Investigation of Inelastic Neutron Scattering on 27Al Nuclei

The results obtained by measuring the angular and energy distributions of gamma rays produced in reactions induced by the inelastic scattering of 14.1-MeV neutrons on ²⁷Al nuclei are presented. The respective measurements were performed by the tagged-neutron method in a beam from the ING-27 compact neutron generator. The angular distributions were obtained for gamma rays emitted from the 844-keV 1/2⁺, 1015-keV 3/2⁺, 2212-keV 7/2⁺, and 3004-keV 9/2⁺ states of ²⁷ Al nuclei.

     

Investigation of γ radiation from 178Hf in the respective (n, n′γ) reaction

The spectra and angular distributions of gamma rays were measured in the reaction ¹⁷⁸Hf(n, n???) induced by a beam of fast reactor neutrons. Data onmultipole mixtures in gamma transitions and a lot of new information about gamma transitions of energy 1.5 to 3.0 MeV were obtained. A comparison of these results with information known from the respective (n, ??) reaction made it possible to refine the schemes of deexcitation of ¹⁷⁸Hf levels at energies above 1.5 MeV, to determine more precisely features of these levels, and to introduce new levels and rotation bands at excitation energies of about 2MeV.     

Inelastic scattering and radiative capture of protons by Cr50 and Cr54

Total yields and angular distributions of gamma rays following the inelastic scattering of protons from Cr⁵⁰ and Cr⁵⁴ have been measured at proton energies below 2.5 MeV. Gamma ray spectra resulting from the proton radiative capture by Cr⁵⁰ and Cr⁵⁴ were obtained. Transitions from most of the Mn⁵¹ and Mn⁵⁵ low-lying levels have been observed. The experimental results are in good agreement with predictions of the statistical compound nucleus theory.     

Low lying levels of Fe55

Angular distributions of 413 and 1,322keV gamma rays from the Mn⁵⁵(p,nγ)Fe⁵⁵ reaction and the (n-γ) angular correlation measurements have been measured at proton energies below 2.50 MeV. The experimental results show agreement with the Sheldon formula. The multipole mixture parameters for the 933 and 1,322 keV gamma rays in Fe⁵⁵ have been determined. The sum coincidence technique is used to give some information about the scheme of excited levels in the Fe⁵⁵ nucleus.     

Investigation of 88Sr in the reaction Sr(n, n′γ)

With the aid of a beam of fast reactor neutrons, the gamma-ray spectrum and angular distributions of gamma rays with respect to the neutron-beam axis are measured in the reaction Sr(n, n′γ) along with the lifetimes of ⁸⁸Sr levels. Values of the multipole-mixture parameter δ are found for gamma transitions between levels.     

Spins of198Au levels from the (n, γ) reaction using polarized neutrons and polarized197Au nuclei

The¹⁹⁷Au(n, γ) reaction was studied with the aid of polarized thermal neutron beams. Two kinds of experimental data were obtained: i) circular polarization of gamma rays of high energy due to capture of polarized neutrons by unoriented¹⁹⁷Au nuclei, and ii) angular distribution of such gamma rays observed after capture of polarized neutrons by polarized¹⁹⁷Au nuclei. Spins of several levels of¹⁹⁸Au could be uniquely assigned or could be restricted. The sign of the hyperfine field of Au in AuFe has been determined to be negative.     

Effects of rotation of fissioning nuclei in the angular distributions of prompt neutrons and gamma rays originating from the polarized-neutron-induced fission of 233U and 235U nuclei

The results of an experiment devoted to searches for effects of rotation of fissioning nuclei in the angular distributions of prompt neutrons and gamma rays originating from the polarized-neutron-induced fission of ²³³U nuclei are presented. The effects discovered in these angular distributions are opposite in sign to their counterparts in the polarized-neutron-induced fission of ²³⁵U nuclei. This is at odds with data on the relative signs of respective effects in the angular distribution of alpha particles from the ternary fission of the same nuclei and may be indicative of problems in the model currently used to describe the effect in question. The report on which this article is based was presented at the seminar held at the Institute of Theoretical and Experimental Physics and dedicated to the 90th anniversary of the birth of Yu.G. Abov, corresponding member of Russian Academy of Sciences, Editor in Chief of the journal Physics of Atomic Nuclei.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Mechanisms of excitation of <Emphasis Type="Italic">T</Emphasis>-odd correlations for prescission gamma rays and structure of these correlations

