RADIOACTIVITY ENOTE

WEEK FOUR

SUBJECT: BASIC SCIENCE

CLASS: GRADE 9

TERM: SECOND

TOPIC: RADIOACTIVITY

MEANING OF RADIOACTIVITY: Inside an atom, there are three kinds of particles: protons, neutrons and electrons.

The nucleus contains protons and neutrons. The region beyond the nucleus is negatively charged and contains electrons that balance out the charge of the protons. There are roughly as many protons as neutrons, which is why the atomic weight is about (but not exactly) twice the atomic number. In the nucleus, there is always a force trying to push the protons apart because like charges repel each other. Provided there are not too many protons in the nucleus, other forces can hold the protons together. But if the ratio of protons to neutrons is not within certain limits, protons may not be held firmly together, and they form an unstable nucleus. This is what makes isotopes of some elements radioactive. For example, carbon, the element found in all living things has the chemical weight of 12 and is written as carbon-12, but the radioactive (isotope) version has two extra neutrons, so the symbol is carbon-14. The radioactive form behaves chemically just like the non-radioactive form, although one never changes into the other.

  Radioactivity is a very interesting phenomenon in nature. It’s a spontaneous and random phenomenon whereby nuclei of certain chemical elements like Uranium, radiate gamma rays (High frequency electromagnetic Radiation), beta particles (Electrons or positrons) and alpha particles (Helium Nuclei). By the emission of these particles and radiation, the unstable nucleus gets converted into a stable nucleus. Through radioactive decay, the nucleus approaches a more stable neutron to proton ratio. The phenomenon was discovered by Henri Becquerel.

RADIOACTIVE ELEMENTS: A radioactive element is an element with an unstable nucleus, which radiates alpha, beta or gamma radiation and gets converted to a stable element. Radioactivity is the spontaneous disintegration of the radioactive elements. Nuclear fission is a form of radioactive decay.

                                                         Energy   Radiation

                                                         Particle   

                               Radioactive Decay

 

  A radioactive element is a fundamental element whose atomic nuclei demonstrate the phenomenon of radioactivity. The name ‘radioactive’ may suggest to you that radioactive elements radiate radio waves, but unfortunately that is not so! The name ‘radioactivity’ has nothing to do with radio waves. Some of the radioactive elements are Radium (Ra), Thorium (Th), Uranium (U), Plutonium (Pu), Americium (Am), Neptunium (Np), Radon (Rn), etc.

TYPES OF RADIATIONS AND PROPERTIES: Radioactive decay releases different types of energetic emissions. The three most common types of radioactive emissions are alpha particles, beta particles, and gamma rays.

A. Alpha (α) Particles: These are fast moving helium atoms. They are positively charged particles. They have high energy, typically in the MeV range, but due to their large mass, they are stopped by just a few inches of air, or a piece of paper. It means they have little penetrating power. These particles are deflected by electric and magnetic fields. Examples of alpha particle emitting radionuclides include ²³⁸U, ²³⁹Pu, and ²⁴¹Am.

₉₂²³⁸U              ₉₀²³⁴Th + ₂⁴He

 ₉₄²³⁹Pu             ₉₂²³⁵U + ₂⁴He

 ₉₅²⁴¹Am             ₉₃²³⁷Np + ⁴₂He

              ₉₄²³⁹Pu                        ₉₂²³⁸U

 

 

                                        α-      

 

                                α particle=⁴₂He

Emission of Alpha Particles

B. Beta (β) Particles: These are fast moving electrons. They are negatively particles. They typically have energies in the range of a few hundred keV to several MeV. Since electrons are lighter than helium atoms, they are able to penetrate further, through several feet of air, or several millimeters of plastic or less of very light metals. It means they have intermediate penetrating power. These particles are deflected in the opposite direction to that of α-particles.

      ₉₈²³⁸Ra     neutrino              ₂₂⁸⁸⁹Ac

         

                                                          

                                                            Β^—particle=⁰_-1e

  Emission of Beta Particles

C. Gamma (γ) Rays: These are photons, just like light, except of higher energy, typically from several keV to several MeV. These radiations are neutral i.e. they do not have any charges.

₉₄²⁴⁰Pu                    ²⁴⁰₉₄Pu  

 

 

 

γ-radiation: high-energy electromagnetic waves

Emission of Gamma rays

  Depending on the energy of those radiations, they can be stopped by a thin piece of aluminium foil, or can penetrate several inches of lead. They have the greatest penetrating power. The radiations are not deflected by electric and magnetic fields as they have no charges.

            Paper Plastic Lead  Concrete

Alpha

Beta

Gamma and X-rays

Neutron

Penetrating power of different particles.

 

X-rays and gamma rays are same, the difference is how they were produced. Gamma rays are electromagnetic radiations with energies higher than X-rays. X-rays are produced when electrons of an atom jump from one orbital location to another. The gamma rays are released when an atomic nucleus releases its excess energy. Nuclear transition involve larger energies than the atomic transitions. γ- rays emission does not change the Mass number or Atomic number. If an atom is in the excited state, it comes to the stable state by emitting  γ-radiation.

Properties of Alpha, Beta and Gamma Radiation

 

Property Composition	Alpha radiation Alpha particle (helium nucleus)	Beta radiation Beta particle (electron)	Gamma radiation High-energy electromagnetic radiation
Symbol	42He	01e or β	γ
Charge	2+	1-	0
Mass	4	1/1837	0
Penetrating power	Low	Moderate	Very High

ASSIGNMENT

OBJECTIVE:

1. The phenomenon of radioactivity was discovered by A. Charles Darwin B. Gregor Mendel C. Francis Galton D. Henri Becquerel E. Michael Faraday   

2. The radioactive elements used in nuclear power plants are plutonium and- A. thorium B. tritium C. radon D. radium E. uranium

3. Radioactivity is used for the following except A. carbon dating B. kill germ C. surgery D. trace chemical reaction E. treat cancer

4. Which of the following statement is true A. electron has a positive charge B. Electron has no charge C. Neutron has a negative charge D. Proton and neutron have negative charge E. Proton has a positive charge

5. Elements in the same group in the periodic table have the same A. atomic mass B. atomic number C. neutrons D. numbers of shells E. number of valence electrons

6. Which of these particles make up the nucleus of an atom? A. electrons B. Electrons and neutrons C. Electrons and protons D. Neutrons and protons E. Protons

THEORY:

1. What is radioactivity?

2. List three types of radiation and state a property of each.

3. What is beta decay?