M.S. Thesis Presentation by Erik L. Swanberg
Tuesday, January 11, 2000

(Dr. John Valentine, advisor)

"Improving the Performance of a Personnel Monitor based on Long Range Alpha Detection"


At Department of Energy facilities where loose alpha emitting radioisotopes are present, personnel are monitored as they exit to detect any alpha contaminants that may be on their clothing or bodies.  Since alpha particles have a range of a few centimeters in air, and since traditional alpha detectors require the alpha particle itself to enter the detector, a detector must get within a few centimeters of every part of the personís body.  This is typically done by hand frisking, a time consuming and inconsistent process.

Long Range Alpha Detection (LRAD) detects the ions created by the passage of an alpha particle through air.  By using a fan to draw these ions into a detector, the detector can be as much as several meters away.  A personnel monitor based on LRAD was constructed at Los Alamos National Laboratory.  This first generation prototype achieved a detection limit (DL) of 3000 decays per minute (dpm).  The second generation prototype reached a DL of 1500 dpm by using better airflow and smaller detector segments.  This thesis focuses on further reducing the DL of the second generation prototype by eliminating or reducing noise.  This was accomplished by isolating components of the system to identify, and subsequently attempt to eliminate, noise sources.  Potential noise sources included vibrations, radon background, air leaks, electromagnetic interference, and leakage current.  Based on these experiments, it was determined that leakage current was the largest source of noise.  Through significant modifications to the LRAD design these leakage currents were nearly eliminated such that the DL was improved to better than 400 dpm.