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  • richardmitnick 12:29 pm on January 15, 2019 Permalink | Reply
    Tags: , NPDGamma Experiment, , , , Precision experiment first to isolate measure weak force between protons and neutrons, Spallation neutron source   

    From Oak Ridge National Laboratory: “Precision experiment first to isolate, measure weak force between protons, neutrons” 

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    From Oak Ridge National Laboratory

    December 19, 2018
    Sara Shoemaker, Communications
    shoemakerms@ornl.gov
    865.576.9219

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    Scientists analyzed the gamma rays emitted during the NPDGamma Experiment and found parity-violating asymmetry, which is a specific change in behavior in the force between a neutron and a proton.

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    They measured a 30 parts per billion preference for gamma rays to be emitted antiparallel to the neutron spin when neutrons are captured by protons in liquid hydrogen. After observing that more gammas go down than up, the experiment resolved for the first time a mirror-asymmetric component or handedness of the weak force. Credit: Andy Sproles/Oak Ridge National Laboratory, U.S. Dept. of Energy.

    A team of scientists has for the first time measured the elusive weak interaction between protons and neutrons in the nucleus of an atom. They had chosen the simplest nucleus consisting of one neutron and one proton for the study.

    Through a unique neutron experiment at the Department of Energy’s Oak Ridge National Laboratory, experimental physicists resolved the weak force between the particles at the atom’s core, predicted in the Standard Model that describes the elementary particles and their interactions.

    Their result is sensitive to subtle aspects of the strong force between nuclear particles, which is still poorly understood.

    The team’s observation, described in Physical Review Letters, culminates decades of work performed with an apparatus known as NPDGamma. The first phase of the experiment took place at Los Alamos National Laboratory. Building on the knowledge gained at LANL, the team moved the project to ORNL to take advantage of the high neutron beam intensity produced at the lab’s Spallation Neutron Source.

    ORNL Spallation Neutron Source


    ORNL Spallation Neutron Source

    Protons and neutrons are made of smaller particles called quarks that are bound together by the strong interaction, which is one of the four known forces of nature: strong force, electromagnetism, weak force and gravity. The weak force exists in the tiny distance within and between protons and neutrons; the strong interaction confines quarks in neutrons and protons.

    The weak force also connects the axial spin and direction of motion of the nuclear particles, revealing subtle aspects of how quarks move inside protons and neutrons.

    “The goal of the experiment was to isolate and measure one component of this weak interaction, which manifested as gamma rays that could be counted and verified with high statistical accuracy,” said David Bowman, co-author and team leader for neutron physics at ORNL. “You have to detect a lot of gammas to see this tiny effect.”

    The NPDGamma Experiment, the first to be carried out at the Fundamental Neutron Physics Beamline at SNS, channeled cold neutrons toward a target of liquid hydrogen. The apparatus was designed to control the spin direction of the slow-moving neutrons, “flipping” them from spin-up to spin-down positions as desired. When the manipulated neutrons smashed into the target, they interacted with the protons within the liquid hydrogen’s atoms, sending out gamma rays that were measured by special sensors.

    After analyzing the gamma rays, the scientists found parity-violating asymmetry, which is a specific change in behavior in the force between a neutron and a proton. “If parity were conserved, a nucleus spinning in the righthanded way and one spinning in the lefthanded way—as if they were mirrored images—would result in an equal number of gammas emitting up as emitting down,” Bowman explained.

    “But, in fact, we observed that more gammas go down than go up, which lead to successfully isolating and measuring a mirror-asymmetric component of the weak force.”

    The scientists ran the experiment numerous times for about two decades, counting and characterizing the gamma rays and collecting data from these events based on neutron spin direction and other factors.

    The high intensity of the SNS, along with other improvements, allowed a count rate that is nearly 100 times higher compared with previous operation at the Los Alamos Neutron Science Center.

    Results of the NPDGamma Experiment filled in a vital piece of information, yet there are still theories to be tested.

    “There is a theory for the weak force between the quarks inside the proton and neutron, but the way that the strong force between the quarks translates into the force between the proton and the neutron is not fully understood,” said W. Michael Snow, co-author and professor of experimental nuclear physics at Indiana University. “That’s still an unsolved problem.”

    He compared the measurement of the weak force in relation with the strong force as a kind of tracer, similar to a tracer in biology that reveals a process of interest in a system without disturbing it.

    “The weak interaction allows us to reveal some unique features of the dynamics of the quarks within the nucleus of an atom,” Snow added.

    Co-authors of the study titled, “First Observation of P-odd γ Asymmetry in Polarized Neutron Capture on Hydrogen,” included co-principal investigators James David Bowman of ORNL and William Michael Snow of Indiana University (IU). The lead co-authors were David Blyth of Arizona State University and Argonne National Laboratory; Jason Fry of the University of Virginia and IU; and Nadia Fomin of the University of Tennessee, Knoxville, and Los Alamos National Laboratory. In total, 64 individuals from 28 institutions worldwide contributed to this research, and it produced more than 15 Ph.D. theses.

    The research was supported by DOE’s Office of Science and used resources of the Spallation Neutron Source at ORNL, a DOE Office of Science User Facility. It was also supported by the U.S. National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation, the PAPIIT-UNAM and CONACYT agencies in Mexico, the German Academic Exchange Service and the Indiana University Center for Spacetime Symmetries.

    See the full article here .


