CALGARY -- An Alberta woman who drugged and killed her nine-year-old daughter has been sentenced to life in prison with no chance at parole for 18 years.

Laura Coward faced an automatic life sentence after she pleaded guilty in a Calgary courtroom last month to the second-degree murder of Amber Lucius.

But it was up to Court of Queen's Bench Scott Brooker to decide how long she'd have to wait before being able to seek parole.

The judge said it was "the ultimate betrayal for Ms. Coward to kill her own daughter."

Coward deserved credit for the guilty plea and her apparent remorse, Brooker said.

But he added that the killing was vile, random and senseless and appeared to have been revenge against Coward's former husband.

Court documents described a bitter divorce between Coward and Lucius. A custody tug-of-war over their daughter had continued up until the girl was found dead.

Amber was reported missing in August 2014 and her mother was arrested two days later near Sundre in west-central Alberta. She was standing outside a burned truck in which her daughter's body was discovered.

Court earlier heard that Coward had had Amber for the weekend and, on a trip to a remote area, gave her a toxic but non-lethal dose of a prescription sleeping medication. She mistakenly thought Amber was dead and filled the truck -- while Amber was unconscious inside -- with paper and plastic totes before setting it on fire with a propane torch and closing the door.

A police officer who came upon the burned vehicle saw a handwritten note on the outside of the driver's door that read: "Help me. It was an accident. Locked keys in."

An autopsy suggested the girl died of a combination of hypothermia, smoke inhalation and carbon monoxide toxicity.

At her sentencing hearing last week, Coward begged the girl's father and her own family and friends to forgive her.

"I want to say to the father, my family, friends that I'm responsible for my choices and I beg for their forgiveness and I plead for your mercy," she told Brooker.

Her former husband, Duane Lucius, read a victim impact statement at that hearing saying he would never forgive Coward for the pain she has caused.

"I feel upset (about) how someone could do something like this to a little, innocent girl with a whole life ahead of her."

The Crown was asking for 20 years in custody before Coward could apply for parole, while the defence suggested 10 to 12 years would be sufficient.