Average number of prompt neutrons from the fission of 233U, 235U and 239Pu by 4 and 15 MeV neutrons

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155

REFERENCES nv D. I. et al., Atomnaya Energiya 2, p. 497 (1957). 1. LEIPUNSK~A. I., BLOKHINTS 2. KALASHNIKOVAV. I., LEBEDEVV. I., MIKAELIANL. A. and PEVZNEXM. I., J. Nucl. Energy 4,67 (1957). 3. GEIGERK. W. and ROSED. C., Cunnd. J. Phys. 32,498 (1954). V. I., LEBEDEVV. I. and SPIVAKP. E., J. Nucl. Energy 5,226 (1957). 4. KALA~HNIKOVA 5. HENKELR. L. and BROLLEYJ. E., Phys. Rev. 103, 1292 (1956). 6. BROLLEYJ. E., DICKINSONW. C. and HENKELR. L., Phys. Rev. 99, 159 (1955). 7. FRASERJ. S., Phys. Rev. 88,536 (1952).

Average number of prompt neutrons from the fission of 233U, 23TJ and 23QPu by 4 and 15 MqV neutrons*t A knowledge of the dependence of v, the number of neutrons emitted in fission, on the energy (,!?) of the neutrons inducing the fission is of considerable interest in the study of the fission process, and is also of practical importance in fast neutron reactor design. Several papers on this subject have been published recently.(l-B) USACHEVand TRUBITSIN,@’ and also Fowler (cited “I), have calculated the energy dependence of v on the assumption that the kinetic energy of the fission fragments is independent of the incident neutron energy. These calculations both predict a linear increase of v with neutron energy according to the equation: v(E) = vr + aE where vr is the average number of neutrons emitted in thermal neutron induced fission and the coefficient a gives the increase in v for a 1 MeV increase in incident neutron energy. For *YJ, Fowler obtained a value.of a = O-125 MeV-r (T = 1.4 MeV), and USACHEVand TRuBrrsIN obtained the value a = 0.145 MeV-1 (T = 1 MeV). The difference arises from the fact that these authors assumed different values of T, the temperature of the excited fission fragments. The energy dependence of v has also been studied experimentally. LEACHMAN@) and KALASHNIKOVA et a1.t4’have compared v for the spontaneous fission of sssPu with v for the thermal neutron induced fission of ZssPu. The ratio v(E)/v~ has been determined at incident neutron energies E = 0.7 and 1.0 MeV for laaU by LEACHMAN(~~~) and, with incident neutrons having a fission spectrum, for *YJ, et al.(6Band by AUCLAIR et al, M) These experiments confnm the zsaU and aaePu by KALASHNIKOVA predicted increase of v with increasing neutron energy. In the work now reported, the ratio v/VT has been determined for the fission of aaaU, *YJ, and as8Pu by incident neutrons having energies 4 and 15 MeV. The sources of these neutrons were the D(d, @He reaction (4 II- 0.3 MeV neutrons) and the T (d, @He reaction (15 = 0.5 MeV neutrons). Thermal neutrons were obtained by placing a paraffin block close to the target. A double fission chamber placed directly in the neutron flux was used in these experiments. Primary neutrons induced fissions in both halves of the chamber. A secondary neutron, originating in a fission in one half of the chamber, was able to induce a fission in the other half. Such events were recorded by a coincidence counting technique, the number of coincidences being proportional to v. The ratio v/vr was determined from the ratio of coincidences recorded when the chamber was exposed to fast and thermal neutrons. In order to increase the efficiency of the chamber to secondary neutrons, the distance between the layers of fissile materials’was minimized by depositing them on both sides of a thin (-30 p) platinum foil and the layers themselves were made of considerable thickness (~2 mg/cm”). Thus, by using a piece of apparatus as simple as a double fission chamber, we were able to count secondary fission neutrons with an efficiency of about 5 x lo-” using a coincidence circuit having a resolving time ~2 x lo-’ sec. It was necessary to deposit considerable quantities of fissile material (50 mg zsaU, 35 mg *%r, and 20 mg ssQPu) on the two sides of the platinum foil in order to obtain a sufficiently high fission rate in the chamber (~300 per set). In-spite of the thickness and high u-activity * Translated from Atomnaya Energiya 4, 188 (1958). f Work done 1955-1956.

