Vijay R. Sharma

Spin distribution measurements in 16O+159Tb system: incomplete fusion reactions

The spin distributions of complete and incomplete fusion residues populated in the 16O+159Tb system have been measured using the particle-γ coincidence technique to investigate the role of high angular momentum values in the production of direct-α-emitting channels. Spin distributions of various αxn channels are measured at ELab ≈ 6.2 MeV A−1. Entirely different de-excitation patterns are observed in direct-α-emitting channels and fusion-evaporation channels. The fusion-evaporation channels are found to be strongly fed over a broad spin range. While narrow range feeding for only high-spin states was observed in the case of direct-α-emitting channels. To have a better insight into the associated angular momenta values in different reaction channels, the present data are compared with similar data from [1, 2]. The mean driving angular momenta involved in the production of direct-α-emitting channels are found to be higher than those involved in the production of fusion-evaporation channels.

Incomplete vs. Complete Fusion at E/A ≈ 4–7 MeV

With a view to study onset and strength of incomplete fusion at low projectile energies (i.e., ~ 4–7 MeV/nucleon) three sets of experiments have been performed in 12C,16O+169Tm systems. In first set of experiments, spin-distributions and feeding intensity profiles for xn,αxn/2αxn-channels have been measured to figure out associated l-values. The spin-distributions for direct-α-emitting channels (associated with incomplete fusion) have been found to be distinctly different than that observed for fusion-evaporation (complete fusion) channels. The mean value of driving input angular momenta associated with direct-α-emitting-channels have been found to be higher than that observed for fusion-evaporation xn/α-emitting-channels, and increases with direct-α-multiplicity in forward cone. The second set of experiments has been performed to understand influence of incomplete fusion on complete fusion at these energies. Incomplete fusion strength function has been deduced from the analysis of experimental excitation functions. The third set of experiments deals with the validation of data reduction procedure used to deduce incomplete fusion fraction, and to confirm the fusion incompleteness at slightly above barrier energies. Forward-recoil-ranges of heavy reaction products have been measured and analysed on the basis of break-up fusion model. More than one linear-momentum-transfer components associated with full- and/or partial-fusion of projectile with target nucleus have been observed. Experimental ranges of forward-recoils are found to be in good agreement with that estimated using range-energy formulation. The relative strengths of complete and incomplete fusion components deduced from the analysis of forward-recoil-ranges and excitation functions complement each other. Result presented in this paper conclusively demonstrate substantial incomplete fusion contribution at energy as low as 7% above the barrier.

Systematic study of projectile structure effect in fusion reactions at low energies

The complete fusion probability has been measured and deduced after the incorporation of recently proposed surface energy coefficient in one dimensional barrier penetration model at a fixed value of diffuseness parameter for the systems 12C+159Tb and 16O+159Tb. Experimental results are compared for several systems involving different projectile and same target (159Tb). The comparison of the results with weakly and strongly bound systems shows a strong dependence of reduced complete fusion probability on the alpha Q-value. In order to study the degree of fusion incompleteness involved in various projectiles, results are also compared with the recently proposed Universal Fusion Function.

SIGNATURE OF PRE-EQUILIBRIUM-EMISSION IN FORWARD-TO-BACKWARD YIELD RATIO MEASUREMENT

In this paper, pre-equilibrium-emission has been investigated using particle-γ coincidence technique. Forward-to-backward yield ratios of different reaction products have been measured in 16O+169Tm system at 5.6 MeV/nucleon. Coincidences between prompt-γ-rays and charged particles emitted in various angular zones have been recorded to identify different reaction products. Several gating conditions have been projected onto the energy spectra to achieve the information about mode of reaction. Yield profiles of fusion/fusion-like channels identified from forward/backward/sideways particle (Z = 1, 2)-gated-spectra have been measured. High yield in forward cone has been observed from the experimentally measured forward-to-backward yield ratios. The enhanced forward cone yield has been attributed to the pre-equilibrium-emission. The maximum observed spin is found to be decreased with the number of proton emitted from the composite nucleus during the equilibrium and/or pre-equilibrium decay.

Investigation of Incomplete Fusion events: Recent Results

The dynamics of incomplete fusion processes has been extensively investigated and is found to compete with complete fusion at low incident energies (i.e. ≤ 7.0 MeV/A). Recent reports suggest that the probability of incomplete fusion depends on Q α of the projectile, and found to be originated from the high input angular momentum provided into the system due to non-central collisions. Apart from the well documented existence of low energy incomplete fusion, a strong contradiction on previously established mass-asymmetry systematics has been noticed recently. It has been found that the incomplete fusion fraction increases with entrance channel mass-asymmetry for the individual projectiles. Further, a strong dependence of incomplete fusion on the Coulomb effect (Z P Z T ) has been observed and is found to increase linearly with Z P Z T , a significantly important approach to understand the breakup fusion reactions. For better insights into the onset and strength of incomplete fusion in terms of various entrance channel parameters and to understand the role of high angular momenta in the onset of incomplete fusion, a quality data have been obtained in a variety of experiments performed at the Inter-University Accelerator Center (IUAC), New Delhi, India. Some of the recent results are presented in this paper, conclusively demonstrating the effect of entrance channel parameters on the onset and strength of incomplete fusion, and the usefulness of incomplete fusion to populate high-spin states in final reaction products.

