15 April 2011
We have generally been conditioned that radiation is a very bad threat to all life on earth; however, we live under a constant bombardment of 搉atural?radiation from the sun, space, and the earth around us. Obviously, as life has existed for millions of years on earth, we have adjusted to the background radiation and it isn抰 a guaranteed death sentence to be exposed to it. (See: http://www.phyast.pitt.edu/~blc/book/chapter5.html)
To start, we need to review the basic atom (I抦 not going into quarks or any other 搉ewer?theories in this discussion). The nucleus of the atom is composed of protons (positive charge of 1) and neutrons (no charge). Electrons (negative charge of 1) surround the nucleus, equal to the number of protons in the atom (atoms normally keep their charge balanced). Electrons are much lighter than protons and neutrons, so almost all of the atom抯 mass is in the nucleus, but the electrons occupy a volume about 10,000 times larger than the nucleus. This allows some things (like a gamma ray or a neutron) to pass through an atom抯 space but not have any interaction with the atom unless it collides directly with an electron or the nucleus. This is important when discussing shielding because gamma rays/neutrons can penetrate further into matter before they may collide with another particle, making them more dangerous. Charged particles like Alpha and Beta particles will generally interact with the electron shells around atoms they are trying to travel through, making them less able to penetrate deeply into matter. Any form of radiation, if it has high enough energy, will be able to penetrate through most any matter, but these cases are unusual in normal radioactive decay.
The simplest atom is the element hydrogen with one proton and one electron, symbol H1. 揌eavy?hydrogen, also called deuterium, has one neutron in the nucleus with the one proton, symbol H2. Tritium, another hydrogen isotope with one proton and two neutrons, (H3) eventually decays into Helium 3, He3. The number of protons determines what type of element the atom is, and the number of neutrons determine the isotope (Carbon 12 and Carbon 14 are different isotopes of the element carbon, with 6 and 8 neutrons, respectively). Neutrons help stabilize the nucleus of atoms; in the lighter elements, the ratio is typically one neutron to one proton. As the elements get larger, the ratio of neutrons to protons increases until it reaches of maximum of around 1.5 neutrons for each proton. For example, Uranium 235 has 92 protons and 143 neutrons; Uranium 238 has 92 protons/146 neutrons. Even at this ratio, the nucleus is not stable and therefore subject to radioactive decay. (See: http://en.wikipedia.org/wiki/Radioactive_decay)
After the element Lead in the Periodic Table, no form of any heavier element is stable and all of its isotopes are radioactive (although some decay so slowly they appear to be stable). Radioactivity is the mechanism by which an element changes itself into a lower energy form by emitting certain types of radiation. (See: http://en.wikipedia.org/wiki/Decay_chain)
This can take many steps until it decays into a stable element like Lead or another lighter element. As the time it takes for an individual radioactive atom to decay can抰 be predicted, but the length of time for half of a sample of radioactive material to decay can be, the term 揾alf-life?is used. Half-life is the length of time for half of a sample of radioactive element to decay. The half-life time does not change, no matter what the size sample. For a half-life of one hour, half of the radioactive atoms in a sample will decay over the 60 minutes, half of the remaining atoms over the hour after that, and so on. Half-lives can range from fractions of a second to billions of years and greater. (See: http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html)
The most common types of radiation given off consist of alpha rays or particles, beta rays or particles, gamma rays (high electromagnetic energy only, no mass, equivalent to X-rays), and neutrons. Elements decaying by alpha or beta decay can also give off gamma rays at the same time, increasing their radiation threat. All of these types of radiation can have different levels of energy, typically, the more energy per particle/ray, the more damage it can cause on a living organism.
