Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
The problem of solar radiations remained one of the most puzzling riddles of science until the discovery of radioactive transformations and the artificial transformations of elements revealed to us tremendous sources of energy hidden in the depths of atomic nuclei. We have seen that practically every element can become a nuclear fuel; liberating tremendous amounts of energy, provided it can be heated up to millions of degrees. Now, while such temperatures are practically unattainable on the face of the earth, they are rather common in the world of stars. In our own sun, the temperature increases gradually from 6000° C at the surface to twenty million degrees in the centre.
Two nuclear physicists, H. Bethe and C. Weizsaecker, simultaneously found out that the nuclear process known, as "carbon cycle" is responsible for the energy production in the sun. This thermonuclear process is not limited to a single nuclear transformation, but consists of a sequence of transformations forming a reaction chain. A most important feature of this process is that it is a closed circuit, returning to the starting point after every six steps. The main participants of the process are the nuclei of carbon and of nitrogen, together with protons with which they collide.
Briefly the cycle is:
- A proton on colliding with an atom of normal carbon (C12) liberates some subatomic energy in the form of gamma rays and transforms the atom to the lighter isotope of nitrogen (N13).
- The nucleus of (N13) being unstable adjusts itself by emitting a positive electron, or positive beta particle and becoming the stable nucleus of the heavier isotope or carbon (C13), which is known to be present in small quantities in ordinary coal.
- This carbon isotope collides with another proton and is transformed into normal nitrogen (N14) with additional release of energy in the form of gamma rays
- In the next step, the nucleus of nitrogen collides with still another proton (third) and gives rise to an unstable oxygen isotope (O15).
- This very rapidly transforms into stable N15 by emitting a positron.
- Finally N15 receiving in its heart the fourth proton splits into two unequal parts one of which is the C12 nucleus with which the process started and the other is a helium nucleus or alpha particle which is composed of two protons and two neutrons.
The same result would have been obtained if the process was started with normal nitrogen atom instead of carbon.
| [1] Incidentally this particular reaction is quite well known to nuclear physicists and has been obtained under laboratory conditions by the use of artificially accelerated high energy protons. |
The net result of the chain of reaction [1] is the formation of one nucleus of helium from the four protons, which entered the process cycle successively accompanied with liberation of energy. The nuclei of carbon and nitrogen in the closed circuit of reactions are forever being regenerated Thus the whole process may be described as the transformation of hydrogen (a proton is the nucleus of hydrogen atom) into helium due to very high temperatures assisted by the catalystic action of carbon and nitrogen.
It is shown that at the temperature of 20 million degrees the energy liberation in the above circuit reaction coincides with actual amount of energy radiated by our sun. Since the astrophysical evidence renders all other possible nuclear reactions inconsistent, it may be accepted that the carbon-nitrogen cycle described above represents the process mainly responsible for the generation of the solar energy. It should also be noted that interior temperature of the sun, the complete circuit requires about five million years.
