CRYSTAL COUNTER

The idea of using an AgCl crystal for counting radioactive radiations belongs to Van Heerden. His work is set forth in a book* which undoubtedly deserves the attention of physicists working in this field.

The energy of β- and γ-radiation is at present measured by a number of methods, of which the principal ones are the magnetic β-spectrograph and the Wilson chamber. For measurements with weak preparations these methods are unsuitable. The energy of α-particles can also be determined with the aid of an ionization chamber. For this it is necessary to amplify the pulse from the particle proportionally, and then its magnitude can be used to judge the energy. The ionization chamber must have such a depth that the entire range of the α-particle is contained within it. The application of such a method to electrons is impossible, since their range in air is not centimeters, as for α-particles, but several meters. A chamber of such large dimensions is not suitable for use.

If it were possible to find substances in which β-particles have an insignificant range and in which the ionization is nevertheless sufficiently large that an individual particle could be observed and the magnitude of the ionization it creates measured, then we would thereby obtain a method for measuring the energy spectra of weak preparations. In addition, it is necessary that, as in a gas, the ionization produced by the particle should be a single-valued function of the energy.

A number of physicists have tried to work with liquid ionization chambers (Jaffé) and with chambers containing gas under high pressure (Kraus and Kleé). Such chambers meet the latter condition, but, because of strong recombination, the ionization current amounts to only 1/3–1/10 of the current in an ordinary chamber for α-particles of the same energy. The charge-collection time in such chambers is large ($10^{-2}$–$10^{-1}$ sec.). Both these circumstances make their application difficult. Observation of pulses from an individual particle in such chambers is altogether impossible. The prospects for using a solid for this purpose were still less favorable. The ionization current observed, for example, when mica was irradiated with α-particles amounted to only 1/1000 of the corresponding current in a gas.

Van Heerden found that, unlike other solids, an AgCl crystal prepared in a suitable manner and cooled to the temperature of liquid air has properties that interest us. The AgCl crystal had a diameter of 4 cm and a thickness of 4 mm. Two of its opposite faces were silvered and served as electrodes. A collecting voltage of about 2000 volts was used. The pulses were recorded on film with the aid of a suitable amplifier.

In the first series of experiments such a crystal was placed in a β-spectrograph as an electron indicator. In this case the crystal was irradiated with monoenergetic electrons, the energy of which could be set. It turned out that the pulses for electrons of a given energy are grouped near a quite definite value, which, in turn, proved proportional to the electron energy. The magnitude of the pulse corresponds to the fact that the electron expends 7.6 eV in transferring an electron from the crystal lattice into the conduction band. The electron-collection time is determined by their mobility in the conduction band. It is of the order of $10^{-6}$ sec. This circumstance advantageously distinguishes the crystal from an ionization chamber and justifies the name “crystal counter.”

A number of circumstances complicate the effect even when monoenergetic electrons are used. They lead to the fact that, in addition to the principal, most intense group of pulses, there are pulses of smaller magnitude. In os-

* The Crystalcounter. A new instrument of Nuclear Physics. By P. J. Van Heerden. N. V. Noord-Hollandsche Uitgevers Maatschappij, Amsterdam, 1945.

Crystal Counter

mainly this occurs because, as a result of the scattering of electrons, their path in the crystal differs. This effect can apparently be eliminated. Thus, in summary, it may be said that for the study of the $\beta$-spectra of weak preparations this method is quite applicable and has the advantage that it makes it possible to use a solid angle of $4\pi$, rather than such small angles as in a $\beta$-spectrograph.

In a second series of experiments the author, using a crystal, studied the $\gamma$-spectra of Ra $(B + C)$ and Th $(B + C)$. The attempt was not crowned with success. The author did not succeed in obtaining reproducible results. This is apparently explained by the fact that $\gamma$-rays produce secondary electrons of different energies. Moreover, the $\gamma$-spectra themselves are sufficiently complex. Thus, although the use of a crystal for measuring $\gamma$-spectra is attractive, it is hardly possible.

When using a crystal for registering $\alpha$-particles and other heavy particles, it is necessary to take into account that for them recombination is considerably more significant than for electrons. The author has shown that the pulses obtained in the crystal from $\alpha$-particles are 4–5 times smaller in magnitude than those from electrons of the same energy.

It is quite obvious that the occurrence of current in a crystal under the action of electrons is explained by the fact that electrons, just like photons in the photoelectric effect, transfer electrons into the conduction band. The book contains a section in which the author attempts to give a theoretical explanation of the fact that not all crystals exhibiting the photoelectric effect (for example, diamond, zinc blende) are sensitive to electrons.

I. Ya. Barit