Competition between the compound and the pre-compound emission processes in α-induced reactions at near astrophysical energy to well above it

The study of pre-compound emission in α-induced reactions, particularly at the low incident energies, is of considerable interest as the pre-compound emission is more likely to occur at higher energies. With a view to study the competition between the compound and the pre-compound emission processes in α-induced reactions at different energies and with different targets, a systematics for neutron emission channels in targets 51V, 55Mn, 93Nb, 121, 123Sb and 141Pr at energy ranging from astrophysical interest to well above it, has been developed. The off-line γ-ray-spectrometry based activation technique has been adopted to measure the excitation functions. The experimental excitation functions have been analysed within the framework of the compound nucleus mechanism based on the Weisskopf-Ewing model and the pre-compound emission calculations based on the geometry dependent hybrid model. The analysis of the data shows that experimental excitation functions could be reproduced only when the pre-compound emission, simulated theoretically, is taken into account. The strength of pre-compound emission process for each system has been obtained by deducing the pre-compound fraction. Analysis of data indicates that in α-induced reactions, the pre-compound emission process plays an important role, particularly at the low incident energies, where the pure compound nucleus process is likely to dominate.

Reconciliation of mass-asymmetry systematics for incomplete fusion

The onset and strength of incomplete fusion (ICF) has been studied in the framework of Morgensterns mass-asymmetry systematics. The fraction of ICF has been deduced in 12C+169Tm system at energies ranging from 1.02Vb to 1.64Vb (Vb = 54.94 MeV from the analysis of excitation functions (EFs). It has been found that the ICF starts influencing complete fusion at noticeably lower, β-values (i.e., 0.025 or 2. 5% of c) than that proposed by Morgenstern (i.e., ≈ 6% of c). The fraction of ICF increases with entrance channel mass-asymmetry for individual projectiles, termed as projectile dependent mass-asymmetry (ProMass-) systematics. The proposed ProMass- systematics has withstood all tests that have been done for fairly large number of systems to verify its validness.

Understanding the onset of incomplete fusion

The entrance channel effect on the onset and strength of incomplete fusion (ICF) has been studied in the present work. Several inclusive experiments have been performed to measure the ICF strength function in 12C,16O+169Tm systems at near and above barrier energies. Data obtained in these experiments suggest the existence of ICF even at slightly above barrier energies where complete fusion (CF) is supposed to be the sole contributor, and conclusively demonstrate strong projectile structure and energy dependence of ICF. The incomplete fusion strength functions for 160,12,13C+159Tb and 160,12,13C+181Ta systems are analyzed as a function of projectile α-Q-value at a constant νrel = 0.053c. It has been found that one neutron (1n) excess projectile 13C (as compared to 12C) results in less incomplete fusion contribution due to its relatively large negative α-Q-value. In order to understand the onset of ICF at such low energies, the driving input angular momenta (l) involved in the production of different evaporation residues have been deduced from the analysis of experimentally measured spin-distributions for the same projectile-target combinations at the same incident energies. Higher l-values, imparted into the system in non-central interactions, are found to be responsible for low energy ICF. The ICF-αxn/2αxn channels display involvement of higher l-values than that observed in CF-xn/pxn/αxn/2αxn channels at the very same projectile energies. It has been observed that the mean value of l increases with successively opened ICF channels and incident energy.

Observation of incomplete fusion reactions at l < l crit

In order to understand the presence of incomplete fusion at low energies i.e. 4-7MeV/nucleon & also to study its dependence on various entrance-channel parameters, the two type of measurements (i) excitation function for 12C+159Tb, and (ii) forward recoil ranges for 12C+159Tb systems have been performed. The experimentally measured excitation functions have been analyzed within the framework of compound nucleus decay using statistical model code PACE4. Analysis of data suggests the production of xn/px)n-channels via complete fusion, as these are found to be well reproduced by PACE4 predictions, while, a significant enhancement in the excitation functions of α-emitting channels has been observed over the theoretical ones, which has been attributed due to the incomplete fusion processes. Further, the incomplete fusion events observed in case of forward recoil range measurements have been explained on the basis of the breakup fusion model, where these events may be attributed to the fusion of 8Be and/or 4He ...

Low energy in-complete fusion: The α-Q-value systematics

In the present work, the in-complete fusion probability has been deduced for 12C+159Tb and 13C+159Tb systems in the energy range ≈ 4-7 MeV/nucleon. The activation technique followed by off-line γ-rays spectroscopy has been used. The results have been compared with the predictions of statistical model code PACE4, which is based on the compound nucleus decay. The ratio of the sum of cross-sections of all measured xn, pxn channels to their corresponding PACE4 predictions are found to be lying close to unity throughout the studied energy range for both the systems, indicating their population via fusion evaporation channels followed by CF processes only. However, the ratio of the sum of cross-sections of all experimentally measured xn, pxn and αxn-channels to their corresponding PACE4 calculations, display significant enhancement over the unity line, indicating the onset and presence of in-complete fusion processes at these energies. The comparison of experimentally measured and systematically deduced in-comp...