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Nuclear Chemistry

Nuclear Chemistry. Chapter 15. Types of Radiation. There are four main types of ionizing radiation: alpha rays: Helium nuclei - 2 protons + 2 neutrons positron rays: Positrons - the antimatter counterparts to electrons; same mass but charge is +1 beta rays: Electrons

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Nuclear Chemistry

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  1. Nuclear Chemistry Chapter 15

  2. Types of Radiation • There are four main types of ionizing radiation: • alpha rays: Helium nuclei - 2 protons + 2 neutrons • positron rays: Positrons - the antimatter counterparts to electrons; same mass but charge is +1 • beta rays: Electrons • gamma rays: High energy photons - ν ≥ 1020 Hz - higher energy than x- rays

  3. Nuclear Reactions • Chemical reactions never alter the identity of the atoms involved • In nuclear reactions, the total numbers of nucleons – protons, neutrons and electrons – remains constant, but chemical identity can change when the particles interchange • Nuclear reactions permit the transmutation of elements Balancing Nuclear Reactions (1) The sum of the mass numbers of reacting nuclei must equal the sum of the mass numbers of the nuclei produced (conservation of mass). (2) To maintain charge balance, the sum of the atomic numbers of the products must equal the sum of the reactants Example of Balancing a Nuclear Reaction Mass number (protons+neutrons) 226  4 + 222 Atomic number (protons only) 88  2 + 86

  4. 7 Classes of Nuclear Reaction: 1 - 2 • Alpha emission In this spontaneous reaction an alpha particle is emitted. 2. Beta emission In this spontaneous reaction an beta particle (electron) is emitted. This implies that the nucleons are transformed during the process, where one of the neutrons is converted into a proton and a high-energy electron is ejected: Note that beta emission increases the atomic number by one.

  5. 7 Classes of Nuclear Reaction: 3 - 4 3. Positron emission This is the opposite of a beta emission where a new particle, called the positron is emitted. Essentially, a proton decomposes to a neutron and a positron which is emitted by the nucleus. This transformation, at the nucleon level, implies the reaction occurs within the nucleus: Positrons and electrons are antiparticles, meaning that If the two were to meet, they annihilate each other releasing two high-energy gamma ray photons. This reaction decreases the atomic number by 1. 4. Electron (K) capture An electron from the atom is incorporated into the nucleus. The electron is from the 1s level also known by the older name of K-level. This spontaneous process is the opposite of beta emission, where a proton takes on an electron to form a neutron This reaction gives same net product as positron emission, decreasing the atomic number by 1.

  6. 7 Classes of Nuclear Reaction: 5 5. Nuclear fission Nuclear fission is always an induced process, where a large nucleus splits into several smaller nuclei. The actual fission process below is the decay of uranium-236. 236U is made by neutron bombardment of 235U. : Fission is accompanied by the release of a lot of energy, and this is the source of the power both of the atomic bomb and nuclear power

  7. 7 Classes of Nuclear Reaction: 6 - 7 6. Nuclear fusion Fusion is the combining of two light elements into a heavier nucleus. A lot of energy is released. (hydrogen bomb - where the hydrogen refers to the heavier hydrogen isotopes deuterium and tritium). 7. Nuclear transmutation The combination of heavier nuclei to produce artificial elements. All the elements beyond uranium are artificial, in the sense that none have stable isotopes. This classification includes all the remaining artificial, i.e. induced, nuclear reactions

  8. Practice Question 1 Example Problem • Complete or balance the following nuclear reaction equations by identifying the missing components. Classify each reaction according to one of the 7 classes of reaction introduced. a) b) c) d) e) f) g)  emission Nuclear fusion Induced α emission,  nuclear transmutation Positron emission Nuclear fission Electron capture Nuclear transmutation

