Nuclear Instruments and Methods in Physics Research 224 (1984) 303-307 303 North-Holland, Amsterdam

M E T H O D FOR FABRICATING LARGE-AREA X-RAY FILTERS

Joe W O N G

General Electric Corporate Research and Development, 1 River Road, PO Box 8, Schenectady, N Y 12301, USA

Received 14 October 1983 and revised form 17 January 1984

A simple, cost effective method for fabricating uniform, large-area filters from powder materials is described. The technique is particularly applicable to making pinhole and crack free, large-area filters for fluorescence measurements of extended X-ray absorption fine structure (EXAFS) using the Stern-Heald type filter-ion chamber assembly.

1. Introduction

X-ray transmission filters are made in a variety of ways [1,2]. Ductile metals may be rolled into thin foils. Non-ductile metals may be ground and polished with abrasive, electropolished or chemically etched. Metals may be electroplated or vacuum-evaporated on thin X-ray transparent substrates. Metal or oxide powder may be slurried with an organic binder and spread on similar substrates or glass, dried and stripped off. Filter papers may be impregnated with solution and then dried. The above methods involving the use of powder materials are applicable for fabricating small-area filters ( - 1 inch diameter) used in conjuction with laboratory X-ray tubes. For utilizing high intensity, highly col- limated synchrotron radiation sources, filters of size 3 inches x 3 inches or larger are required in the case of fluorescence EXAFS measurements, in order to obtain a large solid angle ( - 2~ : / 10 ) of collection of the fluo- rescent signals [3]. This paper describes a simple, repro- ducible technique of fabricating uniform, large-area, pin-hole and crack-free filters from materials that do not have a corresponding ductile metal.

2. Fabrication method

The method is outlined as follows. A predetermined amount of the finely powdered material ( - 400 mesh, or finer) is mixed with a weighed out amount of Duco ® cement [4] in a flat petri dish to form a slurry. The viscosity of the Duco ® cement is lowered by acetone addition to aid dispersion and sedimentation of the particles in the slurry. The slurry is stirred to break up conglomeration of the particles and to enchance thick- ness uniformity. The dish is then placed on a level surface. The acetone is allowed to evaporate slowly and the Duco * cement is allowed to cure. The cast powder

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film is then removed from the flat dish. Details of the method are described below for the case of fabricating large-area Ge filters.

Let d be the diameter (in cm) of the dish and/J'm the mass absorption coefficient of Ge just above the K edge (in cm2/g). For a filter of n absorption lengths thick, the weight of Ge needed can easily be shown to be

w = ,,~rd2/4 ~m' (1)

Thus for d = 13.8 cm and/~m of Ge above its K edge = 1149.9 cm2/g [5], the weights of Oe powder required for filters of various absorption lengths can simply be calculated from eq. (1) and are given in table 1.

The starting Oe powder is - 400 mesh, which is then jet-milled at 70-80 psi, to give a Coulter counter par- ticle distribution with a maximum at - 6/~m and a tail to 20/zm. At an energy just above the K edge of Oe, one absorption length (= 1/ i zmp ) is 8.7/zm, where p is the density of Ge = 5.903 g/cm 3. For fluorescence EX- AFS measurements using the Stern-Heald type filter- ion chamber detector [3], filters of 3-6 absorption lengths are desirable. Since the maximum particle size in

Table 1 Weights of Ge and Duco x cement for fabricating filters of various absorption lengths, n, in a 13.8 cm diameter petri dish.

n Weight of Ge Weight of Duco* Comments [g] [g]

1 0.7681 2 1.536

3 2.304

4 3.072

5 3.804

5

10 5 10 10 12.5 15

no film made 1 good film made, but not reproducible reproducible good films film tears reproducible good film film tears reproducible good films reproducible good films

304

the distribution is only 20 ~m, the jet-milled powder is found to be adequate.

The fabrication steps are as follows. A prede- termined amount of Ge powder is weighed out in an analytical balance. An optimal amount of Duco * ce- ment (see table 1) is then weighed out on a 13.8 cm petri dish (tared) on a top loading balance. Acetone is added to the Duco ® cement in the dish and stirred with a small artist brush equipped with a metal tip to form a uni- form, low viscosity liquid. The weighed out Ge powder is now added to the above liquid and stirred gently to form a dispersion. The whole dish is placed over a bright light box, so that uniformity of the slurry over the whole dish can be visually monitored. More acetone is added as necessary, to maintain a uniform dispersion. Whirling the whole slurry or ultrasonic stirring also helps to achieve a uniform dispersion. The stirring pro- cess takes about 5 min for this size dish.

The dish is now carefully placed on a level table and the acetone is allowed to evaporate gently under room conditions. No forced evaporation (as in a vented hood) should be made, since this may cause cracks in the film, or incomplete sedimentation of the particles. The surface of the drying dispersion is shiny when wet and dull when dry. After the film acquires a dull appearance and becomes non-deformible under fingernail pressure, an x-acto knife is used to detach the film at the meniscus point around the rim of the dish. A pair of teflon tweezers is used to peel the film radially inward from the cut edge.

The weight ratio of Duco ~ cement to Ge powder and the curing time are found to be critical in determining the ease with which the cast film can be detached from the dish and its quality. Films cured overnight or with a low cement/Ge weight ratio usually stick to the dish and tear during the removal process. Table 1 shows the optimal cement/Ge weight ratio for filters of various absorption lengths.

