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If you are divorced, but your marriage lasted 10 years or longer, you can receive benefits on your ex-spouse's record (even if he or she has remarried) if:

• You are unmarried;
• You are age 62 or older;
• The benefit you are entitled to receive based on your own work is less than the benefit you would receive based on your ex-spouse's work; and
• Your ex-spouse is entitled to Social Security retirement or disability benefits.

If you remarry, you generally cannot collect benefits on your former spouse's record unless your later marriage ends (whether by death, divorce or annulment).

See here for details :
http://www.ssa.gov/retire2/divspouse.htm