Spinthariscope

SPINTHARISCOPE of Crookes, an instrument for observing the luminescence caused by alpha (α) rays (see) of radioactive substances. Upon striking a phosphorescent screen, these particles cause the phenomenon of "sparkling" - scintillations. Since each flash is the result of the impact of one alpha particle, the number of luminous points appearing as a result of such bombardment can serve as a measure of the radioactivity of the substance. For counting these sparks, the spinthariscope serves, proposed by Crookes in 1903. At the bottom of a metal tube B (fig.) with blackened inner walls is a phosphorescent screen N, covered with calcium sulfide; in close proximity (about 0.5 mm) from it is placed a metal tip I, carrying at its end a negligible amount of radioactive substance that emits alpha particles. The latter, upon hitting the screen, cause the phenomenon of point flashes - scintillations, which can be observed and counted through a lens C, placed at the bottom of a second movable tube A. Knowing the area of the screen and the amount of substance being tested, one can determine the number of alpha particles emitted per unit time by one gram of substance, and thus numerically express its radioactivity.