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Transcript
Nuclear Reactions vs. Normal Chemical Changes Nuclear reactions involve the nucleus The nucleus opens, and protons and neutrons are rearranged The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy “Normal” Chemical Reactions involve electrons, not protons and neutrons The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom. Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235* Uranium-238 Isotopes Two Categories Unstable – isotopes that continuously and spontaneously break down/decay in other lower atomic weight isotopes Stable – isotopes that do not naturally decay but can exist in natural materials in differing proportions Radioactivity It is not uncommon for some isotopes of an element to be unstable, or radioactive. We refer to these as radioisotopes. There are several ways radioisotopes can decay and give off energy known as radiation. Types of Radioactive Decay Alpha Decay Loss of an -particle (a helium nucleus) 4 2 238 92 U He 234 90 4 2 Th+ He Types of Radioactive Decay Beta Decay Loss of a -particle (a high energy electron) 0 −1 131 53 I 0 or −1 131 54 e Xe + 0 −1 e Types of Radioactive Decay Gamma Emission Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0 0 Penetrating Ability Radioactive Particle alpha Nuclear Symbol 4 He Pertinent Information helium atom without any electrons 2 beta 0 e high energy electron, no mass with a negative charge γ high energy ray with no mass and no atomic number -1 gamma 0 0 positron 0 e +1 mass of an electron but a positive charge Nuclear Reactions Alpha emission Note that mass number goes down by 4 and atomic number goes down by 2. Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved Nuclear Reactions Beta emission Note that mass number is unchanged and atomic number goes up by 1. Write Nuclear Equations! Write the nuclear equation for the alpha decay of radon-222 222Rn 86 218Po 84 4He + 2 Write the nuclear equation for the beta emitter Co-60. 60Co 27 60Ni 28 + 0e -1 Growth/Decay curves for radioactive elements Radioactive Decay Proportion of daughter atoms remaining 120 120 % Proportion of daughter atoms % Proportion of parent atoms remaining Proportion of parent atoms remaining 100 80 60 40 20 0 100 80 60 40 20 0 0 1 2 3 Time 4 5 6 0 1 2 3 4 Time This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 5 6 Half-Life Parent Daughter / Radiation Half-Life (Y) 3H 3He + 0β-1 12.3 14C 14N + 0β-1 5730 87Rb 87Sr + 0β-1 49 X 109 210Pb 206Pb 238U 234Th + + 4α+2 4α+2 20.4 +γ 4.5 X 109 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Element Uranium Uranium- 238 series U-238 4.5*109 y Proactinium Thorium Th- 232 series U-234 245500 y U-235 7.0*108 y Pa-234 1.2 min Th-234 24.1 d Pa-231 32800 y Th-230 75400 y Th-232 1.4*1010 y Actinium Radium U-235 series Th-228 1.91 y Ac-228 6.1 h Ra-226 1600 y Ra-228 5.75 y Th-231 25.5 h Th-227 18.7d Ac-227 21.8 y Ra-224 3.7 d Ra-223 11.4d Pb-208 stable Pb-207 stable Francium Radon Rn-222 3.8 d Astatine Polonium Po-218 3.1 min Po-214 0.00014 s Bismuth Lead Bi-214 19.9 min Pb-214 26.8 min α-decay Z: -2 N: -4 Po-210 138 d Bi-210 5.0 d Pb-210 22.3 y Pb-206 stable β-decay Z: +1 N: +/-0 Decay series of short lived nuclides This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). Detection of Isotopes Existence of ISOTOPES confirmed by Aston using the first mass spectrometer in 1919. Analysis of Neon gas achieved separation of 3 stable isotopes: 20 10 Ne 90.9% 21 10 Ne 0.3% 22 10 Isotope Ratio MS Magnetic sector Ion Source Faraday cups Detection system Ne 8.8% This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chart of the Nuclides 100 90 A nuclide = an isotope Narrow band of stability For stability, N/P ratio rises with mass Number of neutrons, N 80 70 60 β- emission 50 40 30 20 10 0 0 All nuclides outside the band and with Z > 83 are radioactive Elements can have from 0 – 10 stable Isotopes 10 20 30 40 β+ emission 50 60 Number of protons, Z 70 80 90 100 The Clocks in the Rocks In the beginning was the BIG BANG Solar system / earth condensed from dust and gas Radioactive elements decay to daughters (Parent : Daughter ratio changes with time) With half-life can calculate the age of the earth Geologic Time Radioactive Isotopes used in Geologic Dating Parent Daughter half-life (y) U-238 Lead-206 4.5 billion U-235 Lead-207 713 million Thorium 232 Lead 208 14.1 Billion K-40 Argon-40 1.3 billion R-87 Sr-87 47 billion C-14 N-14 5730 Half-life = time it takes for 1/2 of the parent mass to decay into the daughter mass Dating with Isotopes Process: 120 Select suitable isotope Percent parent 100 80 Parent C-14 60 Daughter N-14 40 20 Clearly the 0 system is useless ….. beyond 40,000 years 87Sr 14N – system is fine for 60M to 400G years! 