Radioactivity!

1. Radioactivity!

2. How was radioactivity discovered? In 1896 – The scientist Henri Becquerel left a piece of uranium rock on a photographic plate in his closed drawer. The most unexpected thing happened! A very strong image of the rock was produced on photographic plate! Uranium rock Radiation emitted by the rock. But how is this possible? Where is the energy coming from? A photographic plate produces an image when any radiation falls on it. Henri Becquerel deduced that the radiation of energy must be coming from inside the rock! What is more – The production of the radiation was not affected by temperature, pressure, electric fields or magnetic fields! What was this strange emission? The radiations were called Becquerel rays. Today – the process is known as radioactivity.

3. The atom and the nucleus Radiations from radioactivity come from the nucleus of atoms. This diagram should help you picture the atom and nucleus in your mind. Electron The Atom: (Charged and light) Nucleus Proton Neutron (Charged and heavy) (Uncharged and heavy)

4. What causes radioactivity? It is found by experiment that the radiations from radioactive decay come from the nuclei of unstable atoms. (Nucleus of the Atom) • Only unstable nuclei undergo radioactive decay. • Radioactive decay makes the nucleus more stable. • Radioactive decay is spontaneous (since the products are more stable than reactants) • All elements with atomic numbers 81 to 92 have unstable nuclei – They are all radioactive. So what causes radioactivity? – Instability in the nucleus of atoms

5. Natural Radioactivity and Induced Radioactivity We have discussed that the factor that makes an atom radioactive is instability of the nucleus. There are two ways that a nucleus can be unstable: 1) The nucleus is naturally unstable. When we find these atoms in nature, we find that they are already radioactive. This is called Natural radioactivity. 2) We can make a nucleus unstable by making it absorb extra particles. This is called Induced Radioactivity. Here, we are artificially making a stable nucleus unstable. The official definition of radioactivity: The property of spontaneous disintegration of certain unstable atomic nuclei with the emission of certain radiations, is called radioactivity. Next: We are going to discuss the three types of radiations from radioactivity that are in your syllabus: Alpha radiations, Beta radiations and Gamma radiations.

6. Alpha radiations “Alpha Particle” can be written as: α-particle Alpha radiation is the emission of alpha particles. An alpha particle is a helium nucleus (Which contains 2 protons and 2 neutrons) Since a helium nucleus contains 2 protons (+ve charge) and 0 electrons (-ve charge), alpha particles are positively charged. Since a helium nucleus contains 2 protons and 2 neutrons, alpha particles are heavy.

7. Beta radiations “Beta Particle” can be written as: β-particle The neutrino is another product of beta decay. It is not in your syllabus Beta radiation is the emission of beta particles. A beta particle is an electron (Exactly the same as the electrons that orbit the nucleus). This electron, however comes from inside the nucleus. Since electrons are negatively charged, a beta particle is negatively charged. Since electrons are light, beta particles are light.

8. Gamma radiations “Gamma rays” can be written as: γ-rays Gamma radiation is a form of electromagnetic radiation (similar to light) with very high energy. If light is thought of as particles (photons), we can say that Gamma Photons have a higher energy than even X-ray photons. (VERY high energy) Just like light, Gamma rays are electrically neutral and mass-less. Gamma radiation is emitted when the nucleus is rearranging itself to become more stable. Alpha and Beta radiations are usually accompanied by Gamma radiations.

9. Elements and notations An element is a pure substance => a collection of similar atoms. A specific element is determined by its number of protons in the nucleus. For example, the element with one proton is hydrogen. The element with two protons is helium. A specific form of an element can be written as: A is the mass number (number of protons + number of neutrons in the nucleus). This is because the mass of the nucleus depends on both the protons and the neutrons. Question: Can you explain why the notation of hydrogen (1 proton and no neutrons) is ? Z is the atomic number (number of protons in the nucleus). If the number of protons changes, the element changes. E is the symbol of the element. For example, the symbol of hydrogen is H. The mass number is 2 + 2 = 4. Z= 4. The symbol of Helium is “He”. E = He Helium is an element with 2 protons and 2 neutrons in the nucleus. An alpha particle is a helium nucleus There are 2 protons in the nucleus. Z = 2.

10. Transmutations During alpha and beta decay, a particle is emitted from the nucleus. This means that the number of protons in the nucleus changes. A new element is formed! Changing one element into another is known as Transmutation. Daughter nucleus: More stable than parent nucleus Parent nucleus – Unstable Particle emitted from nucleus (e.g – α-particle or β-particle)

11. Transmutation due to alpha decay Radium is an element with 88 protons and 138 neutrons in its nucleus. Its nucleus is unstable. To become more stable, it undergoes alpha decay. Its nucleus changes and a new element is formed. Which element does it form? We know that an alpha particle is nothing but a helium nucleus. In the alpha particle, 2 protons and 2 neutrons are lost. The new Atomic number (Z) will be: 88 – 2 = 86. We know that Atomic number 86 gives a new element: Radon! (Symbol = Rn). This is found by looking at the periodic table. The full equation for the transmutation can be written as: Or 4 particles (2 protons and 2 neutrons) are lost in the alpha particle. The new mass number (A) is: 226 – 4 = 222.

