PROBING THE IN-COMPLETE FUSION REACTION DYNAMICS USING LIGHT-HEAVY-IONS

Transcription

1 PROBING THE IN-COMPLETE FUSION REACTION DYNAMICS USING LIGHT-HEAVY-IONS ABSTRACT of the thesis submitted for the award of Doctor of Philosophy in Physics (Experimental Nuclear Physics) by ABHISHEK YADAV Under the supervision of PROF. B. P. SINGH DEPARTMENT OF P PHYSICS,, ALIGARH MUSLIM UNIVERSITY, ALIGARH, INDIA 2012

2 i Probing the in-complete fusion reaction dynamics using light-heavy-ions ABSTRACT of the thesis submitted for the award of DOCTOR OF PHILOSOPHY in PHYSICS (Experimental Nuclear Physics) by ABHISHEK YADAV Under the supervision of Prof. B.P.Singh Department of Physics Aligarh Muslim University Aligarh, (UP) INDIA 2012

3 ii The experimental work presented in this thesis has been carried out at the Inter-UniversityAcceleratorCenter(IUAC),NewDelhi-67,India,using 15UD-Pelletron accelerator facilities( An autonomous Research Facility of University Grants Commission (UGC), Govt. of India, New Delhi

4 iii Do not believe in a thing because you have read about it in a book...do not believe in a thing because another man has said it was true...do not believe in words because they are hallowedbytradition...findoutthetruthforyourself. Reason itout...thatisrealization... - Swami Vivekananda

5 1 ABSTRACT The journey to understand the fundamental nature of the matter, which has always been the quest for human being, starts in particular from the 4 th century. Atthattime Democritus believedthateachkindofmaterial could be sub-divided into the smallest indivisible element, invisible to the naked eye, called the atom and theory was referred to as atomism. The idea of atomism remained only a speculation, until investigators in the early 19 th century applied the methods of experimental science to this problem and obtained the evidence needed to raise the idea of atomism to the level of a full-fledged scientific theory. The journey got accelerated with thediscoveryof radioactivity in1896bybecquerel,whileinvestigating phosphorescence in uranium salts, which led to the realization that the radioactive elements spontaneously got transmuted into other elements[1]. The discovery was further verified with the identification of radio-activity in some materials by the Curies in J. J. Thomson [2], a year later, proposed a model of the atom known as plum pudding model in which it is perceived that the atom is like a large positively charged ball with negatively charged electrons embedded inside it. The model could account for the stability of atoms, but could not account for the discrete wavelengths observed in the spectra of light emitted from excited atoms. The existence of the nucleus as the tiny central part of an atom was first proposed by Rutherford in 1911 [3], as a result of the interpretation of famous 1909 α-scattering experiment, performed by Hans Geiger and Ernest Marsden in his guidance [4]. In order to explain the stability of atom and also to explain the emission spectra Niels Bohr, in 1913, gave the model[5], which is a quantum-physics-based modification of the Rutherford s model, known as Bohr s model. Later, as a result of many scattering experiments it wasestablishedthatmatterismadeupofatomswiththenucleusattheir center and electrons as revolving around in almost circular orbits. This The1903NobelPrizeinPhysicswasawardedjointlytoBecquerel,forhisdiscovery, and to Pierre& Marie Curie for their subsequent research into radioactivity. NielsHenrikDavidBohrwasaDanishphysicistwhomadefoundationalcontributions to the understanding atomic structure and quantum mechanics, for which he received thenobelprizeinphysicsin1922.

6 2 tiny central core, called nucleus, is made up of nucleons, the protons and neutrons. Almostallthemassofanatomislocatedinthenucleus,witha very small contribution from the orbiting electrons. From deuteron to uranium, there are almost 1700 nuclei that are found naturally on earth. In addition, a large number of others are created by transmutation in the laboratory and in the interior of stars. The first astounding work on artificial transmutation of the nucleus was performed by Rutherford in 1919, using energetic α-particles as the projectile. In this direction a big boost was given in 1932, with the development of charge particleaccelerator,byj.d.cockroft&e.t.s.walton[6],theassociates of Rutherford. A large fraction of our knowledge on the properties of nuclei is derived from the study of nuclear reactions. In general, a nuclear reaction takes place, when a particularly chosen nuclide(target nucleus) is bombarded by a projectile nucleus of sufficient kinetic energy, to overcome the fusion barrier (V fus ) between interacting partners. As a consequence of such an interaction, the projectile and target nuclei come close to each other within the range of nuclear forces, eventually transforming them into an excited composite system and then to the final stage, consisting of reaction products, like; light ejectile(s) and a residual nucleus followed by the emission of characteristic γ-radiations[7]. There are three broad categories of the study of nuclear reactions based on the energy regions of interest; (i) Low Energy Nuclear Reactions,(ii) Medium Energy Nuclear Reactions, and (iii) High Energy Nuclear Reactions. The present thesis deals with some of the interesting problems associated with low energy heavy-ion(hi) induced nuclear reactions, which has been a topic of resurgence interest for HI nuclear physics community since last decade or so. The heavy-ion(hi) collisions deal with the phenomena that occur when two nuclei(heavier than α-particle) are brought in contact with each other within the range of nuclear forces. In general, when two nuclei are brought in contact, a variety of phenomena can occur. By appropriately selecting the target, the projectile and the incident energy it is possible to excite different degrees of freedom. In HI-reactions the complete fusion(cf), in-complete fusion(icf) and pre-equilibrium(peq)-emission reactions are the dominant reaction processes. In the complete fusion (CF) reactions, for central and

7 near central interactions a composite system is formed after an intimate contact and transient amalgamation of interacting nuclei leading to the formation of an excited composite system with predetermined mass, charge and excitation energy. After the equilibration of this system a compound nucleus is formed, which may decay through the emission of particles and/or γ-radiations depending upon the available excitation energy. However, in case of ICF, for peripheral collisions(at relatively higher l-values) and/or at higher projectile energies the ICF starts competing with CF, where the centrifugalpotential(v cent )becomesrelativelyhigher. Asaresult,thenuclear potential is no more strong enough to capture the entire projectile to form thecompositesystem. Assuch,forl l crit nofusioncanhappenunlessa partoftheprojectileisemittedasaspectator(p s )toprovidesustainable angular momentum [8, 9]. After such an emission, the remnant (participant: P p ) is supposed to have angular momenta less than or equal to its own critical limit for fusion (l eff l Pp +T crit ). After α-particle emission the potentialcurveisobtainedforalowerl Pp +T crit -value, where the pocket exists. Hence,anexcitedIFCsystem(P p +T)isformedwithlessmass/chargeand excitation energy as compared to that formed in CF reactions. This excited composite system then decays via particle and/or γ-emission. Some of the prominent features of ICF reactions, which have emerged from a qualitative inspection of experimental results are summarized below; The fused composite system is formed with less mass and charge as compared to the total mass and charge of interacting partners. The forward recoil velocity of the reaction products formed via ICF hasbeenfoundtobelessthanthosepopulatedviacf. The angular distribution of outgoing projectile-like fragments is found to be peaking at forward angles, where the α-particle(s) are emitted with a velocity centered nearly equal to the projectile velocity. TheICFprocessesmainlyoccurforthel-valuesabovethel crit forcf. The spin-distribution and side-feeding intensity patterns of residues formed via ICF are found to have distinctly different trends than those formed via CF[10]. 3

