The Elusive Neutrino

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Neutrinos, or little neutral ones, are very close to being nothing at all. ttey are tiny, invisible packets of energy with no electric charge and almost no mass, traveling at nearly the velocity of light, ttese subatomic particles are so insubstantial, and interact so weakly with matter, that they streak through almost everything in their path, like ghosts that move right through walls. Unlike light or any other form of radiation, the neutrinos can move nearly unimpeded through any amount of material, even the entire Universe.

Each second, trillions upon trillions of neutrinos that were produced inside the Sun pass right through the Earth without even noticing it is there, tte indestructible neutrinos interact so rarely with the material world that almost nothing ever happens to them. Billions of ghostly neutrinos from the Sun are passing right through you every second, whether you are inside a building or outdoors, or whether it is day or night, and without your body noticing them, or them noticing your body.

tte neutrinos are the true ghost riders of the Universe. As American writer John Updike (1932- ) put it:

Neutrinos, they are very small.

"tteyhave no charge andhave no mass

And do not interact at all.

"tte Earth is just a silly ball

To them, through which they simplypass,

Like dust maids down a draftyhall.13

How do we know that such elusive, insubstantial particles even exist? ttey are required by a fundamental principle of physics, known as the conservation of energy. According to this rule, the total energy of a system must remain unchanged, unless acted upon by an outside force. We know of no process that disobeys this principle.

Nevertheless, in a type of nuclear decay process, called beta-decay, the nucleus of a radioactive atom disintegrates through the release of a beta particle, now known to be an electron, whose energy is less than that lost by the initial nucleus. Careful measurements failed to turn up the missing energy, which seemed to have vanished into thin air, and this suggested that energy might not be conserved during beta-decay. However, it turned out that a mysterious, invisible particle was spiriting away the missing energy. It was the elusive neutrino, whose existence was postulated more than half a century ago by Wolfgang Pauli (1900-1958), abrilliant Austrian physicist (Fig. 3.1).

Pauli proposed a "desperate way out" of the energy crisis. He speculated that a second, electrically neutral particle, produced at the same time as the electron, carried off the remaining energy, tte sum of the energies of both particles remains constant, so the energy books are balanced during beta-decay, and the principle of conservation of energy is saved. As Pauli expressed it in 1933:

"tte conservation laws remain valid, the expulsion of beta particles [electrons] being accompanied by a very penetrating [energetic] radiation of neutral [uncharged] particles, which has not been observed so far.14

FIG. 3.1 Wolfgang Pauli This Austrian physicist, Wolfgang Pauli (1900-1958), predicted the existence of the neutrino to solve an energy crisis in a type of radioactivity called beta-decay. He thought that the invisible neutrino would never be seen, but it was subsequently discovered as a byproduct of nuclear reactions on the Earth and in the Sun. (Courtesy of the American Institute of Physics Niels Bohr Library, Goudsmit Collection.)

FIG. 3.1 Wolfgang Pauli This Austrian physicist, Wolfgang Pauli (1900-1958), predicted the existence of the neutrino to solve an energy crisis in a type of radioactivity called beta-decay. He thought that the invisible neutrino would never be seen, but it was subsequently discovered as a byproduct of nuclear reactions on the Earth and in the Sun. (Courtesy of the American Institute of Physics Niels Bohr Library, Goudsmit Collection.)

When first discovered, the electrons emitted during beta-decay were called beta particles, to distinguish them from alpha particles (helium nuclei) and gamma rays (high-energy radiation) that are also emitted during radioactive decay processes. From their measured charge and mass, we now know that beta-rays are not rays at all but instead ordinary electrons moving at nearly the velocity oflight.

Pauli thought he had done "a terrible thing", for his desperate remedy postulated an invisible particle that could not be detected. Dubbed the neutrino, or "little neutral one" by the Italian physicist Enrico Fermi (1901-1954), the new particle could not be observed with the technology of the day, since the neutrino is electrically neutral, has almost no mass, and moves at nearly the velocity oflight. So the neutrinos were removing energy that would never be seen again. (Even in Pauli and Fermi's time, the observed high-energy shape of the emitted electron's energy spectrum indicated that the mass of the neutrino is either zero or very small with respect to the mass of the electron.)

In 1934 Fermi formulated the mathematical theory of beta-decay in a paper that was rejected by the journal Nature because "it contained speculations too remote from reality to be of interest to the reader." As beautifully described by Fermi, the decay process occurs when the neutron in a radioactive nucleus transforms into a proton with the simultaneous emission of an energetic electron and a high-speed neutrino. When left alone outside a nucleus, a neutron will, in fact, self-destruct in about 10 minutes into a proton, plus an electron to balance the charge and a neutrino to help remove the energy.

As far as anyone could tell, an atomic nucleus consists only of neutrons and protons, so the electron and neutrino seemed to come out of nowhere, ttey do not reside within the nucleus and are born at the time of nuclear transformation. No one knew exactly how the neutrinos were created.

How do you observe something that spontaneously appears out of nowhere and interacts only rarely with other matter? Calculations suggested that the probability of a neutrino interacting with matter, so one could see it, is so incredibly small that no one could ever detect it. To see one neutrino, you would have to produce enormous numbers of them at about the same time, and build a very massive detector to increase the chances of catching it. Although almost all of the neutrinos would still pass through any amount of matter unhindered and undetected, a rare collision with other subatomic particles might leave a trace.

Nuclear reactors, first developed in the 1940s, produce large numbers of neutrinos. Such reactors run by a controlled chain reaction in which neutrons bombard uranium nuclei, causing them to split apart and create more neutrons to continue the chain reaction, thereby producing large amounts of energy with an enormous flux of neutrinos in the process. A similar thing occurs in an atomic bomb, except that the chain reaction runs out of control with an explosive release of energy. If you place a very massive detector near a large nuclear reactor, and appropriately shield the detector from extraneous signals, you might just barely observe the telltale sign of the hypothetical neutrino.

tte existence of the neutrino was finally proven with Project Poltergeist, an experiment designed by Clyde L. Cowan (1919-1974) and Frederick Reines (1918-1998) of the Los Alamos National Laboratory in New Mexico, ttey placed a 10-ton (10,000-liter) tank of water next to a powerful nuclear reactor engaged in making plutonium for use in nuclear weapons. After shielding the neutrino trap underground and running it for about 100 days over the course of a year, Reines and Cowan detected a few synchronized flashes of gamma radiation that signaled the interaction of a few neutrinos with the nuclear protons in water.

tte neutrinos were not themselves observed, and they never have been, tteir presence was inferred by an exceedingly rare interaction. One out of every billion billion, or 1018, neutrinos that passed through the water tank hit a proton, producing the telltale burst of radiation.

In June 1956, Cowan and Reines telegraphed Pauli with the news:

We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta-decay of protons!15

And Pauli promptly sent them a case of champagne in recognition of their accomplishment. Half a century later, Reines was awarded the 1995 Nobel Prize in Physics for this accomplishment, but his colleague Cowan just did not live long enough to share it. tte inverse beta-decay mentioned in the telegram incidentally occurs when a nuclear proton absorbs a neutrino and turns into a neutron, at the same time emitting a positron, the anti-matter counterpart of the electron, which immediately annihilates with an electron and produces the radiation that was detected.

tte ghostly neutrino, which most scientists had thought would never be detected, had finallybeen observed, and thoughts turned to catching neutrinos generated in the hidden heart of the Sun.

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