GCE Physics

Radioactivity

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What are (radio)nuclides?

A nuclide is species of atomic nucleus characterized by the number of protons (Z) and the number of neutrons (N). Nuclides with identical proton number but differing neutron number are called isotopes.

If Z or N changes, the atom becomes a different nuclide. Getting on for 4000 nuclides have been identified. Most of them are radionuclides, meaning they are unstable and undergo radioactive decay.

There are nearly 4000 known isotopes and 580 nuclear isomers.

What are metastable isomers ?

A nuclear isomer that decays very quickly (i.e. has a very short half-life) is sometimes referred to as being metastable and is marked with an "m".

For example, the two isomers of cobalt-58 are:

27-Co-58, half-life 71 days 27-Co-58m, half-life 9 hours

Isomers and metastable states

A nuclear isomer is a long-lived excited state of an atom's nucleus - a state in which decay back to the nuclear ground state is inhibited. (In chemistry, an isomer is one of two or more compounds having the same molecular formula but different structures i.e. arrangements of atoms in the molecule.)

The nucleus of an isomer holds an enormous amount of energy. A nuclear isomer is a metastable or isomeric state of an atom caused by the excitation of a proton or neutron in its nucleus so that it requires a change in spin before it can release its extra energy. Most nuclear isomers are very unstable, and radiate away the extra energy immediately (of the order of 10-12s). As a result, the term is usually restricted to mean isomers with half-lives of at least 10-9 seconds. Quantum mechanics predicts that certain atomic species will possess isomers with unusually long lifetimes even by this stricter standard, and so have interesting properties. Nuclear isomers or metastable nuclear states have been identified in many nuclei throughout the periodic table. Nuclear isomers are usually produced through fusion reactions.

The only stable nuclear isomer is Ta-180m, which occurs naturally in tantalum at about 1 part in 8300. Its half-life is at least 1015 years: it may in fact be entirely stable. The origin of this isomer is mysterious, though it is believed to have something to do with supernovas.

In radioactive processes, particles or electromagnetic radiation are emitted from the nucleus. The most common forms of radiation emitted have been traditionally classified as alpha (a), beta (b), and gamma (g) radiation. Nuclear radiation also occurs in other forms, including the emission of protons or neutrons or spontaneous fission of a massive nucleus.

Of the nuclei found on Earth, the vast majority is stable. This is so because almost all short-lived radioactive nuclei have decayed during the history of the Earth.

There are approximately 270 stable isotopes and 50 naturally occurring radioisotopes (radioactive isotopes). Thousands of other radioisotopes have been made in the laboratory.

Nuclear stability

There are 82 stable elements and about 275 stable isotopes of these elements.

When a combination of neutrons and protons, which does not already exist in nature, is produced artificially, the atom will be unstable and is called a radioactive isotope or radioisotope. There are also a number of unstable natural isotopes arising from the decay of primordial uranium and thorium. Overall there are some 1800 radioisotopes.

At present there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially.

Radioisotopes can be manufactured in several ways. The most common is by neutron activation in a nuclear reactor. This involves the capture of a neutron by the nucleus of an atom resulting in an excess of neutrons (neutron rich). Some radioisotopes are manufactured in a cyclotron in which protons are introduced to the nucleus resulting in a deficiency of neutrons (proton rich).

The nucleus of a radioisotope usually becomes stable by emitting an alpha and/or beta particle (or positron). These particles may be accompanied by the emission of energy in the form of electromagnetic radiation known as gamma rays. This process is known as radioactive decay.

 

Nuclei with even number of protons are more stable that those with an odd number. Similarly, nuclei with and even number of neutrons re more stable than those with an odd number.

So there are some nuclides with odd numbers of both types that can emit both negative and positive beta particles.

In nuclear physics, a magic number is a number of nucleons such that they are arranged into complete shells within the atomic nucleus. The seven known magic numbers as of 2005 are:

2, 8, 20, 28, 50, 82, 126.

Atomic nuclei consisting of such a magic number of nucleons have a lower average binding energy per nucleon than one would expect based upon predictions such as the Semi-empirical mass formula and are hence more stable against nuclear decay. Nuclei which have both neutron number and proton (atomic) number equal to one of the magic numbers are called doubly magic, and are especially stable against decay. For example, Lead 208 is especially stable as it has both 82 protons and 126 neutrons.

