Isotopes of technetium

Technetium (Tc) is one of the two elements in the first 82 that have no stable isotopes (in fact, it is the lowest-numbered element that is exclusively radioactive); the other such element is promethium. The most stable radioisotopes are 98Tc (half-life of 4.2 Ma), 97Tc (half-life: 2.6 Ma) and 99Tc (half-life: 211.1 ka).

Twenty-two other radioisotopes have been characterized with atomic masses ranging from 87.933 u (88Tc) to 112.931 u (113Tc). Most of these have half-lives that are less than an hour; the exceptions are 93Tc (half-life: 2.75 hours), 94Tc (half-life: 4.883 hours), 95Tc (half-life: 20 hours), and 96Tc (half-life: 4.28 days).

Technetium also has numerous meta states. 97mTc is the most stable, with a half-life of 90.1 days (0.097 MeV). This is followed by 95mTc (half life: 61 days, 0.038 MeV), and 99mTc (half-life: 6.01 hours, 0.143 MeV). 99mTc only emits gamma rays, subsequently decaying to 99Tc.

For isotopes lighter than the most stable isotope, 98Tc, the primary decay mode is electron capture, giving molybdenum. For the heavier isotopes, the primary mode is beta emission, giving ruthenium, with the exception that 100Tc can decay both by beta emission and electron capture.

Technetium-99 is the most common and most readily available isotope, as it is a major product of the fission of uranium-235. One gram of 99Tc produces 6.2×108 disintegrations a second (that is, 0.62 GBq/g).

Stability of technetium isotopes
Technetium and promethium are unusual light elements in that they have no stable isotopes. The reason for this is somewhat complicated.

Using the liquid drop model for atomic nuclei, one can derive a semiempirical formula for the binding energy of a nucleus. This formula predicts a "valley of beta stability" along which nuclides do not undergo beta decay. Nuclides that lie "up the walls" of the valley tend to decay by beta decay towards the center (by emitting an electron, emitting a positron, or capturing an electron). For a fixed number of nucleons A, the binding energies lie on one or more parabolas, with the most stable nuclide at the bottom. One can have more than one parabola because isotopes with an even number of protons and an even number of neutrons are more stable than isotopes with an odd number of neutrons and an odd number of protons. A single beta decay then transforms one into the other. When there is only one parabola, there can be only one stable isotope lying on that parabola. When there are two parabolas, that is, when the number of nucleons is even, it can happen (rarely) that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although this happens only in four instances). However, if this happens, there can be no stable isotope with an even number of neutrons and an even number of protons.

For technetium (Z=43), the valley of beta stability is centered at around 98 nucleons. However, for every number of nucleons from 95 to 102, there is already at least one stable nuclide of either molybdenum (Z=42) or ruthenium (Z=44). For the isotopes with odd numbers of nucleons, this immediately rules out a stable isotope of technetium, since there can be only one stable nuclide with a fixed odd number of nucleons. For the isotopes with an even number of nucleons, since technetium has an odd number of protons, any isotope must also have an odd number of neutrons. In such a case, the presence of a stable nuclide having the same number of nucleons and an even number of protons rules out the possibility of a stable nucleus.