Fundamental Physics End Station (FPES)
The problem: The neutron is an excellent laboratory for studying weak interaction physics. No more than a handful of parameters govern the neutron's decay and its interaction via the weak force with other neutrons and protons. During the past 50 years a series of increasingly precise neutron-based experiments have led to an understanding of the role that the weak force plays in the Standard Model of Electroweak Interactions. This model has been phenomenally successful. Today there are proposals for new experiments that are designed to probe the Standard Model with unprecedented accuracy. In some cases one has learned how to reduce the systematic effects present in previous experiments to the point where their precision would be limited by statistics if run again. In order to continue probing the Standard Model one must increase the incident neutron flux (this is critically important considering the length of time required to carry out needed measurements) and better accommodate the increasingly important environmental needs of these experiments, including constraints on acceptable background magnetic fields and radiation, cryogenic requirements, polarization requirements and space requirements. In addition it is important that new capabilities such as a highly efficient ultra-cold neutron source (UCNS) are also developed for a new class of fundamental physics experiments. A suitably designed new beamline will address all of these needs.
The Physics End Station Solution: A new beamline will be constructed using state of the art supermirror and focusing technologies to achieve many fold increase in neutron throughput compared to what is currently available at NG-6 fundamental physics station. Its neutron beam will be well characterized in terms of uniformity and wavelength distribution because some experiments are sensitive to these factors. There will be inline polarization capability for experiment needing polarized neutrons. Just as important as the increased neutron flux will be the environment made available to each experiment. The ambient magnetic field and field gradients will be small enough to accommodate the most sensitive experiment foreseen. There will be sufficient space to incorporate additional shielding should an experiment's background requirements necessitate it. Finally, the experimental area will be designed to accommodate experiments that are physically larger than those that can be accommodated today. This will be accomplished by making more available floor space available and by constructing a pit to accommodate vertically spread experiments. The first experiment expected to be carried out on this beam line is called aCORN which stands for "a CORrelation in Neutron decay". For its ultimate success aCORN will require as much neutron flux as possible, low backgrounds, low ambient magnetic fields, and considerable physical space. The aCORN experiment is expected to be followed by many high profile fundamental physics experiment, such as a search for time-reversal violation in neutron beta decay, neutron spin rotation in deuterium or hydrogen, radiative decay of the neutron, proton asymmetry in neutron beta decay, and the free neutron lifetime. A brief description of aCORN is given below.
The aCORN experiment:
The aCORN experiment is designed to measure the correlation between the outgoing electron and anti-neutrino directions in unpolarized neutron beta-decay (a schematic is shown below). Since the anti-neutrino cannot be measured directly, its direction is inferred by detecting the proton in coincidence with the electron in 180° low solid angle geometry relative to the neutron beam. When the anti-neutrino is traveling towards the proton detector, conservation of momentum implies that the proton momentum is lower than it is for the case where it is traveling towards the electron detector. A histogram of the time delay between proton and electron arrival versus electron energy reveals two groups corresponding to these two cases. The numerical asymmetry between these two groups for a given range of electron energy is proportional to the correlation. A series of collimators and 25 solenoids together serve to limit the maximum accepted transverse momentum for protons and electrons thereby simplifying the analysis. If the new beamline achieves its goals, aCORN has the potential to measure the asymmetry with accuracy comparable to that achieved in experiments requiring polarized neutrons. The systematic effects in these two classes of experiment will be very different, making a comparison of their respective results extremely significant.
A photograph of the aCORN apparatus sitting on the NG-6 beam line where it took data for several months up until the long reactor shutdown. What was learned from the collected data will permit us to move a better functioning apparatus to our new beam line, one that is capable of taking advantage of the new beam's intense neutron flux. It is there that we can achieve our desired statistical uncertainty.
aCORN wishbone data and Monte Carlo. On the y axis is plotted the time of flight between the neutron decay electron and proton; on the x axis is plotted the energy of the electron. Over a large electron energy range, two time of flight groups are seen. The upper (lower) branch corresponds to events where the electron antineutron is traveling in the same (opposite) direction as the proton. The desired asymmetry is simply the number asymmetry in the two branches as a function of electron energy. The Monte Carlo calculation is in excellent agreement with the experimental data.
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