Neutrinos
Lecture 3
Last time, we discussed the fusion reactions that take place in the center of the Sun. One of the byproducts of these reactions is the neutrino, a particle that we will discuss much more today. The neutrino is produced in the first step of the proton-proton chain:
Proton + proton à deuteron + neutrino + positron
The neutrino is formed when one of the protons is turned into a neutron.
What sort of force can make such a strange transition occur?
The Four Fundamental Forces
In nature, there are four forces that are fundamental. By this, we mean that they are inherent to our Universe and cannot be reduced to other forces. The weakest of these forces is gravity, which is also probably the most familiar to you. Gravity is the force that causes all matter to be attracted to all other matter. This attraction of matter to matter is an inherent property of matter and of our Universe. The force of gravity that is exerted by some chunk of matter, such as a star, drops off as you move away from the star, but never completely disappears. We see the effects of this force in the large-scale distribution of material within our Universe, as well as in the fact that we don’t all fall off the Earth!
The electromagnetic force is next. This force causes the attraction of oppositely-charged particles and the repulsion of similarly-charged particles. This is the force that holds electrons and atomic nuclei together, allowing the existence of atoms. While the force exerted by a charged particle is like that caused by matter in that it never completely disappears, because the electromagnetic force can be either attractive or repulsive, these two effects tend to cancel out on larger scales.
Last time, we mentioned the strong nuclear force. This is the force that binds nuclei together. Whenever two protons, two neutrons, or a proton and a neutron get sufficiently close (10-15 m), they are attracted to one another by the strong nuclear force. It is the strongest of all of the four fundamental forces. Without this force, atoms more complicated that hydrogen-1 would not exist. Although this is the strongest force of nature, we have no direct experience with it because it only applies on extremely short distance scales, much smaller than anything we can directly see or intuitively know about.
Nevertheless, it is just as fundamental as gravity to our Universe.
Finally, there is a force called the weak nuclear force. This force is weaker than the strong nuclear force, but stronger than the electromagnetic force. The weak nuclear force is the force that acts to convert protons into neutrons, neutrons into protons, etc. Like the strong nuclear force, the weak nuclear force also acts on extremely short distance scales, putting it, too, beyond the realm of our everyday experience.
Forces and Neutrinos
Neutrinos interact with other stuff primarily via the weak nuclear force. Neutrinos are not charged and they are not nucleons, so the electromagnetic force and the strong nuclear force have no effect on them. Although they do have a very very tiny mass, this mass is so miniscule that gravity is fairly unimportant to these particles.
As the Sun fuses hydrogen into helium via the proton-proton chain, something like
2 x 1038 neutrinos are produced every second! These neutrinos escape the core of the Sun and reach the Earth in just over 8 minutes, moving at speeds just below the speed of light.
How can they pass through the Sun so easily?
Well, the typical size of an atom is about 10-10 meters. To interact with any of the particles in the atom, the neutrino must pass close enough for the weak nuclear force to be relevant. This means that it must pass within 10-18 à 10-15 meters of the nucleus or the electrons within the atom. Otherwise, it “sees” only empty space. Because the neutrino is very unlikely to hit one of these particles just right, in the vast majority of cases, it will fly right through the atom, unaffected.
When they reach the Earth, about 70 billion neutrinos pass through each cm2 every second! Most pass right through the Earth and out the other side.
How do we detect these things?
With great difficulty, we can detect a tiny fraction of the neutrinos that encounter specially-designed experiments.
Homestake gold mine in SD: the first neutrino detector built. It contained 100,000 gallons of dry cleaning fluid; the specific compound is called tetrachloroethylene (C2Cl4). Whenever one of the impinging neutrinos hit the nucleus of a Chlorine-37 atom just right, the atom was turned into Argon-37:
Chlorine-37 + neutrino à Argon-37
(17 protons (18 protons
20 neutrons) 19 neutrons)
This reaction is sort of the reverse of what we see in the sun. Instead of a proton being turned into a neutron:
p à n + neutrino + positron
we see the opposite:
n + neutrino à p + electron
After a suitable time interval, the argon atoms were counted, thus providing a measure of the number of interactions with neutrinos.
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GALLEX (US & European collaboration): a similar concept, except that this detector uses gallium instead of chlorine. Here the reaction is:
Gallium-71 + neutrino à Germanium-71
(31 protons (32 protons
40 neutrons) 39 neutrons)
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Another type of neutrino detector uses water to observe the presence of neutrinos. Both the Super Kamiokande detector in Japan and the Sudbury Neutrino Experiment in Sudbury, Ontario operate on this principle. When an incoming neutrino has a close encounter with one of the electrons in the water, the neutrino gets sped up and it releases a cone of light known as Cerenkov radiation. Very sensitive light detectors line the water tank, ready to detect these faint signals. Based on the time when each light signal is observed and the direction in which the light is traveling, the time of the neutrino encounter and the direction that the neutrino came from can be determined.