12. Transmutation due to beta decay In Beta Decay, an electron is emitted from the nucleus. Electrons usually orbit the nucleus. Isn’t it strange that an electron is emitted from inside nucleus? Where does the electron come from? The electron is produced from the following reaction: Neutron (neutral) Proton (+ve) + Electron (–ve) (Notice how the charge balances) Carbon-14 is a form of the element Carbon with 6 protons and 8 neutrons in its nucleus. Its nucleus is unstable. To become more stable, it undergoes beta decay. (It releases an electron) Its nucleus changes and a new element is formed. Which element does it form? In beta decay, a neutron is converted into a proton (and electron). Therefore the nucleus will have one extra proton. So Z = 6 + 1 =7. The atomic number 7 gives a new element – Nitrogen! (N) The number of particles in the nucleus remains the same: 6 + 8 = 7 + 7 = 14. Therefore, A = 14. The full equation for the transmutation can be written as:

13. Half–life • Did you know? – In theory: Radioactive decay is a reaction that never ends!!! • The reactivity halves as the number of atoms left to react halves. • Therefore, the remaining atoms are slow to decay. • As a result, the time taken for a sample to reduce by half is constant. • The time taken for 50 out of 100 atoms of Carbon to decay to Nitrogen is the same as the time for 25 out of 50 atoms of Carbon to decay to Nitrogen. Half life is represented by the symbol: T1/2 . Radium has a half life of 1600 years. How much of 100g of radium will be left after 3200 years? T1/2 = 1600. Note: 3200 = 1600 + 1600. After the first 1600 years, only half the mass (50g) of Radium will be left. After the next 1600 years, 50/2 = 25 grams will be left. Answer = 25 grams.

14. Graphing Half–life Mass (mg) Half-life of this radioactive material is 0.5 days. Each material will have a unique half life. For example: T1/2 (Radium) = 1600 years. T1/2 (Iodine-131) = 8 days. Reaction goes till infinity. Because line never hits x-axis. Time (Days) Graph of how the mass in milligrams of radioactive material varies with time. From the graph: The time for the mass to halve from 100 milligrams to 50 milligrams = 0.5 days. The time for the mass to halve from 50 milligrams to 25 milligrams = 0.5 days. The time for the mass to halve from 25 milligrams to 12.5 milligrams = 0.5 days.

15. Induced Radioactivity Unstable nuclei are radioactive. In slide 5, we discussed that we can make a nucleus unstable by making it absorb extra particles. This is called Induced Radioactivity or Artificial radioactivity. Forming Phosphorus-30 = Unstable, Radioactive nucleus Aluminium nucleus = Stable nucleus. The neutron is a byproduct. We artificially make an alpha particle collide with aluminium nucleus. (An alpha particle is a helium nucleus) Phosphorus-30 nucleus = Unstable nucleus. Decays into Si-30 Nucleus. Half life = 30. Do you notice how we converted a stable, non-radioactive aluminium nucleus to a radioactive Phosphorus nucleus? This is induced radioactivity. The positron (positively charged electron) is a byproduct. This particle is not in your syllabus. Silicon-30 nucleus is product after decay.

16. Isotopes We have learnt that elements are defined based on their number of protons in the nucleus. What happens if the number of neutrons change? It is still the same element, but is a different form. An isotope is an alternative form of an element with the same number of protons but different number of neutrons. Here are four isotopes of Carbon: The isotopes have different mass numbers, since the number of neutrons in the nucleus differs. For example C-11 has 6 protons and 5 neutrons. C-14 has 6 protons and 8 neutrons. Notice: All isotopes have the same number of protons (6). Therefore, all of these forms are representations of the same element Here are 2 isotopes of Uranium: Uranium-235 is the form that is used as a fuel in reactors. Uranium-238 is the form that is most commonly found in nature.

17. Radio-isotopes Two other radio-isotopes of carbon. (unstable) Non-radioactive isotope of Carbon. Most stable and commonly found Radio-isotope used to determine age of fossils. Radio-isotopes are isotopes of an element that are radioactive. For example, C-12 is the form of carbon that is most abundant. In fact, plants and animals are largely made of Carbon-12. However, Carbon-14, which is less common is radioactive. The difference between C-12 and C-14 is that Carbon-12 contains 6 protons and 6 neutrons. Carbon-14 contains 6 protons and 8 neutrons. The two extra neutrons lead to instability. Did you know: A radioisotope of every element can be made? More than a thousand radioisotopes have been made or found. In fact, we made a radioisotope (Phosphorus-30) in the slide “Induced Radioactivity”. The stable form of Phosphorus is P-31.

18. Uses of Radio-isotopes Carbon-14 can be used to find the age of fossils and other historical objects: All living things have a fixed ratio of Carbon-14 in their bodies. When they die, however, the Carbon-14 begins to decay. Therefore, a fossil of an animal which has died very long ago will have very less Carbon-14.Bymeasuring the amount of Carbon-14, we can determine the age of fossils. Medical uses Radio-iodine is used to treat overactive thyroid glands. Radio-cobalt is used to treat cancer. Radio-sodium is used to study the action of medicines. Industrial uses Agriculture: Radio-phosphorous is used to determine the correct phosphate fertilizer for a particular soil and crop. Manufacturing: Radio–Americium is used to monitor to maintain uniform thickness while manufacturing sheets of steel: By passing radiation through a sheet we can monitor the sheet’s thickness.