Fusion of 6 Li with 159 Tb} at near barrier energies
M. K. Pradhan, A. Mukherjee, P. Basu, A. Goswami, R. Kshetri, R. Palit, V. V. Parkar, M. Ray, Subinit Roy, P. Roy Chowdhury, M. Saha Sarkar, S. Santra
aa r X i v : . [ nu c l - e x ] J un Fusion of Li with
Tb at near barrier energies
M. K. Pradhan , A. Mukherjee , ∗ P. Basu , A. Goswami , R. Kshetri , R. Palit , V. V.Parkar , M. Ray , Subinit Roy , P. Roy Chowdhury , M. Saha Sarkar , and S. Santra Nuclear Physics Division, Saha Institute of Nuclear Physics,1/AF, Bidhan Nagar, Kolkata-700064, India Department of Nuclear & Atomic Physics,Tata Institute of Fundamental Research, Mumbai-400005, India Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India and Department of Physics, Behala College, Parnasree, Kolkata-700060, India
Abstract
Complete and incomplete fusion cross sections for Li+
Tb have been measured at energiesaround the Coulomb barrier by the γ -ray method. The measurements show that the complete fusioncross sections at above-barrier energies are suppressed by ∼
34% compared to the coupled channelscalculations. A comparison of the complete fusion cross sections at above-barrier energies with theexisting data of , B+ Tb and Li+
Tb shows that the extent of suppression is correlatedwith the α -separation energies of the projectiles. It has been argued that the Dy isotopes producedin the reaction Li+
Tb, at below-barrier energies are primarily due to the d -transfer to unboundstates of Tb, while both transfer and incomplete fusion processes contribute at above-barrierenergies.
PACS numbers: 24.10.Eq, 25.70.Jj, 25.60.Pj, 25.70.Mn, 27.70.+q ∗ Electronic address: [email protected] . INTRODUCTION Near barrier fusion is governed by the structure of the interacting nuclei and the couplingto the direct nuclear processes, such as inelastic excitation and nucleon transfer [1, 2]. Fornuclear systems with tightly bound nuclei, the coupling of the relative motion to theseinternal degrees of freedom successfully explains the enhancement of fusion cross sectionswith respect to the 1-D Barrier Penetration Model (BPM) calculations at sub-barrier energies[2]. However, the situation gets more complicated in reactions involving weakly boundnuclei, since they may break up prior to fusion. The interest in understanding the influenceof breakup on fusion and other reaction processes has indeed received a fillip in the recentyears, especially because of the recent advent of the radioactive ion beam facilities in differentlaboratories around the world.Owing to the low intensities of the radioactive ion beams currently available experimentalinvestigation of reaction mechanisms with unstable beams is still difficult, though measure-ments are being increasingly reported [3–11]. On the contrary, precise fusion cross sectionsmeasurements can be carried out with the readily available high intensity beams of weaklybound stable nuclei, , Li and Be, which have significant breakup probabilities. Such stud-ies with weakly bound stable projectiles may serve to be an important step towards theunderstanding of the influence of breakup on fusion process.During the past few years, the effect of breakup of weakly bound nuclei on the fusionprocess has been extensively investigated. In fusion measurements of weakly bound stableprojectiles with heavy targets [11–22], events corresponding to the complete fusion (CF)of the projectile with the target could be separated experimentally from those resultingdue to the incomplete fusion (ICF) process (where part of the projectile is captured bythe target). The works show that the CF cross sections are substantially suppressed atabove barrier energies, compared to the predictions of the 1-D BPM calculations. This hasbeen attributed to the breakup of the weakly bound projectiles, prior to reaching the fusionbarrier.By contrast, fusion measurements for medium and light mass systems [23–31], where CFand ICF products could not be experimentally distinguished, only total fusion (CF+ICF)cross sections were measured. Such measurements show no significant effect of breakup ontotal fusion at above barrier energies. 2ystematic fusion excitation functions measurement carried out by the characteristic γ -ray method, for the systems , B+ Tb and Li+
Tb [19], shows that the CF crosssections at above barrier energies are suppressed for the systems B+ Tb and Li+
Tbby ∼
14% and ∼
26% respectively, with respect to the coupled channels (CC) calculations.Also, the CF suppression was found to be correlated with the α -breakup threshold of theprojectiles. In the context of these results, it appears worthwhile to measure the CF crosssections for the system Li+
Tb, in view of the fact that Li has the lowest α -breakupthreshold (1.45 MeV) amongst the stable projectiles , Li, Be and , B. The present workdeals with the measurement of CF and ICF cross sections for Li+
Tb at energies aroundthe Coulomb barrier, using the γ -ray method. To check the consistency of the present resultswith those of Ref. [19], the reaction Li+
Tb was repeated at a few energies in the presentwork. Some preliminary results of the measurement have been reported in a conferenceproceedings [32].
