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  • Nuclear Chemistry

  • Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are called radiation. When unstable nuclei decompose in nature, the process is referred to as natural radioactivity. When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

  • There are three major types of natural radioactivity:

  • Alpha RadiationAlpha radiation consists of a stream of positively charged particles, called alpha particles, which have an atomic mass of 4 and a charge of +2 (a helium nucleus). When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example: 23892U -> 42He + 23490Th The helium nucleus is the alpha particle.

  • Beta RadiationBeta radiation is a stream of electrons, called beta particles. When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit.For example: 23490Th -> 0-1e + 23491Pa The electron is the beta particle.

  • Gamma RadiationGamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm). The emission of gamma radiation results from an energy change within the atomic nucleus. Gamma emission changes neither the atomic number nor the atomic mass. Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

  • Bombardment ReactionsAlpha, beta, and gamma radiation also accompany induced radioactivity. Radioactive isotopes are prepared in the lab using bombardment reactions to convert a stable nucleus into one which is radioactive. Positron (particle with the same mass as an electron, but a charge of +1 instead of -1) emission isn't observed in natural radioactivity, but it is a common mode of decay in induced radioactivity. Bombardment reactions can be used to produce very heavy elements, including many which don't occur in nature.

  • How to shield against radiationIn general, the shorter the EM wavelength, the thicker and denser the shield material must be. Ultraviolet (UV) can be blocked simply by a sufficiently opaque sheet of plastic. We are all familiar with tinted sunglasses that promise to block some 97% of solar UV rays. Not much additional protection is required in space. X-rays and gamma rays are another matter. Where intense x-rays and gamma rays occur, it requires several inches or centimeters of lead and/or concrete to provide adequate shielding.

  • Blocking Alpha particlesAlpha particles are very large particles. As such they don't penetrate very deeply into many things. In fact, alpha particles will not even penetrate the epidermal (dead) layer of skin, and so present no special hazard to humans. A sheet of reasonably thick paper will block all alpha particles.

  • Blocking Beta particlesBeta particles are very small and can penetrate centimeters into the body. But luckily they're too small to cause much damage if they hit anything. But there's a special problem here. When beta particles hit large atoms, the impact causes those atoms to give off x-rays. Metal atoms are usually quite heavy, and so are especially susceptible to this kind of re-radiation which is known by its German name "Bremsstrahlung". In fact, this is how x-rays are produced intentionally for medical applications.

  • The best materials to shield against beta particles have lots of hydrogen atoms in them. Hydrogen atoms are light, and so absorb the particles without giving off x-rays. Plain old water works very well. In fact, 4 inches (10 centimeters) of water will block almost all background beta particles. But water is impractical for shielding in space, so high-density polyethylene (HPDE, chemical formula CH2CH2...) is frequently used instead. This also effectively blocks protons.

  • Blocking Gamma rays

    Gamma rays are a form of electromagnetic radiation, so they have no mass, carry energy and travel the speed of light.Where intense gamma rays occur, it requires several inches or centimeters of lead and/or concrete to provide adequate shielding.

  • Radiation Source Dose (rems)Chest x-ray0.010 Mammogram0.030 Cosmic rays0.031 annually Human body0.039 annually Household radon0.200 annually Cross-country airplane flight0.005

    Are there any legal limits for radiation exposure? Worker CategoryLegal Limit 18-year old male5 rem/year Pregnant woman0.500 rem

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