Compound Nucleus Contributions Research Articles

Investigation of (n, p), (n, 2n) reaction cross sections for Sn isotopes for fusion reactor applications

The compound Nb3Sn possess superconductivity at suitable temperatures, therefore, it is best suited to be used in the toroidal coils of superconducting magnets which holds the fusion plasma and confine it inside the reactor core. The neutron induced reaction cross-sections are required from threshold to 20MeV for different isotopes of Tin (Sn). Since limited data is available for the reactions with the Sn isotopes. Therefore, we have optimized the (n, p) and (n, 2n) reaction cross-sections for all possible Sn isotopes from threshold to 20MeV with modified input parameters in the nuclear reaction modular codes EMPIRE-3.2.2 and TALYS-1.8. These codes account for the major nuclear reaction mechanisms, including direct, pre-equilibrium, and compound nucleus contributions. The present results from 116Sn(n,p)116mIn, 117Sn(n,p)117In, 117Sn(n,p)117mIn, 118Sn(n,2n)117mSn,120Sn(n,2n)119mSn and 124Sn(n,2n)123mSn reactions calculated with nuclear modular codes: TALYS – 1.8, EMPIRE – 3.2.2 were compared with EXFOR data, systematics proposed by several authors and with the existing evaluated nuclear data library ENDF/B-VII.1, as well. The results from the present study can be used for the future development of ITER devices as well as to upgrade the nuclear model codes.

Dynamical Cluster-decay Model (DCM) applied to 9Li+208Pb reaction

The decay mechanism of 217At⁎ formed in Li9+208Pb reaction is studied within the dynamical cluster-decay model (DCM) at various center-of-mass energies. The aim is to see the behavior of a light neutron-rich radioactive beam on a doubly-magic target nucleus for the (total) fusion cross section σfus and the individual decay channel cross sections. Experimentally, only the isotopic yield of heavy mass residues At⁎211–214 [equivalently, the light-particles (LPs) evaporation residue cross sections σxn for x=3–6 neutrons emission] are measured, with the fusion–fission (ff) component σff taken zero. For a fixed neck-length parameter ΔR, the only parameter in the DCM, we are able to fit σfus=∑x=16σxn almost exactly for 9Li on 208Pb at all Ec.m.'s. However, the observed individual decay channels (3n–6n) are very poorly fitted, with unobserved channels (1n, 2n) and σff strongly over-estimated. Different ΔR values, meaning thereby different reaction time scales, are required to fit individually both the observed and unobserved evaporation residue channels (1n–6n) and σff, but then the compound nucleus (CN) contribution σCN is very small (<1%), and the non-compound nucleus (nCN) decay cross section σnCN contributes the most towards total σfus (=σCN+σnCN). Thus, the 9Li induced reaction on doubly-magic 208Pb is more of a quasi-fission-like nCN decay, which is further analyzed in terms of the statistical CN formation probability PCN and CN survival probability Psurv. For the reaction under study, PCN<<1 and Psurv→1, in particular at above barrier energies.

Estimate of (n,p) Cross Section for Radionuclide 55Fe Using EMPIRE and TALYS

The radionuclide 55Fe (t1/2 = 2.73 years) is one of the radionuclides produced in large quantities inside a fusion reactor. The excitation function of the (n,p) reaction from threshold to 20 MeV and proton emission spectra from the 55Fe target at 14-MeV neutron energy are calculated using optimized input parameters in the nuclear reaction modular codes EMPIRE-3.1 and TALYS-1.4. The codes account for the major nuclear reaction mechanisms, including direct, preequilibrium, and compound nucleus contributions. The present results of 55Fe(n,p)55Mn are compared with the existing evaluated nuclear data libraries ROSFOND-2010, JEFF-3.1, and EAF-2010 along with systematics around 14-MeV neutron energy. The prediction accuracy of the present calculation is considered to satisfy the requirement for fusion reactor applications. The theoretical nuclear model calculations with a reliable parameter set up to 20 MeV are recommended to estimate the cross section of radionuclides or unstable targets in the mass region A ˜ 50 to 60. The present work is an important step to study the cross section of the 55Fe(n,p)55Mn reaction by a surrogate method.

