Abstract
An apparatus for low-level alpha and beta-activity measurements is described. Semiconductor detectors mounted in holders made of carefully selected materials are used for particle registration. The two measurement detectors between which the investigated sample is placed, can work in parallel or in anticoincidence. The anticoincidence shield of the measurement counter is provided by two thick Si(Li) detectors. The passive shield and the detector chamber are make of mercury and plexi, respectively. The samples are in the form of discs of up to 20 mm in diameter and up to 1 mm in thickness. The materials under investigation can be placed in a small container of the given dimensions. A very low background rate was achieved. It was of the order of 0.017 cpm/cm 2 at an 80 keV discriminator level and 0.5 mm detector thickness. The beta-particle counting efficiency was 65% for 50 mg KCl and for a 4π-measurement geometry. The low and constant (over a long period of time) background and high registration efficiency enabled measurements to be made of an activity of 40K at the level of 0.11 pCi in 1000 min and within 10% standard deviation.
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