Nuclear reactions Micro-world Macro-world Lecture 17 Using the strong nuclear force to produce useful energy

Strong Nuclear Force It is very strong –It overcomes the electrical repulsion between positively charged protons that are only m apart. It acts over a very short range –It is not felt by nucleons when they are more than m apart. It is selective –It is felt by neutrons & protons, but not by electrons

Nuclear “bullets” Protons are repelled by electrical the repulsion force of the positively nucleus. Only protons with KE of a few MeV or more can get within the range of the strong nuclear force & produce “nuclear reactions” v F Producing nuclear reactions with protons (or any other charged nuclei) is a challenge

Neutron induced nuclear reactions Neutrons don’t feel the electrical force so even very slow, low-energy neutrons can strike the nucleus & produce “nuclear reactions” v Low energy neutrons are effective nuclear “bullets”

Nuclear fission n + 92 U  56 Ba + 36 Kr + 2n

Energy balance in a fission reaction 141 Ba + 92 Kr + 2n 200 MeV  KE  heat 235 U + n

Chain reaction Use the neutrons produced by one fission to initiate another fission Enrico Fermi

Requirements for A-bomb Fissionable material: 235 U or 239 Pu Critical mass Mechanism

Critical Mass Enriched 235 U50kg 239 Pu10kg M crit

Fissionable Material Fortunately, only certain nuclear isotopes undergo the fission process: 235 U only 0.7% of naturally occurring U (99.3% is 238 U, which doesn’t fission) 239 Pu doesn’t occur naturally, but is produced in nuclear reactors …. There are other fissionable isotopes, e.g. 233 U & 232 Th, but they are very rare

Little boy ( 235 U) (doughnut-like)

Fat man ( 239 Pu)

Devastation Hiroshima Aug :15AM 80,000 people killed immediately; ~100,000 people were exposed to lethal radiation & died painful slow deaths

Hiroshima aftermath

Devastation Nagasaki Aug :45AM 39,000 people killed immediately; ~70,000 people were exposed to lethal radiation & died painful slow deaths

Nagasaki aftermath

Nuclear fusion 2 H + 3 H  4 He + n Two light nuclei fuse together to form a heavier one Here the nuclei have to start out with large energy in order to overcome the electrical repulsion

Energy balance in a fusion reaction 4 He+n 12.3 MeV  KE  heat 2 H + 3 H

Need to overcome electric repulsion + + Protons need ~2MeV energy to get within m of each other (where strong nuclear force can be felt) This requires super-high temperatures (several Million degrees). Such high temperatures exist in the core of the Sun or in an Atomic-Bomb explosion

H-bomb: powered by nuclear fusion Nuclear fission bomb “detonator” produces the high temperature required to initiate fusion processes Nuclear fusion bomb

Brighter than 1000 suns 1000 times the power of an A-bomb!!

Dangers of teaching nuclear physics Oh, and I suppose it was me who said ‘what harm could it be to give the chickens a book on nuclear physics?’

Fusion in the Sun The core temperature is ~14 million degrees Here a tiny fraction of the protons have enough thermal energy to undergo fusion

Solar fusion processes MeV MeV MeV

pp-cycle 6 protons  4 He + 2 protons + 2 “positrons” + 2neutrinos

Energy balance in the pp-cycle 4 He 25 MeV  KE  heat 4 protons + 2 neutrinos

How do we know what goes on inside the Sun?

Superkamiokande

Direction of neutrinos detected in Superkamiokande

Sun as seen by a neutrino detector

Neutrinos come directly from solar core

Neutrinos are everywhere T est test