34 MEASURING TIME EN GEOLOGY 



mass, the spectrum of the masses of atoms, can be detected. The 

 difference between these two types of instrument is that in a mass 

 spectrograph the atoms of different mass fall on different spots of a 

 photographic plate, whereas in the spectrometer they pass through 

 collimator slits and are counted separately by an electronic device. 

 In the spectrograph the relative amount of the various isotopes is 

 measured by the intensity of the dark spots they produce on the 

 photographic plate. So, basically, the instruments are quite similar, 

 but the mass spectrometer is much more sensitive. 



Mass spectrometric methods are described in many textbooks. For 

 our study we will follow the lucid treatment of Russel and Farquar 

 (1960). Mass spectrometers are used mainly to distinguish between 

 isotopes of a single element, or more generally between various kinds 

 of atoms which are very close in their properties. Isotopes of one 

 element, let it once more be stated explicitly, are identical in chem- 

 ical properties. All atoms of the various isotopes together form that 

 element. Their only difference lies in their atomic weight. They are 

 identical in atomic number and in their peripheral electrons, which 

 determine their chemical properties. 



However, because of this very slight difference in atomic weight, 

 isotopes of one element show very slight differences in physical 

 properties, such as temperature and pressure of their boiling point, 

 etc. Most of these differences are, however, too small to form a basis 

 for reliable quantitative determination of relative amounts of isotopes. 



The best technique in use today is the deflection of electrically 

 charged particles passing through a magnetic field. Such particles, in 

 this case the electrically charged individual atoms or ions of the 

 isotopes under study, will undergo a force perpendicular to the 

 magnetic field and to their own path, and proportional to their own 

 mass. In a uniform magnetic field ions of the same mass flying 

 perpendicular to that field with the same velocity, will describe a 

 circular path. 



In practice, a beam of ions of varying mass is sent through a high- 

 vacuum tube, perpendicular to a constant and uniform magnetic 

 field. Upon entering the tube, the ions are accelerated by an elec- 

 trical high-potential field V, from which they all acquire the same 

 energy /a 7nv'^. Their trajectory through the tube will show slight 

 differences in radius, which are related to their mass by the formula 



