14
A. Theoretical and Instrumental Background
After passing through the magnetic field, the separated ions are collected and converted into an electrical impulse, which is then fed into an
amplifier. For relatively large ion currents a simple metal tube (Faraday
cage) is used. It generally consists of a hollow metal tube which is
grounded through a high ohmic resistor. As the ion current passes to the
ground, the potential drop caused in the resistor acts as a measure of the
ion current. If the ion currents are small, then some means must be
provided for magnifying the current strength.
By collecting two ion beams for the isotopes in question, simultaneously and by measuring the isotope ratio directly, a much higher
precision can be obtained than from a single ion beam collection. With
simultaneous collection, the isotope ratios of two samples can be compared quickly under nearly identical conditions. NIER et al. (1947) developed this technique for routine measurements, which is shown, combined with one possible read-out system (after NIELSEN, 1968b) in Fig.6.
The DC feedback amplifiers A and B are coupled to the collectors for the
ion beams of the main isotope (iB) and the rare isotope (iA). The output
signal, - U B, of amplifier B is fed to a precision voltage divider (Kelvin
bridge type) and the fraction, p, of - U B balances the input signal obtained from ion beam iA • Thus, with proper adjustment of p, zero signal
is obtained from amplifier A. In this case,
UA-pUB=O.
The zero adjustment is realized for the standard only, and an output
signal zero is recorded for the unknown sample. Thus, alternating the
Strip chart
-p. VB
Fig. 6. Principle of digital ion beam ratio measurement. (After NIELSEN, 1968b)
A. Theoretical and Instrumental Background
After passing through the magnetic field, the separated ions are collected and converted into an electrical impulse, which is then fed into an
amplifier. For relatively large ion currents a simple metal tube (Faraday
cage) is used. It generally consists of a hollow metal tube which is
grounded through a high ohmic resistor. As the ion current passes to the
ground, the potential drop caused in the resistor acts as a measure of the
ion current. If the ion currents are small, then some means must be
provided for magnifying the current strength.
By collecting two ion beams for the isotopes in question, simultaneously and by measuring the isotope ratio directly, a much higher
precision can be obtained than from a single ion beam collection. With
simultaneous collection, the isotope ratios of two samples can be compared quickly under nearly identical conditions. NIER et al. (1947) developed this technique for routine measurements, which is shown, combined with one possible read-out system (after NIELSEN, 1968b) in Fig.6.
The DC feedback amplifiers A and B are coupled to the collectors for the
ion beams of the main isotope (iB) and the rare isotope (iA). The output
signal, - U B, of amplifier B is fed to a precision voltage divider (Kelvin
bridge type) and the fraction, p, of - U B balances the input signal obtained from ion beam iA • Thus, with proper adjustment of p, zero signal
is obtained from amplifier A. In this case,
UA-pUB=O.
The zero adjustment is realized for the standard only, and an output
signal zero is recorded for the unknown sample. Thus, alternating the
Strip chart
-p. VB
Fig. 6. Principle of digital ion beam ratio measurement. (After NIELSEN, 1968b)