Table 646 i- 2 q 



RADIOACTIVITY 

 GAMMA RAYS 



7 rays are extremely penetrating, nondeviable by electric and magnetic fields, produce 

 ionization of gases, act on the photographic plate, excite phosphorescence. Like X rays, 

 they are similar to light. 7 rays are merely X rays produced in the radioactive atoms. 

 The reflection of X rays and 7 rays from crystals leaves no doubt that the wave theory of 

 light is applicable. There are to be solved the same problems, as indicated by Bragg for 

 the corpuscular theory of X rays. The same difficulties exist as in the case of visible 

 radiation. Theoretical investigations on 7 rays, based on the electromagnetic theory, lead 

 to conclusions not very different from those of a corpuscular theory. 



Emission of gamma rays.— The number of 7 rays per sec. from RaB and RaC in 

 equilibrium with 1 g of Ra, is 1.43 X io 10 and 1.49 X io 10 (Hess-Lawson). The mean 

 value obtained by Kovarik for the number of 7 rays per sec. from Ra(B -f- C) in equilibrium 

 with 1 g of Ra was 7.28 X io 10 , which is nearly (within 2%) one 7 ray per atom dis- 

 integrating. The random emission in time of penetrating 7 rays from radium has been 

 proved. 



Energy and wave length of gamma rays.— The energies and wave lengths of 7 rays 

 have been obtained variously ; much further research is required. The direct experimental 

 determination of 7-ray wave lengths by reflection from a crystal (NaCl) was first 

 made by Rutherford-Andrade for the 7 rays of RaB and RaC. Both surface planes and 

 internal planes were utilized. They showed that certain strong lines of the RaB 7-ray 

 spectrum are identical with characteristic X rays (L series) of nonradioactive lead. The 

 shortest wave length measured was that of a 7 ray of RaC reflected at a grazing angle 

 of 44' having a wave length of about 70 X.U. (1 X.U. = io" u cm= io" 3 A.U.). The 

 counting method was applied by Kovarik to high frequency 7 rays of RaC reflected from 

 calcite. The shortest measured wave length was about 28 X.U. 



The determination of 7-ray wave lengths from mass absorption is made on the supposi- 

 tion that the relation between mass absorption and wave length found for X rays may 

 be applied to 7 rays. For X rays, outside regions of selective absorption, p./p = k\" where 

 X is the wave length and n has a value 2.5 to 3. Rutherford found that as the mass absorp- 

 tion coefficient, p./p, of 7 rays approaches the order of magnitude of the mass scattering 

 coefficient r/p, it varies more slowly with X, probably as the first power ; from his X-ray 

 data he concluded that the very penetrating 7 rays have most probably a wave length 

 between 20 and 7 X.U. Minna Lang applied her work on the absorption of hard X rays 

 to the 7 rays of all known radio-elements and found that many are probably characteristic 

 X rays (K, L, and M series). 



The energies of 7 rays have been obtained also by measuring the energy of p rays 

 " excited " by them in various elements. The velocity of the P particles emitted by the 

 7 rays from the atom of any element depends upon the frequency of the 7 rays and upon 

 the work necessary to separate the emitted electron from the rest of the atom. The 

 photoelectric equation E = hv — IV, is applicable. (E is the energy of the " excited " 

 P ray measured outside the atom, v is the frequency of the exciting 7 rays and W is the 

 work of separation.) The energy E is the value of Hr in magnetic deflection experi- 

 ments, the work IV, the energy corresponding to the appropriate absorption edge in the 

 X-ray spectrum of the atom in the electronic structure of which the p ray arises. The work 

 of separation IV will have different values for different energy levels in the same atom 

 and different values for the same energy level in different atoms. The soft 7 rays of RaB 

 are the L-series X rays of Pb. Some of the 7 rays of radio-elements belong to the K, 

 L, M, or other series of X rays of the atoms concerned in the /3-ray disintegration con- 

 sidered. Evidently, some of the 7 rays are of extra-nuclear source. The most penetrating 

 7 rays can not be so accounted for and must therefore be of nuclear origin. 



Connection between gamma rays and beta rays (or alpha rays).— The more recent 

 work has established: (1) some of the £ rays are of photoelectric origin (extranuclear) 

 "excited" by the 7 rays; (2) some of the 7 rays originate in rearrangements of electrons 

 in the same part of the atom (ordinary X-ray types) ; (3) the change in nuclear charge 

 requires some p rays in disintegration to be of nuclear origin; (4) some of the 7 rays 

 all of the very penetrating rays, are of nuclear origin. The principal point in dispute is 

 whether emission of nuclear p rays precedes or follows the emission of nuclear 7 rays. 

 Smithsonian Tables 



