Radioactivity

The Discovery of Radioactivity

In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. On developing his photographic plates the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

The term radioactivity was actually coined by Marie Curie, andher husband Pierre. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium.

Ernest Rutherford, who did many experiments studying the properties of radioactive decay, named these alpha, beta, and gamma particles, and classified them by their ability to penetrate matter.

Models of the Atom

J J Thompson proposed the plum pudding model, in which electrons were placed like cherries in a matrix of positive charge.  The neutron had not yet been discovered.   

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This was the accepted model.  Nobody had any reason to believe otherwise until Ernest Rutherford, A New Zealand physicist proved otherwise in the early 1900s. Rutherford bombarded a thin layer of gold atoms with alpha particles.  He was using the alpha particles like bullets, expecting to see the atoms burst like watermelons.  He described his experiment as firing artillery shells at tissue paper. To his surprise he found that some of the alpha particles bounced back in the direction they came from.  Other particles went straight though, while other particles were deflected.  This alpha scattering showed some amazing facts about the nucleus:

The nucleus is very small; Most of the atom is empty space; The repulsion of the positively charged alpha particle

showed that the nucleus is positively charged.

 This discovery led to the idea of the nuclear atom.  This was developed further by Neils Bohr, a Danish physicist (and goalkeeper of the Danish Olympic football team).  It is the model of the atom shown at the start of this topic.  The neutron was discovered twenty years later by an English physicist, Chadwick. Since the nucleus is so small, the size of an atom is governed by the size of the electron shells.  Therefore big atoms and small atoms are all roughly the same size, about 10-10 m in diameter.

The Atom

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The Basic AtomAll matter is made up of atoms. The basic atom consists of a nucleus surrounded by electrons going round the nucleus in orbit.  Electrons are negatively charged.  Here is a Lithium atom: 

 The nucleus consists of:

Protons which are positively charged. Neutrons that have no charge.

   The protons and neutrons have very nearly the same relative mass.  The neutron has slightly more mass than the proton, but at this level we are going to say that the relative mass of both the proton and the neutron is 1.  The mass of a proton or neutron in kilograms is about 1.6 × 10-27 kg. The mass of an electron is about 1/1800 the mass of a proton.  The mass of an electron is about 9.1 × 10-31 kg.

Particle ChargeProton + 1 eNeutron 0Electron - 1 e

  The symbol e is often called the electronic charge.  Its value is 1.6 × 10-19 C.

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  The protons and neutrons are the nucleons.  Atoms and IonsElements are often written like this:

A is the total number of nucleons.  This is called the mass number or the nucleon number.

Z is the total number of protons.  This is called the atomic number or the proton number. The number of protons determines the element.  If we change the number of protons in the nucleus from 6 to 7, we change the element from carbon to nitrogen.  This will change the chemistry radically. To work out the number of neutrons we take away the number of protons from the number of nucleons: 

No of neutrons = mass number - atomic number If the number of electrons is the same as the number of protons, the atom carries zero overall charge.  It is described as neutral. The nucleus is very tiny, about 1/10 000 the size of an atom.  It is the equivalent to the size of a pea on the floor of your school dining hall. If we change the number of electrons, the atom is charged.  It becomes an ion:

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Remove an electron, the overall charge is positive.  We have a positive ion.

Add an electron, we have a negative ion.

Ions are NEVER made by adding or taking away protons.   Isotopes    Isotopes have the same number of protons, but different numbers of neutrons.  If we change the number of protons, we change the element completely. Isotopes have the same chemical properties as the normal element.

Examples of isotopes: (e.g. helium-3, carbon-12, iodine-131 and uranium-238).

Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

Hydrogen Deuterium Tritium

Ordinary hydrogen is written 1H1, deuterium is 2H1, and tritium is 3H1.

There are "preferred" combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons

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than protons in order to stick together. Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable.

Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.

Radioactivity

Some isotopes of atoms can be unstable.

They may have:

a) Too much energy or

b) The wrong number of particles in the nucleus.

We call these radioisotopes.

To make themselves more stable, they throw out particles and/or energy from the nucleus. We call this process ‘radioactive decay’. The atom is also said to disintegrate.

The atom left behind (the daughter) is different from the original atom (the parent). It is an atom of a new element. For example uranium breaks down to radon which in turn breaks down into other elements.

The particles and energy given out are what we call ‘radiation’ or ‘radioactive emissions’.

 Three types exist :

Alpha decay; Beta decay ; Gamma radiation.

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Alpha and beta decays result in the emission of a particle.  Gamma radiation is an electromagnetic wave of very short wavelength .

