Scanning Preparation #4: Experimental Apparatus
Overview
Designing in-situ apparatus can be very challenging. The composition and system constraints (height, diameter, and weight) for samples, along with their recommended considerations, are still relevant when using an in-situ apparatus. On top of those constraints, other considerations need to made depending on the nature of the experiment. A non-comprehensive list of typical considerations for in-situ designs are as follows.
Jump To: X-ray Transparency | Rigidity | Mechanical Strength | Thermal Tolerance | Chemical Compatibility | Peripheral Management
1. X-ray transparency
The in-situ apparatus can be made of any material that makes sense for the experiment, but the material within the field of view of the X-ray beam must be X-ray transparent. As described in the sample composition page, image contrast is dependent on the X-ray attenuation of the scanned material. If the X-ray attenuation of the apparatus in the beam path is too high, or the material is too thick, too many X-rays will be attenuated and the image contrast of the scan will be too low to make any meaningful observations or measurements. Below are a list of some materials and their overall ability to transmit X-rays.
Material | X-ray Transparency | Notes |
|---|---|---|
Stainless Steel | Bad | Limited uses in synchrotron systems only |
Titanium | Bad-Poor | Minimal use in laboratory-based CT systems due to X-ray flux limitations |
Aluminum | Poor-Fair |
|
Carbon Fiber | Fair | Machining is difficult without compromising strength |
Polyether ether ketone (PEEK) | Good |
|
Polymers (e.g. PFTE, PFA, PP) or Resins | Excellent | Radiation damage is possible: compromises strength, movement artifacts |
Beryllium composites | Excellent | High toxicity/carcinogenic hazard especially when machining |
2. Rigidity
The apparatus needs to be rigid enough so that there is no perceptible movement during rotation. There are a few things to consider when appraising experimental setups for rigidity:
- Radiation damage - The small deformations due to radiation damage, while imperceptible to the human eye, can cause large movement artifacts in the reconstructed image. For high resolution scans where the in-situ apparatus will be exposed to focused (or repeated) radiation, it would be prudent to not make the system from materials that are prone to radiation damage such as some polymers or resin.
- Top-heavy apparatus - sometimes experimental apparatus require heavy connections at the top of the setup (e.g. fluid lines in a fluid flow experiment). During rotation, this top-heavy weight can cause wobbling of the apparatus and lead to large movement artifacts in the reconstructed image. If the weight of connections is substantial, the system may need to be made from a more rigid material than would normally be needed. Or reinforcement to the system (e.g. support rods) can be added to ensure stability during rotation. It should be noted that, while reinforcement is possible, things like support rods can cause some image artifacts should they move through the field of view during rotation.
3. Mechanical strength
The mechanical strength of the in-situ apparatus is important for experiments that require applied pressures. It is therefore important to know the yield and tensile strengths of the components of the apparatus and whether they can withstand the pressures of the experiment without deformation or failure. Pressure vessels are hazardous to human health and the microCT system. Therefore it is important for the apparatus to be designed following all regulatory requirements and a pressure test will be required before use in the microCT system.
4. Thermal tolerance
Thermal tolerance of the in-situ apparatus is important for experiments that require elevated or reduced temperatures. The material must be resistant to deformation or failure under the temperatures of the experiment, including temperature cycling. The mechanical strength of the system can be largely affected by thermal change and therefore experiments that require high-temperature and high-pressure are very difficult to design with enough strength while also being x-ray transparent, especially for laboratory-based CT systems where X-ray flux is limited.
5. Chemical compatibility
The materials of the apparatus need to be compatible with the experiment. This isn't normally an issue but it can be relevant in some experiments including those that involve flow of reactive fluids. Effort needs be made to ensure that the sample composition does not physically or chemically affect the integrity of the apparatus during experimentation.
6. Peripheral management
Some in-situ experiments require a lot of peripherals that can disrupt the rotation or stability of the apparatus. For example, in-situ fluid flow experiments can include peripherals such as fluid lines, pumps and different sorts of sensors that may require loose wires. Thought will be needed to figure out how all peripherals can be arranged so that they are either incorporated in the flow cell or on the rotation stage so that they are not causing strain on the system during rotation. Or the rotation can be limited to only occur over a specific angle range (at least 180 degrees) but that will come as a trade-off to temporal resolution.