Research article

Received: 6 June 2010 Revised: 4 August 2011 Accepted: 25 August 2011 Published online in Wiley Online Library: 10 October 2011

(wileyonlinelibrary.com) DOI 10.1002/xrs.1367

446

EDXRF with an audio digitizerYasukazu Nakaye* and Jun Kawai

We utilized an audio digitizer in energy-dispersive X-ray fluorescence (EDXRF) with a silicon drift detector and achieved a fullwidth at half maximum (FWHM) of 178 eV at Mn Ka (92-ms peaking time). To confirm the ability of EDXRF with an audio dig-itizer, we also examined energy versus channel number linearity and output count rate. We applied it to EDXRF analysis of(ZnCd)S : Ag and showed a proper energy versus channel number linearity from 5.9 keV (Mn Ka) to 26.1 keV (Cd Kb). And,the maximum output count rate of more than 10 kcps was obtained with 23-ms peaking time (296-eV FWHM). Copyright ©2011 John Wiley & Sons, Ltd.

* Correspondence to: Yasukazu Nakaye, Department of Materials Science andEngineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan.E-mail: Yasukazu@Truemoon.mbox.media.kyoto-u.ac.jp

Department of Materials Science and Engineering, Kyoto University, Sakyo-ku,Kyoto 606-8501, Japan

Introduction

In the previous work, we demonstrated that X-ray spectrometercould be composed of a microphone audio digitizer.[1] We ac-complished an X-ray analysis of X-rays from checking source ofGeiger–Müller counter with the spectrometer and showed thecapability of an audio digitizer as an A/D converter for X-raysignal processing.Conventional digital signal processors (DSP) consist of two

components, i.e. digitizer and signal processor. Analog input signalsfrom the detector-preamplifier system are digitized by an A/D con-verter. Then, the DSP generates spectra using hardware-basedsignal processor (i.e. field-programmable gate array or applicationspecific integrated circuit). These spectra are transferred to acomputer and recorded for further analysis.[2]

The present system also consists of two components. An audiodigitizer is used instead of spectroscopy digitizers in conventionalDSP. Then, digitized signals are sent to the computer where thesoftware performs entire signal processing. The digitized time se-ries data are stored in the computer storage so that they can beanalyzed later with different parameters or algorithms.In the present paper, the method outlined in the previous work

is applied to the energy range of normal energy-dispersive X-rayfluorescence (EDXRF), i.e. energy range of about 2 to 25 keV, withinwhich most EDXRF applications are carried on. This is completedby modifications of the software and the experimental conditionsfrom our previous measurement.[1] This will prove that a softwareanalyzer and an audio digitizer worked perfectly as a DSP in nor-mal EDXRF energy range consequently. However, its applicationis limited to the low count rate experiment (up to about 10 kcps)because of the low sampling frequency of audio digitizers.The microphone digitizer used in the previous work[1] had not

been electromagnetically shielded from high-frequency circuitnoise from the main board of the notebook computer. And, itresulted in a limited energy resolution that it cannot be appliedto the normal EDXRF. To solve this problem, we utilized theelectromagnetically shielded audio interface, instead of thebuilt-in microphone digitizer, and reduced the circuit noise fromthe computer main board. Figure 1 shows the photograph of thepresent EDXRF measurement setup. The output signal elicitedfrom a silicon drift detector (SDD) preamplifier is digitized directlyby an audio interface and sent to a notebook computer for fur-ther analysis by software.

X-Ray Spectrom. 2011, 40, 446–448

Experimental

An SDD (VorteX, 50 mm2 x 300 μm, typically its FWHM is 148 eVat 5.9 keV when used with a VorteX DSP) with a preamplifier wasused for EDXRF. SDDs have shown better energy resolution thanother semiconductor X-ray detectors. The output signals of thepreamplifier were analyzed by software on a notebook computerafter being digitized by an audio digitizer. In the present work, anelectromagnetically shielded audio digitizer was used for signaldigitization to reduce the circuit noise; in the previous work, weused an audio digitizer without electromagnetic shield.[1]

Our software analyzer was modified to accept signals of up to32-bit/sample (the previous one accepted signals of up to 16-bit/sample) and signals of the sampling frequency of up to 4GHz(usually limited to lower frequency by the digitizer or computerspecifications).

We also modified the signal shaping algorithm, from the firstderivative (unipolar) to the second derivative (bipolar), to decreasethe effect of baseline shifts. The pulse height was computed fromthe height: from the local minimum to the local maximum.

