(a) Affirmative Defense. - It is an affirmative defense to a prosecution under any Federal statute that, at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense. (b) Burden of Proof. - The defendant has the burden of proving the defense of insanity by clear and convincing evidence.