Emission of alpha- or beta-particle

—›    gamma- ray commonly emitted, because daughter nucleus produced in excited state

—›    for many types of radionuclide, no gamma- ray emitted, because daughter nuclei normally produced in ground state.

 

Our world is radioactive and has been since it was created. Over 60 radionuclides (radioactive elements) can be found in nature in the following general categories:

  1. Very long-lived radionuclides - e.g. K-40 (τ½ = 1.28 billion years). Some of these were created before or during the formation of the solar system over 4.6 billion years ago, e.g. Rb-87 (τ½ = 48.8 billion years), U-238 (t½ = 447 billion years), and Os-186 (τ½ = 2 million billion years).
  2. Radionuclides still being synthesized. These can be:

Some modes of radioactive decay

Alpha emission
Beta-minus emission
Beta-plus emission
Electron capture
Internal conversion
Spontaneous fission
Proton emission
Neutron emission

General

Alpha-rays

Often the α particle is associated with one or more γ photon. These γ photons have specific energies that uniquely identify the nuclide.

Negative beta-rays

Positive beta-rays

Electron capture

Nuclides

There are about 4100 known nuclides and several thousand more unknown ones. Each nuclide is a variation of one of the 115 known chemical elements. (Thus the average number of known nuclides each element can exist as is about 27. Another way of expressing this is to say that each element has an average of about 27 isotopes.)

About one tenth of the 3000 nuclides are stable (that's 270 of them). About 50 of the rest (which are radioactive) occur in nature, the remaining 2655 being produced artificially.

Isotopes are nuclides that lie next to each other horizontally in this chart. Such nuclides that have the
same number of protons are all of the same chemical element. Example: Carbon-13 and Carbon-14.

Click on animated version of nuclide chart.

Magic numbers

One factor that influences the stability of a nuclide is the ratio of neutrons to protons. (Nuclei that contain either too many or too few neutrons are unstable.) Another factor that affects the stability of nuclides can be understood by examining patterns in the numbers of protons and neutrons in stable nuclides.

Half of the elements in the periodic table must have an odd number of protons because atomic numbers that are odd are just as likely to occur as those that are even. In spite of this, about 80% of the stable nuclides have an even number of protons. Very few elements with an odd atomic number have more than one stable isotope. Stable isotopes abound, however, among elements with even atomic numbers. Ten stable isotopes are known for tin (Z = 50), for example. It is also interesting to note that 91% of the stable isotopes of elements with an odd number of protons have an even number of neutrons. These observations suggest that certain combinations of protons and neutrons are particularly stable.

There are magic numbers of electrons. Electron configurations with 2, 10, 18, 36, 54, and 86 electrons are unusually stable. There also seem to be magic numbers of neutrons and protons. Nuclei with 2, 8, 20, 28, 50, 82, or 126 protons or neutrons are unusually stable. This observation explains the anomalously large binding energies observed for , O-16, and Ne-20. In each case, the nuclide has an even number of both protons and neutrons. Ne-20 has a magic number of nucleons when both protons and neutrons are counted. He-4 and O-16 have magic numbers of both protons and neutrons. The resulting stability of the He-4 nucleus might explain why so many heavy nuclei undergo alpha-particle decay by ejecting an He-4 doubly positive ion from the atom's nucleus.

If nuclei tend to be more stable when they have even numbers of protons and neutrons, it isn't surprising that nuclides with an odd number of both protons and neutrons are unstable. K-40 is one of only five naturally occurring nuclides that contain both an odd number of protons and an odd number of neutrons. This nuclide simultaneously undergoes the electron capture and positron emission expected for neutron-poor nuclides and the electron emission observed with neutron-rich nuclides.

Natural radioactivity

Only 18 radioactive isotopes with atomic numbers of 80 or less can be found in nature.

With the exception of C-14, which is continuously synthesized in the atmosphere, all these elements have lifetimes longer than a billion years. Although these isotopes all undergo radioactive decay, they decay so slowly that reasonable quantities are still present, 4.6 billion years after the planet was formed.

Another 45 natural radioactive isotopes have atomic numbers larger than 80. These nuclides fall into three families. All three families undergo a series of decays and end up with a stable isotope of lead.

