Part 3

Radioactivity ratios for 50 “gold” coins. Above are the silver to gold ratios. There are two groups of genuine coins. Five known forgeries show considerably higher ratios than the genuine coins. Two of the suspect coins also show high ratios but the third, suspect A, shows a ratio that falls into one of the genuine groups. Below are the copper to gold ratios. Again there are two groups of genuine coins. (The same coins make up the two groups here as above.) The five known forgeries again show higher ratios than the genuine ones and again the same two suspects appear to be forgeries. Suspect A, however, shows a ratio similar to one group of the genuine specimens. One therefore concludes that suspect A is genuine and that B and C are not.

For example, suppose for _sample 1_ there are 20,000 counts in the 0.412-MeV peak (gold), 190 counts in the 0.511-MeV peak (copper), and 450 counts in the 0.654-MeV peak (silver). Suppose also that _standard 1_ yielded 10,000, 500, and 400 counts for these three peaks (corrected for decay), respectively. Then the ratio for gold would be (20,000/10,000) = 2.00, the ratio for copper would be (190/500) = 0.380, and the ratio for silver would be (450/400) = 1.13.

Finally, the activity ratio of copper to gold would be (0.380/2.00) = 0.190, and the activity ratio of silver to gold would be (1.13/2.00) = 0.565.

Because each sample was irradiated with an identical standard, and counted in an identical arrangement, the last two ratios will be the same for different samples if, and only if, the concentrations of gold, silver, and copper in those samples are in identical proportions. This will be true no matter where in the reactor or for how long the irradiation took place.

Now the scientist presents the data to you. You immediately see that (a) the good coins fall into two groups, one with a silver to gold activity ratio of approximately 0.56 and a copper to gold ratio of approximately 0.20 and a second group with these ratios approximately 0.51 and 0.18; (b) the coins you were certain were forgeries have distinctly higher ratios ranging from 0.60 to 0.65 for silver to gold and from 0.23 to 0.30 for copper to gold; and (c) of the three suspected coins, two have ratios that fall into the range of the known forgeries, but one, with ratios of 0.552 and 0.198, is probably genuine.

You present the result to the museum director in the form of a graph (see the figure on page 41) and a few weeks later, 43 coins are added to the permanent exhibits of the museum, while 7 are discarded.

In a Criminology Laboratory

_The Problem_

You are a scientist working in the criminology laboratory of a large metropolitan city. A detective brings you a minute sample of paint taken from the clothing of a hit-and-run victim. He has a suspect whose automobile paint seems to match that sample. The suspect was found in his parked automobile, not far from the scene of the accident. He seems to fit the description given by two witnesses, and he is extremely nervous. You scrape a small sample of paint from a recently damaged area of the suspect’s car, and, (with the aid of a microscope) find that the pigment content seems to be the same as that taken from the victim’s clothing. But, are they really from the same paint?

_The Solution_

You know that paint, like almost everything else, contains very small quantities of impurities that are present only by accident and do not affect its properties as a useful material. The trace impurities, as they are called, will vary from batch to batch of the same paint. Very rarely will a match be obtained in both type and concentration of trace impurities in two samples if they are not from the same batch.

By measuring a sufficient number of different elements, the probability of accidentally matching two samples can be as rare as the duplication of fingerprints in two individuals. Matching of trace impurities is often called a “fingerprint” method.

With neutron activation analysis, you can obtain the “fingerprints” of the two samples to see if they match. Although this kind of evidence may be difficult to use as proof in court, a positive match will let the detective know that he is on the right track. Also, the suspect might confess if he is confronted with the evidence and realizes that he is “caught”. On the other hand, a mismatch will clear the suspect completely and the detective will know to look elsewhere for the criminal.

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.

We use the equation from page 12:

A₀ = Nφσ (1 - e^{-λt})

where N is the number of target atoms. (One microgram of arsenic contains (10^{-6} gram/75 grams per mole[12]) × 6.02 × 10²³ atoms per mole which is 8 × 10¹⁵ atoms of arsenic.)

φ is the neutron flux. (This would be known to the reactor operator. It is usually measured by inserting materials of known composition and measuring their activation. In this case, φ = 10¹³ neutrons per square centimeter per second.)

σ is the activation cross section. (Neutron cross sections have been measured and tabulated by scientists. For the activation of arsenic-75 to arsenic-76, the cross section is known to be 4.2 × 10^{-24} square centimeter.)

λ is the disintegration constant for arsenic-76. (Here, λ = (ln 2[13]/t_{½},(in hours); t_{½}, the half-life for arsenic-76, is 26.6 hours so λ = (0.693/26.6) = 0.026.)

t is the time of the irradiation. (Here t is 12 hours.)

Therefore: A₀, the activity of arsenic-76,

= 8 × 10¹⁵ × 10¹³ × 4.2 × 10^{-24} × (1 - e^{-0.026 × 12})

(Note: e is a physical constant, 2.71+)

= 9 × 10⁴ disintegrations per second per microgram

2. Determination of activity of arsenic measured in the sample and corrected back to the time of removal from the reactor.