II. EXPERIMENTAL DETAILS
The experiment was performed using the 14UD BARC-TIFR Pelletron accelerator atMumbai. Beams of Li in the energy range 23-39 MeV and Li at energies of 28, 34 and 37MeV bombarded a self-supporting
Tb foil of thickness 1.59 ± . To monitor thebeam and also for normalization purposes, two Si-surface barrier detectors were placed at ± ◦ about the beam axis inside a spherical reaction chamber of 22 cm diameter. The totalcharge of each exposure was measured in a 1 m long Faraday cup placed after the target. The γ -rays emitted by the reaction products were detected in an absolute efficiency calibratedCompton suppressed clover detector, placed at +125 ◦ with respect to the beam direction. AnHPGe detector having Be window was placed at − ◦ with respect to the beam direction,mainly to detect the low energy gamma rays. Both online and offline γ -spectra were takenduring the runs, using the Linux based data acquisition software LAMPS [33]. The absoluteefficiencies of the γ -ray detectors were determined using the standard calibrated radioactivesources ( Eu,
Ba,
Bi, Co,
Cs) placed at the same geometry as the target. Thetarget thickness was determined using the 137.5 keV (7/2 + → + (g.s.)) Coulomb excitationline of Tb. The same target was used for all the beam exposures. So to minimize theaccumulation of radioactivity in the target, the target irradiations were carried out from the3
Er182.1 keV;
Er162.2 keV;
Tb 198.6 keV;
Er190.3 keV;
Er177.6 keV;
Dy152.4 keV;
Dy143.8 keV;
Er136.5 keV;
Dy132.0 keV;
Dy125.8 keV;
Er 121.0 keV;
Dy106.1 keV;
Er102.0 keV;
Er99.5 keV;
Dy86.8 keV;
Dy84.4 keV;
Er79.5 keV;
Tb69.3 keV;
Er63.68 keV; Tb Counts C hanne l L i + T b ( a ) Er452.8 keV;
Er463.6 keV;
Er 511 keV; Annihilation506.1 keV;
Er281.2 keV;
Er425.1 keV;
Er382.8 keV;
Er360.2 keV;
Er430.1 keV;
Er406.1 keV;
Dy385.7 keV;
Dy 340.0 keV;
Er320.5 keV;
Dy337.5 keV;
Er317.5 keV; Er Er264.1 keV; Er Er218.2 keV;
Dy252.7 keV;
Ho224.7 keV;
Dy 211.1 keV;
Ho297.2 keV; Dy Counts C hanne l L i + T b ( b ) F I G . : ( C o l o r o n li n e ) T y p i c a l γ - r a y s p ec t r u m o b t a i n e d w i t h a c l o v e r d e t ec t o r p l a ce d a t ◦ , f o r t h e r e a c t i o n L i + T b , a t a b o m b a r d i n g e n e r g y o f M e V . l o w e s t b e a m e n e r g y o n w a r d s . A t y p i c a l γ - r a y a ddb a c k s p ec tr u m f r o m t h ec l o v e r d e t ec t o r , a tt h e b o m b a r d i n g e n e r g y o f M e V ,i ss h o w n i n F i g s . ( a - b ) . T h e nu c l e i p r o du ce d i n t h e r e a c t i o n w e r e i d e n t i fi e db y t h e i r c h a r a c t e r i s t i c γ - r a y e n e r g i e s a nd a r e l a b e ll e d i n t h e fi g u