560 Mituo MIWA. [Vol. 22 work and to Mr. S. Takesita for technical assistances. Research Laboratory, Matsuda Division, Tokyo Shibaura Electric Company. (Received June 28, 1940) The Photodisintegration of Deuteron by Radium Gamma Rcays. BY Mituo MIWA. (Read April 2, 1939.) The photodisintegration of deuteron by radium gamma rays was first observed by Szillard and Chalmers(1) and later investigated more thoroughly by Chadwick, Mitchell and others(2). The energy of the photoneutron emitted in this reaction seems to have generally been accepted to be appreciably lower than that of the photoneutron obtained with the combination of radiothorium and deuterium. Halban,(3) however, recently compared the distribution of the density of thermal neutron from a Ra +D source placed in a large water tank with that when radium was replaced by radiothorium, and found that the forms of the distribution curves are hardly distinguishable between the two cases. Hence he concluded that the most of the photoneutrons emitted by Ra -D source is due to a certain line or lines of about 2.6MV in energy and that the well known line of 2.19SMV contribute only little to the reaction if at all. In the present experiment, the mean free path of the Ra +D neutron in paraffin was determined by the scattering method in order to see whether the well known line 2.19SMV is really the most effective in liberating the neutron from deuteron. If such is the case, some inforination,s on the photomagnetic effect in deuteron are expected to (1) Szillard and Chalmers, Nature 134 (1934), 494. (2) Chadwick and Godhaher, Proc. Roy. Soc., A 151 (1935), 479 ; Mitchell, Rasetti, Fink and Perram, Phys. Rev., 50 (1936), 189 ; Kimura, Memoirs Coll. Sci. Kyoto Imp. Univ, A 22 (1939), 237 ; Leipunsky, Rosenkewitsch and Timoshuk, Phys. Zeits. Sowjet- union, 10 (1936), 625. (3) Halban, Compt. rend., 206 (1938), 1170. 1940] The Photodisintegration of Deuteron by Radium .561 be obtained by the determination of the neutron intensity. Experimental apparatus and procedure. The general arrangement of the apparatus of scattering experiment is shown in Fig. 1 diagrammatically. 50cc heavy water(1) of 98% Fig. 1. purity was sealed in a glass tube 3.5cm in diameter and 5cm in height and was irradiated with gamma rays from radium pack containing 2.2 gr radium. Active part of the pack was about 4 cm in diameter and 2 cm in height. The neutrons were detected by the artificial radio- activity of shin sheets of indium (6cm 5cm 0.085mm) placed at 20.3 cm distance from the heavy water in a paraffin sphere 9cm in diameter. As the intensity of neutron was rather weak, the detectors were generally exposed for 90minutes and the activity was measured by an argon-filled Geiger-.Muller counter for 30 minutes. Owing to the inevitable distance of the source and the counting apparatus, the activity of 54 minutes half-life only was utilized. The initial intensity of the activity was of the same order as the natural effect of the counter, i.e., 12~14 per minute against 10~12 per minute of the latter. Three paraffin scatterers with the thickness of 4.97 mm, 7.73mm and 12.5mm were used, each being about 26 cm2 in area. The position of the scatterer was so chosen that it divide the distance between the neutron source and the detector roughly in proportion to the size of the two. In order to reduce unnecessary scattering, each part of the apparatus was laid on the supportor made, of sheet iron and the sup- portors were in turn put on a tall table with an iron plate top. The results of the scattering experiment are shown in Table 1 and also plotted in Fig. 2. In the third column of the table are given the observed transmissions of the total neutrons including those emitted by (1) Heavy water was kindly lent by Dr. Y. Nishina of the Nuclear Physics Labor- atory, LP.C.R, to whom the writer wishes to express his hearty thanks. 