CALCULATION OF THE LIGHT COLLECTION EFFICIENCY IN SCINTILLATING CRYSTALS
Institute for Single Crystals, Alkali
Halide Crystal Department, Lenin Ave. 60, 310001 Kharkov, Ukraine
E-mail: wp@isc.kharkov.com
Keywords: scintillation detectors, light yield, Monte-Carlo
A program which allows to calculate light collection in transparent scintillating crystals was developed. Theoretical analysis results in necessity to solve field equations with complicated boundary conditions. These can be fulfilled only in rare cases. Therefore Monte-Carlo approach is more preferable. In the basis of this method lies tracing of various beams trajectories from the point in which ionizing particle had been absorbed until detecting on the photoreceiver input window or absorption within crystal scope. Only simple forms were considered early: rectangular parallelepipeds and cylinders possessing absolutely smooth or fully diffuse reflecting surface.
By means of the new algorithm various shapes which constitute a combination of arbitrary joined planes also became available. Multiple-window detectors, medical tomographs for example are in this circle too. Scintillation light interaction on a rough surface is considered within the framework of a new treatment. A crystal surface is imagined as a smooth plane some part of which is covered by cylindrical scratches of arbitrary direction and depth. Typical size of the irregularities is taken to be much more greater then light wavelength, so geometric optics laws are valid. The scintillation light is treated as non-polarized. To determine surface roughness some statistical parameters are introduced: average depth of the scratches and relative part of even area with regard to the crystallographic axes orientation. The algorithm takes into account irregular transparency of the crystal which arises from coloration centers appearance after irradiating in some materials.
As a result one can obtain light collection efficiency, pulse shape determined by light expanding within crystal scope and illumination distribution on a photoreceiver input window by different schemes of irradiating. Correctness of the algorithm has been justified by comparison experimental data with calculated ones in cases when measurement of these characteristics is still available. A scheme to improve scintillation parameters of extended cylindrical crystals had been predicted and then justified by experimental implementation. The program turns to be useful for the detectors to be designed.