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 3rd fundamental force Radioactivity Ionizing radiation Nuclear binding – fusion & fission, their applications and (serious) implications

Strong (or nuclear) force: needed to hold the nucleus together. Strongest among the 4 fundamental forces, but very short-ranged: typical nuclear sizes few 10-15 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 42He 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)

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 = mc2 ) Ethermal & Eradiation

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.

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 3H and 3He 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.

Radioactive decay is a prime example of the statistical or probabilistic nature of quantum mechanics and quantum mechanical indeterminacy.

“Half-life” and “exponential” decay

Important: looking at an individual nucleus, can you predict when it will decay, even if you know the isotopes half-life?

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?

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 235U and 1 gram of 238U, which would be more radioactive, i.e. which would emit more α particles per minute?

(a) 235U (b) 238U (c) need more/other info (d) same

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

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!!

Fusion (the fire of stars) & Fission (chapter 15)

Fusion: two light nuclei stick together, releasing Ethermal + Eradiation Fission: one heavy nucleus splits into two (or more) lighter nuclei, again releasing Ethermal + Eradiation

Simple example of fusion: formation of the d(euteron) = 21H, (as atom called “deuterium”). Also a reminder of the concept of Ebinding.

n + p d (= 21H) + Eradiation (a 2.2 MeV photon) So…..how’s md related to mp + mn ?

More relevant for fusion in the sun: p + d 32He + energy Why does this reaction require the intense heat inside the sun?

Even heavier H isotope: 31H (= triton t, as atom “tritium”) d + t 4He + n is the H-bomb reaction (& eventually in fusion reactor, perhaps in your lifetime??)

It’s all in the “nuclear energy curve” – energy per proton or neutron:

<- Fe/Ni are the most stable nuclei

Fusion

Fission