Welcome to SMF Jack. We need a little more info on what you did- such as how long you smoked the brisket, what part of the brisket was it (i.e: point, flat or packer) what was your chamber temps, and did you use a water pan.
Fist piece of advice is do not rely on the door thermometer. They are notoriously inaccurate! Replace it with a tru-temp thermometer or use a dual probe digital thermo such as the Maverick.
Maintain your temps between 225°-250°, make sure that you keep the water pan full and plan on a long smoke. And yes, it perfectly all right to use a pan on the shelf below the brisket in order to catch the drippings. Skim off the fat and serve the au jus along side the meat.
Please stop by Roll Call and introduce yourself to the masses here.
Enjoy the smoke!