Chapter 3 Nuclear Radiation

Download Chapter 3 Nuclear Radiation

Post on 26-Jul-2015




2 download

Embed Size (px)


<p>Nuclear Radiation Chapter 3 1. Atoms consist of electrons, protons, and neutrons. 2. Atoms of elements are distinguished by the number of protons in the nucleus (the atomic number). 3. Isotopes of an element have different numbers of neutrons but the same number of p+ and e-. 4. Isotopes of elements react identically (in most chemical reactions). 5. Traditional chemical reactions focus primarily on interactions in the outer valence electrons of atoms. Review: A nucleus with a specified number of protons and neutrons is a nuclide. E A mass number (number of p+ + number of n) Z atomic number Together, protons and neutrons are called nucleons. A Z Review of Nucleus Nucleus:Particle Properties Protons, neutrons, and electrons are all fermions (spin 1/2). Protons and neutrons are heavy baryons (composed of 3 quarks). [proton = up, up, down quarksandneutron = up, down, down] Electrons are light leptons. ParticleCharge amuSpin Proton+e1.007276 1/2+2.79N Neutron01.008665 1/2 1.91N Electrone5.485810-41/2+1.00B</p> <p>The Nucleus Lets take a look at the nucleus, where the protons and neutrons reside. Breaking these apart can cause a large release of energy. Protons What is Radioactivity? Elements that are RADIOACTIVE are UNSTABLE because they have too many nucleons or too much energy.In an attempt to become STABLE, they give up particles or energy and this is. RADIOACTIVITY From Latin radioto (to radiate) Radioactivity C 12 6 Stable C 13 6 Stable C 14 6 Unstable RADIOACTIVE Radioactivity C 14 6 Unstable RADIOACTIVE The nucleus of this atom is very heavy because it contains two extra NEUTRONS In order to become stable it needs to get rid of some excess weight Radioactivity Because this atom is unstable a NEUTRON begins to break down Neutron Breakdown C 14 6 Unstable RADIOACTIVE Neutron Breakdown Neutrons are made up of positively and negatively charged particles C 14 6 Unstable RADIOACTIVE The positive part of the NEUTRON is actually a PROTON The negative part of the NEUTRON is called a BETA particle An anti-neutrino is also released Neutron Breakdown C 14 6 Unstable RADIOACTIVE = Neutron breaking down The Negative Beta Particle is released | This energy is RADIOACTIVE Having released the| particle the NEUTRON now becomes a PROTON An anti-neutrino is also released Neutron Breakdown C 14 6 Unstable RADIOACTIVE N 14 7 Stable NITROGEN Lose NEUTRON Gain PROTON Neutron Breakdown Is Just one way a element can become stable Accompanied by Beta emission And the conversion of a NEUTRON to a PROTON Carbon-14 is a BETA emitter There are other ways that an element can obtain stability and this results in different types of RADIATION Nuclear Chemistry Introduction Most chemical changes deal with the valence electrons Nuclear chemistry deals with changes in the nucleus, often accompanied by the release of a large amount of energy Unstable nucleus spontaneously emits a particle or energy. Radiation comes from the nucleus of an atom. Radiation is energy in transit in the form of high speed particles and electromagnetic waves Radiation cannot be tasted, felt, or smelt, buthas the potential to do a great deal of damage Radioactivity, or radioactive decay, is the spontaneous change of the nuclei of certain atoms, accompanied by the emission of subatomic particles and/or high-frequency electromagnetic radiation. There are five principal particles or waves of radiation we will learn about: Alpha (o or 4He2+) Beta (| or e-) Positron (|+) Gamma () Neutrons (n) Summary Of Decay Types e e symbolparticleHe 0 1 0 1- particleinneutronsandprotons particlein (NOT Charge here!) protons 4 2 + = Main Types Of Radioactive Decay An alpha (o) particle has the same composition as a helium nucleus (42He): two protons and two neutrons. Beta (|) particles are electrons(-10e). Gamma () rays are a highly penetrating form of electromagnetic radiation(00). Positrons are particles having the same mass as electrons but carrying a charge of 1+ (+10e).A positron and an electron can annihilate each other upon colliding, producing energy as photons: -10e++10e 2 00 Other forms of radioactive decay: Proton emission Neutron emission Electron capture (EC) is a process in which the nucleus absorbs an electron from an inner electron shell, usually the first or second, thus converting a proton into a neutron, along with the release of an X-ray. Radioactivity: Historical Overview 1896: Becquerel accidentally discovered that uranyl crystals emitted invisible radiation onto a photographic plate. 1898: Marie and Pierre Curie discovered polonium (Z=84) and radium (Z = 88), two new radioactive elements. 1903: Becquerel and the Curies received the Nobel prize in physics for radioactive studies. 1911: Marie Curie received a 2nd Nobel prize (in chemistry) for discovery of polonium and radium. 