LETTERS TO THE EDITOR 487 MgO CRYSTALS TABLE I.Obsersed Numbers. Cd SHIELDS Bragg Crossed Background Rate g0ve品in. Rate Run Nx/min. RSORRING 23 60.5±.4 31 55.6±.4 38 43.3±.7 2nd 11 8.6 26.5±.3 3 20.9±.4 PARAFFIN CHAMBER 3rd 12 7+23 12 37.7±，3 28.0±.4 NB-Nx= ,1士，4 ·6CM- F1G1 TABLE II.Relative Numbers. NB-NX Nx NB emerging in the direction of the chamber.These together Run NB-NCd Ncd Nca with those scattered from the general surroundings account With MgO Crystals for the number (Nca)of neutrons counted when the cad- 城 0.40±0.06 1.28±0.02 ,41409 88主83 mium screening of the chamber is completed by a sheet of 1.27±03 With Al Blocks Cd across its front. 3rd 0.01±0.04 The entire removal of the crystals made practically no 134±0.02 140.02 change in the count of these high speed (Cd penetrating) neutrons,and further,the subsequent removal of the Cd from the front of the chamber made no appreciable increase NB-Nx=0.1±0.4 in the count. indicates that the change in geometry could not have NEUTRONS SCATTERED BY SINGLE CRYSTALS changed the incoherent scattering much more than the It thus appears (1),that the crystals do not significantly statistical fluctuations. affect theamount,Ncd,of high speed neutrons counted,and (2),that the slow speed neutrons counted were scattered SUMMARY OF RESULTS from the crystals. When the crystals are in the Bragg positions,the total The greatest statistical precision was obtained for NB number NB of neutrons counted will be the background and Nx,and hence these two values in Table I are the best Nca plus both those regularly reflected and incoherently evidence for the Bragg type of reflection of neutrons.It is scattered by the crystals.The amount of incoherent also interesting to note the relative amount of reflection and scattering should be practically independent of crystal scattering as shown in Table II. orientation,so to observe this without regular reflection the The correction shown takes account of the fact that the crystals were tilted,alternately clockwise and counter- total volume of the aluminum blocks used in the third run clockwise,about 25 from the Bragg position.In this case was 1.63 times that of the crystals (due to the irregular of crossed crystals the total count Nx will be Ncd plus the outline of the crystals,as mentioned above). incoherent scattering.Hence NB-Nx should be a measure On the basis of this evidence it seems reasonable to con- of the number of slow neutrons that are regularly reflected. clude that we have in these experiments observed the In the first run,NB-Nx was eight times that accountable reflection of slow neutrons in accord with the Bragg relation on the basis of statistical fluctuations and in the second run, between the de Broglie wave-length of these neutrons and six times.These results at once indicated the Bragg reflec- the grating space of these crystals tion of slow neutrons.As a check,it seemed necessary to It should be noted that the experimental arrangement determine by actual test whether or not polycrystalline (see Fig.1)permits a sufficiently large angular divergence blocks of about the same size and scattering power would so that the Bragg conditions are satisfied for a large portion due to the change in geometric disposition,scatter more of the velocity range in the Maxwellian distribution!of the slow neutrons in the "Bragg"position than in the crossed thermal neutrons. position. Grateful acknowledgment is made to Mr.Raymond Ridgeway for supplying us with the unusually large single SCATTERING BY POLYCRYSTALLINE BLOCKS crystals of MgO used in this work. In this,the third run,aluminum metal blocks of rec DANA P.MITCHELL tangular size and thickness equal to the rectangular PHILIP N.POWERS boundary of the somewhat irregular single crystals,used in the first two runs,were mounted in place of the crystals Pupin Physics Laboratories. Aluminum was used since it has approximately the same Augu8t1,1380, effective scattering power per unit volume as MgO.The i J.R.Dunning,G.B.Pegram,G.A.Fink,D.P.Whitehall and E. result se学h，.40,453(4936. 704(1935)