reactions.

This was recognised by many people including Eddington, Gamow and others, but it was the paper of Bethe which set out the details of the proton-proton fusion reactions. Every second, the Sun converts 564 million tons of hydrogen to 560 million tons helium, the four million tons deficit is the nuclear binding energy released as radiation.

The first reaction in the cycle is the fusion of two protons into a deuteron, a positron and a mysterious particle called an electron neutrino. These are called pp neutrinos. The neutrino was first postulated in 1930 by Pauli to preserve the conservation laws of energy and momentum in nuclear reactions involving beta-decay where a neutron changes into a proton and electron or a proton changes into a neutron and positron.
It was found that such reactions did not seem to obey the laws of conservation of energy and momentum which is a very basic requirement in all physical processes. To resolve this, Pauli postulated a new particle, carrying away the extra energy, later dubbed the neutrino, as it was expected to have unusual properties like no charge and almost zero mass. However what is really strange about the neutrino is that it hardly interacts with matter and can literally go through light years of lead without any collision.

But detection of these neutrinos would be direct proof that such nuclear reactions are indeed taking place in the Sun’s superhot core. The point is that the neutrinos can come right through the Sun straight from the core as they are produced unlike the protons or radiation quanta. The nuclear reactions in the core produce gamma radiation, and not visible light. These high energy gamma ray photons take some hundred thousand years to traverse from the solar core to the surface, following a very zigzag path, undergoing some trillion collisions.

Detectors employed
The first detector by Raymond Davis used a million gallons of dry-cleaning fluid, a neutrino interaction converting a chlorine nucleus to one of Argon, an inert gas. Only a few events were expected every day.

The Kamiokande Detector in Japan, used several kilotons of ultra-pure water, a neutrino from the Sun, scattered an electron, raising it to high energy so that it produces radiation which can be detected and amplified in arrays of photomultiplier tubes. The above detectors detect the higher energy neutrinos, from a follow up set of nuclear reactions in the Sun to the pp, involving the Beryllium and Boron nuclei.

The pp neutrinos were first detected by the GALLEX and SAGE experiments, which used several tons of the metal gallium as detector. The reaction of the low energy pp neutrinos converted gallium nucleus to germanium which was converted to a liquid and separated to count the number of interacting neutrinos.

All these experiments, interestingly detected only half to one-third of the expected neutrino flux from the solar nuclear reactions. There were two possible main solutions: the first that our understanding of the solar interior was incomplete, maybe the core temperature was ten percent less or had a different chemical composition. The other more interesting idea was that the neutrinos could change their form or oscillate into two other neutrino types (the mu and tau types).

The Sun is in good health, producing thermonuclear energy at the expected rate and will continue to shine for another six billion years. Ironically, to peer deep into the processes going on in the solar core, we have to go deep underground to locate our detectors.
Again, the detection of the mono-energetic Be-7 neutrinos can enable the solar core temperature to be fixed to within half a percent to be compared with the factor of the uncertainty in the core temperature of the Earth.

Thanks to these sophisticated neutrino detectors, we have perhaps a much better understanding of what goes on deep inside the Sun than a few hundred kilometers below the surface of the Earth.