Abstract
A coincidence gamma spectrometer has been used to measure the 134Cs activity in the presence of intense interference from 137Cs and natural radioactivity in environmental sample. The coincidence spectrometer, with a largeBGO scintillator for gating purposes and a HPGe detector for taking the coincident spectrum, is designed to enhance the coincidence photopeak countract of 134Cs and suppress the single photon countrate of 137Cs as well asother natural radionuclidence while the interferring photopeaks is greatly suppressed. Absolute counting efficiency of the coincidence spectrometer is calibrated for environmental sample with activity ratio of A(134Cs)/A(137Cs) down to 5x10-5. The minimum detectable concentration of 134Cs inbeach sand is 0.233Bq/kg. Other applications and the flexibility of the spectroscopic system as well as nuclear instrumentation using HPGe and BGO detectors are discussed.本論文主要是以高純度鍺 (HPGe) 偵檢器與鍺酸鉍 (BGO)偵檢器組合的相符偵測系統, 偵測極微量的銫-134核種. 利用銫-134之605kev與796kev相符加馬射線, 將BGO 偵檢器的能窗設在796kev, 調整時間電路, 使時間電路訊號與放大器之能譜訊號同步進入類比數值轉換器, 以相符模式經由多頻道分析儀記錄相符能譜. 本相符偵測系統對於核爆落塵 (銫-134與銫-137之活性比為10-5) 與環測樣品a銫-134與銫-137之活性比為0.5), 均有優異的偵測能力, 特別是環測樣品含有極低微銫-134的時候, 可以有效壓制 Bi-214 之609kev與Ac-228之795kev之自然背景, 減低銫-134之605kev與796kev譜線干擾. 對於複雜核種存在的樣品, 亦能有效剔除非銫-134核種的譜線, 使得微量銫-134易於偵測, 本相符系統岸砂銫-134之最低可測濃度(MDC)為0.233Bq/kg.本論文對HPGe偵檢器探討偵測大體積環測水樣的幾何最適化的研究, 亦有詳細的探討.