Dynamical approach to heavy ion-induced fission

Transcription

1 EPJ Web of Conferences 91, (2015) DOI: / epjconf/ C Owne by the authors, publishe by EDP Sciences, 2015 Dynamical approach to heavy ion-inuce fission D.Y. Jeung 1, a, E. Williams 1, D. J. Hine 1, M. Dasgupta 1, R. u Rietz 1, b, M. Evers 1, c, C.J. Lin 1,, D.H. Luong 1, C. Simenel 1, an A. Wakhle 1, e 1 Department of Nuclear Physics, Research School of Physics an Engineering, The Australian National University, Canberra, ACT 2601, Australia Abstract. Deep inelastic collisions (DICs) can compete strongly with fusion in collisions of heavy nuclei. However, stanar couple-channels calculations o not take DIC processes into account. As a result, calculations have been shown to overestimate the fusion cross-sections, resulting in a iscrepancy between experimental ata an theoretical calculations, particularly at energies above the fusion barrier. To investigate this iscrepancy, we conucte a series of experiments using the ANU 14UD tanem accelerator an the CUBE 2-boy fission spectrometer to examine the competition between transfer/dic an fusion. In particular, fusion-fission an 3-boy fission yiels have been extracte for 34 S Th an 40 Ca Th systems. This work shows that the transfer-fission probability is enhance relative to fusion-fission for 40 Ca Th, when compare to 34 S+ 232 Th. It is suggeste that the enhancement of this DIC process in 40 Ca Th is linke to an increase in the ensity overlap of the colliing nuclei as a function of the charge prouct an contributes to fusion hinrance. 1 Introuction Deep inelastic collisions (DICs) have been shown to compete strongly with fusion [1] an become increasingly probable as the charge prouct (Z 1 Z 2 ) of the reaction increases [2-4]. However, the quantitative role these processes play in reaction outcomes is not well unerstoo. Therefore, unerstaning the DIC process is crucial for unerstaning fusion ynamics in heavy ion collisions. The stanar couple-channels (CC) moel use for escribing the fusion process has been successful in explaining phenomena like the enhancement of fusion cross-sections at energies below the average fusion barrier [5-6]. However, the observation of fusion suppression, particularly at above-barrier energies, cannot be escribe by these moels [7-8]. This isagreement between CC moels an observation is thought to be linke to the contribution of DICs [7, 9], which are not inclue within the CC framework. In Ref. [7], it was suggeste that an enhancement in DICs with increasing Z 1 Z 2 may be responsible for fusion suppression. Thus, a new approach that inclues both CC effects an energy issipation that results in DICs is require to illustrate any observable outcomes resulting from the onset of DICs. Such a ynamical reaction moel is being evelope at the Australian National University (ANU); the work presente here is aime at proviing experimental inputs to this phenomenological moel. One factor that is consiere to be important in shaping the influence of issipative processes on reaction outcomes is the ensity overlap of colliing nuclei, i.e., increasing nucleon-nucleon interactions that result in a substantial loss of kinetic energy an angular momentum from the relative motion. The relative kinetic energy of the colliing nuclei is issipate into excitation energy (heating) of the heavy prouct, which can be forme by either transfer or fusion reactions. The transfer probabilities, in particular, are significantly affecte by this ensity overlap. However, systematic experimental stuy of the interplay between ensity overlap an transfer probabilities have not been carrie out, although recent experiments are aressing this issue [10-11]. a D. Y. Jeung: ongyun.jeung@anu.eu.au b Current aress: Malmö University, Faculty of Technology an Society, Malmö, Sween c Current aress: Institute of Functional Genomics, University of Regensburg, 93053, Germany Current aress: China Institute of Atomic Energy, Beijing , China e Current aress: National Superconucting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA Article available at or