3. Results

In fig. 1, a Ge film of two absorption lengths thick, which has been detached from the petri dish and put in an A1 frame for insertion into a fluorescence detector for EXAFS measurements is shown. Fig. 2 is a series of optical micrographs of the cross section of Ge films of various absorption-length thicknesses. The large par- ticles are found to form a rather uniform strata at the bottom of the dish by sedimentation, while the finer particles are dispersed throughout the cement layer. Dark holes in the Ge band are due to pullouts during polishing. Variation in the thickness of the Duco ® layers arises from different amounts of Duco ~ cement, used as given in table 1. These filters have recently been used in conjunction with a fluorescence detector assembly [6] to

J. Wong / Fabricating large- area X- ray filters

Fig. 1. Photograph of a two absorption lengths thick Ge filter cast in Dueo R cement inserted in an AI frame ready for use in a fluorescence assemby for EXAFS measurement. The scale marker corresponds to one inch.

measure the Au EXAFS in mineral specimens [7] at the Cornell High Energy Synchrotron Source (CHESS).

Oxide powders, commonly used to fabricate X-ray filters [1,2], are ideally suitable for the case of large-area Cr and Mn filters used for fluorescence measurements of Mn and Fe EXAFS respectively, since both Cr and Mn are non-ductile. Th absorption length of the ele- ment in a corresponding oxide is given by (/~ m fPoxia~)-t where f is the weight fraction of metal in the oxide and Poxide is the density of the oxide. Hence, the weight of oxide required to fabricate a filter of n absorption lengths in a dish of diameter d[cm] is

Woxid e = n~rd2/41Xmf. (2)

Using eq. (2), the weights of a series of MnO z and Cr20 3 filters of various absorption lengths are given in table 2 for d = 13.8 cm. Filters with n = 3, 4 and 5 were

Table 2 Weights of oxide powder and Duco R cement for fabricating filters for various absorption lengths, n, in a 13.8 cm diameter dish.

n MnO 2 Cr203 Duco cement a~ [g] [g] [g]

3 1.517 1.20 10 4 2.023 1.60 10 5 2.528 2.00 10

a) All films were good and reproducible and were removed from the dish soon after the Duco R cement was cured and became non-deformable under finger nail pressure (see text).

J. Wong / Fabricating large-area X-ray filters 305

n=Z

n=3

n=4

Side

n=5

o~Cement Loyer

ss Side

I ! 51ZI~

Fig. 2. Optical micrograph of the cross sections of a series of Ge filters of various absorption length thicknesses, n, dispersed in Duco R cement. Materials above and below the cement layers are used as support during polishing.

fabricated, and optical micrographs of their cross sec- tions are shown in figs. 3 and 4 respectively. The microstructure of the MnO 2 filters is very similar to that

of the Ge filters; both materials being - 4 0 0 mesh particles. The Cr203 powder (Fisher centified reagent) consists of much finer particles and yields very uni-

306 J. Wong ,/ Fabricating hlrge -area X- ray filter~

n-3

n:4

n:5

-Duco~Cemenl Loyer

I I 51Zl~m

Fig. 3. Optical micrographs of the cross sections of a series of MnO 2 filters of various absorption length thicknesses, n, dispersed in Duco R cement.

formly dispersed films throughout the thickness of the cement layer, as seen in the series of micrographs shown in fig. 4.

4. Concluding remarks

The advantages of this fabrication technique are quite evident. It is simple and fast. Any bad films can be reprocessed by merely redissolving in acetone, so that there is no waste of filter material. Also, since

high-purity metal oxides such as those of Ti, V, Cr . . . Zn, Ga, Ge are readily available in fine particle powders ( - 400 mesh), it becomes cost effective even to fabricate large area X-ray filters from oxides of ductile metal, since the rolling of large sheets of ductile foils is costly (capital investments on rollers) and time-consuming in- volving a graded thickness reduction a n d / o r annealing to remove cold work from a previous rolling. As seen in the Ge, MnO 2 and Cr20 examples above, the cast films are uniform and absorption lengths are controllable. In principle, there is no limit to size.

J. 14/ong / Fabricating large-area X-ray filters 307

n=3

n=4

n=5 t Ouco®Cemen! Loyer

Fig. 4. Optical micrographs of the cross section of a series of Cr203 filters of various absorption length thicknesses n, dispersed in Duco s cement.

References

[1] B.W. Roberts and W. Parrish, in: International tables for X-ray crystallography 3 (1968) 77.

[2] E.P. Bertin, Principles and practice of X-ray spectrometric analysis (2nd ed.) (Plenum Press, New York, 1975) p. 400.

[3] E.A. Stern and S.M. Heald, Rev. Sci. Instr. 50 (1979) 1579. [4] A trademark of the DuPont company.

[5] W.H. McMaster, N. Nerr del Grande, J.H. Hallet and J.H. Hubbell, Compilation of X-ray cross sections, Lawrence Radiation Laboratory UCRL-50/74, sect.2, rev. (1969).

[6] F.W. Lytle, R.B. Greegor, D.R. Sandstrom, E.C. Marques, J. Wong, C.L. Spiro, G.P. Huffman and F.E. Huggin, submitted to Nuel. Instr. and Meth. (1984) in press.

[7] J. Wong and K.J. Rao, Sol. State Comm. 45 (1983) 853.