It seemed odd to Dr. Itzhak Brook that someone should be knocking at the back door of his Kensington home at 2 in the morning. So, he got up from his bed and looked outside from an upstairs window.

He saw nothing.

Seconds later, he heard the sound of glass breaking. It seemed to come from the kitchen. Brook pulled a box from under his bed and took out the B-B gun he kept inside.

"I checked the children to see if they were all right. Then I went down to the kitchen, I saw the figure of a man. He had broken the glass in the door," and was headed into the kitchen," Brook said. "I switched on the light and this blinded him."

"I stuck up the B-B gun and told him not to move or I'd shoot him . . . Of course I couldn't really shoot because the gun wasn't loaded," Brook said.

The ruse worked. The doctor then called the police, who arrived and arrested 20-year-old Daniel John DeLacey and charged him with burglary.

Brook, a pediatrician from Israel now practicing in Children's Hospital, said yesterday, "I must admit I wasn't scared I just did it in instinctively . . . It goes to show that citizens can help police and that if you act firm, you can catch criminals."

The B-B gun, Brook said, is something he bought to practice his sharp-shooting. He was an officer in the Israeli Army and served in both the six-day war of 1967 and the 1973 Arab-Israeli was, he said.

"I did take prisoners of war before," Brooke said, "but never in my own home."

DeLacey, of 11915 Parklawn Dr., Rockville, was freed on a personal recognizance shortly after his arrest.