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Radioactivity. Radioactivity : the process by which atoms emit energy in the form of electromagnetic waves, charged particles, or uncharged particles.

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Radioactivity: the process by which atoms emit energy in the form of electromagnetic waves, charged particles, or uncharged particles.

  • In 1896, Henri Bequerel discovered that uranium and other elements emitted invisible rays that can penetrate solid material. These materials are now called “radioactive”

  • The most common unit for radiation is counts per second (known as a Becquerel, Bq)

CBC archives - radioactivity


Applications & Exposure


Natural Sources

  • Exposure to radiation is unavoidable because radioactive elements occur in nature.

  • - some forms of carbon and potassium are absorbed by your body are radiactive.

    • C  600 Bq/kg of body mass

    • K  110 Bq/kg “ “ “

Alps Iceman: 5,300 years old


  • Cosmic rays: high energy radiation coming from space.

    • higher exposure than normal when flying at high altitudes


- Radioactive uranium and radium are found in soil and rocks. When they disintegrate, the produce another radioactive atom: radon gas.

Uranium deposits around the world


Artificial Sources

  • Nuclear power

    • Electricity

    • Submarines

    • - Space probes

February 1, 2005—The U.S. Navy released this photograph last Thursday of the nuclear submarine San Francisco, which crashed headlong into an uncharted undersea mountain near Guam on January 8. Standing more than three stories high and with classified technology veiled by a tarp, the fast-attack submarine is shown awaiting repairs in a Guam dry dock.

The impact shredded the submarine's nose, killed one sailor, and injured 60 more. The sailors were largely protected by the vessel's reinforced inner hull, which did not rupture. After the wreck, the crew quickly ascended and sailed along the ocean's surface back to their base in Guam.


The Cassini space probe is powered by energy released from 28.8 g of radioactive Pu. The radiation is absorbed by ceramic surronding the Pu and the heat is converted ot electricity. Each Kg of Pu emits 556 J each second.


  • There is a lot of radiation released inside nuclear reactors and by the spent fuel (but still less than is emitted by x-ray machines)

  • Some coal-fired power plants emit more radioactivity than nuclear plants (uranium in coal ash)


- Nuclear bombs


In medicine: we use a unit called Sieverts

(10 Sv is a lethal dose for most tissues)

  • Medical applications:

    • X-rays are used for diagnosis

    • Cancer treatment


Effects of Radiation

  • Ionizing radiation carries energy values on the order of 1000’s of eV.

  • Typical chemical bonds can be broken by radiation energy of 5 or less eV.

- Cells do have repair mechanisms, but they are not perfect and they can be overwhelmed.

- Large particle radiation (such as α particles) can do more damage per unit of energy.


  • Effects of Cell Damage:

    • 1) Cell dies: organelles or enzymes can no longer function

  • 2) Cell survives: Damage is passed on to daughter cells in the form of mutations (some mutations can lead to cancer).

  • Cells undergoing division are more susceptible to damage


Radiation Strength

  • Depends on three factors:

    • The kind of particles/EMR emitted

    • Amount of radioactive material present

    • The rate at which atoms disintegrate to emit radiation (1 count/second = 1Bq) – depends on the isotope.


The Nucleus and Nuclear Reactions


Structure of the Nucleus - Review

Which elements are these?

(protons are shown in red and neutrons in white.)

They are both carbon. Both have 6 protons. i.e. they both have an atomic number of 6.

These are two isotopes(varieties) of carbon.

- same chemical properties, but different physical properties (e.g. how they behaving in nuclear reactions)

- different number of neutrons, therefore different atomic masses

In nuclear physics, we often call atoms nuclides.




Mass number = 12

p+ = 6

n0 = 6

Mass number = 14

p+ = 6

n0 = 8

Mass number = #p+ + #no

Atomic number = #p+

















The Strong Nuclear Force

  • Using accelerators, scientists have discovered the forces that hold nuclei together

The big circle marks the location of the Large Hadron Collider (LHC) at the European particle physics laboratory in CERN. The tunnel where the particles are accelerated is located 100 m (320 ft) underground and is 27 km (16.7 mi) in circumference. The smaller circle is the site of the smaller proton-antiproton collider. The border of France and Switzerland bisects the CERN site and the two accelerator rings.


