Speaker
Description
Evaporation Residue (ER) cross-sections and ER gated $\gamma$-ray fold distributions were measured for the $^{32}$S + $^{154}$Sm nuclear reaction above the Coulomb barrier at six different beam energies from 148 to 191 MeV. $\gamma$-ray multiplicity and spin distributions were extracted from ER-gated fold distributions. The measured ER cross-sections are compared with the results of both the Statistical model calculations and the dynamic model calculations. Statistical model calculations have been performed to generate a range of parameter space for both the barrier height and Kramers' viscosity parameter over which ER cross-section data can be reproduced. The calculations performed by the dinuclear system model reproduce the data considering both complete and incomplete fusion processes. Comparison of the ER cross-sections measured in previous work using very different target-projectile combinations with much less mass asymmetry than the present measurement clearly demonstrates the effect of the entrance channel on ER production cross-section.
In the present case, $^{186}$Pt$^*$ compound nucleus was popultaed to measure the ER cross-sections. These measurements were carried out using HYbrid Recoil Mass Analyser (HYRA) in gas mode coupled with TIFR 4$\pi$ spin-spectrometer. $^{32}$S pulsed beam from 15 UD Pelletron + LINAC accelerator facility at IUAC(Inter-University Accelerator Facility), New Delhi with an average current of ∼ 0.5 - 1 pnA was bombarded on $^{154}$Sm target of thickness 118µgm/cm$^{2}$ with carbon capping and backing of 25µgm/cm$^{2}$ and 10µgm/cm$^{2}$ respectively.
Raw fold distributions were ER-gated to remove statistical and non-rotating $\gamma$ rays contributions. Realistic simulations of TIFR 4$\pi$ spectrometer, consisting of 32 NaI(Tl) detectors were carried out using Geant4, and fold distribution for different multiplicities were generated i.e. for a given gamma multiplicity M, distribution in fold k. Fold distribution P(k) probability can be given by:
\begin{equation}
P(k) = \sum_{M_{\gamma=0}}^{\infty}R(k,M_{\gamma}) P(M_{\gamma})
\end{equation}
where R(k, M$_{\gamma}$) is the response function, in other words, it is the probability of firing k detectors out of N detectors for M uncorrelated $\gamma$ rays and P(M$_{\gamma}$) is the probability of multiplicity distribution. Experimental fold data is used to extract multiplicity as well as spin distribution of $^{186}$Pt$^*$. Response function was generated using Geant4 simulations using the exact geometry of the spin-spectrometer. We have convoluted experimental fold data with R(M$_\gamma$,k) to get the multiplicity distribution (with error bars). Theoretical calculations along with experimental results will be presented in the school.