r e . III . D E T E R M I N A T I O N O F C O M P L E T E F U S I O N Y I E L D S T h ec o m p o undnu c l e i E r a nd E r f o r m e d f o ll o w i n g t h e C F o f T b w i t h L i a nd L i, r e s p ec t i v e l y , d ec a y p r e d o m i n a n t l y b y n e u tr o n e v a p o r a t i o n . T h i s i s a l s o p r e d i c t e db y he statistical model calculations done using the code PACE [34]. In the measured energyrange the evaporation of two to five neutrons occurs, resulting in the formation of − Erand − Er evaporation residues (ERs) for the reactions Li+
Tb and Li+
Tb, re-spectively.In determining the ER cross sections, the online spectra were mostly used. But as andwhen required, the offline-spectra were also used. It needs to be mentioned here that in sit-uations where the ERs are stable, only the in-beam γ -ray spectroscopy method can be used.However, in cases where the unstable ERs undergo further radioactive decay to populatethe excited states of their daughter nuclei, which in turn decay to their ground states byemitting γ -rays, one can also use the off-beam γ -ray method, if the situation is favourable.In the present work, this could be done only for the 4 n channel residual nucleus, Er witha half life ( T / ) of 3.21 hours, produced in the reaction Li+
Tb. The off-beam γ -raymethod could not be used for the ER Er ( T / = 75 mins.), as 99.9% of Er undergoesEC decay to ground state of
Ho. Also, as the same target was used for all the irradiations,the off-beam method could not be used for the ER
Er, having T / = 28.58 hours whichis substantially large compared to the data accumulation times (typically ∼ γ -ray cross sections ( σ γ ) were obtained from the relation σ γ = N γ ( ǫ γ N B N T ) (1)where N γ is the number of counts under the γ -ray peak, ǫ γ is the absolute full energy peakdetection efficiency of the detector for the specific γ -ray, N B is the total number of beamparticles incident on the target and N T is the number of target nuclei per cm . The quantityN B was determined by dividing the charge Q collected in the Faraday cup by the equilibriumcharge value ¯ Z e, obtained from Ref.[35]. The total systematic uncertainty in the γ -ray crosssections, arising because of the uncertainties in N B , N T and ǫ γ , is ∼ γ to get the total error in σ γ .For the even-even ERs ( , , Er), the cross sections were extracted from the extrap-5
2n 3n 4n 5n s x n / Ss x n E c.m. (MeV) Li+ Tb FIG. 2: (Color online) Ratio of individual channel cross sections to the total channel cross sectionsas a function of the centre-of-mass energy for the reaction Li+
Tb. The errors are statisticalonly. The dashed lines are drawn to guide the eye.
