The Electromagnetic Spectrum X - The Gamma

Crossposted at Dailykos.com

Well, this will conclude the series. I appreciate all of the comments, and have tried to adapt this series to respond to them, including more links and pictures. Some of them are interesting, and some just help make the point. Unfortunately I was unable to embed any this time for reasons beyond my control.

Gamma EMR is from 10 picometers on down to the mathematical limit of zero (obviously never attaining the limit). Because of the extremely high energies associated with them, gamma rays are difficult to contain.

Gamma is generated by our old friend, blackbody radiation due to thermal excitation in stars and in supernovae, but the temperature requirements are extreme. On earth, gamma is produced by nuclear transitions. Just as visible, UV, and X-rays can be produced by electronic transitions, gamma is produced by transitions involving transitions in nuclear, rather than electronic, quantum states. Because the energetics in nuclear transitions are so much greater than electronic ones, gamma radiation is extremely short in wavelength.

Not only short in wavelength, but hard to confine. Gamma interacts with matter in three main ways: photoelectric effects, Compton scattering, and pair production.

The photoelectric effect arises when a gamma photon encounters an atom and the energy of the interaction causes an electron to be ejected from the atom. The kinetic energy of the electron is related to the energy of the gamma photon, and the speedy electron interacts with other atoms until it is slowed down or absorbed. The energies are so great that such a photoelectric electron can do serious damage to tissue as it it attenuated, causing further ionization and destruction of genetic information, degradation of proteins, and other mischief. This seems to be a significant mechanism of attenuation for lower energy gamma photons.

Compton scattering is similar to the photoelectric effect, except not all of the energy of the gamma photon is absorbed, and both an energetic electron and a lower energy gamma photon are the result. The electron can do what was described just above, the that lower energy gamma photon can repeat the cycle at lower energies. After a cascade, the photoelectric effect essentially disposes of the remaining gamma photon. This is thought to be the mechanism of destruction of higher energy gamma photons, and obviously can cause more damage than the photoelectric effect because the cycle can repeat several times, each causing ionization.

Finally, there is pair production, my personal favorite. Gamma photons with high enough energy (over 1.02 million electron volts (MeV)) sometimes interact with the electric field a a nucleus and are converted, as if my magick, into mass: an electron and a positron, in other words, matter and antimatter. Any energy over the 1.02 MeV is converted to kinetic energy, and the particles are accelerated away from each other. Since both electrons and positrons have a rest mass of 0.51 MeV, we have a physical verification of Einstein’s mass/energy equivalence. Energy does indeed equal mass times the square of the velocity of light.

Now, our pair produced electron is just a fast one, like our photoelectric ones, and does the same damage if its kinetic energy is great enough. But our positron is quite different, because it is destroyed as soon as its encounters an electron. When that happens, two gamma photons, propagating in opposite directions, are formed, each with an energy of 0.51 MeV, plus whatever kinetic energy was incorporated in the positron.

Gamma is useful for several things, but has to be controlled. Thick lead vessels are used for research and industrial quantities of gamma sources that require portability, while thick concrete is used to shield nuclear reactors for power generation, which are not portable. It takes a lot of mass to attenuate gamma, so that is why the shielding is so extensive.

Unlike UV or X-rays, gamma sources cannot be “turned off”. If the power is cut to an X-ray tube, for example, it immediately stops producing the X-rays. Not so with a gamma source, because the energy to produce the gamma already exists in the nuclei of the atoms making up the source, and, short of stopping time, there is no mechanism to prevent those nuclear transitions.

One of the most common gamma sources is cobalt-60. It is make from the naturally-occuring, nonradioactive cobalt-59 by thermal neutron bombardment, either from an elemental neutron source or in a nuclear reactor. It is convenient because of the relatively low cost and energetic gamma photons (over 1 MeV each, and it emits two). This isotope has largely replaced radium as a gamma source due to lower cost and greater ease of handling.

