Long Wave-Length X-Ray Diffraction Crystal

This invention relates to an x-ray diffraction crystal for analyzing long wavelength x-rays, i.e. x-rays having a wavelength of at least 50 Angstroms, and to a method of making such crystals.

X-rays having wavelengths greater than about 10 A are generally known as soft x-rays and are detected using a wavelength dispersive spectrometer with a crystal as a dispersive element. X-ray spectroscopy below 1 keV would allow identification of elements with atomic numbers below 10 and surface analysis at a depth of 10-1,000 A since soft x-ray photons occur below 1 keV, or wavelengths greater than 12.4 A.

Langmuir-Blodgett multi-layer pseudo-crystals are generally used as a dispersive element. Each layer is a monomolecular layer of a heavy metal soap. The distance between the heavy metals is determined by the chain length of the fatty acid. The distance determines the maximum x-ray wavelength which can be resolved. The heavy metal, i.e. the cation, determines the intensity of the diffracted radiation and the intensity is directly related to the atomic number of the cation.

The most commonly used pseudo-crystal is lead stearate wilth a 2d (d being the interplanar distance) spacing equal to 100 A. Longer "d" spacing crystals using lignoceric acid, and the like can be made but are extremely difficult to build.

It is a principal object of the invention to provide a new, multilayer Langmuir-Blodgett crystal capable of diffracting x-rays of at least twice the wavelength of state of the art crystals.

It is another object of the invention to provide a method of making multilayer Langmuir-Blodgett crystals capable of diffracting x-rays of at least twice the wavelength compared to the state of the art crystals.

These and further objects of the invention will appear as the specification progresses.

In accordance with the invention the crystal consists of alternate monolayers of a first, heavy divalent metal soap, and monolayers of either a fatty acid alone or a second, lighter divalent metal soap. Selecting cations with a significant difference in atomic number and dispersing power, such as Pb and Be, Mg, Ca, etc., increases the effective 2d spacing of the crystal since the Pb planes cause the predominant x-ray diffraction. Thus, the terms "heavy" and "lighter", as used above in modifying "divalent metal", mean that the "heavy metal" is chosen to have a higher atomic number than the "lighter metal". The sequence of the fatty acid or lighter metal soap monolayers between the heavy element layers gives the 2d spacing. Thus, for stearic acid or metal-stearate monolayers,

where N represents the number of layers placed between the heavy metal soap layer pairs.

Preferably, the heavy metal soap is a lead salt of a fatty acid such as lead stearate, and the other monolayers, separating the heavy metal soap monolayers, are beryllium stearate, magnesium stearate, calcium stearate or stearic acid.

These crystals are built up in accordance with the invention, for example, by depositing in the desired pattern on a solid substrate, monomolecular layers of Pb-stearate and layers of a lower atomic number metal stearate or a fatty acid. Thus, a fatty acid, for example stearic acid, in, for example, an n-hexane solution is placed on the surface of two separate water solutions containing, respectively, either (a) the heavy metal cation or (b) the lighter metal cation or no metal cation at all. After the n-hexane evaporates, a surface monolayer of the metal stearate or the fatty acid (where no metal cation is in solution) is formed. Subsequently, a floating barrier under sufficient pressure tangent to the surface is applied to the monomolecular layer causing the molecules to come together. Substrates, e.g. glass, are mounted vertically to the liquids and are alternately dipped into and lifted out of the liquids at a constant rate transferring monomolecular layers onto the substrates in the desired pattern.

The invention will be described in greater detail with reference to the accompanying drawing:

FIG. 1, shows a Langmuir-Blodgett pseudo crystal used for reflecting x-rays to a detector;

FIG. 2a, shows in greater detail the structure of the crystal,

FIG. 2b, shows the molecular structure of lead stearate.

In FIG. 1, the Langmuir-Blodgett crystal comprises several metal atom reflecting planes 1 supported by a glass substrate 2 as shown. The metal atom reflecting planes are alternate layers of Pb-stearate, the molecular structure of which is shown in FIG. 2b, separated by layers of Mg-stearate (see FIG. 2a).

X-rays from a sample (not shown) excited to fluoresce, either by an electron beam or x-rays, and emit secondary x-rays characteristic of the elements composing the specimen are reflected by the crystal 1 in accordance with Bragg's relationship

where n is the number of reflecting planes, .lambda. the wavelength of the incident x-rays, d the interplanar spacing, and .theta. the angle of incidence and reflection of the x-rays.

Since d is fixed, the angle .theta. will vary directly in response to the wavelength and thus by positioning the detector and measuring the angle .theta., the wavelength and hence the identity of the element can be determined.

These crystals are built up in accordance with the invention, for example, by depositing in the desired pattern on a solid substrate, monomolecular layers of Pb-stearate and layers of a lower atomic number metal stearate or a fatty acid. Thus, a fatty acid, for example stearic acid, in, for example, an n-hexane solution is placed on the surface of two separate water solutions containing, respectively, either (a) the heavy metal cation or (b) the lighter metal cation or no metal cation at all. After the n-hexane evaporates, a surface monolayer of the metal stearate or the fatty acid (where no metal cation is in solution) is formed. Subsequently, a floating barrier under sufficient pressure tangent to the surface is applied to the monomolecular layer causing the molecules to come together. Such a barrier may comprise, for example, a styrofoam block to which a tangential force is applied by means of cord or wire. The cord is attached at one end to the block and then extends tangentially to an idler roller and then vertically downward to suspend a constant mass attached to the other end thereof. The gravitational force on this mass is thus transferred tangentally to the floating barrier. Due to this force, the molecules on the surface of each solution are compressed between the floating barrier and the walls of the vessel which contains the solution, thus forming a surface monolayer. Substrates, e.g., glass, are mounted vertically to the liquids and are alternately dipped into and lifted out of the liquids at a constant rate transferring monomolecular layers onto the substrates in the desired pattern.