Chapter 265 of the General Laws, is hereby amended by inserting after section 13F the following section:-

 Section 13F1/2.  Whoever commits a rape on a mentally retarded person knowing such person to be mentally retarded shall for the first offense be punished by imprisonment in the state prison for not less than 10 years nor more than 20 years and for a second or subsequent offense by imprisonment in the state prisons for not less then 20 years.  Except in the case of a conviction for the first offense for violation of this section, the imposition or execution of the sentence shall not be suspended, and no probation or parole shall be granted until the minimum imprisonment herein provided for the offense shall have been served.  This section shall not apply to the commission of a rape by a mentally retarded person upon another mentally retarded person.