Presentation on theme: "Problems The ratios e/m of 4 He + and D 2 + ions are very close: m D 2 = 4.0282 amu m He = 4.0026 amu m = 0.0256 amu A high resolution mass spectrometer."— Presentation transcript:
Problems The ratios e/m of 4 He + and D 2 + ions are very close: m D 2 = 4.0282 amu m He = 4.0026 amu m = 0.0256 amu A high resolution mass spectrometer should be required in order to separate the two peaks. D 2 concentration » expected 4 He concentration Capability of on line analysis during the electrolytic experiment
Conventional techniques Dynamic analysis technique (differential pumping or diffusion through large impedance circuits): alter the original composition of the gas mixture; exhibit low sensitivity; do not allow for on line analysis. Static analysis technique (Cryosorption pumps): may erratically trap helium in the condensates; require a long cycle time and are not suitable for on line analysis. Unable to pump hydrogen isotopes to high degree
Innovative solutions Non evaporable getter (NEG) pumps: remove to a high degree all non inert components of the gas mixture (especially hydrogen isotopes); all inert gases are obviously preserved; exhibit very high pumping speeds (especially for hydrogen); the pumping speed for other active gases depends on the operating temperature of the getter alloy; do not require frequent regeneration, allowing periodic analysis of gas samples during the experiment (roughly every 40 minutes). A storage circuit has been purposely designed, which: allows to periodically sample the gas mixture without influence on the electrolytic process; eliminates any alteration of the gas mixture composition; allows to easily compare the results of the analysis of each sample.
Main features High resolution Mass Spectrometer (Balzers QMA 410). Two NEG pumps are used: SAES Capacitorr B 1300 (600 g of ST185 TiV alloy) operating at high temperature (300 - 400°C) SAES GP200 MK4 W (170 g of ST707 alloy) operating at RT. Dynamic pumping: Main pump: Pfeiffer TMU 261 turbo molecular drag pump Backing pump: Pfeiffer TMU 071 + MD4 No cryosorption pumps are used. Automatic operation (Field-Point + LabView). Pressure stabilized storage circuit.
Schematic layout of the gas analysis system QMA UHV chamber Thermostatic Box Cell UHV containment vessel Storage circuit Sample volume HT NEG pump n.o. n.c. n.o. = normally open valve n.c. = normally closed valve Gas mixture inert gases RT NEG pump GV1 GV3GV5
High resolution spectra of a 4 He-D 2 mixture. dwell time 1 s, scan width 10 amu, scan time for each cycle 10s.
Analog spectrum of an air sample 01520355102530404550 Mass (amu) Ion current (x10 -8 A) 0 1 2 3 4 5 6 40 Ar + 40 Ar ++