From Bill: BRAWM can you folks give me an estimate of total Bqs we get internally?

I should clarify a couple of points through some exposure basics. First, the damage from radiation primarily comes as a result of producing oxygen free radicals. These are essentially oxygen atoms that have been ionized (electrons removed) and these are very reactive to the surrounding molecules. If many of these radicals are near DNA molecules in a cell, then a chemical reaction can break one or more strands of the molecule or change one of the nucleotides. Now, this occurs naturally all of the time and our body has defense mechanisms to fight oxidative stress and to repair any DNA damage. During this process cell mutation may occur due to a number of reasons that I will not elaborate on here. I am really simplifying the biological process as it is really complex and not fully understood. (It would take months to just read through all of the literature on this). So, ionizing radiation (alpha, beta, x-, gamma-ray) exposure results in localized and acute oxidative stress within a cell resulting from the ionization of oxygen atoms. How does this occur? Here is where external and internal exposure makes a big difference. First, one must understand the decay process. A radioactive atom is, by definition, unstable, and will decay to a new atom by what is called a decay mode. This is normally alpha or beta decay. Below is an image of the natural uranium decay chain which shows the various decays from uranium in the form of alpha and beta-minus. The alpha means the atom expels a fully ionized helium nucleus (2 neutrons/2 protons, no electrons), or alpha particle. The beta-minus means that the atom decays by changing a neutron to a proton and due to conservation of charge, an electron is emitted. This electron is produced within the nucleus and is not one of the atomic electrons surrounding the nucleus. Both the alpha and electron are emitted at relatively high energies (~Mev) and will interact with surrounding material by ionizing atoms, thus losing energy along the way, and slowing down to "thermal" (~0.025eV) energies. This is the primary internal dose mechanism and it is local to position of the radioactive atom when it decays. This is a more potent exposure mechanism because this localization produces more oxidative stress. Also, alpha particles produce a much higher density of ionizations (higher linear energy transfer (LET)) since the track length is much shorter therefore the localized stress is higher still. The biological effects of alpha relative to electron (beta) is about a factor of 20. Gamma radiation may occur as a secondary process in both alpha and beta decays. This occurs when the decay of one nucleus produces the daughter nucleus in an excited state. Below is an image of the Co60 decay scheme that decays by beta-minus resulting almost always in an excited state of Ni60 that will then emit two gamma-rays during the de-excitation process to the ground state. This is kind of like a ball falling down a hill that get's stuck in a rut before it get's to the bottom. The ball will stay there for some time, then fall the rest of the way. Don't push this analogy too far, quantum physics has no physical analog. The "falling" here is instantaneous and the resulting energy loss from moving from a higher state (higher altitude) to a lower state (bottom of the hill) results (almost always) in a gamma-ray of equal energy. A gamma-ray is an electromagnetic (EM) wave form of radiation exactly the same as visible light, UV, radio, or x-rays. Unlike alpha and beta particles, gamma-rays do not ionize as they penetrate material and the higher the energy the more transparent you are to them. This is why higher energies of gamma-rays may be less dose because they essentially go right through you without interacting. The interactions of gamma-rays result in a single ionization producing a single high-energy electron that will, in-turn, go on to ionize atoms in the same fashion as beta-decay. To summarize: 1) Internal dose has much greater biological effect due, in part, to localization. 2) Internal alpha dose has a factor of 20 greater biological effect due to higher density of ionizations. (higher LET) 3) Internal beta and alpha dose goes up as the energy of these particles go up. 4) External gamma-ray exposure goes down as energy goes up because you become more transparent to the incident rays. Hope this helps. Sorry for the long-winded lecture.

<