Alpha decay (releasing alpha particles) is how a heavy element (like Uranium) becomes lighter, eventually through many steps becoming a non-radioactive element like Lead. (See: http://en.wikipedia.org/wiki/Alpha_particle)
An alpha particle is the nucleus of a Helium atom, 2 protons and 2 neutrons. It will quickly steal 2 electrons from its surroundings and become a harmless atom of Helium. Thus, out in the environment, it cannot travel far (less than a couple of inches), is easily stopped (one sheet of paper, clothing, etc.), and does not exist for any length of time. However, alpha emitters ingested or inhaled inside a living organism can cause massive damage. Think of a cannon firing a cannonball. The cannonball (alpha particle) smashes through things in its pathway, and the heavy cannon (radioactive parent atom) recoiling can 揷rush?things behind it. Inside the body, the cells and their components are receiving the damage ?cellular bonds broken, molecules damaged or parts knocked off, electrons being stripped or knocked free, leaving charged ions behind, etc. The results include the body trying to repair the damage (with complete or partial success); the cell can die, or sometimes go wild and become cancer.
Beta decay covers several mechanisms, usually covering the release or capture of an electron (or its anti-matter opposite, the positron). (See: http://en.wikipedia.org/wiki/Beta_particle)
Neutrons can emit an electron and become a proton in the right circumstances and a proton can combine with an electron and become a neutron in others. Both are types of beta decay pathways. Carbon 12 (6 protons/6 neutrons) is the common stable form of carbon. Carbon 14 (6 protons/8 neutrons) has too many neutrons and is radioactive. One neutron can decay into a proton and release an electron, changing the atom into stable Nitrogen 14 (7protons/7 neutrons, the common stable form of nitrogen). If an atom has too many protons, it can (if it has enough energy) emit a positron (a positively charged particle of anti-matter the size of an electron), and convert a proton into a neutron. The positron will eventually run into an electron and they will annihilate each other, turning into two gamma rays. Another lower energy mechanism is for a proton to capture a nearby electron and convert into a neutron that way.
Outside the body, beta particles are not a great risk. A sheet of thick plastic or aluminum foil can stop them, they can only travel about a meter in the environment, and they don抰 remain free for very long. The anti-matter positrons are even shorter lived and travel less distance (but the gamma rays released are a higher threat). Beta emitters inhaled or ingested inside the body are the human threat. Inside the body, beta particles are like a high-speed bullet to the alpha particles cannonball. They still can cause the same types of damage, but they are smaller and lighter and faster. An example might be that instead of a big hole blasted in the outer wall with an alpha particle, you抎 get a smaller hole drilled through several walls with a beta particle.
Gamma rays are high-energy rays given off either alone or in conjunction with other forms of radioactive decay. (See: http://en.wikipedia.org/wiki/Gamma_radiation)
Currently, gamma rays and X-rays are considered the same type of EM energy, the difference is how they are produced: X-rays are from electron shell interactions and gamma rays from actions in the nucleus being the distinction. They can travel great distances and only cease to exist after interaction with other matter. This is where the amount of shielding (earth, concrete walls, lead, etc.) comes into play. It is the amount of matter between you and the gamma ray that counts ?lead is not special in its ability to absorb gamma radiation, it is just denser (more matter per length) than most other materials. Tungsten and some of its alloys are much better than lead shielding because they are denser than lead; however they are much more expensive than lead, too, so they are not practical as a replacement. So gamma rays are an external as well as internal human threat. However, since they are energy and not particles, they don抰 have the ability to do the same level of damage as an internal alpha or beta particle, but they can penetrate further than the other two types.
