Y. K. Gupta Nuclear Physics Division, BARC, Mumbai Understanding of Diverse Nuclear Phenomena using charged particle emission as a probe

11/10/2012NuStar-Week (2012)2 Quantum Many-body system Exhibits ‘diverse phenomena’ Though, a remarkable progress has been made, much remains still to be understood. ~50% of the nuclear chart is unknown. Cold-Fusion, Hot-Fusion Island of stability This journey of nuclear chart exploration is full of excitement and challenges. Using the stable beams, complete understanding of diverse nuclear phenomena ‘Stepping stone’ Overview

Nuclear-Level-Density (NLD) is one of the basic parameters in nuclear physics which has wide spread applications /implications theoretically as well as experimentally. At higher excitation energy and angular momenta, these are best served by the phenomenological descriptions. Nuclear-Level-Density parameter A key ingredient of the Phenomenological descriptions of NLD The excitation energy dependence of the NLD parameter has been studied quite extensively, but its angular momentum dependence is limited to few nuclei. We performed a series of experiments in this direction, where we measured angular momentum gated α-particles and analyzed in the framework of the statistical model. 11/10/20123NuStar-Week (2012) Statistical aspects: Angular momentum dependence of nuclear level-density parameter

Around shell closure region of Z=50 Y. K. Gupta et. al. Phys. ReV. C78, (2008) The inverse level-density parameter K=A/ã which fits the experimental gross spectra is in the range of 9.0 to Various features in angular momentum dependence of level density parameter, were observed. Beam BGO ; top half BGO ; Bottom half Target ladder 11/10/20124NuStar-Week (2012) On the Shell

5 Mid-shell region of Z~70 Y. K. Gupta et. al. Phys. ReV. C80, (2009) Constant Level-density parameter A/(8.2±1.1)MeV -1 These are new experimental findings which provide crucial inputs for the microscopic theories at higher values of excitation energies and angular momenta. 11/10/2012NuStar-Week (2012)

11/10/20126NuStar-Week (2012) Proposals to NuStar: Nuclear Level density of the exotic nuclei (away from the stability line) Using stable beams there are only limited scopes of populating exotic nuclei. Different shell structure of the exotic nuclei will lead to dramatically different level- density parameters. Possible observables 1.Evaporated particle spectra 2. Particle multiplicities 3. ER Cross sections If confirmed experimentally with RIB’s this result would have strong implications in nuclear astrophysics EXL-NuStar

Heavy-ion reactions  Pre-equilibrium emission  Multi-fragmentation  Nuclear Fission Rather fast Time Scale Long Time Scale Dynamical aspects manifest in terms of nuclear viscosity !!!!! Various probes have been employed to gain information about the nature (one-body vs. two-body) and magnitude of the nuclear viscosity and is still of much current interest. In particular, the question regarding energy dissipation during saddle to scission motion is not clearly understood so far. Scission Saddle 11/10/20127NuStar-Week (2012) Dynamical aspects

Net force A B FAFA FBFB A B FAFA B (a): Symmetric Fission(b): Asymmetric Fission F A < F B F A = F B Scission Axis  Giant Resonance [Peter Paul Ann. Rev. Nucl. Sci, 44, 65(1994)]  Pre-Scission neutron emission [D. Hilchher, H. Rossner, Ann. Phys.Fr., 17, 471(1992)]  Kinetic energy of FFs All these probes are not able to provide a clear picture of nuclear viscosity. Charged particle emission from neck Ternary-fission  Very Sensitive to saddle to scission dynamics  Can be used in a wide energy regime from low energy fission to heavy-ion fission We performed a series of experiments in this direction and analyzed the data along with literature data. 11/10/20128NuStar-Week (2012) Experimental probes to understand the nuclear dissipation