  9. Gamma rays (and neutrinos) • Gamma rays accompany most nuclear reactions. • When a nucleus emits an alpha particle, the resulting nucleus is usually in an unstable, higher energy form - Excited nuclear state • It quickly decays to the nuclear ground state by releasing the excess energy as a gamma ray. • The most dangerous part of a nuclear process, as gamma rays have high penetrating power. • The energy of the gamma rays varies greatly, but is always characteristic of the type of nuclear transformation. • Gamma spectroscopy analyzes the wavelengths of the emitted gamma rays, which can be used to identify the source of the nuclear reaction. • Hence it is possible to detect the kind of radioactive process occurring when radioactivity is detected

  10. Why are some nuclei stable, others not? • Black dots – stable nuclides • Red dots - unstable nuclides • Pink region - all combinations of N (number of neutrons) and Z (number of protons) that cannot exist • Zone of black dots - belt of stability- neutrons are needed to stabilize protons (+ve repulsion) • At low Z, N ≈ Z; at high Z, N > Z • Unstable nuclides surround the belt of stability • Z > N - Nuclides with more protons allowed for stability, attain it, by changing the ratio of N/Z by: • Heavy nuclei - alpha (α) emission • Light nuclei- positron emission or electron capture • N > Z -Nuclides with more neutrons than protons convert neutrons into protons • This is achieved by beta () emission

  11. Binding energy Regionofgreateststability Susceptible to fission reactions All nuclei strive to maximize stability via nuclear reactions Fusion zone Binding energy is the energy change that occurs if a nucleus were formed directly from its component protons and neutrons. The source of the energy is a loss of mass

  12. Calculating binding energy • We use the tabulated masses of free protons and neutrons, and compare them to that of any given nuclide • The difference in mass is converted to energy using the Einstein equation from special relativity • The mass of one mole of “free” protons is 1.0072765 g/mol • The mass of one mole of “free” neutrons is 1.0086649 g/mol • The mass of one mole of “free” electrons is 0.0005486 g/mol Calculate the binding energy of the 16O isotope with mass of 15.994916 g/mol, and the binding energy per nucleon. Note that the isotope mass includes the electrons proton neutron isotope electron

  13. Range of binding energies • In the sample calculation on the previous slide, we calculated the binding energy of the 16O nuclide and also the binding energy per nucleon • A nucleon is either a proton or a neutron • It is found that the binding energies of all the known nuclides falls in the range from:~ 0 – 9.0  108 kJ/mol of nucleon = 0 – 900 GJ/mol of nucleon • The most stable nuclide is 56Fe • Sometime values are given in MeV, which is a measurement per individual nucleon • Chemists most often work out energies in units of kJ for mole quantities of matter, so that we will always work out total and per-nucleon binding energies in kJ/mol • However, the MeV unit is widely used in the nuclear industry. The conversion factor is:1 eV = 96.485342 kJ/mol 1 MeV = 9.6485342  107 kJ/mol ≈ 108 kJ/mol = 100 GJ/mol • The range of binding energies per nucleon in MeV is therefore:~ 0 – 9.0 MeV

  14. Rates of Nuclear Decay Nuclear decay is measured as the number of nuclei that disintegrate in a given period of time. This is termed the activity (A) of a sample and is directly proportional to the number of radioactive atoms (N) in the sample: where k is the rate constant (decay constant). By integrating this equation, we get a first-order rate equation: where A0 and N0 are the initial activity and number of radioactive atoms while A and N are the activity and number of radioactive atoms after time ‘t’. At t1/2, N = ½ N0 and A = ½ Ao. A relationship between k and t1/2 can be found as:

  15. Example 1 – Radioactive Decay

  16. Example 2 – Radiocarbon Dating

  17. Nuclear Decay – The Uranium Series All isotopes of uranium are unstable, but 238U decays extremely slowly.This is the 238U decay series, showing the steps in the decay of this nucleus to eventually produce stable206Pb In this region there are several pathways, but all lead to the same species, 206Pb

