Primary Gamma Transitions Research Articles

Level scheme of 164Dy obtained from 163Dy(n[formula omitted],2γ) experiment

This work presents the nuclear level scheme (NLS) of 164Dy nucleus obtained from the 163Dy(nth,2γ) experiment. The latter has used a γ−γ coincidence spectrometer to measure the two-step gamma cascades corresponding to the decays from the compound state to 9 low-lying states of 164Dy, whose energies are 0.0, 73.393, 242.234, 761.815, 828.215, 915.991, 976.916, 1039.309, and 1122.774 keV. Based on the measured data, the 164Dy NLS, which consists of 64 primary gamma transitions and their corresponding intermediate levels, together with 132 secondary gamma transitions, has been constructed. Among these data, 20 primary gamma transitions, 51 intermediate levels, and 63 secondary gamma transitions are found to be the same as those currently exist in the Evaluated Nuclear Structure Data File (ENSDF) database. The remaining data, which include 44 primary transitions, 13 intermediate levels, and 69 secondary transitions, are therefore, considered as an update for the NLS of 164Dy. In particular, the spin values of 57 observed intermediate levels have been tentatively assigned by means of transition rules. Furthermore, the influence of the present experimental data on the nuclear level density and radiative strength function is also discussed.

Investigating the (n, γf) Reaction as a Way of Studying the Properties of Transitions between Highly Excited States in Heavy Nuclei

The history and current status of investigations of the unique two-step (n, γf) reaction observed in heavy nuclei under the influence of neutrons are discussed using the example of measurements made on the neutron time-of-flight spectrometer GNEIS in Gatchina. It is shown what is already known and what kind of information can be derived from experiments to study the (n, γf) reaction on resonance neutrons for the most important fissionable nuclei $^{235}$U and $^{239}$Pu, including information on the properties of primary gamma transitions between highly excited states near the neutron bonding energy.

Two Methods of Determination of Parities of Low-Lying States in 159Gd from Analysis of the γ-ray Intensities from Reaction 158Gd(nres, γ)159Gd

Energy levels and transitions in 159Gd were studied by means of radiative capture of resonance neutrons at 12 isolated resonances of 158Gd. The time-of-flight technique was used on an enriched target at the IBR-30 reactor at JINR Dubna. A total of 80 primary gamma-transitions were recorded and their absolute intensities were determined resulting in the observation of 1/2±, 3/2± levels up to 2.4 MeV. Parities of found levels were recalculated using two methods: the first method consists in analyzing of intensities averaging in 12 resonances and in the second method individual intensities are analyzed. The second method is described for the first time.

Study of Nuclear Structure Parameters by Using the (nth, 2γ) Reaction

The empirical Dubna model for investigating of the cascade gamma-decay of the neutron resonance allows a simultaneous determination of the level density and radiative strengths in a region up to the binding neutron energy for any nucleus. Data can be obtained by determining the intensities of two-step cascades between a decaying compound-state and a group of low-lying levels of the nucleus and describing these intensities by using the most appropriate models prescribed parametrically. For evaluation and minimization of systematical errors of the models used for the level density and radiative strengths for primary gamma transitions spectroscopic information about quanta sequence for observed resolved cascades must be obtained and at least two different sets of models must be used for verification of derivable nuclear parameters. The step-wise structure, which was observed in the energy dependence of the level density for more than 40 investigated nuclei, shows that the structure of a nucleus modifies by excitation.

Updated level scheme of 172Yb from 171Yb(n[formula omitted], γ) reaction studied via gamma–gamma coincidence spectrometer

This paper provides the updated information on the level scheme of 172Yb nucleus studied via 171Yb(nth, γ) reaction using the gamma–gamma coincidence spectrometer at Dalat Nuclear Research Institute (Viet Nam). The latter is used because of its advantages in achieving the low Compton background as well as in identifying the correlated gamma transitions. We have detected in total the energies and intensities of 128 two-step gamma cascades corresponding to 79 primary transitions. By comparing the measured data with those extracted from the ENSDF library, 61 primary gamma transitions and corresponding energy levels together with 20 secondary gamma transitions are found to be the same as the ENSDF data. Beside that, 18 additional primary gamma transitions and corresponding energy levels plus 108 secondary ones are not found to currently exist in this library and they are therefore considered as the new data.

Basic parameters of the gamma-decay process and properties of the 174Yb nucleus that manifest themselves in radiative resonance-and thermal-neutron capture

An independent analysis of available data on the intensities of primary gamma rays from the capture of ≈2-keV neutrons by a 173Yb nucleus is performed. The distribution of the scatter of these intensities around the average value is approximated in various intervals of energies of primary gamma transitions. An extrapolation of the distributions obtained in this way to zero detection threshold for the intensity of a primary gamma transition makes it possible to estimate, independently of other experimental procedures, the expected number of levels of both parities for spins in the range J = 1–4 and the total possible sum of partial widths with respect to electric and magnetic dipole gamma transitions to levels whose excitation energies extend up to about 4 MeV. The results obtained in this way for the level density and the sum of radiative strength functions confirm the characteristic features of analogous data extracted from the intensities of two-step gamma-ray cascades initiated by radiative thermal-neutron capture by 40 ≤ A ≤ 200 nuclei and also make it possible to assess the sign and magnitude of their systematic uncertainty associated with a very strong dependence of radiative strength functions for cascade gamma transitions on the structure of the excited level, at least for excitation energies below half the neutron binding energy. A comparison with model concepts of the level density reveals that the 174Yb nucleus is in a superfluid states for the bulk of excited levels, at least below 3.5 to 4 MeV.

Partial level density of n-quasiparticle excitations, radiative strength functions and new experimental information on the dynamics of nuclear structure change in the B n range

The most reliable at present values of the level density in the fixed spin window and the sums of radiative strength functions of cascade gamma transitions are obtained from analysis of intensities of two-step cascades excited upon thermal neutron capture for approximately 40 nuclei in the mass range 40 ≤ A ≤ 200. The maximal reliability of these data is provided by the experimental conditions—minimum possible propagation error coefficients and practically unique solution of the problem of determination of gamma decay parameters from measured spectra. The experimental data are approximated by the sum of partial level densities corresponding to excitation of n quasiparticles. Steplike structures in the level density at excitation energies smaller than 3–4 MeV are described with good accuracy as the superposition of two-quasiparticle (three-quasiparticle in odd A nuclei) and vibrational excitations with the coefficient of collective density enhancement K coll ≈ 10−20. They correspond to excitation-energy-correlated maximum enhancement of the radiative strength functions of primary gamma transitions. The level density at larger excitation energies is well reproduced if the breakup of at least two more Cooper pairs of nucleons is taken into account. The increase in the number of excited quasiparticles in the nucleus corresponds to unconditional reduction of the radiative strength functions of primary gamma transitions of the compound state decay. However, the maximum possible value of partial widths of primary transitions increases regularly with decreasing energy. Some ambiguity in the results of approximation and divergence from existing theoretical ideas of the energy dependence of nucleon correlation functions in an excited nucleus point to the possibility of direct extraction from experiment of fundamentally new information on the structure of excited nuclear levels in the range of the neutron binding energy. These are, first of all, the parameters of dependence of nucleon correlation functions on the excitation energy of the nucleus.

Primary gamma transitions in 173,174Yb in neutron capture at isolated resonances

Gamma transitions in 174Yb were investigated in radiative neutron capture at 23 isolated and additional 7 partially resolved summed resonances of 173Yb. The time-of-flight technique was used on an enriched target at the IBR-30 reactor at JINR Dubna. A total of 77 primary gamma transitions are reported populating levels in 174Yb up to 2.8 MeV in the spin–parity range 1 ± , 2 ± , 3 ± , 4 ± . Spin and parity assignments of neutron resonances and of populated levels are proposed. In addition to these results, seven primary gamma transitions in 173Yb were also observed from neutron capture at three isolated resonances of 172Yb.

Primary gamma transitions in 159Gd after isolated resonance neutron capture

Energy levels and transitions in 159Gd were studied by means of radiative capture of resonance neutrons at 12 isolated resonances of 158Gd. The time-of-flight technique was used on an enriched target at the IBR-30 reactor at JINR Dubna. A total of 80 primary gamma-transitions were recorded and their absolute intensities were determined resulting in the observation of 1 2 ±, 3 2 ± levels up to 2.4 MeV. The observed level density is described in terms of the constant temperature Fermi gas model. The neutron binding energy was determined at (5943.1±0.2) keV. The photon strength is derived and compared with a giant dipole resonance of standard Lorentzian shape for E1 and M1 radiation. The sum of observed gamma-ray intensity amounts to about 30% of the total gamma width. The remaining gamma-ray intensity above 2.4 MeV is estimated by extrapolating the level density and photon strength models to high excitation energies.

Measurements of a partial cross section for the reaction 58Ni(n, γ0)59Ni

The partial cross section for radiative neutron capture accompanied by gamma transitions to the ground state of the 59Ni nucleus was measured as a function of energy by a new neutron-spectrometry method that employed the shift of a primary gamma transition in response to a change in the energy of the captured neutron. The reaction 7Li(p, n)7Be was used as source of neutrons for the present measurements. The protons that induced this reaction were accelerated by a Van de Graaff electrostatic generator to energies exceeding the reaction threshold by 60 keV, in which case an appropriate geometry of the experiment permitted irradiation of the sample under study with neutrons whose energy ranged between 10 and 120 keV. The partial widths of some resonances and radiative strength function for hard primary M1 gamma transitions were determined in addition to the above cross sections.

