The U.S. Air Force has a variety of radiation detectors that are used to detect radioactive materials that might be smuggled into the U.S. by terrorists. One detector is small enough to fit in a back pack. Other detectors are designed to be used on aircraft or container ships. When there is a possibility of radioactive materials, these detectors are used to take a "radiological fingerprint" which can identify different types of radioactive materials.
These detectors are looking for gamma rays and neutron emissions from radioactive materials. They utilize helium-3 which interacts with neutrons to produce charged particles that are easily detected. Unfortunately, the supply of helium-3 is very small and it is diminishing rapidly. Most of the U.S. supply of helium-3 has been produced through the decay of tritium.
Tritium is a radioactive isotope of hydrogen. Most hydrogen atoms consists of a proton and an electron. Deuterium is a form of hydrogen that contains a neutron as well as a proton and an electron. Tritium contains two neutrons as well as a proton and an electron. It's nucleus is unstable and will eventually emit an electron (called beta decay) and become helium-3. The half life of tritium is about twelve years. It is only present in nature in tiny trace amounts that are produced when cosmic rays interact with hydrogen in the atmosphere. It is also recovered when nuclear weapons are decommissioned. Most of the U.S. tritium has been produced in this way. As the number of warheads being decommissioned has decreased, the production of tritium has decreased and so has the recovery of helium-3.
The supply of helium-3 is not only important for national security but it is also very important in medical uses such as cryogenics and medical imaging technology. The U.S. stopped producing helium-3 in 1988. Since the 911 attack on the U.S. demand for helium-3 for radiation detectors has been rising as the supply has been shrinking. Methods other than tritium decay can be used to produce helium-3 but they are expensive and complex. A small supply of helium-3 is available from special Canadian reactors but not enough to meet demand. Research on nuclear fusion utilizing helium-3 has been hampered by the low supply of helium-3 and skyrocketing prices.
One possible future source of helium-3 is the dust on the surface of the moon. Millions of years of bombardment of the lunar surface by the solar wind has produced tritium. It is estimated that one tenth of a metric ton of helium-3 exists on the Earth but there may be millions of metric tons of helium-3 on the Moon. The U.S. government is searching for ways of producing helium-3 and limiting the use of helium-3 but no effective solution to the shortage has been identified.
Artist's concept of a lunar helium-3 mining operation:
