P.I.X.E. SPECTRA MEASURED WITH GOOD RESOLUTION

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P.I.X.E. SPECTRA MEASURED WITH GOOD RESOLUTION

F. Folkmann, F. Frederiksen, H. Loft Nielsen

To cite this version:

F. Folkmann, F. Frederiksen, H. Loft Nielsen. P.I.X.E. SPECTRA MEASURED WITH GOOD RESOLUTION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-99-C9-102.

�10.1051/jphyscol:1987915�. �jpa-00227261�

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P.I.X.E. SPECTRA MEASURED WITH GOOD RESOLUTION

F. FOLKMANN, F. FREDERIKSEN and H. LOFT NIELSEN

Institute of Physics, University of Aarhus, DK-8000 Aarhus, Denmark

Abstract : A spectrometer has been built with a curved crystal and a position sensitive detector to measure x rays in a limited region with good resolution and high efficiency. A collimated

5 5 Fe source, electroplated onto a Pt wire, was used for checking the proportional detector. Geometrical considerations for optimating the instrument are discussed. Applications to Particle Induced X-ray Emission are considered, both for diagram lines excited by protons and heavy ion excited satellite lines.

1. Introductian

Particle Induced X-ray Emission (PIXE) has with succes been used for analysis of small amounts of elements in low concentration [I]. To solve the problem of resolving close-lying x-ray lines not separated by a solid state detector, we have built a simple non-scanning crystal spectrometer. It measures x rays in a fixed region of AE/E = 0.1 to 0.25 in the range 2 to 9 KeV using diffraction in a curved crystal and a position sensitive proportional detector.

2. Experimental setup

The position sensitive flowmode proportional counter has a 0.025 mm thick Be window over a 40mm*12mm opening and a Back-gammon posigion readout 123 , and its response is tested by moving a collimated Fe source in front of the detector.

The source had the shape of a 0.1 mm thick wire of active height 4 mm spanned in a collimator with a 0.1 mm opening, 13 mm high, 55 mm from the wire and 10 mm ahead of the detector. The source produced 16 counts/s in the detector. For calibration the source was moved with a screw-rod and monitored by a length gauge, typically in increments'of 1.00 mm across the 40 mm opening of the detector.

SOURCE MOVE

1

PROP. COUNTER

Figure 1 Arrangement for x ray measurement from a beam excited source with a curved NaCl crystal and a position sensitive proportional counter.

gS

movable collimated Fe source in front of the detector is used for calibration and electronical adjustments.

EXPERIMENTAL SETUP

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987915

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C9-100 JOURNAL DE PHYSIQUE

The position is found either by analog pulse addition and division or, with somewhat better resolution, by storing both signals from S1 and S2 via 2 ADC's in list-mode and making digital addition and division. In both cases a gate condition can be set on the total energy improving the resolution. The position signaS5was checked to have a nearly linear relation to the position of the Fe source, but not perfectly, notably at the ends of the detector. An empirical fitted function was used to get the geometrical position from the divided signal, but in most cases a linear relation was sufficient. The calibration was based on the measured position of a diagram line in the center of the detector, using the geometrical determination of angles relative to this point, calculated to the center of the dif- fracting crystal.

For diffraction we used a crystal in Johann geometry, cylindrically curved to radius R, and had the position sensitive detector centered at the Rowland circle (with radius R/2) and perpendicular to the direction to the center of the crystal [3]. We have used a Ge(ll1) crystal with 2d=0.653 nm curved to R=298 mm to measure the radiation around the Ka line of S at Bragg angle @=55.3' and a NaCl(200) crystal with 2d=0.564 nm and R=254 mm centered at @=40° covering a region of interest with K lines of K and Ca, and L- and M-lines of other elements, shown later. However, the radii of curvature of the available crystals were not optimized for this kind of measurement, as discussed later.

The 1 to 5 mm diameter source of radiation, excited by the particle beam, was placed between the symmetric point at the Rowland circle and the crystal. We produced the radiation by focussing an electron or ion beam onto a solid homogeneous target and moved occasionally the beamspot out to 8 mm to each side of the central point on the Rowland circle, see fig. 1.

3. Preparation of the 5 5 Fe wire source

To make a linear collimated source of moderate strength we have de- posited radioactive material onto a wire by the following pro~~dure.

An amount of 0.37 GBq Fe was purified by adsorption of Fe as Fe(III), dissolved in concentrated HC1, on a small anionexchange column. Fe was eluted with 0.1 molar HC1 in one drop; this was contac- ted with a stream of SO gas to reduce yellow Fe(II1) to colourless Fe( 11 )

.