  • Nuclear forces act over very small ranges. (3 x 10-15 m)

  • Over 100 times greater than the electrostatic force.

  • The strong nuclear force is independent of the charge

    • The attraction is the same between:

      • p+ - p+

      • n0 - n0

      • n0 – p+


Unstable (Radioactive) Nulcides

  • Unstable nuclides tend to disintegrate causing:

    •  A different nuclide is to be produced

    • Energy to be released as radiation

  • Unstable nuclides have too few neutrons in relation to the number of protons.

    •  In general, the more protons in a nucleus, the more neutrons that are required to overcome the electrostatic repulsion.

  • All elements with atomic numbers greater than 82 exist only as unstable nuclides.


Types of Radiation

  • Rutherford discovered three types of radiation

  • Also discovered that elements transform into different elements during the process (called transmutation).

  • The original element is called the parent nuclide. The newly formed element is called the daughter nuclide.


Alpha Decay

  • Alpha particles (α) are helium- 4

  • They are ejected at high speeds but can be stopped by aluminum foil


For all nuclear reactions: NUCLEONS AND CHARGE ARE CONSERVED

i.e. The sum of the mass numbers on both sides of the arrow must be equal and the sum of the atomic numbers on both sides of the arrow must be equal

222 nucleons

222 nucleons

charge = +86

charge = +86


Beta Decay

  • A neutron decays into a proton and an electron.

  • The electron is ejected from the nucleus at a high speed – called a beta particle (β).

  • β particles can penetrate several mm of lead.


228 nucleons

228 nucleons

charge = +90

charge = +90


  • Gamma rays can be emitted along with an alpha or beta particle.

  • When a nucleus emits only a gamma (γ) ray, the energy of the nucleus is reduced but the mass number and the atomic numbers stay the same.

  • γ rays can penetrate many cm of lead.

Gamma Decay




  • Often, the same nuclide can undergo different decay modes…


Decay Series

… or go through a series of decays.


Example 1: Complete the balance equation:

Nuclear charge: 83 – 2 = 81



According to my periodic table, that must be

Nucleons: 210 – 4 = 206


What type of radiation is this?

Alpha Decay


Example 2: Complete the balanced equation and identify the radiation type.


Beta Decay


Other Decay Modes

  • Some radionuclides can transmutate by capturing an electron from the lowest energy level.

    • A proton is converted into a neutron

  • Positron emission: (same mass as an electron, but a positive charge)


Fission and Fusion

Nuclear Fission

  • The reaction used in all of the world’s nuclear power plants. The fuel is usually uranium, put plutonium can also be used.

  • Can be used in nuclear bombs.

  • Involves “splitting” an atom into smaller nuclides.

  • Initiated by a slow moving neutron.

Fission Animation

More animations


Example 3: Predict the missing fission product.


Nuclear Fission Chain Reaction

  • The emitted neutrons strike more uranium atoms, causing them to undergo fission.

  • This reaction is very hard to control.


Canada’s CANDU Reactor

  • Canadian Deuterium Uranium Reactor


Nuclear Fusion

  • The process that made the atoms that make you.

  • Two nuclide with extremely high energy collide to form a bigger nuclide.



Example 3: Predict the missing reactant.


Nuclear fusion as an energy source on earth is still experimental


Radioactive Dating

  • A sample of radioactive material consists of vast number of nuclei that don’t all decay simultaneously.

  • We can’t predict when a single nucleus will decay (it is governed only by probability)

  • The decay from parent nuclide to daughter nuclide follows a characteristic decay curve.


Radioactive Decay Curve


  • Rutherford noticed that the radioactivity of a sample of radon gas was reduced by half every ~1 minute.

  • This called the half-lifeof the isotope.

    • half-lives can vary from 10-22s to 1028 s, depending on the isotope.



