Inquiring Minds

Questions About Physics

What is spin? How do you detect neutrinos?

You Wrote:
Hi: I am an old man and would like to know what is spin 1/2 and 2/3 is that two thirds and one half or one two and two thirds or what? is this made by see it in a chamber of some detector? Or maybe is it made up of theoretical spin? How do you determine exactly the spin of a neutrino? Can I make a detector to look for neutrinos from the sun? I have a PMT and can build electronic circuits. Could I help some students. I am 60 %percent disabled but would like something to do in physics. I love particle physics and Richard Feynmann and Doctor Einstien are some of my past guys that I admired before they passed on. I am a ham, KB7IOH and used to rebuild radio stations and sound studios. Then I became an analytical chemist. Now I want to know the nuts and bolts so to speak of this stuff. Can I be of some help? please.

Jim Hardy

Dear Mr Jim Hardy,

Judy Jackson of our Public Information Office passed me your enquiry, and my first reaction is that it could take a book to answer your questions properly! As I don't have time to write a book today (but hope to one day) I must be brief, and hope that you are not too dissatisfied. First, about the "spin" of particles. This is not really like the spin of large objects like the Earth or a top, because a particle like an electron or neutrino has no known finite size, it seems to be like a point to all scales we can probe. However like the Earth, electrons behave like magnets with a North and South pole. If you have them in a magnetic field they can align themselves with their magnetic axis either parallel or antiparallel to the field. In an atom, the atomic nucleus makes a magnetic field and the electrons can orient themselves in both ways with slightly different energies. The effects of this were seen in atomic spectra (light spectra) and explained by supposing that the electrons have what is called an "intrinsic angular momentum" or "spin". The units of angular momentum are the same as Planck's constant h/2.pi and in Quantum Theory can only have values that are integer : 0,1,2,3,... or half integer: one-half, three-halves etc.

All particles come in two classes depending on whether their spins are integer (particles of force fields such as the photon (electromagnetic forces), gluon (strong forces in nuclei), weak bosons W/Z (some radioactive forces)) or half integer (particles of matter such as electrons, protons, neutrons also quarks and neutrinos). All the matter particles found so far have the same spin one-half. The spin of neutrinos was worked out using a law of physics that angular momentum is conserved, i.e. it is the same before and after any process. Neutrinos are emitted in Nuclear beta decay and to conserve angular momentum they had to had this spin of 1/2 (one-half). In fact the conservation of angular momentum and energy laws led to the invention of the neutrino idea long before they were seen to interact, i.e. to be detected.

It is VERY difficult and expensive to detect neutrinos. They interact so weakly with everything that you can shoot millions through the Earth and they generally all come out the other side as if the Earth were not there, and no apparatus put in their way would see any effect. But as their energy goes up they do interact more readily. Fermilab is working on ways to make very intense neutrino beams and detectors weighing thousands of tons so that some interactions can be detected. Deep under a mountain in Japan a huge deep artificial lake of very pure water, viewed by thousands of very large photomultipliers, can detect neutrinos from the Sun and "see" the Sun even when it is dead of night. I am sorry to say that there is no way you would be able to detect neutrinos with one photomultiplier and home made apparatus; these detectors cost tens of millions of dollars and are operated by a hundred or more physicists.

I can imagine that there are other areas of physics that a radio ham might have more of a chance in, but I am not at all an expert. You probably know more than I do about bouncing radio waves off meteor trails. It is conceivable (but I think not yet demonstrated) that very rare and extremely high energy cosmic ray showers in the atmosphere emit a pulse of radio waves. But I imagine that to demonstrate that one would have to correlate the radio data with other detectors, like the so-called Fly's Eye optical detector in Utah or the future "Auger Project".

Scientific American often has very good articles about elementary particles, I can recommend it.

Best regards,
Michael Albrow

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last modified 12/11/1999