Recent experimental studies have reported indications of tetraneutrons in bound and resonant states, the latter being a transient state but with a duration sufficient for experimental detection. However, theoretical analyses suggest that tetraneutrons are unlikely to exist in a bound state under our current understanding of two or three-body nuclear forces.

In response to this conundrum, Associate Professor Hiroyuki Fujioka from Tokyo Institute of Technology spearheaded a research team to examine the possibility of bound tetraneutron emission. Their findings, published in Physical Review C, delve into the potential emission rate of particle-stable tetraneutrons via thermal neutron-induced fission of Uranium-235 (235U) in a nuclear reactor.

Dr. Fujioka elaborated on their approach, stating, "We are aware from previous literature that the dominant thermal fission process for 235U is binary fission, which leads to the emission of two heavy nuclear fragments together with 2.4 neutrons, on average. But there is a 0.2% probability of ternary fission, in which light nuclear fragments are emitted. We, therefore, chose this route for our experiment under the assumption that the hypothetically bound tetraneutron could be a ternary particle in uranium fission."

To investigate this, the team utilized the well-established instrumental neutron activation analysis method. This involved irradiating a target sample of 88SrCO3 for two hours at a thermal power of 5 MW in a nuclear research reactor. The team aimed to detect signals corresponding to a potential tetraneutron emission through X-ray spectroscopy of the irradiated sample.

The hypothesis was that 88Sr nuclei would transform into 91Sr, with a Q value of 20 MeV minus the binding energy of the tetraneutron. The instability of 91Sr, leading to its radioactive decay and subsequent X-ray release, would signal the emission of particle-stable tetraneutrons.

However, the spectroscopy results did not reveal any signs indicative of 91Sr formation. Consequently, the researchers estimated that if particle-stable tetraneutrons do exist, their emission rate is likely lower than 8 + 10^-7 per fission, with a 95% confidence level. They also pointed out that refining sample purity and enhancing experimental sensitivity could improve the detection of subtle tetraneutron signals.

"Our study showed that the instrumental neutron activation method in radiochemistry can be applied to address the open question in nuclear physics. We will improve the sensitivity further to seek for the elusive, charge-neutral system," Dr. Fujioka commented on the implications of their research.

While the team was unable to detect bound tetraneutrons, their investigation has established a solid foundation for future research into tetraneutrons and similar systems. This work underscores the ongoing efforts in nuclear physics to explore the boundaries of our understanding of atomic nuclei and the forces that govern them.

Research Report:Search for particle-stable tetraneutrons in thermal fission of 235U

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