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Radiation and Radioactivity

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Radiation and Radioactivity Radiation and Radioactivity David Hahn “The Radioactive Boyscout” 1976- Known for attempting to make a fast-breeder reactor in his backyard at age 17 How might you distinguish Alpha, Beta and Gamma radiation experimentally? How might you distinguish Alpha, Beta and Gamma radiation experimentally? Magnetic Field β-decay Weak interaction can transform a proton into a neutron or a neutron into a proton (It’s actually happening at the quark level) β- decay: n p + e- e-: electron β+ decay: p n + e+ e+: positron Electronic Conversion: p + e- n β- decay Proton u d u e- W- νe Neutron u d d α-decay U 4 α-particle = He:2 cluster in the nucleus – α: very tightly bound system Tunnel effect (decay probability) r A A-4 4 Z X N Z-2 Y N-2 + 2 He 2 + Q Q-value energy (Q>0, because the decay occurs spontaneously) 2 Q = [mX-mY-mHe].c Radiation and Doses • A number of units are used to describe radiation and its effects on biological material Common Unit SI Unit Description Application 1 Curie (C) 1Becquerel (Bq) Activity Describe amount of 3.7x1010 decays 1 decay material ie 1 μC of 137Cs Rad Gray (G) Absorbed Dose How much Energy 0.01 Joule/Kg 1Joule/Kg is absorbed REM Equivalent Dose Biological Damage Rad x RBE Absorbed x Relative RBE Biological 10-20 Alpha Effectiveness (RBE) 1-2 Beta 1 Gamma Example of a Decay Chain (238U) Scale of Effects of Radiation Exposure Over 5000 rem Death within 1-2 days 1000 – 5000 rem Death within 1-2 weeks 1000 rem 80-90% death rate 500 rem 50% death rate 100 rem Detectable damage to bone marrow 5 rem Annual Occupational Dose Limit INCREASING DOSE 0.2-0.5 rem Typical annual dose (background, lifestyle) .01 rem Chest X-ray The Radium Girls The Radium Girls In the early 1920s, about 70 women were hired at the radium factory in New Jersey where they worked as dial painters and used luminescent paint. They assumed they were not using anything poisonous. Even though some women thought it was strange that when a few of them blew their nose the handkerchief glowed in the dark, they thought the stuff was harmless. Some women even painted their nails and their teeth to surprise their boyfriends when the lights went out. At work, the women were painting glow-in-the-dark radium compounds on the dials of watches and clocks. The women sat at long tables row after row. Dials waiting to be painted sat nearby. Each day they would mix up glue, water and radium powder (a mixture of radium salts and zinc sulfide, ZnS) into a glowing greenish-white paint. With care they would then apply it with camel hair brush to dial numbers. As the brush lost its shape, the paint couldn't be applied accurately, so the women would put the brush in their mouth and use the lips to make a point. The paint has no taste, and the women didn't know it was harmful. In those days most people thought radium was a scientific miracle for curing cancer and other medical problems. Unfortunately, for many of these women it did just the opposite. After several years of working as a dial painter, some women's teeth started falling out and their jaws developed a painful abscess. (…) The glow effect that was seen with the dial painters was the result of ionization of atmospheric gases, producing a blue glow. The blue glow can be enhanced in luminescent paints by the addition of zinc sulfide (ZnS). As the alpha particles strike the ZnS, visible light is emitted in response to the ionizing radiation. By the end of the 1920s, many of the dial painters died from symptoms associated with radium poisoning. From: www.unco.edu/chemist/Bulletin/Chem101/radium.htm Question: Why is ingesting Radium (Ra) so harmful? Why is ingesting Radium (Ra) so harmful? Radium is especially dangerous because the configuration of its outer electrons are similar to that of Calcium. So it can form chemical bonds in the same way that Calcium does. Calcium is one of the elements in bone. Ingested Radium can replace Calcium in bone to become a long lasting internal radiation source. Leukemia and other cancers can result. 222 Ra t1/2 = 1601 years From E-Bay Nov. 29th 2010 “FITRITE RADIUM OUTFIT NOTE!!!!!NOTE!!!!! This is a radium outfit used to put luminous material on watch hands and/or dials. The box says it is easily applied and dries quickly. The directions are on the box, which is original. There are 2 cans of material, one light and one dark and they have been used. There is a metal tool for application.” (Description posted) How to describe radioactive decays ? Radioactive decay represents changes of an individual nucleus. Houston, we have a problem. HOWEVER: quantum mechanics prevents us from describing the decay of a single nucleus !!! Because, usually one looks at the decay of a large number of nuclei (N>>1), one can describe radioactive decay statistically. Decay is proceeding at a certain rate Activity (A) [decays/s] Units: 1 Becquerel = 1 Bq = 1 decay per second 1 Curie = 1 Ci = 3.7 x 1010 decays per second (old unit still widely used) (activity of 1g of radium) Formalism (I) The activity of a certain sample (e.g. source) depends on the number N of radioactive nuclei and on the probability λ for each nucleus to decay: A = λ . N [1/s] [1/s] Evolution with time: dN = -A.dt dN: number of nuclei that decayed during dt dN = -λ.N.dt, which gives: dN/dt = -λ.N Solving the differential equation: Radioactive decay law -λ.t N(t) = N0.e N(t=0) Formalism (II) Similarly, we find that the activity of a source change with time: -λ.t A(t) = A0.e From λ = decay constant, one can define τ = 1/λ, the mean lifetime It is usual to define t1/2, the half-life, the time after which half of the initial nuclei have decayed: t1/2 = ln 2 / λ = τ ln 2 The half-life is characteristic to the decay of a given nucleus. This number (when known) is usually tabulated. Conservation laws • Conservation of Energy • Conservation of linear momentum • Conservation of angular momentum • Conservation of electric charge • Conservation of mass number A e.g. total number of nucleons is conserved, but Z and N can change .
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