15 20 25 30 35 40 4510 no coupling CC CC x s f u s ( m b ) E c.m. (MeV) Li+ Tb FIG. 3: (Color online) Complete fusion cross sections as a function of the centre-of-mass energy forthe reaction Li+
Tb. The error bars indicate the total errors. The dotted and dashed lines showthe uncoupled and coupled channel calculations, respectively, performed with the code CCFULL.The solid line is the coupled channels calculation multiplied by the factor of 0.66. γ -ray intensities (aftercorrecting for the internal conversion) for various transitions in the ground state rotationalband [19]. For the odd-mass ERs ( , Er) the cross sections were obtained by adding thecross sections of the γ -rays corresponding to the transitions from the excited states to theground states of the nuclei, as done by Broda et al. [36]. In such cases, however, direct pop-ulation of the ground states of the nuclei could not be considered. Nevertheless, the directfeedings to the ground states are expected to be substantially small in this mass and energyregion, except at very low bombarding energies. In fact, in the present work this has beenchecked for the ER Er, produced in the reaction Li+
Tb, as both in-beam and off-beam γ -ray method could be applied to measure its production cross sections at low bombardingenergies. It was observed that the cross sections, obtained from the in-beam γ -rays of Er(where direct population of the ground state is not included) and those from the off-beam γ -rays of Ho, following EC decay of
Er, (which obviously includes direct ground statepopulation of
Er) are practically same. This shows the ground state contribution to berather small and can safely be ignored in the evaluation of the CF cross sections. The CFcross sections for both reactions were obtained from the sum of the 2 n − n ER cross sections.Figure 2 shows the individual x n channel cross sections normalized to the CF cross sec-tions (fractional channel cross sections) for the reaction Li+
Tb. The measured CF crosssections, along with the total errors, for the reaction Li+
Tb are plotted in Fig. 3. The CFcross sections for Li+
Tb, measured at a few bombarding energies in the same setup, areseen to agree well with the earlier measurements [19, 36], thus enabling a reliable comparisonof the present results with the earlier ones.
IV. COUPLED CHANNELS CALCULATIONS
To interpret the measured fusion excitation function in a theoretical framework, the re-alistic coupled channels (CC) code CCFULL [37] was used to calculate the fusion crosssections for Li+
Tb. The initial input potential parameters ( V , r , and a ) were obtainedfrom the Woods-Saxon parametrization of the Aky¨ u z-Winther (AW) potential [38], and areshown in Table I. The table also shows the corresponding uncoupled fusion barrier parame-ters ( V b , R b , and ~ ω ). As CCFULL cannot handle shallow potential, a deeper potential wasused. This modified potential was derived keeping the diffuseness parameter fixed at a =0.857m, following the systematic trend of high diffuseness required to fit the high energy part ofthe fusion excitation functions [39]. To obtain the appropriate potential, the parameters V and r were varied accordingly so that the corresponding 1-D BPM cross sections agree withthose obtained using the AW potential parameters at higher energies [19]. The modifiedpotential used for the CC calculations, and the corresponding uncoupled barrier parametersare given in Table I. Using the modified potential parameters, the 1-D BPM calculationswere done using the code CCFULL, in the no coupling limit and the results are shown bythe dotted line in Fig. 3. The CF cross sections at below-barrier energies are seen to beenhanced and the cross sections at above-barrier energies are found to be reduced comparedto the 1-D BPM calculations. The enhancement at below-barrier energies may be becauseof the fact that the target Tb is a well deformed nucleus.The effect of target deformation on the fusion cross sections was calculated by includingcoupling to the ground state rotational band of the target nucleus. As described in Ref.[19], for the odd-A nucleus
Tb, the excitation energies and deformation parameters weretaken to be the averages of those of the neighbouring even-even nuclei
Gd and
