Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.225g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 225
Physics
Scientific paper
A time-of-flight mass spectrometer with a resonance ionization source [1,2] and a cryogenic sample concentrator [3,4] which increases the efficiency of the spectrometer by two orders of magnitude, has been constructed for the isotopic analysis of xenon. In the two-photon excitation, one-photon ionization scheme employed, a wavelength of 249.6 nm excites the 2P(sub)3/26p[1/2](sub)0 level of xenon. A pulsed laser operating at 10 Hz, with a pulse duration of 8 ns and energy of 1.5 mJ is used. The sample concentrator consists of a localised cold spot (<90 K) onto which the xenon sample is condensed. The spot is heated by a Nd:YAG laser (25 mJ, 6 ns), evaporating the condensed sample gas into the ionizing region 1 us (chosen for optimum sensitivity) before the ionizing laser is fired. The mean return time to the cold spot is about 10 seconds in the 450-ml spectrometer and corresponds to an effective spot diameter of 1 mm. With each laser shot, 1% of the atoms released by the heating laser are ionized and, between shots, 1% of the sample recondenses onto the cold spot. Thus at 10 Hz, 1000 atoms produce a signal of 1 cps, which compares with a typical sensitivity of around 15,000 atoms/cps for conventional instruments. Since all isotopes are detected simultaneously, the effective sensitivity is more than two orders of magnitude greater than a single collector conventional instrument. Furthermore the background count rate due to multiplier dark current at a given mass is negligible (1 count per week) and the sole limitation on ultimate sample size is adsorbed xenon on the sample and the spectrometer walls. The velocity distribution of atoms leaving the cold finger has been determined by varying the delay between heating and ionizing lasers, and is Maxwellian with an angular dependence of the form cos^n(q) (n>5) and a characteristic temperature of 350 K. For sampled atoms of velocity v and characteristic temperature T, the theoretical instrumental discrimination [3] is 3/2m-1.66 x 10^27 v^2/2kT per mil per amu and is confirmed by experiment. Nonresonant ionization of hydrocarbons may occur at the high power densities necessary to saturate the two-photon transition of xenon (about 10^-9 Wcm^-2). Residual hydrocarbon effects in the spectrometer are monitored by detuning the wavelength of the ionizing laser away from resonance and are negligible. One side effect of the sample concentrator is that hydrocarbons tend to condense on parts of the cold spot the heating laser does not reach. Moreover most of the hydrocarbons that are evaporated leave the surface with velocities different from those of xenon and do not see the ionising laser beam. The spectrometer is currently being used to analyse xenon in meteorite residues and cosmogenic xenon in terrestrial barite. References 1. C H Chen, G S Hurst, M G Payne (1980) Chem. Phys. Lett. 75, 473-477. 2. J D Gilmour, S M Hewett, I C Lyon, M Stringer, G Turner (1991) Meas. Sci. Technol. 2, 589-595. 3. J D Gilmour, S M Hewett, I C Lyon, I Perera, G Turner, in prep. 4. G S Hurst, M G Payne, R C Phillips, J W T Dabbs, B E Lehmann (1984) J. Appl. Phys. 55, 1278-1284.
Gilmour James D.
Johnston W. A.
Lyon Ian C.
Turner Gary
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