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—— IWND 2026

Presenter ：Yu Yang

Supervisor ：Long Zhu

Collaborators：Fengshou Zhang，Zhong Liu，Cheng Li，Dongsheng Hou

Sino-French Institute of Nuclear Engineering and Technology

SHANXI· TAIYUAN

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The challenge of synthesizing new nuclides

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MNT reaction is an effective way to produce new neutron-rich nuclides.

MNT

Fusion

Theory predicts ~7,000 nuclides (Ref: Erler et al.,Nature 486 (2012) 509 )
~3,300 nuclides have been discovered (M. Thoennessen, Discovery of isotopes project )

136Xe+198Pt @ 7.98MeV/A

208Pb+9Be @ 1GeV/A

PRL 115,

172503 (2015)

Yu. T. Oganessian, et al., Phys. Scr., 92:023003 (2017)

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MNT: Low separation and detection efficiency

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¹³⁶Xe+¹⁹⁸Pt

²⁰⁰Os

Nucl. Phys. Rev. 34, 409 (2017)

Y.X. Watanabe’s talk

Analyze product emission characteristics

Optimize separation and identification
Guide targeted, innovative experiments

Angular acceptance：MNT ≪ Fusion (~30%)

Phys. Part. Nuclei Lett. 19, 693 (2022)

Similar product characteristics.

Covering a wide range of cone angles.

Although multinucleon transfer reactions show great potential, their complex product distributions also bring significant experimental challenges.

First, the excitation functions of transfer reactions are usually broad. This means that, at the same incident energy, a wide range of neighboring isotopes can be produced, making the separation of the target nuclei very challenging. The proportion of 202os is only 0.3%, and the rest are pollutants.

Second, because the angular distributions of the products are broad, the angular acceptance of in-flight separators is usually only a few percent. In comparison, fusion reactions can achieve angular acceptances of up to around 30%.

Under these circumstances, theoretical simulations of product emission characteristics are not only helpful for optimizing nuclear separation and identification strategies, but may also inspire more targeted and innovative experimental designs.

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Motivations for the experiment

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¹³⁶Xe + ²³⁸U

²³⁸U + ²³⁸U

²³⁸U + ²⁴⁸Cm

²³⁸U + ²³⁸U

⁸⁴Kr, ¹³⁶Xe +

¹⁶⁵Ho, ²⁰⁹Bi

Z. Physik A 292, 171-189 (1979)

Eur. Phys. J. A 58, 114 (2022)

4π Coverage!

~ 4 levels

Nucleon diffusion ↑

In early experiments, multinucleon transfer reactions were performing using radiochemical methods. This approach takes advantage of the ability to collect reaction products over nearly the full solid angle.

Experimental results showed that, for producing actinide nuclei, a heavy system such as 238U+248Cm has production cross sections several orders of magnitude higher than the 136Xe+238U system. This demonstrates its great potential for synthesizing neutron-rich actinide nuclei.

However, no new isotopes were discovered in these experiments. The main reason is that radiochemical methods can only detect products with half-lives longer than several minutes, while many unknown nuclei may have lifetimes of only a few seconds or even shorter.

Therefore, some methods that are more sensitive to time need to be adopted. In fact, there have been some related attempts in experiments.

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Motivations for the experiment

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SHIP (GSI)

Designed for fusion reactions

⁴⁸Ca + ²⁴⁸Cm

！Limited theory

hinders wider use

Discrepancy up to a factor of ~10

Angular acceptance ~ 20 msr

Operational only near 0°

In recent years, some experiments have introduced separators originally developed for fusion reactions into multinucleon transfer studies.

Although these separators have relatively small angular acceptance and are not fully compatible with the broad angular distributions of transfer reactions, they offer advantages such as low background and high resolution. Using these devices, several new transuranium isotopes have already been successfully identified.

However, most of these separators have a limited angular acceptance around zero degrees, making theoretical extrapolation essential for extracting total production cross sections from the measured data.

Unfortunately, the small-angle region, particularly near zero degrees, has received relatively little attention in previous theoretical studies, leading to considerable uncertainties in such extrapolations. These uncertainties, in turn, limit the broader application of these separators.

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The research content of this work

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¹³⁶Xe + ²³⁸U

²³⁸U + ²³⁸U

²³⁸U + ²⁴⁸Cm

238U + 248Cm offers strong potential for producing neutron-rich actinides and superheavy nuclei

Small angular acceptance, but excellent background suppression and high sensitivity
Limited theory constrains wider use of the device

Assess emission characteristics and detectability of products near 0°
Guide forward-angle experiments and model refinement

Y. Yang, L. Zhu*, et al., Physics Letters B 862, 139318 (2025)

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Quantum Molecular Dynamics

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Nucleon trial wave function (Gaussian):

Time-dependent variational principle:

Physics Reports 202, 233 (1991)

Hamiltonian:

Phase-space occupation constraint:

Phys. Rev. C 64, 024612 (2001)

The theoretical model used in this work is the Quantum Molecular Dynamics (QMD) model.

