FoundationalConceptsNeuroscience

oxygen, eleven for sodium, twenty for calcium, twenty-six for iron,
eighty-eight for radium, ninety-two for uranium, ninety-nine for
einsteinium, and so forth (see Chapter 3). In addition to protons, all
atomic nuclei contain neutrons—the only exception being the lightest
isotope of hydrogen, consisting of a single proton and no neutrons.
The sum of the number of protons and the number of neutrons for a
given atom is called the atomic mass. Each chemical element, defined
by a specific number of nuclear protons, will have several different
isotopes characterized by different numbers of neutrons in their
nuclei.
Some combinations of protons and neutrons are stable and don’t
decay, and others are unstable and will transform by emitting a high-
energy particle of one kind or another. This is radioactive decay, and
radioactivity is the name given to the high-energy particles emitted
by atoms during the decay process. There are several kinds of radioac-
tive decay: alpha decay, when an unstable nucleus emits a blob made
of two protons and two neutrons; beta decay, when an unstable nu-
cleus emits either an electron or a positron; and gamma decay, when
an unstable nucleus emits a gamma-ray photon.
PET uses unstable elements that undergo beta decay by emitting a
positron. The result is a new element that has the same atomic mass
but is one unit lower in atomic number. This is because the loss of a
positive charge in the form of a positron (e*) effectively converts one
of the nuclear protons into a neutron. PET currently uses radioactive
forms of the elements carbon, oxygen, and fluorine—the isotopes
carbon-11, oxygen-15, and fluorine-18. (The numbers give the atomic
masses of the isotopes. For example, carbon-11 has 6 protons—
because carbon is defined as having 6 protons—and 11 minus 6 = 5
neutrons. The most common isotope of carbon is carbon-12, with 6
protons and 6 neutrons. Carbon-12 is stable and does not decay.) Here