Conditions for the appearance and observation of prescission gamma rays emitted by a fissioning nucleus prior to its separation into fission fragments were investigated within quantum-mechanical fission theory. It is shown that these conditions are realizable in the gamma decay of isovector electric giant dipole resonances in a fissile nucleus that are excited because of nonadiabaticity of the collective deformation motion of the nucleus at the ultimate stages of its prefission evolution. Angular and energy distributions of prescission gamma rays emitted by unpolarized fissioning nuclei are analyzed. Features of T-odd correlations in angular distributions of gamma rays arising in the fission of unpolarized target nuclei that is induced by polarized cold neutrons are investigated, and it is shown that these correlations are similar in nature to T-odd ROT correlations discovered earlier for alpha particles emitted in the ternary fission of nuclei.     

Investigation of multipole mixtures in gamma transitions by means of the reaction 232Th(n, n′γ)

The spectrumof gamma rays from the reaction ²³²Th(n, n′γ) induced by a beam of fast reactor neutrons and the angular distribution of the gamma rays in question with respect to this beam were measured. The multipole-mixture parameter δ was found for many gamma transitions.     

Spins and level widths determined by nuclear resonance fluorescence measurements on75As

The lifetimes of the 822, 865 and 1075 keV levels in⁷⁵As, as well as the spins of the latter two levels, were determined by resonant scattering of gamma rays. The primary radiation from 3000 and 5000 Ci⁶⁰Co sources was Compton scattered to produce gamma radiation of the desired energy for photo-excitation in⁷⁵As. The lifetimes of the three levels were found to be 2.00 ps, 0.60 ps and 0.32 ps respectively. The angular distributions of the resonantly scattered radiation from the 865 keV and 1075 keV levels are consistent with spins of 5/2 and 3/2 respectively. Lifetime values are compared with excited-core model calculations.     

The ideal resolution of a liquid xenon calorimeter

The energy and angular resolution of a liquid xenon calorimeter for gamma rays in the GeV region is studied by a Monte-Carlo simulation. For a size of (70 cm)³, the energy resolution becomes as good as 0.2 % (r.m.s) at 10 GeV. The angular resolution is order of 1° (median angle) at several GeV without using the pair converter separated from the main body of the calorimeter. An effective SΩ of 1 m²sr is attainable. These factors meet the conditions required for observation of line gamma rays emerging from the annihilation of a sort of hypothetical dark matter.     

Mn55(p,p′γ)Mn55 reaction low proton energies

Angular distributions of gamma rays following the inelastic scattering of protons from Mn⁵⁵ have been measured at proton energies below 2.50 MeV. The mixture multipole orders for the transition 983→126 keV and 1,530→0 in Mn⁵⁵ have been obtained.     

Integral rotation of 32− levels in 65Zn and 67Zn

Low-lying levels of ⁶⁵Zn and ⁶⁷Zn were excited by the Ni(α, n) reaction using enriched Ni and Ni₁₀Fe₉₀ targets. From the angular distributions of gamma rays, E2/M1 mixing ratios were obtained. In the iron alloy targets, integral spin precession was observed and used to measure the g-factors of two $\text{3}\text{2}{}^{\mathrm{?}}$ levels of ⁶⁵Zn. The results are discussed in terms of the shell and unified models of the odd Zn isotopes.     

Investigations of Properties of <Superscript>89</Superscript>Y Levels with Reactor Fast Neutrons

By employing a beam of reactor fast neutrons, the spectrum of gamma rays up to an energy of 4.6MeV and their angular distributions with respect to the neutron-beamaxis aremeasured in the reaction ⁸⁹Y(n, n'γ). The multipolarities and multipole-mixture parameters for 34 gamma transitions and the spin–parities J^π of states excited in this reaction are determined. The lifetimes of the lowest 32 levels of 89Y were measured by the Doppler shift attenuation method, and the reduced probabilities for the respective gamma transitions were calculated. Levels of the K^π = +5/2⁺ and K^π = ?7/2⁺ bands associated with, respectively, prolate and oblate deformation shapes are found in ⁸⁹Y at low excitation energies.     