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    ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

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  • richardmitnick 11:38 am on January 5, 2018 Permalink | Reply
    Tags: HANARO research reactor at the Korean Atomic Energy Research Institute South Korea, , , , , Spallation neutron source   

    From physicsworld.com: “Neutrons probe gravity’s inverse square law” 

    physicsworld
    physicsworld.com

    Jan 4, 2018
    Edwin Cartlidge

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    Gravitating toward Newton’s law: the J-PARC neutron facility

    A spallation neutron source used by physicists in Japan to search for possible violations of the inverse square law of gravity. By scattering neutrons off noble-gas nuclei, the researchers found no evidence of any deviation from the tried and tested formula. However, they could slightly reduce the wiggle room for any non-conventional interactions at distances of less than 0.1 nm, and are confident they can boost the sensitivity of their experiment over the next few months.

    According to Newton’s law of universal gravitation, the gravitational force between two objects is proportional to each of their masses and inversely proportional to the square of the distance between them. This relationship can also be derived using general relativity, when the field involved is fairly weak and objects are travelling significantly slower than the speed of light. However, there are many speculative theories – some designed to provide a quantum description of gravity – that predict that the relationship breaks down at small distances.

    Physicists have done a wide range of different experiments to look for such a deviation. These include torsion balances, which measure the tiny gravitational attraction between two masses suspended on a fibre and two fixed masses. However, this approach is limited by environmental noise such as seismic vibrations and even the effects of dust particles. As a result such experiments cannot probe gravity at very short distances, with the current limit being about 0.01 mm.

    Scattered in all directions

    Neutrons, on the other hand, can get down to the nanoscale and beyond. The idea is to fire a beam of neutrons at a gas and record how the neutrons are scattered by the constituent nuclei. In the absence of any new forces modifying gravity at short scales, the neutrons and nuclei essentially only interact via the strong force (neutrons being electrically neutral). But the strong force acts over extremely short distances – roughly the size of the nucleus, about 10–14 m – while the neutrons have a de Broglie wavelength of around 1 nm. The neutrons therefore perceive the nuclei as point sources and as such are scattered equally in all directions.

    Any new force, however, would likely extend beyond the nucleus. If its range were comparable to the neutrons’ wavelength then those neutrons would be scattered more frequently in a forward direction than at other angles. Evidence of such a force, should it exist, can therefore be sought by firing in large numbers of neutrons and measuring the distribution of their scattering angles.

    In 2008, Valery Nesvizhevsky of the Institut Laue-Langevin in France and colleagues looked for evidence of such forward scattering in data from previous neutron experiments. They ended up empty handed but could place new upper limits on the strength of any new forces, improving on the existing constraints for scales between 1 pm and 5 nm by several orders of magnitude. Those limits were then pushed back by about another order of magnitude two years ago, when Sachio Komamiya at the University of Tokyo and team scattered neutrons off atomic xenon at the HANARO research reactor at the Korean Atomic Energy Research Institute in South Korea.

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    HANARO research reactor at the Korean Atomic Energy Research Institute in South Korea

    Time of flight

    In the new research, Tamaki Yoshioka of Kyushu University in Japan and colleagues use neutrons from a spallation source at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, which they fire at samples of xenon and helium. Because the J-PARC neutrons come in pulses, the researchers can easily measure their time of flight, and, from that, work out their velocity and hence their wavelength.


    J-PARC Facility Japan Proton Accelerator Research Complex J-PARC, located in Tokai village, Ibaraki prefecture, Japan

    Armed with this information, the team can establish whether any forward scattering is due to a new force or simply caused by neutrons bouncing off larger objects in the gas, such as trace amounts of atmospheric gases. At any given wavelength, both types of scattering would be skewed in the forward direction and so would be indistinguishable from one another. But across a range of wavelengths different patterns would emerge. For atmospheric gases, the scattering angle would simply be proportional to the neutrons’ wavelength. In the case of a new force, on the other hand, the relationship would be more complex because the effective size of the nucleus would itself vary with neutron wavelength.

    Reactors can also be used to generate pulses, by “chopping” a neutron beam. But that process severely limits the beam’s intensity. Taking advantage of the superior statistics at J-PARC, Yoshioka and colleagues were able to reduce the upper limit on any new forces below 0.1 nm by about an order of magnitude over the HANARO results – showing that their inherent strength can at most be 10^24 times that of gravity’s (gravity being an exceptionally weak force).

    Cost-effective search

    That is still nowhere near the sensitivity of torsion balance searches at bigger scales – which can get down to the strength of gravity itself. As Nesvizhevsky points out, torsion balances use macroscopic masses with “Avogadro numbers” (1023) of atoms, whereas neutron scattering experiments involve at most a few tens of millions of neutrons. Nevertheless, he believes that the new line of research is well worth pursuing, pointing out that many theories positing additional gravity-like forces “predict forces in this range of observations”. Such experiments, he argues, represent “an extremely cost-effective way of looking for a new fundamental force” when compared to searches carried out in high-energy physics.

    Spurred on by the prospect of discovery, Yoshioka and colleagues are currently taking more data. The lead author of a preprint on arXiv describing the latest research, Christopher Haddock of Nagoya University, says that they hope to have new results by the summer. A series of improvements to the experiment, including less scattering from the beam stop, he says, could boost sensitivity to new forces in the sub-nanometre range by up to a further order of magnitude and should also improve existing limits at distances of up to 10 nm.

    See the full article here .

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    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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