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

Nuclei – “Core of Matter, Fuel of Stars”

Strong or nuclear force – a 3rd fundamental forceRadioactivity (incl. a 4th fundamental force, the “weak” one)Ionizing radiation Nuclear binding – fusion & fission, their applications and (serious) implications

Reminder:

Tiny, and yet >99% ofthe (rest) mass of anatom is in the nucleus!

Mp (or Mn ) ~ 2000 Me

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

Strongest among the 4 fundamental forces, but very short-ranged:typical nuclear sizes few 10-15m (remember atomic sizes?)

The 4 fundamental forces (in decreasing strength) & connectionto the 3 classical types of radioactivity:

Strong (or nuclear) – α

(alpha) decay, emission of a 42 He nucleusElectromagnetic – 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 availablein 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 energeticparticles, example: proton cancer therapy)

Radioactive decays & other nuclear reactions are the (medieval)alchemist’s dream – elements can be transformed into each other.

Quiz # 103: Which of these are ionizingelectromagnetic radiation?(a) α

and β

(b) β

and γ(c) γ

and cell phone signals (microwaves)

(d) γ

and x-rays(e) α

and x-rays

Quiz # 104: 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 thestatistical 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 # 105: If a radioactive isotope has a 1-year half-life, whatfraction will remain after 4 years?(a) 1/5 (b) 1/16 (c) ½ (d) ¼ (e) about 40%

Quiz # 106: If you had 1 gram of 235U and 1 gram of 238U, whichwould 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 badaccident!) during the next 70 years among Russians & Europeansexposed to the fallout. But this represents an increase in the cancer

death rate of only 0.003% among that population! (~125 millioncancer 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!!

And, again to put things in perspective:

How many direct radiation deaths from theFukushima-Daichi

nuclear power plant accident (clearly a

bad accident)?

…………a big fat ZERO !!