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of the layers, chambers with satisfactory counting characteristics were obtained for all the materials studied. This was achieved by reducing the pulse width by the addition of 5-10 per cent CO, to the argon, by using low pressure gas fillings (X%200 mm Hg), and by careful design of the electrodes. With a fission rate of 300 per set and a coincidence circuit resolving time of 2 x lo-’ set, the coincidence counting rate was 3-5 pulses/min. The background of accidental coincidences, which was not greater than 30-40 per cent of the number of true coincidences, was determined by using a delay line. A total of 5000-8000 coincidences was recorded for each of the materials studied. TABLE l.-Rssu~n

v(E) AND Av/AE

43

AvIAE

emu

2.55 t 0.06

4 15

1.20 f 0.04 1.73 f 006

3.06 h 0.12 4.42 5 0.17

0.127 & 0.025 0.124 & 0011

zsa*

247 _L 0.05

4 15

1.22 f 0.04 1.82 f 0.07

3.01 f 0.12 4.51 f 0.19

0.136 + 0.025 0.135 f 0.012

=sPu

2.91 f 0.06

4 15

1.18 f 0.03 1.62 It_ @06

’ 3.43 f 0.11 j 4.71 & 0.20

, 0.131 f 0.022 / 0.121 f 0.013

‘,

The following corrections were made to the ratios v/VT obtained in these experiments. (1) A correction which allowed for the difference in the detection efficiency for secondary neutrons from fissions induced by fast and slow neutrons. This difference is caused by asymmetrical angular distribution of the fragments in fast neutron fission”s8) and also by the fact that the fission neutron spectrum depends upon the excitation energy of the nucleus undergoing fission.“‘~lo) (2) Corrections for the presence in the fast neutron flux of neutrons inelastically scattered by the chamber walls, for a background of room scattered epi-cadmium neutrons, and for the presence in the thermal neutron flux of fast neutrons which had penetrated the paraffin block. (3) A correction for the assU content of the uranium layers. The final results obtained for v(E)/v~, after making these corrections, are given in Table 1. Table 1 also gives values of v(E) and Av/AE = v(E) - VT derived from these results using the dam for vT published by HARVEYand SANDERS.“~’ E The values of Av/AE show that, within the experimental errors, v(E) is a linear function of energy, in agreement with the theoretical predictions. (lg3) This linear behaviour is still in evidence at an incident energy of almost 15 MeV, in spite of the fact that at such an energy the reaction (n, n’f> can occur : that is, the excited nucleus can evaporate a neutron before fission occurs. This is explained by the fact that the increase in v caused by neutron evaporation is compensated by a decrease in the excitation energy of the residual nucleus (v for neighbouring isotopes differing very littleu8’). The magnitude of a = AvIAE for *3aU is a little greater than for *=LJ and *saPu. This appears to be due to the lower average binding energy of the last neutron in the assU fission fragments and the softer fission neutron spectrum. Acknowledgements-The authors express their thanks to A. 1. LEIPUNSKIIand 0. D. KAZACHKOVSKII of the Ukrainian SSR Academy of Sciences, for many helpful discussions, and to A. N. SWBINOV, V. A. ROMANOV and the members of the neutron generator group for their assistance in making the measurements. G. N. SMIRENKIN, I. I. B~NDARENKO, L. S. KUTSAEVA, KH. D. MISHCHENKO, L. I. PROKHOROVA and B. P. SHEME~ENKO. REFERENCES 1. LEACHMANR. B., Proceedings of the First International Conference on the Peaceful Uses of Atomic Energy, Geneva, Vol. 2, p. 193. United Nations, New York (1956). 2. LEACHMANR. B., Phys. Reu. 101,1005 (1956). V. P., Report to the USSR Academy of Sciences (1953). 3. USACHEVL. N. and TRUB~TSIN

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V. I., ZAKHAROVAV. P., KRASNUSHKIN A. V., LEBEDEVV. P. and PEVZNERM. I., 4. KALASHNIKOVA

5.

