The neutrino hypothesis has turned out to be completely successful. The neutrino
mass was not expected to be more than a small fraction of the electron mass because
KEmaxis observed to be equal (within experimental error) to the value calculated
from the parent-daughter mass difference. The neutrino mass is now believed to be
the mass equivalent of at most a few electronvolts. The interaction of neutrinos with
matter is extremely feeble. Lacking charge and mass, and not electromagnetic in
nature as is the photon, the neutrino can pass unimpeded through vast amounts of
matter. A neutrino would have to pass through over 100 light-yearsof solid iron on
the average before interacting! The only interaction with matter a neutrino can
experience is through a process called inverse beta decay, which we shall consider
shortly. Neutrinos are believed to outnumber protons in the universe by about a
billion to one.438 Chapter Twelve
A positron emission tomography (PET) scan of the brain of a patient with Alzheimer’s disease.
The lighter the area, the higher the rate of metabolic activity. In PET, a suitable positron-emitting
radionuclide (here the oxygen isotope^15 O) is injected and allowed to circulate in a patient’s body.
When a positron encounters an electron, which it does almost at once after being emitted, both
are annihilated. From the directions of the resulting pair of gamma rays the location of the an-
nihilation, and hence of the emitting nucleus, can be found. In this way, a map that is accurate
to several millimeters of the concetration of the radionuclide can be built up. In a normal brain,
metabolic activity produces a similar PET pattern in each hemisphere; here, the irregular
appearance of the scan indicates degeneration of brain tissue.bei48482_ch12.qxd 1/23/02 12:07 AM Page 438 RKAUL-9 RKAUL-9:Desktop Folder: