Neutron and high-energy gamma-ray emission in 12C Induced reactions on 159Tb

The gamma-ray multiplicity associated with the emission of high-energy gamma rays and neutrons following ¹²C+¹⁵⁹Tb reactions at E_lab= 100 MeV has been measured. The spin decrease deduced for coincident gamma rays within the GDR energy region is found to be in agreement with an estimate based on a statistical model. Neutron spectra measured in coincidence with high-energy gamma rays are explained in terms of two components. The cooler component corresponds to neutrons evaporated from the compound nucleus. The hotter component emanates from a precompound source moving in the center-of-mass system along the beam direction. The multiplicity of gamma-ray cascades, coincident with neutrons from the two different sources, has been deduced. The results show that beside the GDR there appears to be a second source of gamma rays with E_γ≈10MeV. These gamma rays are emitted in more peripheral reactions and before neutron evaporation.     

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.     

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

Measurement of Angular Distributions of Gamma Rays from the Inelastic Scattering of 14.1-MeV Neutrons by Carbon and Oxygen Nuclei

The results obtained by measuring the angular and energy distributions of gamma rays originating from the inelastic scattering of 14.1-MeV neutrons by carbon and oxygen nuclei are presented. The measurements in question were performed by the tagged-neutron method in a beam of an ING-27 standard portable neutron generator. The angular distributions of gamma rays emitted by the 2⁺ state of ¹²С at 4.43 MeVand the 3^? state of ¹⁶O at 6.13 MeV were obtained.     

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.     

The decay of 8.7 min237Pa

The beta and gamma radiations of²³⁷Pa have been investigated employing semiconductor and scintillation spectrometers and coincidence techniques. Sources of²³⁷Pa were obtained after bombardments of²³⁸U with bremsstrahlung and 14-MeV neutrons and subsequent chemical separation. From the total of 18γ-rays following the decay of 8.7±0.2 min²³⁷Pa 17 transitions, representing 99.9% of theγ-ray intensity, could be placed in a level scheme of²³⁷U. AQ _β- value of 2.25±0.1 MeV has been determined.     

Fission process of superheavy nuclei

The process of fusion-fission of superheavy nuclei with Z=102?122 formed in the reactions with ²²Ne, ²⁶Mg, ⁴⁸Ca, ⁵⁸Fe and ⁸⁶Kr ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) using a time-of-flight spectrometer of fission fragments CORSET and a neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions of fission fragments, fission and quasi-fission cross sections, multiplicities of neutrons and gamma rays and their dependence on the mechanism of formation and decay of compound superheavy systems have been studied.     

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 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.     

Observation of a strong correlation betweeng(2+γvib) andg(2+rot) in strongly deformed nuclei

The E2/M1 multipole mixing parameters of cascade transitions inγ-vibrational bands of¹⁵⁴Gd,¹⁶⁶Er and¹⁶⁸Er have been determined byγ-γ directional correlation measurements as: $$\begin{array}{l} \delta \left( {^{154} Gd\left( {3_\gamma ^ + \to 2_\gamma ^ + } \right)} \right) = - 4.3_{ + 2.1}^{ - 9.4} \\ \delta \left( {^{166} Er\left( {5_\gamma ^ + \to 4_\gamma ^ + } \right)} \right) = + 1.94_{ - 0.21}^{ + 0.23} \\ \end{array}$$ and $$\delta \left( {^{168} Er\left( {3_\gamma ^ + \to 2_\gamma ^ + } \right)} \right) = + 1.42_{ - 0.04}^{ + 0.04} $$ (with conversion data [15] taken into account) These data were used to deriveg(2⁺ γvib)?g(2⁺rot). The results, together withg-factors derived from direct measurements by IPAC and Mössbuer spectroscopy [10] or by use of transient fields [9, 31] exhibit a strong correlation between bothg-factors, i.e. ifg(2⁺rot) is largeg(2⁺ γvib) is small and vice versa. The most direct and most simple interpretation is the assumption of a more or less different density distribution of protons and neutrons in the nuclei.     

Decay of the 4.2-min152Pm into levels of152Sm

After bombardments of enriched¹⁵²Sm with 14-MeV neutrons, three isomers of¹²⁵Pm reaction, have been observed with half-lives of 4.2±0.2 min, 7.5±0.2 min and 15.0±1.0 min. Sources of pure 4.2-min¹⁵²Pm were obtained by separating the promethium from its parent 11-min¹⁵²Nd produced by thermal-neutron induced fission of²³⁵U. The beta and gamma radiations of the 4.2-min¹⁵²Pm have been investigated using semiconductor and scintillation spectrometers. Most of theγ-ray transitions could be placed in a level scheme for 152Sm. Especially interesting is a level at 1083 keV for which the assignmentI ^π=0⁺ is proposed. In a 4πβγ experiment, the ground state feeding of¹⁵²Sm in theβ ^?-decay of the 4.2-min¹⁵²Pm has been measured to be 61% of allβ-ray transitions. AQ _β-value of 3.4±0.2 MeV has been obtained.     

