Radiochemical studies of nuclear reactions leading to the formation of π mesons

A study of nuclear reactions leading to the formation of -mesons is of value for understanding nuclear forces. A radiochemical method is used for detecting such reactions and this makes it possible to determine the cross section of reactions of very low probability.Reactions of the type (p,=) and (p,p) have been studied with the nuclei of elements of average atomic weight. It was found that their cross section equals 10^–30 cm²/nucl. at proton energies of 480–660 Mev. A noticeable increase in the cross section was observed in the range of energies 110–480 Mev. Such reactions could not be detected for nuclei of heavy elements. The products of the (p,¦2⁺) reaction with copper could hot be identified.The possibility of a (p.=) reaction in the interaction of a particle with a complex of nucleons in a nucleus is discussed. The information obtained gives a fuller understanding of the processes occurring when high-energy particles react with complex nuclei than can be obtained from the theory of nucleon-nucleon collisions.     

Neutron yield of the reactions Li6(t,n) and Li7(t,n)

The differential cross section is determined for the formation of neutrons at an angle of 0 ° in the reactions Li⁶(t,n) and Li⁷(t, n) in the 0.175–2.4 Mev triton energy range. In the reaction Li⁶(t,n) resonance in the neutron yield is observed at E_t = 1.875 Mev, corresponding to a Be⁹ excitation level of 18.936 Mev.Two resonances are detected in the neutron yield of the reaction Li⁷(t, n): at E_t = 0.765 Mev and E_t = 1.735 Mev, which indicates the presence of Be¹⁰ nucleus excitation levels of 17.78 and 18.46 Mev, respectively. The cross section of the reaction Li⁶(t, n) in resonance is d/d(0 °)_res = 37.2 mb/sr; for the reaction Li⁷ (t, n) the cross section at the first resonance is d/d(0 °)_res = 95 mb/sr; at the second resonance d/d(0 °)_res = 159 mb/sr.The angular distributions of neutrons are given for the interval of angles 0–135 ° (every 15 °) for triton energies of 0.358, 0.559, 1.006, 1.218, 1.370, 1.572, 2.123 Mev.The total cross section for the formation of neutrons at E_t = 2.123 Mev in the reaction Li⁶ (t,n) is equal to 324±32.3 mb in the reaction Li⁷ (t, n) to 1332±83.3 mb.     

Injection and storage of positrons

The authors describe the production of positrons and their injection into a storage ring. The positron beam is oblained by converting electrons in an external target and is shaped by means of special short-focus lenses. The particles are admitted into the storage ring below its medianplane: their capture is single-turn. The authors analyze the conditions required for maximum capture coefficient, which is equal to 1·10^–4 with vertical magnet aperture z = ±37 mm and horizontal aperture R= ±30 mm. They describe the method and results of their system for producing and injecting the positrons, and discuss the optimal conditions for capture and storage.Translated from Atomnaya Énergiya, Vol. 22, No. 3, pp. 206–210, March, 1967.     

Methods for preparing ions of short-lived isotopes

Conclusions A source with thermal ionization in a cavity is extremely convenient for obtaining ion beams with an ultralow quantity of the starting material. Its efficiency with ionization potentials of the starting atoms V_i<6 eV is equal to 100% and drops to 1% for V_i=8 eV [7]. This makes it possible to use it in the study or more than two-thirds of the periodic system of elements, including the actinides. Thus for 10¹² uranium and plutonium atoms ^*9% and 50%, respectively; for 10⁶ curium atoms ^*70%; and, for 10⁵ californium atoms ^*37% [13]. It was shown in [14] that this source can be successfully used in mass spectroscopy to analyze trace quantities of different elements in the solid phase. With its help yttrium isotopes with _1/2=0.4 sec were detected in [16].This source is now widely used in our country, where with its help tens of new isotopes have been discovered [15, 17], and abroad (for example, [16]). The source is effective for short-lived isotopes with _i/_1/2<10 and V_i<7 eV. To study isotopes with _i/_1/2<10 and V_i>7 eV but with _i/_1/2>10 it is desirable to develop sources with thermal and photoionization in the cavity. For elements whose atoms have a long residence time on the surface, these sources, just as the gas-discharge sources, are inefficient. Means must be developed for raising the rate of desorption of the indicated atoms from the surface within the framework of the techniques examined here.Translated from Atomnaya Énergiya, Vol. 60, No. 2, pp. 114–119, February, 1986.     

