Influence of Incomplete Fusion Reaction on Complete Fusion Below 10 Mev / Nucleon Energies

An attempt has been made in the present work to provide an ample opportunity to explore the information about the influence of incomplete fusion (ICf) reaction dynamics on complete fusion in heavy ion induced nuclear reactions. excitation functions for several evaporation residues produced in the interaction of projectile O with target lu have been measured over the wide projectile energy range ≈ 70-100 MeV. the recoil-catcher activation technique followed by the offline γ-ray spectroscopy has been used for the present measurements. In case of precursor decay, we have made use of Cavinato et al. formulation to calculate the independent cross-section of the identified residues. the measured efs are compared with theoretical predictions of statistical model code PACe-2 and any enhancement in the measured cross-section from theoretical prediction may be due to ICf reaction process. An attempt has been made to estimate the ICf contribution of the cross-section from the measured excitation function data and the dependence of ICf cross-section on projectile energy.


IntroductIon
In recent years, the study of heavy-ion (HI) induced nuclear reactions with heavier target nuclei has been the subject of renewed interest for the better understanding of reaction dynamics.It is now well established from the earlier studies that incomplete fusion (ICf) reactions start competing with the complete fusion (Cf) reactions at projectile energies just above the Coulomb barrier.In Cf processes, the entire projectile fuses with the target nucleus in the vicinity of target nuclear field and formation of an equilibrated compound nucleus comes into existence which may further decay by emitting the particles and/ or characteristics γ-rays.However, in case of ICf, the break-up of projectile may takes place into two fragments, near to the target nuclear field.One of the fragments fuses with the target while remnant moves as spectator in the forward direction with approximately beam velocity [2][3][4], [9], [11][12][13][14][15]. Incomplete fusion (ICf) or massive transfer reactions have been studied extensively at lower projectile energies below 10 MeV/nucleon in recent years.However, the study of ICf is still an active area of investigations due to complex nature of incomplete mass transfer mechanism and its ambiguous dependence on various entrance channel parameters.the ICf features first observed by britt and Quinton [5] at lower projectile energies with the break-up of projectiles like 12 C, 14 N, and 16 O into α-clusters.later on Inamura et al. [16] provided the additional but concrete information to understand the ICf reaction dynamics.
Several models have been proposed to explain the ICf reactions, such as break-up fusion model [17] of udagawa and tamura, Sum-rule model of wilczynski et al. [8] and promptly emitted particles (PeP) [7] etc. the existing models have been used to fit the experimental data above 10 MeV/nucleon energies.However, no theoretical model is yet available to explain ICf process data satisfactorily at energies less than 10 MeV/nucleon.thereby, ICf reaction mechanism is still an active area of investigations.Most of the studies on ICf have been centered to medium mass target (A ≤ 100) around beam energies ≈ 4-7 MeV/nucleon.there are fewer studies with heavier targets (A ≥ 150) at lower projectile energies ≈ 4-7 MeV/nucleon.In case of heavier target nuclei, the evaporation of α-particle from the compound nucleus has the less probability due to the high Coulomb barrier, thereby, ICf fraction is observed to be dominating as that of Cf fraction in α-particles emission products.In order to reach on some definite conclusions regarding various parameters on ICf nuclear reaction dynamics, more and more experimental data using α-cluster and non-α-cluster projectile on heavier target nuclei are needed at lower projectile energies.
In the present work, we have measured and analyzed the excitation functions of evaporation residues produced in 16 O + 175 lu system at energies 4 -7 MeV/ nucleon.the experimentally measured efs of various evaporation residues produced via Cf and/or ICf have been compared with theoretical predictions based on statistical model code PACe-2 [1], which takes into account only the Cf contribution.
Influence of incomplete fusion reaction on complete fusion below 10 MeV/ nucleon energies

ExpERIMENtAl pRoCEduRE
for excitation functions (efs) measurement, the experiment was carried out using the facilities of 15uD Pelletron accelerator at Inter-university Accelerator Centre (IuAC), New Delhi.Stacked foil activation technique has been used for efs measurement.the main advantage of the activation technique is the possibility of measuring cross-section for the production of a large number of residues in a single irradiation.Self supporting target foils were prepared using the rolling machine.Keeping in view the half-lives of the interest, two stacks, each having four target foils have been backed by Al-catcher foils were irradiated by 16 O +7 beam with 20 nA current for about 6 hours in the general Purpose Scattering Chamber (gPSC) at energies ~ 100 and ~ 95 MeV. the gPSC has an in-vacuum transfer facility.the energy loss suffered by 5.49 MeV αparticle obtained from 241 Am source, was used to determine the thickness of target and Al-catcher foils.SRIM08 code [6] has been used for determining the thickness from the energy lost measurements.the sample was cut into the pieces and pasted on Al-holders of regular size having concentric holes of 10 mm diameter.the Al-holders were used for rapid heat dissipation produced during irradiation of target.the delay time between the stop of irradiation and the starting of counting may be minimized using the in-vacuum target transfer facility.the total charge collected in the faraday cup, placed behind the target-catcher foils arrangement has been used to calculate the beam flux during the irradiation.In case of activation technique, a large number of residual nuclei may be produced and each radio-nuclide has a number of γ-rays, which provide a specific way for its identification.therefore, in order to have the correct identification of the characteristic γ-rays of evaporation residues, a calibrated HPge detector with high resolution is required.Also, for a given source-detector geometry the detector efficiency must be known.In the present measurements, the activities induced in each catcher foil of the stack were recorded separately using a high resolution (~ 2 keV for 1.33 MeV γ-ray of 60 Co) HPge detector of 100 cm 3 active volume coupled to a PC through CAMAC based fReeDOM software at IuAC, New Delhi.the HPge detector has been calibrated by using the standard 152 eu γray source of known strength.the 152 eu source may decay by emission of various intense and well resolved γ-rays having the energy range from 120 keV to about 1410 keV.
the probability of occurrence of a particular nuclear reaction is generally described by the reaction cross-section.One may measure the cross-section of reaction products in order to get comprehensive information about the process of its formation.the expression [10] is extensively used to calculate the reaction cross-section s r (E) at a given beam energy e; where, A is the total number of counts recorded under the peak in time t 3 , N 0 is the number of target nuclei, f is the incident flux, t 1 is the irradiation time, t 2 is the time lapsed between stop and starting of irradiation, t 3 is the recording time, θ is the branching ratio, λ is the decay constant of the evaporation residue, e G is the geometry dependant efficiency of the detector, K is the self-absorption correction factor of the γ-ray in the target.

