(3)He mass spectrometry for very low-level measurement of organic tritium in environmental samples.
ABSTRACT The design, setup and performance of a mass spectrometric system for the analysis of low to very low-level tritium in environmental samples are described. The tritium concentration is measured indirectly by the (3)He ingrowth from radioactive decay after complete initial degassing of the sample. The analytical system is fully computer-controlled and consists in a commercial helium isotope mass spectrometer coupled with a high vacuum inlet system. A detection limit of 0.15 Bq/kg is routinely obtainable for sample sizes of 20g of water equivalent and an accumulation time of three months. Larger samples (and/or longer accumulation time) can be used to obtain lower detection limits. In addition to the benefit of a lower detection limit, another advantage of this non-destructive method lies in the simplicity of the analytical procedure which strongly limits the risk of contamination. An inter-comparison was successfully performed with the conventional beta counting technique on lyophilized grass samples, in a range of tritium concentrations of environmental interest. It shows that the (3)He mass spectrometry method yields results that are fully consistent with the conventional liquid scintillation technique over a wide range of tritium concentrations.
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ABSTRACT: The efficiency of many noble gas mass spectrometers to ionize gas species is known to be a function of the pressure of gas in the spectrometer. This work shows how the half plate voltage for maximum He or Ar signal depends on the spectrometer pressure and shows that the half plate voltage for maximum 4He sensitivity does not coincide with the half plate voltage for maximum 3He sensitivity. In addition, half plate voltage has a greater control on sensitivity at higher spectrometer pressures. Variations in He and Ar sensitivity and isotopic discrimination as a function of pressure are due, at least in part, to these variations in the position of maximum sensitivity with respect to half plate voltage. The maximum sensitivity settings shift to lower half plate voltage at high spectrometer pressures, irrespective of if the pressure increase is due to the gas being investigated or a different species. Therefore noble gas mass spectrometers should always be tuned at the maximum possible pressure; measurements at higher pressures should be avoided. Significant errors in the spectrometer sensitivity and discrimination can result from improper tuning and calibration of noble gas mass spectrometers.Geochemistry Geophysics Geosystems 01/2000; · 2.94 Impact Factor
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ABSTRACT: Tritium content was measured in snow deposited in Antarctica before the first arrival of artificial tritium (1954), both at Dome C (74 deg 39 min S) and South Pole. The mean tritium values at the time of precipitation are equal to 32 plus or minus 2 and 26 plus or minus 3 TU at South Pole (1939-1942) and Dome C (1951) respectively. A seasonal pattern of natural tritium fallout identical to the artificial pattern (with a winter maximum) is observed in the South Pole profile. The deposition rate of natural tritium is 0.6 and 0.2 tritium atoms per sq cm s at South Pole and Dome C respectively. Its geographical variation over Antarctica appears quite similar to that existing for artificial tritium, clearly showing that the same mechanisms govern the deposition of natural and artificial tritium, in particular tritium injection over polar regions during Antarctic winter. A mean deposition rate of 0.37 tritium atoms per sq cm s, over Antarctica is derived giving an estimated mean global production rate of 0.2 tritium atoms per sq cm s. This strongly suggests that natural tritium results only from cosmic ray production.Geophysical Research Letters 01/1982; 9(10):1191-1194. · 3.98 Impact Factor
Article: Tritium in Nature.Science 02/1951; 113(2923):1-2. · 31.03 Impact Factor