Radiography and Radio-Therapeutics (Volume 2)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 edition. Excerpt: ...to earth, readings are taken when each of the two quantities of radium to be compared stand at some definite distance from one of the lead windows, and a comparison of these results, if one of the radium tubes has been standardised against a known quantity of pure radium salt, will enable the-quantity in the other tube to be determined. It would be scarcely appropriate to go into great detail here as to this matter, but it must be pointed out that several precautions have to be taken. The avoidance of air currents, the allowance for soakage, the correction for the N_ must all be attended to. The charge upon the guard tube should remain constant, and for that purpose a set of 200 Leclanche batteries answers well (No. 3 size). The most important condition of all, however, is that we charge the leaf stem to a sufficient potential to enable it to attract all the ions of opposite sign as quickly as they form in the gas within the apparatus. A good way to test this consists in measuring the ratio of the ionisation produced by Gamma rays from two specimens of radium salt, one of which weighs about twice as much as the other. When brought close to the electroscope the ionisation due to the rays from the larger quantity may be so great that all the ions are not caught before appreciable recombination occ1u's, whereas the lesser tube will give fewer ions, all of which may be attracted to the leaf stem. It is obvious that the readings then will not give the true value for the ratio of the quantities of radium present. At a greater distance, however, the correct result is obtained, and beyond that point no further change in the ratio should be observed. By this means experience will show at what rate the leaf...