8 4 Recently[11, 12, 13, 14], much interest has aroused to study the influence of in-complete fusion (ICF) on the complete fusion (CF) as well as on the total fusion, in HI-interactions in the energy regime 4-7 MeV/A. In order to explain the ICF reactions several approaches viz; SUMRULE model[15], Break-Up Fusion(BUF) model[16], Promptly Emitted Particles (PEP s) model[17], Exciton model[18], HOT SPOT model[19], etc., have been proposed. As a matter of fact, the existing models qualitatively explain the experimental data particularly at E/A 10.5 MeV, however, none of these models is able to provide satisfactory reproduction of the ICF data at lower incident energies 4-7 MeV/A, which triggered a resurgent interest to study the underlying reaction dynamics. In addition to this, the dependence of ICF on the projectile structure, energy, driving angularmomentum(l),bindingenergyand/oralphaq-value(q α ),massasymmetry[µ A =A T /(A T +A P )],deformationofinteractingpartnersetc.,is also required to be explored to develop some kind of systematics. Another important aspect of the HI-interactions is that of PEQ-emission in such reactions. Several experimental and theoretical studies indicate the existence of PEQ-emission at moderate excitation energies[20, 21, 22]. In order to have insight into PEQ-emission, a variety of dynamical models viz; Inter-Nuclear Cascade(INC) model[23, 24], the quasi-free scattering model (QFS)[25], HYBRID model[26], EXCITON model[27], etc., have been proposed to explain the experimental data related to PEQ-emission. Generally speaking, these models have been used to describe the experimental data obtained for light ion (LI)-induced reactions [28, 29]. The experimental data on PEQ-emission using heavy ion(hi)-beams is still limited to a few projectile-target combinations only [30, 31]. It may be pointed out that the distinction of PEQ and EQ-emission is relatively difficult in case of HI-induced reactions than in the LI-induced reactions due to rather large angularmomentacarriedinbythehi-beams,andalsoduetothepresence of in-complete fusion [11, 12] processes. Apart from the above, the data on reactions involving PEQ-emission for various projectile-target combinations may provide a data base for the recently proposed Accelerator Driven Sub-critical-reactor System (ADSS) [32]. Nonetheless, a rich data set on Abhishek Yadavetal.,Phys. Rev. C 78,044606(2008)

9 different modes of reactions may be useful not only to determine the optimum irradiation conditions to produce medically important radio-nuclides butalsoforthedevelopmentofnuclearreactionmodels. Thishasleadtoa renewed interest to the study of nuclear reactions. In the present work, in order to explore some of the important issues related to the HI-reaction dynamics at energies near and above the barrier, several experiments have been performed at the Inter University Accelerator Center (IUAC), New Delhi. In the present thesis the complete fusion, incomplete fusion and pre-equilibrium emission reactions have been studied with the help of following measurements; Excitation functions (EFs): as the preliminary indication of ICF reaction dynamics. Here, the relative contributions of CF and ICF processeshavealsobeendeducedfor 12 C+ 159 Tband 13 C+ 159 Tbsystemsintheenergyrange 4-7MeV/A,andarefoundtobesensitive to the various entrance channel parameters. Forward recoil range distributions(frrds): as a direct proof of fractional linear momentum transfer in the case of CF and ICF processes. Here, a significant fusion incompleteness, associated with fractional degreeoflinearmomentumtransfer(lmt)hasbeenobservedin 12 C+ 159 Tbsystem. TheICFprobabilityhasbeenfoundtodependstrongly on the beam energy. Spin-distribution measurements of residues: to probe the entry state populations in CF and ICF reactions in 12 C+ 169 Tm interactions at energies 5.6and6.5MeV/A,andcomparedwiththespindistributionsofresiduespopulatedin 16 O+ 169 Tminteractionsatenergy 5.6 MeV/A. Forward-to-backward yield ratio [R Y(F/B) ] measurements: to study thepre-equilibriumemissionin 16 O+ 169 Tmsystemat 5.6MeV/A. On the basis of results and analysis presented in this thesis, significant information about CF and ICF reactions and the angular momentum dependence of PEQ-emission processes have been obtained. In the Chapter-2 the 5

10 6 EFs of several radio-nuclides populated via CF and/or ICF in 12 C+ 159 Tb and 13 C+ 159 Tbsystemsmeasuredintheenergyrange 4-7MeV/A.The EFs have been analyzed in the framework of equilibrated CN-decay using statistical model code PACE4[34]. During the decay curve analysis for the identification of different reaction products, it has been found that some of the pxn and αxn-channels have contribution from pre-cursor decay of higher charge isobar. An attempt has been made to deduce the independent production cross-section from cumulative and pre-cursor decay contributions. The experimentally measured EFs of xn/pxn channels have been found to agree reasonably well with the statistical model predictions, indicating their production via CF only. However, in case of all the α-emitting channels, significant enhancement, in the production cross-sections, have been observed as compared to the PACE4 predictions. This enhancement has been attributeddue tothe ICFof 12,13 C with 159 Tb. Ithasbeenobservedthat the probability of ICF processes increases with incident projectile energy, mass-asymmetry and target mass. The results presented are found to follow Morgenstern s mass-asymmetry systematics [33] for individual projectiles separately [11]. On the basis of results and analysis presented, it may be concluded that apart from CF, the ICF is also a process of considerable importance at projectile energies 4-7 MeV/A. The ICF fraction, which isameasureoftherelativestrengthoficftototalfusion,hasbeenfound to follow alpha-q-value systematics. As the alpha-q-value becomes more negative the in-complete fusion contribution decreases. The present results in light of the previously studied systems indicate that the alpha-q-value plays an important role in ICF-reactions. The recoil range distributions of a large number of radio-nuclides populatedin 12 C+ 159 Tbinteractionsatthreeabovebarrierenergieshavealso been measured. The analysis of the measured FRRDs of reaction products presented, strongly reveal a significant contribution from the partial LMT of the projectile associated with ICF in several α-emitting channels. Different partial LMT components are attributed to the fusion of 8 Be and α from the 12 Cprojectiletothetargetnucleus. The SUMRULEmodel[15]calculations are found to highly underestimate the deduced ICF cross-sections. This discrepancy may be due to the assumption in the SUMRULE model