Results and Discussion

Energy versus channel number linearity

The energy versus channel number linearity is essential to con-firm the present measurement method. To reveal this correlation,we conducted an EDXRF analysis of (ZnCd)S : Ag with the presentexperimental setup. And, we also used the data from theEDXRF spectrum of MnCO3. Cd K lines are on the edge of the de-tection efficiency curve. Figure 2 shows the EDXRF spectrum of(ZnCd)S:Ag (the upper figure) and the energy versus channelnumber linearity plot (the lower figure) using the two spectra(EDXRF spectra of MnCO3 and (ZnCd)S : Ag). This linear calibrationof the energy versus channel number was obtained using theK X-ray peaks of Mn, Zn, Ag and Cd.

Copyright © 2011 John Wiley & Sons, Ltd.

Figure 1. A photograph of the practical EDXRF measurement setup. Anaudio interface was used to decrease the circuit noise from the computermain board.

0

5

10

15

0 2 4 6 8 10

23µs

46µs

92µs

Figure 3. Output count rate versus X-ray tube current (as an input countrate) for three different time constants for an audio digitizer and signalprocessing software. The points denote actual experimental measure-ments. The solid line represents the ideal response to full counting (nocounting loss).

EDXRF with an audio digitizer

Throughput rate measurement

To investigate the effect of increasing input count rate, weemployed an acrylic resin to scatter the incident X-ray to the detec-tor and the tube current increased from 0.1 to 9.6mA. A series ofanalyses was conducted at tube voltage of 35 kV. The graphs ofthe output count rate versus X-ray tube current are shown in Fig. 3for peaking times of 23, 46 and 92ms. At the optimum resolutionperformance [full width at half maximum (FWHM) of 178 eV at MnKa] achieved with the 92-ms peaking time, an output count rateof 2600 counts/s (30% dead time) could be obtained. With shorterpeaking times, 5000 counts/s (30% dead time, 46ms and 187-eV

Figure 2. (Top) An EDXRF spectrum of (ZnCd)S : Ag phosphor powder.(Bottom) A plot of energy versus channel number calibration obtainedby using K X-ray peaks of Mn, Zn, Ag and Cd.

X-Ray Spectrom. 2011, 40, 446–448 Copyright © 2011 John

FWHM) and 10 000 counts/s (30% dead time, 23ms and 296-eVFWHM) could be achieved.

Energy resolution

Figure 4 shows the comparison of two EDXRF spectra of MnCO3.As seen in the spectra of Fig. 4, the energy resolution was greatlyimproved in the present result, instead of the equivalence of thetwo audio digitizers in specifications (see figure caption). Figure 4(A) and (B) are spectra obtained by the previous experimentalcondition[1] and by the present experimental condition, respec-tively. The only difference between Figure 4(A) and (B) was a dig-itizer: a built-in microphone digitizer for Figure 4(A) and a USB-connected audio interface for Figure 4(B). We achieved an FWHMof 178 eV at Mn Ka with peaking time of 92ms. Therefore, we pro-pose that EDXRF method with audio digitizers is applicable tonormal EDXRF measurements.

Mn Kα

Mn Kβ

Compton

Cu Kα + Ta LαSum peaks

Figure 4. A comparison of two EDXRF spectra of MnCO3 measured with(A) the previous experimental condition and (B) the present experimentalcondition; with a silicon drift detector, a full width at half maximum of lessthan 180 eV was achieved with an audio digitizer (24-bit, 192 kS/s).

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Y. Nakaye and J. Kawai

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The measurements indicate the applicability of an audio digi-tizer by software to the normal EDXRF measurement. The majorlimitation observed with the spectrometry performance of thepresent method is its output count rate due to the low samplingfrequency and related long peaking time. However, in low countrate X-ray spectrometry measurement (up to 10 kcps), the audiodigitizers have enough energy resolution and have an easyapplication.

Conclusions

We conclude that an audio digitizer is applicable to X-ray spec-troscopy in normal EDXRF energy range instead of an ADC anda DSP. FWHM at Mn Ka X-ray measured by audio recordingachieved 178 eV (92-ms peaking time). EDXRF analysis of (ZnCd)S : Ag was performed to confirm the energy versus channel

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number linearity and showed a proper linearity in the range of5.9 keV (Mn Ka) to 26.1 keV (Cd Kb). Useful output count rate of2.6 kcps was achieved with 92-ms peaking time (178-eV FWHMat Mn Ka) and 5 kcps was achieved with 23-ms peaking time(296-eV FWHM). These experimental data confirm the capabilityof software analyzer with audio digitizer to be used as a DSP ofX-ray detectors.

In the present report, it is shown that an audio digitizer isoperated with our software analyzer as an ordinary EDXRFanalyzer. Without the ambient noise source, energy resolutionin EDXRF improved well. Additionally, recent development ofsmart phones will allow us to develop versatile handy analyzers.

References[1] Y. Nakaye, J. Kawai, X-ray Spectrom. 2010, 39, 318–320.[2] T. Lakatos, Nucl. Instrum. Meth. 1990, B47, 307–310.

X-Ray Spectrom. 2011, 40, 446–448Wiley & Sons, Ltd.