The first family is called the thorium series or the 4n series, because all its members have a mass number that is a multiple of 4. The parent nuclide is Th-232, which decays to form Ra-228, which decays to form Ac-228, which decays to Th-228, and so on, until the stable Pb-208 nuclide is formed.

The second family of radioactive nuclei is called the uranium series or the 4n + 2 series. It starts with U-238 and decays to form the stable Pb-206 nuclide.

The third family, known as the actinium series or the 4n + 3 series, starts with U-235 and decays to Pb-207.

( A 4n + 1 series once existed, which started with Np-237 and decayed to form the only stable isotope of bismuth, Bi-209. The half-life of every member of this series is less than 2 million years, however, so none of the nuclides produced by the decay of neptunium remain in detectable quantities on the earth. ) The neptunium series starts with the artificial isotope plutonium-241, which decays to neptunium-237, and ends with bismuth-209.

See www.eserc.stonybrook.edu/ProjectJava/Radiation for a java applet on the radioactive series.

Induced Radioactivity

In 1934, Irene Curie (the daughter of Pierre and Marie Curie) and her husband, Frederic Joliot, announced the first synthesis of an artificial radioactive isotope. They bombarded a thin piece of aluminium foil with alpha-particles and found that the aluminium target became radioactive. Chemical analysis showed that the product of this reaction was an isotope of phosphorus. In the next 50 years, more than 2000 other artificial radionuclides were synthesized.

Radiation Hazards

Although alpha-rays from an external source are less dangerous in general than beta- or gamma-rays, alpha emission can be associated with dispersion of radioactive material which is itself dangerous in that it can be absorbed in the body.

Protective clothing is worn to prevent contamination of the body by radioactive chemicals rather than as a protection against external sources.


Links

Decay: http://www.walter-fendt.de/ph14e/lawdecay.htm

Nuclear fission - chain reaction: http://lectureonline.cl.msu.edu/~mmp/applist/chain/chain.htm

Isotope half-lives: http://lectureonline.cl.msu.edu/~mmp/kap30/Nuclear/nuc.htm

Alpha decay: http://www.launc.tased.edu.au/online/sciences/physics/alpha.html

Nuclear phyiscs info: http://sol.sci.uop.edu/~jfalward/nuclearphysics/nuclearphysics.html


Stuff which might be added to this page

RADIOACTIVITY is a special attribute recognised more by its outward effect rather than its cause. That effect is the spontaneous and irrepressible emission of radiation.

The majority of the chemical elements that make up the natural world are now known to exist as mixtures of isotopes. Less than 25% of the elements occur in a single isotopic form.

In the vast majority of cases, naturally occurring isotopes are not radioactive and do not emit any form of radiation.

Simply stated, the laws of physics allow certain combinations of protons and neutrons in the atomic nucleus to co-exist in a state of peaceful tranquillity. Isotopes whose nuclei are configured in this manner are called STABLE ISOTOPES.

There are, however, a few chemical elements in the Periodic Table with isotopes in which the arrangement of protons and neutrons is less than ideal. Because of this, these elements exhibit a degree of nuclear instability, which manifests itself as RADIOACTIVITY.

The phenomenon of radioactivity is not only exhibited by elements at the extreme top end of the Periodic Table (eg uranium, thorium, radium and lead). Indeed isotopes of potassium, some rare earths (neodymium, samarium and gadolinium), and hafnium, osmium and platinum have also been found to be slightly radioactive.

Isotopes which spontaneously emit radiation are called RADIO-ISOTOPES.

Natural Radioactivity

The element potassium, a normal constituent of the human body, exists in three isotopic forms -

* potassium-39
* potassium-40
* potassium-41

K-39 and K-41 are stable isotopes and together constitute 99.99% of potassium. Potassium-40 is radioactive.

There being 150 to 200 grams of potassium in the adult human body, some 15 to 20 milligrams of it must always exist as the radioisotope K-20.

Another source of natural radioactivity is the air we breathe.

Bombarded by radiation from the Sun and outer space, atmospheric nitrogen undergoes nuclear reactions to produce the radioisotopes carbon-14 and hydrogen-3 (tritium).

In the form of carbon dioxide, C-14 is fixed by photosynthesis in green plants, which are then consumed by herbivores. Of course, at the head of this food chain is the human animal, which sustains itself by consuming C-14 of both vegetable and animal origin.

