Experiment may help size up neutrinos

Our planet is bombarded every second with a large number
of chargeless, seemingly massless, particles that
originate in nuclear fusion reactions that power the sun.
They’re called neutrinos.

According to The Standard Solar Model — the most
substantiated model of the sun — the sun should emit
around three times more neutrinos than are actually
measured on Earth. They are a source of great interest
for scientists who seek to better understand elementary
particles and the physics of the sun. Indeed, one of
the recipients of this year’s Nobel Prize in Physics
was Raymond Davis, who first drew attention to the
neutrino shortfall.

Three major research efforts (carried out by the
underground large detector complexes at Sudbury
Neutrino Observatory (SNO) in Canada, the U.S. National
Underground Science Laboratory at Homestake and the
Super-Kamikande in Japan) have measured the number of
neutrinos that actually reach Earth as a result of a
specific reaction in the sun (thus the experiments
are sensitive to only a small fraction of the solar
neutrino spectrum). To better understand the shortfall
of neutrinos on Earth, scientists have been trying to
determine precisely how many neutrinos are emitted as
a result of this reaction in the lab, so as to compare
them with the number that actually reach Earth as
measured by SNO, Kamiokande and Homestake.

However, mostly due to difficulties with the preparation
and homogeneity of a central component in the reaction
(the target made of the radioactive isotope of mass 7 of
the beryllium element), large discrepancies persisted.
The present experiment, conducted by Prof. Michael Hass
of the Weizmann Institute’s Particle Physics Department,
uses in a novel way a 2 mm diameter target of the
beryllium 7 nuclei, prepared at the ISOLDE (CERN)
laboratory and brought to the Van de Graaff accelerator
of the Weizmann Institute, Israel, for the measurement
of the reaction. The results of this measurement, with
less than a 4% margin of error, may draw to a close this
reaction’s standing as the largest source of error in
the Standard Solar Model estimates of the measured
neutrino flux.