Synthesis of isotopes of fermium with mass numbers 247 and 246

The isotopes Fm²⁴⁷, Fm^247m and Fm²⁴⁶ were synthesized in the extracted beam of the 310-cm heavy-ion cyclotron in the Joint Institute for Nuclear Research by irradiating Pu²³⁹ with C¹² ions. ]he procedure of collecting the recoil atoms by means of gas jets is used, with subsequent recording of their ~- decay with Si(Au) detectors. For the isotope Fm²⁴⁷, -activities are obtained with two different half-lives - one with two obviously complex groups of -particles E=7.87±0.05MeV (70%) and EE=7.93±0.05 MeV (30%), whose half- life is T_1/2=35±4 sec; the other, with E=8.18±0.03 MeV has T_1/2=9.2±2.3 sec. Data are confirmed concerning the properties of the isotope Fm²⁴⁶(E=8.25±0.03MeV and T_1/2=1.6±0.4 sec), synthesized in 1966 at The Joint Institute for Nuclear Research.Translated from Atomnaya Énergiya, Vol. 22, No. 5, pp. 342–346, May, 1967.     

Coherent effects during the interaction of slow neutrons with liquids

The total cross sections of liquid nitrogen and oxygen and the total cross section of gaseous nitrogen for neutrons with wavelengths in the 4 to 15-A range were determined in the VVR-M reactor of the institute of Physics, Academy of Sciences, Ukr.SSR. The cross sections of gaseous oxygen and nitrogen rise monotonically with increasing wavelength, while those of the liquids begin to fat1 at 5–5.5 A. Apparently this effect is connected with the existence of short-range order in the liquids; in nature, it is similar to the scattering of neutrons in polycrystalline material for wavelengths close to = 2d_max. The existence of coherent effects in the total cross section of heavy water is considered.Translated from Atomnaya Énergiya, Vol. 18, No. 5, pp. 452–455, May, 1965     

14 MeV neutron yields from D-T operation of the MIT Cockcroft-Walton accelerator

As part of our fusion-product diagnostic development program, we have begun a series of experiments with 14 MeV neutrons generated in a Cockcroft-Walton accelerator. Two different detectors have been used to measure the neutron yield: a silicon SBD and a Cu foil. The energy of the emitted neutrons has been determined by using two spectrometers: the SBD and a³He proportional counter. The reaction rate is monitored, with about ±5% accuracy, by detecting the particles from D + T n +. The neutron yields obtained from the Si detector and the Cu activation had associated uncertainties of about ±15% and agreed well with the predicted values from measurements.     

Scattering of fast protons by nonspherical nuclei

The angle distributions of protons scattered by spherical and nonspherical nuclei are considerably different. A comparison of the calculated and experimental results makes it possible to estimate the departure from sphericity of the nuclei. The scattering of fast protons by nonspherical nuclei is analyzed under the assumption that proton energy is large compared with the energy of the Coulomb barrier (E 20 Mev). The black nucleus model is used in conjunction with the adiabatic approximation according to which the calculation of the effective scattering cross sections is reduced to a determination of the scattering amplitudes for a fixed nucleus. It is shown that the amplitude can be given in the form of a sum of amplitudes for diffraction scattering and scattering in the electric field of the nucleus. Both parts of the amplitude are determined under the assumption that the nuclear shape does not exhibit a large departure from sphericity (₂Ze²/v 1). The angle distributions for protons elastically scattered by even-even nuclei are calculated.The author wishes to express his sincere gratitude to B.T. Geilikman who proposed this problem and also to K.A. Ter Martirosyan, B.C. Nosov and V.M. Galitsky for a number of valuable discussions. The author is also indebted to N.M. Trukachev for carrying out the numerical calculations.     

