Neutron-activation analysis offers better prospects

When NAA was developed
      Neutron-activation analysis first became widely available in the 1950s, and the first commercial NAA firm in the United States, General Atomic, began making NAA available in 1960. NAA increased in popularity through the 1960s, and received a big boost when semiconductor Ge(Li) detectors were developed in the mid-1960s. Today it is widely used in many branches of science.

How NAA works
      Neutron activation is a method of determining the elemental composition of a sample. Being a nuclear technique, its results refer only to the number of atoms of various elements present, not their chemical state. Thus, no information about compounds or types of chemical bonding is provided by NAA.
      The name “neutron activation” is ambiguous. It does not mean that neutrons are activated, but rather that neutrons do the activating. The sample to be activated is placed into a concentrated beam of neutrons, which is usually provided by a nuclear reactor, where the fissioning of ²³⁵U to lighter elements produces excess neutrons that move rapidly about until they are absorbed by the nucleus of a nearby atom. The heavier nuclei produced by this neutron-capture process are frequently unstable, and proceed to decay with characteristic half-live and radiation. The radiation is some combination of alpha-particles (nuclei of helium), beta-particles (electrons), and gamma-rays (high-energy photons with no rest mass). The gamma-rays are best for measurement purposes because they have sharply defined energies that are not reduced by passing through matter and can be measured accurately by modern detectors. In contrast, the energies of alpha-rays and beta-rays vary greatly because their electrical charges make them interact more with matter and so lose large and variable amounts of energy.
      After the sample has been irradiated, it is removed from the reactor, repackaged, and taken to a counting room where it is placed near a semiconductor detector. Modern gamma-ray detectors are large single crystals of pure germanium (Ge), sometimes doped with lithium (Li). The latter are referred to as Ge(Li) detectors. Until the late 1960s, gamma rays were counted by scintillation detectors made of sodium iodide doped with thallium [NaI(Tl)]. While more efficient than Ge detectors, NaI(Tl) detectors cannot resolve gamma-ray energies nearly as well as Ge detectors can, and so were largely abandoned when Ge detectors became available. (The FBI used NaI(Tl) detectors, Guinn Ge(Li).)
      The radioactive sample is counted until a suitable gamma-ray spectrum is obtained. A typical gamma-ray spectrum of a complex substance[1] is shown in Figure 2. A gamma-ray spectrum is just a plot of the relative number of gamma counts versus the energy of the gamma rays. A typical spectrum is a series of sharp photopeaks superimposed on a broad, sloping background. Each photopeak represents the decay of a specific radioisotope, although most radioisotopes have more than one photopeak. In this example, strong peaks of iron, scandium, cobalt, zinc, and antimony can be seen. Several other elements are also present, but with smaller photopeaks. A computer program calculates the number of counts above background in each peak, compares it with standard mixtures of elements irradiated and counted similarly, and calculates the masses of each element detected in the sample. Because useful half-lives of elements range from seconds to decades, a full analysis consists of two or more irradiations of lengths ranging from minutes to days, followed by several counts at decay times ranging from minutes to months.

Figure 2. Gamma-ray spectrum of long-lived radionuclides after activating aerosol from a polluted urban area.