We use the equation:

A₁ = (R)/(E × F) e^{λt}

where R is the measured count rate. (In this case, R is the number of counts per second observed in the 0.559-MeV gamma-ray peak, which is 5300 counts in 20 minutes or 4.4 counts per second.)

E is the efficiency of the detector. (In this case, it is the number of counts observed in the 0.559 peak for each 0.559-MeV gamma ray emitted by a radioactive material at the sample distance. This is known for the detector being used by making other measurements and, for the set-up used here, is 0.010.)

F is the average number of 0.559-MeV gamma rays emitted in each disintegration of arsenic-76. (This can be deduced from the decay scheme of arsenic-76. See the decay scheme for manganese-56 on page 13. In the decay of arsenic-76 the number of 0.559-MeV gamma rays emitted per disintegration is approximately 0.41.)

λ is the disintegration constant for arsenic-76. (0.026, see page 49.)

t is the decay time. (This is the number of hours from the time the sample was removed from the reactor to the time it was counted, or 5 hours.)

Therefore, A₁, the activity of arsenic-76 produced in the sample at the time of removal from the reactor,

= (4.4 counts per second)/(0.010 × 0.41) e^{0.026 × 5 hours}

= 1200 disintegrations per second

3. Calculation of arsenic concentration in the sample.

We use the equation:

Concentration in parts per million = (A₁)/(A₀ × W) 10⁶

where A₁ and A₀ were determined above and W is the weight of sample analyzed or 300 micrograms (0.0003 gram).

Therefore the concentration is

(1200)/(9 × 10⁴ × 300) × 10⁶ = 44 parts per million.

FOOTNOTES

[1]There are exceptions. For a few elements there are no stable nuclei. In some cases, there are other differences that make certain atoms radioactive.

[2]These gamma rays (called prompt gamma rays because they are instantaneously produced when the neutron is captured) can also be used for analysis and sometimes are, but we will not be discussing this type of analysis in this booklet.

[3]Sensitivity in this case means how small an amount of an unknown element can be detected.

[4]Nuclide is a general term applicable to all atomic forms of elements. Whereas isotopes are the various forms of a single element (hence are a family of nuclides) and all have the same atomic number and number of protons, nuclides comprise all the isotopic forms of all the elements.

[5]The half-life of a radioactive nuclide is the time it takes for half the nuclei in a large sample to undergo decay. Note that after half of them are gone, a second half-life period will reduce the _remainder_ by one half, leaving one quarter of the original number.

[6]The disintegration constant is related to the half-life, T_{½}, by the expression: λ = natural logarithm of 2/half-life, or

λ = (ln 2)/(T_{½}) = (0.693)/(T_{½})

[7]The detector efficiency is the ratio of the number of gamma rays detected to the number emitted by the sample.

[8]A deuteron is the nucleus of a heavy hydrogen (deuterium) atom and consists of one neutron and one proton.

[9]Not all nuclear reactors are appropriate for this work. For example, reactors designed for electric power production do not have the means built into them for inserting and removing small samples for a “short” period of irradiation.

[10]A scintillation detector is a crystalline device, usually sodium iodide containing a small amount of thallium, which has the property of emitting light when energy is absorbed from nuclear radiation.

[11]After a 10-minute irradiation and a 3-minute delay before counting, corrected for decay to a common time.

[12]One mole is the atomic weight of an atom or molecule expressed in grams, or the weight of 6.02 × 10²³ atoms or molecules per mole.

[13]ln 2 is the natural logarithm of 2.

READING LIST

General Information About Nuclear Science

_Secrets of the Nucleus_, Joseph S. Levinger, McGraw-Hill Book Company, New York, 1967, 127 pp., $0.50.

_Working With Atoms_, Otto R. Frisch, Basic Books, Inc., Publishers, New York, 1965, 96 pp., $3.50.

_The Atom and Its Nucleus_, George Gamow, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1961, 153 pp., $1.95.

_Inside the Nucleus_, Irving Adler, The John Day Company, Inc., New York, 1963, 192 pp., $4.95.

_Radioisotopes and Radiation_, John H. Lawrence, Bernard Manowitz, and Benjamin S. Loeb, Dover Publications, Inc., New York, 1964, 131 pp., $2.50.

_Sourcebook on Atomic Energy_ (third edition), Samuel Glasstone, Van Nostrand Reinhold Company, New York, 1967, 883 pp., $15.00.

The Semiconductor Revolution in Nuclear Radiation Counting, J. M. Hollander and I. Perlman, _Science_, 154: 84 (October 7, 1966).

About Activation Analysis

Popular Level

Neutron Activation Analysis, Vincent P. Guinn, _International Science and Technology, Prototype Issue,_ 74 (1961).

Distribution of Arsenic in Napoleon’s Hair, Hamilton Smith, Sten Forshufvud, and Anders Wassen, _Nature_, 194: 725 (May 26, 1962).

Nuclear Activation Analysis, Richard E. Wainerdi and Norman P. DuBeau, _Science_, 139: 1027 (March 15, 1963).

Neutron Activation Analysis, W. H. Wahl and H. H. Kramer, _Scientific American_, 68: 210 (April 1967).

