Nuclear Physics (chapters 14 & 15) strong societal themes and impact!

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Meredith Shepherd
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1 Nuclear Physics (chapters 14 & 15) strong societal themes and impact! Very brief review of what Bill Miller already covered last week, i.e. ch. 14 on radioactivity Strong or nuclear force a 3 rd fundamental force Radioactivity Ionizing radiation Nuclear binding fusion & fission, their applications and (serious) implications

Strong (or nuclear) force: needed to hold the nucleus together.

types of radioactivity: Strong (or nuclear) α (alpha) decay, emission of a 4 2He nucleus

2 Strong (or nuclear) force: needed to hold the nucleus together. Strongest among the 4 fundamental forces, but very short-ranged: typical nuclear sizes few m (remember atomic sizes?) The 4 fundamental forces (in decreasing strength) & connection to the 3 classical types of radioactivity: Strong (or nuclear) α (alpha) decay, emission of a 4 2He nucleus Electromagnetic γ (gamma) decay, emission of energetic photon Weak β (beta) decay, emission of e - or e + in n(eutron) p(roton) Gravitational (irrelevant in nuclear physics)

3 Strong force strong nuclear binding lots of energy available in nuclear reactions good & bad consequences! Why radioactivity/radioactive decays (or fission/fusion)? Ultimate fundamental physics reason? Achieve a more stable, lower energy state! Excess energy (via E = mc 2 ) E thermal & E radiation Important definitions: 1) Atomic # vs. mass # (# of protons vs. # of protons + neutrons) 2) Element vs. isotope (place in periodic table, i.e. # of protons vs. # of neutrons for a given element) 3) Ionizing radiation α, β, x- and γ-rays (but also other energetic particles, example: proton cancer therapy) Radioactive decays & other nuclear reactions are the (medieval) alchemist s dream elements can be transformed into each other.

4 Quiz # 87: Which of these are ionizing electromagnetic radiation? (a) α and β (b) β and γ (c) γ and cell phone signals (d) γ and x-rays (e) α and x-rays Quiz # 88: How do masses (m) and electric charges (q) of 3 H and 3 He compare? (a) m about the same and q in the ratio of 1 to 2. (b) m about the same and q in the ratio of 2 to 1. (c) m in the ratio of 1 to 2 and q the same. (d) m in the ratio of 1 to 2 and q in the ratio of 1 to 2. (e) m in the ratio of 2 to 1 and q in the ratio of 1 to 2.

5 Radioactive decay is a prime example of the statistical or probabilistic nature of quantum mechanics and quantum mechanical indeterminacy. Important: looking at an individual nucleus, can you predict when it will decay, even if you know the isotopes half-life? Half-life and exponential decay

6 Note the enormous range of half-lives:..and some of the very long half-lives are perhaps the problem with nuclear energy how & where to store such radioactive waste?

7 Quiz # 89: If a radioactive isotope has a 1-year half-life, what fraction will remain after 4 years? (a) 1/5 (b) 1/16 (c) ½ (d) ¼ (e) about 40% Quiz # 90: If you had 1 gram of 235 U and 1 gram of 238 U, which would be more radioactive, i.e. which would emit more α particles per minute? (a) 235 U (b) 238 U (c) need more/other info (d) same

8 Ionizing radiation & risk to humans good summary in 14.6 & 14.7

9 Life is full of risks to live is to risk! So let s get on with it.. Not (at all) to belittle Hiroshima, Nagasaki, Chernobyl, and now (2011) Fukushima. But, always good & instructive to keep things in some perspective: Hobson cites 4000 excess cancer deaths from Chernobyl ( a bad accident!) during the next 70 years among Russians & Europeans exposed to the fallout. But this represents an increase in the cancer death rate of only 0.003% among that population! (~125 million cancer deaths in this population over those 70 years) Even under the controversial linear hypothesis this would increase by (only) a factor of about 4. Another way to look at this, using the microrisk of table 14.6, i.e. 1 in a million risk of death: average Chernobyl risk to Russians & Europeans is about 20 microrisks like smoking 28 cigarettes!!

10 Fusion (the fire of stars) & Fission (chapter 15) Fusion: two light nuclei stick together, releasing E thermal + E radiation Fission: one heavy nucleus splits into two (or more) lighter nuclei, again releasing E thermal + E radiation Simple example of fusion: formation of the d(euteron) = 2 1H, (as atom called deuterium ). Also a reminder of the concept of E binding. n + p d (= 2 1 H) + E radiation (a 2.2 MeV photon) So..how s m d related to m p + m n? More relevant for fusion in the sun: p + d 3 2He + energy Why does this reaction require the intense heat inside the sun? Even heavier H isotope: 3 1 H (= triton t, as atom tritium ) d + t 4 He + n is the H-bomb reaction (& eventually in fusion reactor, perhaps in your lifetime??)

11 It s all in the nuclear energy curve energy per proton or neutron: Fusion Fission <- Fe/Ni are the most stable nuclei

Nuclear Energy. Nuclear Structure and Radioactivity

Nuclear Energy. Nuclear Structure and Radioactivity Nuclear Energy Nuclear Structure and Radioactivity I. Review - Periodic Table A. Atomic Number: The number of protons in the nucleus of an atom B. Atomic Mass: The sum of the mass of protons, neutrons