The RM-2 Radon detector is designed for in situ rapid and accurate analysis of radon (²²²Rn) in soil gas and therefore is well applicable to radon risk mapping and assessment of radon content in foundation soils. Due to its unique characteristics it can be used as well for various geophysical applications such as uranium exploration, assessment of radon at uranium mines and uranium tailings, geochemical mapping and identification of lithological boundaries, locating of faults and fracture zones, radon measurement at oil-fields, and predictions of earthquake and volcanic eruptions.

Uranium and thorium generate by their decay radioactive gases radon (²²²Rn, T_1/2 = 3.82 days) and thoron (²²⁰Rn, T_1/2 = 54.5 seconds), both sources of alpha radiation, which differ by their half-lives T_1/2. The usual radon and thoron activity concentrations in soil-gas range from 10 to 100 kBq/m³, while some anomalies may attain values up to several thousands of kBq/m³. This contrasts to RM-2 high detection resolution 0.1kBq/m³. Radon migrates by diffusion and convection at distances up to tens of metres in the rock environment.

Radon detector RM-2 consists of a set of fifteen ionization chambers IK-250, the reader ERM-3 and a soil-gas sampling set. Samples of soil-gas are collected through hollow steel rods by means of a plastic large-volume syringe and transferred to evacuated ionization chambers. In order to eliminate thoron, ionization chambers with soil-gas samples are measured with specific time delay of 15 minutes after sampling. Alpha radiation of radon in soil-gas sample give rise to ionization current in the ionization chamber, which is measured within 120 s. The pre-programmed, fully automatic reader ERM-3, with installed calibration constant, displays the concentration of radon in units of volume activity (kBq/m³). Estimates of thoron are possible. Main features of the RM-2 radon detector are:

• Modern construction using up-to-date electronics
• PC control via RS232 interface
• Memory and adjustable calibration constant
• Digital data storage into the memory
• Cyclic radon recording
• Easy front panel instrument control
• 120 s pre-programmed exposure time
• Fully automatic reader operation
• 16 digit LC display
• Calibration at radon chamber tied to international standards
• Extremely sensitive - detection resolution 0.1 kBq/m³
• Low power consumption
• Lightweight and rugged
• Easy detector cleaning and maintenance

Cylindrical ionization chambers IK-250 with a detection volume of 250 cm³ are steel pressed pieces with sheet metal thickness of 0.8 mm. The brass axial collection electrode passes through a teflon insulator serving as high performance electrode voltage separator. The described set is fastened in an insulating bushing placed in the upper part of the ionization chamber. A three-way inlet valve provides input of the soil-gas sample into the ionization chamber. The input hose can be equipped with a filter eliminating input of radon decay products into the ionization chamber. The radon detection system operates in two selectable modes; (i) measurement starting 15 minutes after the transfer of the soil-gas sample into the ionization chamber, and (ii) measurement conducted after time delay of 3 hours making the use of alpha radiation of radon and its short-living decay products in radioactive equilibrium. Exposure time 120 s is applied per one sample. Applied voltage between electrodes of the ionization chamber is 40 V (DC). The detector operates in a non-saturated sector of the volt-ampere characteristic of the ionization chamber (interval of proportionality), approximately at 85 % of the level of the saturated current. The ionization current is amplified, electronically and statistically processed, and processed resultant value is subsequently displayed and stored in the memory. The reader ERM-3 enables the current signal recording each 2 s (cyclic mode), that is useful for specific measurements of radon and thoron mixture and other investigations. A standard serial communication interface RS232 serves for connecting with a PC, data transfer, entering the calibration constant and other operating instructions. Basic measuring functions are set-up from the ERM-3 control panel manually.