BS IEC 61577-2:2000 pdf download-Radiation protection instrumentation — Radon and radon decay product measuring instruments.
5.1 Radiation detection unit The geometrical efficiency of the detection unit shall be as high as possible. Contamination of the detector with naturally occurring alpha emitting radionuclides (mainly long-lived daughter products of radon) may result in higher background. Precautions shall be taken to protect the detector surface from airborne contamination when the instrument is not in use. 5.2 Air-pumping device The air-pumping circuit shall provide a total air flow adequate for the measurement method. The air pump shall be capable of withstanding the variations of pressure induced by operating conditions, sampling time, filter types, atmospheric dust mass contributing to blockage, etc. Hoses and connections shall be sufficiently tight to maintain a stable flow rate and prevent leaks. Where an air pump is an integral part of the assembly it is recommended that the air pump be capable of continuous operation between scheduled maintenance operations. The flow rate should be stabilized or measured. 5.3 Aerosol retention device Most instruments use a filter to prevent radon decay products from entering the detector active volume. The manufacturer shall state the type of filter. The filter should not retain radon. It is recommended that the device be designed for use with HEPA (High Efficiency Particulate Aerosol) filter. 5.4 Air-drying device If the detector efficiency is dependent on the humidity of the sampled air, the instrument may be equipped with an air drying device (for example chemical drying agents or electrically operated Peltier-elements). Care should be taken to choose a drying agent which does not absorb radon. When chemical drying agents are used, the life-time of the drying agent shall be clearly stated.
6.4 Statistical fluctuation When using ionising radiation for testing, the random nature of radioactivity is an important part of the instrument indication. A sufficient number of measurements, but at least ten, should be made to guarantee that the average value of these indications is sufficiently precise to assure that a certain characteristic meets the preset requirements. The intervals between such indications should be long enough to guarantee that the indications are statistically independent.