Radioactivity

• The word radiation means the flow of energy through space. There are many forms of radiation.

• Light, radio waves, microwaves, and x-rays are forms of electromagnetic radiation. Many people mistakenly think of radiation as only associated with nuclear reactions.

What is Radiation?

Radiation: Where is it?

Radiation is going

through you at this very moment!

Where does radiation come from? Radiation is generally produced when particles interact or decay. A large contribution of the radiation on earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial). Only a tiny bit is manmade.

Where are Sources of Radioactivity?

• Naturally Occurring Sources:– Radon from the decay of Uranium and Thorium– Potassium -40: Found in minerals and in plants– Carbon 14: Found in plants and animal tissue

• Manmade Sources:– Medical use of radioactive isotopes (X-rays, CAT scans,

etc.)– Certain consumer products (smoke detectors)– Fallout from nuclear testing– Emissions from nuclear power plants

• X-rays are photons, like visible light photons only with much more energy.

• Diagnostic x-rays are used to produce images of bones and teeth on x-ray film.

• Xray film turns black when exposed to x-rays.

Medical Uses for Radiation

Consumer ProductsMany sourcesTelevision sets accelerate electrons to make the picture on the screen and in the process produce a few low energy x-rays.

Smoke detectors emit radiation that is easily stopped even by smoke, and in that way detect the presence of smoke.

Some more products or services: long lasting light bulbs, building materials, and luminous dials, among many others.

Fallout RadiationRadioactivity remaining after

atmospheric nuclear weapons testing

Less than 0.01 mSv (1 mrem)/yr

Long-lived radionuclides:

Cesium-137: 30 year half-lifeMimics potassium - found in muscle

Strontium-90: 29 year half-lifeMimics calcium - found in bones

Radiation Exposure to Americans

Early Pioneers in Radioactivity

Roentgen:

Discoverer of X-rays 1895

Becquerel:

Discoverer of Radioactivity

1896

The Curies:

Discoverers of Radium and

Polonium 1900-1908

Rutherford:

Discoverer Alpha and Beta

rays 1897

How Radiation Works: Isotopes

What’s an isotope? Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are “unstable” and decay to lighter isotopes or elements.

Example: Deuterium and tritium are isotopes of hydrogen. In addition to the 1 proton, they have 1 and 2 additional neutrons in the nucleus respectively.

Another prime example is Uranium 238, or just 238U.

Radioactivity

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles ()

2) Beta particles ()

3) Gamma-rays ()

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles ()

2) Beta particles ()

3) Gamma-rays ()

Where do these particles come from ?

These particles generally come from the nuclei of atomic isotopes which are not stable.

The decay chain of Uranium produces all three of these forms of radiation.

Let’s look at them in more detail…

• In alpha decay, the nucleus ejects two protons and two neutrons.• Beta decay occurs when a neutron in the nucleus splits into a proton

and an electron.• Gamma decay is not truly a decay reaction in the sense that the

nucleus becomes something different.

Alpha Particles ()

Radium

R226

88 protons138 neutrons

Radon

Rn222

Note: This is theatomic weight, whichis the number ofprotons plus neutrons

86 protons136 neutrons

+ nnp

p

He)

2 protons2 neutrons

The alpha-particle is a Helium nucleus.

It’s the same as the element Helium, with the electrons stripped off !

Beta Particles ()

CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+ e-

electron(beta-particle)

We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus.

Gamma particles ()In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

NeonNe20

10 protons10 neutrons

(in excited state)

10 protons10 neutrons

(lowest energy state)

+

gamma

NeonNe20

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the electromagnetic (EM) spectrum.

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the electromagnetic (EM) spectrum.

Gamma Rays

NeonNe20 +

The gamma from nuclear decayis in the X-ray/ Gamma ray

part of the EM spectrum(very energetic!)

NeonNe20

Kinds of Radioactivity

The three main decays are Alpha, Beta and Gamma

Three Common Types of Radioactive Emissions - Penetrability

Alpha particles may be completely stopped by a sheet of paper.

Beta particles can be stopped by aluminum shielding.

Gamma rays, however, can only be reduced by much more substantial obstacles, such as a very thick piece of lead.

• Like most everything in the universe, systems tend to move from higher energy to lower energy over time.

• A ball rolls downhill to the lowest point or a hot cup of coffee cools down.

• A radioactive nucleus decays because the neutrons and protons have lower overall energy in the final nucleus than they had in the original nucleus.

Why Does Decay Happen?

• The radioactive decay of C-14 does not happen immediately because it takes a small input of energy to start the transformation from C-14 to N-14.

• The energy needed to start the reaction is called an energy barrier.

• The lower the energy barrier, the more likely the atom is to decay quickly.

• Radioactive decay depends on chance.

• It is possible to predict the average behavior of lots of atoms, but impossible to predict when any one atom will decay.

• One very useful prediction we can make is the half-life.

• The half-life is the time it takes for one half of the atoms in any sample to decay.

How to Measure Decay

Half-lifeThe half-life of carbon-14 is about 5,700 years.

If you start out with 200 grams of C-14, 5,700 years later only 100 grams will still be C-14.

The rest will have decayed to nitrogen-14.

Another Contribution from Rutherford: Half-life of Radioactive Atoms

The half-life of a radioactive substance, is the time required for one half of it to decay.

Lifetime Not all particles have the same lifetime.

Uranium-238 has a lifetime of about 6 billion (6x109) years !

