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Pulsed Mass Spectroscope*)
Recently the method of mass-spectrographic measurements has acquired great importance not only in scientific, but also in practical research. This has naturally entailed the improvement
Fig. 1. Diagram of the mass spectroscope.
and simplification of the design of mass spectrographs, as well as the search for new principles for measuring atomic masses.
A description of the circuit of a new mass spectroscope has recently appeared in print; its design is simpler than those usually employed and does not require the use of crossed magnetic and electric fields.
The operation of the instrument (Fig. 1) is based on the following idea. A beam of the atoms under study is modulated in the form of short pulses of duration \(0.2\ \mu\mathrm{sec}\). After passing through the diaphragms, the beam of atoms enters the accelerating field. After passing through it, atoms of different
) Helvetica Physica Acta 22*, 386 (1949).
masses acquire different velocities. Therefore the beam of atoms, which initially forms one momentum common to all atoms, breaks up into a series of pulses flying one after another. Light atoms overtake the heavier ones and reach the receiving Faraday cylinder earlier than this occurs with the heavier atoms. As a result, the amplifier connected to the receiving cylinder registers as many pulses as there were kinds of atoms in the original beam. The signals, amplified five hundredfold, are fed to an oscillograph. Synchronization of the moment at which the atomic pulse is emitted with the beginning of the oscillograph sweep is accomplished by means of a
Fig. 2. Mass spectrogram.
two-stage amplifier with cathode coupling, which feeds pulses to the source of atoms from the oscillograph sweep circuit. The pulse repetition frequency is 2500 hertz.
Figure 2 shows a mass spectrogram obtained with a mixture of deuterium with several percent oxygen. Each division of the oscillogram corresponds to \(0.25\ \mu\text{sec}\). In order to obtain sharp maxima on the oscillogram, it is necessary that the duration of the pulse be small in comparison with the ion flight time.
| Divisions . . . . . | 2,8 | 3,9 | 5,5 | 6,8 | 8,0 | 8,9 | 9,7 | 11,2 | 13,1 | 16,1 |
|---|---|---|---|---|---|---|---|---|---|---|
| Time in \(\mu\text{sec}\) . . | 0,70 | 0,98 | 1,38 | 1,70 | 2,00 | 2,22 | 2,43 | 2,80 | 3,27 | 4,02 |
| \((t/t_H)^3\) . . . . . | 1,00 | 1,96 | 3,9 | 5,9 | 8,2 | 10,1 | 12,0 | 16,0 | 21,8 | 33,0 |
| Ions . . . . . | \(H^+\) | \(D^+\) | \(D_2^+\) | \(D_3^+\) | \(O^{++}\) | \(O_2^{++}\) | \(C^+\) | \(O^+\) | \(CO_2^+\) | \(O_2^+\) |
The table gives the results of processing the mass-spectroscopic oscillogram of Fig. 2. In the third row of the table is given the square of the ratio of the flight time of the given ion to the flight time of the proton, i.e., a quantity that should be equal to the ion mass.
The accuracy of the instrument may also be estimated from the following calculations. The flight time of a \(D^+\) ion is determined from the formula
\[ t(\mu\text{sec})=3.2\,\frac{l(m)}{\sqrt{u(\text{kV})}}. \]
Since the first part of the instrument, \(0.38\ \text{m}\) long, is traversed by the ion after acceleration by four kilovolts, and the second part of \(0.38\ \text{m}\) after acceleration by 30 kilovolts, the flight time is equal to \(0.61+0.20=0.81\ \mu\text{sec}\). To this time one must add the time of emergence of the ion from the ion gun, equal to \(0.12\ \mu\text{sec}\). Thus the calculated flight time is \(0.93\), and the measured time is \(0.98\ \mu\text{sec}\). The agreement is quite satisfactory.
It may be assumed that this simple and ingenious method of separating and measuring atomic masses will be improved and put into use.
K. Vulfson