0 5000 10000 15000 20000 25000 30000 35000 Time years 120 100 Percent parent The 87Rb 14C 80 Parent Rb-87 60 Daughter Sr-87 40 20 0 0 100 200 300 400 500 600 Time (billions of years) 700 800 Process: Ideally the crystals in the rock should contain no Sr (eg: Mica in Granite) All subsequent Sr arises from Rb decay Earliest rocks on Earth are 4G years old On melt, the clock is reset when new igneous rock is formed Strontium Dating – the Forensic Application Sr has 4 stable isotopes 0.715 ---- 0.730 ---- As a result of 87Rb decay, 87Sr levels will rise with time 87Sr / 86Sr is highest in oldest rock Sr has similar chemistry to Calcium and ends up in BONE 88Sr 7% 87Sr 10% 86Sr 82% 84Sr 1% 87Sr / 86Sr ratios: 0.703 in young rock 0.750 in oldest rock Strontium Dating – Tracing Adam Sept 2001, torso of 5 yr old boy recovered from Thames Suspected ritual killing Gut contents suggested poisoning and UK as place of murder Bone 87Sr/86Sr was high – PreCambrian levels Police search for relatives in rural area between Benin City and Ibadan, Nigeria…arrest made in Dec 2003 Carbon Dating 14C half life = 5730 yrs Cosmic radiation Excellent for dating organic material from 0-40,000 yrs* Nitrogen-14 Carbon-14 Neutron capture Why is there any 14C left, & how does it work? 14C is absorbed along with and 13C into the tissue of living organisms in a fairly constant ratio 12C Assume 14C is continually being produced at the same rate. Whilst alive, 14C levels remain constant. Only on death does the level start to drop However, calibration is needed to get accurate dates Soil When an organism dies 14C converts back to 14N by beta decay Carbon-14 Beta decay Nitrogen-14 Geologic Time 14Carbon Dating Dating is accomplished by determining the ratio of 14C to non-radioactive 12C which is constant in living organisms but changes after the organism dies When the organism dies it stops taking in 14C which disappears as it decays to 14N Forensic 14Carbon Cases Nuclear testing during 1955-63 put large amounts of 14C into the atmosphere which was incorporated into the enamel of human teeth. Because such testing stopped the 14C input ended and the 14C in the teeth decayed at a fixed rate allowing dating of the teeth Forensic 14Carbon Cases Dead Sea Scrolls – 5-150 AD Stonehenge – 3100 BC Hezekiah’s Tunnel - 700 BC Forensic 14Carbon Cases ● King Arthur’s Table in Winchester Castle, England 14C dated to 13th century AD ● Cave painting at Lascaux, France 14C dated to 14,000 BC ● Rhind Papyrus on Egyptian math 14C dated to 1850 BC Forensic 14Carbon Cases ●The Shroud of Turin was 14C dated 1260-1390 AD which suggests that it is a fake ●However, recent evaluation shows that the sample measured was from a medieval patch and or that it was seriously contaminated with molds, waxes, etc Measuring Radioactivity One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument. Trafficking Nuclear Materials This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Man-made Radioactive Isotopes Smuggled Plutonium – can identify the reactor type in which the fuel was originally radiated and the type of plant where the material was subsequently reprocessed In 1997, two pieces of stainless steel contaminated with alpha-emitters were found in a scrap metal yard in Germany. Source was identified as a fast-breeder reactor in Obninsk, Russia This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Weapons-grade Plutonium The isotopic composition of plutonium can indicate INTENT In 1994, a small lead cylinder discovered in a garage in Tengen on the Swiss-German border was found to contain plutonium metal, isotopically enriched to 99.7% Weapons-grade Pu-239 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Radioactive Fingerprints Preserving the conventional chain of evidence whilst dealing with radioactive samples can be problematic The first ever radioactive fingerprint has recently been identified on an object contaminated with alphaemitting isotopes For example – lifting fingerprints and swiping for radioactive contamination cannot both be carried out This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Nuclear Fusion nuclear fusion is a nuclear reaction in which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy which is released. Fusion is the process that powers active stars. Fusion would be a superior method of generating power. The good news is that the products of the reaction are not radioactive. The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as atomic bombs or atom bombs misnomer because the energy actually comes from nucleus Many fission products are either highly radioactive (but short-lived) or moderately radioactive (but long-lived), and as such are a serious form of radioactive contamination if not fully contained. Fission products are the principal radioactive component of nuclear fallout. Thermonuclear bombs work by using the energy of a fission bomb to compress and heat fusion fuel. When the fission bomb is detonated, gamma rays and Xrays emitted first compress the fusion fuel, then heat it to thermonuclear temperatures Tsar Bomb Nuclear Fission How does one tap all that energy? Nuclear fission is the type of reaction carried out in nuclear reactors. Nuclear Fission Bombardment of the radioactive nuclide with a neutron starts the process. Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction. If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out. Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass. Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator. Nuclear Reactors The reaction is kept in check by the use of control rods. These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.