Neutrino mass: a tritium disagreement.

Neutrino mass: A tritium disagreement

Back around 1930, difficulties in balancingenergy in beta decay of atomic nuclei led Wolfgang Pauli to postulate the existence of a subatomic particle that had no rest mass, which Enrico Fermi named "little neutral one' or neutrino. Today, experiments with beta decay are trying to find out whether perhaps Pauli was a little bit wrong, and the neutrino does have a very small rest mass. A neutrino mass would have two important implications: It is required in some current attempts to unify all of particle physics in a single framework, and it would also be one way to make the universe closed, as a lot of cosmologists would like it to be.

Technical difficulties in evaluating theenergy data in the different experiments have led to a disagreement over whether a mass exists. The two latest results, published in the May 18 PHYSICAL REVIEW LETTERS, still disagree. One says yes, the other a kind of maybe.

In beta decay, a neutron inside anatomic nucleus decays into a proton emitting an electron (which in radiology is often called a beta ray). The beta decay experiments that are searching for neutrino mass all use tritium, the heaviest, rarest and only radioactive isotope of hydrogen, which is also the simplest nucleus susceptible to beta decay. The disagreements seem related to how they take their tritium.

One of the two current experiments,which is being conducted at the Institute of Theoretical and Experimental Physics in Moscow by S. Boris and 10 others, uses tritium that is part of the solid amino acid valine. The other, at Los Alamos (N.M.) National Laboratory, in which John F. Wilkerson, Thomas J. Bowles and R. G. Hamish Robertson are principal participants, uses gaseous tritium. A third experiment, being conducted in Zurich by M. Fritschi et al., uses tritium embedded in solid carbon.

The Moscow group reports a "central'--that is, most probable--value for the neutrino mass of 30 electron-volts (eV), but it says the number could lie anywhere in the range from 17 to 40 eV, depending on assumptions made in the data evaluation. The Los Alamos experimenters say their result contradicts the Soviets' central figure, but does not rule out the lower end of their range--that is, a neutrino mass less than 26.8eV could exist, but the Los Alamos group currently is not capable of determining the existence of a mass below that point.

These are all experiments with goodstatistics, Wilkerson told SCIENCE NEWS. Therefore the disagreement has to arise from how the experiments are set up. One reason for using gaseous tritium, Wilkerson says, is that it makes it easier to account for the effects of the final state of the decayed nucleus (which becomes helium 3). After decay the nucleus does not always go to its ground state or least-energy state, and this can have an effect on the energy spectrum of the beta rays that is similar to the effect a small neutrino mass would have. In the solid, the final-state effects are "impossible to calculate,' Wilkerson says. They are more manageable in the gas.

However, gaseous tritium is in veryshort supply, and so are people who know how to work with it--it is both radioactive and poisonous. Los Alamos does a lot of nuclear fusion work, and so has both tritium and people experienced in working with it. In the experiment, molecular tritium moves down a pipe, 3.7 meters long and 3.8 centimeters in diameter. The entire experiment is inside a room-sized tank that chills it to 160 kelvins to increase the incidence of beta decays as the gas moves along. As electrons are emitted, a magnetic field running along the pipe constrains them to spiral their way down the pipe without bouncing off the walls. As they leave the pipe they are counted by spectrometers.

The experimenters believe the statisticsof the apparatus are so good that, after making some adjustments, they can now try to push the limit on neutrino mass closer to zero, down to 10 eV. As to whether there really is a neutrino mass, Wilkerson says, "We have no bias one way or the other.'