By Ellen Wulfhorst

FORT HOOD, Texas (Reuters) - The widow of a Fort Hood, Texas, worker slain in the November 2009 shooting rampage said in court on Tuesday that she has vowed not to let the killing by convicted gunman U.S. Army Major Nidal Hasan destroy her life and that "he is not going to win."

Joleen Cahill, whose husband Michael Cahill was one of 13 people murdered by Hasan at the central Texas military base, testified at the sentencing phase for the Army psychiatrist convicted of 45 counts of premeditated murder and attempted premeditated murder.

Hasan could be sentenced to death by the military jury of 13 officers who convicted him on Friday and are now weighing his sentence. He killed 13 people and wounded 31 others, most of them unarmed soldiers.

It was the deadliest mass murder ever at a U.S. military base.

An array of more than a dozen family members and shooting victims has testified in the sentencing phase, which started on Monday, recounting heart-wrenching stories of loss, grief and wounds.

Cahill said she and her three children have struggled with emotional, health and work issues since the death of her 62-year-old husband Michael, a civilian employee at Fort Hood. The couple had been married for 37 years.

One night, the widow said, she found herself having a thought that she described as one "you shouldn't have," without elaborating, and realized: "I need to start fighting back."

"The shooting is not going to destroy my life or my children's," she said from the witness stand. "He is not going to win. I am in control."

Once family members and shooting victims finish testifying in the sentencing phase of the court-martial, which could be later on Tuesday, Hasan will have an opportunity to address the jury deciding his fate.

If he chooses to speak, he will not be questioned or interrupted by the prosecution, according to military court procedure.

Hasan, 42, who uses a wheelchair after being paralyzed when shot by police upon his arrest, has spoken very little during the trial, objecting less than a handful of times.

The most Hasan has said was in his opening statement on August 6, when he admitted to being the gunman and said he had switched sides in what he considered to be a U.S. war on Islam.

Hasan opened fire at the U.S. Army base in central Texas, one of the largest in the nation, just weeks before he was to be deployed to Afghanistan.

The mother of shooting victim Sergeant Amy Krueger testified that her daughter, 29, joined the Army after the September 11, 2001, attacks on the World Trade Center.

"She said, 'I'm going in.' I said, 'You can't take on bin Laden all by yourself,'" Jerri Krueger testified. "She said, 'Watch me.'

"The next day she was at the recruiting office," Krueger said.

Krueger was at Fort Hood on November 5, 2009, the day of the shooting, awaiting a second deployment to Afghanistan, her mother said.

Hasan, who has acted as his own defense attorney, has declined to cross-examine any witness in the sentencing phase.

If the jury unanimously recommends death as his punishment, Hasan could face lethal injection, possibly making him the first U.S. soldier to be executed by the U.S. military since 1961.

An American-born Muslim, Hasan told mental health evaluators he wanted to become a martyr and lawyers assisting him said he was actively seeking the death penalty, though Hasan has disputed that claim.

Judge Colonel Tara Osborn has repeatedly reminded Hasan that military-appointed lawyers can represent him but he has declined, choosing instead to represent himself.

Twelve of the dead were active-duty soldiers and one, Michael Cahill, was retired. Of the 31 wounded, 30 were soldiers and one a police officer. Hasan also was charged with shooting at a police officer and missing.

A death sentence would trigger a lengthy process requiring the approval of the Fort Hood commanding general, and the U.S. president, in order for there to be an execution.

If he is sentenced to death, Hasan would become the sixth man on death row at the U.S. Disciplinary Barracks at Fort Leavenworth, Kansas, a maximum security facility for military prisoners.

(Additional reporting by Jana J. Pruet; editing by Paul Thomasch and Matthew Lewis)