_The Problem_
You are a curator working with the ancient coin collection of a large
museum. A donor has just given the museum a group of 50 gold coins
presumably about 1500 years old. After months of careful study, you have
satisfied yourself that most of those coins are genuine specimens of
that period. Judging from your experience, you decide that a small group
of five are definite forgeries.
However, there are three others that you suspect are also fakes, but you
are not quite certain. You know that both genuine minters and forgers
often tried to save money by diluting their gold with less expensive
metals such as silver and copper. Since the chances are slim that the
forger’s product has the same concentration of gold, silver, and copper
as the genuine coins, you realize that a chemical analysis would help
you decide if the doubtful pieces were real or fake.
An accurate chemical analysis would require a sample of such size that
the coin would be ruined as a museum specimen. You need an analytical
method that can be applied to an infinitesimal sample.
_The Solution_
You are not a scientist but you’ve heard about neutron activation
analysis. Therefore, you contact a radiochemist at a local university
who is an expert in this field.
He decides to use a sampling technique developed by scientists at
Brookhaven National Laboratory for sampling metal objects of
archaeological interest. You obtain from him a set of 50 quartz plates
that have been ground on one side. Following his instructions, you
carefully scrape away a small area on the edge of each coin. You then
rub each freshly cleaned area across the ground surface of one plate
leaving a minute streak of metal similar to a pencil mark.
At the scientist’s laboratory, each plate is carefully placed inside a
quartz tube. No attempt is made to weigh the tiny streak of metal since
you wish only to compare the ratios of the metal concentrations.
However, because the samples make a rather bulky package, the scientist
is concerned with the uniformity of the neutron flux that each sample
will “see”. He therefore also places in each tube an exactly equal
weight of a gold—silver—copper alloy wire (of known proportions) to act
as a standard neutron-flux monitor. The tubes are then sealed and taken
to a reactor to be irradiated for 12 hours.
After the samples are removed from the reactor, the scientist carefully
breaks open each of the quartz tubes and places the sample and the
standard piece of wire in separate numbered plastic capsules with lids.
For an accurate comparison, each capsule is prepared in the same manner.
About 4 hours after the samples are removed from the reactor, he begins
the radioactivity measurements.
The sample capsules are loaded into an automatic sample-changing
mechanism that places each one into an identical position above a
lithium-drifted germanium detector. (See the chapter beginning on page
19.) Gamma-ray spectra are collected all day, first from a sample, then
from its accompanying standard. Each count takes 2 minutes, and 3
minutes are required between counts for data printout and sample
changing. A typical gamma-ray spectrum looks like the one in the figure
on the next page. Notice that only gold (gold-198) and copper
(copper-64) show up in this short counting time. Later on, radioactivity
from silver (silver-110_m_) can be measured using a longer counting
time. This can be done because while the activation products from copper
and gold have relatively short half-lives (12.8 hours and 2.7 days,
respectively), that from silver has a half-life of 270 days. To increase
the sensitivity of the analysis for silver, the scientist repackages and
re-irradiates the samples and wires for 100 hours. Silver-110_m_ is one
of _two_ radioactive isotopes of silver that have the _same_ mass. In
this case, one has a higher energy than the other and decays in a
different way. This is known as an isomeric state and it occurs for many
other elements as well as for silver.
[Illustration: Graph: “Counts in one minute per 3.1 KeV channel” _vs_
“Channel”]
Element Counts Channel number
0.158 MsV Au-199 600 40
0.412 MeV Au-198 800 120
0.511 MeV Cu-64 50 150
0.676 MeV Au-198 40 205
(Values estimated from graph)
The spectrum obtained from a streak of metal on a quartz plate after a
3-hour exposure to neutrons in a reactor and a 6-hour delay before
counting. The activation products of gold and copper are obviously
present and are easily measured in only 1 minute.
[Illustration: Graph: “Counts per 100 minutes per 3.2 KeV channel” _vs_
“Channel number”]
Energy Counts Channel Number
0.445 MeV 230 130
0.657 MeV 1200 190
0.687 MeV 170 200
0.715 MeV 220 220
0.760 MeV 230 225
0.820 MeV 60 250
0.884 MeV 500 260
0.937 MeV 190 280
(Values are estimated from graph)
The spectrum obtained from the same streak of metal after re-exposure
to neutrons for 100 hours and a delay of approximately 2 months before
counting. Activation products from gold and copper have decayed away
and the gamma-ray spectrum of silver-110m is now observed. In this
case the sample is closer to the detector than for the earlier
measurement and the measurement takes 100 minutes.
Two months later, the scientist repeats the procedure of counting the
samples and standards, except that this time the plastic capsules are
closer to the detector, each count is for 100 minutes, and the sample
changer operates for about a week. A typical spectrum looks like that in
the figure on page 39.
The scientist can now compute ratios for the three elements in each
sample and compare them with the standard, but he decides that a
computer could do it faster and with fewer errors. The data collected
during the two series of counts are therefore sent to a data processing
center where, in a matter of minutes, a computer does the following for
each of 50 samples: