Proton and α-particle energy spectra were measured in coincidence with fission fragments at different relative angles in $^{16}$O (96 MeV) + $^{232}$Th reaction. The multiplicity spectra were analyzed within the framework of a Moving Source Disentangling Analysis (MSDA) to determine contributions from different emission stages. The MSDA conclusively shows “Near Scission Emission (NSE)” as an essential component in the multiplicity spectra. In contrast to NSE α particles which emit mainly perpendicular (“equatorial emission”), the NSE protons are observed to be emitted perpendicular as well as parallel (“polar emission”) to the fission axis with similar intensities (~20% for each). Thus, around 40% of total pre-scission protons are emitted near the scission stage, whereas the same fraction for α particles is only around 10%. The inevitable presence of “polar” and “equatorial” NSE protons in a heavy-ion induced fission has been observed for the first time. Present results open up a new avenue to study the heavy-ion induced fission dynamics.

India

Pawan Singh

Y.K. Gupta

G.K. Prajapati

B.N. Joshi

V.G. Prajapati

N. Sirswal

K. Ramachandran

A.S. Pradeep

V.S. Dagre

M. Kumar

A. Jhingan

N. Deshmukh

B.V. John

B.K. Nayak

D.C. Biswas

R.K. Choudhury

Observation of near-scission “polar” and “equatorial” proton emission in heavy-ion induced fission

The pre-scission <italic>&#945;</italic>-particle multiplicity ( <math altimg='si1.svg'><msub><mrow><mi>&#945;</mi></mrow><mrow><mi mathvariant='normal'>pre</mi></mrow></msub></math>), measured in 16O (96 MeV) + 232Th reaction, is observed to be significantly lower than the global trend reported earlier. Statistical model calculations also show a much shorter fission-time to reproduce the experimental <math altimg='si1.svg'><msub><mrow><mi>&#945;</mi></mrow><mrow><mi mathvariant='normal'>pre</mi></mrow></msub></math>. By assimilating the previous measurements, it is observed that the <math altimg='si1.svg'><msub><mrow><mi>&#945;</mi></mrow><mrow><mi mathvariant='normal'>pre</mi></mrow></msub></math> makes a changeover from high to a very low value while crossing the Businaro Gallone point in mass asymmetry in the entrance channels in heavy-ion induced fission of 232Th. Similar discontinuous behavior with respect to the entrance channel mass asymmetry was observed earlier in fission fragment angular anisotropy data, and it was attributed to &#8220;pre-equilibrium fission&#8221;. The present results on <math altimg='si1.svg'><msub><mrow><mi>&#945;</mi></mrow><mrow><mi mathvariant='normal'>pre</mi></mrow></msub></math> provide compelling evidence for an admixture of non-equilibrium fission which depends on the entrance channel dynamics.

L.S. Danu

S. Dubey

S. Mukhopadhyay

N. Kumar

K. Mahata

New signature of non-equilibrium fission from pre-scission <italic>&#945;</italic>-particle emission

Quasi-elastic (QEL) scattering measurements have been performed using a $^{28}$Si projectile off a $^{90}$Zr target at energies around the Coulomb barrier. A Bayesian analysis within the framework of coupled channels (CC) calculations is performed in a large parameter space of quadrupole and hexadecapole deformations (<math><msub><mrow><mi>β</mi></mrow><mrow><mn>2</mn></mrow></msub></math> and <math><msub><mrow><mi>β</mi></mrow><mrow><mn>4</mn></mrow></msub></math>) of $^{28}$Si. Our results unambiguously show that $^{28}$Si is an oblate shaped nucleus with <math><msub><mrow><mi>β</mi></mrow><mrow><mn>2</mn></mrow></msub></math>=-<math><mn>0.38</mn><mo>±</mo><mn>0.01</mn></math> which is in excellent agreement with results from electromagnetic probes. The sign and magnitude of quadrupole deformation along with a precise value of hexadecapole deformation (<math><msub><mrow><mi>β</mi></mrow><mrow><mn>4</mn></mrow></msub></math>=+<math><mn>0.03</mn><mo>±</mo><mn>0.01</mn></math>) of $^{28}$Si have been determined for the first time using QEL scattering. A remarkable agreement is obtained between the experimental and calculated <math><msub><mrow><mi>β</mi></mrow><mrow><mn>4</mn></mrow></msub></math> values of $^{28}$Si based on Skyrme-Hartree-Fock method. The present results demonstrate the strong sensitivity of the quasi-elastic scattering to the sign and magnitude to the ground state deformation parameters, thus affirming its suitability to be used for rare exotic nuclei using low intensity RIBs.

V.B. Katariya

Japan

K. Hagino

D. Patel

V. Ranga

United States

U. Garg

A. Pal

V.V. Desai

S. Panwar

R. Sariyal

I. Mazumdar

Precise determination of quadrupole and hexadecapole deformation parameters of the -shell Nucleus, Si

A high-spin study of the shape transitional nucleus 188 Pt reveals the unusual coexistence of both shape- and K -isomeric states. Reduced B(E2) transition probabilities for decays from these states inferred from the data clearly establish their hindered character. In addition to other excited structures, a rotational band built upon the K isomer is identified, and its configuration has been assigned through an analysis of alignments and branching ratios. The shape evolution with spin in this nucleus has been inferred from both experimental observables and cranking calculations. The yrast positive parity structure appears to evolve from a near-prolate deformed shape through triaxial at intermediate excitation, and eventually to oblate at the highest spins.