Outgassing properties of some materials used to assemble gaseous detectors and gas systems

based on Ageing of gaseous detectors: assembly materials and procedures by Mar Capeans Garrido

Aging of gaseous detectors is known as the degradation of their performance under the exposure to ionizing radiation. It is a complex phenomenon that depends on many parameters. Among others, aging depends on the gas mixture and may be enhanced by the presence of pollutants in the gas. The origin of the impurities is diverse and includes outgassing from assembly materials and the gas system components, and contamination of the detector during the assembly process.

List of tables

• Sealants used for fixing small gas leaks
• O-rings
• Plastic pipes
• Rigid materials
• Low outgassing epoxy compounds curing at room temperature
• Epoxy compound curing at room temperature for which the GC detects som pollutants at the ppm level
• Investigated epoxy compounds curing at temperatures above 50 ° C
• Outgassing tests of conductive epoxies
• Gas Analysis of adhesive tapes

Gas analysis

A method to determine the outgassing properties of certain material is the analysis of a gas sample that is in contact with the material. A basic test consists of flushing clean gas through a box where the material under investigation has been introduced and analyzing the gas as it flows out. The surface of each individual sample and the gas flow should be kept constant to allow comparative studies. The material can be heated up in order to increase the rate of possible outgassing, so that relative large amounts of pollutants can be produced in a few days, thus increasing the sensitivity of the measurements (unfortunately the scaling factor is unknown). Still impurity detection has the intrinsic limitations of the analysis station: i) the incapacity to detect substances below some given quantity, and ii) the inability to separate, thus identify, certain compounds. Another drawback is that even if outgassing is detected, the particular pollutant might not be harmful for the gaseous detector operated at high rate. Data presented here has been obtained analyzing the gas composition and possible pollutants using a Gas Chromatograph1 (GC) and two associated detectors: a Mass Spectrometer2 (MS) and an Electron Capture Device3 (ECD). The GC is simply an oven with a capillary column capable of separating gas substances depending on their interaction properties with the column. A signal appears for each separated compound at some retention time, defined by the column and temperature profile. In the MS detector a 70 eV electron source ionizes and fragments molecules. A quadrupole mass filter sorts the resulting ions according to their mass over charge (M/Z) ratio. Signal appears as ion abundance as function of retention time in the column or M/Z ratio. As a result, identification of each molecular compound is possible, having detection sensitivity up to the ppm level. The ECD is the second detector connected to the GC. Its operation is based on the fact that at normal temperature and pressure some gases behave as perfect insulators. Consequently, the presence of very few charged species (electronegative molecules such as halogens and halocarbons freons-) can be readily observed by amplification in an electric field. There is no information other than signal amplitude versus retention time, and a specific calibration has to be made for each compound to attain pollutant identification. Its advantage is the extreme detection sensitivity, better than ppb.

Gas analysis and aging test

A positive result obtained with the method described in the previous section should be considered as necessary although not sufficient for a material to be used for assembling a detector. The definite test should consist of an aging test. Comparable from a qualitative point of view is the answer obtained monitoring the gas gain of a clean, validated gaseous detector, which is connected downstream the outgassing box where the material under test is introduced, and strongly irradiated. This test allows correlating the presence of impurities in the gas with aging effects in the detector. In this case the response is more extensive than the simple gas analysis, but unfortunately the irradiation conditions (high dose rates) make it difficult to extrapolate the results to the final running conditions and lifetime scale of the experiments. Thus existing data obtained either from systematic outgassing studies or experience gained with detectors has only a pre-selective character when designing new detectors.

Outgassing tests carried out for some sealants used for fixing small gas leaks in chambers and gas systems

"?" = Even if some outgassing is detected one should note that this components are usually employed in very small quantities to fix small gas leaks. This would explain why for instance the pollution outgassed from DOW CORNING R4-3117 RTV does not affect the response of the irradiated detector where few leaks have been potted with few milligrams of this sealant.
"-" = has not been tested

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Results of tests carried out for some commonly used O-rings

"-" = has not been tested

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Outgassing properties of some plastic pipes

* water at <100 ppm level is detected due to water diffusion through the pipe walls. This has to be taken into account specially for detector using F-containing gases in high radiation environments.
"-" = has not been tested

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Rigid materials tested for outgassing

"-" = has not been tested

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Low outgassing epoxy compounds curing at room temperature

Epoxy compounds that polymerize at room temperature are very attractive because they make it possible to assemble detectors having materials with very different expansion coefficients.

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Epoxy compounds curing at room temperature for which the GC detects some pollutants at the ppm level

"-" = has not been tested

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List of investigated epoxy compounds curing at temperatures above 50 ° C

*Epotek H72, used for the assembly of MSGC+GEM detectors presently running in Hera-B, slightly pollutes the operating gas in the firsts hours of contact with it. Though the irradiated counter shows very stable behaviour under irradiation, indicating that the detected pollutant might be volatile and it is easily removed from the system thanks to the gas flow.

"-" = has not been tested

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Outgassing tests of conductive epoxies

"-" = has not been tested

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Gas analysis of adhesive tapes

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