Neutron-Activation Analysis and the John F. Kennedy Assassination
Kenneth A. Rahn
30 November 1995
To be revised and expanded during spring 2000
Almost since the moment of the JFK assassination, it has
been realized that the chemical composition of the bullets and fragments could
offer unique information on important questions such as the number and types of
bullets that hit JFK and Governor Connally. After the single-bullet theory was
developed, the compositions of the fragments from Connally’s wrist and the
“pristine bullet” from Parkland Hospital offered the only strong evidence
concerning whether the bullet had actually passed through at least Connally’s
body (i.e., whether the infamous single-bullet theory (SBT) was true).
The very night of the assassination, the
FBI laboratory in Washington analyzed all available fragments by emission
spectroscopy, and submitted a report to Dallas Police Chief Jesse Curry on the
23rd of November. For such tiny fragments (milligrams to tens of milligrams),
emission spectroscopy is only semiquantitative. J. Edgar Hoover reported to
Curry, and later to the Warren Commission, that the fragments were “similar in
composition” and that “no significant differences were found within the
sensitivity of the spectrographic method.” He refused to release the actual
data, however. Only one sentence about these analyses appears in the entire 26
volumes of the Warren Report.
We now know why Hoover kept the data
secret until the mid-1970s, when they were obtained by various Freedom of
Information lawsuits. The data are exceedingly poor, all elements being either
undetectable, “trace,” or when they could be quantified, having
order-of-magnitude uncertainties. Thus they are useless.
Within a few months after the Warren
Report was released, critics began calling for the samples to analyzed by
neutron activation. At that time, neutron-activation analysis (NAA) was a
fledgling technique with great promise. The critics incorrectly saw NAA as
giving absolute answers that would allow it to disprove the SBT once and for
all. They did not realize that the FBI had already analyzed the samples by NAA,
in May 1964 at Oak Ridge National Laboratory in Tennessee. Hoover. Hoover not
only kept these results secret as well, but the fact of the analyses too. The
existence of the FBI’s NAA only came to light in 1973. Once again, Hoover had
refused to release the data because they were inconclusive, or so the FBI
thought. They turned out to be very wrong, but that’s another story.
In 1977, the House Select Committee on
Assassinations had Dr. Vincent Guinn of UC Irvine reanalyze the lead specimens.
Guinn was the country’s leading practitioner of forensic analysis of bullets
and fragments by NAA, and had been interested in this topic since the early days
of NAA. His studies had shown that three elements were most characteristic of
bullet lead, and could be used to discriminate among bullets: antimony (Sb),
silver (Ag), and copper (Cu). Antimony was the most important, because it varied
the most among the bullets. The reason was because antimony is added as a
hardener to bullet lead, in concentrations of 1–4% (10,000–40,000 ppm).
Unhardened lead typically contains 10–1000 ppm Sb, and virgin lead contains
less than about 10 ppm. (See plot of Sb from handouts.) Silver varies less
because it is not deliberately added to lead. Copper is less useful because it
is easily contaminated by the jacketing of military bullets. (See table for
nuclear characteristics of these elements during NAA.)
During the previous decade, Guinn had
analyzed many bullets and had acquired a considerable data base on their
compositions. Even before he was approached by the HSCA, he had analyzed some
Mannlicher-Carcano bullets, and had learned to his great surprise that their
compositions were fundamentally different from nearly all other types of bullets
he had tested. Whereas the compositions of most bullets of a given type were
extremely reproducible within a box or within a production lot (“little carbon
copies,” Guinn called them) MC bullets differed greatly from bullet to bullet.
(This is shown most clearly on the plot of Sb in bullets, from the handout.) The
range of Sb in MC bullets, 20–1200 ppm, is nearly the same as the range of Sb
in all other unhardened bullets (15–1000). This offered Guinn the opportunity
to determine how many bullets had hit the two men (providing they had left
fragments somewhere), something that could not have been done with bullets of
other types.
There was a catch, though (Isn’t there always,
when something big is at stake?). Differences between individual MC bullets can
only be detected when variabilities within bullets are much smaller.
Within-bullet variations (heterogeneities) don’t have to be zero, but they do
have to be markedly smaller than differences between bullets. In this regard,
Guinn noted that individual MC bullets were “fairly homogeneous in Sb and
Ag,” and that they can “usually be distinguished from one another.” He
also wrote that “…you simply do not find a wide variation in composition
within individual WCC/MC bullets.” In his testimony to the HSCA, he said that
MC heterogeneities in Sb were about 6%. We’ll see.
