Let m1, m2 be the masses of the two daughter nuclei, and v1, v2 be the daughter nuclei’s respective velocities. Let m be the parent nucleus’s mass.
After disintegration, total linear momentum = m1v1 +m2 v2.
The nucleus is at rest prior to disintegration. As a result, before disintegration, its linear momentum is zero.
Using the law of conservation of momentum as a guide,
Before disintegration, total linear momentum Equals After disintegration, total linear momentum
m1v1 + m2v2 = 0
v1=-m2v2/m1
The minus symbol denotes that v1 and v2 are moving in opposing directions.