Application of a two-step dynamical model to calculating properties of fusion-fission reactions

The two-step model of fusion-fission reactions that was proposed previously by the present authors is extended in such a way as to describe the multiplicity of light particles emitted from nuclearfission fragments. Calculations are performed for the reaction induced by ⁴⁸Ca + ²⁴⁴Pu collisions. The mass distribution of fragments, their mass-energy distribution, and the total multiplicity of neutrons and gamma rays are obtained for E _lab = 230, 238, 249, and 255 MeV. It is shown that the model reproduces qualitatively relevant experimental data. In order to attain quantitative agreement, it is necessary to take into account the angular momentum carried away by particles from the nucleus undergoing fission and various types of gamma rays emitted by the nucleus and its fission fragments.     

Angular momentum misalignment in deep inelastic processes and angular distribution of sequentially emitted particles and gamma rays

The angular momentum misalignment for fragments produced in deep inelastic scattering is discussed in terms of the thermal excitation of angular-momentum-bearing modes in the intermediate complex. Analytical expressions for the in- and out-of-plane angular distributions are obtained for sequentially emitted particles and fission fragments. The angular momentum dependence of the ratio between particle and neutron decay widths is explicitly treated and found to be quite important. Similarly angular distributions are obtained both for dipole and quadrupole gamma decay. The theoretical results are compared with experimental angular distributions of sequential fission fragments, sequential alphas and gamma rays, and a good agreement is found.     

Scission gamma rays

Gamma rays probably emitted by the fissioning nucleus ²³⁶U* at the instant of the break of the neck or within the time of about 10⁻²¹ s after or before this were discovered in the experiment devoted to searches for the effect of rotation of the fissioning nucleus in the process ²³⁵U($ \vec n $ \vec n , γf) and performed in a polarized beam of cold neutrons from the MEPHISTO Guideline at the FRM II Munich reactor. Detailed investigations revealed that the angular distribution of these gamma rays is compatible with the assumption of the dipole character of the radiation and that their energy spectrum differs substantially from the spectrum of prompt fission gamma rays. In the measured interval 250–600 keV, this spectrum can be described by an exponential function at the exponent value of α = −5 × 10⁻³ keV⁻¹. The mechanism of radiation of such gamma rays is not known at the present time. Theoretical models based on the phenomenon of the electric giant dipole resonance in a strongly deformed fissioning nucleus or in a fission fragment predict harder radiation whose spectrum differs substantially from the spectrum measured in the present study.     

Excited levels and isobaric analogue states in55Co investigated with the54Fe(p, γ)55Co reaction

The yield curve for the⁵⁴Fe(p, γ)⁵⁵Co reaction in the energy regionE _p=1.5–2.9 MeV has been recorded and the decay of ten resonances has been investigated. For six of the resonances the strengths have been determined. The angular distributions of the gamma rays have been recorded for four resonances to obtain spins or spin limitations for excited levels. In addition the lifetimes of 10 bound levels were determined with the Doppler Shift Attenuation Method. The isobaric analogues of the ground state and five excited states in⁵⁵Fe are discussed.     

Fast neutron radiative capture in silicon

Using the²⁸Si(n, γ)²⁹Si reaction, transitions to the ground state and first excited state in²⁹Si have been studied in the neutron energy range 3–14 MeV with improved neutron energy resolution (of about 100 keV). The 90° cross sections show considerable structure in the entire neutron energy range. Comparison with theoretical calculations shows that compound-nucleus and direct-semidirect processes account for the non-resonant part (smoothly varying part) of the cross section. A microscopic model is, however, required to describe the resonance structure. Continuum shell-model calculations have proven to be a very promising means towards a better understanding of the capture process in, and below, the giant resonance region in light nuclei. The angular distributions of gamma rays in the neutron energy range 8–14 MeV indicate that the capture reaction is mainly of direct character and that the effect of interference between the electric dipole and isoscalar quadrupole resonance is weak.