In September 1977, armed guards from the
National Archives brought the bullet fragments to Guinn’s laboratory in
Irvine, CA. Over a period of three days he analyzed them by NAA. The guards then
returned them to Washington. For our purposes, Guinn analyzed five important
samples of lead, the stretcher bullet from Parkland (CE 399), two metal
fragments from Connally’s wrist, a fragment from the front seat of the
presidential limousine, two fragments from the president’s head, and three
small fragments from the rear floorboard carpet (see table in handout). These
were the same samples, or were tiny pieces cut from the same samples, as the FBI
had analyzed in 1964, even though many critics erroneously claim otherwise
(another long story generated by misunderstanding how NAA operates). Guinn’s
results were essentially identical to the FBI’s, once he had discovered
systematic errors in their analysis and corrected for them.
Guinn’s data for copper proved not very
useful. Much better were the data for Sb and Ag, as shown on one of the tables
in the handout. According to the Sb data, the samples fall neatly into two
groups: two samples with concentrations of about 820 ppm and three with
concentrations nearer 620 ppm. The Ag data show the same groups, but with much
smaller differences.
One’s pulse quickens when it is noticed
that the compositions of the groups are physically meaningful—the stretcher
bullet and the wrist fragments in the first group, and the fragments from brain,
front seat, and rear floor in the second group. Could it be so simple and
obvious, that the first group represents the single bullet through the bodies
and the second group represents the head shot, where major fragments of the
emerging bullet bounced off the windshield and chrome strip and plopped into the
front seat, and minor fragments flew less energetically forwards (and other
directions) and fell right in front of JFK? The data surely point in that
direction.
If so, think what we would have: the
strongest evidence to date for the SBT (including proof that the hospital bullet
was real rather than being planted), extremely strong evidence for only one
head shot (giving the lie to critics who claim that JFK was hit nearly
simultaneously by bullets from front and rear), and evidence for two and only
two bullets hitting in total. But
there’s more. Note that within each group of fragments is one large fragment
that was traced by ballistic markings to Oswald’s rifle to the exclusion of
all other rifles. Since the chemistry of the other fragments in each group ties
them to the large fragment from Oswald rifle, all five fragments are then tied
to that rifle. Stated most baldly, the NAA data tie every
recovered bullet fragment to Oswald’s rifle (to a high probability). There
is no chemical evidence that anyone else hit Kennedy or Connally that infamous
day. (Of course, someone could have fired and missed completely, but until some
hard evidence is produced to support that contention [none has emerged so
far], the idea may not be accepted.)
This entire interpretation depends on the
two groups being analytically separate, which in turn requires that their
difference in concentration (about 30%) be much larger than the uncertainty of
each group (about 1% for each of the fragments). Obviously, 30% far exceeds 1%.
But for some unfathomable reason, Guinn chose to publish his results with the
smallest possible analytical uncertainties attached. That 1% represents only the
“counting standard deviation,” from summing the number of gamma-ray counts
under each of the photopeaks of Sb in the samples. The details here are not
important, except to note that the full analytical uncertainties of the
neutron-activation process are about 2–3 times larger, or 2–3%. To this must
be added the heterogeneities of Sb within the bullet’s lead core—the
critical factor in this discussion. Guinn testified that according to his
experience, the total uncertainties of his Sb data (including heterogeneities)
were about six times the counting uncertainties, or about 6%. By this figure,
the two groups of fragments remain clearly separated, and all those juicy
conclusion from the previous paragraph remain. Based on this 6%, Guinn
testified, to great acclaim from the HSCA and the media, that the fragments were
“clearly distinguishable,” that the wrist fragments differ
“considerably” from the fragments in the other group, that the stretcher
bullet matched the wrist fragments “so closely” that “CE 399 did cause the
injuries to Governor Connally’s wrist, and that the results “were
consistent with” the single-bullet theory. Heady stuff indeed.