Dy. Theenergy states, in the ground state rotational band of the corresponding average spectrum( β =0.344 [40] and β =+0.062 [41]), upto 12 + were included in the calculations. Projectileexcitation was not included in the calculations. It needs to be mentioned here that Li has aground state with non-zero spin (1 + ) and spectroscopic quadrupole moment of − ,and has an unbound first excited state (3 + ) at 2.186 MeV. But coupling to the unbound firstexcited state of Li with such ground state properties, along with the rotational coupling tothe target excited states could not be included in the CCFULL calculations.The dashed line in Fig. 3 shows the CC calculations that include rotational coupling tothe inelastic states of the target. The calculations, though reproduce the low energy part ofthe data reasonably well, overestimate the high energy part of the data. The little differencethat can be seen at the lowest energy could be due to the projectile effect, which could notbe considered in the calculations, as mentioned. At above barrier energies, where couplingis not expected to play any significant role, the CF cross sections are found to be suppressedcompared to the CC calculations.As CC model cannot yet separate CF and ICF, the measured CF cross sections can onlybe compared with the calculated total fusion cross sections. So in order to have an estimate ofthe extent of CF suppression compared to the total fusion cross sections, the CC calculations8
ABLE I: The parameters for AW and modified CC potentials, along with the correspondingderived uncoupled barrier parameters V b , R b , and ~ ω .System Potential V r a V b R b ~ ω (MeV) ( f m ) ( f m ) (MeV) ( f m ) (MeV) Li+
Tb AW 46.40 1.18 0.62 24.89 10.60 4.85CC 128.0 0.98 0.85 24.48 10.53 4.15 Li+
Tb AW 46.43 1.18 0.62 24.70 10.69 4.48 B+ Tb AW 54.54 1.18 0.64 40.71 10.79 4.68 B+ Tb AW 54.54 1.18 0.64 40.34 10.89 4.42 for Li+
Tb were scaled so as to reproduce the high energy part of the measured CFexcitation function. Agreement could be achieved only if the calculated fusion cross sectionsare scaled by a factor of 0.66, and the resulting scaled calculations are shown in Fig. 3 bythe solid line. The CF suppression factor (F CF ) for the system is thus 0.66 ± ±
5% has been estimated, resulting from the overall errors in the measuredfusion cross sections. The CF suppression of 34 ±
5% thereby obtained at above barrierenergies for Li+
Tb agrees with the value reported for the heavier systems Li+
Bi [14]and Li+
Pb[16] and is also in close agreement with the suppression of 32 ±
5% reportedfor Li+
Sm [22].
V. COMPARISON OF SUPPRESSION WITH OTHER SYSTEMS
The F CF for Li induced reactions on different targets are compared in Fig. 4(a), usingthe present data and those reported in the literature [14, 16, 22]. The dotted line has beendrawn in the figure only to guide the eye. It appears that the F CF for Li induced reactionsare almost independent of the atomic number ( Z T ) of the target nucleus, in the heavy massregion. However, more values of F CF for Li induced reactions, especially with targets oflower Z T are required before drawing any definite conclusion. Figure 4(b) compares F CF for the reactions Li+
Tb, Li+
Tb [19] and B+ Tb [19] as a function of the α -separation energies (S.E. α ) of the projectiles. Like Li+
Tb, a ±
5% uncertainty has also9 Li+ Sm Li+
Tb [present work] Li+ Pb Li+ Bi F C F ( % ) Z T Li+
Tb [present work] Li+ Tb B+ Tb F C F ( % ) S.E. a (MeV) FIG. 4: (Color online) (a) CF suppression (%) as a function of atomic no. Z T of target for the Li-induced reactions involving different targets. The reactions considered are Li incident on
Sm[22],
Tb (present work),
Pb [16], and
Bi [14]. The dotted line is drawn to guide the eye.(b) CF suppression(%) as a function of α -separation energies (S.E. α ) of the projectiles in reactionswith the target Tb. The reactions considered are B+ Tb [19], Li+
Tb [19] and Li+
Tb(present work). been estimated for F CF of Li+
Tb and B+ Tb reactions. The plot shows that thereis a correlation between F CF and S.E. α . But more such measurements, including reactionswith unstable projectiles, are needed to understand the nature of the correlation.Figure 5 compares the reduced fusion cross sections σ fus / R b as a function of E c.m. / V b for different projectiles in logarithmic scale (a) and linear scale (b). The parameters V b and R b used for the reduction are those deduced from the AW potentials, and are listed inTable I. The CF cross sections for , B+ Tb and Li+
