In order to carry out the experiment, first the self-supporting target foils of isotopically pure (99.97%) ⁵¹V of thicknesses ranging from ≈1.19 - 1.50 mg/cm² and Al-degrader foils of thicknesses ranging from ≈1.47 - 1.64 mg/cm² were prepared at Target Lab of VECC, Kolkata. The thickness of each target and degrader foil was measured separately by two methods. Firstly, by weighing the ⁵¹V target and Al-degrader foils, and secondly, by measuring the energy loss suffered by 5.485 MeV α-particles from the ²⁴¹Am source while traversing the target and degrader medium. The measured thicknesses of ⁵¹V target and Al-degrader foils using these two methods were quite in agreement with each other. The prepared target foils were cut into square pieces of 1.5 ´ 1.5 cm² and placed into the Aluminum holders of standard size having concentric holes of 10 mm in diameter. Each target foil was backed by an Al foil of appropriate thickness (hereafter called the target catcher foil assembly) to trap the heavy recoiling products produced in the reaction. The use of stack-foil activation technique, i.e., by using a set of successive target-catcher foil arrangement, helps in reducing the energy of the beam on the subsequent targets and ensured a relatively wide energy range for measuring the excitation function (the energy dependence of cross section). The stack-foil activation technique has this considerable advantage of allowing the simultaneous determination, in a single irradiation, of the excitation functions of a large number of exit channel products. This technique has been extensively utilized in the study of nuclear reactions using the stable ion beams and the results were found to be quite satisfactory and in agreement with the predictions of theoretical models [26] [27] [32] .

The stack was placed normal to the beam direction with ⁵¹V target facing the beam and it was irradiated with ²⁰Ne⁶⁺ ion beam at. Keeping in view the half-lives of interest, irradiation has been carried out for ≈11 hrs. The weighted average beam current of about ≈40 nA behind the target assembly was measured with an electron suppressed Faraday cup, using a current integrator device, for every 2 min, so as to correct for the variation, if any, in the beam intensity during the irradiation, which are particularly important for short lived radionuclides. Despite of precautionary measures to keep the beam current constant, there exist some fluctuation in the beam current and the error arising due to the beam current fluctuation was found to be less than 6%. The incident energy of ²⁰Ne⁶⁺ ion beam on each target foil, present in the stack, was calculated from the energy degradation of the initial beam energy using stopping power software SRIM [33] . The incident energy on each target foil has been taken as the energy at half of the target thickness. Beam flux was measured by two methods. Firstly, by measuring the time-weighted beam current, and secondly, by measuring the total charge collected in Faraday cup. The beam flux calculated using these two methods were found to agree well with each other.

After irradiation, the γ-ray activities produced in each target foil of the stack was recorded using 60 cm³ HPGe detector coupled to a PC based data acquisition system at VECC, Kolkata. The resolution of HPGe detector was found to be 1.9 keV for 1.33 MeV γ-ray from ⁶⁰Co source. The geometry dependent photo-peak detection efficiency of the HPGe detector at various source-detector separations were measured using the ¹⁵²Eu source of known strength. The γ-ray spectroscopy software package RADWARE [34] has been used for analyzing the γ-ray spectrum. A typical γ-ray energy spectrum obtained from the irradiation of ⁵¹V target by ²⁰Ne⁶⁺ ion beam at is shown in Figure 1. Various peaks in the observed γ-ray spectra were assigned to different ERs on the basis of their characteristic γ-radiation as well as by their measured half-lives. The measured half-lives of the observed ERs were found to be in good agreement with their literature values. The nuclear spectroscopic data used in the evaluation and measurement of the cross sections were taken from the Radioactive Isotopes Data Table of Brown and Firestone [35] and are listed in Table 1 for ready reference.

The reaction cross section of identified ERs populated via different possible channels were estimated using standard formulation [8] , given by

where A is the total number of counts recorded under the peak in time t₃, λ is the decay constant of the product nucleus, N₀ is the total number of nuclei present in the target foil, μ is the branching ratio of identified γ-ray, A is the incident beam flux, ε_G is the geometry-dependent efficiency of the HPGe detector, t₁ is the irradiation time of the stack, t₂ is the time elapsed between the termination of the irradiation and start of the counting, t₃ is the

counting time and is the self-absorption correction factor for the target material of thickness

“d” with the absorption coefficient “μ”. The correction factor for the decay of the induced activity due to the delay time t₂ between the stop of irradiation and the start of the counting is taken as and the correction factor due to decay of the irradiated sample during the data accumulation time t₃ is taken as

. During the irradiation, a factor takes care of the decay of ERs and is known

as the saturation correction factor.

