densities and temperatures that allow the production of large amounts of radioactive isotopes. For example, the bottom of the helium burning layer in the 25M$_\odot$ star is much hotter than the other progenitors. Although the conditions are not as extreme as the core, this region also produces a range of radioactive isotopes. Because much of the core in the 25M$_\odot$ falls back, many of the radioactive isotopes produced in the core are not ejected. For this progenitor, the shell burning layers dominate the radioactive yields.

The 20M$_\odot$ star’s core similar to the 15M$_\odot$, but has a lot of fallback, so does not eject a lot of radioactive isotopes in its innermost ejecta. Figure \[fig:expparams\] shows that the innermost ejecta of this particular star have much lower peak temperatures than our other progenitors. In addition, it also has lower density/temperatures in its outer layers than the 25M$_\odot$ star. So this also limits its production of certain radioactive isotopes. These isotopes can be used to detect the specific structure of the 20M$_\odot$ star.

![image](K43_EnergyLines_Mass.pdf){width="0.32\linewidth"} ![image](CA47_EnergyLines_Mass.pdf){width="0.32\linewidth"} ![image](FE59_EnergyLines_Mass.pdf){width="0.32\linewidth"}

Observations of some of the radioactive isotopes can be used to differentiate the structure of the 20M$_\odot$ star from our other progenitors. For example, because of 