Scientific instruments and imaging devices are benefiting from advances in detectors. Selecting the right cooling technology improves performance and minimizes operational costs for any precision detector.
Chris Rebecchi, Helix Technology Corp.
Thermally induced noise — called dark-current noise in CCDs and Johnson noise in x-ray and gamma-ray detectors — can degrade the signal-to-noise ratio of a detector, impeding detection of weak signals. Cooling a detector reduces the leakage current, significantly enhancing sensitivity for many applications.
CCDs are integrated circuits that convert light into electrons. They are installed in cameras that convert the charge stored in the CCD pixels into digital images. High-end CCD cameras, used in the most demanding scientific applications, require cooling to help detect dim light originating from as far away as distant stars and from as nearby as laboratory-controlled experiments.
X-ray and gamma ray detectors typically are single-crystal semiconductor materials that require cryogenic cooling to function properly. They help to quantify and characterize x-ray and gamma-ray spectra in such diverse applications as semiconductor inspection, materials science, transportation security and nuclear physics.
X-ray diffraction spectra differ for Gouda (top) and Semtex H (bottom). Conventional dual-energy technology has difficulty distinguishing the plastic explosive and the cheese because they have similar densities. The detector in the Yxlon 3500 baggage screener that collected these spectra was cooled by a CryoTiger mixed-gas refrigeration system from Helix Polycold Systems Inc. Courtesy of Yxlon International Security GmbH.
Drug discovery is one of the most important applications for cooled CCD cameras. High-throughput screening enables pharmaceutical researchers to rapidly discover drugs and bring them to market while minimizing development costs. It offers the ability to simultaneously test hundreds of drug combinations that are tagged with luminescent, fluorescent or radiometric tracers to indicate a positive interaction. The technique would not be possible without high-speed cooled CCD cameras that can resolve in a single exposure the faint light emitted from more than 1500 individual chemical reactions on a microplate.
Highly sensitive cooled CCD cameras also measure the light emitted from bioluminescent sources within a living mammal. Using this imaging method, researchers can observe long-term biological processes inside an animal without sacrificing it. Rather than dissecting a succession of animals, they can track drug efficacy in the same animal over an extended period, simultaneously improving test accuracy and reducing the number of lab animals needed.
Energy dispersive x-ray analysis nondestructively measures the elemental composition of unknown samples with cooled x-ray detectors. X-ray microanalysis uses the technique to collect data over a very small sample area. This procedure is commonly used for failure analysis, including identifying unknown defects in semiconductor wafer fabrication processes.
The trend is toward combining multiple analytical technologies on a single inspection tool. For example, one semiconductor equipment manufacturer builds an instrument that allows researchers to find their way around a defective chip using scanning electron microscopy, to burrow into the chip with focused ion beam cross sectioning, and to examine the defects beneath the surface with x-ray microanalysis.
Conventional baggage inspection techniques usually are based on dual-energy technology, which analyzes the density of scanned objects. Unfortunately, many products have densities similar to some explosive materials, leading to false-positives. Today, more than 30 percent of all checked baggage must be forwarded to another level for further inspection (usually by the monitor operator) or must be searched by hand, a process that is expensive, time-consuming and subject to human error.
Explosives have an x-ray diffraction pattern that is distinct from many other materials commonly found in checked baggage. Suitcases failing the primary dual-energy scan are conveyed automatically to secondary x-ray diffraction equipment featuring cryogenically cooled x-ray detectors. X-ray diffraction decreases false-positives and hand searches to the single-digit range, cutting inspection costs, improving the safety of air travel and helping maintain on-time departures.
A number of cooling technologies have been developed to improve the performance of these precision detectors. Liquid nitrogen was one of the earliest. Others include closed-loop mixed-gas refrigerators, thermoelectric coolers, pulse tube refrigerators, Gifford-McMahon refrigerators, Joule-Thomson coolers and Stirling refrigerators. Each has its advantages and disadvantages, depending on the particular application, and when selecting a detector cooler, it is helpful to consider convenience, performance and the effects of vibration.
The detector cooler should be easy to install, utilize and maintain, and should provide safe, uninterrupted cooling. All cooling technologies based on mechanical refrigeration must reject heat to the environment. Both air- and water-cooling are commonly employed. Air-cooled compressors are more convenient because they do not require water hookups, making them typically easier to install and move. Closed-loop refrigerators and solid-state devices have fewer maintenance requirements than liquid nitrogen and open-loop Joule-Thomson coolers because there is no depleted cryogen to refill. Some refrigeration technologies require more costly and more frequent maintenance than others.
Another factor to consider is cold-end temperature, understanding that many cooled CCD cameras operate between 260 and 2120 °C. There is only a minimal benefit from cooling to lower temperatures than is necessary, and extremely low temperatures may damage some parts of the camera. By contrast, many x-ray and gamma-ray detectors require cooling below about 2170 °C for proper operation. (Some newer types require cooling to only 230 °C.)
Remember to include heat leaks when calculating the required cooling capacity for a specific application. Usage of 1 liter of liquid nitrogen per hour is equal to 45 W of cooling. If liquid-nitrogen cooling has already been used for the same application, it is easy to estimate the total heat load by multiplying the liquid nitrogen consumption (in liters per day) by two to determine the required cooling capacity (in watts).
The speed of the cooldown is important so that imaging can begin in a timely manner. Although many coolers remain cold once they are switched on, cooldown time becomes a factor if they are ever warmed up. A larger cooling capacity will contribute to a faster cooldown.
Another consideration is vibration. Some applications can tolerate vibrations better than others, but the cooling technology must not produce vibrations that adversely affect the detector resolution. In general, more moving parts in the cold end result in more vibrations in the detector. A compact cold end that is insensitive to orientation also is desirable when selecting a cooler for detector applications.
Meet the author
Chris Rebecchi is a senior applications engineer with Helix Polycold Systems Inc. of Petaluma, Calif., a subsidiary of Helix Technology Corp.; e-mail: firstname.lastname@example.org.