Tb were obtained from Refs.[19,36]. It can be seen from Fig. 5(a), that at the lowest energies the CF cross sections of , Li+
Tb are enhanced compared to those of , B+ Tb reactions. This enhancement,10 -2 -1 (a) B+ Tb : Mukherjee et al. (2006) B+ Tb : Mukherjee et al . (2006) Li+
Tb : Broda et al . (1975) Li+
Tb : Mukherjee et al . (2006) Li+
Tb [present work] Li+
Tb [present work] s f u s / R E c.m. / V b (b) FIG. 5: (Color online) A comparison of the reduced complete fusion excitation functions for thesystems , B+ Tb [19] and Li+
Tb [19, 36] with those of the present measurements for , Li+
Tb. The errors are statistical only. which could be due to the effect of the projectiles , Li, was also observed while comparing themeasurements with CCFULL calculations [Fig. 3 and Ref.[19]]. For the reaction Li+
Tb,this has already been discussed in Sec. IV. For the reaction Li+
Tb, the deformation of Lineeds to be considered in the calculations [19], but both projectile and target deformationscan not be included simultaneously in the CCFULL calculations.Figure 5(b) shows that as one moves from the projectile B to Li, i.e. as the projectile α -breakup threshold decreases, the CF cross sections are observed to be more and moresuppressed. A comparison with the CCFULL calculations has shown that the measuredCF cross sections for B+ Tb, Li+
Tb [19] and Li+
Tb are suppressed by ∼ ∼
26% and ∼
34% respectively. This certainly shows that the CF suprression is correlatedwith the α -breakup threshold of the projectile. Lower the α -breakup threshold, larger isthe CF suppression. Thus the CF suppression can be attributed to the loss of flux fromthe fusion channel due to the breakup of the loosely bound projectiles, and hence at least11 major part of this suppression should be the ICF cross sections of the reactions. Also,if one looks carefully into Fig. 5(b), it appears that higher the α -breakup threshold ofthe projectile, higher is the energy where the CF suppression starts. However, more suchsystematic measurements, especially with unstable beams, are required for confirming thisobservation. VI. INCOMPLETE FUSION
In order to have a complete picture of the fusion process in the reaction Li+
Tb, besidesCF cross sections, it is also important to measure the ICF cross sections. As discussed inthe previous section, a major part of the observed reduction in CF is expected to be due tothe ICF process.In the γ -ray spectra, besides the γ -ray lines of the Er nuclei resulting from CF, the γ -ray lines corresponding to Dy and Ho isotopes produced via the ICF processes were alsoobserved. In the reaction Li+
Tb, the Dy nuclei are produced by the capture of the lighterprojectile fragment, d , following Li breakup, by the target
Tb and subsequent emissionof neutrons. Similarly, the Ho nuclei are formed by the capture of the heavier projectilefragment, α , by Tb, followed by neutron emission. The ICF cross sections are shown inFig. 6. The cross sections of the ICF products were determined in a similar way as that forthe CF residues. The α n , α n and α n channels, following the capture of d by Tb, are seento be the dominant ICF channels. On the other hand, only γ -lines corresponding to Honucleus resulting from the α + Tb ICF process, followed by 2 n emission, could be identifiedin the spectra. However, the ICF contribution of Ho, plotted in the figure, partly includesthe contribution of
Ho produced via the EC decay of
Er CF residue. Nevertheless, itis clear that the contribution of
Ho formed in the ICF process is relatively much lesscompared to Dy isotopes. A possible explanation of this could be given on the basis of Q-values of the reactions. It is to be noted that the Q-value for the reaction
Tb( Li, α ) Dyis +10 . − . Tb( Li, d ) Ho. This indicates thatthe former channel corresponding to ICF process, where the α -particle is emitted with the d being captured by the target is more favoured compared to the latter. Our measurementon the systems Li+
Tb and B+ Tb reported earlier [19] also showed similar result.It needs to be mentioned here that the ICF cross sections for Dy isotopes also include12 Dy ( a n) Dy ( a Dy ( a Ho (d2n) s I C F ( m b ) E c.m. (MeV) Li+ Tb FIG. 6: (Color online) The ICF/transfer cross sections measured for the reaction Li+
