Absolute measurement of $^{60}Co$ radioactivity with anticoincidence counting method

Abstract

Coincidence counting method is, in general, widely used in absolute measurement of activity. However, this method cannot be applied to the nuclides which have relatively long delay time (micro-second order) between the emission of $\beta$-particle and $\gamma$-ray, and also the correction for accidental coincidence counting is required. Anticoincidence counting method has been developed to overcome the problems. However, dead time correction for the anticoincidence channel is not so well understood and needs to be solved. If the live times of $\beta-,\; \gamma-$, and anticoincidence channel are measured, above problem can be resolved. The live time measurement is performed by combining $\beta$-and $\gamma$-channel signals to produce a common live time clock channel. In this experiment, extending deadtime generator, standard-frequency divider, and anticoincidence logic circuit are fabricated for the anticoincidence counting system which consists of two NaI(Tl) scintillation detectors, $4\pi$ pressurized proportional counter, and counting equipments. Low energy threshold level variation method is used to adjust the $\beta$-detector efficiency. The sample used in this experiment is $^{60}Co$ solution source which was calibrated by K-SRI $4\pi \beta-\gamma$ coincidence counting system. The pressure of counting gas is at 1.31 MPa and optimum voltage was set to 5,500 V from plateau curve. Disintegration rate of $312.43 \pm 1.06$ KBq/g was obtained on Oct.-08-84 (Oh UT).