Encyclopedia
Radioactive decay is the set of various processes by which unstable
atomic nuclei emit
subatomic particles . Decay is said to occur in the
parent nucleus and produces a
daughter nucleus. This is a random process, i.e. it is impossible to predict the decay of individual atoms.
The SI unit for measuring
radioactivity is the becquerel . If a quantity of radioactive material produces one decay event per second, it has an activity of one Bq. Since any reasonably-sized sample of radioactive material contains very many atoms, one becquerel is a tiny level of activity; numbers on the order of gigabecquerels are seen commonly. For example the curie, which was originally defined as the radioactivity of one gram of pure radium, is 37 gigabecquerels .
Explanation
The
neutrons and
protons that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The
electrostatic force is also significant. Of lesser importance is the
weak nuclear force. The
gravitational force has no influence on nuclear processes.
The interplay of these forces is very complex. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could fall into a lower-
energy arrangement. One might draw an analogy with a tower of
sand: while friction between the sand grains can support the tower's weight, a disturbance will unleash the force of
gravity and the tower will collapse.
Such a collapse requires a specific
activation energy. In the case of the tower of sand, this energy must come from outside the system, in the form of a gentle prod or swift kick. In the case of an
atomic nucleus, it is already present.
Quantum-mechanical particles are never at rest; they are in continuous random motion. Thus, if its constituent particles move in concert, the nucleus can
spontaneously destabilize. The resulting transformation alters the structure of the nucleus; thus it is a
nuclear reaction, in contrast to
chemical reactions, which involve changes in the arrangement of the outer
electrons of atoms.
Discovery
Radioactivity was first discovered in 1896 by the
French scientist
Henri Becquerel while working on
phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in
cathode ray tubes by
X-rays might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent
minerals on it. All results were negative until he tried using
uranium salts. The result with these compounds was a deep blackening of the plate.
However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken.
At first it seemed that the new radiation was similar to the then recently discovered X-rays. However further research by Becquerel,
Marie Curie,
Pierre Curie,
Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur.
For instance, it was found that an
electric or
magnetic field could split such emissions into three beams. For lack of better terms, the rays were given the
alphabetic names
alpha,
beta, and
gamma, names they still hold today. It was immediately obvious from the direction of electromagnetic forces that alpha rays carried a positive charge,
beta rays carried a negative charge, and
gamma rays were neutral. From the magnitude of deflection, it was also clear that alpha particles were much more massive than beta particles. Passing alpha rays through a thin glass membrane and trapping them in a
discharge tube allowed researchers to study the
emission spectrum of the resulting gas, and ultimately prove that alpha particles are in fact
helium nuclei. Other experiments showed the similarity between beta radiation and
cathode rays, and between gamma radiation and X-rays.
These researchers also discovered that many other
chemical elements have
radioactive isotopes. Radioactivity also guided Marie Curie to isolate radium from barium; the two elements' chemical similarity would otherwise have made them difficult to distinguish.
The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when an Serbo-Croatian-American electric engineer
Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to the X-rays. Fortunately his injuries healed later.
The genetic effects of radiation, including the effects on cancer risk, were recognized much later. It was only in 1927 that Hermann Joseph Muller published his research that showed the genetic effects. In 1947 he was awarded the
Nobel prize for his findings.
Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as
patent medicine; particularly alarming examples were radium
enema treatments, and radium-containing waters to be drunk as tonics.
Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood . By the 1930's, after a number of cases of bone-necrosis and death in enthusiasts, radium-containing medical products had all but vanished from the market.
Modes of decay
Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with positive charge
Z and atomic weight
A is represented as .
