Section 20G. A witness who has been granted immunity as provided in section 20E shall not be prosecuted or subjected to any penalty or forfeiture for or on account of any transaction matter, or thing concerning which he is so compelled, after having claimed his privilege against self-incrimination, to testify or produce evidence, nor shall testimony so compelled be used as evidence in any criminal or civil proceeding against him in any court of the commonwealth, except in a prosecution for perjury or contempt committed while giving testimony or producing evidence under compulsion, pursuant to section 20C or 20E.
A witness who has been granted immunity shall be given a certified copy of the transcript, if he so requests, of any testimony that he furnished in compliance with an order of the court to testify, and shall be given a copy of all documents he has furnished in compliance with such order.
A transcript of said testimony and copies of said documents shall be maintained by the supreme court, appeals court or superior court, and shall be available at the request of such witness in any subsequent proceeding involving the witness.