Various factors are likely to introduce error and uncertainty in the measured reaction cross sections. Some of the potential sources of error in the present work are as follows: 1) Fluctuation in the beam current lead to variation in the flux. Proper care has been taken to keep the beam current constant. However, error arising due to this factor was found to be less than 6%. 2) Non-uniformity of the target thickness leads to an uncertainty in determining the number of target nuclei. In order to check the uniformity of the target, the thickness of each target foil has been measured at different position by α-transmission method as well as by weighing the individual foils using a microbalance. Error due to the uncertainty in target thickness was found to be less than 3%. 3) Errors arising due to the geometry-dependent detector efficiency, caused by the statistical uncertainty in the counts under the peak, were estimated to be less than 5%. 4) Error contributed due to the dead time of the spectrometer was kept below 8% by suitably adjusting the sample-detector separation. Efforts were made to minimize the uncertainty and the overall error estimated in the present work does not exceed 22%.

Complete fusion of ²⁰Ne projectile with ⁵¹V target lead to the formation of excited compound nucleus ⁷¹As^*. The dominant decay mode of ⁷¹As^* through the emission of neutron and proton lead to the formation of isotope ^66,67Ge via pxn (x = 3, 4) channel. The remaining ERs have too short/long half-lives and difficult to extract from the measured gamma lines. The experimentally measured fusion cross sections of the pxn channel residues were given in Table 3 and plotted in Figure 2(a) and Figure 2(b). As can be seen from Figure 2(a) and Figure 2(b), the experimental EFs of the residues populated through pxn channel were in good agreement with their theoretical values calculated using the code PACE4 for level density parameter K = 8. The values of K = 10, 12 also reproduces the experimental EFs of pxn channel residues as is evident from Figure 2 but K = 8 gives a slightly better fit. A good agreement between the theoretical and experimental EFs confirms the production of these

residues through the de-excitation of fully equilibrated compound nucleus formed through the CF reaction. The pxn (x = 3, 4) channel ERs observed in the present work have very little contribution to CF cross sections. The remaining CF residues could not be detected due to their too short/long half-lives. The unaccounted CF cross section due to the formation of stable/unstable ERs in the present reaction, were accounted using the statistical model code PACE4 as it was done earlier in Ref. [41] [42] . By using the code PACE4, ratio

(x = 3, 4) is calculated and using this ratio experimental fusion cross section is calculated as.

The values of experimental fusion cross sections and the ratio R thus obtained are listed in Table 3 along with the experimentally measured reaction cross sections for the p3n and p4n channels. As can be seen from Table 3, the value of R decreases with the increase in energy indicating a fall in contribution arising from pxn channel residues with the increase in beam energy.

In order to understand the formation mechanism of low Z-residues, experimentally measured EFs of α-channel residues along with their PACE4 values are plotted and as a representative case the EFs of ⁶³Zn(αp3n) and ⁶⁰Cu(2α3n) are shown in Figure 3(a) and Figure 3(b). α-channel residues were expected to be populated via CF and/or ICF processes. As can be seen from Figure 3, there is a significant enhancement in the experimentally measured reaction cross sections over the theoretically calculated values. This enhancement in experimental reaction cross section over the predicted PACE4 values is attributed to the contribution of ICF in the formation of α-channels residues. It is also observed that degree of ICF contributions, in the formation of ERs populated through α-emitting channels, are varying from residue to residue. For example, the trend of EF measured for ⁶⁶Ga(αn) residue reflects the interplay between the contributions of CF and ICF processes through two different decay channels:

1) CF: The excited ⁷¹As^* nucleus formed through CF reaction may decay via the emission of an α-particle and a neutron (αn channel) as

Here E_thr. is the threshold energy for the given reaction.

2) ICF: Only a part of the incident projectile (i.e., ¹⁶O) fuses with the ⁵¹V target to form an incompletely fused composite system ⁶⁷Ga^*, while the α-particle moves in the forward direction as a spectator. The excited ⁶⁷Ga^* nucleus further decay via the emission of a neutron (n) as

(α-particle as a spectator),

Furthermore, in order to have a better representation, the sum of experimentally measured reaction cross sections of all identified α-emitting channel residues (i.e., the sum of experimentally measured reaction cross sections of ⁶⁶Ga, ^60,61Cu, ^61,55Co, ⁵⁶Mn and ^62,63Zn residues), have been compared with those estimated theoretically using the statistical model code PACE4 in Figure 4(a). As can be seen from this figure, the sum of experimentally measured reaction cross sections of α-emitting channel residues are significantly higher than corresponding PACE4 predictions for the level density parameter that has been used to reproduce CF residues in the present work. Since the statistical model code PACE4 does not take ICF reactions into account, therefore, the observed enhancement in the experimentally measured EFs over the theoretically predicted values point towards the contribution of ICF in the production of ERs populated through α-emitting channels.

