Rp001 introduction to ionizing radiation

Editor's Notes

• #17: When atoms throw off energy in an attempt to re-balance themselves that process is called decay. Half-life is the time it takes for half of the nuclei in a substance to undergo radioactive decay. Decay is a random process. Background radiation fluctuates up and down all day long. Important to take background readings of the entire port. Radionuclides found in nature typically have long half-lives; medical radionuclides typically have short half-lives. Uranium (natural) – long half-life of 4.5 billion years Medical nuclides – short half-lives – Carbon 11 has a half-life of 20 minutes - that’s why for some medical radionuclides it literally has to be made down the hall from the person receiving it, otherwise the material would be rendered useless. 6X half-life for a substance to be decayed down to 1%
• #25: If you were to inadvertantly wander through the tunnel of the VACIS or an x-ray machine while it was scanning, you would be irradiated. However, once the source of x-rays is turned off - you would no longer be irradiated. However, if you were to get a radioactive substance (powder, liquid, etc.) “on” you - you would be contaminated. In other words, that source of contamination “goes with you” no matter where you (and can contaminate others as well) until you are decontaminated.
• #27: Alpha ( ) particles are helium nuclei (4He). They consist of 2 protons + 2 neutrons, forming a very stable atom. -particles are quite heavy particles (atomic mass = 4), i.e. 4 times the mass of the hydrogen nucleus. The electrical charge is + 2. Usually -particles are emitted from heavy atoms, with high energy of several MeV (Mega = million electronvolt). If -particles penetrate matter they cause heavy ionization, loose their energy fast and are stopped by few centimeters of air or millimeters of solid material (e.g. one sheet of paper). In biological tissue they create a lot of damage by killing many cells in a small volume.
• #29: Beta () radiation consists of electrons emitted from the nucleus. Normally electrons are negatively charged. If they are positively charged they are called “positrons”. The mass of the electron is much (~1/8000th) smaller than that of alphas, therefore ionization is much weaker, penetration through matter much deeper. Betas penetrate meters of air and centimeters of solid material. They are stopped by some cm of plastic (e.g. plexiglass), or few mm of glass (goggles). Note: Never handle any Beta sources without eye protection - goggles!!
• #36: Gamma () radiation is a kind of electromagnetic radiation, similar to light or radiowaves, only with much higher energy. It has no mass and no charge. Gamma “particles” are called “photons”. Ionization of matter by -radiation is very weak, penetration through matter very deep and dependent on the energy. The higher the energy, the deeper the penetration through matter. For low energetic -radiation (up to ~ 0.5 MeV) high-Z materials (lead) provide better shielding. For higher energies only the product of density and thickness counts. High energy -radiation can penetrate kilometers of air, meters of water or concrete. Typical shielding materials are several 10 cm of lead or up to 1 m of concrete. -radiation completely penetrates the body, causing a deep dose. X-rays have physically the same nature as gamma rays, i.e. they are electromagnetic radiation. However, while -rays are emitted from the nucleus of the atom, X-rays are generated in the shell, by electrons moving from a higher to a lower energy level (“characteristic X-rays”). X-rays can also emerge when charged particles are slowed down, called “Bremsstrahlung”
• #38: Neutron radiation exists in nature only as part of cosmic radiation impinging on the earth from outer space. Neutrons are mostly produced in nuclear reactors and accelerators. Only a few man-made “Trans-uranium” elements (e.g. Pu, Americium, Californium) undergo “spontaneous fission” and emit neutrons. Neutrons have no charge and mass 1 (same as protons). They give up their energy by bouncing on other nuclei, like billiard balls. If a neutron hits a proton, all the energy from the neutron may be transferred to the proton, which will then transfer it to the surrounding matter (“indirect ionization”). Hydrogen (consisting of protons), e.g. in form of water or plastics, is therefore the most efficient shielding material for neutrons.
• #42: Shielding alpha and beta particles Due to the fact that alpha and beta particles deposit so much energy over such a short distance they are very easy to shield. Alpha particles require little or no shielding as they travel only a very short distance in air. For beta-emitting radioisotopes, low atomic number material such as plastic is recommended. Plastic will adequately shield the betas and keep bremsstrahlung production to a minimum. The thickness of the shield will depend on the range of the beta in the chosen material. A 3/8" plexiglass shield is recommended when working with P-32.
• #43: Due to their weak penetration -radiation is not dangerous in view of external exposure, - particles are stopped in the outmost layer of the skin. However, if -emitting radionuclides are incorporated in the body, e.g. inhaled in the lung, they can be very dangerous and destroy the inner organs. Shielding of -radiation is very easy, a plastic wrapper or piece of paper is sufficient to completely stop all -particles. They will not penetrate clothing or goggles. Note: always carefully avoid incorporation of  - emitting radionuclides !!
• #44: Beta radiation penetrates the skin up to a few cm of tissue. It can heavily damage (burn) the skin and is particularly dangerous for the eye. Note: never touch beta sources with your fingers, always use tweezers or tongs and wear goggles to protect your eyes !!
• #45: The intensity of -radiation decreases with the square of the distance from the source (“1/R2 law”). So if you retreat from the source from 10 cm to 1 m the intensity and your exposure will decrease by a factor of 100 ! NOTE: never take a gamma source in your fingers, use at least tongs or tweezers of some length ! The first few centimeters make a lot of difference !
• #46: Neutrons penetrate through the body like gammas, leading to a deep dose. It takes several meters of concrete or metals to stop neutrons, but only some 10 cm of water, paraffin or plastic (hydrogen). Best shielding for fast neutrons is first to slow them down in a moderator containing hydrogen, e.g. water of paraffine, then to absorb them by some particular elements, such as boron or cadmium.