Nuclear Physics

Describe in words and diagrams the evolution of a very massive star. Provide a short description of each stage.

1-2-3-4: Cloud, Protostar, Main sequence, Super giant Very massive gas clouds contract much faster and form very luminous main sequence stars that have surface temperatures higher than 20,000K radiating hundred of thousands of times more light than the sun. Since massive stars have 5-50 times the mass of the sun but radiate up to one million times more energy, they spent their nuclear fuel at a disproportional faster rate and evolve much faster through the protostar, main sequence and red (actually, yellow super) giant stages. 5: Supernova At the supergiant state, the fusion in very massive stars continues up to the point that an iron core is formed. There is a fusion to nuclei that requires additional energy, and there is a collapse that generates a tremendous amount of energy that produces a supernova. 6: Final stage: Neutron star or Black hole The core of the star that remains after a supernova explosion can be a neutron star, a tiny, a few kilometres in radius and greatly compressed star consisting of neutrons. The presence of a black hole can be determined by its gravitational attraction on a nearby star. When the black hole is part of a binary star system, gas pulled from the other star and falling into the black hole emits X-rays due to the high acceleration of the falling atoms.

Describe in words and diagrams the evolution of an average mass star like the sun. Provide a short description of each stage.

1. Gas cloud: Stars are born out of clouds of mostly hydrogen gas. 2. Protostar: Pulled by gravity, the cloud contracts into a dense, glowing gas of about 2000k temperature. 3. Main sequence: As the star continues to contract, the density of its core increases and its temperature exceeds 10 million degrees. At that high temperature, protons collide at very high speeds and fuse into Deuterium, Tritium and finally, Helium nuclei. 4. Red giant: When Hydrogen has been depleted in the core, there is no more energy production and the mostly helium core of the star shrinks. 5. Planetary nebula: When the star has burned most of its energy, it contracts, but the surface of the star becomes extremely hot, since hydrogen still fuses under it and a substantial amount of gas evaporates from the star. 6. White Dwarf: Finally, all the fuel in the star has been spent and the star contracts under the gravitational force becoming extremely small and hot, a white dwarf, in which atoms are crushed under the enormous gravitational force.

Describe in words and diagrams why a chain reaction can happen in a mass of Uranium-235.

A Uranium-235 nucleus hit by a neutron breaks up emitting three more neutrons which may hit and break up three more Uranium nuclei, which will produce nine neutrons, etc. This fission process with increasing number of break ups is called chain reaction.

Describe in words and with a schematic diagram the structure of a Hydrogen bomb and explain how it explodes.

A hydrogen bomb explosion is due to fusion of hydrogen nuclei. First a small atomic bomb detonates around a mixture of Deuterium and Tritium, compressing it and raising its temperature to about 100 million degrees. Deuterium and Tritium nuclei fuse into Helium nuclei and this process releases high and energy photons. The great amount of released energy heats up the air that expands suddenly creating the tremendous explosion of the hydrogen bomb.

Describe the neutron induced fission of a Uranium-235 nucleus into a Barium-142, Krypton-91, etc.

An important fission reaction is the break up of the Uranium-235. This nucleus is unstable and can break up into two lighter nuclei - e.g., Barium and Krypton - plus three neutrons plus high energy photons. The fission reaction releases 200,000,000 eV of energy.

Explain why and how a massive star undergoes a supernova explosion.

At the supergiant state, the fusion in very massive stars continues up to the point that an Iron core is formed. Now, fusion to nuclei heavier than iron, instead of releasing energy, requires additional energy. Thus, under the tremendous gravitational pressure, within seconds, electrons and protons fuse into neutrons. This process absorbs tremendous amounts of photons, the outer layers of the star lose their support (outward photon pressure) and collapse. This collapse generates a tremendous amount of energy that produces a gigantic detonation that throws most of the stellar matter out in space. This cataclysmic explosion is called supernova. During a supernova explosion, pressure waves and high temperatures in the stellar mass fuse light elements to heavier ones and so all chemical elements up to Uranium are created. Most of the mass of the star is thrown out into space, while the core of the star implodes to a tiny size. Heavy elements blown out in supernova explosions mix up with Hydrogen gas creating second-generation stars, like the sun. Most of the atoms that make earth and out bodies have been fused inside a star that exploded about 5 billion years ago and its heavier elements mixed with the hydrogen cloud that contracted and formed the solar system.

Describe the cause of beta radioactivity. Describe the beta radioactivity of Carbon-14.

Beta (electron) emission is the result of the decay of a neutron inside the nucleus into a proton (that remains inside the nucleus) an electron and an antineutrino that fly away. Beta decay increases the number of protons in the nucleus by one. For example, a Carbon-14 nucleus undergoes beta decay and converts into a Nitrogen-14 nucleus.

