38.15 - Radioactivity and radiation
Radioactive decay: A nucleus spontaneously
emits particles or high-energy photons and
either changes identity or becomes less excited.
Transmutation: Changing of one element to
another after Į or ȕ radiation is emitted.
Fission is not the only way that a nucleus with an unstable combination of protons and
neutrons can change into a more stable configuration. An unstable nucleus may instead
spontaneously emit a charged particle or a high-energy photon in order to reach a more
stable state. It changes via a nuclear reaction. Isotopes that change (decay) like this are
said to be radioactive. It is possible for the isotope to become a different chemical
element after the decay, a process known as transmutation.
The outgoing radiation can be classified as alpha rays,beta rays, or gamma rays. (They
are represented by the Greek letters Į,ȕ, and Ȗ.) Different decay processes result in
different forms of radiation. Alpha and beta rays consist of matter particles, while
gamma rays are photons (light particles).
In an alpha decay, the initial radioactive isotope decays into a different element by
emitting an Į particle. An Į particle is made up of two protons and two neutrons. It is a
helium-4 nucleus.
The initial isotope is known as the parent. Because it loses two protons, its atomic
number is reduced by two (the mass number is reduced by four). Since the nucleus now
has a different number of protons, it becomes another element í it has transmuted. The
newly-formed nucleus is known as the daughter.
Alpha decay occurs most commonly in heavy nuclei whose ratio of protons to neutrons,
Z/N, is too large, making them unstable. An Į particle is a very stable particle, and the
daughter nucleus that is left behind is more stable (tightly bound) than the parent. To
put it another way, the net result of the radioactive decay is a reduction of the ratio Z/N.
Beta decay is characterized by the emission of an electron or antielectron. There are
two types of beta decay, negative and positive. Negative ȕ emission is represented in
Concept 4. This occurs when a neutron inside the nucleus decays into a proton, an
electron (the beta ray), and an almost zero-mass, uncharged particle known as an
antineutrino. The antineutrino’s interaction with matter is so weak that it is very hard to
detect, and so it is customarily left out of nuclear equations.
The emitted electron did not exist in the nucleus beforehand, and is not one of the
orbital electrons in the parent nucleus. When a neutron inside the nucleus turns into a
proton, electron, and neutrino (and then emits the electron as a negative beta ray), its
number of protons increases by one. The mass number stays the same. The released
electron usually zips away, leaving behind a daughter atom with a net positive charge.
In positive ȕ emission, the nucleus emits an antielectron (also called a positron).
Antielectrons are essentially the same as electrons, except they have a change of
positivee. The decay process is shown in Concept 5. A proton inside the atom decays
into a neutron, an antielectron (the beta ray), and a neutrino. Like an antineutrino, a
neutrino has no charge and almost zero mass. Its interaction with matter is also weak
and it too is customarily left out of nuclear equations.
In emitting either type of ȕ ray, the initial isotope changes atomic number, so ȕ decay
results in transmutation.
In gamma decay, the radioactive isotope emits a high-energy photon, also known as a Ȗ
ray.
Since the atomic number stays the same, the atom is the same chemical element after the decay. In this case it is the nuclear energy that
changes in the decay. A nucleus can have different energy states. When a nucleus changes from an excited, high energy state to a lower one,
a photon is emitted.
This is similar to the case when an electron falls from one energy level to another. A notable difference is that nuclear energy levels are much
more widely spaced, on the order of millions of electron volts as opposed to say five or ten. This means the emitted photon, called a gamma
ray, is much more energetic than the photon emitted by an excited atom.
A gamma decay is represented in Concept 6. An excited nucleus is denoted by an asterisk “*” after the usual symbol. How does the nucleus
get into an excited state? This usually happens after another kind of decay. In many cases of Į and ȕ decay, the product nucleus is in an
excited state, after which it emits a Ȗ ray and transitions to a lower state or to the ground state. Because the photon is electrically neutral,
transmutation does not occur during gamma radiation.
Unstable nuclei
Too massive or wrong ratio of protons to
neutronsRadioactive decay with Į or ȕ ray
Nucleus decays by emitting charged
particle
·Radioactive element has transmuted
(changed to another element)Alpha radiation
Nucleus emits alpha particle (2 protons,
2 neutrons)
Number of protons in nucleus decreases
by 2Mass number decreases by 4
·Transmutation