Resonance scattering of γ-rays in Mg24

Resonance scattering of -rays With energies E₁=1.38 Mev, corresponding to the transition to the ground state in Mg²⁴. have been observed in metallic magnesium. The energy given off by the -ray (E₁=1.38 Mev) in emission and collision with the nucleus, is compensated for by the energy obtained due to recoil associated with the emission of the preceding -ray with an energy E₂ = 2.76 Mev. Using a fast coincidence method and amplitude discrimination, coincidences were recorded between the -rays with energies E₁=1.38 Mev and E₂=2.76 Mev. Scattercrs of magnesium and aluminum were alternately placed in the path of the 1.38 Mev -rays. The source was radioactive Na²⁴ in a water solution of NaOH. At an angle of 120 ° between the -rays a strong attenuatlon of the 1.38 Mev -rays was observed; this is attributed to resonance scattering. When the angle between the -rays was varied by 5 °. the strong attenuation of the flux disappeared. The width of the level at 1.38 Mev in Mg²⁴ has been estimated at > 1.6· in^–4 Mev.     

Equilibrium and stability of plasma in axially symmetric toroidal systems

Conclusions In the case when the magnetic surfaces have circular normal cross sections in the neighborhood of a magnetic axis (=1), the stability criterion reduces to a limitation on the ratio of the current density to the magnetic field on the magnetic axis j₀R/B₀<2. On the assumption of a uniform axial current, this criterion is equivalent to the Shafranov-Kruskal stability condition RB (a)/aB(a)<1/m for the first mode of oscillation m=1. If the current density falls off on going away from the magnetic axis, the stability criterion when written in the form of the Shafranov-Kruskal condition will correspond to m>1. For example, in the case of the parabolic current density distribution j=j₀ (1–₂/a ²), it turns out that m=2.To investigate the effect of the ellipticity of the magnetic surface cross sections, a study was made of the two cases when the external magnetic surface had an elliptical or semielliptical cross section. As can be seen from Figs. 2 and 4, the limiting value of j₀R/B₀ decreases rapidly when the semi-axis ratio is decreased below unity and decreases more smoothly when is increased above unity: (=l _z/l _r).In the case when toroidal aspects of the problem are very strongly expressed, when the external boundary of the plasma has a semi-elliptical cross section with a semi-axis ratio =1/2 (which corresponds to =1), the limiting ratio of the axial current in the plasma to the current in the windings of the solenoid producing the axial magnetic field amounts to J/I1/3, which corresponds to a ratio of plasma pressure to magnetic pressure 1/2.To investigate the effect of the toroidal geometry on the stability, we also investigated the case of a plasma torus in which the external magnetic surface was of rectangular cross section. As can be seen from Fig. 6, in the case when the axial magnetic field decreases on moving away from the magnetic axis (b/a>0) the limiting value of the ratio N=j₀R/B₀ is reduced somewhat with increasing toroidality, this reduction in N being associated with a corresponding increase in .In summary, as far as the conditions of hydromagnetic stability for a prescribed magnetic field are concerned, an increase in toroidality should lead to an increase in the limiting plasma current and pressure.Translated from Atomnaya Énergiya, Vol. 24, No. 5, pp. 453–459, May, 1968.     

Direct measurement of the energy variation of η for U233, U235, and Pu239

A technique is described that measures directly the energy variation of , the number of fission neutrons produced per neutron absorbed. When combined with total cross sections, the method is capable of giving fission cross sections as well. Results are presented in the energy region near thermal, of importance to reactor design, for U²³³, U²³⁵, and Pu²³⁹. Comparison with computed from total and fission cross sections shows good agreement for U²³³ and U²³⁵ but a disagreement outside experimental error for Pu²³⁹. An auxiliary experiment demonstrates that , the number of neutrons per fission, is constant with energy in the region of interest for Pu²³⁹, hence that the discrepancy cannot be ascribed to a variation.Work performed under contract with U. S. Atomic Energy Commission.[The following is a reproduction of the original American paper, and not a re-translation from Russian — Publisher's note].     

