How long do free neutrons exist before they decay? This question has sparked much debate, as different measurement methods yield varying results. A potential new explanation has been offered: all observed results can be accounted for by the existence of different neutron states, each with its own lifetime.
Neutrons are essential components of matter. Within a stable atomic nucleus, they can survive indefinitely. However, free neutrons exist for much shorter periods — typically about fifteen minutes on average before decaying.
Interestingly, contradictory results regarding the average lifespan of free neutrons have arisen based on the method of measurement, whether in a neutron beam or stored in a ‘bottle’. A research team at TU Wien has proposed a possible explanation: there might be formerly unrecognized excited states of neutrons. This suggests that some neutrons could be in states of slightly higher energy and possess different decay times. Such a hypothesis could clarify the previously noted discrepancies. The team has also started exploring methods to detect these neutron states.
Two measurement approaches, two outcomes
Neutrons have the capability to decay spontaneously, in line with quantum theory. This process transforms a neutron into a proton, an electron, and an antineutrino. This decay is particularly likely to occur with free neutrons, while neutrons that combine with other particles can remain stable within atomic nuclei.
Measuring the average lifespan of free neutrons is surprisingly complex. “For nearly thirty years, physicists have grappled with contradictory findings on this issue,” remarks Benjamin Koch from the Institute of Theoretical Physics at TU Wien, who examined this conundrum alongside his colleague Felix Hummel. They are collaborating closely with Hartmut Abele’s neutron research team at the Atomic Institute at TU Wien.
Typically, a nuclear reactor is used as the source of neutrons for such measurements. “Free neutrons are created through radioactive decay in the reactor,” Benjamin Koch elaborates. “These free neutrons can be directed into a neutron beam for precise measurement.” Researchers measure the initial neutron count in the beam and how many protons result from decay. By accurately determining these values, the average lifespan of the neutrons in the beam can be calculated.
Alternatively, neutrons can be stored in a type of ‘bottle’, often using magnetic fields. “This method indicates that neutrons from the neutron beam have an average lifespan about eight seconds longer than those in a bottle,” notes Benjamin Koch. “An average lifespan of just under 900 seconds indicates a significant difference — too large to be simply due to measurement errors.”
An undiscovered state?
Benjamin Koch and Felix Hummel have now demonstrated that this inconsistency may be explained by the concept of excited neutron states — previously unidentified states with slightly greater energy. Such excited states are known in atomic physics and are fundamental to lasers, for example. “Determining such states for neutrons is decidedly more challenging,” Benjamin Koch states. “Nevertheless, we can hypothesize the characteristics they ought to possess to account for the varying neutron lifespan measurement results.”
The researchers propose that when free neutrons emerge from radioactive decay, they exist in a mix of different states: some are ordinary neutrons in their ground state, while others exist in an excited state with slightly more energy. Over time, these excited neutrons transition to their ground state. “It’s akin to a bubble bath,” Felix Hummel explains. “By adding energy and agitating it, you create a lot of bubbles — effectively putting the bubble bath into an excited state. However, given time, the bubbles pop, and the bath returns to its initial state.”
If this theory regarding excited neutron states holds true, it implies that a neutron beam contains a variety of different neutron states in notable quantities, while a bottle primarily contains ground-state neutrons. After all, the process of capturing neutrons in a bottle requires time, allowing most to revert to their ground state.
“Our model suggests that a neutron’s decay probability is significantly influenced by its state,” claims Felix Hummel. Consequently, this leads to differing average lifetimes for neutrons in the neutron beam versus those in the neutron bottle.
Need for further experiments
“Our model indicates the parameters we need to explore,” Benjamin Koch comments. “The decay time of the excited state must be under 300 seconds, or else we can’t explain the discrepancy. It also needs to exceed 5 milliseconds, or the neutrons would revert to their ground state before reaching the beam experiment.”
The theory regarding the existence of undiscovered neutron states can be investigated by re-evaluating data from prior experiments. However, additional experiments will be required for robust confirmation. Such experiments are already being planned. The researchers are collaborating with teams at TU Wien’s Institute for Atomic and Subatomic Physics, as their PERC and PERKEO experiments align well with this endeavor. Research groups from Switzerland and Los Alamos, USA, have also expressed interest in employing their measurement techniques to verify this new hypothesis. Technically and conceptually, the necessary measurements are feasible, raising hopes that answers to this long-standing physics dilemma may soon be revealed.