      The advantages of neutron activation are its sensitivity, specificity, freedom from common interferences, number of elements analyzable, and ease of preparation of the samples. In many cases, neutron activation is purely instrumental; that is, samples need not be treated chemically before or after irradiation. Depending on the nature of the sample, up to 50 elements can sometimes be determined. Measuring 50 elements requires little extra effort than required to measure 5 elements. Neutron activation is free from effects of sample size that plague other nuclear techniques; because the entering neutrons are uncharged (they pass right through the sample) and the exiting gamma rays are energetic enough not to be attenuated by large samples. The technique is specific because the gamma-ray energies of the photopeaks can be measured so accurately. Finally, neutron activation is advantageous because of its great sensitivities. In some cases, elements can be detected in amounts as low as micrograms to nanograms (millionths to billionths of a gram). As discussed below, there is great confusion in the JFK literature on this point. Low detection limits are not always the same as highly precise analysis: neutron activation may measure a tiny amount of an element, but with an uncertainty of 50% or more. One of the main causes of NAA’s analytical uncertainties is the randomness of radioactive decay. When N counts are measured under a photopeak, the uncertainty (standard deviation) is N^½, or √N. Thus a photopeak containing 100 counts would have to be considered as 100 ± 10 counts, i.e., its uncertainty is 10%. The results of neutron activation are not absolute (free of uncertainty), as many JFK writers seem to think. Typical analytical uncertainties are 1%–10% for the biggest peaks, and up to 50%–100% for small peaks on high backgrounds.

The critics press for NAA of the fragments
      Soon after the Warren Report appeared and the public could sense that the chemical tests of the fragments were inconclusive, critics and forensic scientists alike began to call for better analyses of the fragments that remained in the National Archives. They began to mention NAA in particular. Warren Commission critic Sylvia Meagher raised her voice as early as 1967:[2]

      …Yet there was open to the Commission a scientific and conclusive method for determining whether a metallic fragment recovered from Connally’s wrist had originated in the stretcher bullet. That method was the neutron activation analysis—the same scientific test the Commission utilized in an abortive attempt to reverse the negative result of the paraffin test of Oswald’s face…
      Neutron activation analysis can determine to the millionth of a part the composition of a metal fragment and establish whether or not it is identical with another sample. Such analysis would have eliminated all need for guesswork. But the Commission presented dubious and slanted arguments for insisting, despite contrary expert testimony, that the stretcher bullet had caused all of Connally’s wounds, and declined the opportunity to prove its claim by neutron activation analysis.…
      The physical evidence, including the metal fragments and scrapings, presumably still exists and can still be subjected to neutron activation analysis.

    (Being a staunch critic, Meagher of course expected that such analyses would vindicate her view that the Warren Commission’s lone-gunman scenario was unfounded. She turned out to be very wrong.)
      Unfortunately, this passage is full of basic scientific errors and misrepresentations. But that didn't stop the critical community from accepting it as gospel and using it content, even its phrases, in their later writings. Presumably they accepted Mrs. Meagher's writings on NAA because she was "one of theirs." This is a wonderful example of arguing from false authority.
      Archcritic Dr. Cyril Wecht called repeatedly for new analyses, particularly neutron activation. He wrote in 1972 that:

      It is important to note here that two tests would answer some of the most urgent questions. Although spectrographic analysis was ordered—and presumably done— the results have never been made available. Also, neutron activation analysis—a test that was not performed—would enable us to match fragments of infinitesimal size with a known object. This could be done with the bullet (Exhibit 399) and the fragments still in the Archives. All this is vital information.[3]

      Like so many critics, Wecht overstated the powers of neutron activation. First, it cannot be used on particles of “infinitesimal size.” NAA is good, but not that good. Second, Wecht equated composition and origin, which we have seen is not the case.
      In 1974, Wecht repeated the call, this time linking NAA directly with testing the single-bullet theory:[4]

      If it had been found that the composition of the lead in the fragment recovered from Governor Connally's wrist wound was indistinguishable from the composition of the lead in the nearly whole bullet found at Parkland Hospital, that fact alone would lend strong support to the single bullet theory.

      However, when testifying before the HSCA in September 1978, as the reality of such tests loomed large,[5] Wecht’s stance suddenly stiffened, and he posed apparently impossible conditions for any analysis that would cause him to rethink his opposition to the single-bullet theory:[6]

      Extensive, detailed, complete neutron activation analyses of all the bullets and all the fragments, including those which had not been removed, including the one which was in Governor Connally's left thigh, including the one which I understand, according to sworn affidavits, was removed from the Governor's chest, given to a nurse, who gave it to it to a policeman, who gave it to an FBI agent, if all those things were done, Mr. Cornwell, then I would be prepared to discuss the impact of the NAA findings on the single bullet theory.
      In the absence of all NAA tests, I am not prepared to accept a piecemeal presentation of a few tests that some people in some Government agencies felt could be done and others which would not necessarily have to be performed.

