| Popis: |
Two methods were developed for determining the residual ionization in brass chambers and the absolute ionization of the hard and the soft component of cosmic radiation in the atmosphere at sea level, without secondaries from the walls of the chambers. The portable Gish-Sherman ionization meter in connection with four geometrically similar cylindrical vessels of widely different size was used. One method is based on the use of all four chambers, the other requires only one large chamber in which the pressure is varied between about 0·1 and 1·0 atm. These measurements were carried out on the campus of Fordham University in a garden. It was found that, on the average, the hard component (filtered through 10 em of iron) produces an ionization of 1·51 I while the total intensity (hard+soft component) amounts to 1·88 I . The effect of the gamma rays from the radioactive substances floating in the atmosphere was determined by a new method, described in this paper. This “ air radiation ” amounts to about 0·15 I , varying considerably from day to day. Another set of experiments was carried out to find the ionization produced by the alpha rays from radon, thoron, and their subsequent products. Samples of air from outdoors were filled into a large ionization chamber and the variation of ionization was observed for several days. In this way it was possible to determine the ionizing effect of the radon and thoron products separately. It was found that radon and its subsequent products give 0·93 I , while thoron and its products give 0·83 I . The latter, therefore, contribute 47% of the total ionization by alpha rays. The radon content itself was 46 × 10 − 18 curie per cm 3 (average of November and December). The ionization by gamma rays from the ground, at Fordham. amounted to 3·0 I . The contribution by beta rays from radium, thorium and their decay products in the ground and in the atmosphere was estimated as 0·4 I . The total ionization. on the point of observation, 1 m above ground, consisted therefore of the following four components: Alpha rays 1·76 I Beta rays 0·40 I Gamma rays 3·15 I Total: 7·21 I Cosmic rays 1·90 I Using the linear law of recombination of small ions ( E. Schweidler, J. J. Nolan and others) which holds for air polluted with a great number of nuclei, one can compute the mean number of small ions per cm 3 if one knows the average lifetime of such ions. Taking this lifetime as 10·6 sec (from a recent determination at the same location), one would get less than 100 small ions per cm 3 present in the stationary state. This is to be expected in the vicinity of a big city, where the number of nuclei of condensation ordinarily exceeds 40000 per cm 3 . |
