Development of an active detector for radon detection in air

Tese de mestrado integrado, Engenharia Física, Universidade de Lisboa, Faculdade de Ciências, 2019In this thesis, we developed and tested an active device for radon in air detection. It was intended to operate in pulse counting mode for real-time measurements without the need of an active gas pumping system. The prototype presented is relatively simple, low-cost and reproducible, made of inexpensive and easy to acquire components. We used as a sensor the commercial low-cost Si-PIN planar photodiode SLCD-61N5 with an area of 9.67x9.67 mm2, sensitive to alpha particles. An Arduino UNO was attached to the electrical circuit of the device to count the logic pulses and connected to a computer to send and save the data. The initial prototype is comprised of a commercial light-tight metal box with 22.2x14.5x5.5 cm3 with lateral perforations where the gas enters by natural diffusion. Electrical pulse signals corresponding to the hit of the alpha particles in the sensor, distinguishable from noise, were observed in a test using a 241Am source. Indeed , this showed an excellent response to alpha particles detection. We performed three different experiments to estimate the order of magnitude of radon concentration using passive alpha-trackCR-39, the commercial monitor Ramon2.2and a sodium iodide detector employing rocks sample from Nisa uranium mine as radon source. A new alpha-track counting technique, using the free CellProfiler software, was further applied and tested to replace the visual counting method during theCR-39 test. From the comparison between the two techniques, deviations of 1-14% were obtained, which are quite satisfactory, demonstrating the feasibility of use for rapid and cheap analysis of alpha-track detectors. We also performed simultaneous measurements in radon atmosphere to test the prototype comparing its performance with a commercial Geiger given as reference. The total number of counts was recorded in 10 minutes intervals. We observed an increase of the sensitivity response of the prototype for a higher reverse voltage applied to the photodiode. We carried out the same test for a month, keeping a fixed voltage, in order to study the prototype’s stability for long-term measurements. A positive high correlation was obtained. As a future work on it, our study suggests the possibility of making a fully portable detector powered only by batteries and data recording on a memory card attached to the Arduino. This will allow operating in areas where the electricity supply is not stable. Furthermore, despite the ability of the prototype to detect the presence of radon in air calibration study are necessary in order to obtain exact concentrations values