In this model, the trial wave function of each nucleon is represented by a Gaussian wave packet, which is characterized by two parameters: the coordinate center and the momentum center.

The equations of motion can be derived using the time-dependent variational principle, and their form is similar to the classical canonical equations.

By performing event-by-event simulations with the QMD model, we can obtain information about each reaction product, including its proton number, mass number, emission angle, and kinetic energy.

By accumulating a large number of events, statistical quantities such as angular distributions and production cross sections can be calculated.

For the de-excitation process of the primary fragments, the Gemini++ code is coupled with QMD to describe the subsequent decay process.

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Validation of the QMD Model

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Model reproduces data trends

Exp: Phys. Rev. C 88, 054615 (2013)

Y. Yang, L. Zhu*, et al., Physics Letters B 862, 139318 (2025)

Neutron-rich yields vary

Product angular distributions

To validate the reliability of the model, we simulated the product distributions of the 238U+248Cm system at an incident energy of 898 MeV and compared the results with experimental data.

As shown here, the model can reasonably reproduce the overall trend of the experimental distribution, indicating that it has a certain capability to describe this reaction system.

It is worth noting that the experimental incident energy is much higher than the Bass barrier.

For actinide nuclei with relatively low fission barriers, the excitation energy plays a crucial role. The excitation energy is directly influenced by the incident energy.

Therefore, we also calculated the cross-section distributions at 800 MeV.

The results show that the maximum production cross section of the final products is not very sensitive to the incident energy.

However, the yield of neutron-rich nuclei increases significantly as the incident energy decreases.

This indicates that lower incident energies are more favorable for producing neutron-rich actinide nuclei.

This naturally raises an important question:

At what angles are these products emitted?�Is it possible to detect them near zero degrees in the laboratory frame?

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Studies of Actinide Production near 0°

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+6p

Langevin model

Phys. Rev. C 83, 044618 (2011)

+4p

PLF

TLF

QMD model

To answer this question, we take fermium isotopes as an example and present their angular distributions at different incident energies.

As shown in the figure, the angular distribution changes significantly with incident energy.

At 898 MeV, the initial distribution of the products is approximately isotropic. However, after the de-excitation process, it evolves into a single-peaked distribution concentrated at larger angles.

The main reason is that only peripheral collisions can maintain relatively low excitation energies, allowing the products to survive during de-excitation.

At 800 MeV, the overall excitation energy is reduced. Therefore, even products emitted at smaller angles can survive effectively.

As a result, the angular distribution becomes broader, and a clear enhancement appears near zero degrees.

It is worth mentioning that this zero-degree enhancement trend is also observed in other theoretical models, suggesting that it maybe a model-dependent phenonmenon.

Overall, with an appropriate incident energy, performing multinucleon transfer experiments near zero degrees is experimentally feasible, and the produced nuclei are more likely to be neutron-rich.

This provides important guidance for extending fusion-evaporation separators to multinucleon transfer reactions.

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Studies of Actinide Production near 0°

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The fraction emitted near θ_lab = 0° increases with atomic number

Different theoretical models yield similar results

Physics Letters B 862, 139353 (2025)

Relative detection probability

To further understand the properties of products emitted near zero degrees, we present an analysis based on the normalized yield.

This quantity is obtained by integrating the angular distribution within the range of 0 to 6 degrees, and it represents the relative probability of detecting a certain isotope within this angular region.

As shown in the figure, this ratio increases gradually with increasing proton number.

In other words, heavier nuclei have a stronger tendency to be emitted in the forward direction near zero degrees.

This can be understood as follows:

The zero-degree region corresponds to more central collisions, where the interaction time is longer. Therefore, more extensive multinucleon exchange can occur, leading to the formation of products with higher proton numbers.

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Validation of the QMD Model near 0°

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Reproduces forward-angle data
Underestimates forward yields

V. F. Comas, S. Heinz. et al., Eur. Phys. J. A 49, 112 (2013).

H. M. Devaraja, S. Heinz . et al., Eur. Phys. J. A 56, 224 (2020).

Langevin

⁶⁴Ni + ²⁰⁷Pb @ E_c.m. = 244.4 MeV

QMD

~0° MNT experiment at SHIP (GSI)

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Production of actinide nuclei near 0°

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Products emitted at θ_lab = 0–6° reach unknown neutron-rich actinides with cross sections above 1 nb
Existing in-flight separators could detect new actinide nuclides in MNT experiments

Y. Yang, L. Zhu*, et al., Physics Letters B 862, 139318 (2025)

Predicted cross sections of forward-angle products

Known boundary

Drip line

N

Z

HIAF can deliver the world’s highest-intensity ²³⁸U beam

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Summary

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This work investigates forward-angle heavy products from the ²³⁸U + ²⁴⁸Cm reaction, with particular emphasis on emission near θ_lab = 0°.
At E_c.m. = 800 MeV, yields at θ_lab = 0° are enhanced, and production cross sections of some unknown neutron-rich actinides reach the nanobarn scale.
These results provide a theoretical basis for optimizing MNT experiments and demonstrate the potential of gas-filled recoil separators and velocity selectors for MNT studies.

Thank you for your attention!