6. 7. 8. 9.

Conference of the USSR Academy qf Sciences on the Peaceful Uses of Atomic Energy, Moscow, Physical Sciences Division. Consultants Bureau, New York (1955). KALASHNIKOVA V. I., LEBEDEVV. P. and SPIVAKP. E., J. Nucl. Energy 5,226 (1957). AUCLAIRJ. M., LANDONH. H. and JACOB M., Physica 22, 1187 (1956). BROLLEYJ. E., DICKINSONW. C. and HENKELK. L., Phys. Rev. 99, 159 (1957). HENKELR. L. and BROLLEYJ. E., Bull. Am. Phys. Sot. Ser. ZZ, 2, 308 (1957). KOVALEVV. P., ANDREEVV. N., NIKOLA~VM. I. and GUSEINOVA. G., Zh. eksp. teor. jiz. 33,

1069 (1957). 10. GRUNDL J. A. and NEUER J. R., Bull. Am. Phys. Sot. Ser. ZZ, 1,95 (1956). 11. HARVEYJ. A. and SANDERSJ. E., Progress in N&ear Energy, Ser. I, 1, 1 Pergamon London (1956). 12. KUZ’MINOVB. D. and SMIRENKIKG. N., Zh. eksp. teor.jiz. 34, 503 (1958).

Press,

14 MeV fission cross-sections of 232Th and 237Np* (Received 31 August 1957)

THE only source of information about the fission cross-sections of Z3aTh and ,,‘Np for 14 MeV neutrons is the compilation of neutron cross-sections by HUGHES and HARVEY.(~) However, these cross-sections have been taken from unpublished work and the experimental errors are not given. ‘The purpose of the work reported here was to obtain more complete data on these cross-sections. The source of fast neutrons in our experiments was the T(d, #He reaction, in which 175 KeV deuterons bombarded a thick zirconium tritide target placed at an angle of 45” to the beam. The effective deuteron energy, after allowing for the energy loss of the deuterons in the target and the cross-section of the reaction as a function of energy, was 118 KeV.‘z’ The neutron output was monitored by using a proportional counter to count the a-particles emitted at 90” to the deuteron beam. The fissile materials were electroplated upon platinum disks to form targets 20 mm in diameter which were placed in an ionization chamber at a distance of 6.2 cm from the neutron source. The energy of the neutrons inducing fissions in these targets was 14.6 MeV. In order to simplify the geometry and to obtain greater accuracy, the deuteron beam was defined by a diaphragm to give a beam spot on the tritium target less than 5 mm in diameter. The neutrons could therefore be regarded as originating from a point source. With this assumption, and allowing for the anisotropic angular distribution of the cc-particles and neutrons in the laboratory system of co-ordinates,@) the fission. cross-section u can be calculated from the following formula:(3) CT=

4m,2kwiNf

In (1 -1.;i)NNa .where r,, is the radius of the fissile deposit, R, is the distance from the neutron source to the deposit, .w, is the acceptance solid angle of the a-particles, K is a coefficient which allows for the anisotropic angular distribution of the cc-particles and neutrons, N is the number of atoms of the fissile isotope in the deposit, N, is the number of fissions, and N, is the number of cc-particles recorded by the proportional counter. Particular care was taken over the determination of the weight and purity of the fissile isotopes. .To facilitate the determination of the weight of eszTh in the thorium target, the specific activity was increased by adding a trace amount of zsOTh(decay period: 7 = 8.0 x lo* years). The chemical purity of the preparation was first examined spectrographically. It was then heated to a temperature of 1400°C to ensure the complete oxidation of the thorium to ThOa, and weighed to an accuracy of 0.5 per cent. The oxide was then dissolved in distilled water and the solution successively diluted. This solution was also weighed very accurately. In order to determine the specific activity of the mixture of thorium isotopes (i.e. the number of &-particles counted per unit weight of thorium), * Translated from Ammaya

Energiya 4, 190 (1958).