2 EPJ Web of Conferences One way of examine the influence of issipative processes is to stuy fusion-fission (or calle as full momentum transfer (FMT) fission) an transfer-fission reactions. FMT fission correspons to fission that occurs when a projectile has been completely absorbe into a target an forme a thermally equilibrate composite nucleus before unergoing fission. In contrast, transferfission correspons to fission from a target-like (heavy) nucleus, forme following nucleon exchange between colliing nuclei, that has an excitation energy higher than the fission barrier. In this paper, FMT fission an transfer-fission yiels have been extracte for 34 S Th an 40 Ca Th reactions. The comparison between these reactions will she light on the effect of the DIC process on heavy ion collisions an explore the necessity of explicitly taking energy issipation into account in the escription of the fusion process. 2 Experimental Details The experiments were carrie out at the Heavy Ion Accelerator Facility at the ANU. The 14UD electrostatic accelerator was use to prouce a pulse beam of ~1.3 ns with at intervals of ~106 ns. For the 40 Ca Th reaction, the ANU superconucting linear postaccelerator was use to increase the beam energies above the fusion barrier. Table 1 provies target etails an energy ranges for the reactions stuie. Reaction Table 1. Reaction parameters. Target Thickness (μg/cm 2 ) Backing, Thickness (μg/cm 2 ) Beam Energy Ranges (MeV) 34 S Th 80 C, configuration as seen from above. The position of etectors is efine by their istance from the target (), azimuthal angle (ϕ) an polar angle (θ) relative to the beam axis [12]. The two etectors were place 180 mm away from the target. The front etector was centre at θ 2 = 45, proviing an angular coverage from 5 to 80. We use two ifferent ranges of angular coverage for the back etector. It was centre at θ 1 = 90 with either 55 to 130 or 50 to 125 coverage, epening on the orientation of the back etector. X an Y anoe planes, mae up of a gri of 20 μm gol plate tungsten wires, provie position information, an the centre foil provie timing an energy loss information. The coincient etection of fragments allowe the etermination of the fission fragment mass ratio an velocities through kinematic reconstruction base on twoboy kinematics [13]. 3 Data Analysis The ata analysis proceure is largely ivie into three steps. Each step of the analysis is explaine below, using the 40 Ca Th reaction at E lab = MeV as an example. 3.1 Selecting the Fission Events There are two types of ata that nee to be eliminate in orer to select the fission events clearly; (1) those corresponing to elastically scattere particles an (2) coincient events occurring ue to reactions with lighter elements in the target an etection of one fission fragment in coincience with a beam-like particle. 40 Ca Th 80 or 330 C, 10 or Al, Coincient fission fragments were etecte in the CUBE etector, which consists of two large-area position-sensitive multi-wire proportional counters (MWPCs) [12]. A schematic view of the experimental setup is given in Figure 1. MWPC 1 MWPC 1 y φ x MWPC 2 x z Target θ MWPC 2 Beam Figure 2. Scatter plot of mass ratio M R vs v par /v cn. The scale of counts is given by a colour scheme in the right panel. The rectangular gate (black lines) is use to select fission events an remove the elastically scattere events. Figure 1. Experimental setup of the CUBE binary fission spectrometer. Labels 1 an 2 correspon to the back an front etectors respectively; shows the azimuthal positioning where the beam is going into the page, an shows the The parallel velocity component (v par ) for the elastic scattering events is the same as the compoun nuclear velocity (v cn ) an thus these events can overlap with the fusion-fission events. The elastically scattere events were clearly ientifie in the mass ratio plot, M R vs p.2

3 Heavy Ion Accelerator Symposium 2014 v par /v cn, as the two high intensity regions near M R ~ 0.15 an M R ~ 0.85 shown in Figure 2. To simplify the next step in the analysis, these events were eliminate using a rectangular gate as shown. fusion-fission events for 34 S Th an 40 Ca Th systems but can lose events following transfer fission. This is because as the projectile becomes heavier, a larger momentum is transferre to the target, leaing to a larger recoil velocity. The fragments following the fission of a composite systems forme in such reactions will have a foling angle that goes beyon the CUBE etector geometry, meaning that only one of the fragments is etecte. To make a irect comparison between fusionfission an transfer-fission yiels, a careful selection of the etector angles is require in the analysis to avoi missing such 3-boy events. To ensure that we ha