1. Nuclear Physics and Bombs

2. Topics  Elements and atoms  Radioactive decay of atoms  Fission and fusion  Nuclear energy  Nuclear bomb designs  Nuclear explosion phenomena  The Big-Bang theory

3. Basic Knowledge about Atom  The smallest particle (10 -10 m or 0.1 nm) of an element that still retains the characteristics of the element.  An atom has a very small size (10 -15 m) nucleus surrounded by negatively charged electrons.  Nucleus consists of protons (positively charged) and neutrons.  Proton and neutron have about the same mass that is very larger than electron’s. So an atom's mass is essentially the mass of its nucleus.

4. Periodic Table –The number of protons (the atomic number) determines which element it belongs to. Atoms with the same atomic number but different number of neutrons are called isotopes, e.g., 235 U and 238 U.

5.  Our Universe: 92% H; 7% He  Most abundant elements in the Earth: O, Mg, Si, Fe Most other light elements have escaped.  Peak at iron.

6. Radioactive Decay of Atoms  Some elements will spontaneously turn into other elements. This is called radioactivity and was discovered in 1896.  It happens randomly and the probability only depends on the structure of the nucleus (isotope).  Scientists use half-life to describe the probability of decay.

7. Radioactivity Example  Example: 238 U  206 Pb + 8 4 He + 6ß  The half-life T 1/2 of 238 U is about 4.5 billion years.  The graph shows the number of 238 U and 206 Pb at time t N 238 (t) = N 0 /2 t/T N 206 (t) = N 0 -N 238 (t)

8. Radioactive Dating  This can be used in the opposite way: if we can count how much daughter isotope in the sample as compared to the parent isotope we can get the age of the sample t = T log 2 ( 206 Pb/ 238 U + 1)  This method is called radioactive dating.  By using it, we find that Earth is ~4.5 Ga old. 206 Pb/ 238 U = 2 t/T - 1

9. Fission Process Heavy elements are split into lighter ones.

10. Fusion Process Light elements are combined into more heavy ones

11. Nuclear Energy Nucleons in the nucleus are bound tightly together by the so- called “strong force”. The nuclear binding energy is the energy required to break them apart. Therefore it is possible to release large amount of energy by breaking an nucleus to form a different nucleus that is bound more tightly, i.e., has higher binding energy.

12. Nuclear Energy The left figure shows the binding energy per nucleon of different atoms. Iron is the most stable element. Large amount of energy (yield) is released by fusion of light elements into heavier elements or by fission of heavy elements into lighter elements.

13. Yield of a 50 kg Uranium Bomb?  Fission of one 235 U atom releases energy of 235 * 1 MeV/nucleon = 3.8 10 -11 J.  235 gram of 235U has 6.0 10 23 atoms (Avogadro’s number). A 50 kg U-bomb has a yield of 50,000/235*6.0 10 23 *3.8 10 -11 = 4.8 10 15 J.  The yield of 1 kt of TNT is 4.2 10 12 J. So the yield of the 50 kg nuclear bomb is 1100 kt.  The first Uranium bomb, the Little Boy, is ~15 kt. The first generation A-bombs are not efficient.  US electric power consumption is about 3 10 18 J in 2000.

14. Chain Reaction  For 235 U and 239 Pu, one fission takes one neutron and produces three neutrons on average. The produced neutrons can be used to split more 235 U and start a chain reaction.

15. Nuclear Power Plants and Bombs  To sustain the chain reaction, it is necessary to have many fissionable atoms around to catch neutrons. The smallest amount of fissile material is called critical mass.  The critical mass depends on the density of the material and how easy for 235 U to capture a neutron.  Inside a nuclear power plant, the reaction is controlled by absorbing neutrons and moderating their speed. Slow neutrons can be captured more easily so nuclear power plants can use low-grade uranium (a few per cent in 235 U) as fuel.  In contrast, nuclear explosions are uncontrolled chain reaction. The fission material need to be supercritical and high grade (enriched).  The minimum mass is 47 kg for 235 U and 16 kg for 239 Pu in normal condition.  The mass can be reduced substantially by using tamper and increasing material density (e.g. the Fat Man bomb only used 6 kg of 239 Pu).

16. Fission Bomb Designs A fission device: 1.a subcritical system that can be made supercritical quickly; 2.a strong neutron source to initiate the supercritical system.

17. Fission Bomb Designs

18.  Boosted weapon: a small amount of deuterium- tritium mixture is placed in the center of the sphere of fissile material.  When the primary explosion happens, it produces enormous pressure and temperature at the center. This causes the deuterium and tritium mixture to undergo fusion and releases lots of neutrons in the center of the fissile sphere, greatly increasing the overall fission energy release.  Boosting can increase yields by a factor of ten (400 kt).