Mon i tor CsI (Tl) detectors 32 strip silicon Beam C1:105 0 c3:70 0 C2:130 0 GasTelescope:140 0 Beam Target ladder NuStar-Week (2012)9 γ d α p t PLFs Y. K. Gupta et. al. Nucl. Instrum. Meth. A629, 149 (2011) 11/10/2012

Measured α-particles in coincidence of fission fragments Employing the Moving source fit, α-particle multiplicities corresponding to different emission stages are determined. Statistical theory also predicts that particle emission width increases as Q α increases. Prescission emission is a statistical process Prescission Multiplicity 11/10/201210NuStar-Week (2012)

Near-scission Multiplicity The fraction α nse is ~10% of the total pre-scission emission, irrespective of the Z 2 /A and excitation energy of the fissioning system. NSE α-particles in heavy-ion fission, are same as ordinary pre-scission α- particles. Y. K. Gupta et. al. Phys. ReV. C84, (2011) 11/10/201211NuStar-Week (2012)

Beam 8 Be 232 Th α 8 Be   p p n n   FF Scission axis α- transfer 8 Be Breakup 236 U Target Fission Det. α α CsI Detector α 12 C Enhanced multiplicity in 12C + 232Th reaction: γ d/t 2α2α p PLFs Energy (arb. units) ZCT (arb. units) α Transfer- breakup process Y. K. Gupta et. al. Phys. ReV. C86, (2012) 11/10/201212NuStar-Week (2012)

Fast Neck rupture Heavy-ion Fusion-Fission: Low Energy/ Sp. Fission: Dynamical Emission Statistical Emission Nuclear collective motion during scission exhibits a change over from super - fluid to viscous nature as excitation energy is increased. NuStar-Week (2012)13  Low Viscosity  High Viscosity Slow Neck rupture 11/10/2012 So what do we learn from here about NSE ?  Coordinate dependence of nuclear viscosity  Temperature dependence Qualitatively we are address about:

11/10/2012NuStar-Week (2012)14 (i) From theoretical model simulations it is shown that the neck joining the nascent FFS is highly neutron rich. There is no experimental verifications of it. Using stable beams there are only limited scopes of populating CN having same Z but varying N/Z. Using the RIBs from NuStar this task can be completed. (ii) Look for the turning point from dynamical to statistical emission of ternary particles Excitation Energy Initial CN (MeV) 236 U 240 Pu 239 Np Ternary Light Charged particle s(× ) From ‘Thomas: 1966’ Through fusion route it is difficult to populate the CN in this excitation energy range Using relativistic heavy (Z>50) beams on actinide targets, Coulomb fission provides an avenue to study the near scission emission process at very low excitation energies. Proposals to NuStar: Satpathy et al., Pramana J. Phys. 70, (2008)87 Satpathy et al., arXiv:nucl-th/ by Guth and Wilets

11/10/201215NuStar-Week (2012) (ii) To investigate the effect of widely varying N/Z ratio for same Z on Coulomb Fission Proposals to NuStar: x Sm x Sn ….will influence the potential energy surfaces and hence the fission characteristics. 235U Target Projectile b>R P + R T Fission RIB Below and Near barrier energies Study of Coulomb fission at relativistic energies is a wide open area, where very little experimental work exists. The strong interplay of Coulomb and nuclear effects…….

11/10/2012NuStar-Week (2012)16 Thank -You References to Coulomb Fission: Theoretical- Experimental-  J. W. Norbury Phys. Rev. C 43, R368 (1991).  V. E. Oberacker et al. Rep. Prog. Phys. 48, 327(1985).  H. Kruse et al., Phys. Rev. C 22, 2465(1980).  V. E. Oberacker Phys. Rev. C 20, 1453(1979).  H. Holm and W. Greiner, Phys. Rev. Lett. 26, 1647 (1971).  H. Backe et al., Phys. Rev Lett., 43, 1077(1979).  G. Franz et al., Z. Phys. A291, 167(1979).  D. Habs. et al., Z. Phys. A283, 261(1977). …………….very little work………. NuStar