During our lifetime we participate in natural processes involving C-14 absorption and excretion and so the C-14 in our tissues gradually increases to an equilibrium level. On a much longer time scale, the levels of C-14 in our tissues decrease due to radioactive decay. Because the half-life of C-14 is 5730 years, the effect of decay is not noticeable while we are alive. Only after we are dead and have stopped assimilating the radioisotope is an age effect measurable. Then an accurate measure of the residual C-14 becomes a very sensitive gauge of the age of an object that was once alive.

The technique of measuring C-14 content is possibly the most important tool available to archaeologists for dating historical artefacts.

The most significant of the naturally occurring radioisotopes are radon-222 and radon-220. These radioactive gases seep out from rocks containing uranium and thorium to be responsible for 50 - 80% of background radiation.

The concentration of radon in air is highest in localities where igneous rocks are prevalent. It can be trapped in poorly ventilated buildings.

Much higher levels of natural radioactivity have existed throughout the history of the Earth. Fortunately, the planet is old enough to have allowed the original intense radioactivity to all but disappear.

However, we should not lose sight of the fact that all species of plants and animals have evolved to their present life-forms in this radiation environment. No plant or animal species has ever known absolute protection from radiation.

Artificial Radioactivity

The first major advance inproducing artificail radiactivity occurred in 1934 with the invention of the cyclotron by Ernest Lawrence in Berkeley, California. With this electrical machine being used to accelerate DEUTERONS (ions of the stable hydrogen isotope, H-2) to very high speeds, it became possible to create the nuclear instability that we now know is a pre-requisite of radioactivity.

By directing a beam of the fast-moving deuterons at a carbon target, Lawrence induced a reaction which resulted in the formation of a radioisotope with a half life of 10 minutes.

Radioactivity had been created by disturbing the natural balance of 6 protons and 6 neutrons in the nucleus of the carbon atom with the insertion of another proton.

The product atom now had 7 protons in its nucleus, was no longer carbon and, in fact, had been changed to a nitrogen species. The neutron count in the nucleus of the product atom, however, stayed at 6; this was insufficient to stabilise the 7 protons that were now present in the nucleus.

The overall effect of bombarding carbon with deuterons in the cyclotron was not only to convert the carbon to nitrogen, but also to ensure that the new species was radioactive.

Following this experiment, the path was opened to the discovery and production of many more radioisotopes from the cyclotron.

In the same year as Lawrence experimented with the cyclotron, Enrico Fermi in Rome started systematically exposing the elements in the Periodic Table to beams of neutrons.

In the course of this work, Fermi identified around 40 new radio-active species and thus was able to show how neutrons that had been slowed down prior to interacting with the targets gave rise to much higher levels of radioactivity.

However, the significance of his most important experiment initially eluded him. That was when uranium was exposed to neutrons and several new radioactive species were produced as a consequence.

Accounting for the multiplicity of products induced by the neutron bombardment of uranium occupied many brilliant minds for the next four years.

Repeating Fermi's experiments, Otto Hahn, Lise Meitner and Fritz Strassman in Berlin concluded that the only explanation was nuclear fission - a process in which uranium nuclei are split into barium, krypton and smaller amounts of other highly radioactive disintegration products, all accompanied by the release of enormous amounts of energy.

The possibility that nuclear fission could be developed into a bomb of enormous power was not overlooked.

Mindful of the perils this latest scientific discovery imposed on world peace, Meitner secretly slipped out of Germany to Sweden where she explained nuclear fission. The discovery was published in Nature in January 1939.

In view of the darkening war clouds in Europe, Fermi, who by now was resident in the USA, was moved to draft a letter (with the collaboration of Leo Szilard and Eugene Wigner), which was signed by Albert Einstein and then delivered on 11 October 1939 to US President Roosevelt to alert him to the danger.

The President reacted immediately to initiate the Manhattan Project which ultimately led to the creation of the first nuclear reactor then to the nuclear weapons that brought World War II to a conclusion.

The development of the cyclotron had provided the scientific world with a prolific source of artificial radioisotopes from which were facilitated enormous advances in the field of biochemistry.

However, the nuclear reactor's capability of producing copious quantities of radioisotopes completely eclipsed the cyclotron.