562 Mituo MIWA. [Vol. 22 the gamma ray source itself, and in the fourth and fifth columns the contributions of the latter neutrons to the primary and transmitted intensities respectively. As is seen in the table, the gamma ray source itself emits an appreciable amount of neutron. This is due to the cir- cumstance that the radium pack contains 0.7 gr radium sealed in glass tubes. Fig. 2. Fig. 3. For computing the scattering coefficient or the mean free path from the results obtained above, we must further take the following two corrections into account: the correction for the stray neutron from the wall, floor, etc., and that for the scattering deficiency. The latter comes from the circumstances that some of the neutrons go to the detector even after being scattered because of the large size of the paraffin sphere and that, some are newly directed to the paraffin sphere by the scatterer. The intensity of the stray neutron was estimated as usual by extrapo- lating the distance-total intensity curve to the infinite distance. The results are shown in Fig. 3, from which the intensity of the stray neutron is estimated to be 8 2 and 16 4% of the observed intensities at 20.3 and 30.3 cm distances from the source respectively. The correction for the scattering deficiency was so determined that the two values of mean free. path calculated from the transmissions observed at 20.3 cm and 30.3 cm distances from the source coincide. Observed transmissions for 7.73mm paraffin scatterer were 1940] The Photodisintegratiov of Deutron . by Radium 563 and On the basis of the assumption that the angular distribution of the neutron scattered by proton follows the "cosine" law (uniform scattering in the system fixed to the centre of gravity), the scattered intensities observed at 20.3 cm and 30.3 cm distances from the neutron source are expected for the given geometrical condition to be 40 and 20,%(1) less than the true intensities. On the basis of the assumption of uniform Table 1. Ra: Radium, D: Heavy water, P: Paraffin scatterer. angular distribution, the corresponding values are 12 and 6% respec- tively. Because of the multiple scattering and the scattering by carbon nuclei the actual values are expected to be a little lower than the former values, but higher than the latter. The final values estimated were 26 7 and 13 3% for 20.3 and 30.3cm distances respectively. They are just equal to the mean values of the two extreme ones given above. The true transmissions for 7,73mm scatterer thus corrected were and Owing to the weak intensity of Ra +D neutron, the estimation of the two corrections described above was done with Ra +Be source. In our case such a provisional method seems to be justified in the first approximation, because the average energies of the neutrons of the two sources are nearly equal as will be seen later. The stray neutron of the former source may, however, be a little less than that of the (1) For neutrons of a few MV energy still low values are expected from the recent results of Kikuchi, Aoki and.Wakatsuki [Proc. Phys. Math. Soc. Japan, 21 (1939), 40]. 564 Mituo MIWA. [Vol. 22 latter, because the most of the neutrons of the former source are emitted in the direction perpendicular to the gamma ray beam while the neu- trons of the latter source are emitted in all directions nearly uniformly. The mean free path of Ra +D neutron may therefore be a little greater than given below, but would never be smaller. Applying these correc- tions we finally found the mean free paths of Ra +Be and Ra +D neutrons in paraffin: In the second experiment the Ra +D neutron source was placed in a large paraffin block and the neutron intensity was compared with that when radium was replaced by radiothorium(1). 300mg radium scaled in platinum tubes was used in this case. As the geometrical arrangements of the two gamma ray sources and the energies of their effective gamma rays were nearly equal, the comparison was done only at a fixed distance from the sources. Results and Discussions. a) The energy of gamma rays. From the mean free paths obtained above, we find the collision cross-sections of Ra +Be and Ra +D neutrons for proton: The cross-sections for carbon nucleus are assumed to be (2.0 0.5) 10-24 cm2 in both cases. The mean free path of resonance neutron in paraffin or water was carefully determined by several investigators(2) and for water 9mm seems to be the most probable value. The corresponding cross-section for porton is 14.8 10-24 cm.* On the basis of the modified Wigner's formula(3) the average energy of Pa +D neutron, therefore, is estimated (1) The writer is indebted to Dr. R. Sagane of Tokyo Imperial University, who has kindly lent radiothorium of 6.1 mg Ra equivalent. (2) Bethe, Rev. Modern Phys., 9 (1937), 127; Simons, Phys. Rev., 55 (1939), 792. Kittel end Breif, Phys. Rev., 56 (1939), 744. Note added in proof. According to the recent result of Hanstein [Phys. Rev., 57 (1940), 1045],the collision cross-section of proton for Indium resonsnce neutron is 20.3~ 21.6 10-24cm2. If it is the case, the energy of Ra +D neutron would be 0.25 MV or so and the energy of the gamma-ray 2.7 MV. 1940] The Photodisintegration of Deuteron by Radium 565 to be 0.21 0.10MV.