40 ILCS 5/11-139
The widow of an employee whose death occurs in service before age 65 shall be entitled to an annuity of an amount provided on a single life annuity basis from the total sum accumulated to his credit for age and service annuity and widow's annuity, plus the credit as of the date of death in the service for prior service annuity, and widow's prior service annuity if he was a present employee; but no part thereof representing contributions by the city shall be used to provide an annuity in excess of that which she would have had if the employee had lived and remained in service at the rate of his final salary until he became age 65, and the widow's annuity were fixed on a reversionary annuity basis as provided in this Article. The annuity shall be computed as of the date of the employee's death.
40 ILCS 5/11-139