Low energy deuteron-induced reactions on Fe isotopes

The activation cross sections for production of $^{51}\mathrm{Cr}$, $^{52,54,56}\mathrm{Mn}$, $^{59}\mathrm{Fe}$, and $^{55,56,57,58}\mathrm{Co}$ radioisotopes in deuteron-induced reactions on natural Fe were measured at deuteron energies up to 20 MeV. Then, within an extended analysis of deuteron interactions with ${}^{\mathrm{nat}}$Fe, all processes from elastic scattering until the evaporation from fully equilibrated compound system have been taken into account. Following the available elastic-scattering data analysis that supports the deuteron optical potential for reaction cross sections calculations, increased attention is paid especially to the breakup (BU) mechanism and direct reactions (DR). The deuteron activation cross-section analysis is completed by consideration of the preequilibrium and compound-nucleus contributions, corrected for decrease of the total reaction cross section due to the leakage of the initial deuteron flux towards BU and DR processes. The overall agreement of the measured data and model calculations validates the description of nuclear mechanisms taken into account, particularly the strong effects of direct interactions that have still not been appropriately considered within previous deuteron activation evaluations.

Pre-equilibrium effects on (n,p) reactions of Gd and Dy isotopes from threshold to 20 MeV

The excitation functions of (n,p) reactions for the 13 stable isotopes of rare earth Gd and Dy elements are calculated using TALYS-1.4 code from threshold to 20MeV. The optimization of input parameters has been done before running the code. The global optical model potential for neutron and proton proposed by Koning and Delaroche has been adopted in the calculations. The cross-sections are calculated using Hauser Feshbach statistical model for compound nucleus contribution with constant temperature and Fermi gas nuclear level density model. The effect of pre-equilibrium has been checked by invoking different pre-equilibrium models in the code. A good agreement between experimental data (EXFOR) and the calculated cross-sections using two component exciton model with optical model for collision probabilities (Preeqmode 3) has been found. It is also found that the pre-equilibrium contribution dominates in these (n,p) reactions.

Compound and non-compound nucleus contributions in the evaporation residue cross-sections of 20Ne + 181Ta → 201Bi⁎ reaction at and 162 MeV

Abstract The simultaneous existence of Compound Nucleus (CN) and Non-Compound Nucleus (NCN) processes in the decay of a nuclear system, provide a comprehensive picture of nuclear reaction dynamics and related phenomenon in low energy region. The CN and NCN based Evaporation Residue (ER) cross-section data is available for 20 Ne + 181 Ta → 201 Bi ⁎ reaction at two energies i.e. E lab = 150 MeV and E lab = 180 MeV , and the same is tested in the framework of Dynamical Cluster Decay Model (DCM). Experimentally, the NCN component is reported to have major contribution from Incomplete Fusion (ICF) channel and the Deep Inelastic Collision (DIC) component is shown to be negligibly small. The ER formed in CN and NCN (ICF in present case) are nicely reproduced in the framework of DCM. The available CN based ER cross-sections are fitted for spherical, β 2 deformed and β 2 – β 4 deformed fragmentation paths, which in turn provide a nice description of deformation effects in the decay of 201 Bi ⁎ formed in 20 Ne induced reaction. The role of barrier modification and angular momentum dependence is also addressed in addition to deformation and orientation effects.

Deuteron-induced reaction mechanisms at low energies

An extended analysis of the reaction mechanisms involved within deuteron interaction with nuclei is presented. An increased attention is devoted to the breakup mechanism, all its components, namely the elastic, inelastic (fusion), and total breakup being carefully considered. Next, the direct reactions represented by the stripping and pick-up mechanisms are discussed. Finally, the pre-equilibrium and compound-nucleus contributions, corrected for the breakup, stripping, and pick-up decrease of the total reaction cross section, complete the deuteron-induced reaction cross section analysis. The overall agreement between the measured data and model calculations validates the description of nuclear mechanisms taken into account for the deuteron-nucleus interaction.

