Highly sensitive, electrostatic collection chambers have been developed for low-level radon measurements using CR-39 plastic track detectors. Chambers with outer electrodes shaped as a cone and a hemisphere with lines of electric force that were focused on the center of the collector electrode gave alpha-track distributions similar to a 2-dimensional Gaussian with a FWHM of less than 3 mm. The sensitivities, defined as the track density for a radon exposure of 1 Bq h/m3, were more than 4 tracks/cm2 using a chamber with a volume of about 300 cm3. The dependencies of the sensitivity and the track distribution on the collector voltage and humidity were investigated.
A 12-stage Cu–Be electron multiplier was constructed for detecting very feeble ion current. General characteristics of the multiplier coupled with a vibrating reed electro-meter were investigated by measuring the output of a high vacuum mass spectrometer. The stability of the multiplier gain was sufficient for the measurement of isotope ratios. For example, the 20Ne/22Ne ratio was found to be 10.563±0.006 (un-corrected for mass discrimination).
At an ultimate pressure of 10-8 Torr in this mass spectrometer, background mass spectra up to M/e 150 were recorded, and some hydrocarbon fragment peaks corresponding to 10-16–10-18 amperes were found in C2–C7 groups. Mass peaks below 10-16 amperes fluctuate considerably, but such statistical fluctuations may be smoothed out by using an amplifying system with a sufficiently long time constant and a low scanning speed. Under such conditions, an ion current down to 1×10-19 amperes was distinguishable over the noise level of the amplifying system.
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