Bettelheim, Brown, Campbell and Farrell Chapter 9 Nuclear Chemistry Bettelheim, Brown, Campbell and Farrell Chapter 9

Nuclear Chemistry Background Types of radiation Nuclear Equations Half-Lives Units Uses Medical Other

Nuclear Reactions Involve changes in nuclei Protons and Neutrons----NOT Electrons

Nuclear Chemistry- the study of the properties and reactions of atomic nuclei

Introduction Nuclear radiation: radiation emitted from a nucleus during nuclear decay Results from an unstable nuclei alpha particle (a): a helium nucleus, He2+; contains two protons and two neutrons, has mass of 4 amu, and atomic number 2 beta particle (b): an electron; has a charge of -1, and a mass of 0.00055 amu gamma ray (g): high-energy electromagnetic radiation; positron (b+): has the mass of an electron but a charge of +1

Representing Isotopes 12C 6 14C 14N + 0e 6 7 -1 Example of a nuclear equation

Nuclear Radiation There are more than 300 naturally occurring isotopes 264 are stable More than 1000 artificial isotopes have been made in the laboratory; all are radioactive

Alpha Emission in alpha emission, the new element formed has an atomic number two units lower and a mass number four units lower

Beta Emission beta emission: decomposition of a neutron to a proton and an electron emission of a beta particle transforms the element into a new element with the same mass number but an atomic number one unit greater Problem: carbon-14 is a beta emitter. When it undergoes beta emission, into what element is it converted?

Positron Emission positron emission: decomposition of a proton in the nucleus to a give a “positive electron” (and a neutron) in positron emission, the new element formed has an atomic number one unit lower but the same mass number

Electron Capture electron capture: electron near the nucleus is “captured” and combines with a proton to form a neutron) in electron capture, the new element formed has an atomic number one unit lower but the same mass number 12553I + 0-1e → 12552Te

Gamma Emission In pure gamma emission, there is no change in either the atomic number or the mass number of the element a nucleus in a higher-energy state emits gamma radiation as it returns to its ground state (its most stable energy state) Usually accompanies  or  emission 60Co 27

Half-Life half-life of a radioisotope, t1/2: the time it takes one half of a sample of a radioisotope to decay Amount of radioactive material left is given by Rt = (1/2n)Ri where Ri is initial amount of radioactivity, Rt is the amount of radioactivity at time t, and n is the number of half-lives Half-Life = 8 days

Amount left

If you start with 50 Curies of P-32, how much is left after 28. 6 days If you start with 50 Curies of P-32, how much is left after 28.6 days? (t½ = 14.3 days)

Two factors determine how dangerous different kinds of radiation are Ionizing power: Ability to cause damage Penetrating power: How far radiation will travel into the body

Ionizing Power Ionizing power is the ability to knock off electrons and thus cause damage Alpha particles have highest ionizing power Beta particles have moderate ionizing power Gamma rays have least ionizing power

Ability to penetrate sample

Comparison of Radiation Types Ionizing Power (do damage) How far will it penetrate? Alpha High Stopped by piece of paper Beta Medium Stopped by thin sheet of metal or plexiglass Gamma Low Pass through tissue easily

Radiation Dosimetry Curie (Ci) or millicurie (mCi): measure of the number of radioactive disintegrations occurring each second in a sample. (1Ci = 3.7 x 1010 dps) Roentgen (R): amount of radiation delivered by a radiation source Radiation absorbed dose (Rad): a unit for measuring the energy absorbed per g of material exposed to a radiation source Roentgen-equivalent-man (Rem): measures the tissue damage caused by radiation Preferred for medical purposes

Radiation Dosimetry Average exposure to radiation from common sources

Measuring Devices Film Badge Geiger-Mueller Counter Scintillation counters

Geiger-Müller Counter

Geiger-Müller Counter

Measurement of Radioactivity and Radioactive Exposure Curie: amount of radioactivity which gives 3.7 x 1010 dps dps = disintegrations per second Disintegration = decay of a single atom

Measurement of Radioactive Exposure Roentgen = amount of radiation that produces ions which have 2.56 x 10-4 coulombs/kg Radiation absorbed dose (Rad) = energy absorbed per gram of material exposed to a radiation source REM = Roentgen Equivalent Man Rem is measure of the effect of radiation when one Roentgen is absorbed

Medical/Research Uses Experimental Tracers Basic biochemical and medical research Diagnostic Uses Organ scans involving preferential uptake of isotopes I-131 concentrates in thyroxine in thyroid gland Medical Imaging PET Scan – positron and electron →2 gamma rays MRI—imaging of soft tissue, such as brain, spinal cord

Medical/Research Uses Radiation Therapy Aim high energy radiation at cancer cells Radiation affects rapidly growing cells more Cobalt-60 often used for brain tumors Actinium- 225 attached to monoclonal antibody targets prostate cancer (binds to PSA on cell surface)

Fission and Fusion Fusion Combining smaller nuclei to form a larger nucleus Fission “Splitting” of a larger nucleus to form smaller nuclei Energy is released in both fusion and fission

Nuclear Fission “Split” a larger nucleus into smaller nuclei. Used in nuclear power plants Used in the Atomic bomb Energy is released.

Chain Reaction Chain reaction: Self-sustaining reaction in which the products of one reaction event initiate another reaction event Critical Mass: Minimum amount of an isotope needed to sustain a chain reaction

Chain Reaction 04-13 Title: Nuclear Chain Reaction Caption: The process by which a chain reaction develops when uranium-235 undergoes fission. Note the different fission products. Notes: Uncontrolled chain reactions lead to powerful explosions.

Nuclear Power Plants Utilize “controlled” Fission Reaction Fuel rods contain radioactive material Moderator slows speed of neutrons (water or graphite) Control rods contain neutron-absorbing material such as Cd or B Control rods can be raised or lowered to control the number of neutrons available for the reaction

Nuclear Power Plant Figure: 21-20C

Nuclear Reactor Control rods can absorb neutrons Control rods can be lowered to absorb more neutrons (slow reaction) Control rods can be raised to absorb fewer neutrons (increase reaction) Figure: 21-19

Use of Nuclear Power % of total electricity France 75 Sweden 47 Europe 40-50 US 20 Canada 13

Comparison of Nuclear and Fossil Fuel Power Plants Radioactive material NOT connected to outside world Does NOT pollute air Fossil Fuel: Smoke stack open to air DOES emit air pollutants

Major Problem Fuel rods need to be replaced periodically Disposal of “spent” rods--nuclear waste Currently dry cask storage on site Yucca Mountain proposed as nuclear repository site

Recycling of spent fuel Possible to reprocess spent fuel to concentrate Pu-239 and U-235 Pu-239 potentially used for nuclear weapons US does not currently reprocess spent fuel

Problems with Plant Operation Three Mile Island 1979 failure of water pump partial core meltdown Chernobyl 1986 poor design only graphite moderator (burns) inadequate reactor containment

Fusion Occurs in sun Theoretically wonderful source of energy Lots of water and H sources available Have not yet achieved fusion Requires very high temperatures