What is Polycapillary Optics?

Polycapillary optics are a type of X-ray optics that consist of thousands of hollow glass capillaries arranged in various shapes and configurations. They operate by capturing a large solid angle of X-rays from an X-ray source and redirecting them to a micron-sized focal spot or a highly collimated beam. 

This is achieved by exploiting the phenomenon of total external reflection, which occurs when X-rays hit the inner walls of the capillaries at grazing incidence angles. Polycapillary optics have several advantages over other X-ray optics, such as being achromatic (i.e., independent of the X-ray wavelength), having high flux (i.e., intensity), low divergence (i.e., spread), and small spot size (i.e., resolution). These features make polycapillary optics suitable for a wide range of applications in various fields of X-ray analysis.

Types of Polycapillary Optics

Applications of Polycapillary Optics

Polycapillary optics have been used in a variety of applications and are integral components in many state of the art instruments. Some of the fields that benefit from polycapillary optics are:

• X-ray astronomy: Polycapillary optics can collect and focus X-rays from distant celestial sources, such as stars, galaxies, and black holes, and enhance the signal-to-noise ratio and spatial resolution of X-ray telescopes.

• Wafer analysis: Polycapillary optics can produce large area collimated beams that can scan semiconductor wafers for defects, impurities, and thickness variations using techniques such as X-ray reflectometry and X-ray topography.

• Protein crystallography: Polycapillary optics can provide small focused beams that can illuminate protein crystals with low power X-ray sources and enable high-resolution structure determination using techniques such as X-ray diffraction and X-ray scattering.

• Scanning fluorescence imaging: Polycapillary optics can generate microbeams that can scan samples for elemental composition and distribution using techniques such as X-ray fluorescence (XRF) and X-ray absorption spectroscopy (XAS) .

• Surface analysis: Polycapillary optics can enhance the sensitivity and spatial resolution of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) by focusing X-rays onto small areas of interest.

Specifications of Polycapillary Optics

The specification of a polycapillary optic is a set of parameters that describe its physical and optical properties, such as:

• Optic type: This parameter indicates the shape and function of the polycapillary optic, such as focusing, collimating

• Optic diameter: This parameter measures the outer diameter of the polycapillary optic. It depends on the number and arrangement of the capillaries.

• Capillary diameter: This parameter measures the inner diameter of each capillary. It determines the critical angle of total external reflection and the transmission efficiency of the optic.

• Optic length: This parameter measures the length of the polycapillary optic. It affects the number of reflections and the beam divergence of the optic.

• Focal length: This parameter measures the distance from the exit of the polycapillary optic to the focal point of the X-ray beam. It depends on the shape and tapering of the capillaries.

• Entrance size: This parameter measures the size of the X-ray beam incident on the polycapillary optic. It determines the capture angle and the transmission efficiency of the optic.

• Exit size: This parameter measures the size of the X-ray beam emitted by the polycapillary optic. It determines the focal spot size and the beam divergence of the optic.

• Capture angle: This parameter measures the maximum angle of incidence that can be accepted by the polycapillary optic. It depends on the entrance size, optic diameter and capillary diameter of the optic.

Performance of Polycapillary Optics

To evaluate the performance of polycapillary optics, you need to measure some parameters that characterize their optical properties, such as:

• Transmission efficiency: This parameter measures the ratio of the X-ray flux transmitted by the optic to the X-ray flux incident on the optic. It depends on the X-ray energy, the capillary material, the capillary diameter, the capillary length, the capillary shape and the alignment of the optic with the source and the detector.

• Focal spot size: This parameter measures the size of the X-ray beam at the focal point of the optic. It depends on the X-ray energy, the source size, the optic diameter, the optic shape and the focal length of the optic.

• Beam divergence: This parameter measures the angular spread of the X-ray beam emitted by the optic. It depends on the X-ray energy, the source size, the optic diameter, the optic shape and the focal length of the optic.

• Spatial resolution: This parameter measures the ability of the optic to distinguish two adjacent features on a sample. It depends on the X-ray energy, the focal spot size, the beam divergence, the detector size and the detector resolution.

To measure these parameters, you need to use some techniques and instruments, such as:

• Pinhole camera: This technique uses a small hole in a metal plate to project an image of the X-ray beam onto a detector. By measuring the intensity and shape of the image, you can calculate the transmission efficiency, focal spot size and beam divergence of the optic.

• Knife-edge scan: This technique uses a sharp edge to scan across the X-ray beam and measure its intensity profile. By applying a mathematical algorithm to the profile, you can calculate the focal spot size and beam divergence of the optic.

• Edge response function: This technique uses a sharp edge on a sample to measure its contrast on a detector. By applying a mathematical algorithm to the contrast curve, you can calculate the spatial resolution of the optic.

• These are some ways to evaluate the performance of polycapillary optics. Of course, there may be other methods or instruments that can be used for this purpose.

Selection of Polycapillary Optics

To choose the best type of polycapillary for a specific application, you need to consider several factors, such as:

• The X-ray source: The type and power of the X-ray source will affect the performance and efficiency of the polycapillary optics. For example, a low-power tube source may require a focusing polycapillary to increase the X-ray flux density, while a high-power synchrotron source may require a collimating polycapillary to reduce the beam divergence.

