Microscopic quasifission dynamics of the ${}^{54}\text{Cr}+{}^{243}\text{Am}$ reaction

Using fully microscopic time-dependent Hartree-Fock simulations, this study reveals that the quasifission dynamics in the $^{54}\text{Cr}+^{243}\text{Am}$ reaction are governed by a complex interplay between collision geometry and incident energy, where specific energy windows can suppress shell effects to potentially enhance the fusion probability for synthesizing superheavy element 119.

The Gist

Here is an explanation of the paper, translated into everyday language with some creative analogies.

The Big Picture: Trying to Build a "Super-Heavy" Lego Tower

Imagine you are trying to build the tallest, heaviest Lego tower possible. In the world of atoms, these "super-heavy" towers are called Superheavy Elements (SHEs). Scientists have managed to build towers up to a certain height (Element 118), but trying to build the next ones (Elements 119 and 120) is incredibly difficult.

Why? Because when you try to smash two smaller Lego blocks together to make a big one, they often don't stick. Instead, they bounce off each other or shatter immediately. In physics, this shattering is called Quasi-fission (QF). It's like trying to merge two balls of clay, but instead of becoming one big ball, they just split back apart before they ever really mix.

This paper is a computer simulation study trying to figure out exactly why this shattering happens and how we can stop it to successfully build Element 119.

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The Experiment: The "Crash Test" Simulation

The scientists used a super-advanced computer program (called Time-Dependent Hartree-Fock theory) to simulate a specific crash test:

• The Projectile: A Chromium-54 atom (like a speeding car).
• The Target: An Americium-243 atom (like a stationary wall).

They wanted to see what happens when these two smash together. But here's the catch: atoms aren't perfect spheres; they are often shaped like rugby balls or even weird, pear-shaped blobs. So, the scientists had to simulate crashing them together at every possible angle.

Analogy 1: The Rugby Ball Crash

Imagine two rugby players trying to tackle each other.

• Side Collision: If they tackle each other broadside (side-to-side), they have a lot of surface area touching. They might get stuck together, roll around, and eventually separate.
• Tip Collision: If one player tackles the other head-on (tip-to-tip), they might bounce off quickly or slide past each other.

The paper found that how you aim the crash matters more than you think.

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Key Discovery 1: The "Magnetic" Pull of Atomic Shells

The most surprising finding is that the atoms aren't just random blobs of matter; they have an internal "skeleton" made of energy levels called shells. Think of these shells like the rungs on a ladder. If a rung is perfectly filled, the atom feels very stable and "stiff."

• The Side Collision (The Sticky One): When the atoms hit side-to-side, they get pulled by these "magnetic" shell rungs. The heavy part of the split wants to become an atom with 82 protons (a very stable number, like a perfect rung on a ladder). The light part wants to become an atom with 52–56 neutrons (another stable spot).

  • Result: The atoms get "stuck" in these stable shapes, but because they are so rigid, they snap apart quickly. It's like trying to bend a stiff metal rod; it resists bending and then snaps. This happens fast, and the atoms don't fuse.

• The Tip Collision (The Bouncy One): When they hit tip-to-tip, these "magnetic" pulls are much weaker. The atoms don't get stuck in those specific stable shapes as easily. They exchange fewer particles and the interaction is less dramatic.

The Takeaway: The "side" crashes are actually the ones that dominate the failure rate because the atoms get too attracted to their own internal stability and refuse to merge into a new, bigger element.

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Key Discovery 2: The Speed of the Crash (Energy Matters)

The scientists also changed the speed of the crash (the energy). They found that the rules of the game change depending on how hard you hit.

• Low Energy (The "Octupole" Zone): At certain speeds, the atoms act like they are wearing a specific type of armor (octupole deformation). They stabilize in a weird, pear-like shape.
• High Energy (The "Spherical" Zone): As you hit harder, that weird armor melts away, and the atoms start acting like perfect spheres again, seeking the stable "82 proton" shape.
• The "Sweet Spot": There is a specific range of energy (around 226 MeV) where these "magnetic" pulls seem to turn off or get confused.

Analogy 2: The Dance Floor
Imagine the atoms are dancers.

• At low energy, they are doing a slow, rigid waltz where they get stuck in specific poses (shells).
• At high energy, they are doing a fast, chaotic dance where they spin into different poses.
• At the sweet spot, the music stops, the rigid rules disappear, and the dancers might actually have a chance to hold hands and merge into a new couple (fusion) instead of spinning apart.

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Why Does This Matter?

For years, scientists have been trying to build Element 119, but the "Quasi-fission" (the shattering) has been winning every time. The yield (the number of new atoms created) is so low it's almost zero.

This paper suggests a new strategy: Don't just pick any speed.

If we can tune our particle accelerator to hit that "sweet spot" energy where the atoms' internal "magnetic" pulls are suppressed, we might be able to reduce the shattering. If we reduce the shattering, the atoms have a better chance of sticking together and fusing into the new, super-heavy element.

Summary in One Sentence

By simulating how two atoms crash at different angles and speeds, this study found that the atoms' internal "stability zones" usually cause them to shatter, but if we hit them at just the right speed, we might be able to trick them into fusing instead.

Technical Summary

Here is a detailed technical summary of the paper "Microscopic quasifission dynamics of the 54Cr + 243Am reaction":

1. Problem Statement

The synthesis of superheavy elements (SHEs) beyond Oganesson ($Z=118$), specifically elements 119 and 120, faces a critical bottleneck: the quasifission (QF) process. In heavy-ion fusion reactions, the captured dinuclear system often re-separates into two fragments before reaching a fully equilibrated compound nucleus. This QF channel dominates over fusion, drastically suppressing the evaporation-residue cross-sections to the femtobarn level or lower.