Pre-equilibrium reactions on the stable tungsten isotopes at low energy

Excitation functions were measured for the 180 W ( n , 2 n ) 179mW, 182 W ( n , p ) 182Ta, 182 W ( n , p ) 182m2Ta, 182 W ( n , α ) 179m2Hf, 183 W ( n , p ) 183Ta, 183 W ( n , α ) 180mHf, 184 W ( n , p ) 184Ta, 184 W ( n , α ) 181Hf, 186 W ( n , p ) 186Ta, 186 W ( n , α ) 183Hf, 186 W ( n , n ′ α ) 182mHf, and 186 W ( n , 2 n ) 185W reactions from 13.4 to 14.9 MeV. The data of these reactions that are dominant for neutron-induced activation in tungsten were in most cases either defective or incomplete. The experimental results are compared with the results of calculations including all activation channels for the stable W isotopes up to a neutron incident energy of 20 MeV, using two global- and one local-parameter approaches which take into account direct reaction, pre-equilibrium emission (PE), and compound nucleus contributions. The sensitivity of the calculated cross sections to various model approaches and parameters is derived and discussed also in connection with the deviation from unity of calculated-to-experimental activity ratios from benchmark experiments with natural W irradiated by D–T fusion neutrons. The particular importance of the charged-particle emission in this atomic-mass range for PE model validation is discussed and the need for additional data for incident energies above 15 MeV is demonstrated.

A dispersive optical model for n + 120Sn from −15 to +80 MeV and properties of neutron single-particle and single-hole states

Data for σ(θ) and Ay(θ) previously obtained at the Triangle Universities Nuclear Laboratory for 120Sn(n, n) are combined with other measurements of σ(θ) and Ay(θ) to create an elastic-scattering database from 9.9 to 24 MeV. In addition, relatively recent high-accuracy measurements of the neutron total cross section σT for Sn from 5 to 80 MeV are combined with earlier σT data to form a detailed σT database from 0.24 to 80 MeV. All of these data are analysed in the framework of a dispersive optical model (DOM). The DOM is extended to negative energies to investigate properties of single-particle and single-hole bound states. The DOM also is used in calculations of compound-nucleus contributions to σ(θ), so that DOM predictions can be compared to σ(θ) measurements. Excellent agreement is obtained for the entire set of scattering data from 0.4 to 24 MeV, and for σT values from 0.05 to 80 MeV. Calculations of bound-state quantities are compared to values derived from experiment for energies down to −15 MeV. Reasonable agreement for the binding energies is achieved, while the predicted spectroscopic factors disagree somewhat with the values found in stripping and pickup experiments. Finally, the DOM is modified to investigate two features (volume absorption that is asymmetric about the Fermi energy and zero absorption in the vicinity of the Fermi energy) that have been ignored in many DOM models. These modifications have little effect on the agreement of the calculations with the scattering data or with the bound-state quantities.

Compound-nucleus contributions to 6Li+12C scattering.

Statistical compound-nucleus contributions to enhanced back-angle cross sections for $^{6}\mathrm{Li}$ + $^{12}\mathrm{C}$ elastic scattering and for inelastic scattering to the channels, $^{6}\mathrm{Li}$(${3}^{+}$, 2.18 MeV) + $^{12}\mathrm{C}$(g.s.) and $^{6}\mathrm{Li}$(g.s.) + $^{12}\mathrm{C}$(${2}^{+}$, 4.44 MeV), are studied. Differential cross sections and vector analyzing powers in the range ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\approxeq}130\ifmmode^\circ\else\textdegree\fi{}--165\ifmmode^\circ\else\textdegree\fi{} have been measured at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$ = 20 MeV by detecting recoil $^{12}\mathrm{C}$ nuclei from $^{6}\ensuremath{\rightarrow}$ elastic scattering and inelastic scattering to the $^{6}\mathrm{Li}$(${3}^{+}$, 2.18 MeV) state. Adding a statistical compound-nucleus contribution is shown in general to reduce the magnitude, and to leave unaffected the sign, of theoretical calculations of vector analyzing powers. Analysis, including Hauser-Feshbach calculations, of the above data along with previous data indicates that compound-nucleus contributions are unimportant in the elastic and inelastic $^{6}\mathrm{Li}$(2.18 MeV) scattering channels, and are insufficient to resolve discrepancies between current coupled-channels calculations and large-angle data for inelastic $^{12}\mathrm{C}$(4.44 MeV) scattering. \textcopyright{} 1996 The American Physical Society.

