Writing Gamma Decay (Emission) Equations

A Brief Tutorial About Writing Nuclear Symbols

Gamma decay equations are also called gamma emission equations.

Example #1: Write the equation for the gamma decay of the metastable form of 43-Tc-99.

Solution:

${}_{\mathrm{ 43}}^{\mathrm{99m}}\text{Tc}$ ---> ${}_{43}^{99}\text{Tc}$ + γ

Sometimes, the energy of the gamma ray is included in the equation. As follows:

${}_{\mathrm{ 43}}^{\mathrm{99m}}\text{Tc}$ ---> ${}_{43}^{99}\text{Tc}$ + γ (0.143 MeV)

MeV stands for mega electron-volts. You also see it as Mev and, sometimes (incorrectly), as mev. MeV is the preferred (and correct) way.

Example #2: Write the equation for the gamma decay of the metastable form of 38-Sr-87.

${}_{38}^{87}\text{Sr^*}$ ---> ${}_{38}^{87}\text{Sr}$ + γ (0.388 MeV)

Note the presence of the asterisk. It is another way to indicate a metastable condition. The use of 'm' is preferred, but the asterisk is in wide-spread use, so you need to know what it indicates.

Example #3: Write the equation for the gamma decay of the metastable form of 53-I-125.

${}_{\mathrm{ 53}}^{\mathrm{125m}}\text{I}$ ---> ${}_{\mathrm{ 53}}^{125}\text{I}$  +  ${}_0^0$γ

Notice the use of zero for the atomic number and mass number. This is an older way to show a gamma. It has fallen into wide-spread disuse, but it does show up in reference materials on an occasional basis.

This use of zeros was also used with neutrinos and anti-neutrinos, but its usage has also fallen off almost completely.

Example #4: 27-Co-58 has two different metastable states. How might these be shown?

Solution:

${}_{\mathrm{ 27}}^{\mathrm{58m1}}\text{Co}$ ---> ${}_{27}^{58}\text{Co}$ + γ (0.025 MeV)

${}_{\mathrm{ 27}}^{\mathrm{58m2}}\text{Co}$ ---> ${}_{27}^{58}\text{Co}$ + γ (0.053 MeV)

The amounts of energy must be looked up.

Example #5: Give the notation for a gamma particle and briefly describe what occurs during a gamma emission.

The notation for a gamma particle is to use the Greek lower-case letter delta:

γ

I might add that the word photon is more correct than particle but, in this area, the term 'gamma particle' is often used. I have seen the word ray used (as in 'gamma ray'), but not usually for talking about writing gamma emission equations. More often, it is used in talking about detecting gamma, as in 'gamma ray detector.'

In a gamma emission, a nucleus has some excess energy. In order to become more stable, the nucleus releases some of this energy, in the form of one (or more) gamma particle emissions. After release of the energy, the nucleus is now more stable.

By the way, the nucleus doesn't have to become stable after release of some gamma. It may have decayed by alpha or beta, then release a gamma particle in a separate decay. Then, the nucleus might continue on to another decay (beta, positron, whatever) after which there might even be more gamma releases.

Bonus Example: Write the equation for the gamma decay of 92-U-238m

Note that 'm' (for metastable) is not used. Also, note where the gamma is written. Finally, the word energy is used where, in my above examples, I placed the Greek letter gamma.