The Use of Bulk Semiconductor Material for Absorbed-Dose Measurements

IN STUDYING TRANSIENT EFFECTS PRODUCED BY BURSTS OF IONIZING RADIATION, IT IS ESSENTIAL TO KNOW THE AMOUNT OF ABSORBED ENERGY. A CUSTOMARY PROCEDURE IS TO MEASURE THE EXPOSURE DOSE IN ROENTGENS AND TO CONVERT IT TO THE ABSORBED DOSE IN RADS, KNOWING THE ENERGY SPECTRUM OF THE INCIDENT RADIATION AND THE ABSORPTION CONSTANT OF THE MATERIAL. THIS IS A TEDIOUS AND OFTEN INACCURATE PROCEDURE. SINCE MOST OF THE PRACTICAL SITUATIONS INVOLVE SEMICONDUCTOR DEVICES, IT IS ADVANTAGEOUS TO MEASURE THE ABSORBED ENERGY DIRECTLY IN THE PERTINENT SEMICONDUCTOR MATERIAL, GERMANIUM OR SILICON. WE HAVE PERFORMED ENERGY ABSORPTION MEASUREMENTS IN THREE FLASH X-RAY ENVIRONMENTS USING SILICON. THE TECHNIQUE INVOLVES MEASURING THE CHANGE IN CONDUCTIVITY PRODUCED BY PULSED IRRADIATION OF BULK SAMPLES. IT IS ESSENTIAL THAT THE MINORITY CARRIER LIFETIME BE LONG COMPARED TO THE RADIATION PULSE. (IF THE PULSE IS LONG COMPARED TO THE LIFETIME, THIS TECHNIQUE CAN BE EMPLOYED, BUT IT THEN REQUIRES MEASUREMENT OF THE LIFETIME). TO PROVIDE CONFIDENCE IN THE TECHNIQUE, MEASUREMENTS HAVE BEEN PERFORMED USING A VARIETY OF SAMPLE CONDITIONS. THE RESISTIVITY, GEOMETRY, CONTACTS, AND SURFACE CONDITIONS HAVE BEEN VARIED. LEAD SHIELDING ELIMINATED CONTACT EFFECTS. EFFECTS OF AIR IONIZATION, SECONDARY ELECTRON PRODUCTION, SAMPLE HOMOGENEITY, AND X-RAY SCATTERING WERE ALSO DETERMINED. INTERNAL X-RAY ABSORPTION WAS MEASURED. THE RESULTS INDICATE THAT SIMPLE APPLICATION OF THIS PROCEDURE LEADS TO MEASUREMENT OF ABSORBED DOSES WITH A PRECISION OF THE ORDER OF 5 TO 10%.