aequate soli angle coverage for both fusion-fission an transfer-fission, we looke at the position istribution of fission fragments in the front etector (see Figure 4 ) for ifferent gates in the back etector angle plot (see Figure 4 ). Figure 4 is an example of fission fragments following transfer reactions etecte in the front etector after restricting the back etector angular coverage. The etail of selecting transfer-fission events is explaine in the following section. If the fission fragments lie close to the black line bounary that correspons to the ege of the front etector angular coverage, some of fission fragments might be misse. Therefore, we systematically searche for the back etector (1) position gates that kept fission fragments away from the front etector (2) eges. Figure 3. A polygonal gate is use to remove coinciences between beam-like nuclei an one fission fragment. Scatter plot of v 1 vs v 2 for 40 Ca Th reaction at E lab = MeV. Scatter plot of M R vs TKE/TKE Viola for 34 S Th reaction at E lab = 182 MeV. The scale of counts is given by a colour scheme in the right panel. In aition to the elastics, events ue to ranom coinciences between beam-like nuclei an one fission fragment can be present. These were separate by putting an aitional gate in the v 1 -v 2 plot as shown in Figure 3. For the 34 S Th reaction analysis, an aitional polygonal gate was applie to the scatter plot of M R versus relative total kinetic energy (RTKE) in orer to further exclue unwante events. This gate is shown in Figure 3. RTKE is efine as TKE exp /TKE Viola where TKE exp is the total kinetic energy etermine from the experimental ata an TKE Viola is the kinetic energy erive from the Viola systematics [14]. 3.2 Angular Acceptance Ranges In aition to fusion-fission, transfer-fission becomes an important process for heavier projectile an target systems. The CUBE etector can efficiently etect the Figure 4. Scatter plot of fission fragments observe in the back an front etectors for the 40 Ca Th reaction at E lab = MeV. The scale of counts is given by a colour scheme in p.3

4 EPJ Web of Conferences the right panel. The top rectangular box represents fission fragments having negative velocities while the bottom rectangular box represents those having positive velocities. Black line is a bounary of the front etector angular coverage. The events at the centre of the plot in correspon to pairs of fission fragments caught following a transfer-fission reaction. Applying these gates in gives the fission fragments from the transfer-fission seen in. In the 34 S Th reactions, pairs of the fission fragments coul be etecte efficiently in both etectors by efining one rectangular gate between 75 < θ 1 < 105 an < ϕ 1 < in the back etector an looking for the complimentary fission fragment in the front etector, ensuring fission fragment events were clustere away from the eges. However, the above metho i not give efficient etection of the fission pairs for the 40 Ca Th system. Hence, two sets of θ-ϕ gates were put on the back etector. In the first case, a gate incluing the angular ranges 75 < θ 1 < 105 an 135 < ϕ 1 < 165 was put on the back etector: this correspone to positive perpenicular velocity of the fission pair. In the secon, for the same angular coverage in θ 1, 195 < ϕ 1 < 225 was selecte. This gate correspone to negative velocity of the fission pair. The total fission events were the sum of fission pairs extracte respectively from the two sets of gates shown in Figure 4. well as the position/time resolution of the etector. Therefore, the high intensity region in the centre of the plot correspons to fusion-fission. This region is boune within an elliptical shape. The lengths of the major an minor axes of the ellipse were efine from the withs of a 1-D Gaussian fit to the projection of the v par -v cn versus v perp plot. Outsie this region, the 3-boy events corresponing to fission following a transfer reaction are locate. These events are associate with reactions resulting in a projectile-like nucleus an two fission fragments. These fragments are constraine by kinematic limits, an thus they are sprea insie a circle of raius v cn, centre at v par - v cn, = 0 v perp = 0. However, some of these events cross the kinematic limits ue to some instrumental effects or resolution issues. Therefore, an elliptical shape bounary was use for selecting transfer-fission events. 