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Radioactive Physics - Alpha, Beta & Gamma Radiation

Nucleur model of an atom

Scientist used to believe that the atom was a neutral charged object which had negative charges evenly spaced

This was known as the plum pudding model

However, Scientists realised that the atom follows a different model and this was proven by Rutherford's alpha scattering experiment

A diagram of rutherford's alpha scattering experiment which shows how alpha particles are deflected

Alpha particles where ejected towards a thin gold leaf and there were different observations

The observations of this experiment must be remembered.

  • Most of the particles passed straight through the gold leaf
  • This is because, the atom is mostly empty space and this also shows that the positive & negative charges are not evenly spaced.

  • Few were deflected by a slight angle of less than 90°
  • This is because when the alpha particles passes near the nucleus they are deflected by a bit. This shows the nucleus is positively charged and so the alpha particles +2 experiences a repulsive force

  • Very few only deflected completely or by an angle over 90°
  • This is around 1/20000 alpha particles and this shows that the nucleus of an atom is so tiny and densed at the center and positively charged. When alpha particle collide head on or sideways to the nucleus, they will deflect by an angle more that 90°

    Deflection diagrams

    You will need to know how to draw some of these deflections

    deflection of alpha particles in a atom and by the plum pudding model

    1. Most pass through without any deflection

    2. Some pass with little deflection

    3. Very few pass with a large deflections especially when the alpha particle collides with the tiny positive nucleus

    Also compare the idea of the plum pudding model and Rutherford's experiment.

    Basic Definitions

    You will need to know some basic terms of an atom

    Let us first see the structure of an atom

    structure of an atom with electrons and nucleus. A labelled atom


    It contains both neutrons and protons of the atom and so it's positively charged. Why? This is because, neutrons are neutral and protons are positively charged. The nucleus is very small and highly densed and is at the center of the atom.


    These are known to be fundamental particles which orbit around the nucleus of an atom in energy levels. If you need to know more go to the leptons section.

    The Key

    A period key to identify the atomic number and mass number of an atom

    Atomic Number ( Z )

    The total number of protons in the nucleus of an atom

    The proton number of an atom identifies the element. This is a fundamental fact and this proves that isotopes are of the same element

    Mass Number ( A )

    The total number of protons and neutrons in the nucleus of an atom

    It is the same as the nucleon number

    Particles in the nucleus of an atom are called nucleons. So if there are 7 protons and 8 neutrons, it has 15 nucleons.


    Atoms of the same element with the same number of protons but, different number of neutrons

    The definition defines only atoms and not for ions

    So really we usually compare atoms rather than ions so that's why isotopes usually have the same chemical properties because, they have the same number of electrons in the valence shell

    However, the physical properties will be different such as the melting point and the density

    This is because, the Physical properties depends on the nucleur properties of an atom and the number of electrons determine the chemical properties

    So isotopes are always neutral and have the same proton number but, have different neutron or mass number

    Nucleur density

    It is the mass of the nucleus per unit volume of the nucleus of the atom

    Nucleur density = Mass/Volume

    To calculate the mass of the nucleus, we must find the total number of neutrons and protons (nucleon number). We also need to remember that the mass of a proton is same as the mass of a neutron and it is 1.67*10-27kg

    So we Multiply to find the total mass

    Mass of nucleus = Nucleon number * 1.67*10-27kg

    To find the volume of the nucleus, we need to to know the diameter of a typical nucleus is 10-15m. Most of the time they will give it.

    Volume = 4/3 * πr3

    Nucleur density = Mass/volume

    You will notice something. That the density is extremely high. This is because, the nucleus is strongly packed using strong nucleur force. But why is the real density way less than the nucleur density?

    This is also because, the atom is mostly empty space and so combining the density of the empty space and the tiny densed nucleus will give an overall density which is alot less

    Radioactive decay

    It is the spontaneous random decay of a unstable nucleus by emiting ionising radiation inorder to be stable

    These release radiations such as alpha beta and gamma radiation.

    We will need to know some terms

    • Random
    • The time taken for a particle to decay is unknown or random. However, as an overall picture, the average time can be found by using the Decay constant

    • Spontaneous
    • The rate of the decay is not affected by external factors such as temperature

    • Ionisation
    • The ability to strip electrons from a gas molecule to form gaseous ions

      Remember that a particle can ionise another atom if it has sufficient energy or charge

    • Penetration

    The ability for a radiation to pass through materials

    Laws conserved during decay

    So what laws are conserved during decay:

    • Law of conservation of charge and nucleon number
    • Law of conservation of Mass-energy
    • So at both sides the mass-energy is conserved. The products usually have less mass than the reactants but, the fast moving Beta particle(or any radiation) is created. The energy due to this kinetic energy compensates this loss in mass. This is called the mass-energy conservation.

    • Law of conservation of momentum
    • Usually we don't see this in normal equations but, momentum is always conserved.

      Types of radiation

      There are 3 types of radiation we must know and their characteristics

      Type Nucleur symbol Nature of radiation
      Alpha radiation He - 4 or 42α It's a helium nucleus with 2 protons and 2 neutrons
      Beta radiation - e or 0-1β It is a fast moving electron
      Gamma radiation γ It is a high frequency electromagnetic wave or a gamma photon

      Now we will check their ionisation and penetration

      Type Ionisation Penetration
      Alpha radiation Greatest ionisation Lowest penetration
      Beta radiation Medium ionisation Medium penetration
      Gamma radiation Least Ionisation Highest penetration

      Alpha has the lowest penetration and greatest ionisation as it has the highest charge of 2+ and it is also the slowest. So it has a greater ionisation effect and because it ionises more it loses more energy and thus can't penetrate far. Also alpha particles are stopped by a few cm of air or a sheet of paper or skin

      Beta has medium penetration and ionisation as it is charged but, way faster. So it has greater penetration. It is stopped by few cm of aluminum

      Gamma has the least ionisation and due to this, it has the greatest penetration. It is never truly stopped but, reduced almost completely by some cm of lead or few meters of concrete

      The pentrations of alpha and beta and gamma radiation. Gamma is stopped by a block of lead. Beta is stopped by a block of aluminium. Alpha is stopped by a sheet of paper

      We will see some more details

      Type Speed in terms of c KE
      Alpha radiation 5%c More than β
      Beta radiation 99%c Range of KE as due to range of speeds
      Gamma radiation c High - depends on the frequency- usually a specific value. Whereas, beta always changes.

      Alpha decay

      a radioactive equation where alpha particle is released. The proton number decreases by 2 and the mass number decreases by 4. Total Nucleon and atomic number is conserved

      Always remember that during alpha decay, the nucleon number decreases by 4 and the proton number decreases by 2.

      And a helium - 4 nucleus is formed

      Beta decay

      Remember that there are 2 types of beta decay so we will evaluate each:

      • Beta(-) decay
      • This is usually the default decay whenever they talk about beta decay

        an example of a beta minus decay where an electron and a antineutrino is formed

        Always in beta(-), an neutron becomes an electron and a proton and so The Nucleon number or mass number never changes but only the proton number increases by 1 and so it becomes the next element. Also an electron antineutrino is formed.

        A weak interaction which converts down quark to an up quark

        We need to know that a down quark is turned into an up quark due to the weak force during this process

        Neutron → Proton

        ddu → uud

        d → u

        Why? was an electron antineutrino formed rather than a neutrino?

      • Beta(+) decay
      • In this a proton is converted to a neutron and a positron and a electron neutrino

        a beta positive decay or a positron emission where an neutrino is formed

        Always the Mass number remains constant in any beta decay but in beta (+) the atomic number decreases to form the stable product.

        Also you need to know that an up quark turns into a down quark during this process

        Proton → Neutron

        uud → ddu

        u → d

      Gamma radiation

      There is no change in Mass number or proton number as no particles are released except energy. So this allows particles to release excess energy.

      a gamma decay where gamma photons is released

      Magnetic fields and electric fields

      As gamma radiation is neutral, it is not deflected by an electric field or a magnetic field

      Beta and alpha are deflected in an electric field

      the deflection of radiation in electric fields

      Always as alpha is +2 it deflects towards the negative plate, whereas the beta particle deflects towards the positive plate. However, as beta particle has a lower mass to charge ratio, the deflection is relatively higher than alpha particle.

      So when we usually compare the deflection of the same charge to find which one has the greater deflection we use this equation

      Deflection ∝ 1/(mass/charge)

      For example, a proton has a mass of 1 and a charge of +1

      It has a mass to charge ratio of 1.

      Another particle of charge +2 and a mass of 4 has a mass to charge ratio of 2

      So relative to the first one the alpha particle has twice the mass-charge and so twice as less deflection or further apart

      Alpha and beta rays are both deflected in a magnetic field but, gamma rays are not deflected.

      The deflection of radiation in an magnetic field
      Magnetic field into the page.

      Just imagine that alpha is current as current is the flow of positive charge.

      Then use Fleming's left hand rule and find the direction of the motion:

      1. The thumb is the deflection direction

      2. The index is the direction of the magnetic field(in or out of the page).

      3. The middle finger is the direction of the alpha particle.

      So to find beta decay, always remember that beta deflects in the opposite direction of alpha particles and also by a larger angle

      Elementary particles

      All things are made from the smallest indivisable particles known as elementary particles

      You will only need to know these particles:

      • Quarks
      • There are 6 types of quarks and you need to remember them all but, there is an easier way to remember there charges

        Charge = +2e/3 Up Quark Charm Quark Top Quark
        Charge = -1e/3 Down Quark Strange Quark Bottom Quark

        So what this table shows us that the particles in the first row have the same charge of +2e/3 where as the 2nd row has charges of -1e/3

        Also remember that a quark has another similar but, opposite name like top and bottom quarks

        Remember this table!

