Thermal Ion Source Research Articles

Mass spectrometer for the isotopic analysis of lithium

A mass spectrometer has been designed for the routine analysis of lithium. Microgram samples of lithium nitrate are ionized in a thermal ionization source. A 60°sector magnet analyses the two ion beams which are collected simultaneously on two separate collectors. The source is pulsed and the two signals are a.c. amplified and then substracted from one another. The difference is displayed on a cathode-ray tube and the ratio is given rapidly by reading attenuation dials after adjusting for a null deflection. A calibration technique corrects for unequal transmission through the spectrometer and for inequalities in the circuitry. The precision of a single measurement of isotope ratio is ±0.7%.

The cross-section of the reaction 134Cs(nγ)135Cs and the half-life of 134Cs

The isotopic composition of caesium samples which had been subject to intense reactor neutron irradiation was measured with a 6 in. radius, 60° sector field, mass spectrometer fitted with a triple filament thermal ionization source. The cross-section of the reaction 134Cs(nγ)135Cs was calculated from the observed isotopic composition and found to be 134 ± 12 barns. By measuring the change with time of the 134Cs/(133Cs + 135Cs) ratio using the mass spectrometer the half life of 134Cs was found to be 2·15 ± 0.040.08 years. In a subsidiary experiment the cross-section of the reaction 133Cs(nγ)134Cs was found by activation to be 30·4 ± 1·7 barns.