This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility:


peer-reviewed publication

trusted source


Hunting for elusive tetraneutrons with thermal fission

Hunting for the elusive tetraneutrons with thermal fission
Credit: Physical Review C (2023). DOI: 10.1103/PhysRevC.108.054004

The possible emission rate of particle-stable tetraneutron, a four-neutron system whose existence has been long debated within the scientific community, has been investigated by researchers from Tokyo Tech. They looked into tetraneutron emission from thermal fission of 235U by irradiating a sample of 88SrCO3 in a nuclear research reactor and analyzing it via γ-ray spectroscopy.

Tetraneutron is an elusive atomic nucleus consisting of four neutrons, whose existence has been highly debated by scientists. This stems primarily from our lack of knowledge about systems consisting of only neutrons, since most are usually made of a combination of protons and neutrons. Scientists believe that the experimental observation of a tetraneutron could be the key to exploring new properties of atomic nuclei and answering the age-old question: Can a charge-neutral multineutron system ever exist?

Two recent experimental studies reported the presence of tetraneutrons in bound state and resonant state (a state that decays with time but lives long enough to be detected experimentally). However, indicate that tetraneutrons will not exist in a bound state if the interactions between neutrons are governed by our common understanding of two or three-body nuclear forces.

Intrigued, a team of researchers led by Associate Professor Hiroyuki Fujioka from Tokyo Institute of Technology set out to investigate the feasibility of bound tetraneutron emission. In their recent study published in Physical Review C, the team explored the possible emission rate of particle-stable tetraneutron via thermal -induced fission of 235U (Uranium-235) in a nuclear reactor.

"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," explains Dr. Fujioka.

The team adopted the well-known instrumental neutron activation analysis method, where a trace element in a chosen sample is irradiated and activated by the capture of thermal neutrons. For this study, 88SrCO3 was chosen as the target sample and was irradiated for two hours at a thermal power of 5 MW in a nuclear research reactor. The team also performed γ-ray spectroscopy for the irradiated sample to detect signals corresponding to a possible tetraneutron emission.

The 88Sr nuclei were expected to convert into 91Sr with a Q value (change in mass between the initial and final states of a reaction expressed in terms of energy units) of 20 MeV minus the binding energy of the tetraneutron. Since 91Sr is unstable, its followed by the release of γ-rays would indicate the emission of particle-stable tetraneutrons.

The γ-ray spectroscopy results for the irradiated 88Sr sample, however, did not show any photopeak corresponding to the formation of 91Sr. Based on this, the team estimated that if particle-stable tetraneutrons exist, their emission rate might be lower than 8 × 10-7 per at the 95% confidence level. They also suggested that improving the purity of samples and increasing the sensitivity of experimentation could help with the detection of subtle signals arising from tetraneutrons.

Dr. Fujioka says, "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."

While the team was not able to detect bound tetraneutrons, their work has laid a solid framework for future studies on the elusive tetraneutrons and other such systems.

More information: Hiroyuki Fujioka et al, Search for particle-stable tetraneutrons in thermal fission of U235, Physical Review C (2023). DOI: 10.1103/PhysRevC.108.054004

Journal information: Physical Review C

Citation: Hunting for elusive tetraneutrons with thermal fission (2024, January 4) retrieved 17 July 2024 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Explore further

Chi-Nu experiment ends, bolsters nuclear security and energy reactors


Feedback to editors