The 2824HR and 2824HS are high speed, low noise CMOS X-ray detectors with 50 µm and 100 µm pixel pitch respectively and an active area of 280 x 240 mm2.
These indirect X-ray detectors use a 2 x 2 array of waferscale CMOS image sensors bonded to a fibre optic faceplate (FOP) with a CsI(Tl) or Gadox scintillator. Spectrum Logic has now released water-cooled versions of these products to further improve their market leading performance. In the cooled detectors water circulates between a chiller unit (provided by the customer) and a heat exchanger in the detector housing. This cools the detector and maintains a stable temperature which is configured at the chiller. The stable temperature reduces dark noise and stability of the temperature of the sensor array means that the offset correction is stable and does not suffer from variations in the dark image due to temperature changes in the CMOS sensors in the detector.
CMOS sensors have dark noise and one of the components of this noise is leakage current. Leakage current is a function of temperature, so if the temperature of the CMOS image sensor increases, then the dark noise will also increase. Furthermore, if the temperature of a CMOS detector increases during a long CT scan, then the dark image used for offset corrections will need to be refreshed. Cooling can mitigate both issues reducing dark noise and improving stability.
With approximately every 7°C increase in the temperature of a CMOS sensor, the dark current, measured in electrons per pixel per second, doubles.
2824HR Water Cooled Detector
To address this problem, we have developed a water-cooled enclosure for our 2824HR and 2824HS detectors in which temperature-controlled water passes through the detector via a heat exchanger in the heat sink located directly under the CMOS sensor array substrate.
Water-cooled 2824HR detector internal view
Test results (outlined below) show that the 2824HR with water-cooling has a lower dark current and fixed pattern noise than an uncooled detector of the same type. Due to the thoughtful mechanical and electrical design by the Spectrum Logic engineering team, our water-cooled detectors reach a stable temperature within 20 minutes and remain at this temperature while in continuous use. The water temperature used for cooling should be selected to avoid condensation in the ambient temperature and humidity. We used 14°C.
The results from our tests on the 2824HR with water-cooling demonstrate that the cooled detector has 5X lower dark current than the standard enclosure when used in an ambient environment of 22 °C. This innovative design offers significant advantages for scientific, synchrotron, micro-CT and pathology applications where longer exposure times and long scans are used. For example, some high resolution pathology micro-CT studies can run for eight hours. Also, cooling will help with imaging in locations with high ambient temperature, e.g. outdoor weld inspection in India in the summer.
The water-cooling system is modular in nature and can be used in conjunction with any of our standard 2824 models.
Experimental set up for testing the 2824HR with water-cooling enclosure:
• CW-5200 Industrial Chiller
• Chiller water temperature controlled by constant mode (Case 2)
• 14 ± 0.2°C (F0: 14°C, F2: 0.2°C)
• Ambient temperature 22°C
• Let detector temperature stabilise to water cooler temperature before test (>= 10 minutes)
• FW: 2824hrv2-50u-0.7.3.afp (v1.10.181)
• SDK: v2.0.2
Tests:
• Dark current
• Single image at 1s and 10s exposure time
• Periodic images taken after every 10 mins for 3 hours.
• Offset drift.
• 50ms, 100ms, 200ms, 400ms and 800ms exposure time single image after every 10 mins for 3 hours
Results:
Dark Current measured in (ADU/sec)
ADU: Analog Digital Unit (Grey Levels)
Dark current
Dark current over time
• Cooling reduces the dark current
• Cooling keeps dark current constant after about 20 minutes
• Dark current increases as the detector warms up over time without cooling
• Roughly 5 times higher than with cooler at 14°C
Offset Drift
Offset drift. Mean ADU over time
• Cooling prevents the dark current from drifting higher, shown by the flat curves of the mean ADU values.
• ~70 ADU difference after 3 hours without cooling at 800ms exposure time.
• No difference in ADU for all exposure times with cooler.
• The dark current drift is more noticeable at longer exposure times without cooling.
Conclusion
Water cooling of CMOS X-ray detectors using a water chiller and a specially designed detector enclosure, incorporating a heat exchanger, provides reduced dark noise and enhanced stability, which improves offset correction. This is especially beneficial for applications that require long exposure times such as ultra high-resolution micro-CT.