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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Electronic radon monitoring with the CMOS System-on-Chip AlphaRad

Abstract

The development of the integrated circuit AlphaRad as a new System-on-Chip for detection of α-particles has already been reported. This paper deals with electronic monitoring of atmospheric radon, which is one of the promising applications of the chip. The future electronic radon monitor (ERM) is designed to be compact, inexpensive, operating at low voltage and fully stand-alone. We present here the complete electronic board of the future ERM: it is made of three independent AlphaRad chips running in parallel, mounted on a small printed-circuit board which includes a numeric block for data treatment based on a Xilinx programmable gate array. The maximal counting rate of the AlphaRad chip has been pushed to at least 3×106  α-particles   cm−2. The complete system for detection of the solid aerosols will be published separately, and this paper will focus on the electronic board alone. Already 20 times faster than our first measurement with a CMOS pixel sensor, the system was tested at low and high activities, showing an excellent linearity for 222Rn levels up to 80   kBq   m−3.

Introduction

Health concerns about public exposure to radon emanations in houses and public buildings are ever growing [1] and existing monitoring systems are still not satisfactory. Expensive electronic devices are available only to professionals and passive integrating films like track detectors (SNTD), relying on chemical pre- and/or post-processing, are unable to deliver real-time information, as for instance the efficiency of classroom ventilation. The latest studies for lung cancer [1] suggest that even the level 400   Bq   m−3 should not be considered as harmless, and the health authorities in several countries are ready to define 200   Bq   m−3 as the new maximal value for public safety. Obviously, a sensitive, real-time and inexpensive system would be useful to permit an efficient survey of radon exposure for the general public. Our first electronic measurement of radon activity [2] with a CMOS pixel sensor was published in 2004. The present paper is the direct continuation of this work.

All along this paper, we refer equivalently to a concentration ("radon level") or an activity ("radon activity concentration in air") both expressed in Bq   m−3.

Section snippets

Description of the device

Following our first measurement with a pixel sensor [2], the development of a dedicated System-on-Chip for fast counting of α-particles was achieved in Strasbourg [3], with applications in sight for neutron dosimetry and radon monitoring. We present here a complete prototype of the electronic board (EB) that will be inserted in a portable and stand-alone system for electronic radon monitoring (referred to as ERM).

Purely electronic measurements of the AlphaRad chip have been published elsewhere

Experimental procedure

A first series of tests of the three-chip EB took place in the radon chamber of the RaMSeS group at the IPHC. This radon chamber is a cylindrical aluminum container of 0.224   m3 operated at atmospheric pressure [2]. Gaseous 222Rn has to be left in contact with a standard 226Ra source for at least 10 days to reach secular equilibrium which always gives the same activity (per unit volume) of about 1100   Bq   m−3 as measured by our ionization chamber (IC) monitor. The total volume of radon is injected at

High-activity measurements

The radon chamber available in Strasbourg has three limitations, a single value of radon activity, long counting times and a 2-week long delay to regenerate the radium source between two runs. To check the linearity over a larger set of activities, we have tested our device in a more powerful test bench.

Conclusions and perspectives

A new electronic radon monitoring (ERM) device is under development. The electronic part of the system, based on the CMOS sensor AlphaRad, was tested successfully in radon atmospheres in the range 1–80   kBq   m−3. Even in a fully passive counting mode, the AlphaRad system proved that it can compete with existing electronic monitors. The system's response is highly linear, and reasonably precise measurements are available within a delay of only 10   min in the atmosphere to be monitored. The future

Acknowledgments

We are grateful to the LPMA/IRSN staff in Saclay, and especially to Sylvain Bondiguel for his kind help on the BACCARA facility. Our thanks also go to Pierre-Yves Meslin, for valuable discussions on the aerosol problem, and to Arthur Pape for his rigorous comments on the paper.

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