Uses of gamma are many, but boil down to imaging and gaging applications and cell destruction applications, with a catch all of “everything else”, like laboratory radiation experiments.

The uses for imaging and gaging include such applications like passing the gamma photons through materials to detect defects, such as voids in metal castings. While it is difficult to focus gamma, a detector can be passed back and forth across the surface of a casting while the casting is being irradiated from the other side, and a map of intensity can be developed. Any region that is unexpectedly transparent to the gamma photons is apt to be a flaw in the form of an air void. This is particularly useful for very large objects, like ship propellers. As a thickness gage, for instance in precision metal rolling, a gamma source can be placed on one edge of a conveyor and a detector on the other. The amount of gamma photons passing through to the detector is a function of the thickness of the material on the conveyor.

But it is killing cells that gamma does best. Gamma photons just simply wreck DNA by ionizing it, and also wreck other cell components such as proteins and lipids the same way. The damage is proportional to the exposure, and this is key to uses. One use is sterilization of medical devices, which can be done by gamma irradiation without significant damage to metallic objects. Another use is irradiation of foodstuffs to kill bacteria and prolong shelf life. This is controversial, and I will elaborate in a bit. The other main use is in cancer treatment.

As I mentioned about X-rays, cancer cells are more sensitive to ionizing radiation than most normal cells because rapid division allows damage to be passed onto successive generations of cells before repair mechanisms can fix them, so fatal flaws are more likely to be passed on in rapidly dividing cells. One of the best examples is the “gamma knife” wherein multiple beams of gamma photons are sent through the body, converging at the point of the tumor. Each single beam is relatively low in intensity, and so does relatively little injury, but where the multiple beams converge produce a much larger flux of gamma photons, “zapping” the tumor with large amounts of radiation. By the way, when you hear of someone having radium therapy, usually it is cobalt-60 therapy, but radium is still used in certain procedures.

Whilst on the subject of medical uses, I would mention that PET (positron emission tomography) scans use the word “positron”, the actual positrons are not detected, since they never leave the body. In this procedure, the subject ingests or in injected with a radioisotope that decays by positron emission, and the resulting mutual annihilation of the emitted positrons with electrons in the body produce gamma photons, which are detected. The mean free path of a positron in the condensed phase of matter is much shorter than the size of the human body, but the gamma photons resulting from the positron/electron interactions easily do. This is essentially the opposite of pair production, vide supra.

Food preservation takes advantage of the cell destroying properties of gamma photons. It is used for the “cold pasteurization” of meat to kill pathogens. It can also be used to increase the shelf life of root crops by destroying enough DNA to prevent sprouting in, for example, potatoes. This is controversial, because there certainly are degradation products present in the foods after such high exposures. There are also anecdotal reports that the taste of the food is adversely affected, but taste is so subjective that I hold no opinion on that. I will say that my personal opinion, shaped by my scientific background, is that it is NOT a good idea to irradiate foods as a matter of course. There may be certain, specific situations where the benefits outweigh the potential hazards, but rather than wholesale irradiation I would advocate on case-by-case hazards analyses and benefit-to-risk studies.

Gamma detectors are now used at ports and trucking terminals to check for clandestine shipments of nuclear materials. Since gamma photons are so penetrating, detectors can be passed around the outside of a cargo container to detect them. While it is not perfect, it is one tool for detecting of materials that we really should keep out, but the hype is much greater than the actuality, since very few actual containers and trucks are really scanned.

To wrap it up, we have just launched a gamma telescope to observe stellar and galactic phenomena. This exciting development will certainly allow us to make new discoveries as to the nature of the universe.

Thus ends the long series on the electromagnetic spectrum. I am open to suggestions for the next topic, and am leaning to a discussion of the phases of matter. Any ideas are welcome! Warmest regards, Doc.

Tags: Compton scattering, Electromagnetic spectrum, Food preservation, Gamma, Medical imaging, Pair production, Photoelectric effect, Teaching