Part 7

You seal each sample in a tiny polyethylene bag about ½ inch square. One sample is taken from the victim’s clothing and the second, about the same size as the first, taken from the damaged area of the automobile. In preparing these samples, you handle all the materials with clean forceps because you realize that the most minute dirt from your fingers will be detected in the analysis.

The two bags are irradiated together for 1 hour in a nearby reactor and 2 hours later you begin counting the samples with a high-resolution, lithium-drifted-germanium, gamma-ray spectrometer. This will give you a match (or mismatch) for elements that yield radioisotopes of fairly short half-life such as manganese (2.56 hours), copper (12.8 hours), sodium (15 hours), arsenic (27.7 hours), etc. You plan on “counting” the samples again later on, if the first counts match, so that you can check on radioisotopes with longer half-lives such as iron (45 days), chromium (27 days), silver (270 days), cobalt (5 years), etc.

The two gamma-ray spectra you obtain look like those in the figure on the opposite page. The gamma rays from the irradiated paint taken from the victim’s clothing indicate the presence of the common elements sodium, potassium, and copper, but gold, lanthanum, and europium are also conspicuously present. The gamma rays from the other sample also reveal sodium, potassium, and gold but in rather different proportions. More striking is the absence of copper and the two rare earths, and the presence of manganese and arsenic, which were not indicated in the first sample.

The paint samples definitely do not match. Therefore, you inform the detective that his suspect is innocent after all. You’ve solved your problem, but he still has his. Perhaps the same technique will provide positive proof when he finds the real culprit.

Element Channel number .122 MeV Eu (Europium) 35 .328 MeV La (Lanthanum) 95 .344 MeV Eu 96 .412 MeV Au (Gold) 110 .486 MeV La 120 .511 MeV Cu (Copper) 145 .815 MeV La 240 .837 MeV Eu 245 .961 MeV Eu 290 1.37 MeV Na (Sodium) 410 1.53 MeV K (Potassium) 455 1.60 MeV La 475

(channel numbers estimated)

Element Channel number 0.412 MeV Au (Gold) 110 0.511 MeV Na 145 0.559 MeV As (Arsenic) 160 0.657 MeV As 195 0.847 MeV Mn (Manganese) 250 1.21 MeV As 370 1.37 MeV Na (Sodium) 410 1.53 MeV K (Potassium) 455

(channel numbers estimated)

Gamma-ray spectra of two samples of paint. These two spectra are obviously different and, therefore, could not have come from the same source.

SUMMING UP: WHAT LIES AHEAD

These five situations are intended to show why neutron activation analysis is used, when it can be applied, and how it works.

In the real world, there are often many reasons why this kind of analysis is used. As in the situations described here, it may be the only workable method. Sometimes there may be a choice of methods, but activation analysis is used because it has certain peculiar advantages or because it happens to be the most convenient. There are other times, however, when other analytical methods can and should be used. Such situations arise when the element sought is not easily activated, or when a satisfactory alternative method exists that is more economical or more convenient. The points to remember about the use of activation analysis are that:

1. In many cases, no elaborate sample preparation procedure is required.

2. For many elements, it is the most sensitive analytical technique known.

The diversity of applications in which activation analysis is used is enormous and will probably continue to be. The examples given here represent only a tiny fraction of circumstances in which the method has been used. Consider that it has been used successfully:

1. In the microscopic world of biology and medicine;

2. For meteorites arriving from the vast reaches of space;

3. In the production lines of consumer products;

4. For precious samples of moon rocks;

5. In the most “down-to-earth” business of hunting for new mineral sources;

6. For exploring the causes of Napoleon’s death nearly 150 years earlier (see photograph on next page). Today, there is virtually no field of science and technology that is untouched by this method.

The illustrations of procedures used in the situations described in this booklet are typical of some in use today. There are many other situations that require still other techniques. One of the most exciting, which will be used with increasing frequency in the future, involves the use of computers. It has been shown that data collected by high-resolution gamma-ray spectrometers can be “fed” directly to a computer. The computer can be programmed to identify unknown components and to determine the concentrations of elements of interest to the analyst. It is entirely possible to include corrections for radioactive decay, possible interferences from other elements present, and many other factors. It appears quite likely that the kinds of analyses described here (as well as others) may someday be accomplished automatically, with far smaller chances for error and probably more economically.

Samples of Napoleon’s hair. Neutron activation analysis of these hairs revealed that he had been poisoned with arsenic. (He died, however, not from arsenic poisoning, but from acute mercury intoxication.)

Other newer techniques that may find increased usage in the future are exemplified by the method for activation analysis of the whole human body. The use of neutrons produced by nuclear machines (such as cyclotrons or other particle accelerators) or produced by compact, portable isotopic sources will make neutron activation analysis even more versatile. Isotopic sources produce neutrons as the result of a nuclear reaction. One such reaction uses alpha particles emitted by polonium-210 (or some other alpha emitter) to bombard the element beryllium. A different kind of isotopic source is the man-made radioisotope californium-252 that decays by fissioning (splitting) spontaneously and produces neutrons in the process. (One milligram of californium-252 will spontaneously produce over 10⁹ neutrons per second.) While californium-252 is quite expensive at present, it is likely that production costs will be significantly reduced in the future.

With computers, more convenient radiation sources, and continuing improvements in the technology of gamma-ray detectors and nuclear electronics, neutron activation analysis will become more and more a routine tool of the analyst.

APPENDIX

Calculation of arsenic concentration with no standard for comparison.

1. Determination of arsenic-76 activity produced from 1 microgram of arsenic at the time it comes out of the reactor.