| Activity of radioactive source is a fundamental physicalquantity, measurement of which plays very important role in experimentalnuclear physics and nuclear technology applications. There are twomethods for measurement of Activity of radioactive source: relative andabsolute measurements. The method of small solid angle is one kind ofabsolute measuring methods for Activity of radioactive source, applicablenot only toβ-source, but also to thinα-source, obviously to disc source.Takeβ-source as an example, if the isotropic radioactive source radiatesβ-rays, the detector only can detectβ-rays in the solid angleΩin thesmall solid angle device. Therefore, it is required that relative small anglerevision factor -the geometry efficiency is introduced. The geometryefficiency is defined by the rate of solid angle value of detector tosource and 4π, when a point source is considered. While the geometryefficiency is equal to mean value of geometry efficiency of all pointsource of this plane source if a plane source is involved, with itscalculation being complex. It is required in experiment that the detectormust be parallel to the source one, and that the axis of detector must bethe same as the axis of source. In the actual operation, however, it is possible due to the error of operation that two detector and source planesare not parallel each other, and that the axis of detector and the axis ofsource don't superpose each other, leading to a small angle. It isinstructive significance for experiment operation that geometry efficiencyof two kinds of situations as indicated above has been investigated.Monte Carlo method, called random simulation method, may be used forcalculation of geometry efficiency in experimental nuclear physics andnuclear technology applications. In this paper, geometry efficiency of discdetector to parallel disc source in the situation that C≠0 (the distancefrom axis of detector and axis of source is called as C) is calculated bybasic Monte Carlo method and ellipse observation factor weight MonteCarlo method, respectively, and then compared with that of disc detectorto disc source in the case that C=0. We investigate the influence of thedistance C from two axes on the geometry efficiency. Geometryefficiency of disc detector to disc source in the situation thatα≠0 (theangle from axis of detector and axis of source is called asα) iscalculated by basic Monte Carlo method and ellipse observation factorweight Monte Carlo method, respectively, and then compared with that ofdisc detector to disc source in the case thatα=0. We investigate theinfluence of the angleαfrom two axes on the geometry efficiency.According to the discussion results, the experimental operationrequirement has been proposed. A few main results have been obtained as below:(1) Relative error of (?) of the geometry efficiency rate between thenon-same-axis and the same-axis and 1, RSD(x) of the geometryefficiency rate between the non-same-axis and the same-axis increasewith C andαincreasing when the radius of the resource is fixed.(2) The influence of the distance C orαon geometry efficiency for alarge radius resource is bigger than for a small one.(3) Results from ellipse observation factor weight Monte Carlo methodare advantageous to those from basic Monte Carlo method.(4) When C＜0.1Rs andα＜10°, Relative error of (?) and 1 Can becontrolled by 0.2%, RSD(x) can be controlled by 0.6% in this paper.
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