Once the war was over, the US authorities lost little time in making radioisotopes available "for peaceful and humanitarian ends".

Radioisotopes in Nuclear Medicine

In NUCLEAR MEDICINE a radio-isotope is administered to a patient either to aid the diagnosis of disease or for the treatment of disease.

The radioisotopes used in DIAGNOSTIC nuclear medicine are selected on the basis of their ability to provide useful clinical information (usually by providing an image of an internal structure in the human body or by visualising various stages in the function of an organ) while exposing the patient to only minimal radiation.

To ensure this, certain selection criteria are applied :-

For example the radioisotope should -

* possess a short half-life(hours) which is commensurate with the duration of the investigative procedure
* not emit alpha or beta radiation, because these particles would be trapped in the patient's tissues and not be detected externally
* emit gamma radiation of an energy which will allow its origin to be efficiently assessed
* be available in the highest possible specific activity, so that it will not invoke either a toxic or pharmacological response in the patient.

On the other hand, in THERAPEUTIC nuclear medicine, a different set of criteria apply :-

* the half life should not be the cause of an extended stay in hospital for the patient
* the radioisotope should emit particulate (alpha or beta) radiation of sufficient energy to penetrate to all parts of the lesion
* it should, in addition, emit gamma rays to facilitate the assessment that the appropriate region of the body has been targeted.

From a possible population of more than 2300, only a handful of radioisotopes come close to satisfying the selection criteria for use as a diagnostic agent. Of these, reactor-produced technetium-99m (99mTc) is pre-eminent, being used in more than 80% of the estimated 100,000 patient studies that are performed world-wide each day.

After (99mTc), a series of cyclotron produced radioisotopes, such as thallium-201(201Tl), gallium-67 (67Ga), indium-111(111In) and iodine-123(123I), are the next most popular.

A different group of radioisotopes is used for therapeutic purposes.

Well-established examples are iodine-131 (131I), phosphorus-32 (32P) and yttrium-90 (90Y) but several others are being investigated for possible application.

Examples of these are samarium-153 (153Sm), rhenium-186 and rhenium-188 (186Re, 188Re), dysprosium-165 (165Dy) and holmium-166 (166Ho).

The various radioactive substances found naturally in terrestrial materials are very ancient remnants of the time when the Earth was formed. None of them satisfy the nuclear medicine selection rules and consequently are not used clinically.

Reactors and Cyclotrons

A popular misconception, which has been particularly evident in the public debate that has continued since the reactor disaster at Chernobyl, is that the cyclotron is an alternative to the nuclear reactor for the production of artificial radioisotopes.

Like most other misconceptions, this one also contains an element of truth.

It is important in promoting the benefits that radioisotopes have to offer society, that the respective roles of the reactor and the cyclotron are known and properly understood.

For the vast majority of radioisotopes, including almost all those used medically, the cyclotron complements the reactor - it does not replace it.

As described earlier, radioactivity is the end-result from disturbing the balance between neutrons and protons in the atomic nucleus.

In theory, radioactivity can be achieved by :-

1. adding a neutron to the nucleus, or
2. removing a proton from the nucleus,or
3. removing a neutron from the nucleus,or
4. adding a proton to the nucleus.

In practice, effects (1) and (2) can be achieved through reactions only available via a nuclear reactor - giving rise to a family of radioisotopes which are described as neutron rich.

On the other hand, effects (3) and (4), leading to the neutron deficient family of radioisotopes, are achievable only in a cyclotron.

Therefore the decision on which nuclear effect to exploit will direct whether a reactor or a cyclotron must be used.

Hence the cyclotron is complementary to the reactor, providing the producer with access to an even greater variety of radioisotope products.

Rarely do circumstances exist which allow either a reactor or a cyclotron to be used to produce the same radioisotope. The prime example is 99mTc.

As indicated above, this radioisotope has the greatest impact on our welfare since it is used in more than 80% of nuclear medicine studies.

Those interested in minimising the use of nuclear reactors, strongly promote the cyclotron alternative for99mTc. However, for this goal to be achieved a number of severe practical difficulties must be overcome.

