One of the more interesting encased coin is the
Irradiated Dime. Irradiated dimes were created as a way to
demonstrate atomic energy principles and as a souvenir. A visitor to the American Museum of Atomic Energy could place a dime from their
pocket in to the "dime irradiator", where it was subjected to radiation. The coin was returned in a plastic encasement.
The original pieces were encased in a round 25mm holder with a rolled rim and a plastic convex cover over the dime. The reverse of the dime does not show through the back of the encasement making collecting by mint mark impossible without removing the coin.
The dimes were prepared ahead of time or visitors were allowed to supply their own dime and have it irradiated. The prepared coins were encased in a circular holder of aluminum with a plastic cover protecting the dime. The inside edge of the dime, underneath the cover, was a label that read "American Museum of Atomic Energy - Neutron Irradiated" or "Oak Ridge National Laboratory - Neutron Irradiated." The dime went into the chamber, was irradiated and came out. It's radioactivity was detected with a Geiger counter. When sealed the holder and the dime were a finished souvenir.
However since the visitor could supply the dime, there is a wide variety of dates available for encased irradiated dimes. the irradiator was build in the 1950's so Roosevelt dims were most common, however many Mercury (Winged Liberty) dimes were irradiated as well. There are collectors who collect these encased dimes by date.
One factor to take into account is that with our very sophisticated equipment it is impossible to tell if the dime is original and has been irradiated. The coins were easily removed and spent by many a young person with an eye to their sweet tooth than to historical significance.
The Irradiated dime was produced from 1949 to 1954. The American Museum of Atomic Energy gave out 250,000 souvenir samples. By 1967, the number exceeded one million. The program ended when the US Mint ceased making silver dimes in 1964. In 1965, the dimes were made out of nickel clad copper. For a period of time, the Museum purchased rolls of dimes and searched them for silver dimes to keep the popular program going.
The Oak Ridge Graphite Reactor was during the 1950's the leading source for radionuclide's. The availability of radionuclide's revolutionized medicine and other branches of science. Radionuclides were produced in several ways, but neutron activation was perhaps the most important. In order to demonstrate this process the American Museum of Atomic Energy in Oak Ridge, Tennessee built the dime irradiator. It provided a dramatic demonstration of neutron activation.
At the time that the dime irradiator was in operation, the Museum was managed by the Oak Ridge Institute for Nuclear Studies (ORINS). ORINS is now Oak Ridge Associated Universities (ORAU).
1954 press release from the American Museum of Atomic Energy:
One of the most popular exhibits in the American Museum of Atomic Energy is adime irradiator. To date, more than 250,000 dimes have been irradiated, encased in plastic and returned to their owners as souvenirs. The irradiator works as follows: A mixture of radioactive antimony and beryllium is enclosed in a lead container. Gamma rays from the antimony are absorbed by the beryllium atoms and a neutron is expelled by the beryllium atom in the process.
These neutrons, having no electrical charge, penetrate silver atoms in the dime. Instead of remaining normal silver-109, they become radioactive silver-110. After irradiation, the dime is dropped out through a slot in the lead container and rests momentarily before a Geiger tube so that its radioactivity may be demonstrated. It is then encased in the souvenir container. Radioactive silver, with a half-life of 22 seconds, decays rapidly to cadmium-110 (In 22 seconds, half of the radioactivity in each dime is gone, in another 22 seconds half the remainder goes, and so on until all the silver-110 has become cadmium). Only an exceedingly minute fraction of the silver atoms have been made radioactive."
The irradiated dimes were 90% silver and 10% copper. The silver consisted of equal amounts of Ag-107 and Ag-109. The absorption of the neutrons by the silver converted some of the Ag-107 to Ag-108m (400+ year half-life) and Ag-108 (2.39 min half life). Similarly, a few atoms of the Ag-109 were converted to Ag-110m (250 d half-life) and Ag-110 (25 s half-life). The bulk of the activity that was produced was due to the Ag-110 and, to a lesser extent, the Ag-108. Both of these radionuclides decayed away very quickly, within minutes. By the time the visitor was leaving the Museum, there was almost nothing left. The only activity that might be detected today would be due to trace quantities of Ag-108m, but it would require long counts and relatively sophisticated equipment.
Quote taken from ORAU - complete story at Orau.org