Induced gamma emission

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Induced gamma emission (IGE) is a process where an excited nucleus emits gamma rays after being stimulated by gamma photons. It’s similar to fluorescence, but it happens in the nucleus instead of in electrons around an atom. Nuclear isomers can store energy for relatively long times, which makes them interesting as potential “nuclear fluorescent” materials. There are many known isomers, but most are too radioactive to use safely. As of 2006, tantalum-180m and hafnium-178m2 were the two most promising candidates for safe IGE fluorescence.

How it works
A gamma photon hits a nucleus in an isomer and puts it into a higher excited state. The nucleus then relaxes by emitting gamma rays. The process is resonant: the incoming photon must have a very specific energy that matches the difference between two nuclear states. There is no threshold in this case, and the incident photon’s energy is absorbed and put into the nucleus. This is closely related to how fluorescence works at the atomic level, just with nuclear energy levels.

History and experiments
The idea goes back to 1939, when Bruno Pontecorvo and André Lazard reported the first example of this kind of reaction using indium. They irradiated 115In and produced 115mIn, a long-lived isomer. The experiment showed that such resonant, photon-driven transitions could occur. It took until 1988 for the resonant nature of these reactions to be clearly demonstrated. In that year, researchers showed that exciting a nuclear isomer could proceed through specific resonant states, confirming the IGE concept.

In 1988, Collins and colleagues reported the first excitation of IGE from a nuclear isomer by using x-rays from an external accelerator. They targeted tantalum-180m and, after careful analysis, identified the resonant states involved. The results were surprising at first but could be explained once those resonant states were understood.

Possible applications and ideas
Because IGE depends on very specific photon energies, it could lead to energy-specific dosimeters. By using several different nuclides together, it might be possible to calibrate the full spectrum of a radiation beam and better predict how different energies deposit dose in tissue. This could be useful in radiation therapy and in calibrating complex X-ray beams.

Controversy and speculative ideas
In 2003, a New Scientist article discussed the idea of an IGE-powered airplane based on hafnium-178m2, described as a “quantum nucleonic reactor.” The notion is highly speculative and controversial. Critics worry about the potential for rapid, large energy releases and the possible use of stored nuclear energy as a weapon or in propulsion. The safety and nonproliferation implications make practical development of such ideas very challenging.

Overall, IGE sits at the crossroads of nuclear physics and quantum electronics. It offers interesting physics and potential applications in dosimetry and medical physics, but practical and safe use remains a topic of ongoing research and debate.