        Protons and neutrons are made out of elementary particles called quarks

        Quarks both up and down quarks. Up quarks have a charge of 2e/3 whereas down quark has a charge of -e/3

        Remember that neutrons has 2 down quarks and 1 up quark

        Remember that protons have 2 up quarks and 1 down quarks

        If you do forget, use this method below!

        Charge of a proton is +1e

        +1e = 2e/3 +2e/3 - 1e/3

        So we can find the number of up and down quarks

      • Electron and Neutrinos
      • An electron is considered to be an elementary particle but, there are many types of electrons(actually 3 types):

        1. Muon

        2. Tau

        3. Electron

        They all have the same charge but, we only need to know electrons

        And for each particle there is a corresponding neutrino

        1. Muon Neutrino

        2. Tau Neutrino

        3. Electron Neutrino

        Mostly, we talk about electrons and the electron neutrinos in beta decay only.

        Also, Neutrinos are considered to be massless and chargeless so they don't affect the decay equation but, must be still added. This is because, it still carries a very little energy

      • Photons
      • These are known to be called gamma photons as they are responsible for gamma radiation. Actually, gamma radiation are not just waves. According to Heinsberg's uncertainty principle, waves could be particles at the same time.

        This radiation is responsible for pair production and annihilation of antimatter and matter


        So for each elementary particle, you have seen there is a corresponding antiparticle which has the same mass but, Opposite charge and spin. When representing them we put a dash above the original symbol

        We will give some examples

        Charge = -1 Electron
        Charge = +1 Positron

        We will also check the antiparticle of neutrinos

        Charge = 0 Electron Neutrino In beta(+) decay
        Charge = 0 Electron antineutrino In beta(-) decay

        Also the antiparticle has the opposite properties

        Classes of Particles

        Particles can be classified into 2 main groups

        • Hadrons
        • These are large particles such as protons and neutrons and are made out of quarks

          These particles are affected by strong nucleur force and the weak forces. Actually all types of forces.

          Hadrons can be divided in two more groups

          • Baryons
          • These are particles which contain 3 quarks together

            For example the proton - uud

          • Mesons
          • These contains 2 quarks together. Usually an antiquark and a quark

            These includes phi+ meson - up quark and antidown quark

          We will need to know one more thing. The baryon number in an equation is always conserved and antiparticles have the opposite baryon number. But more information is not necessary...

        • Leptons
        • These are electrons and neutrinos and their antiparticle

          These particles are affected by all other forces except the strong nucleur force

          The below part is not necessary but, it can help you with identifying if a neutrino or antineutrino is produced

          During decay, the lepton number is conserved. The antiparticles have the opposite properties, which means that an antineutrino and a positron has a lepton number of -1 whereas, neutrinos and electrons have a lepton number of 1

          An example of beta decay. where an electron and antineutrino is formed

          So on the left side, it only contains neutrons and protons. It has no leptons so it is 0. On the right side, it has a electron with a lepton number 1 and another lepton must be present which is a antineutrino of lepton number -1 to cancel the other lepton and equate to 0

          0 = +1-1

          Fundamental Forces

          There are 4 forces which govern the universe:

        • Gravitational Force
        • Forces between masses

          This is the weakest force in the universe but, it also has the largest range. In fact, the range is said to be infinite!

        • Strong Nucleur Forces
        • Attraction forces which holds up the positive nucleus together. As the nucleus is made out of protons - positively charged. The strong nucleur force overcomes the repulsion forces between the protons.

          You will need to know some details of this force:

          1. This is the strongest force in the universe

          2. This doesn't affect leptons but, only Hadrons and Quarks.

          3. This force is highly short ranged and doesn't extend to the outer shell of an atom.

        • Weak interaction or Weak force
        • All you have to know is that this force is necessary for both types of beta decay as we have seen it changes neutrons to protons

          The weak force is used to convert upquarks into down quarks

          To change the identity, this force is required. So this is not technically a force but, an interaction

        • Electromagnetic Force( Electrostatic force )
        • These are forces between charged objects. Anything that has a charge, creates an electric field around it and causes other charges to experience a force


          annihilation of antimatter and matter release gamma radiation and energy

          When a particle and the corresponding antiparticle meets, they annihilate and release energy in the form of Gamma radiation or two gamma photons

          So Momentum, charge and mass-energy is conserved

          Pair Production

          This is the reverse of annihilation.

          When a Gamma ray is passed through a nucleus, a particle and the antiparticle is formed. The nucleus is essential to conserve momentum.

          pair production where gamma ray is coverted to antimatter and matter


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