How long does a neutron live?
The neutron lifetime is so fundamental and important to understand
The difference of 8-9 seconds is four times greatermeasurement error of two seconds. The chance that they agree with each other is about 60 in 1 million, which is practically impossible. These seconds constitute the mystery of the neutron lifetime.
Two methods, two results
So, scientists used two methods to determine the life of a neutron. How do they work?
- Bottle method
In the bottle method, neutrons can besealed in a vacuum bottle made of neutron-safe material or held by magnetic fields and gravity. They have extremely low kinetic energy and move at a speed of several meters per second. They are called ultracold neutrons (UCNs). Physicists separate neutrons from the nuclei of atoms, put them in a bottle, and then count how many of them remain there after a while. As a result, scientists conclude that neutrons decay radioactively in an average of 14 minutes and 39 seconds.
- Ray method
Radiation experiments use machineswhich create neutron fluxes. Scientists measure the number of neutrons in a certain volume of the beam. They then direct the flow through a magnetic field into a particle trap formed by the electric and magnetic fields. The neutrons decay in a trap, where physicists measure the number of protons remaining. In such experiments, they determine the average neutron lifetime at 14 minutes 48 seconds.
results
There are seven results so farhigh-precision bottle measurements with different settings and only two - beam measurements. In both beam measurements, the same method was used - the Penning trap. The decay product, protons, is captured by it and counted by a well-calibrated detector.
The Penning trap itself representsis a device that uses a uniform static magnetic field and a spatially inhomogeneous electric field to store charged particles. This type of trap is often used to make precise measurements of the properties of ions and stable subatomic particles that have an electrical charge.
There is no doubt that more experiments are required for comparison and verification, not only with the beam, but in general.
Are there other ways?
In the beam method, physicists determine how muchneutrons undergo beta decay. Let us recall that neutron beta decay is the spontaneous transformation of a free neutron into a proton with the emission of a β-particle (electron) and an electron antineutrino.
Precision measurements of beta decay parametersneutron (lifetime, angular correlations between particle momenta and neutron spin) are important for determining the properties of the weak interaction. This is a fundamental interaction, responsible in particular for the processes of beta decay of atomic nuclei and weak decays of elementary particles, as well as violations of the laws of conservation of spatial and combined parity in them. This interaction is called weak, since the other two interactions, significant for nuclear physics and high-energy physics (strong and electromagnetic), are characterized by much greater intensity. However, it is much stronger than the fourth of the fundamental interactions, gravitational.
Antineutrino detection is difficult.The world's leading detectors are often gigantic and target an intense source of flux such as the Sun or a nuclear power plant. However, only a few events happen in a year. So antineutrino won't help here.
What about the proton?Until now, all results with the best accuracy in the ray method have been obtained by registering protons. Now active work is underway to improve the method. For example, a modernized BL3 experiment is under preparation at NIST, USA. Researchers at J-PARC recently announced their preliminary neutron lifetime result by detecting beta decay electrons using a time projection chamber (TPC). Such chambers are a combination of drift and proportional chambers. They are the most versatile instrument in high-energy physics, since they allow one to obtain a three-dimensional electronic image of a track with a comparable spatial resolution in all three coordinates. The work of Japanese scientists is a revival of an experiment first proposed by Kossakowski et al. In 1989. They are now working to improve its accuracy.
After decades of effort, it can be assumed that all possible pathways of the ray method should be carefully investigated.
Or are there more options?
Superfluid helium time
Recently, in his article “New experimenton the lifetime of a neutron with the decay of a beam of cold neutrons in superfluid helium-4,” published in the Journal of Physics G: Nuclear and Particle Physics, Dr. Wanchun Wei proposed a new approach. Namely, to use a superfluid helium-4 scintillator to detect the decay product of a neutron—an electron. The author of the study received his PhD in physics from Brown University, USA and completed his postdoctoral fellowship at Los Alamos National Laboratory. He currently works as a research engineer at the Kellogg Radiation Laboratory, California Institute of Technology, USA (Caltech).
Experiment at UNCtau at Los Alamos using the bottle method to measure the neutron lifetime
Wei's idea sounds unusual, and here's why.
Most lifetime experimentsneutrons are carried out in a high vacuum to eliminate the scattering of neutrons on gas particles. An exception is the J-PARC experiment, where the TPC requires a working gas to amplify the beta decay charge of an electron to a detectable current. Sophisticated analysis is required to identify and eliminate background events caused by scattered neutrons.
The new method will work thanks to amazingproperties of superfluid helium, quantum liquid. It forms a macroscopic quantum wave function, and most of it condenses into the ground state. Elementary excitations in a quantum fluid were predicted by Landau in 1947 and confirmed by inelastic neutron scattering.