Properties of RadiationThe table shows some properties: Radiation

Description

Penetration

Ionising Power

Effect of Electric or Magnetic field

Alpha ()

Helium nucleus2p + 2n Q = + 2 e

Few cm air Thin paper

Intensely ionising

Deflection as a positive charge

Beta () High speed electron Q = -1 e

Few mm of aluminium

Less than alpha

Deflection in opposite direction to alpha.

Gamma ()

Very short wavelength em radiation

Several cm lead, couple of m of concrete

Weakly ionising

No effect.

 

Alpha particle

This consists of a helium nucleus. If we send alpha particles through the poles of a magnet (a magnetic field), we find

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that they are deflected.  This means that they are charged.  If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the negatively charged plate.  This means they are positively charged.

Alpha particles are stopped by a few cm of air.  This means that an alpha source can be used safely with minimal shielding.  Your skin will stop alpha particles. Alpha particles are intensely ionising.  Being quite big and moving fast, they collide frequently with other atoms, knocking off electrons, causing ionisation.  They rapidly lose their energy.  Eventually they stop and then pick up two stray electrons to become helium atoms.  All the Earth's helium atoms are thought to come from alpha decay.  Beta particle

This consists of a fast moving electron . If we send a beta particles through the poles of a magnet (a magnetic field), we find that they are deflected in the opposite direction to alpha particles.  This means that they are charged.  If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the positively charged plate.  This means they are negatively charged.

Gamma Radiation Gamma rays are very short wavelength and highly energetic electromagnetic radiation. They are given off by very energetic or excited nuclei when some other decay has occurred. Cobalt-60 is a common source of gamma rays.

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 Gamma radiation does not in itself alter the nucleon and proton numbers. Gamma rays are not affected by electric or magnetic fields.

 Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting–several centimeters of lead or a meter of concrete is needed to stop most gamma rays.

Radioactivity

It is found that nuclei with mass numbers greater than about 100 spontaneously decay into other types of nuclei. Such nuclei are said to be radioactive.

alpha decay, which occurs by emission of an alpha ( 4

2 He) nuclei. An example of this is the decay of Uranium:

23892 U 234

90 Th + 42 He

beta decay, which occurs by emission of a beta particle (electron or positron). An example of this is the decay of Nitrogen:

127 N 12

6 C + 01 e

The notation 0 1e denotes an electron/positron - a

positron is identical except it has a positive charge .

gamma decay, which occurs by emission of gamma particles (photons, or quanta of light). An example of this is the decay of a Carbon atom in an excited state:

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Beta Particle

126 C * 12

6 C + where denotes the photon.

Alpha Decay

When a nucleus decays by alpha decay, it ejects a helium nucleus (NOT atom).  The nucleus recoils, just like a canon firing a canon ball.

 

       

A helium nucleus consist of 2 PROTONS AND 2 NEUTRONS . So when an alpha particle is emitted the atomic number goes down by 2, because 2 protons are lost from the nucleus.  The mass number goes down by 4 because 4 nucleons are lost. Beta Decay

A beta particle is a high speed electron which is ejected from the nucleus.  A neutron turns into a proton and the electron is ejected.  It has nothing to do with the electrons surrounding the atom.   

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Alpha Particle

The atomic number goes up by 1, so a new element is formed, but the mass number stays the same.   The electron comes out of the nucleus, NOT the electron shells. 

Gamma Decay

When a nucleus decays by gamma decay, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay.

Measuring Radiation

In the old days, radiation was detected by exposing a sheet of photographic film to the radioactive source.  Each decay caused the deposit of a grain of silver, and it was possible measure the density of the deposits when the film was developed.  This method is still used today with film badges that people wear if they are working with radioactive materials. 

To get a real-time measurement, we measure the radiation from a radioactive sample using a radiation detector

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called a Geiger-Müller tube.  This is connected to a ratemeter. 

 The radioactive decay is measured by the number of counts per second.  A computer can act as a ratemeter and store the results.  It will also plot a graph. When we take readings it is important that we measure the background count.  There is radioactivity all around us; it's a natural part of the environment.  So we find out what the background count is, then we take that away from the count we get with the source. 

cloud chamber, device used to detect elementary particles and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air. When ionizing radiation passes through the vapor, it leaves a trail of charged particles (ions) that serve as condensation centers for the vapor, which

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condenses around them.

ALPHA PARTICLES PRODUCE STRAIGHT LONG LINES

BETA PARTICLES PRODUCE STRAIGHT WEAK LINES

GAMMA RAYS LOOK LIKE TINY CURLY STRANDS OF HAIR

The Gold Leaf Electroscope Dry air is normally a good insulator, thus a charged electroscope will stay that way, as the charge cannot escape. When an electroscope is charged, the gold leaf sticks out, because the charges on the gold repel the charges on the metal stalk.