Tb. Thecross sections corresponding to the α n , α n and α n channels, following d -capture by the target,and the cross sections corresponding to the d n channel, following α -capture by the target areshown. contributions from transfer of d from projectile Li to the higher excited states of the targetsince in the present γ -ray measurement it was not possible to distinguish between the twoevents. Also, the single-proton stripping reaction Tb( Li, He)
Dy, with Q-value +2.836MeV, if occurs will lead to the same
Dy nucleus. Hence the contribution from
Dy nucleivia p -transfer, if any, is also included in the α n channel cross section.A careful insight into Fig. 6 shows appreciable cross sections for Dy nuclei, even atenergies below the barrier where CF shows no suppression (Fig. 3). This is perhaps becauseof the fact that at below-barrier energies, it is essentially the transfer of d to the unboundstates of Tb (one-step process), followed by the emission of neutrons, that produces the Dyisotopes. In a simplistic picture, this can be understood by considering the optimum Q-value(Q opt ) associated with a transfer reaction. The ground state Q-value (Q gg ) for the d -transferreaction Tb( Li, α ) Dy is +10.2 MeV, and Q opt for the transfer process, say at E c.m. =22 MeV and 25 MeV are calculated [42] to be − − ǫ ∗ ) in Dy to which the d -transfer is energetically favoured is given by13 gg − Q opt . Thus at E c.m. = 22 MeV and 25 MeV, ǫ ∗ = 17.3 MeV and 18.3 MeV, respectively,thereby showing that the d -transfer to Tb will energetically favour the production of
Dy nuclei in the unbound states. Unlike transfer, at below-barrier energies, the breakupfragments may not have sufficient energy to overcome the Coulomb barrier and get furthercaptured by the target (two-step process). By contrast, at above-barrier energies the breakupfragments will have sufficient energy to undergo further fusion with the target and hence atsuch energies the ICF (breakup-fusion) process, along with d -transfer, lead to the productionof Dy nuclei. It is mainly the ICF (breakup-fusion) yield (which could not be separated fromtransfer in the present measurement) that contributes to the reduction of CF at above barrierenergies. Similar argument also holds true for the Ho nuclei. Unfortunately, only one Hoisotope, namely Ho could be identified in the present work and that too had an admixturedue to the contribution from
Ho nuclei resulting from the EC decay of
Er residue. Sonothing conclusive could be said about Ho nuclei. Detailed exclusive measurements aimedat disentangling ICF and transfer yield, though difficult, are indeed necessary to see howmuch of the reduction in CF is accounted for by the ICF process.
VII. SUMMARY
The CF cross sections for the reaction Li+
Tb have been measured at energies aroundthe Coulomb barrier, using the γ -ray method. CC calculations using the code CCFULL weredone to calculate the total fusion cross sections. The calculated fusion cross sections hadto be scaled by a factor of 0.66 ± Li nucleus. The CF suppression of ∼
34% for Li+
Tb when compared tothe values of ∼
26% and ∼
14% for Li+
Tb and B+ Tb [19] respectively, convincinglyshows that the CF suppression is correlated with the α -separation energy of the projectile.Lower the α - breakup threshold of the projectile, larger is the CF suppression. At energiesbelow the barrier, enhancement of CF cross sections could be reasonably well reproducedby considering the deformation of the target.The nuclei produced via the ICF process in the reaction Li+
Tb were also identifiedand their cross sections have been determined. Similar to B+ Tb and Li+
Tb [19],the present measurement also shows that the α -emitting channel is the favoured ICF process14n reactions of projectiles, having low α -breakup thresholds, with Tb target.At below barrier energies, the Dy isotopes are primarily produced by the d -transfer tothe unbound states of Tb, while at above barrier energies both transfer and ICF processescontribute to their production.Further investigation of the light particles emitted in reactions involving loosely boundprojectiles, in conjunction with the results presented here, may lead to a better understand-ing of the mechanisms involved in such reactions.
Acknowledgments
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