Nuclear Reactions Fisson, Fusion IB Physics Core Unit 6.3

Artificial Transmutation In artificial transmutations the nucleus is bombarded with high-energy (kinetics energy) particles to induce transmutation. There are two reactants or the nucleus that is being bombarded and the high-energy particle. Note: High-energy particles are accelerated in accelerators, by the application of electric and magnetic fields. Neutrons cannot be accelerated in accelerators using electric and magnetic fields because - WHY? their charge is 0.

Example The first artificial transmutation was accomplished in 1919 by Rutherford who bombarded nitrogen-14 with α particles. Later, in 1930, Irene Curie and her husband Frederic bombarded stable Al-27 with alpha particles.

Problem for You Find what X is in the following nuclear reactions

Fisson A fission nuclear reaction is the splitting of a heavy nucleus into two or more lighter nuclei. Example U-235 is bombarded with slow neutrons to produce Ba-139, Kr-94, or other isotopes and 3 fast moving neutrons

Fisson Chain Reactions A nuclear chain reaction is a reaction in which an initial step, such as the reaction above, leads to a succession of repeating steps that continues indefinitely. Nuclear chain reactions are used in nuclear reactors and nuclear bombs.

Fusion A fusion nuclear reaction is the combination of very light nuclei to make a heavier nucleus. Extremely high temperatures and pressures are required in order to overcome the repulsive forces of two nuclei. Examples

Star Light Star Bright Consider the fusion nuclear reaction taking place in stars. Mass of reactants = 4 (mass of H-1) = 4 (1.00718 amu) = 4.02872 amu Mass of products = 1 (mass of He-4) = 1 (4.00150amu) = 4.00150 amu The difference between the mass of the reactants and products is 4.02872 amu - 4.00150 amu or 0.02722 amu This difference is called the mass defect and it is converted into energy according to the formula E = mc2 (we will look at this later) E = energy m= mass c = speed of light (3 x 108 m/s) Since the speed of light is a large number a small mass change corresponds to a large amount of energy.

Therefore Energy released during nuclear reactions is much greater than the energy released during chemical reactions. Energy released in a nuclear reaction (fission or fusion) comes from the fractional amount of mass converted into energy. Nuclear changes convert matter into energy.

There are benefits and risks associated with fission and fusion reactions. Fission reactions Benefits: Large amount of energy is released Production of electricity in nuclear plants Development of nuclear weapons (Atomic bomb, depleted uranium bullets) Creations of new elements in accelerators (Americium-241 used in smoke detectors) Risks: Dangerous nuclear waste are produced Accidents can release radiation into the environment

Fusion reactions Benefits: Release of larger amount of energy than fission. However, they are not a practical source of energy because the technical problems of high temperature, pressure, and containment of reaction are enormous. Risks: Relatively low - (unless you are close when it blows!)

Unified Mass Unit The unified atomic mass unit (u), or Dalton (Da) or, sometimes, universal mass unit, is a unit of mass used to express atomic and molecular masses. It is the approximate mass of a hydrogen atom, a proton, or a neutron. The precise definition is that it is one twelfth of the mass of an unbound atom of Carbon 12 (12C) at rest and in its ground state. 1 u = 1/NA gram = 1/ (1000 NA) kg (where NA is Avogadro’s No.) 1 u = 1.660538782(83)×10−27 kg = 931.494027(23) MeV The atomic mass unit is an older name for the same thing, which differs slightly in definition, and differs in value by one part in 1000. 1 u = 1.000 317 9 amu (physical scale) = 1.000 043 amu (chemical scale). Since 1961, by definition the unified atomic mass unit is equal to one-twelfth of the mass of a carbon-12 atom.

Binding Energy Nuclear binding energy is derived from the strong nuclear force and is the energy required to disassemble a nucleus into free unbound neutrons and protons, strictly so that the relative distances of the particles from each other are infinite (essentially far enough so that the strong nuclear force can no longer cause the particles to interact). A bound system has a lower potential energy Ep than its constituent parts; this is what keeps the system together. The usual convention is that this corresponds to a positive binding energy.

Mass Defect Because a bound system is at a lower energy level than its unbound constituents, its mass must be less than the total mass of its unbound constituents. For systems with low binding energies, this "lost" mass after binding may be fractionally small. For systems with high binding energies, however, the missing mass may be an easily measurable fraction.

The energy given off during either nuclear fission or nuclear fusion is the difference between the binding energies of the fuel and the fusion or fission products. In practice, this energy may also be calculated from the substantial mass differences between the fuel and products, once evolved heat and radiation have been removed.

The measured mass deficits of isotopes are always listed as mass deficits of the neutral atoms of that isotope, and mostly in MeV. Of course, when nuclear decay happens to the nucleus, the properties ascribed to the nucleus will change in the event. But for the following considerations and examples, you should keep in mind that "mass deficit" as a measure for "binding energy", and as listed in nuclear data tables, means "mass deficit of the neutral atom" and is a measure for stability of the whole atom.

Specific quantitative example: a deuteron A deuteron is the nucleus of a deuterium atom, and consists of one proton and one neutron. The experimentally-measured masses of the constituents as free particles are mproton = 1.007825 u; mneutron= 1.008665 u; mproton + mneutron = 1.007825 + 1.008665 = 2.01649 u. The mass of the deuteron (also an experimentally measured quantity) is Atomic mass 2H = 2.014102 u.

The mass difference = 2.01649−2.014102 u = 0.002388 u. Since the conversion between rest mass and energy is 931.494MeV/u (See Appendix 1 sheet), a deuteron's binding energy is calculated using E=mc2 to be: 0.002388 u × 931.494 MeV/u = 2.224 MeV. Thus, expressed in another way, the binding energy is [0.002388/2.01649] x 100% = about 0.1184% of the total energy corresponding to the mass.

In the periodic table of elements, the series of light elements from hydrogen up to sodium is observed to exhibit generally increasing binding energy per nucleon as the atomic mass increases. This increase is generated by increasing forces per nucleon in the nucleus, as each additional nucleon is attracted by all of the other nucleons, and thus more tightly bound to the whole.

The region of increasing binding energy is followed by a region of relative stability (saturation) in the sequence from magnesium through xenon. In this region, the nucleus has become large enough that nuclear forces no longer completely extend efficiently across its width. Attractive nuclear forces in this region, as atomic mass increases, are nearly balanced by repellent electromagnetic forces between protons, as atomic number increases.

Finally, in elements heavier than xenon, there is a decrease in binding energy per nucleon as atomic number increases. In this region of nuclear size, electromagnetic repulsive forces are beginning to gain against the strong nuclear force. At the peak of binding energy, nickel-62 is the most tightly- bound nucleus, followed by iron- 58 and iron-56 (This is the basic reason why iron and nickel are very common metals in planetary cores, since they are produced profusely as end products in supernovae).

The most tightly bound isotopes are 62Ni, 58Fe, and 56Fe, which have binding energies of 8.8 MeV per nucleon. Elements heavier than these isotopes can yield energy by nuclear fission; lighter isotopes can yield energy by fusion.