  18. Nucleosynthesis by transmutation • All the elements beyond 92U are synthetic. They are made either as a by product of operating a nuclear reaction (neutron source) or by deliberate transmutation • Latest to be recognized: It was made by a nucleosynthesis reaction: • Roentgenium is the most recent of the transuranium elements which start with 93Np, complete the actinides and most of the 6d series of the PT • Early efforts at transmutation date back all the way to Rutherford. For example, he performed the reaction: • Alpha-particles: high repulsive forces experienced when the 2+ alpha nucleus comes near a strongly positive charged nucleus • Neutron: uncharged particle reaches target much easier, and efforts at transmutation switched to neutron bombardment • The discovery of fission was a result of such studies: neutron bombardment of

  19. Periodic Table with Element Names(using the 1-18 group nomenclature) A new element is recognized (IUPAC Bureau decision)

  20. Nucleosynthesis by transmutation • Successful neutron bombardment: an example is the “manufacture” of plutonium, which is a two-stage process: • This is the process that is used in “breeder” reactors to convert unusable 238U into fissionable 239Pu • This method works well up to element 101, Mendelevium, but beyond this larger “bullets” must be used, as for example in: • Such reactions suffer from very strong nuclei-nuclei repulsion, and require great effort and very specialized apparatus • Many transuranium elements from the Actinides have been made in sufficient quantity to have some of their basic chemical properties investigated • The “super-heavy” elements of the 6d block are usually only made a few atoms, or “events” at a time and nothing is known of their chemistry. Roentgenium is “heavy” gold!

  21. Energy ranges of the main types of nuclear radiation and detection of radiation • a particles - energy range 3.5 - 10 MeV • b particles(and positrons) - energy range 0.18 - 3.6 MeV • g radiation - energy range 0.008 - 7.11 MeV • Radiation may be detected by a Geiger counter. • This device is suitable for any type of radiation capable of ionizing argon gas, causing a current to flow and a needle or loudspeaker to sound. • This includes those we have talked about, a, b, and g, as well as high-energy X-rays. As a group, these are called "ionizing radiation".

  22. Rates of Nuclear Decay Nuclear Decay – The Uranium Series The stability of an isotope is measured by its half-life, t1/2, i.e the time required for half of a sample to decay Isotopes with long half-lives are more stable than those with short half-lives. An isotope with a long half-life emits lower levels of radiation but for a longer period of time An isotope with a short half-life emits higher levels of radiation for a short period of time. Two important factors to consider in assessing the relative risk of radioactive isotopes are: 1. sample size, and 2. type of particle(s) emitted (and their energy!)

  23. Radiation Safety • The unit of radioactivity: Curie Ci; 1 Ci = 3.70  1010 events per second (Becquerel, Bq) The Curie measures the rate of nuclear decay - similar to half-life. For a given isotope, with a known half-life, the Curie indicates much of that isotope is present • The unit of radiation intensity:rad = radiation absorbed dose = amount of radiation that results in absorption of 1  10-5 J g-1 of absorbing material. It varies with the type of radiation and its source. All sources of nuclear radiation are distinguishable by type and energy. • The Röntgen equivalent for man: Unit of exposure – related to riskrem ; 1 = 1 rad  1 RBE This measures the relative effect of different types of radiation (a, b, g or X) on humans, which is expressed as the RBE, relative biological effectiveness. Multiplying the RBE by rad results in a unit that tells just how much a certain radioactive source, with a certain intensity, will harm you.

  24. Nuclear Power -The fission process • Typical reaction is 235U fission is illustrated by the equation: E = –2.1  1010 kJ / mol • This is not the only possible reaction: a variety of daughter isotopes are produced (As, Br, Sr, Zn, and Zr), some of which are stable, but most of which are radioactive themselves (e.g. as b-, b+ or g emitters). • These reaction can release 1, 2 or 3 neutrons, and on average 235U fission releases 2 neutrons for every one captured. • To be self-sustaining, a nuclear reactor needs to control the fast neutrons produced in fission. • The right number of fast neutrons must be slowed down to a speed where they can be captured by 235U nuclei. • This is accomplished by using a moderator. Properties of a moderator include: a) must not absorb many neutrons, since these are required to sustain a nuclear chain reaction b) must be light in mass so that the neutrons are not slowed too much c) must not react with neutrons to form radioactive species d) must have a high probability of collision with a neutron

  25. Nuclear chain reactions • The splitting of 235U requires first that it is hit by a neutron to make 236U, which then spontaneously breaks into many pieces, including two lighter units and several neutrons • The substances which have been used as moderators are water, D2O (heavy water) and graphite. Water and D2O are favoured because they are liquids, and can double as heat transfer agents by circulation through the reactor and a heat exchanger.

