The Nobel Prize in Physics 2002


The Nobel Prize in Physics 2002
  Questions and Answers Related to the Prize

What is the mass of the neutrino?

We do not know today if the neutrino has a non-zero mass or not. Recent results from experiments seem to indicate that neutrinos do have a very small but non-zero mass.

What is the flux of neutrinos from the sun at the earth?

About 60 billion (6×1010) per second per cm2. Only one in 1000 billion solar neutrinos interact during the way through the earth.

How many kinds of neutrinos are there?

There are three kinds of light neutrinos, the electron neutrino (n), the muon neutrino (m) and the tau neutrino (t). For every neutrino there is also an antineutrino.

Who proposed the neutrino and why?

Wolfgang Pauli (Nobel Prize 1945) proposed the existence of the neutrino in 1930. In nuclear beta-decay when a nucleus increases its number of protons with one and decreases its number of neutrons with one, a negatively charged electron is emitted. Pauli suggested that a neutral particle, the neutrino, must be emitted together with the electron in order to explain that the electron energy was not the same in all decays.

Why did it take so long to detect the neutrino?

The neutrino interacts only very weakly with matter and a very large neutrino flux is needed in order to register a neutrino interaction in an experiment. Nuclear reactors that became available in the ’50s produce very high fluxes of neutrinos, of the order of 1015 per second per cm2, and in 1955 the neutrino was detected in an experiment by F. Reines (Nobel Prize 1995).

What is a neutron star?

A Supernova explosion can result in either a neutron star or a black hole. A neutron star is 1000 billion times more dense than a normal star and consists of closely packed neutrons. The neutron star has a diameter of 10 – 15 km and a mass of several solar masses.

What are X-rays?

Like normal light X-rays are electromagnetic waves but with much shorter wavelength, in the range 0.01 – 1 nm (visible light typically 500 nm).

Why are X-rays called X-rays?

When discovered, X-rays were very mysterious – one did not know what they were. Therefore the name X-rays.

How are X-rays produced?

When charged particles meet strong electromagnetic fields X-ray photons can be emitted. Such fields exist close to the atomic nuclei, and most X-ray sources are made from electrons hitting a piece of metal. Magnets can also have strong fields and charged particles passing through a magnetic field can produce X-ray photons.

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