Topic 6 : Atomic and Nuclear Physics. 6.2 Radioactive Decay Topic 6 : Atomic and Nuclear Physics.

Natural Radioactive Decay Naturally occurring radioactive materials are common in the environment and in the human body. These materials are continuously emitting ionizing radiation. Ionizing radiation from outer space (cosmic radiation) bombards the earth constantly. Collectively, the ionizing radiation from these and similar sources is called background radiation. Human activities, such as making medical x-rays, generating nuclear power, testing nuclear weapons, and producing smoke detectors which contain radioactive materials, cause additional exposure to ionizing radiation.

The percentage of the average annual radiation exposure contributed by each major source is illustrated in Figure 1. About 82 percent is from nature, and 18 percent is from industrial, medical, and consumer sources. The values given are averages for the United States. Actual values vary depending on where people live and how they spend their time.

Radioactivity 6.2.1 Natural Radioactive Decay Many nuclei are radioactive. This means they are unstable, and will eventually decay by emitting a particle, transforming the nucleus into another nucleus, or into a lower energy state. A chain of decays takes place until a stable nucleus is reached.

Radioactive decay and conservation principles. During radioactive decay, principles of conservation apply. Some of these we've looked at already, but the last is a new one: conservation of energy conservation of momentum (linear and angular) conservation of charge conservation of nucleon number Conservation of nucleon number means that the total number of nucleons (neutrons + protons) must be the same before and after a decay.

6.2.2 Types of Radioactive Decay There are three common types of radioactive decay, alpha (α), beta (β), and gamma. The difference between them is the particle emitted by the nucleus during the decay process.

Alpha Decay In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable. An example of an alpha decay involves uranium-238: The process of transforming one element to another is known as transmutation. Alpha particles do not travel far in air before being absorbed; this makes them very safe for use in smoke detectors, a common household item.

Beta Decay A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. An example of such a process is: In terms of safety, beta particles are much more penetrating than alpha particles, but much less than gamma particles.

Gamma Radiation The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level. Similar to the energy levels for electrons in the atom, the nucleus has energy levels. The concepts of shells, and more stable nuclei having filled shells, apply to the nucleus as well.

Gamma Decay (cont) When an electron changes levels, the energy involved is usually a few eV, so a visible or ultraviolet photon is emitted. In the nucleus, energy differences between levels are much larger, typically a few hundred keV, so the photon emitted is a gamma ray. Gamma rays are very penetrating; they can be most efficiently absorbed by a relatively thick layer of high-density material such as lead.

6.2.3 Ionising properties of radiation When the nucleus of a radioactive isotope gives up its extra energy, that energy is called ionizing radiation. Ionizing radiation may take the form of alpha particles, beta particles, or gamma rays. Ionizing radiation is of concern because it may cause adverse health effects

Alpha Ionisation When alpha particles travel through solid material, they interact with many atoms (ie bash into them) within a very short distance. They create ions and use up all their energy in that short distance. Most alpha particles will use up their energy while travelling through a single sheet of ordinary notebook paper. The primary health concern associated with alpha particles is that when alpha-emitting materials are ingested or inhaled, energy from the alpha particles is deposited in internal tissues such as the lungs.

Beta Ionisation A beta particle is an electron that is not attached to an atom. It is small, over 7000 times lighter than an alpha particle. The beta particle travels farther through solid material than an alpha particle. For example, a very high-energy beta particle will travel about 1.25cm through plastic before it uses up all its energy. Like alpha particles, beta particles lose energy with every interaction and no longer produce ions once all their energy is spent. Health concerns associated with beta particles arise primarily when beta-emitting materials are ingested or inhaled.

Gamma Ionisation Gamma radiation (gamma rays) can pass completely through the human body. Thus gamma rays emitted outside of the body may cause ionization, and possible health effects, in any organ in the body. But once a gamma ray loses all its energy, it can no longer cause damage. Most high energy gamma rays will lose all their energy in a 0.6m of soil, 1m of concrete, or 30cm of lead.

6.2.3 How Can You Detect Radiation? Radiation cannot be detected by human senses. A variety of handheld and laboratory instruments is available for detecting and measuring radiation.

The figure below illustrates the basic principle used by portable instruments in the detection and measurement of ionizing radiation. The detector tube (i.e. Geiger counter) is simply a gas filled, cylindrical tube with a long central wire that has a 900-volt positive charge applied to it and is then connected, through a meter, to the walls of the tube.

Radiation enters the tube and produces ion pairs in the gas. The electron part of the ion pair is attracted to the positively charged central wire where it enters the electric circuit. The meter then shows this flow of electrons (i.e. the number of ionizing events) in counts per minute (cpm).

The only prerequisite for the detection of radiation with a survey meter is that the radiation must have sufficient energy to penetrate the walls of the detector tube and create ionizations in the gas. Particulate (alpha and beta) radiations have a limited range in solid materials so, radiation detectors designed for these radiations must be constructed of thin walls that allow the radiation to penetrate. The most common types of portable radiation survey meters used in research labs are the thin window Geiger-Mueller (GM), Low energy Gamma (LEG), and Ion Chamber survey meters.

Geiger-Mueller (GM) Survey Meters GM survey meters are radiation detection devices used to detect radiation or to monitor for radioactive contamination. GM detectors usually have "window" either at the end or on the side of the detector to allow alpha or beta particles to enter the detector. These detectors may have a variety of window thicknesses but, if the radiation cannot penetrate the "window" it cannot be detected. Depending upon the "window" thickness, GM systems can detect x-ray, gamma, alpha, and/or beta radiation. Common radioactive materials that emit these types of radiation (e.g. 22Na, 32P, 35S, 51Cr, 137Cs) can usually be detected using GM survey meters.

Low Energy GAMMA (LEG) or Scintillation Survey Meters LEG survey meters are radiation detection systems used to monitor radionuclides that emit low energy gamma radiation (e.g., 51Cr, 125I). They can not detect alpha particles nor low energy beta particles but they can detect radionuclides that emit high energy gamma (22Na) and/or high energy beta (32P) radiation. The meter is usually read in counts per minute.

Ion Chamber Survey Meters Ion chamber survey meters are radiation detection devices designed to collect all of the ion pairs produced in the detector tube and then measure the current flow. These meters are primarily used to measure X- and gamma-ray exposure in air and the readings are usually expressed as milliroentgen per hour (mR/hr) or roentgen per hour. Ion chambers are most often used for measuring high levels of X- or gamma radiation exposure and are not often used in research labs.

6.2.4 Stable or Unstable Nuclei