It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article.
The details are best pursued in a dedicated text like McDougall and Harrison. But the decay of potassium has multiple pathways , and detailed information about each of these pathways is necessary if potassiun-argon decay is to be used as a clock. But geologists project a much smaller distance between the points at the time of the impact because of measured sea floor expansion. The measurements were done by the argon-argon method. These each have 19 protons and 21 neutrons in their nucleus. A third piece of evidence came from age measurements of shocked zircon crystals which were found in the K-T layer as far away as Colorado and Saskatchewan.