Fifteen years passed, and then Wallace
Milam entered the picture. Wallace Milam is a high-school social-studies teacher
from Dyersburg, Tennessee, a long-time assassination critic who lectures widely
on the subject. He just loves to catch
the big boys saying things they can’t defend. A couple years ago, Milam turned
his attention to the NAA data, and started writing articles accusing Guinn of
duplicity at best and perjury at worst. In a word, Milam claimed that Guinn’s
data show that MC bullets are actually much more heterogeneous than he stated,
so much as to invalidate his main conclusion about two separate groups. In
Milam’s view, Guinn found only one group. Gone is the evidence for two and
only two bullets. Gone is the strong support for the single-bullet theory. Gone
are all the accolades heaped upon Guinn by Robert Blakey of the HSCA, and many
others. The Guinn who remains is a charlatan in a scientist’s white coat.
Never one to understate, Milam used phrases like “Guinn’s testimony is
filled with inconsistencies and contradictions, and his laboratory data is
shocking in both its quantity and its results,” “…he presented findings
from his own laboratory tests which contradicted the very hypothesis which is
the basis of his work: that fragments from the same Mannlicher bullet exhibit a
high degree of homogeneity,” “…Dr. Guinn’s work represents, at best, an
invalid scientific hypothesis based on inadequate research. At worst, it is a
scientific charade meant to lend an aura of legitimacy to the single bullet
theory,” and finally, “He raised serious questions about his own
integrity.” Whew!
What got Milam’s dander up so much? My
first reaction was to dismiss him as just another crank critic talking before
thinking. But when I read Milam’s articles carefully and checked Guinn’s
data myself, I had to admit that Milam had found something that we all had
missed. I tip my hat to Wallace Milam for daring to challenge the big boy whom
we all accepted uncritically because of his reputation.
What Milam found was very simple. He noted
that whereas Dr. Guinn said that the heterogeneity of Sb in MC bullets was 6% (recall the
famous figure from above), his published
data showed its heterogeneity to be far greater—so great that the two groups
of fragments actually coalesced into one big, meaningless blob (my words).
Guinn’s published data on heterogeneities consist of four samples from each of
three MC bullets, from different production lots (see table in handouts). In the
bullet from lot 6001, all four concentrations of Sb were quite similar, and had
a standard deviation of 6% of the mean (just as Guinn had testified). But in the
bullets from lots 6002 and 6003, one of the four samples differed greatly from
the others. As a result, the standard deviations (heterogeneities) of those
bullets were 36% and 29%, respectively. When the three standard deviations are
combined, they average to 24%, a far cry indeed from the 6% that Guinn claimed.
With a standard deviation of 24%, two groups of fragments that differ by only
30% are no longer separate—they overlap completely.
What is going on here? How can someone
claim a heterogeneity of 6% but publish 24%? Guinn’s 6% came from—he
said—a larger set of analyses that he was then preparing to publish. But he
didn’t show the data then, and he hasn’t published them in the 17 years
since. Guinn was not following the rules of science, which say that conclusions
must be based on published, verifiable data. In effect, he was saying, “Trust
me, fellas, it is 6%.” While trust has no real role in science, one can offer
some latitude to a recognized expert, particularly in matters of little
consequence. But the 6% was of huge
consequence. It held the key to establishing two groups of fragments and thereby
to supporting the single-bullet theory far more strongly than otherwise
possible. It was, as Milam wrote, Blakey’s “linchpin” to the HSCA’s
endorsing the single-bullet theory. And in the final analysis, the linchpin came
down only to trust.
So which is it, 6% or 24%, two groups or
one? On this question literally hangs the entire single-bullet theory, which
many writers have judged the key to separating conspiracy from nonconspiracy in
the JFK assassination. The answer is not easy to find, and in the end becomes a
matter of probabilities rather than conclusiveness, but at least it can be taken
that far. Here are the steps.
Consider first the bizarre heterogeneities
of MC bullets. They violate everything we think we know about normal materials.
In well-mixed solutions containing impurities, the larger the sample size, the
smaller the differences in composition ought to be, because larger samples will
afford more opportunities for regions of different composition to be averaged
out. But MC bullets behave oppositely—the larger
the sample size, the greater the differences. Specifically, Sb in subfragments
(masses of 1–40 mg) has heterogeneities of about 5% (FBI data), Sb in
fragments (10–50 mg) has heterogeneities of 9%, Sb in Guinn’s
“quarter-bullets” has homogeneities of 8% with the two outliers removed but
24% with all 12 quarters considered, and Sb in whole MC bullets (Guinn’s data)
has heterogeneities of 90%. What’s going on here?
