Irradiation and Semantics
Most high school projects are dim memories, but I will never forget an assignment given by Mrs. Page, my Semantics teacher. She required us to watch television at least two hours per day for a week, paying close attention to the commercials. We then had to write a paper about the ways that the advertisers chose their words and pictures to sell a product. The lesson I learned was that any marketer worth a paycheck knows how to paint a desirable picture of a product while leaving a negative impression of a competitor.
I thought about that long ago assignment as I prepared to write this column. One might think that a country threatened with biological agents that have contaminated important office buildings, disrupted billions of individual mail deliveries and killed half a dozen people would be able to engage in a factual investigation of alternative solutions to the problem.
However, there are some people that immediately see a market in any problem. Where there is a market, there are marketers seeking economic advantage. Though that is not a bad thing, the rest of us need to understand that so we can attempt to cut through the slanted words to discover the truth.
Several proven ways exist to kill bacteria; many of which we practice on a regular basis. Heat, chemical agents, and even radiation – in the form of sunlight – have all been used to kill germs ever since humans realized they existed. Unfortunately, most common germ fighting methods can damage the articles that we want to clean.
Scalding water or steam can kill all bacteria; most of us choose to keep the skin on our hands and accept a lesser degree of cleanliness. Anti-bacterial chemicals also do a fine job; however, many of us prefer a slightly germ ridden couch over one that is bleached to the point of letting the stuffing out.
One effective way to kill bacteria on food, medical instruments, and possibly mail packages and office surfaces is to use ionizing radiation. It is a process that has been under development since the 1940s; there is a wealth of information about the amount of radiation needed to eliminate numerous types of bacteria and about the effect that the energy will have on many common materials and structures.
Over the years, a tiny industry has developed, mostly to sterilize medical instruments and certain spices. Within the irradiation industry, there are two camps that split based on the type of radiation that they recommend (translation: there are two different kinds of radiation systems for sale, each has its backers.)
Once faction likes to produce their ionizing radiation in particle accelerators. These machines use electricity and magnetism to speed up electrons. Sometimes these machines are called cyclotrons. The rapidly moving electrons are physically identical to beta particles that are released by radioactive isotopes except they do not come from atomic nuclei.
Accelerated electrons can deposit enough energy on the surface of most materials to kill all bacteria without damaging the base material. Depending on the specific energy level achieved, they can penetrate some objects up to a depth of 3 inches. They cannot penetrate atomic nuclei and do not cause the material to become radioactive.
The artificial radiation crowd can also aim their accelerated electrons onto high atomic number material – like tungsten – to cause a reaction that releases energetic photons called x-rays. These rays are physically identical to gamma rays but they do not come from atomic nuclei. They are called x-rays when artificially produced. When the accelerator is turned off, x-rays stop immediately. In the most efficient x-ray machines, only about 7% of the input power ends up in the energy beam, the rest is wasted as heat.
The other camp in the radiation sterilization industry uses radioactive isotopes like Cobalt 60 or Cesium 137. These materials have a natural need to emit gamma rays. No matter what anyone does or does not do, these materials emit energetic rays on a schedule that cannot be altered. It is such a predictable process that it can be used as a reference for extremely accurate clocks.
Some isotopes are produced in nuclear reactors; others are produced by aiming accelerated protons onto targets. Proton accelerators can be several miles in circumference and often cost a billion dollars or more. The materials produced by proton interactions are not the same as those produced in a nuclear reactor.
People can control the emissions from isotopes by the simple expedient of surrounding the sources with radiation sponges – also known as shields – that soak up most of the energetic emissions before they can cause harm. Gamma rays, like accelerated electrons, cannot make other material radioactive.
The difference between high-energy electrons and gamma rays is substantial; electrons interact with all types of matter and rapidly lose their energy. Gamma rays penetrate all materials and are merely attenuated as they pass through.
When the items to be sterilized can be presented to the radiation source as a moving, thin stream, high-energy electrons have significant advantages. Their low penetration capability is a benefit; all of their energy is released in a concentrated area. The achievable dose rate is very high, leading to short exposure times and high production rates.
When the items needing sterilization are thick, irregularly shaped, packaged, or pre-assembled onto transportation pallets, gamma radiation seems to be the winner. It will penetrate deeply into most materials; all it needs is sufficient time for a deadly dose to accumulate in the hardest to reach spots. High overall sterilization rates are possible in a batch-processing mode.
Gamma radiation is often preferred for medical instruments because it can sterilize a packaged instrument that can be guaranteed to remain sterile as long as the package is intact. With electron beam irradiation, there is a slight risk of recontamination in the period between radiation application and packaging.
Beyond the efficacy of the radiation itself, the two different means of producing ionizing radiation have some additional differences in their characteristics.
