Here's why:

* Photosynthesis is the process plants use to convert light energy into chemical energy in the form of sugars. This process relies on the absorption of light by chlorophyll and the subsequent chemical reactions involving water, carbon dioxide, and other molecules.

* Radioactive isotopes are atoms of an element that have an unstable nucleus. They emit radiation as they decay into a more stable form. This radiation can be harmful to living organisms.

While radioactive isotopes aren't used directly in photosynthesis, they are used as tracers to study the process. Here's how:

* Carbon-14 (¹⁴C): This radioactive isotope of carbon is used to trace the path of carbon atoms during photosynthesis. Scientists can introduce ¹⁴C into a plant and then track its movement through the plant's tissues, providing valuable information about the steps involved in carbon fixation and sugar production.

* Other isotopes: Radioactive isotopes of other elements like phosphorus (³²P) and sulfur (³⁵S) are also used to study specific aspects of photosynthesis, such as nutrient uptake and enzyme activity.

Therefore, while radioactive isotopes are not part of the normal process of photosynthesis, they are powerful tools that scientists use to gain a deeper understanding of how photosynthesis works.