Some subatomic particles have lifetimes that are less than 1x10-12 sec !

Given a batch of unstable particles, we cannotsay which one will decay.

The process of decay is statistical. That is, we can only talk about either,

1) the lifetime of a radioactive substance, or2) the “probability” that a given particle will decay.

Not all particles have the same lifetime.

Uranium-238 has a lifetime of about 6 billion (6x109) years !

Some subatomic particles have lifetimes that are less than 1x10-12 sec !

Given a batch of unstable particles, we cannotsay which one will decay.

The process of decay is statistical. That is, we can only talk about either,

1) the lifetime of a radioactive substance, or2) the “probability” that a given particle will decay.

Lifetime (IV) Given a batch of 1 species of particles, some will decay within 1 lifetime (1, some within 2, some within 3and so on…

We CANNOT say “Particle 44 will decay at t =22 min”. You just can’t !

All we can say is that: After 1 lifetime, there will be (37%) remaining After 2 lifetimes, there will be (14%) remaining After 3 lifetimes, there will be (5%) remaining After 4 lifetimes, there will be (2%) remaining, etc.

• Most radioactive materials decay in a series of reactions.

• Radon gas comes from the decay of uranium in the soil.

• Uranium (U-238) decays to radon-222 (Ra-222).

• Many satellites use radioactive decay from isotopes with long half-lives for power because energy can be produced for a long time without refueling.

• Isotopes with a short half-life give off lots of energy in a short time and are useful in medical imaging, but can be extremely dangerous.

• The isotope carbon-14 is used by archeologists to determine age.

Why Isotopes are Helpful

• Living things contain a large amount of carbon.

• When a living organism dies it stops exchanging carbon with the environment.

• As the fixed amount of carbon-14 decays, the ratio of C-14 to C-12 slowly gets smaller with age.

Calculating with isotopes

• A sample of 1,000 grams of the isotope C-14 is created.

• The half-life of C-14 is 5,700 years.

• How much C-14 remains after 28,500 years?

• The intensity of radiation measures how much power flows per unit of area.

• When radiation comes from a single point, the intensity decreases inversely as the square of the distance.

• This is called the inverse square law and it applies to all forms of radiation.

Measuring Radioactivity

• Radiation becomes harmful when it has enough energy to remove electrons from atoms

• The process of removing an electron from an atom is called ionization

• Visible light is an example of non-ionizing radiation

• UV light is an example of ionizing radiation

Effects of Radiation

Units of Radiation Exposure Roentgen (R)The roentgen is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. One roentgen is equal to depositing in dry air enough energy to cause 2.58E-4 coulombs per kg.

Rad (radiation absorbed dose)The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of the different radiations.

Rem (roentgen equivalent man)The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem, or mrem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of incident radiation.

• Ionizing radiation is a natural part of our environment.

• There are two chief sources of radiation you will probably be exposed to:– background radiation.– radiation from medical procedures such as x-rays.

• Background radiation results in an average dose of 0.3 to 0.5 rem per year for someone living in the United States.

How Much Radiation is Safe?

How Much Radiation is Safe?

• People who work with radiation use radiation detectors to tell when radiation is present and to measure its intensity.

• The Geiger counter is a type of radiation detector invented to measure x-rays and other ionizing radiation, since they are invisible to the naked eye.

Radiation Detectors

Nuclear Reactions and Energy

• A nuclear reaction is any process that changes the nucleus of an atom.

• Radioactive decay is one form of nuclear reaction.

• If you could take apart a nucleus and separate all of its protons and neutrons, the separated protons and neutrons would have more mass than the nucleus did.

• The mass of a nucleus is reduced by the energy that is released when the nucleus comes together.

• Nuclear reactions can convert mass into energy.

Where Does the Energy Come From?

Fusion Reactions• A fusion reaction is a

nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus.

• It is difficult to make fusion reactions occur because positively charged nuclei repel each other.

•Fusion powers stars and may also be used in thermonuclear bombs (60 megatons)

• A fusion reaction is a nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus.

How Fusion Reactions Work

• A fission reaction splits up a large nucleus into smaller pieces.

• A fission reaction typically happens when a neutron hits a nucleus with enough energy to make the nucleus unstable.

•Fission is used in nuclear power plants and powered the first atomic bomb (21 kilotons)

Fission Reactions

• The average energy of the nucleus for a combination of molybdenum-99 (Mo-99) and tin-135 (Sn-135) is 25 TJ/kg.

• The fission of a kilogram of uranium into Mo-99 and Sn-135 releases the difference in energies, or 98 trillion joules.

How Fission Reactions Work

Nuclear Reactors (Fission at Work)

Conservation Laws• There are conservation laws that apply to the type of particles

before and after a nuclear reaction.

– Protons and neutrons belong to a family of particles called baryons.– Electrons come from a family of particles called leptons.

Rules for Nuclear Reactions

• Nuclear reactions obey conservation laws.

• Energy stored as mass must be included in order to apply the law of conservation of energy to a nuclear reaction.

• Nuclear reactions must conserve electric charge.

• The total baryon number before and after the reaction must be the same.

• The total lepton number must stay the same before and after the reaction.

Calculating Nuclear Reactions

• The nuclear reaction above is proposed for combining two atoms of silver to make an atom of gold.

• This reaction cannot actually happen because it breaks the rules for nuclear reactions.

• List two rules that are broken by the reaction.