For example, the cyclotron method is totally dependent on the availability of a rare and expensive starting material, a highly enriched stable isotope of molybdenum,100Mo. Cyclotron production inevitably gives rise to impurities in the 99mTc which increase the radiation exposure of each patient and could be responsible for inferior clinical images. It is also probable that cyclotron produced 99mTc would be considerably more expensive than its reactor produced counterpart. Finally, current pharmaceutical regulations, specifying minimum drug quality, would prohibit the use of cyclotron 99mTc in humans.

Research continues in an attempt to ameliorate these problems.

Meanwhile, around the world, 99mTc is exclusively produced in nuclear reactors where there exists the option of two different technologies.

Most commonly, 99mTc is obtained as the result of the fission of uranium(235U). From this rather complex process 99mTc is produced in a highly purified state and at a reasonable price. There are, however, difficult waste products to be contended with.

Radiopharmaceuticals

Every developed nation needs access to one or more sources of artificially produced radioisotopes in order to provide its community with what has come to be expected as the full range of medical services.

When a radioisotope is designed for use in a medical application, it is usual for it to be presented in a form which directs in which part of the body it will concentrate.

This type of presentation is usually referred to as a RADIOPHARMACEUTICAL.

One particular radioisotope may be presented in a number of active forms, each designed to target specific tissues in the human body.

It is implicit in the formulation of a radiopharmaceutical that it possesses all the necessary attributes of purity, stability and safety.

Like any other drug, it must be proved safe and clinically effective through a series of clinical trials.

A radiopharmaceutical is the end-product of extensive chemical processing of a substance after it has been irradiated in a cyclotron or nuclear reactor.

Most commonly, the preparation of a radio-pharmaceutical involves :-

* extracting the radioisotope from the bulk of the target substance
* purification from undesirable chemical and radioisotopic impurities
* chemical conversion to a biologically specific form (may be more than one active form, each targeting specific groups of tissues in the human body)
* making the preparation suitable for administration to patients
* testing the quality of the final product.

While radiopharmaceuticals must comply with the normal requirements of drugs, their production must also contend with the special problems of radiation safety and short half-lives.

In Australia, radiopharmaceuticals have been routinely produced by the Australian Nuclear Science and Technology Organisation (ANSTO) for over a quarter of a century. ANSTO uses both its reactor, at Lucas Heights, and its cyclotron, located on the campus of Royal Prince Alfred Hospital, to create a range of artificial radioisotopes for a domestic and export market.

Special laboratory facilities exist to ensure that the products from these two major nuclear resources are processed into medical products essential to the continued well-being of the Australian community.

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Radioactivity is a natural part of our environment. Present-day Earth contains all the stable chemical elements from the lowest mass (H) to the highest (Pb and Bi). Every element with higher Z than Bi is radioactive. The earth also contains several primordial long-lived radioisotopes that have survived to the present in significant amounts. K-40, with its 1.3 billion year half-life, has the lowest mass of these isotopes and beta decays to both Ar-40 and Ca-40.

Many radioisotopes can decay by more than one method. For example, when actinium-226 (Z=89) decays:

83% of the rate is through β--decay  Ac-226 -> Th-226 + e- + antineutrinl
17% is through electron capture      Ac-226 + e- -> Fr-226 + neutrino
0.006% is through α decay  Ac-226 -> Fr-222 + He-4

Therefore, from 100,000 atoms of actinium, one would measure on average 83,000 beta particles and 6 alpha particles (plus 100,000 neutrinos or antineutrinos)

Three very massive elements, 232Th (14.1 billion year half-life), 235U (700 million year half-life), and 238U (4.5 billion year half-life) decay through complex "chains" of alpha and beta decays ending at the stable 208Pb, 207Pb, and 206Pb respectively. The ratio of uranium to lead present on Earth today gives us an estimate of its age (4.5 billion years). Given Earth’s age, any much shorter lived radioactive nuclei present at its birth have already decayed into stable elements. One of the intermediate products of the 238U decay chain, 222Rn (radon) with a half-life of 3.8 days, is responsible for higher levels of background radiation in many parts of the world. This is primarily because it is a gas and can easily seep out of the earth into unfinished basements and then into the house.

Some radioactive isotopes, for example 14C and 7Be, are produced continuously through reactions of cosmic rays (high energy charged particles from outside Earth) with molecules in the upper atmosphere. 14C is useful for radioactive dating. Also, the study of radioactivity is very important to understand the structure of the Earth because radioactive decay, it is believed, heats the Earth’s interior to very high temperatures.