(n) a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead, Syn. protoactinium, atomic number 91, Pa
. The radioactive chemical element, of atomic number 91; Atomic symbol Pa; at. wt. of longest-lived isotope, 231 (T1/2 = 32, 500 yrs.) Also called brevium, Uranium X2 and UX2. [ PJC ]