4 Results an Discussion This paper focuses on stuying the ratio of 3-boy events (comprising one projectile-like nucleus an two fission fragments arising from fission of the target-like nucleus) to FMT fission following capture. For simplicity, we enote this as the ratio of transfer-fission to fusionfission. 3.3 Separation of Fusion-fission an Transferfission Figure 5. Fission fragment velocity components parallel an perpenicular to the beam. Fusion-fission events lie in the centre of the figure (re line), while transfer-fission events are sprea out within an ellipse (black line). The scale of counts is given by a colour scheme in the right panel. Figure 5 shows a scatter plot of the measure v par -v cn against v perp, where v perp is the velocity component perpenicular to the beam axis. For fission following a fusion reaction, fission fragments travel with the same centre-of-mass velocity as the parallel component of the compoun nucleus velocity an zero perpenicular velocity. A small sprea in fusion-fission events results from the evaporation of light particles such as n, p, α particles from the nuclei either before or after scission, as Figure 6. Ratio of transfer-fission to fusion-fission with respect to the reuce energy. Figure 6 shows the ratio of transfer-fission to fusionfission events as a function of the beam energy normalise by the proximity barrier V B [15]. The errors on the ratios were obtaine from the error propagation of the statistical errors. As Figure 6 shows, for reactions involving heavy projectiles, a higher ratio is observe as compare to reactions involving light projectiles for the full range of measure energies. The results show that with increasing Z 1 Z 2, transferfission is more probable than fusion-fission process. This is consistent with the iea that an increasing ensity p.4

5 Heavy Ion Accelerator Symposium 2014 overlap between the colliing nuclei results in an increase in transfer reactions, which in turn competes with fusion reactions. Thus, an increase in the transfer probability leas to an increase in fusion suppression ue to loss of flux from the fusion channel. This result is consistent with Ref. [7], which also escribes an increase fusion suppression in reactions between heavier nuclei in comparison to those involving light nuclei. 13. D. J. Hine, M. Dasgupta, J. R. Leigh, J. C. Mein, C. R. Morton, J. O. Newton, an H. Timmers, Phys. Rev. C 53, 1290 (1996) 14. V. E. Viola, K. Kwiatkowski, an M. Walker, Phys. Rev. C 31, 1550 (1985) 15. W. J. Świątecki, K. Siwek -Wilczyńska, an J. Wilczyński, Phys. Rev. C 71, (2005) 5 Conclusions Transfer-fission events have been well separate from fusion-fission events for 34 S Th an 40 Ca Th reactions using the kinematic coincience metho. The measurements show that transfer-fission reactions increase with an increase in Z 1 Z 2 relative to fusionfission. This relative increase in transfer with the charge prouct may be linke to an enhancement in the ensity overlap for collisions between heavier nuclei. This observation provies important evience supporting the iea that DICs shoul be taken into account in moels escribing the fusion process in heavy-ion reactions. Acknowlegments The authors are grateful for the contribution of D. C. Weisser, N. Lobanov, T. Kibei, an all the accelerator staff at the ANU accelerator facility. This work is supporte by the Australian Research Council Grants No. FL , FT , DP , DP , DE References 1. E. L. H. Wolfs, Phys. Rev. C 36, 1379 (1987) 2. W. J. Swiatecki, Phys. Scr. 24, 113 (1981) 3. S. Bjornholm an W. J. Swiatecki, Nucl. Phys. A 391, 471 (1982) 4. J. P. Blocki, H. Felmeier, an W. J. Swiatecki, Nucl. Phys. A 459, 145 (1986) 5. M. Beckerman, Rep. Prog. Phys 51, 1047 (1988) 6. W. Reisorf, J. Phys. G: Nucl. Part. Phys. 20, 1297 (1994) 7. J. O. Newton, R. D. Butt, M. Dasgupta, D. J. Hine, I. I. Gontchar, an C. R. Morton, Phys. Rev. C 70, (2004) 8. D. J. Hine, M. Dasgupta, an A. Mukherjee, Phys. Rev. Lett. 89, (2002) 9. J. G. Keller, B. B. Back, B. G. Glagola, D. Henerson, S. B. Kaufman, S. J. Saners, R. H. Siemssen, F. Viebaek, B. D. Wilkins, an A. Worsham, Phys. Rev. C 36, 1364 (1987) 10. M. Evers, M. Dasgupta, D. J. Hine, D. H. Luong, R. Rafiei, an R. u Rietz, Phys. Rev. C 84, (2011) 11. L. Corrai, G. Pollarolo, an S. Szilner, J. Phys. G 36, (2009) 12. R. u Rietz, E. Williams, D. J. Hine, M. Dasgupta, M. Evers, C. J. Lin, D. H. Luong, C. Simenel, an A. Wakhle, Phys. Rev. C 88, (2013) p.5