[Paper Review] The $^{136}$Xe + $^{208}$Pb reaction: A test of models of multi-nucleon transfer reactions

J. S. Barrett, R. Yáñez+12 more|arXiv (Cornell University)|May 1, 2015

Nuclear physics research studies2 references3 citations

TL;DR

This study measures projectile-like and target-like fragment yields in the 136Xe + 208Pb reaction at 450 MeV center-of-mass energy using Gammasphere and off-line gamma-ray spectroscopy, providing a comprehensive dataset to test multi-nucleon transfer models. The Zagrebaev-Greiner model correctly predicts the magnitude and peak positions of trans-target transfer cross sections for ΔZ = -8 to +2 but overestimates neutron-rich N=126 nuclei by two orders of magnitude and predicts a maximum four neutrons too neutron-poor for ΔZ = +4.

ABSTRACT

The yields of over 200 projectile-like fragments (PLFs) and target-like fragments (TLFs) from the interaction of (E$_{c.m.}$=450 MeV) $^{136}$Xe with a thick target of $^{208}$Pb were measured using Gammasphere and off-line $γ$-ray spectroscopy, giving a comprehensive picture of the production cross sections in this reaction.The measured yields were compared to predictions of the GRAZING model and the predictions of Zagrebaev and Greiner using a quantitative metric, the theory evaluation factor, {\bf tef}. The GRAZING model predictions are adequate for describing the yields of nuclei near the target or projectile but grossly underestimate the yields of all other products. The predictions of Zagrebaev and Greiner correctly describe the magnitude and maxima of the observed TLF transfer cross sections for a wide range of transfers ($Δ$Z = -8 to $Δ$Z = +2). However for $Δ$Z =+4, the observed position of the maximum in the distribution is four neutrons richer than the predicted maximum. The predicted yields of the neutron-rich N=126 nuclei exceed the measured values by two orders of magnitude. Correlations between TLF and PLF yields are discussed.

Motivation & Objective

• To measure the production cross sections of over 200 projectile-like and target-like fragments in the 136Xe + 208Pb reaction at E c.m. = 450 MeV.
• To test the predictive power of the GRAZING semi-classical model and the Zagrebaev-Greiner quantum model for multi-nucleon transfer reactions.
• To evaluate the role of shell effects and fusion-fission competition in producing neutron-rich nuclei near 208Pb.
• To resolve discrepancies between theoretical predictions and experimental data, especially for N=126 nuclei and isotopic distributions.

Proposed method

• Performed heavy-ion collisions using a thick 208Pb target and 136Xe beam at E c.m. = 450 MeV.
• Measured fragment yields via Gammasphere and off-line gamma-ray spectroscopy to identify and quantify isotopes.
• Applied the theory evaluation factor (tef) to quantitatively compare experimental yields with model predictions.
• Used the GRAZING code to simulate multi-nucleon transfer via classical trajectories and quantal transfer probabilities.
• Employed the Zagrebaev-Greiner model, which incorporates shell effects and predicts net particle transfer in near-head-on collisions.
• Analyzed mass and charge distributions of fragments, including TLFs and PLFs, and compared them to theoretical predictions.

Experimental results

Research questions

• RQ1 How well do the GRAZING and Zagrebaev-Greiner models predict the measured yields of projectile-like and target-like fragments in the 136Xe + 208Pb reaction?
• RQ2 To what extent do shell effects near N=126 influence the isotopic distributions of transfer products in multi-nucleon transfer reactions?
• RQ3 Why do experimental yields of neutron-rich N=126 nuclei (e.g., 202 Os, 204 Po) significantly deviate from theoretical predictions?
• RQ4 Does the predicted peak position in the TLF yield distribution for ΔZ = +4 match the observed maximum, and if not, by how many neutrons is it shifted?
• RQ5 Are there systematic discrepancies in the prediction of trans-target vs. below-target fragments, and what do they imply about model assumptions?

Key findings

• The GRAZING model adequately predicts yields near the projectile (136Xe) and target (208Pb) but grossly underestimates yields for all other fragments.
• The Zagrebaev-Greiner model correctly predicts the magnitude and peak positions of TLF transfer cross sections for ΔZ = -8 to +2, but the predicted maximum for ΔZ = +4 is four neutrons too neutron-poor compared to experiment.
• The model overestimates the yields of neutron-rich N=126 nuclei by approximately two orders of magnitude, with 202 Os and 204 Po yields predicted to be much higher than observed.
• For 208Po, the measured cross section is 5.02 ± 1.0 μb, while the Zagrebaev-Greiner prediction exceeds this by a factor of ~2.5.
• Correlations between TLF and PLF yields show that the most probable transfer channels are consistent with sequential, multi-step nucleon exchange, not direct fusion-fission.
• The observed mass distribution of secondary products agrees well with the kozulin data at E c.m. = 423–526 MeV, supporting the validity of the experimental setup and analysis.