Accelerators are complex electro-mechanical devices, industrial models can require hundreds of kilowatts of electrical power, and they can require the services of specialized technicians to maintain them. On the plus side, they do not require any shielding when the electric power supply is removed and they can be focused into tight beams of concentrated power.
Radioactive isotopes do not require any power supply; they give away their energetic rays in all directions without any external stimulation. The isotopes must be shielded in all unwanted directions. Since isotopes continuously decay, the strength of the field that they produce also steadily drops, requiring a replacement scheme or increased exposure times for the same dose. For Cobalt 60, approximately 12% of the material would need to be replaced each year to keep the doses constant.
When the discussion gets beyond the above facts (though I did throw in some slanted phrases to see if anyone was paying attention) the marketers and financial folks take over from the technical people. Companies that have spent a lot of time and money refining particle accelerators like the fact that they are complex devices; they are depending on their patents to prevent competition that might spoil a potentially lucrative market. Business school texts frequently advise aspiring company leaders to “erect barriers to entry” and stock analysts give companies high praise if they manage to position themselves in a market that others cannot easily penetrate.
It has to concern accelerator investors when they realize that very small quantities of easily produced material can often out perform their complex machines. It might take a million curies of Co-60 to stock a gamma irradiator that can treat a few million tons of material a year. A million curies is a big number until you find out that there can be approximately 400 curies per gram in freshly produced cobalt; a million curies is only about 2.5 kilograms of material.
With today’s limited market, all the Co-60 the world needs can be produced in a single nuclear reactor; however, if there was a larger demand it could be met with the simple expedient of using a few more reactors. It would not be technically difficult to modify even large electricity production reactors to produce vast quantities as a side business. Today’s supply – demand equation does not support such a modification.
It must be even more troubling to the accelerator salesmen to know that vast quantities of Cesium 137 already exist and more is being produced every day. They must be banking on the fact that it will always be considered an expensive waste material from nuclear reactor operation and that no one would even think of extracting it to use as a low cost source of sterilizing radiation. They must be hoping that no one ever mentions the fact that extracting the cesium not only results in valuable material, but it also makes disposing of the rest of the reactor by-products a bit simpler.
Maybe that is why the accelerator company marketing departments devote so much ink to emphasizing the fact that their machines are not radioactive and do not rely on nuclear reactors for their base materials. As I read some of their literature, I began to understand how people with less nuclear education and experience might get the idea that isotopes and nuclear reactors are a BAD thing. Of course, depending on where an accelerator operates, it might depend heavily on nuclear reactor produced electricity.
The marketing propaganda from accelerator companies discusses the difficulty involved in transporting and tracking isotopes, and often emphasizes the enormous regulatory burden placed on companies that use millions of curies for their normal production process. The propaganda does not mention that the difficulty is man made; if regulations were based solely on health and science concerns it would be far simpler to shield and move a million curies – 2.5 kilograms — of radioactive cobalt that to move a 500 kW particle accelerator.
Accelerator salesmen even try to make an issue of the fact that about 80% of the Co-60 in commerce comes from a single foreign source. They fail to mention that the source is a respected, reliable Canadian corporation and they never acknowledge the fact that all nuclear reactors are capable isotope factories that could produce irradiation sources if there was a sufficient market.
Isotope irradiators can improve food safety, reduce spoilage, sterilize large packages, and possibly even decontaminate historically significant office buildings. Building decontamination is a challenge that might be met by putting a source with a remotely controlled shield in the middle of a room. After everyone has been moved a safe distance away, the source could be exposed to send out radiation in all desired directions. After the most remote point has received sufficient exposure, the shield would be closed and the source moved to the next room. Similar devices already exist for industrial non-destructive testing of pipes and for treating certain types of cancers.
Accelerators can eliminate pests from rapidly moving streams of grain and sterilize letters and packages moving on a conveyor system as long as they are arranged in such a way as to be less than about 3 inches thick. Because of their size, power requirements, narrowly focused beam and complexity, they probably cannot perform large area decontamination.
Both types of irradiation devices have a role to play in potentially important and lucrative markets; the rest of us must critically evaluate the information provided to ensure we can separate the facts from the marketing pitch.
In case you did not realize it while you were reading, I am biased. I like the idea that there might be some valuable uses for the waste products of nuclear reactors and that there are some products that can be produced in the reactor while it is operating to produce electricity, heat or motive force. It is kind of neat to think of ways to begin using the old sausage maker’s dictum of “using everything but the moo,” and letting nothing go to waste.
I especially like the fact that isotopes could be used to sterilize stuff in places where electricity is rare, expensive and unreliable. Millions of people each year get sick and die from food and water born germs, most of them in those less developed places. It would be really something to use isotope irradiators to tackle that problem!