Properties and Uses of Radiation from Unstable Isotopes Chemistry Tutorial

Key Concepts

Isotopes of an element have
⚛ the same atomic number (Z)

⚛ different mass numbers (A)

Isotopes can be either
⚛ stable (do not undergo spontaneous nuclear deacy)

⚛ unstable (those that undergo spontaneous nuclear decay)

Unstable isotopes are also known as radioisotopes (or radio-isotopes, the contraction of radioactive isotopes).

The time taken for half of the radioisotope to decay is called its radioactive half-life.

When a radioisotope undergoes nuclear decay, radiation is emitted in the form of:
⚛ alpha-particles (α-particles, positive charge, low penetrating power)

⚛ or beta-particles (β-particles, negative charge, moderate penetrating power)

⚛ and/or gamma radiation (γ-rays, no charge, high penetrating power)

The use we can make of a radioisotope depends on the
⚛ radiation emitted during nuclear decay

⚛ half-life of the radioisotope

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Properties of Radiation

Henri Becquerel discovered radioactivity in 1896.¹

A few years later, Ernest Rutherford found 2 distinct types of radiation from uranium:

one that was easily absorbed which he called alpha (α)

one that was more penetrating which he called beta (β)

In 1900, Villard identified a third radiation from radium. This radiation was much more penetrating than alpha and beta, and he called it gamma radiation (γ).

Experiments looking at how these radiations interact with magnetic fields and electrically charged plates indicated that:

alpha particles (α-particles) had a positive charge

beta particles (β-particles) had a negative charge

gamma rays (γ-rays) were not charged

Alpha-particles (α-particles) have more mass than beta-particles (β-particles).
Gamma rays (γ-rays) have no mass, they are part of the electromagnetic spectrum.

The identity of each type of radiation is known to be:

alpha particle is a helium nucleus

beta particle is a high energy electron

gamma ray is electromagnetic radiation

The most dangerous form of radiation to living things are gamma rays (γ-rays) because they have the greatest penetrating power and are the most energetic. Gamma-rays (γ-rays) can penetrate the tissue of living organisms and are therefore useful in sterlisation because they kill bacteria.
Less energetic beta particles (β-particles) can also be harmful if they come into contact with living things, burning skin.
Less energetic alpha particles (α-particles) can be harmful if ingested.

The properties of alpha, beta and gamma radiation are summarised in the table below:

Name

Symbols

Identity

Relative Charge

Relative Mass

Penetrating Power

Interaction with Charged Plates

Hazards

alpha

4	He
2

helium
nucleus

low

Stopped by a sheet of paper

⚛ Attracted to negative plate.
⚛ Deflected by positive plate.

Harmful if ingested

beta

0	e
-1

electron

1
2000

moderate

Passes through paper and ½ mm aluminium.
Stopped by ½ mm lead.

⚛ Attracted to positive plate.
⚛ Deflected by negative plate.

Skin burns.
Harmful if ingested, particularly iodine-131 in thyroid and strontium-90 in bones.

gamma

electro-
magnetic
radiation

high

Only stopped by several centimetres of lead or many centimetres of concrete.

Unaffected

Most dangerous as these are the most penetrating, as a consequence, gamma rays can be used to sterilize materials and destroy bacteria in food.

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Uses of Radioisotopes

All the elements beyond bismuth (Bi) in the Periodic Table of the Elements display radioactivity.
There are natually occurring radioactive isotopes of many of the other elements as well.

Naturally occurring radioisotopes can be used to date :

rocks.
The radioisotope needs to be present in the rock, and, have a very long half-life, one measured in billions of years.

archeological artefacts.
A half-life measured in thousands of years can suffice ( see Carbon-14 dating ).

Radioisotopes that emit low-penetrating alpha particles and that have a relatively long half-life have found use in domestic settings such as smoke detectors.

To be useful, radioisotopes with very short half-lives, such as those measured in seconds, hours, or days, are produced in nuclear reactors or cyclotrons close to the where they will be used.
These are man-made, or synthetic, radioisotopes.
These radioisotopes are particularly useful in medical applications.
A short half-life, such as a few hours, means that the radiation is reduced to harmless levels quickly.
A longer half-life, such as hundreds or thousands of years, means that the radioisotope continues to emit harmful radiation for a very long period of time.

The table below gives the emitted radiation, half-life, and uses for a selection of useful isotopes in order of decreasing half-life.

