Chapter II: Radioactivity PDF

Chapter II Radioactivity Chapter II Radioactivity...

Chapter II Radioactivity Chapter II Radioactivity 𝑨 CHAPTER II: RADIOACTIVITY 𝒁𝑿 is called the parent nuclide (le nucléide parent) 𝑨 𝟒 𝒁 𝟐𝒀 is called the daughter nuclide (le nucléide fille) In the previous chapter, we discussed the binding energy per nucleon and its relation to The previous nuclear reaction occurs and carry out a gamma energy (photon of light) symbolized the stability of the atomic nucleus. We remind that the nucleus is more stable when its EBn is 𝟎 𝟎𝜸. greater. For the existing elements of the periodic table, the binding energy per nucleon is increasing with the increase of the atomic mass till a maximum reached with the Iron-56 isotope. This nuclear reaction is frequent for chemical elements with a heavy nucleus having The later possesses the most stable nucleus among all elements. A=Z+N>200. In stable nuclides, the nucleus do not undergoes change. However, in an unstable one, the Example: nucleus spontaneously undergoes change. This change involves emission of radiation from the nucleus, a process by which the unstable nucleus can become more stable. The radiation emitted Bismuth-211 decay to give Thallium-207: from unstable nuclei is called radioactivity. 𝟐𝟏𝟏 𝟒 𝟐𝟎𝟕 𝟎 𝟐𝟏𝟏 𝟒 𝟐𝟎𝟕 𝟖𝟑𝑩𝒊→ 𝟐𝑯𝒆 + 𝟖𝟏𝑻𝒍+ 𝟎𝜸 𝟖𝟑𝑩𝒊→ 𝟐𝑯𝒆 + 𝟖𝟏𝑻𝒍 II.1. DEFINITION Uranium-238 decay to give a daughter nuclide Thorium-234: The radioactivity is the emission of radiations by the nucleus that is changing from a higher 𝟐𝟑𝟖 𝟒 𝟐𝟑𝟒 𝟎 energy level to a lower energy level. The released energy is emitted as a radiation. The 𝟗𝟐𝑼→ 𝟐𝑯𝒆 + 𝟗𝟎𝑻𝒉+ 𝟎𝜸 radioactivity is a result of the decay of an unstable isotope. As we can see for 211Bi or 238U, the number of protons is important (Z>83). Thus, the ratio (N/Z) is 1.54 and 1.59, respectively. In both nuclides, the repulsive electrostatic force II.2. TYPES (rising from the repulsion of protons) in the nucleus is more important than the strong nuclear II.2.1. Natural Radioactivity (Main types of radiation) force. The latter is not enough to counter the repulsion force, thus the nucleus search to reach a Some naturally occurring isotopes of certain elements have nuclei that spontaneously more stable level. This is achieved by breaking down the nucleus into less heavy one by rejecting disintegrate (breakdown), and we have no control over these processes. These include most some neutrons and protons. The estimation of the mass defect in both cases will reveal realizing isotopes with atomic numbers greater than 83 (Z>83). Each of these disintegrations occurs with of a quantity of energy in form of gamma rays. emission of one of the three types of nuclear radiation: alpha, beta particles and gamma ray as well as a considerable quantity of energy. II.2.1.2. Beta particles II.2.1.1. Alpha particles ( 𝟒𝟐𝑯𝒆, 𝟒𝟐𝜶) The beta particles are realized when a nucleus has an excess of protons or neutrons. Therefore, two types of Beta particles can arise: The particles are identical to a Helium-4 nucleus, which has 2 protons and 2 neutrons. Alpha particle has a mass number A=4, an atomic number Z=2 and a charge “e=2+”. a- Beta negative ( 𝟎𝟏𝜷) or ( 𝟎𝟏𝒆 )or (β-) The Beta negative particle has a charge and mass identical to those of an electron (high In short