Determination of α-widths in 19F relevant to fluorine nucleosynthesis

Abstract Nucleosynthesis of fluorine in the context of helium burning occurs through the 15 N(α,γ) 19 F reaction. At temperatures where fluorine formation takes place in most astrophysical models, the narrow resonance associated with the 4.378 MeV level of 19 F is expected to dominate the reaction rate, but its strength is not known. We used a 15 N confined gas target to study this level by means of the transfer reaction 15 N 7 Li,t) 19 F at 28 MeV. Reaction products products were analysed with a split pole magnetic spectrometer and the angular distributions for the first 16 levels of 19 F were extracted. These distributions are fairly well reproduced by FR-DWBA calculations in the framework of an α-cluster transfer model with a compound-nucleus contribution obtained by Hauser-Feshbach calculations. α-spectroscopic factors were deduced and, for unbound levels, the α-widths were determined and compared with the existing direct measurements. The α-width of the level of astrophysical interest ( E x = 4.378 MeV) was found to be Γ α = 1.5 × 10 −9 eV, a value 60 times smaller than the commonly used one. The astrophysical consequences for 19 F production in AGB stars are discussed.

Elastic and inelastic scattering in the 6Li, 9Be + 12C systems: excitation of unbound excited states and cluster transfers

Abstract For the system 9Be + 12C, angular distributions at Ec.m. = 10.86 MeV and excitation functions in the c.m. energy range 3–16 MeV have been measured for the elastic scattering and inelastic scattering leading to the unbound state 9 Be( 5 2 − , 2.43 MeV ) and to the state 12C(2+, 4.44 MeV). For the system 6Li + 12C, angular distributions at Ec.m. = 13.33 MeV and excitation functions in the c.m. energy range 2–16 MeV have been measured for the elastic scattering and inelastic scattering to the unbound state 6Li(3+, 2.186 MeV) and to the state 12C(+, 4.44 MeV). The wide angular range of the inelastic-scattering angular distributions, 20° ≲θc.m.≲ 170°, is obtained by changing appropriately the roles of the projectile and target nuclei. The 9Be + 12C data are analyzed using first-order EFR-DWBA calculations that include 3He-cluster transfers in order to describe the enhancement in the elastic and inelastic scattering at backward scattering angles. In addition to the 3He transfers, compound-nucleus contributions are found also to be significant at the backward scattering angles. Reorientation and coupling effects in the 9Be + 12C system are studied using an optical potential with a double-folded real part, resulting in a good description of the 9Be( 5 2 − , 2.43 MeV) angular distribution without the need for a renormalisation of the M3Y nucleon-nucleon interaction, but with the elastic 3He transfer still required to account for the backward-angle enhancement in the elastic-scattering angular distribution. The 6Li + 12C data are analyzed using coupled-channels calculations; here the inclusion of compound-nucleus contributions is found to be significant for inelastic scattering at backward scattering angles.

Inelastic scattering in the [formula omitted] system leading to unbound excited states of either nucleus

Complete angular distributions for elastic scattering and inelastic scattering to the first excited state (3+, 2.186 MeV) of 6Li and the second excited state (52−, 2.43 MeV) of 9Be in the system 6Li + 9Be are presented for energies Ecm. = 7, 10 and 12 MeV. The inelastic-scattering cross sections for these particle-unbound excited states are measured by detecting the corresponding recoil nucleus. The wide range of the inelastic-scattering angular distributions (30° ≲ θc.m. ≲ 170°) is achieved by interchanging appropriately the role of the projectile and target nuclei. Excitation functions for the studied elastic and inelastic scattering covering the energy range 4 ≲ Ec.m. ≲ 12 MeV are reported also. In the analysis, the channels with the nucleus 6Li excited and with the nucleus 9Be excited are coupled to the ground states in one coupled-channels calculation, using an optical potential with a real double-folded part and a Woods-Saxon imaginary part. The inelastic-scattering data are fitted using deformation lengths that exhibit some energy dependence and that approach at lower c.m. energies more closely those deduced from the B(E2) values. Compound-nucleus contributions to the observed cross sections are calculated using the Hauser-Feshbach formalism, but are found to be rather small in this system, except at backward scattering angles. In addition, the effect of elastic and inelastic triton transfers is examined by performing finite-range DWBA calculations and such contributions are found also to be unimportant.