Why is it very difficult to obtain, commercially, energy from (controlled) nuclear fusion?

Controlled, slow fusion, for generation of useful electric energy is technologically very difficult to achieve. Maybe, some decades from now, nuclear reactors fusing Deuterium into Helium will be in operation and provide great amounts of energy without radioactive byproducts.

When is fusion exothermic, that is, releases energy? When is fusion endothermic, that is, requires additional energy?

Energy is released when light nuclei fuse to give Iron or a lighter nucleus. (Since the heavier nucleus has a greater amount of binding energy, it is lighter than the sum of the fusing nuclei.) Thus, additional energy is required to fuse to nuclei heavier than Iron. As a result, heavy nuclei is required to fuse to nuclei heavier than iron. So, heavy nuclei can break up to lighter ones and also release energy if the binding energy of the produced nuclei is more than that of the parent nucleus.

Describe the nucleon composition of Hydrogen, Carbon, Nitrogen and Uranium-238 nuclei.

Hydrogen: 1p Carbon: 6p 6n Nitrogen: 7p 7n Uranium-238: 92p 146n

Describe in words and with a schematic diagram the structure of a Uranium atomic bomb and how it explodes.

In an atomic bomb explosion, neutrons produced by the fission of Uranium hit other Uranium nuclei and cause them to break up and a chain reaction occurs. However, this chain reaction will happen only if there is enough Uranium mass (above a critical mass) so that the produced neutrons do not escape but almost certainly hit other Uranium nuclei. The detonation happens as follows: A small TNT bomb explodes and compresses two or more pieces of Uranium together into one piece whose mass then exceeds the critical mass. Then, the chain reaction occurs and trillions of Uranium nuclei break up in a tiny fraction of a second. The energy released in such a process is tremendous. One kg of Uranium-235, when it breaks up into a chain reaction, produces, in an instant, energy equal of burning 2500 tons of coal.

Describe in words and with a schematic diagram the structure of a Plutonium bomb and how it explodes.

In another type of atomic bomb, the fissionable mass of Plutonium is less than the critical. At the moment of detonation, a chemical bomb explodes around the mass of Plutonium. The higher density the Plutonium mass sustains chain reactions and explodes. Implosion atomic bomb: The mass of plutonium is below critical, but the implosion increases its density, plutonium bomb explodes.

Describe hydrogen fusion in the cores of stars.

In the core of the sun and stars, hydrogen nuclei (protons) collide at high speeds and fuse into a deuteron, a positron (e+) and a neutrino. The energy released by the fusion of two protons is 2.2 MeV, about one million times more than the energy released when two atoms bind chemically. The fusion chain continues with the production of tritium and finally helium. Altogether, in the core of the sun and stars, hydrogen nuclei fuse to helium nuclei and this process releases a tremendous amount of energy.

What are the isotopes? What is common and what is different in isotopes?

Isotopes are what we call atoms that have the same number of protons but different number of neutrons (same chemical elements but different masses). The word isotope refers to the fact that they occupy the same location in the periodic table of elements, meaning they are the same chemical element.

What is the approximate ratio of neutrons to protons in light and heavy nuclei? Why are there more neutrons than protons in light and heavy nuclei?

Light nuclei have about equal numbers of neutrons and protons, but heavier nuclei have a greater number of neutrons. The neutron to proton ratio rises from 1 for light nuclei to about 1.5 for the heaviest. Because the attractive nuclear force has a very short range heavier stable nuclei have a greater number of neutrons so protons are farther apart reducing their repulsion.

Describe the inner structure and main source of energy of a star in the main sequence stage.

Main sequence: As the star continues to contract, the density of its core increases and its temperature exceeds 10 million degrees. At that high temperature, protons collide at very high speeds and fuse into Deuterium, Tritium and finally into Helium nuclei. The greater the mass of the star the hotter and more luminous it becomes.

Explain how a nucleus can emit a gamma ray photon.

Nuclei are composite particles consisting of protons and neutrons. Consequently, nuclei can be in excited states in which a proton or a neutron has more energy than its lowest possible energy. Then when a nucleon falls from a higher into a lower energy level, it emits a gamma ray photon that carries away the energy lost by the nucleon. The process is similar to the falling of an atomic electron from an excited to a lower state.

Explain why Hydrogen fusion can happen at 15 million degrees but Helium fusion requires at least 100 million degrees temperature.

Since helium nuclei have two protons, they repel each other four times more strongly than hydrogen nuclei, therefore, they must collide at much higher speeds to overcome their stronger electric repulsion and fuse. Such high speeds can happen only above 100 million degrees temperature.

Describe in words and with a schematic diagram how energy is produced and converted to electricity in nuclear power plants.

