MadSci Network: Medicine

Re: What are the downsides of radioisotopes and gamma radiography?

Date: Fri Dec 19 01:26:40 2008
Posted By: Michael Kay, President and Consultant AMBRY, Inc., and
Area of science: Medicine
ID: 1229135849.Me


I think the reason that you didn't find any downsides for the processes you mentioned--Gamma Radiography and Gamma Sterilization--using radioisotopes is they are very good at producing quality radiographs where no other method is practical (for example, examining welds on a pipeline in the Alaska wilderness) and sterilizing heat sensitive materials.

What is missing are the requirements for the safe handling of radioactive materials. Google "Interaction of Radiation with Matter" and "Biological Effects of Ionizing Radiation" and you will have hundreds of thousands of results. Wikipedia and the Health Physics Society sites give good easily understandable information on this topic. Any book in you school library on Nuclear Chemistry or Nuclear Science will also have this information. The goal of radiation protection, or Health Physics, is to minimize any of these effects over the lifetime of the worker.

There are two classes to consider in your question; these are sealed sources of radioactive material (such as those used in Gamma Radiography and Gamma Sterilization) and unsealed sources of radioactive materials (described further down).

Sealed sources of radioactive material can be very hazardous if a person is exposed to them outside their integral shields. When they are inside their shields, they are not a hazard to people. The sources used for gamma sterilization are so radioactive that they are kept in a well, usually under water, in a special room with thick walls--sometimes more than one meter thick-- designed to absorb the radiation before it can reach people in the area. The materials to be sterilized move on conveyor belts through specially designed entrances to the room so that there is never an unshielded place where the radiation from the source can get out (radiation travels in a straight line and cannot go around curves). Imagine the conveyor belt goes through one wall, makes a right turn in front of a second wall as thick as the first, goes a distance then makes a "U"-turn around the second wall, and now there is a short thick wall on the right about 3 meters long. After the conveyor belt has passed that last wall, it is now in the gamma sterilization chamber. The source, or sources, are arranged so that the conveyor spends enough time in a very high radiation field to gamma sterilize the materials in the boxes on the conveyor belt-- typically disposable medical equipment is sterilized this way. Since this is all mechanical, it will require servicing--personal will have to enter this chamber. The doors are electrically interlocked so that they cannot be opened from the outside while the source is raised. There are standard procedures for entering the sterilizer when the source is (supposedly) in its underwater shield. Radiation readings from equipment inside the cell and from calibrated instruments carried by those entering must indicate safe levels of radiation before entering the sterilization room. Personnel Overexposures only happen when these procedures are not followed, and there is a failure of an in-cell radiation detector.

It is a bit different with gamma radiography. These sources are not anywhere near the strength of those in a gamma sterilizer, but they are much more likely to cause personnel overexposures. These sources are in stored position when they are inside their shield. The source is attached to a strong braided metal wire. In use, a guide tube is attached to the front of the shield, and the source is pushed down the guide tube by un- reeling the wire from a spool. The source is placed opposite the area to be radiographed, and left there for the time necessary to produce a good image given the thickness and material of the pipe or article to be radiographed. While the source is out of its shield, all personnel must be kept out of the area. When the time is up, the radiographer rewinds the source back into its shield. Radiation measurements are taken to make certain the source is safely stowed. However, every year there are several incidents where the source was not properly secured in its shield due to human or mechanical errors and personnel overexposures occur.

Both methodologies are safe when the standard operating procedures are followed each and every time. Time--as short as practical in the area of a radioactive source; Distance--the intensity from a source drops as the square of the distance from the source (a 1/r-squared dependance); and Shielding--radiation absorbing materials between you and the source are the three keys to safety from sealed sources.

When you ask "What are the downsides of radioisotopes?", other uses of radioisotopes need to be considered. Since we have looked at high level sealed sources, we can group all other sealed-sources of lesser radioactive strength in that group and apply the same rules to them. They are safe for personal operating equipment as long as they follow the procedures.