Mechanism of reactions induced by 7 MeV deuterons on9Be [(d, p), (d, d), (d, t), (d,4He)

Angular distributions of protons, deuterons, tritons and alpha-particles emitted from the reactions in thed+9Be-system atE d =7 MeV as well as excitation functions at selected angles in the energy rangeE d =6.5–7.5 MeV (LAB) were measured. The potential part of the elastic scattering is described by the phenomenological optical model. The compound nucleus contribution to all exit channels is determined using the Hauser-Feshbach model. The collective excitation of the 2.43 MeV excited state of9Be and transfer processes are analysed within the DWBA formalism. The analyses suggest a significant contribution of five-nucleon transfer to the (d,4He) channel.

Analysis of primary electric dipole gamma rays from slow-neutron capture by Ca isotopes.

We have analyzed the available data on the primary $E1$ transitions in the thermal-neutron capture by even isotopes of calcium with the aim of determining the relative importance of direct and compound-nucleus mechanisms of capture in these nuclides. In some of these cases, the thermal-neutron scattering length, as well as the spectroscopic factors of the final states, is known, and an absolute estimate of the direct capture cross sections can be made. In these cases ($^{40}\mathrm{Ca}$, $^{44}\mathrm{Ca}$) the estimates made from a specialized optical model (which reproduces the scattering length precisely) and a global optical model with a valency contribution from a local level are in agreement and are also generally close in absolute value to the experimental cross sections. The detailed deviations can be accounted for as a compound-nucleus contribution to the transitions, which fluctuates, but has an overall magnitude in agreement with Cameron's semiempirical relationship for this mechanism. In both nuclides the compound-nucleus contribution to the capture can plausibly be attributed to a bound local level. For $^{48}\mathrm{Ca}$ there is some indirect evidence for the value of the thermal-neutron scattering length, and the use of this value in the calculations of direct capture leads to agreement with the data. There is no evidence for an important compound-nucleus contribution in this magic number nucleus. In $^{42}\mathrm{Ca}$ and $^{46}\mathrm{Ca}$ the thermal-neutron scattering length can be adjusted to "best fit" the calculation of direct capture from the optical model to the experimental data. After this adjustment the detailed agreement between theory and data is very good, indicating the major predominance of the direct mechanism in capture at thermal-neutron energies by these nuclides.

Elastic and inelastic scattering of nucleons from 16O

Measurements of differential elastic and inelastic cross sections for neutron scattering from 16O at incident energies 18 to 26 MeV are presented. In addition to cross sections for neutron scattering differential cross sections for proton scattering up to 66 MeV are described in terms of phenomenological optical model potentials. At 24.5 MeV incident energy inelastic scattering up to 11.5 MeV excitation was measured. The elastic and inelastic compound nucleus contributions were examined. Direct inelastic scattering from the normal parity states was calculated using the DWBA and coupled-channel formalisms. The inelastic scattering cross section from non-normal parity state 2 − was calculated using the coupled-channel formalism via multi-step processes. Cross sections due to inelastic scattering from some of the states, which are thought to be members of an excited state rotational band were calculated using both vibrational and rotational approaches and were compared.

Nuclear structure and direct reaction mechanism in neutron scattering on 28Si in the energy range 6.8 to 14.8 MeV

The structure of excited states in 28Si as well as the energy dependence of the reaction mechanism are investigated. Angular distributions related to the 0 1 +, 2 1 +, 4 1 +, 0 2 +, 3 1 +, 2 2 + + 2 3 +, 3 1 − + 4 2 + levels in the 28Si(n, n′) reaction were measured at incident energies 6.8, 7.0, 8.0 … 12.0 MeV. The analysis was extended up to 14.8 MeV bombarding energy. The experimental cross sections are described by means of a coupled-channel calculation including compound nucleus contributions. The low-lying excited states can be interpreted by a rotational model with prolate but also with oblate g.s. deformation. For the higher-lying states various coupling schemes have been tested. Especially, a second rotational band could be described adopting an oblate deformation.

