Section 70. Where, upon a trial by jury, a person is convicted of a crime which is punishable by death, a sentence of death shall not be imposed unless findings in accordance with section sixty-eight are made. Where such findings are made and the jury finds that the death penalty shall be imposed, the court shall sentence the defendant to death. Where such findings are not made or not unanimously made or where a sentence of death is not a unanimous finding by the jury, the court shall sentence the defendant to life imprisonment as provided in section two of chapter two hundred and sixty-five.