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Fission mode identification in the 180Hg region: derivative analysis approach

D. T. Kattikat Melcom, I. Tsekhanovich, F. Guezet, A. Andreyev, K. Nishio

Abstract

Experimental setups commonly used to study fission properties of nuclei in the exotic neutron-deficient 180Hg region are based on the time-of-flight technique for the fission-product identification. The nuclei of interest are created via fusion reactions at excitation energies of several tens of MeV and identified with limited mass resolution. The deduced final fission-fragment mass distributions are in general structureless, which makes the identification of fission modes, along with their properties, ambiguous and author-dependent. The standard functional-analysis technique applied to the simulated limited-resolution fusion-fission data appears to provide consistent results on the number and parameters of fission modes, even in cases of strong symmetric-mode dominance, i.e. for Gaussian-like fission-fragment mass distribution shapes. The method is shown to work also on data sets with limited statistics (real experimental data with integral of a few tens of thousands of events).

Fission mode identification in the 180Hg region: derivative analysis approach

Abstract

Experimental setups commonly used to study fission properties of nuclei in the exotic neutron-deficient 180Hg region are based on the time-of-flight technique for the fission-product identification. The nuclei of interest are created via fusion reactions at excitation energies of several tens of MeV and identified with limited mass resolution. The deduced final fission-fragment mass distributions are in general structureless, which makes the identification of fission modes, along with their properties, ambiguous and author-dependent. The standard functional-analysis technique applied to the simulated limited-resolution fusion-fission data appears to provide consistent results on the number and parameters of fission modes, even in cases of strong symmetric-mode dominance, i.e. for Gaussian-like fission-fragment mass distribution shapes. The method is shown to work also on data sets with limited statistics (real experimental data with integral of a few tens of thousands of events).
Paper Structure (5 sections, 2 equations, 3 figures, 3 tables)

This paper contains 5 sections, 2 equations, 3 figures, 3 tables.

Table of Contents

  1. Introduction
  2. FFMD and TKE data analysis methodology
  3. Results
  4. Conclusions
  5. Acknowledgments

Figures (3)

  • Figure 1: FFMDs (left column) and their $2^{nd}$ derivatives (middle column) for $^{240}$Pu at two different excitation energies: $E^{*}=10$ MeV (upper row), and $E^{*}=50$ MeV (lower row). Black, blue and red lines depict, correspondingly, the simulated and the 2u- and 4u-resolution affected data sets. The FFMD components are relevant to the fit of the simulated data set (red line) made with free (dotted lines) or constrained (dashed lines) fit function parameters. SL/S1/S2/S3 mode is shown in cyan/magenta/green/brown. The right column displays the mass-TKE matrices for $E^{*}=10$ MeV : simulated (c) and resolution-broadened data (f) (resolution 4 u for mass and 8 MeV for TKE). Here, red contour lines represent the resultant 2D fit with function from Eq.\ref{['eq2']}.
  • Figure 2: Four first rows: simulated FFMDs (black symbols, left column) affected by resolution of 2 u and their 2$^{nd}$ derivatives (right column) for different combinations of $S$ and $A$ modes, refer to Table \ref{['table_2']} for input data. Panels (a,c,e): sum results of the 3G function FFMD fit (red line), panels (a—f): fission modes components shown with black and green broken lines for the $S$ and $A$ modes, respectively. Resolution effect is demonstrated with the 2$^{nd}$ derivative data: blue/red symbols stand, correspondingly, for the initial and resolution-affected (where a random function was used to simulate the resolution impact) data sets. Broken black, green and blue lines in (g-j) are respectively the $S$, $A1$ and $A2$ mode components from the fixed parameter fit. Two bottom rows: pure $S$ and pure $A$ mode FFMDs and their 2$^{nd}$ derivatives.
  • Figure 3: Experimental FFMD for $^{180}$H at two different excitation energies taken from Ref. nishio2015excitation and their derivatives calculated with window length of 3 u (panels (b,f)) and 5 u (panels (d,h)). The minima identified at $A$=78, 90 and 102 u in the 2$^{nd}$ derivative data determine the fit function composition as $S$ plus one $A$ mode. Panels (a,e) and (c,g) display, respectively, the individual mode components from the FFMDs fit with free and fixed $A$-mode peak positions. Blue/green lines indicate the $S$/$A$-mode components, red line is their sum. The mass resolution was 2.4 u.