  26. Energy considerations in nuclear power • Nuclear reactors have all sorts of technical difficulties associated with handling radioactive fuels and by-products. Why bother? • A small amount of nuclear fuel can release a large amount of energy. The origin of this energy is the nuclear binding energy. • The nucleons in the daughter isotopes have much less total binding energy than the parent. This excess binding energy is released and converted into heat. The amount of heat released is given by Einstein's equation: E = mc2 • For this fission reaction the total mass change is 0.19 u per 235U nucleus. • This converts to 7.3  1010 J or 73 GJ per gram of 235U. • In consumer terms, where 1 kW hr of power is equivalent to 3.6 MJ, each gram of 235U can supply 20,000 kW hr of heat energy. (average household uses ~10 GJ per Month) • This is the maximum possible of course, and there are significant losses in electric power generation by any process, but consider the situation with fossil fuels

  27. Environmental factors for nuclear power • The major difficulty with non-nuclear power generation is simply the huge scale of these operations. • A 2000 MW coal-fired power station releases 42,000 tons of CO2, 600 tons of SO2 (and related acid gases) and 10 tons of fly ash per day. • Scrubbers can reduce the acid gases and fly ash considerably, but nothing can be done about the CO2 greenhouse gas • Power in Alberta is generated near Edmonton from large coal thermal generating stations • It must be transmitted great distances, with considerable losses during transmission • Nuclear reactors are usually located close to residential areas: thus they overcome the large losses from long-distance transmission

  28. The CANDU reactor for electric power generation • Schematic diagram of a CANDU nuclear reactor as used in a typical electric generating station • This is a unique Canadian design that has been used in several countries including Canada • There are none in Western Canada, but several in Ontario, Quebec and New Brunswick • The name stands for CANada Deuterium Uranium. It is unique in its ability to burn normal uranium dioxide instead of needing enriched fuels

  29. Application of Nuclear Reaction in Medecin Imaging methods The pattern of radiation emitted from a nuclear reaction in the body can be reconstructed to make images of parts of the body. An agent that undergoes a nuclear reaction must be administered (invasive techniques) Chemical Identification An agent that undergoes a nuclear reaction can be attached to a drug molecule that binds to a specific area in the body of interest. Ex) cancer cells, blood clots, diseased cells, receptors specific to an organ.( in the brain). Imaging methods are used to observe the phenomenon of interest. Treatment In radiation therapy radiation produced from a nuclear reaction can be used a destroy diseased tissue. Sometime a drug is administered to make the tissue more susceptible to radiation damage.

  30. Imaging A common positron emitter is 18F, PET – Positron Emission Tomography It is incorporated into [18F]-dopamine. The dopamine accumulates in the putamen of a normal human brain (right). A Parkinson’s disease patient shows much lower accumulation in the putamen (left) The emitted positron immediately encounters an electron, which undergoes an annihilation event releasing two photons. The detector senses the photons and where they come from which is used to reconstruct an image. A whole-body scan (photographic process) undertaken with a phosphate complex labeled with 99mTc that accumulates in bones. 99mTc emits gamma radiation with is detected.

  31. Concepts of Chapter 23 • Types of radiation - α, β, γ and positrons) • balancing nuclear reactions • 7 classes of nuclear reactions • α emission • β emission • positron emission • electron capture • nuclear fission • nuclear fusion • nuclear transmutation • radioactive decay series • nuclear binding energy and nuclear stability • kinetics of nuclear decay • activity • half-life • decay constant

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