  • Half-lives are always a uniform interval of time for a particular isotope.





  • More examples of half-lives:

  • - Polonium-214 ---1.6 x 10-4 s

    • Carbon-14 --------5730 years

  • If you have 10 g of carbon-14 when an organism dies, after 5730 years, you’ll have 5 g. After another 5730 years, you’ll have 2.5g.

  • The age of a material can be determined using radioactive dating


  • An equation that describes half-life

Original amount of parent nuclide

Amount or mass of the parent nuclide remaining

Number of half-lives that have passed


Effect of Strontium-90 on Squamous Cell Carcinoma in an Eastern Box Turtle (Terrapene carolina); Discussion of Alternative Treatment Modalities

Cheryl B. Greenacre, DVM, Dipl. ABVP - Avian and Royce Roberts, DVM, MS, Dipl. ACVR

Example 1:

If a 2.00 g sample of strontium-90 is produced in a reactor, how much will remain after 10.0 years have passed. (The half-life of Sr-90 is 29.1 years.)

1.58 g


Example 2:

A baby mammoth found frozen in a glacier is found to contain one quarter of its original carbon-14. Determine its age if the half life for the radioactive decay of carbon-14 is 5.73 x 103 years.

1.15 x 104 years


Extension example:

A pregnant ichthyosaur fossil is located just below a volcanic ash layer containing a ratio of uranium-235 to lead-207 of 4:1. Determine the minimum age of the fossil in years. (The half-life of U-235 is 7.13 x 108 a)

230 million years


Mass-Energy Equivalence

  • The mass of a nucleus is always less than the mass of all the separate nucleons (protons and neutrons)

  • This difference in mass is called the mass defect

  • Energy is required to make a nucleus (called the binding energy)

  • The binding energy is related to the mass defect by the equation E = mc2

E = mc2


Example 1

Determine the mass defect of an alpha particle.

alpha particle mass (2 protons, 2 neutrons) = 6.65 x 10-27kg

massprotons =2(1.67 x 10-27kg) = 3.34 x 10-27 kg

massneutrons = 2(1.67 x 10-27kg) = 3.34 x 10-27 kg

total mass of separate nucleons = 6.68 x 10-27 kg

mass defect = - = 0.03 x 10-27kg

  • In nuclear reactions, mass is converted to energy or energy is converted to mass

E = mc2


Example 2:

Calculate the energy produced in the reaction

mass2H = 3.34341 x 10-27 kg

mass3H = 5.00661 x 10-27 kg

masstotal = 8.35002 x 10-27 kg

massα = 6.6463 x 10-27 kg

massn = 1.6749 x 10-27 kg

masstotal = 8.3212 x 10-27 kg

Mass defect = 8.35002x10-27 kg – 8.3212x10-27 kg

= 2.882 x 10-29 kg

2.882 x 10-29 kg

E = mc2

E = (2.882 x 10-29 kg)(3.00 x 108 m/s)2

E = 2.59 x 10-12 J




In a CANDU reactor, 1 kg of fuel (natural uranium) produces 3.4 x 105 MJ of heat that is converted to electricity.

In oil and coal power plants, 1 kg of fuel produces about 4 MJ of heat


  • Energy may create matter through the process called pair production. The process must produce 2 particles whose total charge is zero, since charge must be conserved. Pair production requires a very high energy photon.

  • A particle and its antiparticle (antimatter) are often produced. Example: an electron and anti-electron (positron) have the same mass, but opposite signs.


Example 3:

A 8.50 x 1020 Hz photon produces an electron and an anti-electron. Determine the total kinetic energy of the particles.

Law of Conservation of Energy:

Photon energy = energy to make 2 particles + Ek

Ephoton =Eelectron + Eantielectron + Ek

hf = mc2 + mc2 + Ek

hf = 2(mc2) + Ek

Ek= hf – 2(mc2)

Ek= (6.63 x 10-34 J•s)(8.50 x 1020 Hz) – 2(9.11 x 10-31kg)(3.00 x 108 m/s)2

Ek= 4.00 x 10-13 J

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