An X-Ray Spectrometer with a Position= Sensitive Wire Detector (PSD)

H. Ebel, M. Mantler, N. Gurker and J. Wernisch Institut fiir Angewandte und Technische Physik, Technische Universitat Wicn, Karlsplatz 13, 1040 Wien, Austria

A new crystal-type x-ray spectrometer is described, which employs a position-sensitive detector for simultaneous radiation detection in a certain energy range given by the spectrometer design. The main specilications are (a) better energy resolution and (b) longer data collection times compared with an energy-dispersive spectrometer.

INTRODUCTION POSITION-SENSITIVE DETECTORS

X-ray spectrometers are usually classified as (a) se- quential crystal spectrometers, (b) simultaneous crystal spectrometers or (c) energy-dispersive spectrometers.

Crystal spectrometers employ one or more crystal- detector pairs and require well-collimated parallel beams in order to define the Bragg angle accurately. The spectrometer as described in this paper basically consists of one crystal and one PSD in fixed geometry. In contrast to conventional spectrometers, the fluores- cence radiation passes divergently from the specimen to the crystal (see Fig. 1). According to the various incidence angles on the crystal, a certain range of wavelengths are reflected, coresponding to Bragg's equation, into the PSD. Therefore the photon- detection position within the detector is directly re- lated to the photon energy and a portion of the x-ray spectrum from the specimen can be accumulated simultaneously by means of a multichannel analyser. Close-up views of spectral portions with improved resolution or changes of the energy range, however, require careful positioning of the detector and/or different crystals.

A M- / \

mu Y

\ /

specimen

Figure 1. Principal geometry. T=x-ray tube; specimen with point P irradiated by primary x-rays; crystal; mp=mirror plane: M = mirror point of P: PSD= position-sensitive detector; I = distance between mirrorpoint and detector wire.

Most commercially available PSDs are either based upon arrays of optical diodes that are equipped with surface layers to convert high energy photons to opti- cal photons, or they are similar to conventional gas counters. The latter type of detector gives the infor- mation on the position as follows: wire and housing form together an R-C network. Depending on the position, where the absorbed photon causes a charge transfer to the detector wire, the rise time of the according pulses differs on the two ends of the wire.

The basic performance of the latter detectors is very dependent upon detector wire material, gas pressure, geometrical design and the electronic signal processing circuits.

Some detectors are equipped with a high pressure gas cell and a carbon coated quartz fibre detector wire. A geometical resolution of 50-60 pm over an active wire length of 50mm can be achieved.

Other detectors employ a gas cell under atmos- pheric pressure and a metal wire, that can be exposed to primary x-rays without deterioration or memory effects. They allow a geometrical resolution of 70- 150 pm over wire lengths between 25 and 50 rnm. Both detectors allow pulse height discrimination as well as sufficient count rate capabilities. Photon detec- tion efficiencies are shown in Fig. 2.

Figure 2. Detector efficiencies for two detector types. a=Xe under 1 bar; 0.25 mm Be window; b = Ar under 11 bar; 0.4 mm Be window.

0 Wiley Heyden Ltd, 1983 X-RAY SPECTROMETRY, VOL. 12, NO. 1, 1983 47

H. EBEL, M. MANTI,ER, N. GURKER AND J. WERNISCH

THE IDEAL SPECTROMETER

As mentioned in the introduction, the spectrometer consists of one crystal and a PSD. For the following discussion, the illuminated area of the specimen is assumed to be small, that means it is in principle a point source of fluorescent radiation.

The geometrical arrangement of specimen, crystal and detector defines the energy resolution as well as the observed energy range. A decreasing distance I between mirrorpoint M and the PSD increases (ac- cording to Fig. 3) the detectable wavelengths range of the fluorescent radiation and decreases the energy resolution. A rotation of the PSD around an axis normal to the plane of Fig. 1 does not improve the resolution and due to this fact the PSD is kept perpen- dicular to the central beam in order to provide a maximum registered energy range.

Figure 3. Effect of changing the distance I from the mirror point to the detector wire (compare Fig. 1) on the spectra measured with a LiF crvstal.

ABERRATIONS DUE TO FINITE SPOT SIZE

An explanation of the resulting broadening of the measured lines is given in Figs 4 and 5. A rectangular area (1,2,3,4) of the specimen is irradiated by the

specimen signal

Figure 5. Line shape resulting from a rectangular specimen area emitting monochromatic x-rays.

primary x-rays from the spectroscopy tube. This area includes the “ideal” point P and the “ideal” circle c. Circle c is obtained from the intersection of a cylinder (with its axis in v having a radius identical with the distance of P from v ) with the specimen surface plane. From Fig. 5 follows the correlation of points (circles) from the rectangular specimen area (1,2,3,4) emit- ting fluorescent radiation to points on the detector wire. The ideal circle is depicted into the reference position D on the wire (black bar in reference posi- tion). There are two more circles shown in Fig. 5 on the specimen area, which are depicted to the detector wire into positions shifted with regard to the reference position. The signal heights (lengths of the signal bars) are directly proportional to the lengths of the arcs of the according circles on the specimen area. As all these contributions are added, the signal on the right side of Fig. 5 is the result of a monochromatic radiat- ing rectangle from the specimen. As a rough approxi- mation is the measured line width directly propor- tional to the width of the rectangle. It is an interesting feature that primarily the length of the rectangle defines the signal height whereas the width merely causes broadening.

y;; “- <....

specimen

a

FINAL REMARKS

The advantage of the described spectrometer is its resolution of less than 100 eV as compared to energy dispersive detector systems, its simple mechanical de- sign, and finally the fact, that there is no coolant needed. The disadvantage is, that its data collection

Figure 4. Monochromatic radiation from all points of circle c is depicted into the detector wire point D.

channel position

I 1000

Figure 6. Spectrum obtained from a steel specimen.

48 X-RAY SPECTROMETRY, VOL. 12, NO. 1, 1983

AN X-RAY SPECTROMETER WITH A POSITION-SENSITIVE WIRE DETECTOR (PSD)

2d (nm)

0 , )r(nm) 0 0 2 04 0'6 0'8 so

Figure 7. Observable spectral region for various crystals. Edch of the three sections corresponds to a region, and can be observed simultaneously; in order to adapt the system for the observation of another section, the detector position has to be varied.

time, which is, as compared to energy dispersive sys- tems about one order of magnitude higher. One typi- cal spectrum measured with the described spectrome- ter is given in Fig. 6. It shows the spectrum of a steel specimen, that directly compares to the theoretical spectra of Fig. 3. The K a lines of elements 2 are clearly separated from the KP lines of elements 2- 1. This is not only of advantage for a qualitative interpre- tation, but also increases the accuracy of quantitative results, as no deconvolution of lines is required.

Quantitative analysis follows the same principles as for conventional spectrometers. It can be done by means of empirical coefficients after having deter- mined the spectrometer function. The method of fun- damental parameters is also applicable. Figure 7 shows the wavelength range which is covered by the spec- trometer for a given geometry (distance 1 ) and length of the detector wire as one changes the analyser crystal.

Received 19 June 1982; accepted 10 September 1982

X-RAY SPECTROMETRY, VOL. 12, NO. 1, 1983 49