The giveaway is Guinn’s data from his
heterogeneity tests of the three bullets. Note how each of the two outliers
differs completely from the other
three samples of the bullet. In bullet 6002A, the outlier’s Sb is 358 ppm,
nearly three times lower than the other three values of 880–980. In bullet
6003A, the outlier’s value is 667 ppm, about 1.5 times greater than the other
three values of 363–441 ppm. Recall that samples of smaller size (fragments
and subfragments) have Sb that is much less variable—5% to 9% only. It’s
almost as though Sb either varies greatly or not at all—it varies greatly on
scales of entire bullets, but is nearly constant on all smaller scales. This
implies that the vats of lead from which MC bullets were made had not been
completely mixed before the lead was poured into the molds. Given that MC
bullets were made with lead from various sources, such a situation is entirely
plausible. Imagine making a marble cake at home. Into the bowl go the brown
batter and the yellow batter. The circular mixing motion of your wooden spoon
produces whorls that remain for a long time unless you are unusually persistent.
Thick zones of brown coexist with thick zones of yellow. Within each zone the
color is nearly constant; only right at the border does it change noticeably.
The same situation in a vat of lead could easily produce zones with very
different concentrations of impurities, completely invisible and consequently
impossible to know when they were gone. These zones need only be of the size of
a bullet to produce the pattern of heterogeneities found in MC bullets. It’s
like having two different materials in the same vat—two different colors in
the same cake pan. Most samples drawn randomly from the vat (i.e., bullet-sized
units poured into the mold) will contain some of each material, each well-mixed
within itself but very different from the other. Most small fragments will be
drawn wholly from one material, and so will produce homogeneous subfragments.
But one fragment may be very different from the other, and create the observed
heterogeneities within the MC bullets.
Since Guinn’s data on heterogeneities
are the only ones we have, we must base all our conclusions about separability
of groups on them. What do his data show? That of 12 zones within bullets, only
two differed markedly from the “pack.” The other ten zones varied from one
another by only 8% on the average—about the same as Guinn’s anecdotal 6%.
Thus for Mannlicher-Carcano bullets as a whole, 83% of the time (10 of 12 cases)
we may expect bullets to produce highly reproducible fragments (heterogeneities
of only 8%), and 17% of the time (2 of 12 cases) we will find much larger
variations. Since variations of 8% will keep Guinn’s two groups of fragments
separate to a very high confidence limit (between 95% and 99%), we may conclude
that there is an 83% chance that the two groups are actually separate to the
95–99% confidence level, and a 17% chance that they are not separate. This is
the probabilistic result referred to earlier.
How should we view an 83% probability of
having separate groups, strong support for the SBT, and all those other good
things? Obviously, it’s not the 95% probability that a scientist would like,
nor the 90–95% that a jury needs in order to convict in a criminal trial. But
it’s stronger than any other evidence for the SBT, and far better than any
evidence against it. And so it controls the issue. Maybe some day someone will
analyze a much larger suite of MC bullets for heterogeneity. Maybe Dr. Guinn
will release his large set of analyses from so long ago. In either case, we
could have more confidence in our probabilities. But until then, we may state
that there is an 83% probability that the hospital bullet had first passed
through Connally’s wrist, that all the other fragments came from a single head
shot, and that only those two bullets—both fired from Lee Harvey Oswald’s
Mannlicher-Carcano rifle to the exclusion of all other rifles—hit Kennedy and
Connally that day.
We may also state—and this must never be
forgotten—that no strong evidence exists for any contrary view.
Thus the neutron-activation evidence
provides an extremely simple picture of the assassination—one rifle, two
bullets—an explanation that has not been refuted by any evidence of similar
strength. An eerily complementary picture is provided by rigorous
physical/ballistic analysis of Kennedy’s movements as recorded by the Zapruder
film—the double head motion is consistent only with a single shot from the
rear.
So the assassination was actually the
result of two single bullets, not just
the one of SBT fame. The first single bullet wounded two men, and is revealed
starkly by neutron-activation analysis. The second single bullet created two
head motions—a quick snap forward followed by a longer lurch backward—and is
revealed by classical physics/wound ballistics applied carefully and rigorously.
All other views are speculation. All theories involving multiple shooters and
diffuse conspiracies are based on weak evidence. They are nothing more than
illusion, created by well-intentioned people (mostly) who are fooling themselves
and the American public because they refuse to face the consequences of dealing
with weak evidence. They think that somehow unreliable evidence can lead to
reliable conclusions. They are wrong, and the country is the worse for it. It is
time to end this misguided charade.