Nuclear Properties of Material
Radioactivity, interaction of radioactivity and material, and nuclear reactions are nuclear properties of material. The emission of a subatomic particle or high energy photon from the atomic nuclei is called radioactivity. The most general types of radioactivity are alpha particles, beta particles, and gamma photons. When bombarded by subatomic particles, elements undergo nuclear reactions, changing their elemental identities. Interactions of radioactivity with material cause chemical and physical changes. Nuclear reactions also cause changes that affect properties of materials.
Three types of particles are emitted from the atomic nuclei, and these have been called alpha, beta and gamma rays. Alpha particles are the atomic nuclei of helium, and beta particles are electrons or antielectrons. Gamma rays are high energy photons.
Alpha particles are helium nuclei, consisting of 2 protons and 2 neutrons. This nuclide is very stable, and helium is a rather abundant element at a universal scale. Many heavy elements such as 238U and 239Pu emit an alpha particles, 4He2. For example:
238U92 = 234Th90 + 4He2 + Energy
235U92 = 231Th90 + 4He2 + Energy
239Pu92 = 235U92 + 4He2 + Energy
226Ra88 = 222Rn86 + 4He2 + Energy
Most of the energy is carried away by the alpha particles in the form of kinetic energy. The particles move at high speed when emitted. Most alpha emitters are elements heavier than lead. Some rare earth elements also emitts alpha particles.
Beta particles are electrons, e-, emitted from the atomic nuclei. As a result, one neutron of the nucleus is converted to a proton. This type of conversion takes place for nuclei containing too many neutrons. For example, carbon-14 is a typical beta particle emitter:
14C6 = 12N7 + e- + energy
60Co27 = 60Ni28 + e- + energy
Nuclides too rich in proton emit positrons, e+, which are antiparticles of electrons. For example,
10C6 = 10B5 + e+ + energy
Another mode of reducing the positive charge is by electron capture,
10C6 + e- = 10B5 + energy
Usually, electrons in the inner shell are captured by the nuclei, and when outer shell electrons fill the vacancies, X-rays are emitted. X-rays are high-energy photons.
A third type of radioactivity is the emission of high-energy photons, hv, from the highly energetic atomic nuclei. These excited nuclei become stable after emitting the photons. For example, the beta-emitting 60Co27 may leave Ni nuclei at excited state which we designate as 60*Ni28. These nuclei emit gamma rays.
60Co27 = 60*Ni28 + e- + energy
60*Ni28 = 60Ni28 + hv
Wavelengths of gamma ray photons are usually shorter than those of X-rays.
Nuclear reactions are usually induced by bombardment of a nuclide A using energetic particle a.
A + a = B + b
The products are another nuclide B and other light particle b. Identity of the element change in nuclear reactions. For example, Rutherford observed the proton in this reaction:
14N + 4He = 17O + 1H.
The neutrons do not carry any charge, and they approach atomic nuclei with ease. They are often used to induce nuclear reactions. For example, natural cobalt atoms have mass 59, and they are converted to Co-60 by neutron capture reaction,
59Co + n = 60Co.
The radioactive cobalt is used in medical applications as well as a source for gamma rays in food processing.
A heavy nuclide splits into two fragments with the emission of some light subatomic particles is called nuclear fission. For example, fission of uranium isotope 235U can be induced by neutrons, n, bombardment.
235U + n = 100A + 133B + 3 n.
The masses of A and B vary with high probability for masses of 100 and 135. Usually more neutrons are produced. Due to the release of neutrons, they can induce further fission, leading to a chain reaction for nuclear power generation or atomic explosion. Aside from 235U, another well-known nuclide for fission is 239Pu.
At 10,000,000 K, light nuclides combine into a heavy nuclide. The following reaction has been employed for hydrogen bombs.
2D + 3T = 4He + n + energy
A neutron, n, is the byproduct in this reaction. A lot of energy is released in fusion reactions. Fusion reactions provide the energy to power the Sun. There are several types of reactions. The following reactions make up the hydrogen burning cycle in the Sun.
H + H = D + e+ + hv
D + H = 3He + hv
3He + 3He = 4He + 2 H.
Multiplying the first two reaction equations by 2 and adding all three equations give the overall reaction. The intermediates, D and 3He are canceled when the appear on both sides.
4 H = 4He + 2 e+ + 4 hv + Energy
The net result is that 4 protons combine to give a helium nucleus with the emission of 2 positrons and some gamma photons.
Interaction of Radiation with Matter
Interaction of radiation with matter
Hi-energy subatomic particles such as neutrons, alpha and beta particles, and gamma rays strip electrons off atoms and molecules, ionizing them. Thus, these are called ionizing radiation. They produce ion pairs in the air.
N2 = N2+ + e-
Irradiating bulk material causes chemical bonds to break, leading to the formation of ions, free radicals, and molecular fragments. These fragments are very reactive chemically, and they cause secondary effect on the biology of the species. Since there are so many modes of interaction, the effects are also very diverse.
Although nuclear reactions between subatomic particles and biological tissues are possible, but ionizing radiation does not cause significant nuclear reactions.
When animals and human beings are exposed to low doses of ionizing radiating, the effect may not show for several years or decades. The uncertainty is the major cause of fear.
High doses of ionizing radiation causes burned symptoms. If damage to the gastrointestinal and central nerve systems are detected in a few days, the doses are rather high. Higher doses cause immediate injury or death. Damages to bone morrow and DNA may not have detectable symptoms, but these damages lead to anemia, cancer, birth defects, and mutations. Radiation damages affect the young more than to the old. Dividing cells are particularly vulnerable to radiation, and people younder than 18 years old are usually not allowed to work in environment where radiation is present.
- What are alpha and beta particles? What are gamma photons?
- What are radioactivities?
Discuss the particle emissions of unstable isotopes of one of the following elements: hydrogen, carbon, oxygen, lead, and uranium. For this part, reference material is required.
- Describe fission and fusion reaction.
Find out how fission nuclear reactors are designed and work. This is a research activity.
- Find out the current research activities of fusion.
Chung (Peter) Chieh (Chemistry, University of Waterloo)