3He cluster structure of 12C.

The reaction $^{9}\mathrm{Be}$${(}^{6}$Li,t${)}^{12}$C was studied at 32 MeV. Selective population of states was observed up to the 14.1 MeV state in $^{12}\mathrm{C}$. The high probability for three or more body processes resulted in a large continuum yield which prevented the observation of the population of states in the 17--22 MeV excitation region of $^{12}\mathrm{C}$. Hauser-Feshbach calculations show that the statistical compound nucleus contribution to the cross section is small. The experimental angular distributions are well reproduced by distorted-wave Born approximation calculations. The extracted relative spectroscopic strengths are in good agreement with theoretical values. The T=1 15.1 and 16.1 MeV states were not observed. A measurement of the $^{9}\mathrm{Be}$${(}^{6}$Li${,}^{3}$He${)}^{12}$B cross sections to their analog states shows that these states would have a peak height of only 10% of the continuum yield, making it difficult to determine if these states are populated in the present reaction.

The Inelastic Scattering of Neutrons by 232Th

The method previously used to analyze the inelastic scattering of neutrons by 238U by separating the cross sections into compound nucleus and direct interactions is applied to the corresponding data for 232Th. The fission and capture channels are now included explicitly, resulting in a 5% increase in the compound nucleus contribution. Other data and theoretical techniques are discussed briefly. The excitation functions for 24 states up to 2.5 MeV are analyzed, together with two angular distributions at 2.5 MeV.

Thermal neutron capture gamma rays from sulfur isotopes: Experiment and theory.

We have carried out a systematic investigation of $\ensuremath{\gamma}$ rays after thermal neutron capture by all stable sulfur isotopes ($^{32}\mathrm{S}$, $^{33}\mathrm{S}$, $^{34}\mathrm{S}$, and $^{36}\mathrm{S}$). The measurements were made at the internal target facility at the Los Alamos Omega West Reactor. We detected a larger number of $\ensuremath{\gamma}$ rays: \ensuremath{\sim}100 in $^{33}\mathrm{S}$, \ensuremath{\sim}270 in $^{34}\mathrm{S}$, \ensuremath{\sim}60 in $^{35}\mathrm{S}$, and \ensuremath{\sim}15 in $^{37}\mathrm{S}$. Before developing detailed level schemes, we culled and then consolidated the existing information on energies and ${J}^{\ensuremath{\pi}}$ values for levels of these nuclides. Based on the current data, we have constructed detailed decay schemes, which imply that there are significant populations of 26 excited states in $^{33}\mathrm{S}$, 70 states in $^{34}\mathrm{S}$, 20 states in $^{35}\mathrm{S}$, and 7 states in $^{37}\mathrm{S}$. By checking the intensity balance for these levels and by comparing the total intensity of primary transitions with the total intensity of secondary $\ensuremath{\gamma}$ rays feeding the ground state, we have demonstrated the relative completeness of these decay schemes. For strongly populated levels, the branching ratios based on the current measurements are generally better than those available from previous measurements. In all four cases, a few primary electric dipole ($E1$) transitions account for a large fraction of the capture cross section for that particular nuclide. To understand and explain these transitions, we have recapitulated and further developed the theory of potential capture. Toward this end, we reviewed the theory relating off-resonance neutron capture to the optical-model capture. We studied a range of model-dependent effects (nature and magnitude of imaginary potential, surface diffuseness, etc.) on the potential capture cross section, and we have shown how experimental data may be analyzed using the expression for channel capture suitably modified by a factor that takes into account the model-dependent effects. The calculations of cross sections for most of the primary transitions in the sulfur isotopes are in good agreement with the data. Some discrepancies for weaker transitions can be explained well by an interfering compound-nucleus contribution to capture. This contribution is of the magnitude expected from statistical surveys of resonance capture data. Estimates of the cross section due to the valencecapture mechanism in $s$-wave resonances show that this cross section should dominate the more complicated compound-nucleus contributions.