1938: Hahn (1944 Nobel prize) and Strassmann discovered nuclear fission -Lisa Meitner played a key role! 1938: Enrico Fermi received the Nobel prize in physics for producing new radioactive elements via neutron irradiation, and work with nuclear reactions. Three Main Types of Radiation Radioactivity All elements have at least one radioactive isotope. All isotopes with atomic number greater than 83 are radioactive. Artificial and Natural sources exist. Radioactive isotopes have same chemical properties as non-radioactive isotopes. Stable and Unstable Nuclei Everyday Radiation Exposure Alpha ( ) Particles Symbol: 24He or o; Equivalent to the Helium Atom It is composed of 2 protons, 2 neutrons, has a mass of 4 amus and a charge of 2+ Since they are so large they can cause great damage if they strike tissue. But, they cannot travel very far because of their weight and theyre low energy Travel 3-4 inches in air and can be blocked by a sheet of paper Cannot penetrate the epidermal layer of the skin More of an internal hazard than an external hazard Once ingested they are usually within 3-4 inches of a vital organ 4 inches Emission of Alpha Particle 3-4 inches Because Alpha particles are so large, they are the most damaging.The probability of them coming into contact with other particles is great Beta ( ) Particles Symbol: -10e or |; A high energy electron Can be either positively or negatively charged Usually given off when a neutron is converted to a proton or when protons convert to neutrons. Very small and can travel up to 100 feet in air Can penetrate the skin Can be stopped by a thin piece of metal or 2-3 inches of wood Since they are so small the likelihood of them striking biological tissue is much less than an Alpha particle. 100 Feet If particle strikes damage will occur Particle may pass through without touching any matter A neutron in the nucleus breaks down 11 0 n H + e 0 1-1 Gamma ( ) Particles and X-Rays For all practical purposes Gamma and X-rays are identical. Gamma particles are produced by atomic disintegration X-rays are produced by machines and Electron Capture Both are pure energy and travel at the speed of light 3 x 108 m/s Can travel great distances without striking other particles. If collision takes place, damage will occur Because it is electromagnetic radiation, it is deeply penetrating Takes several feet of concrete or many inches of lead to stop them It has no mass or charge Very high energy There are very few pure gamma emitters, although gamma radiation accompanies most o and | decay In radiology one of the most commonly used gamma emitters is Tc 4399mTc 4399Tc + A gamma decay will have no change in the atomic number or atomic mass Much energy will pass through without any effect on biological matter Some energy may cause ionization Neutron Radiation Symbol:01n It has a mass of one, no protons, and no charge Very rare but very lethal Generated in the explosion of nuclear weapons Neutron Bombs Since this type of radiation is so specialized it is not usually discussed in lectures such as this Types of nuclear radiation Radiation Type of Radiation Mass(AMU) ChargeShielding material Alpha Particle42Paper, skin, clothes Beta Particle1/18361Plastic, glass, light metals Gamma Electromag -netic Wave00 Dense metal, concrete, Earth Neutrons Particle10Water, concrete From: Nuclear Physics General Rules: 1)o emitted to reduce mass, only emitted if mass number above 209 2)| emitted to change neutron into proton, happens when have too many neutrons 3)|+ emitted (or electron capture) to change proton into neutron, happens when have too few neutrons 4) emitted to conserve energy in reaction, may accompany o or |. 5) Neutrons and protons emitted due to bombardmentBombardment Reaction Bombardment reaction-bombarding 2 stable atoms together, creating a radioisotope All of the known elements whose atomic number is greater than 92 were created from bombardment reactions Nuclear Equations Basic principle in writing a nuclear equation : charge, mass number, and atomic number must be conserved in a nuclear reaction. The two sides of a nuclear equation must have the same totals of atomic numbers and mass numbers. Balancing Nuclear Eqns: reactants and products Atomic numbers must balance and Mass numbers must balance Alpha decay Beta decay 234Th 234Pa+ 0e 90911 beta particle Gamma radiation No change in atomic or mass number 11B11B+ 0 5 5 0 boron atom in ahigh-energy state Learning Check NR1 Write the nuclear equation for the beta emitter Co-60. Solution NR1 Write the nuclear equation for the Beta emitterCo-60. 60Co 60Ni + 0 e</p> <p>27 28-1</p> <p>Producing Radioactive Isotopes Bombardment of atoms produces radioisotopes = 60= 60 59Co +1n 56Mn +4H e</p> <p>27 0 252= 27= 27 cobalt neutronmanganesealpha atomradioisotope particle Learning Check NR2 What radioactive isotope is produced in the following bombardment of boron? 