Gamma rays, also known as gamma photons, are a shorter-wavelength version of visible light. Visible light, or photons that our eyes can detect, has a wavelength of 380 to 700 nanometers (a nanometer is one-billionth of a meter). Gamma photons have a much shorter wavelength: 0.3 to 0.003 nanometers. We cannot see them, and have to detect them using special equipment. The shorter the wavelength, the higher the energy. Both gamma and visible photons are massless packets of electromagnetic radiation, but gammas carry energies upwards of a million times those of visible photons.
Most of the visible photons bouncing around here on earth come from the sun, and began life as gamma photons, the result of the annihilations of the positrons and electrons—annihilations that were themselves the product of the weak nuclear force interactions that fuel the sun. After being produced from the electron-positron annihilation, those high energy gamma photons banged around inside the sun, lost a lot of their energy, and finally emerged as visible photons before travelling their eight-minute, 150-million-kilometer journey through space to the earth. The photosynthesis that made the collard greens I had for dinner yesterday is driven by light (visible photons) that used to be gamma photons.
Gamma photons carry so much energy that they can knock electrons out of atoms, or bump electrons into higher energy states, or even cause subsequent annihilations (this is called pair production).
But contrary to what a lot of people think, gamma rays are not irredeemably dangerous. They are ubiquitous; turn on a Geiger counter and unless you are underwater or in a lead-lined concrete room you will probably receive a reading. We are exposed to them all the time: they are part of the natural background radiation that comes from rocks and minerals in the ground, building materials made of those rocks and minerals, and outer space. Since humans discovered in the 1930s ways to make artificial radiation sources, some small amounts of anthropogenic gamma radiation have been added to the natural background; most of that was fallout from nuclear weapons tests. Click here for a good web resource on background radiation sources.
I commented the other day about the importance of the electric grid in our history. My focus in that article was on the emancipating effect the grid had on women, but the effect was really on everybody. The grid literally made our modern world. It is interesting that the expansion of the grid coincided with the introduction of practically everything else that makes up modern life, including mass personal vehicle transportation, air travel, and nuclear medicine. And it was not actually coincidental. The transportation economy depends on liquid fuel. Manufacturing, transporting, distributing, and retailing that liquid fuel is impossible without electricity.
It is obvious that modern life is pretty much impossible to imagine without the mass personal vehicle transportation and air travel. Nuclear medicine though is much less visible. But that does not mean it is not significant. Canada had a crisis a few years ago because the reactor that makes a particular medical isotope, molybdenum-99, had to sit idle while the federal regulator bickered with the licensee. It was not just a Canadian crisis, it was an American crisis, because American hospitals administer millions of procedures every year involving Mo-99 and almost all of the Mo-99 used in the U.S. comes from that Canadian reactor.
Molybdenum-99 transforms into technetium-99, which is injected into patients, where it emits a gamma ray that allows physicians to diagnose diseases. Because their wavelength is orders of magnitude shorter than even x-rays, gamma rays provide orders-of-magnitude higher resolution when it comes to medical diagnosis. It is interesting that while Tc-99 is hugely useful, there are other gammas that provide even better resolution. These come from positron-emitting isotopes like carbon-11, which decays into boron-11, one atomic number lower in the periodic table (Mo-99, Tc-99’s “parent,” is a negatron emitting isotope, i.e. the decay results in an element one atomic number greater). Positrons collide with electrons, annihilating both particles and turning them into gamma rays travelling in opposite directions. This is what happens in a PET (positron emission tomography) scan, and is also exactly the same annihilation that creates sunlight.
Gamma rays from other man-made materials, such as cesium-137 and cobalt-60, are used all over the world and particularly in western countries to protect the blood supply. When blood or blood products such as platelets are transfused in infants or immuno-deficient patients, they must be sterilized in order to protect the patients from a lethal transfusion-related disease. The only way to properly sterilize blood and make it safe for vulnerable patients is to treat it with gamma radiation.
Cesium-137 and cobalt-60 are made in nuclear reactors. Cesium-137 is a fission product, formed when a uranium atom splits into lighter fragments after being struck by a neutron. Cobalt-60 is made when a neutron is absorbed in cobalt-59.
Canada is the world leader in Co-60 production. Much of Canada’s Co-60 is made in CANDU reactors, including some of the CANDUs in Ontario that at eight a.m. this morning were generating nearly half the province’s power (see Tables 1 and 2 in the left-hand sidebar).
We in the modern world cannot live without electricity. Nor could we live without radioactive isotopes: they are fundamental to the technological processes on which we depend. And the world as we know it would not exist without gamma radiation.