Experimentally measured reaction cross sections of the individual ERs populated through α-emitting channel in ²⁰Ne + ⁵¹V reaction are listed in column 2 - 9 of Table 4. In Table 4, column 10 gives the sum of ICF cross sections. Column 11 presents the total fusion cross sections given by the algebraic sum of experimental fusion cross section and ICF cross section i.e.,. The

last column of Table 4 gives the percentage fraction of ICF (F_ICF) calculated as. As mentioned above, ICF cross section is given by the difference in experimental and PACE4 value of the α-emitting channel residues, so the error in value is arising mainly from the values. In an attempt to extract more information about the degree of ICF contribution in the formation of ERs populated through α-emitting channels, systematically deduced ICF cross sections are plotted, along with the experimental fusion cross sections and total fusion cross sections, as a function of incident projectile energy in Figure 4(b). Since the values of are too low as compare to and, so the data points in Figure 4(b) are shown by multiplying the original values by a factor of 3. As shown in Figure 4(b), the increasing separation between and with incident projectile energy indicates the energy dependence of ICF.

Moreover, in order to have a better insight into the onset and influence of ICF, F_ICF has been deduced from the analysis of the given data. To have an idea about the projectile structure effect on the underlying reaction dynamics, the values of F_ICF (%) are plotted in Figure 5(a) as a function of reduced projectile energy (E_c.m/V_b) for the present system along with those of ¹⁶O + ⁵¹V and ¹²C + ⁵¹V systems from Ref. [29] [30] . The ICF strength function F_ICF, is taken as an empirical definition of the probability of ICF at different projectile energies. As

shown in Figure 5(a), the value of F_ICF was found to be 2.97% at 1.84V_b and increases smoothly up to 22.13% at 3.22V_b for ²⁰Ne + ⁵¹V system whereas for ¹⁶O + ⁵¹V system it rises from 3.15% at 1.99V_b to 11.13% at 2.65V_b and in the case of ¹²C + ⁵¹V system it was 3.65% at 2.23V_b and reaches a maximum value of 11.88% at 3.33V_b. The observed increasing trend of F_ICF (%) with E_c.m/V_b suggests that break-up probability of incident projectile increases under the influence of increasing input angular momenta. The incident projectile, under the influence of target’s field, may break-up into energetically favored α-fragments to pave the path for the ICF reactions. ²⁰Ne projectile is likely to break-up into α + ¹⁶O whereas ¹⁶O projectile is expected to break-up into α + ¹²C and ¹²C projectile into α + ⁸Be. Moreover, the effect of E_B.U values of these α-cluster projectiles on their break-up probability was quite evident from the observed trend of F_ICF (%). As can be seen from Figure 5(a), the values of F_ICF (%) for the ²⁰Ne + ⁵¹V reaction was maximum, followed by ¹⁶O + ⁵¹V reaction, and minimum in the case of ¹²C + ⁵¹V reaction throughout the energy range. This particular trend of F_ICF (%) is associated with the magnitude of E_B.U value of the incident projectile. Lower E_B.U value of a projectile makes it more susceptible to the break-up process and hence leading to a higher value of F_ICF.

The α-cluster projectiles with lower E_B.U value, as in the present case, may break-up before undergoing fusion with the target and in turn give rise to ICF. Similar results were also observed for strongly bound α-cluster projectiles, having relatively higher E_B.U values, e.g., ¹⁶O and ¹²C, but at slightly higher incident energies [43] [44] . In order to elaborate the observed trend of F_ICF (%) with E_c.m/V_b (Figure 5(a)) in a better way, the three most energetically favorable break-up channels for the ²⁰Ne, ¹⁶O and ¹²C induced reactions on ⁵¹V target have been calculated and listed in Table 5. The other break-up channels, not shown in Table 5, require energy ≥ 20 MeV. For these three projectiles, break-up into α-fragments are energetically most favorable as evident from Table 5. Experiments with ⁶Li and ⁷Li projectiles incident on ²⁰⁸Pb [45] [46] target also supports the break-up of incident projectiles into energetically favored partitions of α + ²H and α + ³H, respectively. Similar results were also reported from the study of reactions using ¹²C on ²⁰⁸Pb, ¹³C on ²⁰⁷Pb [47] , ^12,13C on ¹⁵⁹Tb [6] [48] and ^10,11B on ²⁰⁹Bi [5] . The F_ICF (%) for ²⁰Ne, ¹⁶O and ¹²C induced reactions on ⁵¹V target are shown in Figure 5(b) as a function of E_B.U value of the projectiles. As it can be inferred from this figure, F_ICF (%) decreases linearly with increase in E_B.U value of the incident projectiles, suggesting a major role played by E_B.U value of the projectiles on ICF. The break-up probability of incident projectile is expected to be influenced by its E_B.U value. The larger F_ICF (%) for ²⁰Ne as compared to ¹⁶O and ¹²C is related to lower E_B.U value of 4.73 MeV for ²⁰Ne as compared to 7.16 MeV and 7.36 MeV for ¹⁶O and ¹²C, respectively.

Moreover, a systematic variation of F_ICF with mass asymmetry was suggested by Morgenstern et al. [20] which has been observed experimentally in some recent reports [3] [4] with the inclusion

of projectile structure dependence. Morgenstern et al. suggested that for a fixed relative velocity (V_rel/c), given by

where m is the reduced mass of the projectile-target system and V_b is the fusion barrier, the fraction of incomplete fusion increases almost linearly with increasing mass asymmetry.