Radiation Measurements 40 (2005) 275–278www.elsevier.com/locate/radmeas

MCNPXsimulation for�-particle detection byCMOSactive pixelsensor�

A. Nourreddinea,∗, A. Nachaba, D. Hussonb, S. HigueretaaInstitut de Recherches Subatomiques, 23 rue du Loess BP 28 F-67037, Strasbourg Cedex 2, France

bLaboratoire d’Electronique et de Physique des Systèmes Instrumentaux, 23 rue du Loess BP 28 F-67037, Strasbourg Cedex 2, France

Received 27 August 2004; accepted 16 February 2005

Abstract

This work assesses the performance of the CMOS electronic system for�-particle detection from sources and in a222Rnatmosphere and compares the experimental results with those obtained from conventional methods. For the measurements,a special acquisition system with a digital mother board and a daughter board supporting the chip was developed. Firstmeasurements were reproduced with a simulation using the Monte Carlo N-Particles MCNPX code.© 2005 Elsevier Ltd. All rights reserved.

Keywords:Solid-state detectors; CMOS; MCNPX code;222Rn; � spectroscopy

1. Introduction

Themain objective of this work carried out collectively bythe Radioprotection et Mesures Environnementales (RaM-sEs) Group of the Institut de Recherches Subatomiques(IReS) and the Laboratoire d’Electronique et de Physiquedes Systèmes Instrumentaux (LEPSI) was to develop aprototype electronic dosimeter based on a Complementarymetal oxide semiconductor (CMOS) sensor for the contin-uous measurement of222Rn activity in the atmosphere. Inthis paper the first test results of the CMOS response to a241Am source and to a222Rn atmosphere are presented. Averification of the222Rn results was carried out with ionchambers and track detectors (Durrani and Bull, 1987) andcompared with a simulation calculation.

� Presented at the 22nd International Conference on NuclearTracks in Solids Barcelona, August 23–27, 2004.

∗ Corresponding author. Tel.: +33388106576;fax: +33388106508.

E-mail address:abdelmjid.nourreddine@ires.in2p3.fr(A. Nourreddine).

1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2005.02.012

2. Description of the CMOS setup

The CMOS active pixel sensor (APS) is an integrated cir-cuit device (Turchetta et al., 2001), with four independentmatrices (Fig. 1), each one comprising 64× 64 pixels witheach pixel 20×20�m2 in size. This detector was manufac-tured using a technology that provides an epitaxial Si layeron request. The chemically pure Si layer, of high crystallinequality located just below the oxide, is the true detectingzone (Deptuch, 2002).For a fully dedicated system, we have developed a ver-

satile new acquisition chain described briefly here. Thesystem consists of a main board equipped with two ADCs(8 bits) and programmable logic circuits for control of theinput/output signals. A driver for distant signals (>2m) isalso available. This main board is connected directly (or viaan intermediate card for long-distance signals) to the daugh-ter board of the CMOS chip. The main function of thislocally fabricated printed circuit board is to bring in all thepolarization signals. The sensor delivers four analog outputs(one for each matrix). The present system allows acquisi-tion of two matrices at the same time. After conversion, the

276 A. Nourreddine et al. / Radiation Measurements 40 (2005) 275–278

Fig. 1. Left: CMOS sensor with its four matrices. Right: the graphic acquisition interface for two CMOS matrices. The two-dimensionalprojection of a typical event registered in the CMOS detector is shown.

main board sends the digital signals to a PC via a digitalacquisition card (National Instruments PCI-6534) with 32inputs and a maximal rate of 80 MB/s. For low-rate ex-periments, the acquisition frequency was set to 615 kHzper pixel, which is the lowest rate to still avoid satu-ration due to leakage currents in the CMOS chip. Thewhole system is controlled on-line and displayed underLabView.

3. Results and discussion

3.1. Response of the CMOS sensor

To study the response of our detector, we exposed it to241Am point sources at a distance of 1.5 cm. They were elec-troplated onto the extremity of cylindrical Ta rods 1mm indiameter. For one source and irradiation times of 5–45min,we counted�-particles detected by the CMOS sensor by tak-ing for each duration the average number of events on fivedata files.Fig. 2shows the number of counts registered as afunction of exposition time. One can see that the detector re-sponse is linear.Fig. 3gives the number of counts registeredwith the sensor for five different241Am point sources pre-pared in our laboratory. These activities were measured bya Si(Li) detector system calibrated by an�-particle sourcecontaining238Pu,241Am and244Cm.