Study of the zirconium apex op the Zr-Ta-Nb phase diagram

Metallographical examination thermal analysis and electrical resistance measurements have been applied to a study of the zirconium apex, up to 82% zirconium and a temperature of 1200C, of the ternary system Zr-Ta-Nb, with limited solubility of tantalum and niobium in -zirconium ( phase), limited solubility and complete solubility of niobium in -zirconium, with eutectoid decomposition of the \ solid solution and three-phase eutectoid equilibrium + between - and-zirconium. In the investigated portion of the Zr-Ta-Nb phase diagram, the following phase regions were found: a) two one-phase regions and ; b) three two-phase regions + , + and + : c) one three-phase region + + ; the region contracts as the temperature falls below 1200 C.The solubility of tantalum and niobium in -zirconium in the system Zr—Ta—Nb is about 0.5%. On passing from Zr—Ta to Zr—Nb, the + and + regions are displaced toward lower temperature and high niobium concentrations; the boundaries of the + and + + regions are lowered from 790 for Zr—Ta to 612 C for Zr-Nb. Passing between the + and + regions is a binary eutectoid line which, from Zr-Ta to Zr-Nb is displaced toward lower temperatures and higher niobium concentrations. The solubility of niobium in ot zirconium in the Zr-Nb system is about 0.5%by weight. Eutectoid decomposition in the Zr-Ta system shifts the maximum of the martensitic-like transformation to the left and results in an increase in the stability of the phase at room temperature in quenched alloys.     

Effect of a Boron Screen on the Sensitivity of Neutron Fission Chambers

It is suggested that absorbing screens with ¹⁰B be used to maintain constant sensitivity under prolonged irradiation of fission chambers with natural uranium. The transmission factor T (E) of boron screens with various thicknesses ( = 0.1–2 ge/cm²) for a wide neutron energy range and attenuation of a spectrum of the type _e/E are estimated. The group and average group constants of the transmission factor of boron are calculated for neutron fluxes in 25 energy groups of the neutron cross sections library.The contribution of ²³⁸U and ²³⁵U to the signal of a fission chamber with natural uranium is analyzed as a function of the boron screen thickness. ²³⁹Pu accumulation and ²³⁸U burnup are estimated using ²³⁸U group capture cross sections, ²³⁸U and ²³⁹Pu fission cross sections, and the group values T (E)E/E obtained by the authors. It is shown that in the absence of a boron screen for thermal-neutron fluence 10¹⁷ cm^–2 the sensitivity of a fission chamber with natural uranium increases as a result of the formation of ²³⁹Pu. A boron screen with = 1 g/cm² makes it possible to maintain the sensitivity of the fission chamber constant up to thermal-neutron fluence 5·10²² cm^–2.     

Effective cross sections for fission of U233, U235, Pu239 and Pu240 by neutrons with energies from 30 kev to 5 Mev

Measurements have been made of the absolute values of the effective fission cross sections in U²³³, U²³⁵, Pu²³⁹ and Pu²⁴⁰, using as photoneutron sources Sb + Be ( 30 kev), Na + D₂O ( 250 kev), Na + Be ( 900 kev), a source with a simulated fission-neutron spectrum and Po--Be. The fission cross sections for 30 kev-neutrons are 3.06 ± 0.16, 2.21 ± 0.12 and 1.79 ± 0.11 barns for U²³³, U²³⁵ and Pu²³⁹, respectively. As the neutron energy Is increased from 30 kev to 250 kev the cross sections in U²³³ and U²³⁵ fall off by approximately 35% and then remain almost constant while the cross section in Pu²³⁹ falls off by 12% and then increases. The effective cross sections for fission in Pu²⁴⁰ for 900-kev and 5-Mev neutrons is 1.4 barns and the threshold for fission in Pu²⁴⁰ lies within the limits 250 and 900 kev.In conclusion the authors wish to express their gratitude to P. E. Spivak for help in formulating the problem and a discussion of the results.     

The neutron spectrum in a mixture of two non-interacting gases heated to different temperatures

In the heavy-monatomic-gas approximation the neutron spectrum in an infinite nonabsorbing medium filled with a mixture of gases which do not interact with each other and which have been heated to different temperatures is a Maxwell distribution with a temperature averaged over the different gases with a weight _s, where is the mean logarithmic energy loss and _s is the macroscopic scattering cross section. It is found that even for light scattering nuclei where the temperature of one gas is several times the temperature of the other, the neutron spectrum is also Maxwellian, with an accuracy sufficient for approximate calculations.The author thanks M. V. Maslennikov and L. V. Maiorov for their useful comments and E. S. Kuznetsov for his constant interest in the work.     

