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March 20, 2013 at 1:21 PMComments: 0 Faves: 0

When Atoms Fall Apart

By Lydia from SLN More Blogs by This Author

Ever since the splitting of the atom in the 1940s, we have been fascinated and horrified by radioactive materials. They have had good uses, such as noninvasive medical scanning and controversial ones, such as generating electricity. We are still nervous about negative side effects like mutations and cancer, and with good reason. The tiny, dense center of an atom stores large amounts of energy, so care has to be taken when trying to harness it.

Here is an explanation of the structure of atoms  I wrote previously. Radioactive properties come from the nucleus, specifically an unstable one. As the nucleus grows larger, it becomes less stable. This occurs because the number of protons increases. Since protons have positive charges, they repel each other.

Neutrons can dilute the repulsion, while the strong nuclear force can overcome that repulsion to force protons into proximity. According to Sam Kean, the strong force is like the T.Rex because it is strong but has short arms. That is, the strength of the interaction drops off fast with increasing distance, so that as a nucleus swells, the strong force is less able to hold particles together, and so the nucleus splits in a process called fission or emits particles in some kind of decay. This explains why all atomic nuclei above atomic number (number of protons) 92, uranium, are radioactive. A nucleus that is simply too big often sheds mass by ejecting something called an alpha particle. The alpha particle is a helium nucleus, two protons and two neutrons with a charge of positive 2. This happens when uranium-238 (238U) decays to thorium-234 (234Th).

Helium atom

Nuclei can also be unstable if the ratio of neutrons to protons is too high or too low. Normally, there is roughly a 1:1 ratio of the particles in the nucleus. This grows to about 1.5:1 with larger nuclei to dilute the increasing repulsive force between protons. If there is too great a number of neutrons, a neutron is converted into an electron, proton, and a nearly-impossible-to-detect particle called an antineutrino. This process is called beta decay. The nucleus is transformed into the next one up on the table because a proton is made. For example, cobalt-60 (60Co) undergoes beta decay to become nickel-60 (60Ni).
If there are too many protons in the nucleus, an electron close to the nucleus will be pulled in to neutralize the charge. This decreases the atomic number (number of protons) by one, turning the atom into one of the element before it. For example sodium-22 (22Na) captures an electron to become the more-stable neon-22 (22Ne). A proton-rich nucleus might also decay by positron emission. In this process, a proton is converted to a neutron by release of a neutrino and a particle called a positron. A positron has the same mass as an electron but the opposite charge. This happens when carbon-11 (11C) decays to boron-11 (11B).  Electron capture occurs more frequently because it uses less energy.

Another variety of radioactivity doesn’t alter the identity of the atom. An atom might be at a higher-energy, more unstable state called a “metastable state,” denoted by an “m” next to the mass number or star next to the element symbol. For example, 60Ni is an excited state, and so it would be expressed as either 60mNi or 60Ni*. If given a push over the energy barrier, the nickel nucleus will release the excess energy and settle down at a lower state. The energy emitted is a gamma ray, the most energetic possible form of light.

There are only so many atoms of radioactive material. Eventually every atom will decay to more stable product(s). This rate of depletion is of interest to many, chiefly for reasons of health and safety. The rate is quantified in the idea of the half-life. A half life is simply the time it takes for half the atoms in a radioactive sample to decay. The range of half-lives is stunning. Many of the first 92 elements are stable, having essentially infinite half-lives. Some of the ultraheavy artificial elements, like roentgenium and ununitrium, hang out for a few seconds or even fractions of seconds or milliseconds. Polonium-214 lasts for a scant 164 microseconds (millionths of a second)!

The half-life is a cause of significant concern. After a nuclear accident, how long will it take for a region to become habitable? Will a medical procedure give me cancer down the line? The half-lives of many elements have been investigated, especially those used for medical purposes, and doctors and scientists have worked to figure out optimal exposure. Isotopes used for medical purposes have short half-lives, from two (15O) to 110 (18F) minutes, so they are eliminated in weeks. Other isotopes, like plutonium-239 have half-lives of thousands, millions, or even billions of years, so they don’t cause large-scale contamination. The intermediate-length half-lives, like 5.67 years for 60Co, cause the greatest concern because the contamination will last for decades, possibly even centuries, affecting several generations.

Chernobyl

Radiation can be scary, but that does not mean it can’t be properly used. An application in the medical field is positron emission tomography, or PET. A radioactive tracer, such as 18F (Fluorine-18), is attached to a sugar molecule, and the solution is injected into a person. The tracer emits positrons, which a detector records and transforms into images. A brighter image means the body uses more glucose in that area; this is especially useful for recording brain activity.

Smoke detectors make use of americium-241 (241Am), with a half-life of 432 years.  A small amount of americium dioxide, AmO2, containing 0.29 millionths of a gram of americium is placed in a special chamber in the detector. The americium decays by alpha and gamma emission, and the alpha particles collide with oxygen and nitrogen molecules, giving them a charge. The battery sends a charge out, herding the nitrogen and oxygen ions into a line to keep the current going between the electrodes. When smoke particles enter the detector, they latch onto the ions, neutralizing them and interrupting the flow of current, which in turn sets off the alarm.

smoke alarm

As with everything that we ingest, dosage determines toxicity with radiation. There are limits as to what we can absorb without the high possibility of corporal damage. There is background radiation wherever we go, and that does not cause us problems. Some radioactive sources, like smoke detectors, are so well-shielded that they contribute less radiation than the background to our intake.

Should you ever be around radioactive sources, it’s a good idea to know how to protect yourself from them. Alpha particles are tanks in comparison to beta and gamma emissions, traveling at five percent of the speed of light initially. Because of this, they have a range of a few centimeters in air and can be blocked by a piece of paper or our skin. Beta particles travel at about six-tenths the speed of light. This, coupled with their small size, means they penetrate farther than alpha particles and require more shielding, like a sheet of aluminum one centimeter thick or several centimeters of plastic. Gamma emissions are very energetic light particles, and so are very penetrating; they need lots of dense material, like lead or concrete, to stop them. You are unlikely, however, to be exposed to large sources of gamma radiation, because scientists are well aware of its danger.

I find it interesting that something as small to us as an atom can, on atomic-size scales, still be big and unstable. We can predict how an atom will decay and figure out how best to use it or deal with it.  Although the atom was first split fairly recently, we have figured out many uses for fission. The atom is small but has great and potentially lethal power.

References:

"Cobalt-60." HyperPhysics. N.p., 08 Apr 1999. Web. 7 Mar 2013.

The Encyclopedia of Science. N.p., n.d. Web. 7 Mar 2013.

"Smoke Detectors and Americium." World Nuclear Association, n.d. Web. 7 Mar 2013.

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