40 ILCS 5/18-120

An eligible judge who is not a participant shall become a participant beginning on the date he or she becomes an eligible judge, unless the judge files with the board a written notice of election not to participate within 30 days of the date of being notified of the option. A person electing not to participate shall thereafter be ineligible to become a participant unless the election is revoked as provided in Section 18-121.