For the record, this is Cassius' view and is not shared by all. The sources that use ataraxia include Letter to Menoeceus:
There is no doubt that the term ataraxia is used occasionally and in certain contexts; that is not the issue. The issue is whether we should draw the conclusion that "ataraxia" is correctly identified as equivalent to a specific type of pleasure, or as a unique "highest pleasure," which I contend is not the case, nor do those cites establish that point. The goal of life stated over and over again by Epicurus and others is pleasure, not "ataraxia." Pleasure is the overriding ultimate term, ataraxia is a subordinate concept just like aponia.
This is true even in the letter to Menoeceus - all of these references here are to "pleasure," not to "ataraxia":
"And for this cause we call pleasure the beginning and end of the blessed life. For we recognize pleasure as the first good innate in us, and from pleasure we begin every act of choice and avoidance, and to pleasure we return again, using the feeling as the standard by which we judge every good. And since pleasure is the first good and natural to us, for this very reason we do not choose every pleasure, but sometimes we pass over many pleasures, when greater discomfort accrues to us as the result of them: and similarly we think many pains better than pleasures, since a greater pleasure comes to us when we have endured pains for a long time. Every pleasure then because of its natural kinship to us is good, yet not every pleasure is to be chosen: even as every pain also is an evil, yet not all are always of a nature to be avoided."