The peculiarity of superfluid helium-4 is that it flows without friction over any surface, flows through very small pores, obeying only its own inertia.
Liquid helium is in a superfluid phase.While it remains superfluid, it creeps along the wall of the cup in a thin film. It descends from the outside, forming a drop that will fall into the liquid below. Another drop will form — and so on until the cup is empty
If passing a neutron beam through a gas is problematic, why consider a liquid?
Yes, neutrons are scattered in superfluid helium,but only on elementary excitations. And the condition of conservation of energy and momentum must be met. Cohen and Feynman showed in their article published in 1957 that scattering does not occur if the neutron wavelength exceeds 16.5 angstroms. This means that low-energy, long-wavelength neutrons can travel through superfluid helium-4 as if it were a vacuum. In turn, this confirms the proposal for a new beam experiment with a superfluid helium-4 scintillator.
Superfluid helium-4 as a scintillator
The first scintillation detector wasa screen covered with a layer of zinc sulfide (ZnS). The flashes that occurred when charged particles hit it were recorded using a microscope. It was with such a detector that Geiger and Marsden conducted an experiment in the scattering of alpha particles by gold atoms in 1909, which led to the discovery of the atomic nucleus. Since 1944, light flashes from the scintillator have been recorded by photomultiplier tubes (PMTs). Later, photodiodes were also used for these purposes.
The scintillator can be organic (crystals, plastics or liquids) or inorganic (crystals or glasses). Gaseous scintillators are also used.
Superfluid helium-4 is well studied as a candidateto the scintillation detector of neutrinos and dark matter. When charged particles with high kinetic energy collide with superfluid helium-4, the helium atoms are ionized, excited and emitted scintillation light. The process is quite complex, but in general, the number of emitted photons is linearly proportional to the energy of a charged particle. The released electron carries kinetic energy in the range from zero to 782 keV from the released nuclear energy in beta decay. Thus, the number of decayed neutrons can be calculated from the scintillation frequency.
In the meantime, it is necessary to control the neutron fluxpulsed beam. This can be done with the isotope helium-3, which captures a neutron, turns into a proton and a triton, and releases 764 keV of energy. The rate of such capture events is proportional to the beam flux. These events represent the kickback of cores. On the contrary, decay is the donation of electrons. Consequently, capture and fade events have a different set of signatures in the scintillation signal. In an instant glow, a capture event produces far fewer photons per unit of energy input than a decay event. The capture event has a short stopping range of tens of microns, while the decay event has a long trail of up to 2 cm. By analogy, one looks like a supernova, and the other like a meteor. In addition, they have a distinct behavior in the decay rate of the persistence.
Ultimate accuracy
The key to solving the mystery of the neutron lifetime is high accuracy. A new experiment only makes sense if the accuracy can reach 0.1% or less than 1 second.
It is almost impossible to register allbeta decay electrons, because some of them have too low an energy to obtain adequate scintillation light. But there is a way out. On the one hand, the proposed detector will provide positional resolution along the beam axis. Only events in the central area will be used for highly accurate data analysis. On the other hand, you can collect as much light as possible. The detector is designed to cover more than 96% of the solid angle of events in the central region, so that the energy of beta decay electrons can be accurately recovered. A large number of these events make up the exact β-decay spectrum, which is well described by Fermi theory. The lower part of the spectrum may be missing due to low flicker.
In addition, suppression of background events is important,especially related to scattered neutrons. The absence of neutron beam scattering by superfluid helium is already a good start. All parasitic neutrons scattered from the volume windows will be captured by neutron absorbers surrounding the detector to minimize neutron activation.
The detector will also see Comptonevents caused by instantaneous emission of gamma radiation during the capture of neutrons at the entrance and exit windows. It will appear as two bright bursts in a time sequence and can be used as a time and intensity reference to reconstruct the position of signal events, calibrate the detector, and characterize the spectrum of the beam.
What's the bottom line?
This new method is fundamentally different fromexisting beam experiments. Does not require a strong magnetic field. It uses a pulsed beam with much lower energy neutrons. And the superfluid helium scintillation detector offers a clear set of systematic effects. Of course, there are many technical difficulties to overcome. In his article describing the new approach, Wei, an experimenter in the study of particles in superfluid helium, said that he was confident that the new idea would ultimately help solve the mystery of the neutron lifetime and provide new opportunities for discovering new physics.
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US National Institute of Standards and Technology
J-PARC - proton accelerator complex forneeds of high-energy physics, hadronic and neutrino physics, materials science. Located near Tokai, Japan, a joint project of the KEK National High Energy Physics Laboratory and the JAEA Atomic Energy Agency.
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