PSC404, Spring 2001
Answers to assignment 6
Neutron-activation analysis and the assassination

Read: Relevant sections of Neutron-Activation Analysis and the John F. Kennedy Assassination.

Answer these questions:
      1. What was the main goal of the neutron-activation analysis of bullets and fragments from the assassination? To see whether all the little fragments could be paired up chemically with the two big fragments, both of which came from Oswald’s rifle. If so, all the fragments could be attributed to his rifle, and there would be no evidence for conspiracy. If not, there would have been a second shooter and conspiracy.
      2. Explain briefly how NAA works. The sample to be analyzed is put into a clean container and inserted into a nuclear reactor, where it is bombarded with neutrons for a predetermined period. Some nuclei of some elements absorb neutrons and become artificially radioactive. The sample is then removed from the reactor, allowed to decay for some period, and then placed near a Ge(Li) detector that together with the proper instrumentation records the gamma radiation from the sample. The energy, abundance, and half-life of the gamma rays allow the experimenter to calculate the abundance of various elements in the sample.
      3. Explain how the FBI analyzed the fragments with NA. They followed the above procedure at Oak Ridge National Laboratory, Tennessee, in May 1964. The FBI’s chief spectroscopist, John Gallagher, went there and actually did the work but under the guidance of two ORNL employees. What did they officially conclude? That the fragments were generally similar but could not be grouped beyond that. What did they unofficially conclude? That there was solid but inconclusive evidence for the same two groups of fragments that Vincent Guinn later found. List one or two strengths and weakness of their approach and their results. The main weakness was that they somehow let in a systematic error that made their data hard to interpret. A secondary weakness was that they analyzed far too few background bullets. The main strength was that they analyzed all the fragments in replicate, which later turned out to hold the key to properly understanding the lack of heterogeneity in the evidence fragments.
      4. How did Vincent Guinn’s later results compare with the FBI’s earlier results? Once the FBI’s results were adjusted for the systematic error, they became extremely similar to Guinn’s. Specifically, the FBI found the same two groups at the same two concentrations of antimony that Guinn later found.
      5. Briefly recap the problem of heterogeneity of Sb as expounded by Wallace Milam. Antimony in four quarters of three bullets was heterogeneous enough (24%) to wipe out the difference between the two groups of evidence fragments (i.e., to make them overlap into one big schmeer). Why is it potentially so significant? It destroys the two-bullet, all Oswald interpretation of the five fragments and removes all meaning from the NASA results.
      6. Explain why the measured heterogeneity and the measured groupings of fragments are incompatible. The heterogeneity says there is only one group of fragments. Probability says these groups could not have arisen by chance, and so must be real. Both cannot coexist.
      7. How can we resolve the incompatibility in (6)? By recognizing that the heterogeneity on the scale of quarter bullets does not apply to most fragments from shattering FMJ bullets. This is both predicted theoretically and confirmed by ballistic understanding.
      8. What are the main conclusions about the JFK assassination offered by the NAA? All the fragment came from two bullets fired from Oswald’s rifle that day. No second shooter hit the men. No fragments were planted. No physical reason to consider a second shooter.
      9. Explain how NAA fixes a three-point path of the head shot from Oswald’s rifle to the fragments recovered from the front seat of the car. Q2 (front seat) is traceable ballistically to Oswald’s rifle. Q4,5 (brain) is traceable chemically to Q2. Thus the bullet started from Oswald’s rifle, left fragments in JFK’s brain, and wound up (at least in part) on the front seat. Why is this result so important? Because it definitively ties the fragments in the front seat to the head shot for the first time. Because it removes the burden of fighting over the exact entrance and exit of the bullet to JFK’s head.
      10. What are some of the objections to the results of the NAA? Are any of them valid? The main objections are that fragments were tampered with after the fact and that the heterogeneity obscures the apparent groupings. Neither is valid.

N.B. For more details about these question, see the new monograph on NAA.

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