Isotope	Radiation Emitted	Half-life	Use
Oxygen-18	stable		Biological tracer for example, in studies of photosynthesis
Rubidium-87	beta	48 × 10⁹ years	Rubidium-Strontium radiometric dating (millions of years)
Uranium/Lead	U-238 alpha & gamma	4.5 × 10⁹ years	Radiometric dating (millions to billions of years)
Potassium-40	positron emission	1.26 × 10⁹ years	Potassium-Argon radiometric dating (100 000 to several billion years)
Uranium-235	alpha, gamma	7.1 × 10⁸ years	Enriched as a fuel for most nuclear reactors
Chlorine-36	beta	3.01 × 10⁵ years	Measurement of sources of chloride and determining the age of water up to about 2 million years old. Naturally occurring radioisotope.
Plutonium-239	alpha, gamma	24 400 years	Fuel for most "fast-breeder" nuclear reactors
Carbon-14	beta	5 730 years	Radiometric dating Determination of age of carbon-containing artifacts up to about 70,000 years. Also used as a biological tracer, for example, in studies of photosynthesis. Naturally occurring radioisotope.
Americium-241	alpha	432 years	Domestic smoke alarms and neutron gauging. Americium-241 is a decay product of plutonium-241 formed in nuclear reactors.
Caesium-137	beta	30.2 years	Radiotracing to identify sources of soil erosion and depositing. Also used in thickness gauging.
Lead-210	beta	22.3 years	Date layers of soil and sand deposited up to about 80 years ago. Naturally occurring radioisotope.
Tritium	beta	12.32 years	Measure the age of 'young' groundwater up to about 30 years old. Naturally occurring radioisotope. Titriated water is used to study sewage and liquid wastes.
Cobalt-60	beta, gamma	5.3 years	Cancer treatment as tumour cells tend to be more susceptible to radiation than other cells.
Manganese-54	gamma	312.2 days	Used to predict the behaviour of heavy metals in effluents from mining waste water.
Zinc-65	gamma	244.26 days	Used to predict the behaviour of heavy metals in effluents from mining waste water.
Gadolinium-153	gamma	240.4 days	Used in X-ray fluorescence and bone density guages for osteoporosis screening.
Iridium-192	beta	73.83 days	Supplied as a wire for use as an internal radiography device.
Iron-59	beta, gamma	46.3 days	Used in blood studies. When incorporated into steel it is used to determine the amount of friction in machinery.
Chromium-51	alpha	27.7 days	Labelling of red blood cells and quantifying gastro-intestinal protein loss. Isotope prepared in a nuclear reactor.
Phosphorus-32	beta	14.28 days	Treatment of excess red blood cells. Isotope prepared in a nuclear reactor.
Iodine-131	beta, gamma	8.1 days	Medical tracer to study and treat the thyroid gland. Used in the diagnosis of adrenal medullary and for imaging suspected neural crest and other endocrine tumours. Isotope prepared in a nuclear reactor.
Gallium-67	gamma	3.3 days	Tumour-seeking agent. Isotope prepared in a cyclotron.
Thallium-201	gamma	3 days	Locating damaged heart muscle. Isotope prepared in a cyclotron.
Ytterbium-169	gamma	3 days	Brain scans. Isotope prepared in a nuclear reactor.
Molybdenum-99	beta	2.75 days	Used as the 'parent' in a generator to produce technetium-99m, the most widely used radioisotope in nuclear medicine. Isotope prepared in a nuclear reactor.
Gold-198	beta	2.69 days	Trace factory waste causing ocean pollution and to trace sand movement in river beds and on ocean floors.
Yttrium-90	beta	2.67 days	Liver cancer therapy. Isotope prepared in a nuclear reactor.
Samarium-153	beta	1.93 days	Used in the treatment of pain associated with bony metastases of primary tumours. Isotope prepared in a nuclear reactor.
Potassium-42	beta & alpha	22 hours	Determination of exchanged potassium in blood flow. Isotope prepared in a nuclear reactor.
Sodium-24	beta, gamma	15 hours	Location of leaks in water pipes, studies of body electrolytes. Isotope prepared in a nuclear reactor.
Iodine-123	gamma	13.2 hours	Used in imaging to monitor thyroid function and detect adrenal dysfunction. Isotope prepared in a cyclotron.
Copper-64	gamma	12.7 hours	Dtudying genetic disease affecting copper metabolism. Isotope prepared in a nuclear reactor.
Technetium-99	beta	6 hours	Medical tracer used to locate brain tumours and problems with the lungs, thyroid, liver, spleen, kidney, gall bladder, skeleton, blood pool, bone marrow, salivary and lacrimal glands and heart blood pool and to detect infection. Isotope prepared in a nuclear reactor.
Magnesium-27	beta, gamma	9.5 minutes	Location of leaks in water pipes.
Krypton-81	gamma	13 seconds	Lung ventilation studies. Isotope prepared in a cyclotron.

Positron emitters such as carbon-11, nitrogen-13, oxygen-15 and fluorine-18 are produced in a cyclotron and are extremely short lived isotopes.
They are used in positron emission tomography (PET) for studying brain physiology and pathology for epilepsy and dementia.

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Footnotes:

1. Henri Becquerel discovered X-rays a year earlier, in 1985.

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