The active solbtion was transferred to an electroplating cell consisting of a 4 mm diameter, 10 mm high cylindrical cup made from a piece of polyethylene tubing with a Pt anode, heat-sealed in the bottom. A 0.1 mm diameter Pt wire, forming the cathode, was stretched along a diameter on the top of the cell. Since the surface of the solution curved above the brim of the cell, the wire was immersed in the solution over 4 mm of its length. Fe was electroplated on that part of the wire with a 10 mA current for about 30 min. The electroplating yield was sufficient to produce a source of 3 MBq.

4. Results

We have chosen our setup to cover a region of interest for applied PIXE work between 3 and 4 KeV with K lines-of K and Ca, L lines

of

^Ag,

Cd, In, Sn and Sb and M lines of Th and U. For this range we show in fig. 2 data with characteristic K- and L- X rays taken with our NaCl crystal bombarding solid targets with a few nA protons for 100 sec.

The resolution is around 15 eV FWHM and the spectra are best reproduced in the central region. The observed non-ideal intensity distribution among the lines in a group is effected not only by the limited acceptance of the position sensitive detector but also by the position of the source. To illustrate the source dependence, two spectra are shown for In with the source focussed 3 mm to the right and 3 mm to the left of the target centre, giving different intensities.

To compare with traditional spectroscopy we show in fig. 3 a similar spectral region for the same targets and analyzing crystal but

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Figure 2 X-ray spectra with the divided position result for solid targets after impact of 2.5 MeV protons, using a NaCl crystal.

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38 37 36 35 31 33 32 31 30

WAVELENGTH lnml

r I , I I 1 r l , ' \ L , l r t 8 I

Figure 3 X-ray wavelength spec- tra measured with a scanning cur- ved NaCl crystal spectrometer af- ter excitation by 3 MeV protons.

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C9-102 JOURNAL DE PHYSIQUE

measured with a scanning spectrometer. This wavelength-geared setup had two 0.5 mm slits on the Rowland circle and the source 18 mm outside [4], and shows clearly somewhat better resolution and in most cases more reasonable intensity ratios. However mechanical scanning is required and a longer exposure, with typically 10 times as high beam intensity and 3 sec/channels over a shown scan of 880 channels.

We have measured spef$ra induced by 10 KeV electrons, 2.5 MeV and 3 MeV protons and 22 MeV 0 ions to separate either close-lying diagram lines or satellite structures of S, K or Cr Ka-lines.

5 . Discussion

Our aim is to avoid scannina and to obtain a sianal like in fia. 2 similar to the energy output from a Si(Li) detector: but with biker resolution. We are concerned about the influence of an extended source due to different response for various parts of the exposed area. That the peak centroid shifts a little is not so severe. A worse problem is that we are not sure of getting the same result for all parts of a large source. This may be cured by a source closer to the crystal.

To reduce background from stray-electrons a thin Al-coated 6 pm thick mylar foil was placed between the source and the crystal. This foil will also protect the crystal from vapourized or sputtered material caused by the beam bombardment and separate the spectrometer vacuum chamber from the scattering chamber.

The counting rate is limited by the division procedure and by dead- time in the ADC (10-50 ps), but in practice our limit is 200-2000 c/s.

If a major line is present, one can consider shielding it geometrically by putting a movable beam dump in front of it.

To explain the observed lineshape and position of lines, in dependence of the crystal and location of the source spot, we have perfor- med calculations with the ray-trace program of Morita 153, see fig. 4.

With another setup with a Si(ll1) crystal of 2d=0.626 nm and R=600 mm around 0=34.5' we plan to get better resolution. We shall be able to place the source further away from the point at the Rowland circle, but we will then cover a smaller spectral range.

References

[I] F. Folkmann, J. Phys. E 8 (1975) 429

[2] B.P. Duval, J. Barth, R.D. Deslattes, A. Henins and G.G. Luther, Nucl. Instr. Meth.

222

(1984) 274

[3] H.F. Beyer, R.D. Deslattes, F. Folkmann and R.E. LaVilla, J. Phys. B 18 (1985) 207

141 F. ~ o l k m a n n T ~ . F . Beyer, R. Mann and K.-H. Schartner, Nucl. Instr. Meth.

181

(1981) 99

[51 S. Morita, Jap. J. Appl. Phys.

22

(1983) 1030

Figure 4 Theoretical calculation of the lineshape of an x-ray line in a position sensitive detector as used in our setup, for various radii of curvature R of a LiF crystal. The graph was made with the ray-trace procedure and para- meters of [5]. A linear extended source is assumed. A centroid shift is seen and tails are more pronounced than in fig. 6 of [5].

The left tail is due to the close geometry and source elements dis- placed from the central position.

-0.5-OL -0.3 -0.2-01 0 0.1 0.2 0.3 0.L 0.5 PERPENDICULAR DISTANCE (mm)