Speaker
Description
Due to the worldwide scarcity of the extremely rare isotope helium-3 and its high procurement costs, it is economically reasonable to recycle contaminated gas from discarded detectors.
Therefore, we are frequently faced with the task to clean and to reuse this gas for the construction of new detectors or to recondition the gas within existing detectors in order to maintain long term measurement quality.
Typically, the detectors are filled with a gas mixture consisting of 75% helium-3 and 25% tetrafluoromethane (CF4) at an absolute pressure of approximately 4 bar.
Over the course of operation, however, undesired gaseous contaminations, for example due to desorption from the detector surfaces, due to contamination during gas refilling procedures or due to activation induced by neutron irradiation lead to a degradation of the detector performance.
To address this issue, a setup was designed and constructed enabling the purification of helium-3 detector gas from existing detectors as well as the recovery of helium from any gas mixtures of unknown purity for subsequent reuse.
The design of the system was optimized to both minimize the total pipe volume and to provide a helium leak tightness better than 1 × 10⁻⁸ mbar l/s. In addition, emphasis was paid to high robustness and operational safety.
Furthermore, the system allows for precise quantitative in-situ gas analysis by means of a quadrupole mass spectrometer. The purification unit consists of a three-stage cryogenic trap. The first and second stages are cooled by LN2 (77K), the third one by LHe (4.2K). When the gas flows through the first LN2-cryogenic trap without an adsorber, the majority of the CF4 is frozen out. The second LN2 cryogenic trap is filled with activated carbon and, due to its large surface area, is intended to bind the atmospheric oxygen contained in the gas. The LHe-cold trap, the last stage, is needed to remove all impurities below 77K. Experiments showed that high gas purities could be achieved corresponding to a final contamination level of less than 10 ppm for all impurities with condensation temperatures above 5K. However, it is not possible to separate helium-4.
After being purified the helium-3 can be compressed with negligible losses into external storage vessels up to a maximum pressure of 15 bar by means of a membrane compressor.
The poster describes the technical implementation of the helium-3 recovery system, the gas purification process, and the results of the performed gas analyses.