REsults ANd dIsCussIoN
efs of various evaporation residues (eRs) produced via Cf and/or ICf have been measured for the system 16 O + 175 lu at energies ranging from 4-7 MeV/ nucleon.Special care has been taken to separated out the precursor decay contributions in the production of several evaporation residues to get the independent production cross-sections of the residues [11].the measured efs are then compared with the predictions of statistical model code PACe-2 [1], which gives only the Cf contribution.So any enhancement in the measured cross-section from theoretical prediction may be due to ICf reaction process in addition to Cf process.Independent cross-section for the production of daughter nucleus b in the sequential isobaric decay A→b is given by; where, T A and with branching ratios P A and P B , relation (2) takes the form; efs of residues 186 Au(5n), 187 Pt(p3n), 186 Pt(p4n) and 184 Ir(α3n) are shown in fig.1(a).the experimental values match well with theoretical predictions of statistical model code PACe-2 for level density parameter constant K = 10, which shows that these residues are populated through Cf process as expected.Moreover, from the measured ef of 184 Ir(α3n) reveals that this residue is also formed by the complete fusion of projectile 16 O with target 175 lu. the compound Influence of incomplete fusion reaction on complete fusion below 10 MeV/ nucleon energies 51 system 191 Au * may decay via statistical evaporation of one α-particle and three neutrons leaving behind the residue 184 Ir, which may be represented as: It is noteworthy here that the ef of 184 Ir does not show any enhancement even 184 Ir is populated via the emission of α-channel.the absence of enhancement indicates that the excitation energy of the incompletely fused composite (IfC) system 187 Ir * (in the fusion of fragment 12 C with the target) is insufficient for emission of three neutrons.the sums of cross-sections for all measured Cf channels (Ss CF ) are plotted along with the sum of cross-sections for all Cf channels obtained from PACe-2 as a function of projectile energy.It may be pointed out that sum of cross-sections for measured Cf channels matches with the sum of theoretical prediction as shown in fig.1(b).for the residue 183 Ir(α4n), as shown in fig.2(a), we have observed an enhancement from the theoretical predictions, which is the signature of contribution from ICf process along with the Cf process, where the projectile breaks up into fragments [ 12 C + 4 He(α)], one of the fragment, 4He(α) moves forward with same angular momentum as that of the projectile and the other fragment fuses with the target 175 lu, forming the excited compound system 187 Ir * , which decays by emitting four neutron to from 183 Ir. this can be represented as,  Influence of incomplete fusion reaction on complete fusion below 10 MeV/ nucleon energies 53 Some other residues produced via αxn/2αxn channels like 187 Ir(α), 182 Re(2αn) etc. are expected to be populated either by Cf and/or ICf processes; however, PACe-2 values for these residues are either very small or negligible.thus, it may be concluded that these residues are populated by ICf process only and Cf process does not contribute in the population of these residues.the ICf fraction (fICf), ) has been deduced for the residue 183 Ir(α4n) and plotted as a function of projectile energy and is shown in fig.2(b).It may be pointed out from this figure that fICf increases with projectile energy in case of 183 Ir(α4n). the ICf contribution of individual break-up α-emission channels (ICf products) for 16 O + 175 lu system has been deduced by subtracting the theoretically calculated cross-section by PACe-2 from the experimentally measured cross-section at each projectile energy.the sum of cross-sections from all measured ICf channels Sσ ICF ( ) has also been plotted against the projectile energy and shown in fig. 3.

suMMARY ANd CoNClusIoNs
In the present work, the efs of several evaporation residues produced via Cf and/or ICf have been measured and analyzed in the framework of statistical model code PACe2 for the system 16 O + 175 lu in the energy regime 4-7 MeV/ nucleon.the experimentally measured efs of xn/pxn channels have been found to be reproduced reasonably well with the theoretical predictions based on statistical model code PACe2, indicating their population via Cf only.On the other hand, a significant enhancement is observed in the measured crosssections of α-emitting channels.It may be concluded that ICf contribution i.e. break-up probability of the projectile into α-clusters [i.e.break-up of 16 O into 12 C + 4 He and/or 8 be + 8 be and/or 4 He + 12 C] in general, increases with projectile energy.
the half-life of parent and daughter nuclei, P A is the branching ratio, σ cum B is the cumulative cross-section of the daughter nucleus,σ ind Bis the independent cross-section of the daughter nucleus and σ ind A is the independent cross-section of the parent nucleus.In case of two precursor isobars i.e.A→b→C with half-lives T T

Figure 1 :Figure 2 :
Figure 1: (a) excitation functions of the evaporation residues 186 Au(5n), 187 Pt(p3n), 186 Pt(p4n) and 184 Ir(α3n) produced in the 16 O + 175 lu system.Solid circles represent experimental data.the solid line represents the polynomial fit to the PACe-2 predictions and (b) the sum of cross-sections from all measured Cf channels ( ) ∑ σ CF are plotted along with the sum of cross-sections for all Cf channels obtained from PACe-2 as a function of projectile energy.

Figure. 3 :
Figure.3: Sum of incomplete fusion cross-sections (Ss ICf ) as a function of projectile energies for the 16 O + 175 lu system.