Nuclear spectroscopy with slow neutrons

The aim of nuclear spectroscopy is to study nuclear excitations with a view to improving our knowledge of the forces play within the nucleus. Many complementary techniques have been developed to unravel the complex interplay of these forces and it is usually the combination of several experimental approaches such as the study of radioactive decay, transfer reactions, neutron capture, inelastic scattering, Coulomb excitation and other charged particle reactions which allows us to construct detailed level schemes and interpret them. We will concentrate, within the frame of this Workshop on the advanced instrumentation of the neutron capture gamma-ray spectroscopy, which plays an important and specific role in the comprehensive testing of nuclear models. Neutron capture gammaray spectroscopy now employs very sophisticated techniques to study nucear structure. The combination of precise measurements of primary gamma transitions with the precision obtained to data with curved and flat crystal and conversion electron spectrometers has considerably extended the possibilities in the study of nuclear structure. The nearly perfect resolution obtained with the most modern double flat crystal spectrometers allows the measurement of lifetimes of excited nuclear states in the range < 10−12 s via the observation of the Doppler broadening of gamma rays due to the recoil of the nucleus following emission of a primary gamma ray.

Neutron resonance parameters and thermal-neutron capture by 43Ca.

Neutron transmission measurements have been carried out on a small sample (205.9 mg) of ${\mathrm{CaCO}}_{3}$ with a $^{43}\mathrm{Ca}$ enrichment of 49.1%. The parameters (spins and neutron widths) of eight resonances below the neutron energy of 20 keV have been determined. This information was combined with a previously measured thermal-neutron coherent scattering length to deduce the separate scattering lengths for the two spin states present in s-wave scattering. These values were used to calculate the direct (potential + valence) capture cross sections of the primary electric-dipole gamma transitions in $^{44}\mathrm{Ca}$ using optical-model potentials with physically realistic parameters. For those states below 6.2 MeV for which the l=1 (d,p) spectroscopic factors are known, the experimental cross sections are consistent with our current understanding of the direct and compound-nuclear capture processes. The processes leading to states above 6.2 MeV require further experimental and theoretical studies.

Identification of all intrinsic excitations below 2 MeV in168Er

The levels and gamma decay in168Er have been investigated following thermal neutron capture on enriched targets of 167Er2O3. Precision measurements of the gamma spectrum with curved-crystal spectrometers revealed over 700 transitions up to an energy of 2.5 MeV, with intensities spanning five orders of magnitude. A list of primary gamma transitions populating levels in 168Er up to an excitation energy of 3.1 MeV resulted from measurements with the pair spectrometer. Information on the gamma spectrum in the energy region 1400<E<2600 keV was obtained using a Ge(Li) spectrometer. The conversion electron spectrum up to 2.2 MeV electron energy was measured with a beta spectrometer, whence multipolarities of many of the transitions were derived. Average resonance capture measurements at neutron energies of 2 keV and 24 keV were made which ensured that the set of spin 2-5 levels below about 2.2 MeV was complete. From all these data, employing the Ritz combination principle, a level scheme was developed and the levels were arranged into a complete set of 20 low-K rotational bands. This is the most complete and extensive test of the limits of applicability of models for low-lying collective excitations.

G9/2isobaric analogue resonances in the56Fe(p,γ)57Co reaction

Resonances consistent with fragmented g9/2 analogue resonance have been found in 57Co corresponding to the Ex=2.454 MeV level in the 57Fe parent nucleus. The resonance was located through the 56Fe(p, gamma )57Co reaction. Total gamma -ray spectra were measured with a Ge(Li) detector in 2 keV steps in the expected proton energy range. Excitation functions for the different primary gamma transitions were constructed. The g9/2 IAR were found in two components at Ep=3728 and 3735 keV. The angular distributions of gamma rays were measured at these two bombarding proton energies. The two resonances were suggested to have spin and parity Jpi =9/2+ based on the gamma -ray excitation functions and angular distribution measurements. The gamma -branchings and absolute transition probabilities were also determined. The Ml gamma transitions from the g9/2 IAR components going to the possible anti-analogue level at Ex=4.585 MeV excitation energy have absolute transition strengths of 1.8*10-2 Wu and 1.2*10-2 Wu.

Study of isobaric analogue states in52Cr

A detailed investigation of the gamma decay of four analogue resonances in the reaction 51V(p, gamma )52Cr has been performed in the proton energy range Ep=0.7-2.0 MeV. From (p, gamma ) angular distributions, mixing ratios of primary gamma transitions depopulating the analogue resonances and Jpi values of some states populated in the gamma decay have been obtained. Decay schemes, branching ratios, partial gamma widths and resonance strengths have been determined for these IARs. Upper limits of M1 transition rates from analogue states to low-lying states are found to be of the order of few mWu. The analogue states are seen to decay significantly to a group of states in the region of excitation 3.4 to 6.0 MeV.

Spin assignments in low-energy neutron-capture reactions using polarized neutrons and oriented target nuclei

In this paper the combined use of polarized low-energy neutrons and oriented nuclear targets in the study of (n, γ) reactions is discussed in detail. The determination of spins of levels populated by primary gamma transitions is considered. In addition the contribution of I+ 1 2 and I− 1 2 capture, I being the target spin, to individual transitions can be obtained. Necessary corrections are derived from transmission and polarization measurements of the neutron beam; hence, detailed knowledge of the relevant neutron cross sections is not necessary. The orientation parameters and final-state spins are obtained with the aid of a simultaneous fitting and iterative procedure applied to all observed primary gamma transitions. This is elucidated with experimental results of the 59Co(n, γ) reaction.