the alpha particles can be represented as α, 4α, 𝟒𝟐𝜶 , 4He, or 𝟒𝟐𝑯𝒆 energy electron e= -1 and A=0.00055 amu). The general equation representing the alpha particle decay of a nuclide 𝑨𝒁𝑿 to give new The beta particle forms because of breakdown of a neutron into proton and electron as stable one 𝒀 is: 𝑨 𝟒 follow: 𝑨 𝒁𝑿 → 𝒁 𝟐𝒀 + 𝟒𝟐𝑯𝒆 + 𝟎𝟎𝜸 𝟏 𝟏 𝟎 𝟏 𝟏 𝟎 Parent Nuclide Daughter Nuclide Alpha particles Energy 𝟎𝒏→ 𝟏𝒑 + 𝟏𝒆 or 𝟎𝒏→ 𝟏𝒑 + 𝟏𝜷 The resulting proton remains in the nucleus and the energy will be released as beta Note: (The nuclear reaction must be balanced against the atomic number and mass particles. In short, the beta negative particles can be represented as β-, 𝟎𝟏𝜷 or 𝟎𝟏𝒆. number) Prepared by A. HAMMA Prepared by A. HAMMA 18 19 Chapter II Radioactivity Chapter II Radioactivity The general equation representing the Beta negative particle decay of a parent nuclide We notice that nuclear reactions release different quantities of energy or gamma rays. 𝑨 𝒁𝑿 to give a daughter nuclide of a new element 𝒀 is: Example: 𝑨 𝑨 𝟎 𝟎 𝒁𝑿 → 𝒁 𝟏𝒀 + 𝟏𝜷 + 𝟎𝜸 Technetium-99m is the unstable isotope of Technetium (m indicate that is metastable or Nuclide parent Nuclide daughter Beta negative Energy exited). By emitting energy in the form of gamma rays, the nucleus become more stable. 𝟗𝟗𝒎 𝟗𝟗 𝟎 𝟒𝟑𝑻𝒄→ 𝟒𝟑𝑻𝒄+ 𝟎𝜸 Example: Same for Tin-119: 𝟓𝟎𝑺𝒏→ 𝟏𝟏𝟗 𝟏𝟏𝟗𝒎 𝟎 𝟓𝟎𝑺𝒏+ 𝟎𝜸 Beryllium decay to give Boron: 𝟏𝟎 𝟎 𝟏𝟎 𝟎 II.2.2. Artificial Radioactivity 𝟒𝑩𝒆→ 𝟏𝜷 + 𝟓𝑩+ 𝟎𝜸 The artificial radioactivity is produced by bombarding some stable and nonradioactive isotopes, Thorium decay to give Palladium: 𝟐𝟑𝟒 𝟎 𝟐𝟑𝟒 𝟎 with high-speed particles (a particle projectile that can be a protons, neutrons or small nuclei). 𝟗𝟎𝑻𝒉→ 𝟏𝜷 + 𝟗𝟏𝑷𝒂+ 𝟎𝜸 When one of these particles is absorbed, the stable nucleus is converted to a radioactive isotope b- Beta positive or Positron ( 𝟎𝟏𝜷) or (𝟎𝟏𝒆+) or (β+) and usually some types of radiation particles. The Beta positive particle is a particle that is charged positively and has no mass. The The general nuclear reaction which could represent the artificial radioactivity is : positron forms as a result of breakdown of a proton into neutron and electron as follows: 𝑨 𝑨𝟏 𝑨 𝑨𝟐 𝒁𝑿 + 𝒁𝟏 𝑷 → 𝒁𝒀 + 𝒁𝟐 𝑷 𝟏 𝟏 𝟏𝒑→ 𝟎𝒏 + 𝟎𝟏𝒆 or 𝟏 𝟏 𝟏𝒑→ 𝟎𝒏 + 𝟎𝟏𝜷 Target nucleus Projectile New Nucleus Radiation 𝑨 𝑨 The resulting neutron remains in the nucleus and the energy will be realized as beta positive The abbreviated writing of the nuclear reaction is : 𝑨𝒁𝑿 ( 𝒁𝟏𝟏 𝑷, 𝒁𝟐𝟐 𝑷) 𝑨𝒁 𝒀 particles. In short, the beta positive particles or positron can be represented as β+, 𝟎𝟏𝜷 or 𝟎𝟏𝒆. II.2.2.1. Fission In the nuclear fission reaction, a heavy nucleus (A>200) is bombarded by neutrons and The general equation representing the Positron emission of a parent nuclide 𝑨𝒁𝑿 to give a became unstable. The later splits and gives rise to a lowest nuclei (72 we replace it in the decay law: N = N. e => 𝐀 𝐭 = 𝐀𝟎. 𝐞 8- Mass of the source at this life-time: 𝐴 = 𝜆. 𝑁 => 𝑁 =... m = m.e  m = 4.8742 ∗ 10.e. = 9.7489 ∗ 10 Example: Cesium-137 ( 𝐶𝑠) is a radioactive isotope undergoing β- decay. The initial activity of a radioactive sample source containing this isotope is A0= 1.5*105 Bq. The cesium has a half- life of 30.2 years. 1- Write the corresponding nuclear reaction. Prepared by A. HAMMA Prepared by A. HAMMA 26 27