Radioactive Decay Atomic Physics

Becquerel • In 1896 Henri Becquerel discovered that certain uranium compounds would fog photographic plates as if exposed to light. • He discovered that a magnetic field could deflect the radiation that caused the fogging.

Pierre and Marie Curie Marie and Pierre Curie investigated further. • They discovered that thorium is also radioactive, and discovered two new elements: radium and polonium (named for Marie’s native Poland). • Marie coined the term radioactivity.

Three Radiation Types Alpha: the emission of a helium nucleus (an alpha particle): • Beta: the emission of a high energy electron: • Gamma: the emission of a high energy photon:

Radiation in magnetic field Heads Up! Guaranteed exam Question

Conservation In Nuclear Reactions Charge: Net charge remains constant: total charge of the reactants = total charge of the products. Atomic mass number: The total atomic mass number for the products = total atomic mass number for the reactants.

Example Charge: (p+ are +) +90 = +88 + +2 (charge is balanced) A. Is this reaction possible? Charge: (p+ are +) +90 = +88 + +2 (charge is balanced) 2. Atomic mass number: 230 = 226 + 4 (N is balanced) The reaction is possible.

Example Charge: +27 ≠ +28 + 0 (charge not balanced) B. Is this reaction possible? neutron Charge: +27 ≠ +28 + 0 (charge not balanced) 2. Atomic mass number: 60 = 60 + 1 (N not balanced) The reaction is not possible.

1. Alpha Decay alpha (α) – spontaneous emission of an alpha particle by a large, unstable nucleus. occurs when the electromagnetic force within a nucleus is as great or slightly greater than the strong nuclear force during the decay, momentum, energy ( mass energy) and charge are conserved

Alpha Decay Eg) consists of 2 protons and 2 neutrons cannot penetrate more than a sheet of paper ionize atoms they collide with low biological hazard Mass is conserved 238=238 Eg) Charge is conserved 92=92

Note: In a large nucleus, the electrostatic force pushing p+ apart is nearly as strong as the attractive strong force, making the nucleus unstable. An alpha particle (2 p+ and 2 n°) is a very stable configuration, so alpha particles are emitted not individual p+or n°. In general, Z decreases by 2, and A decreases by 4.

Alpha reactions In general: Example: Parent element Daughter element

Example Write the alpha decay process for polonium- 210. Name the parent and daughter elements. Parent element: polonium Daughter element: lead

Energy in alpha decay Mass defect, and mass-energy equivalence can be used to determine the maximum Ek an emitted alpha particle will have.

Example Write the alpha decay process for the decay of radium-226. Use the data on page 881 to determine the mass defect in the reaction, and its energy equivalence. This energy will be the maximum Ek an alpha particle could have in the reaction.

Decay reaction Parent element: daughter element: radium radon

Mass defect Note: if Δm were negative, this means no energy was released, so the decay would not happen.

Energy equivalence Most of this energy will be Ek of the alpha particle.

2. Beta Decay –spontaneous emission of a high energy electron A. Beta Negative Decay A neutron emits a beta particle (an e¯) and becomes a proton. penetrate up to 3mm of aluminum, 500 sheets of paper Produces burns to skin or tissue

Beta Negative Decay Beta particle Z increases by 1, and A does not change. The transformation of a neutron into a proton involves the weak nuclear force and antimatter Beta particle

Beta decay B. Beta Positive Decay β+

Antimatter The antimatter of a neutrino (v) is an antineutrino ( ) Eg)an electron The antimatter of the electron is a positron, The antimatter of a neutrino (v) is an antineutrino ( )

Note: -stable isotopes have the same number of protons and neutrons -elements with more than 83 protons are unstable and transmutate into stable nuclei with higher binding energies

Text: p. 796 Binding energy VS atomic mass No Text: p. 796 Binding energy VS atomic mass No. (see that the binding energy decreases)

3. gamma(γ) – emission of a high energy photon most harmful because it penetrates through most matter penetrates the living cells, breaks down DNA bond resulting in cancer growths, mutations in offspring uses – radiation therapy, irradiate foods, fruits

Gamma Decay (γ-decay) Nuclei energy levels change, which correspond to different configurations of nucleons within a nucleus When the nucleus makes a transition to a lower energy level, it emits a gamma ray photon

Uranium-238 146 n 92 p Gamma radiation n = 1 n = 2 e

Example:

What are the electrical properties of the rays in the diagram below?