In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type. The total absorption shows an exponential decrease of intensity with distance from the incident surface:. These machines are advertised to be able to scan 30 containers per hour. The first confident observation occurred in High-energy nuclear physics. Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones , and is often used to change white topaz into blue topaz. This is a similar mechanism to the production of high-energy photons in megavoltage radiation therapy machines see bremsstrahlung. A number of different gamma-emitting radioisotopes are used.
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