3.2. MCNPX test

The MCNP code has been used successfully for mak-ing self-absorption corrections to�-ray spectrometricresults used and has given evidence already as for theresults (Nachab et al., 2004a). It is a general-purpose,

Fig. 2. CMOS counts as a function of exposition time for a241Amsource.

Fig. 3. CMOS response to different241Am sources.

continuous-energy, generalized geometry, time-dependent,coupled neutron/photon/electron Monte Carlo transportcode (Briesmeister, 1997). It can be used in several trans-

A. Nourreddine et al. / Radiation Measurements 40 (2005) 275–278 277

Fig. 4. Comparison of the MCNPX simulation with the measuredspectrum for a238Pu, 241Am and244Cm �-emitting source.

port modes: neutrons only, photons only, electrons only orany combination of the three. In this work we have used anextended new version of MCNP called MCNPX that canfollow charged particles as well as�-rays.We assume a small irradiation chamber under vacuum

with a circular flat �-particle source containing238Pu,241Am and 244Cm. Each isotope emits three�-particlegroups but for the simulation, only the two strongest groupswere taken into account because they were the only onesvisible during the measurement. After a counting time, weobtain both peaks for each of the three isotopes. From this,a calibration and a determination of the diode resolution aredone. This latter parameter is necessary for the MCNPX in-put file and it plays a prominent role in the characterizationof the peaks resulting from the simulation.In Fig. 4, we observe small differences between simu-

lated andmeasured peak heights. This difference appears be-cause the detector and the source characteristics were poorlyknown by modifying these parameters. It would have beenpossible to obtain perfect peak correspondences, but the pur-pose of this part of the study was to see whether MCNPXis reliable for following�-particles. Then calculations for222Rn were carried out.

3.3. Experiment and simulation of CMOS response for222Rn

The measurements were made after injecting222Rn inequilibrium with a standard226Ra source into a closedtank (vol. = 224 l). Because of the relatively low activityand small detector areas, one has to wait several days tocollect data with acceptable statistics. The average activi-ties registered by the conventional techniques were 3578±209 (Bq/m3) for SSNTDs and 1170± 83 (Bq/m3) for the

AlphaGUARD1 and E-PERM2 chambers. Inside the Rntank, radioactive equilibrium between222Rn and its218Poand214Po daughters is established within a matter of hours.Thus, the track detectors LR115 and CR39, whose resultsdo not discriminate on�-particle energy, show more eventsthan the AlphaGUARD and E-PERM chambers that detectonly 222Rn �-particles. In the following, we distinguish theactivity ATOT of 222Rn plus Po daughters from the activityA222 of 222Rn only. Our different detectors allows to mea-sure the ratioR = ATOT/A222= 3.06± 0.28 (Nachab etal., 2004b), which shows that a radioactive equilibrium isestablished between222Rn and its first two daughters in theatmosphere inside the tank.The calculational study consists of simulating the number

of events registered by the CMOS sensor placed in the Rntank. For 75h time counting and 3578Bq/m3, the MCNPXresult is MMC� = 1.96± 0.23 10−2�m−2 s−1 (Bq/m3)−1.Comparing this value to the measured value of MExp

� =1.75 ± 0.3210−2�m−2 s−1 (Bq/m3)−1 (Nachab et al.,2004b), we deduce an APS detection efficiency (mea-sured/calculated) of 89± 27%.

4. Conclusion

The development of fast new Monte Carlo algorithms andthe availability of more powerful computers, as well as theoptimization of the 3D reconstruction algorithm, should of-fer the possibility of performing very fast on-line calcula-tions during irradiation in the future. In this work, with theMCNPX code we were able to obtain a good fit of the en-tire measured spectrum for a three-isotope�-emitting sourceand to reproduce the CMOS response for a known222Rnactivity. The CMOS sensor gives quite pure�-particle sig-nals, almost free of�-ray contamination, with the advan-tages of an on-line measurement, no chemical processingand low-voltage operation (5V). A complete dosimeter sys-tem is under development with an electrostatic collector anda dedicated chip.

Acknowledgements

We thank Dr. A. Pape for his pertinent comments on themanuscript.

References

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1AlphaGUARD Portable Radon Monitor, Genitron Instru-ments, Frankfurt, Germany

2 E-PERM Passive Environmental Radon Monitor System,Genitron Instruments, Frankfurt, Germany

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