20 CFR 216.66: Who is an employee's surviving divorced spouse.

An individual who was married to the employee is the deceased employee's surviving divorced spouse if he or she:

(a) Was married to the employee for a period of at least 10 years immediately before the date the divorce became final, and applies for an annuity based on age or disability; or

(b) Applies for an annuity based on having a “child in care” and either:

(1) Is the natural parent of the employee's child;

(2) Was married to the employee at the time the employee or the surviving divorced spouse adopted the other's child who was then under 18 years old; or

(3) Was married to the employee at the time they adopted a child who was then under 18 years old.