The dominant decay mode gives the useful gamma ray at 140.5 keV. Some aspects of the complex decay of this radioisotope are shown in the diagram below. With such a long half-life for the excited state leading to this decay, this state is called a metastable state, and that is the reason for the designation 99m. This is extremely long for an electromagnetic decay - more typical is 10 -16 seconds. The technetium isotope 99mTc is unusual in that it has a half-life for gamma emission of 6.03 hours. For medical purposes, the 99mTc is used in the form of pertechnate, TcO 4. Since 99Mo is a fission product of 235U fission, it can be separated from the other fission products and used to generate 99mTc. This process permits the production of 99mTc for medical purposes. The resultant 99Mo decays with a half-life of 66 hours to the metastable state of Tc. Technetium -99m is produced by bombarding molybdenum 98Mo with neutrons. A further advantage is that the gamma is a single energy, not accompanied by beta emission, and that permits more precise alignment of imaging detectors. The fact that both its physical half-life and its biological half-life are very short leads to very fast clearing from the body after an imaging process.
It's gamma ray energy of about 140 keV is convenient for detection. Technetium-99m is a widely used radioactive tracer isotope in Nuclear Medicine.
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