can a person catch radiation through sex from a person on chemo with cancer ?

Chemotherapy is not the same as radiotherapy.

Chemotherapy is where a liquid is injected in the person’s veins, that kills of cancer cells (as well as other cells)

Radiotherapy is where the tumour gets blasted with radioactive rays in an attempt to shrink or kill the cancer cells.

It is highly unlikely that a person going through a cycle of either treatment would want sex though.

If a person is having radiotherapy, they do not become radioactive. The exception may be if medical personnel have implanted radioactive rods near the tumour site.