Radioactivity and particles - Shawon Notes

Physics > Section 7: Radioactivity and Particles

a) Units

7.1 use the following units: Becquerel (Bq), centimetre (cm), hour (h), minute (min), second (s).

Unit of radioactivity: Becquerel (Bq)
Unit of length: centimetre (cm)
Unit of time: hour (h)
Unit of time: minute (min)
Unit of time: second (s)

b) Radioactivity

7.2 describe the structure of an atom in terms of protons, neutrons and electrons and use symbols such as (14/6)C to describe particular nuclei

An atom is a tiny particle with nucleus in the centre and electrons orbiting it. A nucleus is made up of proton and neutron.

An atom is presented in this way = YXZ
Z=Symbol of the atom
X=Mass Number
Y=Atomic Number

7.3 understand the terms atomic (proton) number, mass (nucleon) number and isotope

Atomic Number: Atomic number is the number of protons in an atom

Mass number: Mass number is the addition number of protons and neutrons

Isotope: Isotope is an element which have the same atomic number as the original atom but different mass number.

7.4 understand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random process

When a unstable nuclei decay they give out ionising radiation. Ionising radiation causes atom to gain or lose electrons to form ions. Basically there are three types of ionizing radiation: alpha, beat and gamma.

Alpha radiation: Alpha radiation consists of fast-moving helium nucleus.

Beta radiation: Beta radiation consists of fast-moving electron.

Gamma rays: Gamma ray is an electromagnetic wave.

7.5 describe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating power

Radiation Ionising power Penetrating range in air Example of range in air Radiation stopped by
Alpha,α strong weak 5-8cm paper
Beta, β medium medium 500-1000 cm Thin aluminium
Gamma, γ weak strong Virtually infinite Thick lead sheet

7.6 describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiation

Alpha decay: In alpha decay, alpha particles takes away 4 nucleons with itself which reduce the mass number of the element by 4. Alpha particles have 2 protons with it, which reduce the atomic number of the element by 2.

Beta decay: Beta particle practically has no mass, so it doesn’t affect the mass number of the element. As beta particles has a charge of -1, the elements atomic number is increased by +1.

Gamma decay: Gamma ray is an electromagnetic wave and doesn’t affect the mass number or atomic number of the element.

7.7 understand how to complete balanced nuclear equations

In a nuclear equation, in the left hand side the total mass number should be equal to the mass number in the right hand side. And the atomic number should be equal in both sides.

Here, Uranium experienced an alpha decay:

Here, Lithium faced a beta decay:

7.8 understand that ionising radiations can be detected using a photographic film or a Geiger-Muller detector

Photographic film is a traditional way to detect ionising radiation. Ionising radiations imprints photographic plates.

Geiger Muller tube is used to measure the level of radiation. It is a glass tube with an electrically conducting coating on the inside surface. The tube has a thin window made of mica. The tube contains low pressured gases. In the middle of the tube, there is an electrode which is connected to a high voltage supply via a resistor. When ionising radiation enters the tube through the glass, it causes the low pressured gas to form ions. As ions are charged particle they allow to flow a pulse of current in the electrode which is detected by an electronic circuit.

7.9 explain the sources of background radiation

Background radiation have many sources. Billions of years ago, when the earth formed, it contained many radioactive isotopes. Some of them are still decaying in the Earth’s crust.

Violent nuclear reaction in stars are producing very energetic cosmic rays which continuously bombard the Earth.

Some of the radioactive isotopes are inside our body which formed from nuclear reactions in stars at the very beginning of the Universe. Also we breathe small amount of carbon-14.

There is also sources of artificial radiation from nuclear stations and bombs, and use of radioactive materials in industry and medicine.

7.10 understand that the activity of a radioactive source decreases over a period of time and is measured in Becquerels

Radioactive substance keeps decaying in a random process. As it decays, its activity is reduced over a period of time. The unit of Radioactivity is Becquerels.

7.11 understand the term ‘half-life’ and understand that it is different for different radioactive isotopes

“Half-Life” is the amount of time taken for the activity of any radioactive substance to reduce to half. Each radioactive isotope decays in different speeds. So half life is different for different types of isotopes.

7.12 use the concept of half-life to carry out simple calculations on activity

Plot the activity of the graph against time. Point out the half of the activity and draw line to match the time as done in the figure. The time is your half-life.

7.13 describe the uses of radioactivity in medical and non-medical tracers, in radiotherapy, and in the radioactive dating of archaeological specimens and rocks

Radioactive isotopes are used as tracers to help doctors indentify diseased organs. The tracer is swallowed by the patient. Inside the body, it emits gamma ray which is traced outside by GM-tube and the disease can be detected.

Radioactive isotopes are also used in treatment of cancer. Cancerous cells are targeted and emitted beta ray which kills the cancer cells inside the body.

Every substances contains little amount of radioactivity. Ancient object’s radioactivity is measured and archeologist tells the age of the object.

7.14 describe the dangers of ionising radiations, including:
  • Radiation can cause mutations in living organisms
  • Radiation can damage cells and tissue
  • The problems arising in the disposal of radioactive waste
and describe how the associated risks can be reduced.

Ionising radiation can damage the molecules that make up the cells of living organism and damages cell and tissues. Eventually, later cell might behave in an unexpected way called mutation. Once mutation occurs, it transfers from parent to children and so on for hundreds of generation.

Radioactive waste might damage environment. So they are stored in sealed thick containers that is capable of containing the radioactivity for a long period of time.

c) Particles

7.15 describe the results of Geiger and Marsden’s experiments with gold foil and alpha particles

Geiger and Marsden made an experiment with alpha particle. They shoot alpha radiation to a thin gold foil. The gold foil was surround by zinc sulphide screen which detected the presence of alpha particles. The result was, few alpha particles went straight through the foil, some bended a little bit and some deflected at high angles.

7.16 describe Rutherford’s nuclear model of the atom and how it accounts for the results of Geiger and Marsden’s experiment and understand the factors (charge and speed) which affect the deflection of alpha particles by a nucleus

After the experiment of Geiger and Marsden, Rutherford came to a conclusion that most of alpha source went through the foil was through empty space of atoms. Other deflection occurred because of the nucleus which is positively charged and so is alpha particle. That’s why they repelled each other and deflected. Rutherford gave the evidence that an atom is consisted of a positive nucleus in the centre and rotating electron outside in the mostly empty space.

7.17 understand that a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy in the form of kinetic energy of the fission products

Fission: If unstable nuclei split up to form stable nuclei, the process is called Fission. Urnaium-235 can also be split up by collide a neutron with the nuclei. As it is done so, the nucleus becomes unstable and split up to form Krypton and Barium (stable atoms) and three nucleus and gamma rays.

7.18 understand that the fission of U-235 produces two daughter nuclei and a small number of neutrons

When a radioactive isotope splits it forms a stable nuclei which is called daughter nuclei. Uranium-235 produce two daughter nuclei of barium-144 and krypton-89 and three neutron.

7.19 understand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei

When a U-325 splits, it gives out three neutrons. This three neutrons again hit other uranium nucleus and gives out nine neutrons. These nine neutrons hits other nuclei and keep on continuing fission reacted. This type of reaction is called chain reaction.

7.20 understand the role played by the control rods and moderator when the fission process is used as an energy source to generate electricity.

In a chain reaction, huge amount of energy are produced which is used to generate electricity. In the chain reaction if the neutron behaves in unexpected way, explosion might occur. Therefore, moderator are used to slow neutrons and slow the fission reaction to a steady and safe way. Control rods are used to absorb neutrons completely to shut down the fission process.

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