From Current Literature

Mass Spectrometer for Cosmic Radiation

It is known that almost all methods currently used for determining the mass of charged particles that make up cosmic radiation are based on the assumption that a charged particle loses its energy only through ionization. In this case the well-known Bethe–Bloch formula makes it possible to determine the velocity of the particle. However, other, more direct methods are also possible for determining the mass of a charged particle. One of these methods is, for example, the measurement of the particle momentum \(p = mv\) from its deflection in a magnetic field and the direct determination of the particle velocity \(v\) from the interval of time during which the particle traverses a given distance. Another well-known method, used in mass spectroscopy, consists in the simultaneous deflection of a charged particle in electric and magnetic fields.

A mass spectrometer of this type was built in 1948 at the Physics Institute of the University of Łódź*) (Poland). The principle of its operation is clear from the figure. In this figure four rows of Geiger counters are visible. Each row consists of two layers of counters, with the axes of the counters in both rows mutually perpendicular. The radio-engineering circuit controlling the operation of the mass spectrograph is arranged in such a way that, when a particle passes through rows 1, 2, 3, and 4, it records exactly which counters of these rows the particle passed through (a hodoscopic system).

The electric field is produced by capacitor plates (plate length \(5\) m, distance between them \(45\) cm at the upper ends of the plates and \(110\) cm at the lower ends), between which a potential difference of \(10^6\) V is applied. The magnetic field, of strength 300 oersteds, produced in

) Cosmic Radiation*. Colston papers, p. 155, London, 1949.

region 1 m long, situated under the upper plates of the condenser, directed parallel to the electric field. To reduce the error in determining the initial direction of motion of the charged particle, arising from scattering of the particles in the counter walls, the counters of the second row are made thin-walled (glass 30 μ thick, coated with aquadag).

The distances between rows of counters 1 and 2 are 10 m, and between 2 and 3, 5 m. A fourth row of counters, located at a distance of 2 m from row 3, serves to check the correctness of the trajectory. The greater part of the mass spectrometer, including the second row of counters, is in vacuum.

The spectrometer is intended for investigations in the region of low cosmic-radiation particle energies. Thus, for example, for positively charged mesons (mass \(200\,m_e\)) the instrument covers a momentum interval from 85 to 135 MeV. The mean deflection in the electric field is then \(6^\circ\). The expected intensity is 5 particles per day. The authors of the instrument described believe that the accuracy of the mass measurement that can be attained with it is 1%.

A. E.