When a radioactive source comes near, the air is ionised, and starts to conduct electricity. This means that the charge can "leak" away, the electroscope discharges and the gold leaf falls. Half-LifeRadioactive decay is a random process.  If you look at a nucleus, it might decay within ten seconds, or twenty two million years.  Since there are many billions of nuclei, a random decay pattern is seen. 

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 What is half-life?

Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically. As they decay the atoms change to daughter atoms, until eventually there won’t be any of the original atoms left.

Different substances decay at different rates and so will last for different lengths of time. We use the half-life of a substance to tell us which substances decay the quickest.

Half-life – is the time it takes for half of the radioactive particles to decay.

It is also the time it takes for the count-rate of a substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time.

The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph.

This would also work if you plotted the number of parent atoms against time.

The longer the half-life of a substance the slower the substance will decay and the less radiation it will emit in a certain length of time.

Each radioactive isotope decays in its own way and has its own half-life which is defined as: the time taken for half the original number of atoms

to decay. 

This is shown on the graph:

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.If it takes 4 days for half the atoms to decay:

after 4 days, 1/2 are left over; after 8 days,  1/4 are left over; after 12 days, 1/8 are left over.

This is called exponential decay.   Some half lives are extremely short, much less than 1 second.  Some are very long, about 4500 million years. 

 Using radioactivityDifferent radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are the main uses you will be expected to know about:

1. Uses in medicine to kill cancer –

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radiation damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body. Sources of radiation that are put in the body need to have a high count-rate and a short half life so that they are effective, but only stay in the body for a short period of time. If the radiation source is outside of the body it must be able to penetrate to the required depth in the body. (Alpha radiation can’t travel through the skin remember!)

2. Uses in industry –

one of the main uses for radioactivity in industry is to detect the thickness of materials. The thicker a material is the less the amount of radiation that will be able to pass. Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness. Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal.

They can even use this idea to detect when toothpaste tubes are full of toothpaste!

3. Photographic radiation detectors –

these make use of the fact that radiation can change the colour of photographic film. The more radiation that is absorbed by the film the darker the colour it will go when it is developed. This is useful for people working with radiation, they wear radiation badges to show them how much radiation they are being exposed to.

4. Dating materials –

The older a radioactive substance is the less radiation it will release. This can be used to find out how old things are. The

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half-life of the radioactive substance can be used to find the age of an object containing that substance.

There are three main examples of this:

i) Carbon dating – many natural substances contain two isotopes of Carbon. Carbon-12 is stable and doesn’t disintegrate. Carbon-14 is radioactive. Over time Carbon-14 will slowly decay. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object.

ii)Uranium decays by a series of disintegrations that eventually produces a stable isotope of lead. Types of rock (igneous) contain this type of uranium so can be dated, by comparing the amount of uranium and lead in the rock sample.

iii) Igneous rocks also contain potassium-40, which decays to a stable form of Argon. Argon is a gas but if it can’t escape from the rock then the amount of trapped argon can be used to date the rock.

5. Smoke Detectors and Americium-241

6. Agricultural Applications - radioactive tracers

Radioisotopes can be used to help understand chemical and biological processes in plants.

7. Food Irradiation

Food irradiation is a method of treating food in order to make it safer to eat and have a longer shelf life.

 Uses and Hazard of Radiation Radiation

Use Hazard

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Alpha () Used in smoke detectors If taken in to the body (ingested), alpha emitters can do immense damage to living tissues

Beta () Checking the thickness of paper sheet in manufacture.Radioactive tracers in medical research and diagnosis

Some risk of tissue damage, although nowhere near as dangerous as alpha.

Gamma ()

Medical research.Non-destructive testing of castings.

Can cause genetic damage and cancer.

  Background radiationThere is a certain amount of radiation around us (and even inside us) all the time. There always has been – since the beginning of the Earth. It is called Background radiation.

Background radiation comes from a huge number of sources. Cosmic radiation Radiation from rocks Radioactive waste

In most areas, Background radiation is safe. It is at such a low level that it doesn’t harm you. You need to be exposed to many times the normal background level before you notice any symptoms.

Dangers of handling radioactive substances

Each type of radiation that can be emitted can be absorbed by different materials and ionises different amounts. They are equally dangerous but for different reasons.

Alpha particles:

Although alpha particles cannot penetrate the skin, if it gets into the body it can ionise many atoms in a short distance.