• The sample: The size, shape and composition of the sample will determine the optimal spot size and intensity of the X-ray beam. For example, a small or thin sample may benefit from a focusing polycapillary to achieve high spatial resolution and signal-to-noise ratio, while a large or thick sample may need a collimating polycapillary to cover a wider area and avoid absorption effects.

• The measurement technique: The type and purpose of the X-ray analysis technique will influence the choice of polycapillary optics. For example, X-ray fluorescence (XRF) may require a focusing polycapillary to enhance the detection sensitivity and selectivity of elements, while X-ray photoelectron spectroscopy (XPS) may need a collimating polycapillary to preserve the angular distribution and energy resolution of electrons.

• The cost and availability: The price and availability of polycapillary optics may vary depending on the manufacturer and the specifications. For example, a custom-made polycapillary optic may be more expensive and take longer to produce than a standard one.

some examples of how to choose polycapillary optics for different applications. Here are some scenarios:

1. X-ray astronomy: In this application, the goal is to focus a parallel beam of X-rays from a distant source onto a detector. A polycapillary conic collimator can be used to achieve this, as it can collimate X-rays from a point source with high efficiency and low background. The optic must be curved to match the curvature of the detector and have a large diameter to collect more X-rays.

2. Wafer analysis: In this application, the goal is to produce a large area collimated beam of X-rays to scan a semiconductor wafer for defects or impurities. A collimating polycapillary can be used to transform a divergent beam from a tube source into a quasi-parallel beam with a low divergence. The optic must have a rectangular shape and a large length to cover the entire wafer.

3. Protein crystallography: In this application, the goal is to provide a small focused beam of X-rays to irradiate a protein crystal and obtain its structure. A focusing polycapillary can be used to increase the X-ray flux density and spatial resolution on the sample surface. The optic must have a circular or elliptical shape and a short focal length to achieve a small spot size or high intensity.

Challenges of Polycapillary Optics

Polycapillary optics, despite their advantages, also face some challenges and limitations that need to be addressed. Some of the main challenges are:

• Alignment: Polycapillary optics require precise alignment with the X-ray source and the detector to achieve optimal performance. Any misalignment can result in loss of flux, increase of divergence, and degradation of resolution.

• Transmission efficiency: Polycapillary optics have a finite transmission efficiency that depends on the X-ray energy, the capillary diameter, the capillary length, and the capillary shape. The transmission efficiency can be reduced by factors such as absorption, scattering, and leakage of X-rays inside the capillaries.

• Damage threshold: Polycapillary optics have a limited damage threshold that depends on the material and the coating of the capillaries. The damage threshold can be exceeded by factors such as high power X-ray sources, high energy X-rays, and thermal stress. Damage can cause cracks, deformations, and delaminations of the capillaries, which can affect the optical properties and the lifetime of the optic.

• Background noise: Polycapillary optics can generate background noise that can interfere with the signal of interest. The background noise can originate from factors such as fluorescence emission from the capillary material or coating, scattering from dust or impurities inside the capillaries, and leakage of X-rays from neighboring capillaries.

FAQs

Intensity Gain of Polycapillary optics

Intensity gain for polycapillary optics is a parameter that measures the ratio of the X-ray flux transmitted by the optic to the X-ray flux transmitted by a conventional pinhole collimator with the same aperture size. It indicates how much the polycapillary optic can enhance the X-ray intensity on the sample surface or the detector compared to a simple hole.

Intensity gain for polycapillary optics depends on several factors, such as:

• X-ray energy: Intensity gain for polycapillary optics decreases with increasing X-ray energy, because the critical angle of total external reflection becomes smaller and more X-rays are lost due to transmission or absorption.

• Optic length: Intensity gain for polycapillary optics decreases with increasing optic length, because more X-rays are subject to multiple reflections and scattering inside the capillaries.

• Focal length: Intensity gain for polycapillary optics decreases with decreasing focal length, because more X-rays are subject to higher incidence angles and lower transmission efficiency at the exit of the optic.

• Optic alignment: Intensity gain for polycapillary optics decreases with poor alignment of the optic with the X-ray source and the detector, because more X-rays are missed or blocked by the optic aperture or capillary walls.

Intensity gain for polycapillary optics can be measured by comparing the intensity of the X-ray beam transmitted by the optic to the intensity of the X-ray beam transmitted by a pinhole collimator with the same aperture size. This can be done using a detector or a pinhole camera.

Intensity gain for polycapillary optics can be very high, up to 10,000 times or more, depending on the conditions and specifications of the optic. This can significantly improve the detection sensitivity, spatial resolution and measurement speed of X-ray analysis techniques.

What is a Pinhole Collimator

A pinhole collimator is a type of X-ray or gamma-ray optics that consists of a small hole in a metal plate that can project an image of a radiation source onto a detector. A pinhole collimator can magnify the image of a small object or organ, such as the thyroid or a joint, by placing it close to the hole and the detector far away from the hole1.

A pinhole collimator is used for X-ray or gamma-ray imaging in various applications, such as nuclear medicine, small animal imaging, industrial inspection and security screening. A pinhole collimator can provide high spatial resolution and contrast, but it also has some limitations, such as low transmission efficiency, small field of view and high sensitivity to alignment errors.