The fission of the Uranium-235 can be induced by firing neutrons at it. This is how nuclear energy is released in power plants. A controlled beam of neutrons hits steel rods that contain Uranium-235. From the fission of these nuclei, energy is released that heats up the surrounding water. This water circulates inside another tank of water, heating it up and the produced steam rotates the electric generators. Nuclear reactor: In nuclear plants, steel rods containing Uranium-235 atoms are exposed to a neutron beam that causes Uranium nuclei to break up releasing energy heating up the surrounding water. This hot water circulates inside pipes within another tank of water that boils up becoming the steam that turns the turbines of the electric generators.

Discuss the role and properties of the nuclear force.

The force that binds nucleons was called nuclear as well as strong force and has two outstanding characteristics. First it is very strong, 2000 times stronger than the electric force. Second, while the electric force has infinite range, the nuclear force has a very short range of about 10^-15m from each other, no nuclear force acts between them. To see that the nuclear force is much stronger than the electric force, consider that a photon carrying 14eV of energy can break up the electron and photon held by the electric force in a Hydrogen atom. However, it takes a gamma ray photon carrying at least 2,200,000 eV of energy to split the proton and neutron held by the nuclear force in a deuteron.

Explain why the nuclear force has a very short range.

The nuclear force has a very short range because unless two nucleons are very close to each other, less than 10^-15 m, will not interact strongly; unless 2 nucleons are very close to eachother, no pion exchange is possible. This is why strong interactions are completely absent between atoms. Nuclear forces do not play any active role in chemical bonds, they just hold protons and neutrons inside the nucleus.

Describe in words and diagram why and how alpha radioactivity happens, e.g., the alpha decay of a Uranium-238 nucleus.

The radioactive alpha decay of nuclei is the spontaneous emission of a helium nucleus. The alpha particle escapes from the nucleus through tunneling - a quantum process - and that explains why it happens with specific probability. For example, the nucleus of Uranium-238 emits an alpha particle and it transforms to the nucleus of Thorium-234.

Describe the inner structure and main source of energy of a star in the red giant stage.

This gravitational contraction raises the temperature of the core to 100 million degrees and at that temperature, helium nuclei fuse - at a very fast rate -into carbon nuclei. The energy released from the Helium fusion is tremendous and causes the outer laters of the star to expand. The star becomes a red giant having radius 10 to 100 times the radius it had in the main sequence stage.

In the beginning, about 14 billion years ago, the universe consisted only of hydrogen, deuterium and helium. How and where were all the other elements formed?

Three minutes after the Big Bang explosion (13.8 billion years ago), the universe was a hot soup of photons, electrons, protons, deuterons and alpha particles. There were no heavier elements like those that make up most of the Earth because the photons at that time were so energetic that they would blast apart any heavier nuclei. After one million years of expansion, the temperature of the initial cloud decreased to 3000K so electrons and nuclei combines to neutral atoms that could not be broken up by the much less energetic photons at that stage. The expansion of mostly hydrogen gas continued. Ten million years after the Big Bang, the huge cloud that was the universe started breaking up into smaller ones that became galaxies. Within galaxies, smaller clouds of hydrogen gas condensed into stars; new stars are born even now.

Where were the heavy atoms that make up Earth produced?

Three minutes after the big bang explosion the universe was a hot soup of photons, electrons, protons, deuterons and alpha particles. There were no heavier elements like those that make up most of the earth because the photons at that time were so energetic that would blast apart any heavier nuclei. Most of the atoms that make earth and our bodies have been fused inside a star that exploded about 5 billion years ago and its heavier elements mixed with the hydrogen cloud that contracted and formed the solar system.

Write down the number of protons and neutrons in Uranium-238 and Uranium-235.

Uranium 238: 92 protons, 146 neutrons Uranium 235: 92 protons, 143 neutrons

What is nuclear fusion? Why can nuclear fusion only happen at very high temperatures?

We call fusion the binding of light nuclei to heavier ones. For example, two proton (hydrogen molecule) can collide and fuse to produce a deuteron, as the Deuterium nucleus is called. The nuclear force that binds protons and neutrons in the nucleus is very strong, consequently, protons and neutrons binding into nuclei lose a substantial amount of energy that is released by emitted gamma rays (photons), electrons, positrons, neutrinos, etc. For a nuclear fusion reaction to occur, it is necessary to bring two nuclei so close that nuclear forces become active and glue the nuclei together. Nuclear forces are small-distance forces and have to act against the electrostatic forces where positively charged nuclei repel each other. This is the reason why nuclear fusion reactions occur mostly in high density, high temperature environment.

What is (nuclear) fission? When does fission release energy?

When nuclei heavier than iron break up to lighter ones energy may be released. The break up of a nucleus called fission which can happen spontaneously but also with a bit of help, e.g., collision with neutron.