The TRIUMF-ISAC gamma-ray escape suppressed specrometer, TIGRESS

The TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer is a high energy-resolution, high efficiency γ-ray detector array for radioactive beam experiments. It consists of high-purity germanium detectors with scintillator suppressors and waveform digitization. TIGRESS can operate in modes optimized for either maximum efficiency or for high peak-to-total, and can switch between these modes within a day. The digitized waveforms provide 6 mm FWHM position resolution for incident ¹³⁷Cs γ rays. The array hosts a suite of auxilliary detectors for charged particles and neutrons.     

Gamma ray and π0 production from hydrogen and complex nuclei by 270–375 MeV gamma rays

Measurements are reported on the differential cross section for secondary gamma ray production on hydrogen and nuclear targets at 90° in the laboratory through the interactions of primary gamma rays in the energy range 270–375 MeV. A difference method using bremsstrahlung beams at different end point energies was employed. The gamma ray detector was a high resolution Nal(Tl) spectrometer and time of flight discrimination was used to reject neutrons. For incident gamma rays in the range 270–375 MeV the first pion nucleon resonance is strongly excited and the observed gamma ray yields are consistent with those expected due to an admixture of coherent and incoherent π⁰ photoproduction in the resonance region. The experiment marks the first successful use of a large Nal(Tl) crystals as a gamma ray spectrometer at a high energy electron linear accelerator, despite the low duty cycle and its accompanying difficulties for such detectors.     

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.     

Nuclear spectroscopy of neutron-richA=143 nuclei

A study of the beta decay of¹⁴³Cs and¹⁴³Ba fission products was undertaken by the use of two on-line mass-separators OSTIS and OSIRIS. Level schemes for¹⁴³Ba and¹⁴³La are deduced from gamma and conversion electron spectra,γ-γ andβ-γ coincidences.Q _β values and ground state beta feedings were also measured. The nuclei¹⁴³La and¹⁴³Ba are tentatively inserted in a systematic of odd-even nuclei aroundA=143 and even-odd nuclei with 87 neutrons respectively.     

Cross sections and barriers for nuclear fission induced by high-energy nucleons

The cross sections for the fission of ²³²Th, ^235,238U, ²³⁷Np, and ²³⁹Pu target nuclei that was induced by 20- to 1000-MeV neutrons and protons were calculated. The respective calculations were based on the multiconfiguration-fission (MCFx) model, which was used to describe three basic stages of the interaction of high-energy nucleons with nuclei: direct processes (intranuclear cascade), equilibration of the emerging compound system, and the decay of the compound nucleus (statistical model). Fission barriers were calculated within the microscopic approach for isotopic chains formed by 15 to 20 nuclei of the required elements. The calculated fission cross sections were compared with available experimental data. It was shown that the input data set and the theoretical model used made it possible to predict satisfactorily cross section for nuclear fission induced by 20- to 1000-MeV nucleons.     

强辐射环境下高能脉冲裂变中子探测方法

在研究通道衰减、探测方法分离和探测器中子/射线本征分辨的基础上,研究了测量高能脉冲裂变中子数目的探测技术。基于电流型Si-PIN探测器,设计了减本底的背靠背探测结构,给出了测量强射线和低能散射中子干扰信号及有效扣除强辐射本底的实现方法,最终实现了高n/n和n/分辨测量和强裂变中子、射线混合场中的高能脉冲裂变中子数目探测,探测系统的信号/辐射本底比可达到10倍以上。     

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.     

Calculated beta-ray spectra from decay of fission products produced by thermal-neutron fission of 235U

Beta-ray spectra from decay of fission products created by thermal-neutron fission of ²³⁵U have been calculated using summation techniques and using an independently created beta-ray data file. The calculated beta-ray spectra are in very good agreement for E_β ? 1 to 8 MeV with recently measured specta of beta rays for two very different exposure and counting time conditions: (a) an “instantaneous” spectrum following a 1.5 day exposure, and (b) an 8 s counting-time spectrum obtained 1.7 s following a 1 s exposure of ²³⁵U by thermal neutrons.