By Scott Malone

BOSTON(Reuters) - Dzhokhar Tsarnaev, convicted of carrying out the Boston Marathon bombing, would spend most of his days alone in his cell if a U.S. jury decides to send him to prison instead of sentencing him to death, a penal expert testified on Thursday.

The same jury found Tsarnaev, 21, guilty last month of killing three people and injuring 264 others at the race's crowded finish line on April 15, 2013, in one of the highest-profile attacks on U.S. soil since Sept. 11, 2001.

The U.S. District Court jury in Boston can only sentence him to death or life in prison without possibility of release.

Tsarnaev's lawyers are trying to convince the jury that he should get a life sentence and have been painting that punishment as a fate worse than death.

If Tsarnaev goes to prison, he would be alone most of the time at the U.S. government penitentiary in Florence, Colorado, a former federal prison warden, Mark Bezy, told jurors.

The prisoners at the super-maximum security, or "Supermax," penitentiary have no cellmates, and recreation is solitary, too, according to Bezy.

"They sit in their cells, locked in their cells every day," he said.

Tsarnaev, however, would be allowed up to five visitors a month, as well as phone calls and letters from friends outside prison, Bezy said. Even in their isolation, inmates manage to communicate from cell to cell by passing notes and speaking through pipes, he said.

Tsarnaev's attorneys argue that his older brother, Tamerlan, was the driving force behind the bombing, with Dzhokhar playing a willing but secondary role.

Over the past two weeks, the defense team has called more than 40 witnesses. Many of them described the young ethnic Chechen as a mild-mannered teenager who, even as his college grades slipped and his family fell apart, remained the kind, well-liked youngster he had been as a child.

Investigators found al Qaeda propaganda on computers belonging to both men, and a note written by Dzhokhar casting the bombing as retribution for U.S. military campaigns in Muslim lands.

Prosecutors, who are seeking death for Dzhokhar, have argued that he and Tamerlan were equal partners in the attack. Tamerlan, 26, died following a gunfight with police three days after the bombing.

Killed in the attack were Martin Richard, 8, Chinese exchange student Lu Lingzi, 23, and restaurant manager Krystle Campbell, 29. The Tsarnaev brothers also shot dead Massachusetts Institute of Technology police officer Sean Collier.

(Additional reporting by Richard Valdmanis; Editing by Christian Plumb and Jonathan Oatis)