Disintegration of copper by 680-Mev protons

The results of a radlochemical study of the disintegration products in copper disintegration induced by 680-Mev protons is presented; the yields of nuclei which were not directly observed are obtained by extrapolation. This procedure makes it possible to obtain a complete picture of the residual products and to draw certain conclusions with respect to the basic characteristics of the process; 1) the total cross section for the disintegration of copper is 0,6 · 10^–24 cm² corresponding to 65% of the geometric cross section; 2) the main contribution in the total cross section for the production of disintegration products in copper is due to cobalt, nickel and copper (60%); 3) the isotope yield is increased as the region of stability is approached; 4) in the disintegration process, protons and neutrons are emitted in almost equal numbers _n/_p= 1.3; 5) the emission of an -particle is more probable than the subsequent emission of four nucleons; 6) nuclear structure has no effect on the disintegration of nuclei by high energy particles. By appraising the characteristic features of the disintegration of copper by 680-Mev protons in conjunction with the data obtained in studies of the products of copper disintegration induced by particles with energies from 130 Mev to 2.2 Bev, it is possible to evaluate the effect of the energy of the bombarding particles on the nature of the disintegration process in this element.     

Interpretation of Threshold Fission Chamber Signals

The total thermonuclear-neutron fluxes in the two energy groups 1.4–6.5 and 6.5–14.2 MeV are determined by mathematical analysis of the signal from a fission chamber with ²³⁸U using two proposed values of the effective fission threshold cross sections _eff1 and _eff2 in these groups and the generally accepted effective threshold cross section _eff encompassing both of these energy groups.It is shown that for the real ITÉR spectrum and for model spectra of thermonuclear neutrons, where the signals from a fission chamber with ²³⁸U remains constant and are distinguished from one another by the ratio of the fluxes in these groups, the two effective threshold cross sections _eff1 and _eff2 remain constant, while the generally accepted effective threshold cross section _eff varies from one model spectrum to another.Using two fission chambers, one with ²³⁸U and the other with ²³⁷Np, makes it possible to expand the energy range and to determine the total neutron fluxes in the three energy groups.     

Silicon counters for nuclear spectrometry

Laboratory models of surface-barrier and diffusion-type silicon n-p counters were developed which were suitable for nuclear spectrometry. The counters have linear pulse height-energy dependence for particles with 60 paths (in particular, for -particles with energies up to 10 Mev), and 0.5 % resolution for an E of 5.5 Mev (with units of small area).The authors consider it a pleasure to thank engineer G. V. Khozov, technicians I. A. Lebedeva and G. D. Gusarina, and machinist P. I. Gorshkov for helping greatly in this work.     

Neutron yield in inelastic collisions of 14-Mev neutrons with nuclei and cross section for the (n, 2n) reaction

A measurement has been made of , the number of neutrons produced in one inelastic scattering event between a neutron and a number of elements of natural isotopic composition: Fe, Cu, Mo, Cd, Sn, Sb, Hg, Pb, Bi and U. The measurements were performed by determining the relative change in the total neutron flux and the attenuation of the primary neutrons after passage through samples of the materials being investigated. It was also possible to obtain data on the cross section _in for inelastic collisions of neutrons with the above-mentioned nuclei. The values of and _in, in conjunction with the known cross sections for neutron capture, were then used to compute the cross section for the (n, 2n) reaction (averaged over the isotopic composition) in nonfissile nuclei.This work was completed in 1952.The authors wish to thank A.A. Malinkin for comparing the neutron yields from sources used to measure the dependence of counter sensitivity on neutron energy.     

Distribution of the ratio of radiation capture and fission cross sections for Pu239 with respect to height in the BR-5 reactor

A study was made of the distribution of the ratio of radiation capture and fission cross sections for Pu²³⁹ with respect to height in the BR-5 reactor. The distribution of neutron capture reactions was measured by determining the concentration of Pu²⁴⁰ from the rate of spontaneous fissions in plutonium samples irradiated in the reactor with an integral flux of 10²¹ to 10²² neutrons/cm². As initial material for irradiation, almost isotopically pure Pu²³⁹ (Pu²⁴⁰ content 5·10^–3% was used. The values of obtained rose from 0.1 to 0.8 on moving away from the center of the reactor. The data corresponding to equilibrium neutron spectra in the active zone and the outer region of the reflector agree with the results of measuring and _f in monoenergetic neutrons.Translated from Atomnaya Énergiya, Vol. 16, No. 6, pp. 497–500, June, 1964