1. Sensor calibration at ENEA-INMRI
Functional test of a Radon sensor based on a high-resistivity-silicon BJT detector
G.-F. DALLA BETTA1,2, V. TYZHNEVYI1, A. BOSI2, C. ANGELINI3, G. BATIGNANI3, S. BETTARINI3, F. BOSI3, F. FORTI3, M.A. GIORGI3, A. LUSIANI8, F. MORSANI3, E. PAOLONI3, G. RIZZO3, J. WALSH3, R. CIOLINI4, G. CURZIO4, F. D'ERRICO4, A. DEL GRATTA4, L. BIDINELLI5,2, M. BONAIUTI5,2, L. ROVATI5,2, D. SAGUATTI5,2, G. VERZELLESI5,2, L. BOSISIO6, I. RACHEVSKAIA6, M. BOSCARDIN7, G. GIACOMINI7, A. PICCIOTTO7, C. PIEMONTE7, N. ZORZI7, M. CALAMOSCA9, S. PENZO9, F. CARDELLINI10
1University of Trento and INFN; 2RSens srl; 3University of Pisa and INFN; 4University of Pisa; 5University of Modena and Reggio Emilia; 6University of Trieste and INFN; 7FBK Trento; 8Scuola Normale Superiore, Pisa, and INFN;
9ENEA-IRP, Bologna; 10ENEA-INMRI, Casaccia.
Abstract - A battery-powered, wireless Radon sensor has been designed and realized using a BJT, fabricated on a high-resistivity-silicon substrate, as a radiation detector. Radon daughters are collected on the detector surface electrostatically. Thanks to the BJT internal amplification, real-time alpha particle detection is possible using simple readout electronics which records alpha particle arrival time and charge. Tests demonstrated a good sensitivity up to 4.9 cph/(100 Bq m-3) and a 0.05 cph at zero Radon concentration.
The alpha-particle detector is a BJT fabricated on a high-resistivity-silicon substrate. Radon daughters are electrostatically collected on the detector surface. Real-time alpha particle detection is possible using simple readout electronics which records alpha particle arrival time and charge spectrum. Readout is based on a DDC112 (Burr-Brown) chip, providing charge integration and 20-bit A/D conversion coupled to a MSP430F2274 microcontroller (Texas Instruments). The system can keep trace of particle arrival times thanks to an ISL12026 real-time clock (Intersil). Since WSN (Wireless Sensor Network) is of great appeal for wide-area monitoring, environmental tests and also building measurements, the readout system was provided with wireless communication capabilities suitable for star-arranged networks (single-point-to-multi-point). The RF module implements a communication system based on a commercial standard (SimpliciTITM, Texas Instruments), having advantages in terms of a simple interface with the MSP430-family controllers, low development costs and low power consumption.
System description
Sensor optimization
Count rate vs. time (top) and charge spectrum (bottom) obtained during a 18-h Radon exposure test carried out at ENEA-IRP for two sensors with (HV) and without (no HV) electrostatic collection of charged Radon daughters. The Radon concentration (Ref.) was simultaneously measured using the IRP chamber reference detector, calibrated by comparison with SSNTD responses. A sensitivity of 1.8 cph/(100 Bq m-3) was obtained for the HV sensor.
Count rate vs. time (top) and charge spectrum (bottom) obtained during a 100-h Radon exposure test carried out at the University of Pisa for two sensors incorporating a BJT detector with (Al) and without (no Al) an Al layer deposited on top of passivation as a light shield. Both sensors use electrostatic collection. The Radon concentration (Ref.) was simultaneously measured with a reference monitor. A sensitivity of 0.7 and 1.2 cph/(100 Bq m-3) is obtained with and without the Al shield, respectively.
Count rate vs. time obtained during Radon exposure tests carried out at ENEA-INMRI under constant Radon concentration (top) and decaying (bottom) Radon concentration for two sensors having BJT detectors of different geometry. Both sensors use electrostatic collection. The Radon concentration (Ref.) was simultaneously measured with a reference monitor. A sensitivity of cph/(100 Bq m-3) is obtained.
Conclusions
A battery-powered, wireless Radon sensor has been realized using a BJT on high-resistivity Si as a radiation detector, electrostatic collection of charged Radon daughters, and simple IC-based readout electronics. Options that have been taken into consideration during the system optimization phase include (i) presence/absence of the electrostatic collection, (ii) presence/absence of a light shield on top of the BJT detector, (iii) different geometries of the BJT detector. Optimized sensors yielded a sensitivity of 4.9 cph/(100 Bq m-3), 0.05 cph at zero Radon concentration, and Radon daughter alpha spectroscopy capability.
Sensor calibration at ENEA-INMRI
Radon concentration provided by 22 calibrated sensors during a 48-h Radon exposure test carried out at ENEA-INMRI under a constant Radon concentration of 4170 Bq m-3, as measured by a reference sensor. The average calibration constant (sensitivity) is 4.9 cph/(100 Bq m-3).
Radon concentration provided by 4 different sensors during a 19-h test at nominally zero Radon concentration carried out at ENEA-INMRI. The Radon concentration simultaneously read by a reference sensor is included (Ref.). The average count rate is 0.05 cph.