When a scientist uses radiometric dating to determine the age of a rock specimen, would he use a radioactive element with a short half life, say a half-life of less than 10,000 years, or a radioactive element with a long half-life, say a half-life greater than 1 billion years?

He will use a radioactive element with a long half life of greater than 1 billion years

Explanation:

Rocks are an aggregation of solid mass which is usually rich in minerals. The age of rocks is established using radiometric dating method.

According to Oxford Dictionary, radiometric dating is "a method of dating geological specimens by determining the relative proportions of particular radioactive isotopes present in a sample."

The age of rocks can only be ascertained using isotopes that have very long half lives because many of these rocks are very old (aged billions of years) hence their age can only be determined using elements that has a half life greater than 1 billion years.