That leaves the category of unsealed radioactive materials. This covers the use of radiopharmaceuticals in medical situations, and other unsealed radioactive materials in research and analytical laboratories. When you look at the Biological Effects of Ionizing Radiation, you will find that radioisotopes give off alpha, beta, and gamma rays. Alpha particles can be stopped by a piece of paper; beta particles can be stopped by aluminum a centimeter thick; gamma rays require lead shielding.

In a laboratory, open containers with radioactive materials are usually handled in chemical hoods for ventilation, containment, and control. However the risks are different. Although alpha particles can be stopped by a piece of paper, they are very dangerous when the radioactive material is in the body. That is the main risk when working with alpha emitting radioactive materials. The alpha particles can't get through the walls of any container, cannot travel more that a few centimeters in air, but can get into the body from contaminated gloves, hands, or escaped aerosol particles. Hoods with measured air flow and good hygiene practices make working with alpha emitters quite safe. If greater containment is needed, glove boxes (sealed "boxes" having windoes, air locks to bring materials into the box, and long gloves sealed to ports into the box for the experimenter's hands will help to avoid internal contamination. Small quantities of radioactive materials are often handled with syringes. The one thing gloves will not protect against is an accidental needle-stick through the glove from careless handling of a syringe.

Beta emitters are less harmful than alpha emitters internally, but are still harmful internally and externally in large doses. Here, time, distance, and shielding are all used to reduce dose to the person to "As Low As Reasonably Achievable" (ALARA the acronym for radiation protection).

Gamma ray emitters in small doses are less dangerous internally because often the gamma ray will exit the body without causing ionization. However from outside the body in very large doses they can cause radiation burns, radiation sickness, and even death. Since gamma rays are higher-energy X-rays, both X- and gamma rays can be considered together-- I will just call them gamma rays. Gamma rays are much harder to stop that alphas or betas because they go much farther before their ability to cause ionization lessens. Lead is the most common material for shielding from gamma rays because it is dense and less expensive than the denser metallic elements such as irridium, tungsten, or osmium. When working with gamma emitting radionuclides, distance (even using tongs or other holders to move a test tube a few centimeters from your hand will greatly reduce the dose to your hand) and shielding (shielded test tube or flask holders) along with practicing procedures with non-radioactive materials until they can be performed in a quick, orderly manner will reduce the time of exposure. The last precaution for working with any radioactive material is to use the least amount possible for the experiment or analysis at hand.

One very positive aspect of working with radionuclides is their ease of detection. Alpha particles are the hardest to detect because their range is so short and their penetrating power so low. Alpha detectors must have a very thin window into the detection chamber--usually very thin mylar. But once in the chamber, they are easily detected electronically. Beta particles are detected by the familiar Geiger (or Geiger-Mueller or G-M) Counter. Gamma rays can be detected by a G-M counter, but with much decreased efficiency. Solid state crystals, such as high-purity germanium, have much better efficiency for detecting gamma rays, and the older Thallium activated Sodium Iodide crystal detectors are very efficient for detecting gamma rays. Alpha and Beta particles can also be detected by adding them to a special solution of material that gives off light when an alpha or beta particle "hits" the scintillator molecule. The screw-capped vial is put into a liquid scintillation counter. The vial goes down an elevator to a counting chamber with light-sensitive phototubes around the chamber. As the vial descended, shutters to block light from coming down the elevator tube close off the tube behind the vial. The flashes of light (scintillations) from the interactions are detected by the photomultiplier tubes and then electronically counted. Many biologically importand elements have beta emitting radioisotopes (hydrogen, carbon, sulfur, phosphorus, and calcium to name some), and liquid scintillation counting is used to determine the results of experiments, or analyses of body chemicals such as enzymes.

I hope this picture of how radioactive materials are used with the accompanying safe handling requirements will help fill out the whole picture for you.

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