Technical Level

_Activation Analysis Handbook_, Robert C. Koch, Academic Press, Inc., New York, 1960, 219 pp., $8.00.

Neutron Activation Experiments in Radiochemistry, K. S. Vorres, _Journal of Chemical Education_, 37: 391 (August 1960).

Radioactivation Analysis, H. J. M. Bowen and E. Gibbons, Oxford University Press, London, England, 1963, 295 pp., $8.00.

_Neutron Irradiation and Activation Analysis_, Denis Taylor, Van Nostrand Reinhold Company, New York, 1964, 185 pp., $8.95.

_Guide to Activation Analysis_, William A. Lyon (Ed.), Van Nostrand Reinhold Company, New York, 1964, 186 pp., $5.95.

_Advances in Activation Analysis_, Volume 1, J. M. A. Lenihan and S. J. Thomson (Eds.), Academic Press, Inc., New York, 1969, 233 pp., $9.50.

_Activation Analysis; Principles and Applications_, J. M. A. Lenihan and S. J. Thomson (Eds.), Academic Press, Inc., New York, 1965, 211 pp., $8.50.

_Modern Trends in Activation Analysis_, Volumes 1 and 2, J. R. DeVoe and P. D. LaFleur (Eds.), National Bureau of Standards Special Publication Number 312, U. S. Government Printing Office, Washington, D. C., 1969, 2005 pp., $8.50.

_Pottery Analysis by Neutron Activation_, I. Perlman and F. Assaro, _Archaeometry_, 11: 21 (1969).

Bibliographies

_Activation Analysis: A Bibliography_, G. J. Lutz, R. J. Boreni, R. S. Maddock, and W. W. Meinke (Eds.), National Bureau of Standards Technical Note 467, U. S. Government Printing Office, Washington, D. C., 1969, $8.50.

_Forensic Science: A Bibliography of Activation Analysis Papers_, G. J. Lutz (Ed.), National Bureau of Standards Technical Note 519, U. S. Government Printing Office, Washington, D. C., 1970, $0.50.

_Determination of Light Elements in Metals: A Bibliography of Activation Analysis Papers_, G. J. Lutz (Ed.), National Bureau of Standards Technical Note 524, U. S. Government Printing Office, Washington, D. C., 1970, $0.75.

_Pollution Analysis: A Bibliography of the Literature of Activation Analysis Papers_, G. J. Lutz (Ed.), U. S. Government Printing Office, 1971, $0.45.

_14-MeV Neutron Generators in Activation Analysis: A Bibliography_, G. J. Lutz (Ed.), U. S. Government Printing Office, 1971, $1.00.

_Oceanography: A Bibliography of Selected Activation Analysis Literature_, G. J. Lutz (Ed.), U. S. Government Printing Office, Washington, D. C., 1971, $0.50.

MOTION PICTURES

Available for loan without charge from the USERDA-TIC Film Library, P. O. Box 62, Oak Ridge, TN 37830.

_The Nuclear Witness: Activation Analysis in Crime Investigation_, 28 minutes, color, 1966. This film illustrates the application of activation analysis to the investigation of criminal cases involving murder, burglary, and narcotics peddling.

_Nuclear Fingerprinting of Ancient Pottery_, 20 minutes, color, 1970. Animated sequences are used to explain several of the analytical techniques. Part of the film shows how the research is actually done in the laboratory.

_The Atomic Fingerprint_, 12½ minutes, color, 1964. The principles of neutron activation analysis are explained and the machines used in this work are shown. Some of its applications in crime detection, geology and soil science, analysis of art and archaeological objects, oil refining, agriculture, electronics, biology and medicine, and the space sciences are illustrated.

_Neutron Activation Analysis_, 40 minutes, color, 1964. This film describes the nature, potentialities, and applications of neutron activation analysis. The kinds of neutron sources used and the counting techniques are shown. Examples of applications in crime detection, geology and geochemistry, agriculture, medicine, the petroleum and chemical industries, and the semiconductor industry are shown.

Photo Credits Cover Federal Bureau of Investigation 2 Smithsonian Institution 30 & 31 University Hospital, University of Washington 47 Dr. Sten Forshufvud

The Author

Dr. Bernard Keisch received his B.S. degree from Rensselaer Polytechnic Institute and his Ph.D. from Washington University. He is now a Senior Fellow at the Carnegie-Mellon Institute of Research at Carnegie-Mellon University in Pittsburgh. He is presently engaged in a project that deals with the applications of nuclear technology to art identification. This is sponsored by the National Gallery of Art and in the past has also received support from the U. S. Atomic Energy Commission and the National Science Foundation. Previously he was a nuclear research chemist with the Phillips Petroleum Company and senior scientist at the Nuclear Science and Engineering Corporation. He has contributed articles on art authentication to a number of journals. For ERDA, in addition to this booklet, he has written _The Mysterious Box: Nuclear Science and Art_, _Lost Worlds: Nuclear Science and Archaeology_, and _Secrets of the Past: Nuclear Energy Applications in Art and Archaeology_.

A word about ERDA....