From Current Literature

Secondary Emission Properties of Alkaline Earth Metals

Until 1937 it was considered reliably established by the work of Copeland¹ and Warnecke² that pure alkaline earth metals (Be, Ba), unlike all other metals, possess values of the secondary-emission coefficient at the maximum ($\sigma_{\max}$) of several units. In 1937, after the appearance of a preliminary communication by Bruining and de Boer³, this incomprehensible discrepancy in the properties of different metals was explained. It proved to be nonexistent, since the alkaline earth metals indicated, in a really pure form (obtained by evaporation in vacuum), were found to have $\sigma_{\max}$ less than unity. This circumstance was also discovered independently by other authors, who established that the cause of the discrepancies lies in the fact that these metals cannot be freed from gas by annealing in vacuum sufficiently to restore to them the properties of pure metals⁴.

A more detailed investigation of the influence of gas (oxygen) on the secondary emission of beryllium was carried out by Kollath⁵, who studied both massive beryllium plates and layers of this metal (obtained by evaporation in vacuum) subjected to various treatments. For massive beryllium he found $\sigma_{\max}$ equal to $4 \text{—} 5$, at primary-electron energies of about 500 V, i.e. the very same values that Copeland had published at one time. As for layers obtained by evaporation in vacuum, it was established here that simple exposure to an atmosphere of oxygen (or air) gives $\sigma_{\max}$ about 2, as does treatment by a discharge in oxygen ($\sigma_{\max}$ up to 2.2). Other authors obtained by the same methods even larger $\sigma_{\max}$ (about 3). More effective proved to be the method of treatment consisting in heating the evaporated Be layer to 350° in an oxygen atmosphere ($\sigma_{\max}$ up to 4). On annealing in high vacuum a Be layer deposited by evaporation on a metallic substrate (temperatures up to 900°), values of $\sigma_{\max}$ reaching 4 were also obtained (at $V_p$ about 450—500 V). Thus, high values of $\sigma$ are obtained under the simultaneous action of oxygen and high temperature. This leads the author to believe that, along with the formation of compounds, structural changes of the layer also play a role in the change of secondary-emission properties, and that this latter factor is even the principal one.

Kollath’s experimental data on the beneficial influence on $\sigma_{\max}$ of alkaline earth metals of simultaneous action by oxygen and heating to a high temperature have found confirmation in the recent experiments of Timofeev and Afanas’eva⁶ and of Timofeev and Aranovich⁷. In the first of these works it was found that, for a layer of Mg deposited by evaporation in vacuum and having $\sigma_{\max} \simeq 1$ (at $V_p \simeq 200$ V), after heating to 600° in an oxygen atmosphere $\sigma_{\max}$ proved equal to 6.7 (at $V_p \simeq 900$ V). Annealing this layer in vacuum up to 800° gave some increase of $\sigma_{\max}$ without any noticeable change in the position of the maximum of the curve $\sigma = H V_p$. With the same treatment, Timofeev and Aranovich found for Be $\sigma_{\max} = 6.5$ at $V_p = 500$ V, and for Mg $\sigma_{\max} = 7.5$ at $V_p = 500$ V.

It is highly significant that such emitters, along with high $\sigma$, also possess great temperature stability. This makes it possible to increase the output powers of electron multipliers and thereby broaden the range of their possible applications.

N. Khlebnikov, Moscow

References

1. P. L. Copeland, Phys. Rev., 46, 167, 1934.
2. R. Warnecke, J. Phys. Rad., 7, 270, 1936.
3. H. Bruining and J. H. de Boer, Physica, 4, 473, 1937.
4. N. S. Khlebnikov, Journal of Technical Physics, 8, 994, 1938.
5. R. Kollath, Ann. Physik, [5], 33, 285, 1938.
6. P. V. Timofeev and A. V. Afanas’eva, Journal of Technical Physics, 10, 28, 1940.
7. P. V. Timofeev and R. M. Aranovich, Journal of Technical Physics, 10, 32, 1940.