11 that a substantial contribution to ICF comes from the collision trajectories withl>l crit. Inthepresentstudy,substantialamountofICFcontribution hasbeenreportedatthestudiedenergies,wherel max <l crit,thuscollision trajectories with l < l crit significantly contribute to ICF processes. More dataonsuchreactionsisneededtoexploretheaboveaspects,sothatthe assumptions of the SUMRULE model for energies near the barrier, where l<l crit,maybeimprovedupontoexplaintheexperimentaldata. In order to obtain information about the angular momentum dependence of ICF reaction processes, the measurement of spin distributions and feeding intensity profiles of different CF and ICF products populated via xn/pxn/αxn/2αxnchannelsinthe 12 C+ 169 Tmsystemhavebeencarriedout attheenergies 5.6and6.5MeV/A.Thespindistributions of ICF-(αxn /2αxn)-channels are found to be distinctly different than those observed for CF-(xn /pxn/αxn)-channels, and show entirely different de-excitation patterns for CF and ICF products. The spin distribution(s) of CF products are foundtoreflectstrongfeedingoverabroadrangeofspintowardstheband head. However, the spin distribution(s) associated with ICF channels are found to arise from a narrow spin population. This indicates the competitionfromsuccessivelyopenedicfchannelsforeachvalueoflabovel crit for normal fusion(cf) at respective projectile energies. Moreover, the populationoflow-spinstatesarefoundtobehinderedand/orlessfedinthecaseof ICF. It has been observed that the direct α-multiplicity in the forward cone increases with the l value at a particular projectile energy. Hence, it may be concluded that the peripheral interactions essentially contribute to open up direct-α-emitting channels at relatively high l-values. It may further be pointed out that the higher l -values associated with 2α-emitting channels may be considered to originate from higher impact parameters than that associated with the production of single direct-α-emitting channel. One of the important observations of the present work is that ICF can populate high-spin states in final reaction products, which is not possible to be achieved via CF at a given projectile energy. In the forward(f)-to-backward(b) yield ratio measurement a significant enhancement over the unity line has been observed, which indicates significant contribution from PEQ-emission in 16 O+ 169 Tm system at as low as 5.6 7

12 8 MeV/A.Themaximumobservedspin(J max obs )isfoundtodecreasewiththe number of proton emitted from the composite nucleus during EQ and/or PEQ-decay. As such, on the basis of results presented, it may be concluded that PEQ-emission in HI-induced reactions is a process of importance at energies as low as 5.6 MeV/A, which may be investigated by yield ratio measurements. Further, in order to have better understanding of underlying processes andtohaveperfectmodelingforicfreactiondynamics,itwouldbequiteinteresting to perform more detailed experiments for various projectile-target combinations. However, additional information about the PEQ-emission can be obtained by measuring the multiplicity, the angular distribution, and energy spectra of emitted light nuclear particle(s), which may serve as extra degrees of freedom to enhance the understanding of underlying processes. = = = =

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16 Do not believe in a thing because you have read about it in a book... Do not believe in a thing because another man has said it was true... Do not believe in words because they are hallowed by tradition... Find out the truth for yourself... Reason it out... that is realization... - Swami Vivekananda WORK DONE AT INTER UNIVERSITY ACCELERATOR CENTER, NEW DELHI 2012