Development of a system for online measurement of ~(220)Rn (Thoron) by gamma ray spectrometry method

摘要

Regulatory requirements for thorium fuel cycle demand measurement of ~(220)Rn (Thoron) in theworkplace and stack exhaust towards evaluation of internal exposures due to inhalation of the progeny ofThoron gas. ~(220)Rn occurs in the Thorium series in nature and in the thorium fuel cycle, the gas is beingcontinuously generated by the decay of ~(224)Ra that occurs in the decay chain of ~(228)Th. This manuscriptpresents the discussions regarding feasibility of a simple method based gamma spectrometry using gammascintillation detectors to estimate the air-borne activity concentration of ~(220)Rn in the workplace environsand in the gaseous effluent exhaust points. Air borne thoron is estimated by directly measuring the ~(208)Tl andother gamma emitters present in the chamber air after correcting for decay and build up of thoron daughtersin sampling chamber both due to air flow and radioactive decay phenomena. Scintillator pulse heightcontributions from wall deposited thoron daughter particulate activity at various times after start of air flowthrough the chamber are also considered in the simulation to aid in detector background correction. Pulseheight distribution of simulated gamma spectrum corresponding to uniform distribution of ~(220)Rn activityinside the air-flow chamber and calculated values of detector sensitivity and efficiency are presented in thispaper. Scintillation materials NaI(TL), LaBr3 and BGO were tested for full energy peak area responseusing Monte Carlo simulation techniques. The system sensitivity works out to 0.08 cps/TWL at thorondaughter product equilibrium, when one uses a 30 liter airflow chamber volume system without an inletfilter and 4”x6” NaI (TL) crystal. For typical composite gamma photon energy spectrum from thorondaughters, LaBr3 and BGO are found to be 4 - 6 times more sensitive than NaI (TL), given the same activescintillator volumes. The airflow chamber can be used either with an inlet filter or not depending onwhether one is interested in measuring the airborne concentration of thoron gas or ambient thoron workinglevel respectively. Thus the system is adaptable for use in occupational work milieus, for onlinemeasurement of thoron and thoron working level (TWL) as well.