Radioactivity is "the propensity for radioactive material to emit radiation."
We can't measure radioactivity because the radioactivity is "a characteristic".
Generally, "radiation" and "radioactivity" are confused and used interchangeably on the news and in daily conversation,
but actually both are words with a totally different meaning.

It may be difficult to understand by this explanation.
We can look at radioactitive material in the image of a flashlight.
Please think of the light of the flashlight as radiation,
and the corresponding radioactivity as "the ability to create light" of the flashlight.
An near by flashlight emitting light is very bright,
and it becomes dark as we become distant from flashlight.
Like that, we absorb much radiation as we approach radioactive material.
Conversely, if we are separated from radioactive material, the influence from the radioactive material decreases.