10B +4He ? +1n</p> <p>5 2 0 Solution NR2 What radioactive isotope is produced in the following bombardment of boron? 10B +4He 13N+1n</p> <p>5 270nitrogen radioisotope Half-Life of a Radioisotope The time for the radiation level to fall (decay) to one-half its initial value decay curve 8 mg 4 mg2 mg 1 mg initial 1half-life 2 3 HalfLife (t1/2) The half-life (t1/2) of a radioactive nuclide is the time required for one-half the nuclei in a sample of the nuclide to decay. The shorter the half-life t1/2, the larger the value of (decay constant) and the faster the decay proceeds. The time required for one-half of the unstable nuclei to decay. (t1/2) A0 A = -------- 2n</p> <p> A0 = original amount n = number of elapsed half lives 1 half life 1/2 original amount left (50%) 2 half lives1/4 original amount left (25%) 3 half lives1/8 original amount left (13%) 4 half lives1/16 original amount left (6.3%) Selected Nuclide Half-lives Learning Check NR3 The half life of I-123 is 13 hr.How much of a 64 mg sample of I-123 is left after 26 hours? Solution NR3 t1/2= 13 hrs 26 hours = 2 x t1/2 Amount initial =64mgAmount remaining=64 mgx x =16 mg Radiocarbon Dating Carbon-14 is formed at a nearly constant rate in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic radiation. The carbon-14 is eventually incorporated into atmospheric carbon dioxide. Carbon-14 in living matter decays by |emissions at a rate of about 15 disintegrations per minute per gram of carbon. When the organism dies, no more carbon-14 is integrated into the system. Ratio of 14C to 12C tells how long the item has been dead. The half-life for carbon-14 is 5,730 years. This dating method works well if an object is between 5,000 and 50,000 years old. Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14N+1on---&gt; 14C+1H The C-14 is oxidized to CO2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t1/2 = 5730 years. Activity of a sample can be used to date the sample. NUCLEAR vs. CHEMCIAL REACTIONS Nuclear reactionsChemical reactions 1.Atomic numbers may change1.Atomic numbers do not change 2.Isotopes of an element have2.Isotopes of a given element different properties behave almost identically. 3.There is a small but significant3.There is no significant change mass change; matter isin the total quantity of matter converted to energy. in the reaction 4.Individual atoms are usually4.Mole quantities are usually used in calculations used in calculations. Summary The five types of radioactive nuclides involve emission of alpha (o) particles, beta (|) particles, gamma () rays, positrons, and electron capture. All known nuclides with Z &gt; 83 are radioactive, and many of them occur naturally as member of four radioactive decay series. In the formation of an atomic nucleus from its protons and neutrons, a quantity of mass is converted into energy. Synthetic Nuclides For centuries, alchemists tried - without success - to change one element into another alchemy turn lead into gold. The process of changing one element into another is calledtransmutation. Modern scientists have learned to do this. Rutherford, in 1919, was able to convert nitrogen-14 into oxygen-17 plus some extra protons by bombarding the nitrogen atoms with o particles. This is a naturally occurring isotope of oxygen and is not radioactive. 147N + 42He 178O + 11H Phosphorous-30 was the first synthetic radioactive nuclide. Since its discovery, scientists have synthesized over a thousand others. Transuranium Elements In 1940, the first of the transuranium elements - elements with a Z &gt; 92 - was synthesized by bombarding uranium-238 nuclei with neutrons. This first element is plutonium. 23892U + 10n 23992U 23992U 23993Np + 0-1e 23993Np 23994Pu + 0-1e Nuclear Stability About 160 stable nuclides have an even number of protons and an even number of neutrons. About 50 stable nuclides have an even number of protons and an odd number neutrons. About 50 stable nuclides have an odd number of protons and an even number neutrons Only four stable nuclides have an odd number of protons and an odd number of neutrons. The magic numbers of protons or neutrons for nuclear stability are 2, 8, 20, 28, 50, 82, and 126. Stability of Nuclides All the stable nuclides lie within the belt of stability (as do some radioactive ones).Nuclides outside the belt are radioactive.Their modes of radioactive decay are indicated. Energetics Of Nuclear Reactions While working out the details of the theory of special relativity, Einstein derived the equation for the equivalence of mass and energy: E = mc2. In a typical spontaneous nuclear reaction, a small quantity of matter is transformed into a corresponding quantity of energy. Nuclear energies are normally expressed in the unit MeV (megaelectronvolt). 1 u = 931.5 MeV : one atomic mass unit contains energy equivalent to 931.5 megaelectronvolts. 1 amu = 1 u Nuclear Binding Energy The energy released in for...</p>


View more >