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This makes it potentially extremely dangerous. A radioactive substance that emits just alpha particles can therefore be handled with rubber gloves, but it must not be inhaled, eaten, or allowed near open cuts or the eyes.

Beta particles:

Beta particles are much more penetrating and can travel easily through skin. Sources that emit beta particles must be held with long handled tongs and pointed away from the body. Inside of the body beta particles do not ionise as much as alpha particles but it is much harder to prevent them entering the body.

Gamma waves:

These waves are very penetrating and it is almost impossible to absorb them completely. Sources of gamma waves must also be held with long handled tongs and pointed away from the body. Lead lined clothing can reduce the amount of waves reaching the body. Gamma waves are the least ionising of the three types of radiation but it is extremely difficult to prevent them entering the body. 

Units of Radioactivity

The number of decays per second, or activity, from a sample of radioactive nuclei is measured in becquerel (Bq), after Henri Becquerel. One decay per second equals one becquerel.

An older unit is the curie, named after Pierre and Marie Curie. One curie is approximately the activity of 1 gram of radium and equals (exactly) 3.7 x 1010 becquerel. The activity depends only on the number of decays per second, not on the type of decay, the energy of the decay products, or the biological effects of the radiation .

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Nuclear Energy

Sixty years ago the power of nuclear energy was demonstrated to the world.  Little Boy, a free-fall atomic bomb was dropped on Hiroshima in August 1945.  A few days later an aerial mine, Fat Man, was detonated over Nagasaki.  The destruction and carnage caused by these bombs is well known.  The energy released was due to the conversion of 20 grams of nuclear material to heat. The conversion of nuclear energy to heat is at the heart of nuclear energy.  Scientists have learned to control the process so that instead of an explosion, a steady heat source is achieved.  A nuclear reactor can boil water to steam to turn a steam turbine.  In the early days, nuclear energy was greeted with unbounded optimism.  All sorts of nuclear powered devices were conceived, including railway engines and aeroplanes.  It was hoped that nuclear power would be so cheap that it would not be necessary to meter electricity.  However this proved not to be the case.  We will look at the processes that release nuclear energy.  Fission

Very large nuclei tend to be rather unstable.  This means that they are radioactive.  Some nuclei, for example, Uranium-235 and Plutonium-239, can be made so unstable that they split into two or more nuclei of more stable elements.  This is called fission.  The nuclei are called fissile. These fissile nuclei are isotopes of more stable elements (e.g. Uranium-238).  If left alone, they decay radioactively by emitting alpha particles. Fission is not a spontaneous process.  It has to be started by injecting a neutron into the nucleus.    

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The neutron has to be injected at the right speed:

too fast, the neutron will pass right through, or knock out another neutron.

too slow, the neutron will bounce off the nucleus.

Many pictures show the neutron smashing the nucleus like a bullet.  This is wrong.  It's more like that the neutron "tickles" the nucleus. The nucleus is not a neat array of protons and neutrons.  It is very active , changing shape all the time.  It's like a "wobbly drop".  When the extra neutron is taken into the nucleus, the wobbly drop goes dumbbell-shaped like this:   The weak spot at the neck makes the nucleus fly apart to form two or more new nuclei.  A lot of energy is released.  Nuclear energy gives off far more heat energy than chemical reactions.    Also two or three (or more) neutrons are released.  These can go on to be absorbed by other nuclei to cause a chain reaction, which is shown in the picture below. 

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  If the chain reaction is not controlled, a nuclear explosion will occur.  In a nuclear reactor, only one neutron is allowed to pass on to be incorporated into one nucleus. Reactors in nuclear power station do the same job as the boiler; they boil water to steam.  They also can be used to make radioactive isotopes for medical purposes.    Fission has NOTHING whatever to do with radioactivity.  Alpha and beta particles are NOT emitted during fission.  However many of the new daughter nuclei are radioactive.  

Fusion

This involves light nuclei, two isotopes of hydrogen, deuterium and tritium. 

  If two helium nuclei are forced together, they join together or fuse to form a helium nucleus, giving off lots of energy, more than in fission.     The two nuclei have to be slammed together by heating them to temperatures of millions of degrees Celsius before they fuse. The vast amounts of energy can be released in a massive explosion.  The amount of hydrogen involved in a hydrogen bomb explosion is tiny; it would fill a party balloon. 

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Achieving controlled fusion has proved more challenging, and commercial fusion power stations remain a distant prospect. Fusion is the process that fuels stars.  In the Sun, four million tonnes of hydrogen fuel is consumed every second.  This sounds a lot, but the Sun has enough fuel to keep burning for another 4500 million years, by which time we will all be long gone and forgotten.

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