Devendra P. Singh

Investigation of incomplete fusion in 20Ne + 55Mn reactions at 3–8 MeV/nucleon

The excitation functions of radio-nuclides have been measured for the 20Ne + 55Mn system in the energy range ≈51–164 MeV by employing the recoil catcher technique followed by off-line gamma-ray spectrometry. It has been observed that some of the residues produced directly via complete and/or incomplete fusion (ICF) give rise to independent yield while some of the residues are produced in the decay of higher charge precursor isobars through β+ and/or electron capture and give rise to cumulative yield. Independent yields of radio-nuclides 71, 70As, 69, 67Ge and 67, 66Ga have been separated out from the measured cumulative yields. The measured excitation functions are then compared with statistical model code PACE-2. It has been observed that excitation functions of the measured complete fusion channels are in good agreement with theoretical predictions. However, a significant enhancement in the measured excitation functions for the residues, produced in the break-up of the projectile into α-clusters, has been observed. This enhancement may be attributed to the ICF process. Moreover, ICF-fraction has also been calculated by dividing the ICF-cross-section to the total fusion cross-section and found to increase with the projectile energy as well as with the entrance channel mass-asymmetry of the interacting partners.

Complete and Incomplete Fusion Reactions in 16O+45Sc System: Measurement and Analysis of Excitation Functions below 7 MeV/Nucleon

Complete and incomplete fusion reactions study in heavy ion induced reactions, by 16 O ions on 45 Sc at incident energies ranging between 39 and 114 MeV have been done using the activation technique followed by γ-ray spectrometry. The excitation functions for eleven reactions have been measured using this technique. A comparison of the experimental data has been made with the calculations based on statistical model. Excitation function measurement of various reactions, for the above projectile–target system is being reported for the first time. Complete and incomplete fusion reaction channels have been identified in the production of various residual nuclei, by analysing the experimental data. The results indicate the occurrence of incomplete fusion involving breakup of 16 O into 4 He+ 12 C and 8 Be+ 8 Be followed by fusion of one of the fragments with target nucleus 45 Sc.

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.

Calibration of thermocouples for low temperature applications

In this communication, concept of enhancement of instrument accuracy in providing measuring values is been discussed. One way to increase instrument accuracy is through calibration technique. K-type thermocouple wire as got advantage of low cost of operation but with decreased accuracy. To overcome this disadvantage, Oil bath, a standard instrument for maintaining constant temperature is considered as reference and calibration of K-type thermocouples is carried out. This study led to the observation that variation in the values of oil temperature and thermocouples varies linearly with respect to each other at every particular temperature, thus utilizing mathematical procedure of linear regression analysis appeared suitable to find out the error in thermocouple readings. Linear regression method provides close approximations (at least within a given range of interest) to relationships which would otherwise be difficult to describe in mathematical terms. This study concluded that similar procedure can be adopted for calibration of other doubtful electrical and electronic instruments, thus increasing their accuracy.

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.

An overview on incomplete fusion reaction dynamics at energy range ∼ 3-8 MeV/A

The information of ICF reaction has been obtained from the measurement of excitation function (EF) of ERs populated in the interaction of 20Ne & 16O on 55Mn, 159Tb & 156Gd targets. Sizable enhancement in the measured cross-sections has been observed in α-emitting channels over theoretical predictions, which has been attributed to ICF of the projectile. In order to confirm the findings of the measurements and analysis of EFs, the forward recoil range distributions of ERs populated in 20Ne+159Tb (E ∼165MeV) and 16O+156Gd (E ∼ 72, 82 & 93MeV) systems, have been measured. It has been observed that peaks appearing at different cumulative thicknesses in the stopping medium are related with different degree of linear momentum transfer from projectile to target nucleus by adopting the break-up fusion model consideration. In order to deduce the angular momentum involved in various CF and / or ICF reaction products, spin distribution and side-feeding intensity profiles of radio-nuclides populated via CF and ICF cha...

Investigation of complete and incomplete fusion dynamics of 20Ne induced reactions at energies above the Coulomb barrier

Experiment has been performed to explore the complete and incomplete fusion dynamics in heavy ion collisions using stacked foil activation technique. The measurement of excitation functions of the evaporation residues produced in the 20Ne+165Ho system at projectile energies ranges ≈ 4-8 MeV/nucleon have been done. Measured cumulative and direct cross-sections have been compared with the theoretical model code PACE-2, which takes into account only the complete fusion process. The analysis indicates the presence of contributions from incomplete fusion processes in some α-emission channels following the break-up of the projectile 20Ne in the nuclear field of the target nucleus 165Ho.