The specific reaction system $^{54}\text{Cr} + ^{243}\text{Am}$ is a primary candidate for synthesizing element 119. However, predicting the optimal experimental conditions is challenging because QF dynamics are governed by a complex interplay of:

• Entrance channel degrees of freedom (collision energy, impact parameter).
• Nuclear deformation and orientation of the reactants.
• Quantum shell effects in the emerging fragments.
• Neutron-to-proton asymmetry.

Existing macroscopic models often lack the microscopic detail required to disentangle these factors, necessitating a fully microscopic approach to guide future experiments at facilities like the upgraded HIRFL-CAFE2.

2. Methodology

The authors employed the Time-Dependent Hartree-Fock (TDHF) theory, a fully microscopic, self-consistent framework that evolves the nuclear many-body wave function without empirical reaction parameters.

• Energy Density Functional: Calculations utilized the Skyrme SLy5 parametrization. Crucially, the authors implemented a modified version of the Sky3D code to explicitly include the squared spin-current tensor density ($J^2$ terms), ensuring consistency with the SLy5 functional which is often omitted in standard implementations.
• System Setup:
  • Nuclei: Deformed projectile $^{54}\text{Cr}$ ($\beta_{20} \approx 0.218$) and target $^{243}\text{Am}$ (complex deformation including octupole $\beta_{30}$ and hexadecapole $\beta_{40}$ components, resulting in a pear-like shape).
  • Initial Configurations: Systematic simulations covered a broad range of initial orientations:
    • Projectile: Tip ($\theta_P = 0^\circ$) and Side ($\theta_P = 90^\circ$).
    • Target: Nine orientations ($0^\circ$to$180^\circ$in$30^\circ$ steps) to account for the target's reflection symmetry breaking.
  • Energy Range: Simulations spanned incident center-of-mass energies ($E_{c.m.}$) from below to well above the Coulomb barrier ($200$–$350$ MeV).
• Dynamics: The evolution was tracked until the fragments separated ($>26$ fm). Key observables included fragment mass/charge distributions, scattering angles, and contact time (defined by neck density $> 0.03 \text{ fm}^{-3}$).

3. Key Contributions

• Microscopic Mechanism of Orientation Dependence: The study provides the first comprehensive microscopic mapping of how projectile orientation dictates the QF pathway in the $^{54}\text{Cr} + ^{243}\text{Am}$ system.
• Shell Effect Dynamics: It elucidates the transition between different shell-driven regimes (octupole-deformed vs. spherical) as a function of incident energy.
• Contact Time Anomaly: The paper identifies a counter-intuitive relationship between nucleon transfer and interaction time near magic numbers, attributing it to the "stiffness" of shell-stabilized fragments.
• Energy Optimization Strategy: It proposes a method to identify "optimal" incident energies where shell effects suppressing fusion are minimized.

4. Key Results

A. Orientation Dependence (Fixed Energy $E_{c.m.} = 251.9$ MeV)

• Side Collisions ($\theta_P = 90^\circ$): These dominate the total QF yield. The fragment distributions are strongly driven by shell effects:
  • Heavy fragments accumulate around the spherical shell closure $Z=82$.
  • Light fragments are anchored by the deformed shell gap in the $N=52\text{--}56$ region.
  • Contact Time: As the heavy fragment approaches $Z=82$, the average contact time decreases or plateaus, contrary to classical diffusion expectations (which predict longer times for more mass transfer). This is attributed to the enhanced rigidity of the shell-stabilized fragment, which accelerates neck rupture.
• Tip Collisions ($\theta_P = 0^\circ$): These exhibit significantly weaker shell influence. The nucleon transfer is lower, and the fragment yields are dispersed in the intermediate region between shell closures and quasielastic peaks.

B. Energy Dependence (Tip-Tip Configuration)

The study reveals a complex, non-monotonic evolution of QF dynamics with increasing energy:

1. Low Energy ($< 218$ MeV): Dominated by quasielastic scattering.
2. Near-Barrier ($218\text{--}226$ MeV): Sharp increase in nucleon transfer.
3. Intermediate Energy ($226\text{--}240$ MeV): A reversal occurs where nucleon transfer decreases.
4. **Octupole-Stabilized Regime ($240\text{--}280$MeV):** The system stabilizes around **$Z \approx 88$and$N \approx 136$** (corresponding to$^{224}\text{Ra}$), indicating dominance by octupole shell gaps.
5. High Energy ($> 280$ MeV): The system transitions to a spherical shell-driven regime. Heavy fragments shift to $Z=82$ and light fragments to $Z=36, N=52$. This suggests that thermal damping at higher excitation energies suppresses the octupole effects, allowing the more robust spherical shell closures to dictate the dynamics.

C. Suppressed Shell Influence

A critical finding is the existence of specific energy windows (e.g., around $E_{c.m.} \approx 226$ MeV for tip-tip collisions) where the steering influence of shell effects appears suppressed. In these windows, the competition between QF and fusion may be more favorable for fusion.

5. Significance and Conclusion

This work demonstrates that the manifestation of shell effects in QF is not static but a dynamical outcome sensitive to both initial geometry and incident energy.

• Experimental Guidance: The results suggest that synthesizing element 119 can be optimized by carefully selecting the incident energy to avoid windows where strong shell effects (like the octupole stabilization at $Z \approx 88$) drive the system toward rapid QF.
• Strategic Insight: By operating in energy windows where shell influence is suppressed, experimentalists may reduce the QF barrier, thereby enhancing the fusion probability and improving the prospects for discovering new superheavy elements.
• Theoretical Advancement: The study validates TDHF as a predictive tool for disentangling the complex interplay of deformation, orientation, and quantum shell effects in heavy-ion reactions, providing a roadmap for future microscopic investigations of SHE synthesis.