19. Fusion  To achieve fusion enormous temperatures are required. It takes about 50,000,000 degrees to get deuterium ( 2 H) to fuse with tritium ( 3 H). The required temperatures are higher for heavier elements.  At present, it appears that only a fission device is capable to initial fusion in a H-bomb. Other technology, such as laser, may be possible.  Once started, the energy released by fusion will sustain the process if enough fusion material are available.

20. Thermonuclear Bomb

21. Material for Nuclear Bombs  Usable materials are 235 U, 239 Pu, 2 H, and 3 H.  235 U is less than 1% in natural uranium. It can be enriched by using gas-diffusion or gas-centrifuge.  239 Pu is virtually nonexistent in nature and can be obtained by bombarding 238 U with neutrons in nuclear reactors.  2 H can be enriched by conventional methods.  3 H can be produced by neutron irradiation of lithium.

22. Making a Nuclear Bomb  Making a nuclear bomb requires a high degree of competence in various disciplinary.  Given enough time and supply, any nation or group with competent people should be able to produce a crude, heavy nuclear device.  Delivery the weapon on a sophisticated platform is not easy. Nuclear tests are likely to be needed in order to reduce the size and weaponize.  Other possibilities are to steal or purchase existing nuclear weapons, technology, or experts.

23. Nuclear Explosion Phenomena  The explosion happens in micro-seconds.  Can you use the nuclear physics you've learned to explain what's happening here?

24. The forms of nuclear energy  The energy released in the fission/fusion reactions are in the forms of kinetic energy of particles (neutrons and other produced elements) and high- energy gamma rays (photons).  In the atmosphere, the particles collide with air molecules to raise the temperature to 10 million degrees (thermal energy)

25. Nuclear Explosion Phenomena  Hot air radiates electro-magnetic waves in a wide spectrum from infra-red to visible and to X and gamma rays (thermal radiation and EM pulse).  The EM waves travel at speed of light.  The high temperature also causes the pressure of the air around the explosion to increase to a million atmospheric pressure (bar).  The highly-pressured air expands at speeds larger than the sound speed and generates shockwave.  For atmospheric explosions, shockwave takes about 50% of the yield and thermal radiation 35%.

26. Nuclear Explosion Phenomena  The fireball rises through the atmosphere in the form of a “mushroom” cloud.  Radioactive products of the nuclear explosions deposit around the area.  Some enter the upper atmosphere with the plume and fall to the ground over a large area.

27. New Generation Weapons  Neutron bomb: enhance neutron radiation with minimum radioactive fallout and shockwaves.  Reduced radiation weapons (RRW): maximize the electromagnetic pulse to destroy electronic equipment.  “doomsday bomb”: It has been hypothesized to produce a Cobalt bomb (coat the outside a thermonuclear device with a tamper of Cobalt). Neutrons produced in the fusion reaction will change Cobalt to Cobalt-60, which is a very radioactive atom with a half-life of 5.6 years. Such a bomb with enough Cobalt may kill all life by spreading dangerous radioactive material over the world.

28. Where Are Elements Made?  The light elements hydrogen and helium were created during the Big Bang and the elements between hydrogen and iron can be fused together inside stars. Heavier elements form during massive stellar explosions called supernovae.

29. Fusion in Stars  Stars like the Sun were mostly made of H initially and have a temperature of ~10 7 K. So, H to He fusion reaction is going on in stars (main sequence).  How did it get started in the first place?

30. The Birth of a Star As hydrogen clouds condense, pressure and temperature at the center increase. This lead to the ignition of H fusion.

31. The Fate of a Star  When most H is fused into He, fusion stops and and the star starts to collapse under gravity.  For stars with mass less than the Sun, they become brown dwarf and eventually end up as cold, dead bodies in space.  For stars like the Sun, the gravitational force can squeeze the center and make it hot enough to start fusion of He. Star starts to swell into a red-giant.  Elements from Li up to Fe are produced.  Eventually all fusion fuel are burnt out. It collapses to form a white dwarf.

32. Supernova  More Massive stars tend to explode in a supernova.  Elements heavier than Fe are produced in explosion.  A small central core remains to form a neutron star.  If the mass is is large enough, the core continue to shrink to form a black hole The crab nebula

33. How Much Time Do We have?  The Sun is radiating energy at a rate of 4 10 26 W.  It has a mass of 2 10 30 kg ( 1 H).  The main fusion process in the Sun is combining 1 H to 2 H, which releases 1.4 MeV of energy. The amount of H in the Sun can keep the fusion process last for about 10 Ga.  At a age of 4.5 Ga, the Sun is in its middle age (no crisis yet).