The Photodisintegration of Deuteron by Radium Gamma Rcays

Home , Photodisintegration

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% respectively. 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 neutrons 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 corrections 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. If we take the energy of the ground state of deuteron as 2.189MV, the energy of the corresponding gamma ray is (2.189 +0.21 2=) 2.6 0.2MV. This is definitely higher than 2.198MV, the energy of the well-known line of RaC. Thus it seems certain that there exist some RaC lines (or a line at least) of higher energy, as was already pointed out by Halban(1). As the energy of the gamma rays obtained above was, however, accidentally equal to that of ThC" gamma rays, i.e. 2.62 MV, there arose a question that they may be due to the decay product of mesothorium impurity contained in radium. But it was experimentally established that the intensity of neutron does not change at all even though radium is replaced by radon. The latter does not, of course contain active deposit of thorium at all even if there were mesothorium impurity in radium. It is therefore quite certain that the gamma rays in question are emitted by radium C or by active deposit of radium at least. To facilitate the comparison, experimental and theoretical collision cross-sections of proton for Pa +Be and Ra +D neutrons and the energies of corresponding gamma rays are listed in Table 2 and 3 respectively.

Table 2. Theoretical and Experimental Scattering Cross-Section of Proton for Ra +D Neutron

Table 3. Theoretical and Experimental Scattering Cross-Section of Proton for Ra +Be Neutron

(b) The intensity of gamma rays.

(1) Halban, l.e. 566 Mituo MIWA. [Vol. 22

In the second experiment the intensity of Ra +D neutron was found to be about 1/25 of that of RaTh +D neutron having equal gamma ray intensity. The intensity of the gamma rays of radiothorium relative to radium was determined by several investigators(1) and found to be 0.7~0.9 for equal number of disintegrating atom. Remembering that the most of the gamma rays of the former come from thorium C", of which branching ratio is 0.34, and that thorium C" emits one quantum of 2.62 MV per disintegration, we find that the intensity of the gamma ray in question is 1.7 quanta per 100 disintegrating atoms of radium C. In this calculation it is assumed that the disintegration cross-section of the gamma ray in question is equal to that of thorium C". According to Ellis and Wooster the intensity of 2.19S MV line is 7.4 per 100 disintegrating atoms of radium C. The relative intensity of the two lines therefore is 4.3: 1. There are a few evidences(2) for the existence of the gamma rays of radium C, the energy of which is higher than 2.198 MV. Only little, however, is known as to the intensity. Our results may be compared with that of Alichanov(3):

The importance of these gamma rays of weak intensity but of higher energy in the photodisintegration of deuteron seems to be justified by the results of Chadwick, Feather and Bretscher(4) and Halban(5). According to the theory of deuteron(6), the disintegration cross-sections of deuteron expected for the gamma rays in question are(7)

Cross-Section in 10-28 cm2 Unit

(1) Rutherford, Chadwick and Ellis, Radiation from Radioactive ,Substances, Cham- ridge (1930), p. 500; Winand, Journ. Phys. Radium, 10 (1939), 361. b (2) Rutherford, Chadwick and Ellis, l.c., p. 381 ; Steadman, Phys. Rev., 36 (1930), 460. (3) Cited by Leipunsky, Rosenkewitsch and Fimoshuk, Phys. Zeits. Sowjetuniou, 10 (1936), 635. (4) Charhwiek, Feather and Bretscher, Proc. Roy. Soc. A 154 (1937), 366. (5) Halban, Nature, 141 (1938), 644. (6) See Bethe and Bacher, Rev Modern Phys., 8 (1936). (7) The numerical values given here are computed by assuming 2.189 MV and 0.105 MV for the energies of ground state and 1S state of deuteron, and 4.72 nuclear magnetons for the difference of the magnetic moments of proton and neutron. 1940] The Photodisintegratiott of Deuteron by Radium 567

where the upper figures for the total and photomagnetic cross-sections correspond to real 1S level and the lower to virtual 1S level. Chadwick and others, however, showed that the contribution of the photomagnetic transition to the total cross-section for 2.62 MV gamma ray is only 5 per cent at the most, instead of 17 or 33 per cent expected theoretically. If the same is true for the gamma ray of lower energy, 2.62 MV (or 2.47 MV) gamma ray quantum would be more than 15 (or 10) times as effective as 2.198 MV one in disintegrating the deuteron. This is in good agreement with the observation of Halban(1), who found no neutron of low energy. Moreover, as is seen from the numerical values given above, the former gamma ray quantum is expected to be about 3 times as effective as the latter, even if the 1S level of deuteron were virtual(2). Even in the worst case, therefore, the number of photoneutrons liberated by the gamma rays of higher energy would be nearly equal to that liberated by the 2.198 MV line(3). Finally, the angular distribution of Ra +D neutrons was studied by observing the relative number of photoneutrons emitted in the directions of incident gamma rays and perpendicular to the beam. A conspicuous asymmetry in the angular distribution would be expected for the gamma rays of the higher energy, for which the photoelectric effect has the main contribution to disintegration. On the contrary, the angular distribution of the photoneutron liberated by 2.198 MV gamma ray would be nearly uniform, because the photomagnetic transition is far more favourable for the gamma ray. Unfortunately, owing to the weak intensity of Ra + D neutrons and the large size of the sources, the results obtained were too rough and qualitative to decide the question conclusively. The general trend, however, seemed to indicate the predominance of the neutron emitted in the direction perpendicular

(1) Halban, Compt, rend., 206 (1938), 1170. (2) Experimental evidences from the mean life of thermal neutron in water and the scattering of thermal neutron by orthe-anrl para-hydrogen are in favour of virtual S level. Frish, Balban and Koch, Nature, 140 (1937), 895 ; Dunning, Manley, Hoge 1 and Brickwedde, Phys. Rev., 52 (1937), 1076 ; Halpern, Esterman, Simpson and Stern, Phys. Rev., 52 (1937), 142. (3) in the scattering experiment, the relative contribution of photoncutrons of different energy to the observed intensity depends upon the spectral sensitivity of the system of detector and paraffin sphere. Although the estimation of the spectral sensitivity is very difficult, it may safely be said that the sensitivities for 0.01 and 0.20 MV neutrons do not differ from each other by a factor 2. 568 Mituo MIWA. [Vol. 22

to the gamma ray beam. This again is in favour of the existence of RaC gamma ray of higher energy and more effective in producing the disintegration of deuteron than 2.198 MV line.

Summary.

The mean free path of Ra +D neutrons in paraffin was determined by the scattering method and found to be 12.7 2.0 mm. This value is definitely higher than that expected for 2.198 MV line of RaC. Hence it was concluded that there exists a RaC line or lines having effective energy of 2.6 MV and more effective in liberating the neutron from deuteron than 2.198 MV line. The yield and the angular distribution of photoneutron were also studied and were found to be in favour of this conclusion. RaC gamma rays of 2.198 MV are definitely less effective in disintegrating deuteron than expected for a virtual 1S level. This is in good agreement with the observation on the angular distribution of RdTh +D neutrons, but is contrary to evidences obtained with thermal neutrons.

In conclusion, the writer wishes to express his hearty thanks to Prof. S. Nishikawa of Tokyo Imperial University for his kind advice and criticism and to Prof. M. Nagayo, the President of the Japanese Foundation for Cancer Research and Dr. H. Yamakawa, the head of the radiological department of the Foundation for their encourage- ments.

Radiological Department, .Japanese Foundation for Cancer Research, Tokyo.

(Received June 20, 1940).