      By calling for analysis of all fragments, explicitly including those that had not been removed from the bodies, Wecht was demanding that President Kennedy’s lost brain be found, so that the remaining 40 or so tiny fragments could be removed. He was also insisting that the alleged fragment from Connally’s chest, never seen since the initial rumor about it, surface before Wecht would agree to neutron activation. Dr. Wecht was therefore imposing conditions that he knew could not be met. His commitment to NAA was an empty gesture that allowed him to avoid dealing with Guinn’s new findings.
      Among the other forensic scientists who called for new chemical tests was Dr. Vincent P. Guinn,[7] who was later chosen to do the second round of NAA tests.

Secret NAA by the FBI revealed
      The surprising news that the FBI had used NAA a decade earlier first came out in 1973, when, according to Henry Hurt, “…a volume of Warren Commission correspondence was released by the National Archives.”[8] Among the items was the letter of 8 July 1964 from Hoover to Rankin that was described in part in the earlier section on spectroscopic analysis. Whereas the first paragraph of this letter described the FBI’s spectroscopic analysis and its indeterminate results, the second and third paragraphs went on to reveal that the FBI had also run NAA on a suite of the fragments, but that these results were also indeterminate. Here is the letter in its entirety:

UNITED STATES DEPARTMENT OF JUSTICE
FEDERAL BUREAU OF INVESTIGATION

Washington 25, D.C.
July 8, 1964
By Courier Service

Honorable J. Lee Rankin
The President’s Commission
200 Maryland Avenue, Northeast
Washington, D.C.

Dear Mr. Rankin:

      As previously reported to the Commission, certain small lead metal fragments uncovered in connection with this matter were analyzed spectrographically to determine whether they could be associated with one or more of the lead bullet fragments and no significant differences were found within the sensitivity of the spectrographic method.
      Because of the higher sensitivity of the neutron activation analysis, certain of the small lead fragments were then subjected to neutron activation analyses and comparisons with the larger bullet fragments. The items analyzed included the following: C1—bullet from stretcher; C2—fragment from front seat cushion; C4 and C5—metal fragments from President Kennedy’s head; C9—metal fragment from the arm of Governor Connally; C16—metal fragments from the rear floor board carpet of the car.
      While minor variations in composition were fund by this method, these were not considered sufficient to permit positively differentiating among the larger bullet fragments and thus positively determining from which of the larger bullet fragments any given small lead fragment may have come.

                                    Sincerely yours,
                                    J. Edgar Hoover (signed)

      In other words, Hoover was reporting that all the fragments—large and small alike—had about the same chemical composition, and could not be told apart even by the ultraprecise neutron activation.
      The critics had a field day. They jumped on the FBI’s secrecy and its interpretation of the results, claiming that the FBI had kept the data secret because they failed to support the official single-bullet theory. Many of the critics even claimed that the FBI’s results actually disproved its single-bullet theory. At least one critic cried cover-up. The reactions of the critics are described at length elsewhere.[9]
      The critics’ judgments were premature, however. They failed to consider at least two legitimate reasons why J. Edgar Hoover may have chosen to keep the NAA information secret. First, he may have wished to avoid the ill-informed discussion that indeterminate technical results would inevitably create (witness George Michael Evica in 1978). Subsequent events vindicated this concern.[10] Second, Hoover may simply have been concerned for the image of the FBI—a mixed institutional/personal vanity linked to the strong popular perception that Hoover was the FBI, and vice versa.[11] We will probably never know just why Hoover stonewalled.

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