The correct sentence would be "employed at." For example, "She is employed at the bank."

The prefix of "employed" is "em-".

if you are talking about your boss specifically or if you are working with a sole employer you say employed by. If you are talking about a business as a whole you say employed at.

25 percent are self-employed, about 33 percent are employed by general building contractors, 20 percent are employed by specialty trade contractors, and 12 percent are employed in heavy construction.

Yes, alpha radiation is an ionizing radiation.