This brings us to neutrons. (See: http://en.wikipedia.org/wiki/Neutron_radiation and http://www.safetyoffice.uwaterloo.ca/hse/radiation/rad_laboratory/interaction/neutron_interactions.htm)
Although not the common way for an element to adjust its proton/neutron ratio (energetically, spitting out an alpha particle beats eliminating either a single proton or neutron in most cases), it does happen. When a heavy element splits by fission or light atoms combine by fusion, free neutrons are usually released also (the lighter fission products need a lower proton/neutron ratio than the parent, so excess neutrons can be released). As neutrons have no charge, they don抰 interact much with the electron cloud around other atom抯 nuclei, so they can travel much farther than alpha or beta particles. This ability to penetrate makes them a threat external to the body, also. But they will only last about 10-15 minutes in the environment, decaying into a proton and electron (becoming a hydrogen atom), unless absorbed by another atom抯 nucleus first. Because they can penetrate deeply in tissues, they can cause damage first by colliding with cells (smaller and less damaging than an alpha particle, but much bigger and much more damaging than a beta particle). Or they can be absorbed by an atom抯 nucleus, and possibly convert it into a radioactive isotope, starting off another decay pathway, changing the atom into another element, and causing the internal radiation damage described above, depending on how the new isotope decays. This makes neutron radiation more of a threat than the other particles, since you don抰 have to inhale or ingest it for it to cause internal damage. Also, since some of the elements have a toxicity associated with them (heavy metals, anyone?), as the original atom decays into other elements in the body, they may cause problems from poisoning as well as the radioactivity damage. Effective shielding against neutrons uses items with lots of light nuclei to collide with or absorb the neutrons, like hydrogen atoms. Such things as water tanks, concrete (contains water), and certain plastics/waxes (lots of hydrogen atoms) have been used.
Reactor vs. Nuclear Explosion:
In an atomic nuclear explosion (whether the weapon itself or the trigger for a fusion weapon), you are trying to split all of the heavy Uranium or Plutonium before the force of the explosion spreads the critical mass apart, so it is never a 100% complete fission reaction. Thus you will have the leftover heavy element(s) and the wide array of fission products. The fissionable Uranium/Plutonium (and any other fissionable element) can split into two parts in many different ways, although they tend to cluster around one lighter range and one heavier range of products. (See: http://en.wikipedia.org/wiki/Radioactive_waste
Of course, these vary widely in how fast they decay into stable isotopes. Many decay quickly in hours or days; eventually Cesium 137 and Strontium 90 dominate because they have longer half-lives than most other radioactive fission isotopes, but they are still radioactive enough to be dangerous and they can be biologically utilized by the body and be an increased threat.(See: http://en.wikipedia.org/wiki/Fission_products )
Cesium is chemically similar to Potassium and Strontium is chemically similar to Calcium, and the body can use them in similar fashion if they are ingested, which is why they can be retained for long periods of time. Some fission products are quickly eliminated because they aren抰 absorbed/used in biological pathways. This makes them less of a threat even if they are very radioactive, since they won抰 stay in you for long.
A reactor fuel rod release would be different for several reasons. Because you won抰 have the nuclear explosion, the spread of radioactive fallout would be less. You won抰 have the amount of free neutrons that could be absorbed into the nuclei of surrounding material and making it radioactive. The fuel rod Uranium is in its oxide form (already combined with oxygen, and thus it can抰 burn), so it抯 already a heavy ash which isn抰 as easy to spread and would tend to settle out of the air quickly. The controlled rate of fission in a reactor will also create different ratios and types of fission products than an explosion as well as different isotopes of long-lived 揾eavy elements (Plutonium 238, Americium 241, etc) that don抰 typically form from an explosion. Since the rods are used for long periods of time, much of the short half-lived radioactive isotopes have decayed and won抰 be present in any significant quantity. So there will be potentially more radioactive material with a long half life from reactor core material that won抰 have the same rapidly falling radiation level as from an explosion, but hopefully less total radioactive material and less wide-spread than from a nuclear explosion.
Radioactive elements for medical use pose another source of potential exposure. (See: http://en.wikipedia.org/wiki/Nuclear_medicine)
However, the quantities available for a 揹irty?weapon?should be smaller, and they are generally very short half life isotopes (days at most). Two sites (one Canadian, one European) produce most of the materials, which are then distributed to the various medical facilities. On the other hand, security around the hospitals, etc. is probably no where near as high as a nuclear plant.
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