| Mode of decay | Participating particles | Daughter nucleus |
|---|
| Decays with emission of nucleons: |
| Alpha decay | An alpha particle emitted from nucleus | |
| Proton emission | A proton ejected from nucleus | |
| Neutron emission | A neutron ejected from nucleus | |
| Spontaneous fission | Nucleus disintegrates into two or more random smaller nuclei and other particles | - |
| Cluster decay | Nucleus emits a specific type of smaller nucleus larger than an alpha particle | + |
| Different modes of beta decay: |
| Beta-Negative decay | A nucleus emits an electron and an antineutrino | |
| Positron emission, also Beta-Positive decay | A nucleus emits a positron and a neutrino | |
| Electron capture | A nucleus captures an orbiting electron and emits a neutrino | |
| Double beta decay | A nucleus emits two electrons and two antineutrinos | |
| Double electron capture | A nucleus absorbes two orbital electrons and emits two neutrinos | |
| Electron capture with positron emission | A nucleus absorbs one orbital electron, emits one positron and two neutrinos | |
| Double positron emission | A nucleus emits two positrons and two neutrinos | |
| Transitions between states of the same nucleus: |
| Gamma decay | Excited nucleus releases a high-energy photon | |
| Internal conversion | Excited nucleus transfers energy to an orbital electron and ejects it | |
Radioactive decay results in a loss of
mass, which is converted to energy according to the formula . This energy is released as kinetic energy of the emitted particles.
Decay chains and multiple modes
The daughter nuclide of a decay event is usually also unstable, sometimes even more unstable than the parent. If this is the case, it will proceed to decay again. A sequence of several decay events, producing in the end a stable nuclide, is a
decay chain.
Many radionuclides have several different observed modes of decay. Bismuth-212, for example, has three. Thus a given nuclide may lead to several different decay chains.
Of the commonly occurring forms of radioactive decay, the only one that changes the number of aggregate protons and neutrons contained in the nucleus is alpha emission, which reduces it by four. Thus, the number of nucleons modulo 4 is preserved across any decay chain.
Occurrence and applications
According to the
Big Bang theory, radioactive isotopes of the lightest elements were produced very shortly after the emergence of the universe. However, these nuclides are so highly unstable that virtually none of them have survived to today. Most radioactive nuclei are therefore relatively young, having formed in
stars and during ongoing interactions between stable isotopes and energetic particles. For example, Carbon-14, a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and Nitrogen.
Radioactive decay has been put to use in the technique of radioisotopic labelling, used to track the passage of a chemical substance through a complex system . A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events.
On the premise that radioactive decay is truly random , it has been used in hardware random-number generators and is an invaluable tool in estimating the absolute ages of geological materials and young organic matter.
Radioactive decay rates
The
decay rate, or
activity, of a radioactive substance are characterized by:
Constant quantities:
- half life - symbol - the time for half of a substance to decay.
- mean lifetime - symbol - the average lifetime of any given particle.
- decay constant - symbol - the inverse of the mean lifetime.
Time-variable quantities:
- Total activity - symbol - number of decays an object undergoes per second.
- Specific activity - symbol - number of decays per second per amount of substance. The "amount of substance" can be the unit of either mass or volume.)
These are related as follows:
where
is the initial amount of active substance - substance that has the same percentage of unstable particles as when the substance was formed.
Activity measurements
The units in which activities are measured are: becquerel = number of disintegrations per second; curie = disintegrations per second. Low activities are also measured in disintegrations per minute .
Decay timing
See also: exponential decay
As discussed above, the decay of an unstable nucleus is entirely random and it is impossible to predict when a particular atom will decay. However, it is equally likely to decay at any time. Therefore, given a sample of a particular radioisotope, the number of decay events –dN expected to occur in a small interval of time dt is proportional to the number of atoms present. If N is the number of atoms, then the probability of decay is proportional to dt:
Particular radionuclides decay at different rates, each having its own decay constant . The negative sign indicates that N decreases with each decay event. The solution to this first-order differential equation is the following function:
This function represents exponential decay. It is only an approximate solution, for two reasons. Firstly, the exponential function is continuous, but the physical quantity N can only take non-negative integer values. Secondly, because it describes a random process, it is only statistically true. However, in most common cases, N is a very large number and the function is a good approximation.
In addition to the decay constant, radioactive decay is sometimes characterized by the mean lifetime. Each atom "lives" for a finite amount of time before it decays, and the mean lifetime is the arithmetic mean of all the atoms' lifetimes. It is represented by the symbol , and is related to the decay constant as follows:
A more commonly used parameter is the half-life. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. The half life is related to the decay constant as follows:
This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 1024 years for very nearly stable nuclides, to 10-6 seconds for highly unstable ones.
See also
External links
