This prototype of the detector consists of stainless steel box (with volume of 600 l), which is divided into four subdetectors of different shapes and volumes (2 x hemispheres with volume of 50 l each; 2x rectangular box with volume of 100 l each). All subdetectors are made of wire net. Electrostatic field is formed between wire net and Si PIN diode. To reach high efficiency of the Rn decay progenies collection, Si PIN diode should be on the top of the electrical feed-through and the supporting frame. High voltage is tuneable in the range from 5 to 12 kV. The stainless steel box is airtight; air is flushed through two inlets.