Rapid Screening for Radiation
In a major radiation emergency, medical workers would need to quickly screen thousands of people to determine if they have inhaled or ingested significant amounts of radioactive material. Such screening would indicate if treatment is needed or, more likely, help allay peoples’ fears that they have received a damaging dose of radiation.
However, existing screening methods are too slow and costly to be used in such an emergency. A person’s internal radiation dose is often estimated by measuring the amounts of radioactive uranium, plutonium, and americium in that person’s urine, which traditionally takes from 8 hours to 5 days and costs about $1,000.
A simple method developed at Los Alamos by Dom Peterson and co-workers could yield results in as little as 5 minutes for about $10. For this reason, the Centers for Disease Control and Prevention (the CDC)—a key responder in a radiation emergency—is seriously interested in the technique.
The radioactivity of a urine specimen is often determined by measuring the rate(s) at which the radioactive isotopes (existing as ions in the urine specimen) emit alpha particles. Many radioactive isotopes are alpha emitters, including those that could be used to make dirty bombs or that could be widely dispersed in other sorts of radiation emergencies—such as an accident at a nuclear power plant or the explosion of an improvised nuclear device or stolen nuclear weapon. Measuring alphaparticle emission rates normally takes only a few hours, but preparing the specimen for that measurement takes time.
First, a chemical is added to the specimen to precipitate all the ions present—including those of the alpha emitters. Then the precipitate is dissolved in acid. The resulting solution is then divided into several portions, and each portion is put through a separate “ion-exchange” column, each packed with small resin beads that absorb alpha-emitting ions on their surfaces. (Ion-exchange columns are also used to soften water by selectively absorbing the ions that harden it.)
The ions collected in a column are flushed from it in solution and deposited on a surface that is then dried and placed in an instrument that counts the emission rate for the particular ion absorbed in that column. Different solutions are used on different columns to flush out specific ions. To assay for different alpha-emitters, the flushing/ counting procedure is repeated for each column.
Radioactive ions are first trapped by small resin molecules deposited on a thin planchette and then assayed by alpha spectroscopy.
The new Los Alamos technique is much simpler. First, a “planchette” (French for “little plank”)—a thin stainless-steel disk about an inch and a half in diameter, coated on one side with a thin mixture of ion-exchange resin and plastic—is stirred in a solution of urine and nitric acid or sodium nitrate. During stirring, some of the radioactive ions in solution form chemical bonds with resin molecules deposited in the planchette’s thin film. After a rinse in deionized water and air drying, the planchette is placed in an alphaspectrometer, which measures the energy distribution of the alpha particles emitted by the radioactive ions in the thin film.
The energy distribution for a particular alpha-emitter has a narrow peak at a unique energy. The position of the peak identifies the alpha-emitter, and the height of the peak measures the amount of the alphaemitter present. Thus, alpha spectroscopy of a planchette allows one to quickly and simultaneously assay all of the alphaemitters present in a specimen. Moreover, alpha spectroscopy is relatively inexpensive to do, so hundreds of spectrometers could be operated in parallel to analyze the thousands of planchettes resulting from a radiation emergency.
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