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Transcript
Chapter 30 30.1 Radioactivity The atom is the smallest unit of achemical element, such as gold or carbon, which has the physical and chemical properties of the element. Nucleus Atoms have a dense center called a nucleus where you can find protons and neutrons. The nucleus is surrounded by orbiting electrons. A atomic nucleus is instable when he is to heavy or when a balance is missing between the protons and the neutrons. There are three types of decays: alpha decay, beta decay and gamma decay. 1- Alpha decay (α) 4 2 • Two neutrons and two protons (helium nuclei • • • Charge of +2 Emitted from nucleus of radioactive atoms Transfer energy in very short distances (few cm in air) Shielded by paper or layer of skin Primary hazard from internal exposure • • He ) • Alpha emitters can accumulate in tissue (bone, kidney, liver, lung, spleen) causing local damage 2- Beta Decay (β) There are two types of the beta decay. The one is the βdecay and the other is the β+ decay. a- β• neutron decays into a proton • it has the same charge as electron • it has the same mass as electron • it can penetrate with few meters in air. 2 or 3 cm of wood are enough to protect oneself. b- Β+ • • a proton decays into a neutron it has the same charge as electron but negative charge • it has the same mass as electron • it can penetrate with few meters in air. 2 or 3 cm of wood are enough to protect oneself. 3- Gamma decay (γ) • • • • High energy electromagnetic radiation It has no charge It has no mass Highly penetrated. You need a big wall of lead to protect yourself from gamma radiation. 30.2 Half-Life It is found that, if a given radioactive substance at a certain time contains N0 nuclei, then at a short time t later a certain number N have decayed which is given by N = N 0e −λ t Where λ is the decay constant (s-1) It is sometimes convenient to introduce the half-life, , T1/2 , of a substance, defined as the time over which e exactly xactly one half of a substance remains. N0 −λ t = N 0e 1/ 2 2 1 −λ t = e 1/ 2 2 1 −λ t ln( ) = ln(e 1 / 2 ) 2 − ln(2) = −λ t1 / 2 t1/ 2 = ln(2) λ = 0.693 λ 30.9 Radioactive Decays Standard nuclear notation shows the chemical symbol, the mass number and the atomic number of the isotope isotope: A = Z = number of protons + number of neutrons number of protons A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number A Z X → AZ−−mnY + mny It is found that nuclei with mass numbers greater than about 100 spontaneously decay into other types of nuclei. Such nuclei are said to be radioactive, and there are three main types of such decays. 1- Alpha (α) decay is the spontaneous emission of an alpha particle (identical to a helium nucleus) from the nucleus A Z X → ZA−−42Y + 24He + Q Example: 238 92 4 U → 234 90 Th + 2 He 2- Beta (β) decay Beta decay occurs when a neutron changes into a proton and an electron. a- β1 0n 1 → 1p + 0–1e (β– emission) Example: 14 6 C →147 N + β − b- β+ 1 1p → 1 0n + 01e (β+ emission) Example: 13 7 N →136 C + β + 3- Gamma decay A nucleus (which is unstable) changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons) (called gamma rays). A Z X * → ZA Y + 00 γ