Characterization of a personal exposure dosimeter
Résumé
To be able to perform occupational exposure assessment in an airport area with personal dosimeters, two models of personal dosimeters were indeep characterized. Frequency response, linearity, cross-over, reproducibility, isotropy were measured, and results are presented in view of practical considerations for daily use. Many radiocommunication systems are used within Paris airport utilities (ADP) for occupational use or service to passengers. ADP would assess the exposure of personnel and passengers. A cartography of sources has been achieved, showing some places with electric field levels above the ambient level. In view to establish a methodology to systematically characterize personnel exposure, two models of dosimeter have been evaluated. Detailed characterization of 2 models of dosimeter: EME SPY 121 and EME SPY 140: 1. Correctness of the field measurement at any frequency, and rejection in other frequency bands. 2. Response in the frequency domain. 3. Assessment of exposure to 2 or 3 simultaneous frequencies of a same source or of different sources. EME SPY dosimeters are selective isotropic and wearable dosimeters, recording electric field of 12 different sources for the EME SPY 121 and 14 sources for the EME SPY 140. Sources are identified by their frequency band and include: modulation frequency radio broadcasting (FM - 87.5-107.5 MHz), TV3 now abandoned since the all digital TV (174-223 MHz), TETRA (cellular phone for emergency and safety services - 380-400 MHz), TV4&5 (470-830 MHz), GSMtx where tx means the phone terminal, the device with which we can call or be called, and GSM is the GSM 900 in the 900 MHz frequency band (880-915 MHz), GSMrx, where rx means the base station antennas (925-960 MHz), the DCSrx and tx, where DCS is the GSM1800 system in the 1800 MHz frequency band (1710-1785 MHz for the terminal devices tx, 1805-1880 MHz for the antennas rx), the wireless phone, domestic or occupational DECT (1880-1900 MHz), the 3G data communication system also called UMTS (1920-1980 MHz for tx, 2110-2170 MHz for rx), the WiFi "2G", where 2G does not mean 2nd generation but the 2 GHz frequency band, and for the EME SPY 140, two more recent services are WiMax (3400-3800 MHz) and WiFi 5G (around 5 GHz: 5150-5850 MHz). Measurements were performed in a GAM T20 anechoic chamber, with generators Marconi 2023, Rohde & Schwartz SLM03 and SMBV100A. Ophir 5303069 and ifi CMCE300 amplifiers were used; antennas were Electro-metrics EM6917 and Schwarzbeck BBHA 9120. Electric field level was calibrated with a calibrated EMR300 fieldmeter and another one NBM550. For mobile phone emission, a dual band Motorola phone was used, and for DECT emission, a commercial DECT phone. Only one orientation of the dosimeters in front of the emitting antenna was achieved, and the two polarisations were tested. Most measurements were correct within less than 3 dB. GSM modulation was found to correctly proportionate the peak to rms value of a factor of SQRT(8): dosimeters indicate the peak value, as the fieldmeter indicates the rms value. With the EME 121, values were found higher for GSM1800 antennas and 3G handsets. With EME 140, values were found lower for WiMax and WiFi 5G. This can be explained because this measurement is made in one orientation only, and the calibration of the dosimeters is achieved for an average level on 360° for a better isotropy. No crossover was observed. The frequency response of dosimeters was good, with a slight shift to upper frequencies for FM with the EME 140. Cumulative exosure to 2 different frequencies of a same source lead to lower values of FM and TV4&5 with the dosimeters. When an antenna and a mobile handset emit at the same time, there results in higher values by the EME 140, as the emission of 2 handsets in GSM 1800 was lower. Wireless DECT emissions are either higher either lower. When 3 devices emit at the same time as for example when simultaneously using a GSM handset, a wireless phone and a WiFi tablet, which is not a realistic situation, there is an higher value of the exposure. Alltogether, these results indicate that the measurements by the dosimeter are balanced, and that the actual exposure to the most frequent sources is lower than indicated by the dosimeters. Then dosimeters can reliably be used to assess conservative personal exposure of employees or passengers: the most useful information will be on duration of exposure to different sources (exposure distribution or histogram), and the electric field levels can be used as indicative and comparative among different places in the airport, or at different times of the day, the week, or the year. A recalibration is necessary at least every two years.