Undetected Electron Backscattering in PERKEO III

The beta asymmetry in neutron beta decay is used to determine the ratio of axial-vector coupling to vector coupling most precisely. In electron spectroscopy, backscattering of electrons from detectors can be a major source of systematic error. We present the determination of the correction for undetected backscattering for electron detection with the instrument PERKEO III. For the electron asymmetry, undetected backscattering leads to a fractional correction of $5\times 10^{-4}$, i.e. a change by 40% of the total systematic uncertainty.Comment: Proceedings of the International Workshop on Particle Physics at Neutron Sources PPNS 2018, Grenoble, France, May 24-26, 201

The point spread function of electrons in a magnetic field, and the decay of the free neutron

Experiments in nuclear and particle physics often use magnetic fields to guide charged reaction products to a detector. Due to their gyration in the guide field, the particles hit the detector within an area that can be considerably larger than the diameter of the source where the particles are produced. This blurring of the image of the particle source on the detector surface is described by a suitable point spread function (PSF), which is defined as the image of a point source. We derive simple analytical expressions for such magnetic PSFs, valid for any angular distribution of the emitted particles that can be developed in Legendre polynomials. We investigate this rather general problem in the context of neutron beta decay spectrometers and study the effect of limited detector size on measured neutron decay correlation parameters. To our surprise, insufficient detector size does not affect much the accuracy of such measurements, even for rather large radii of gyration. This finding can considerably simplify the layout of the respective spectrometers.Comment: 24 pages, 12 figure

Das Spektrometer PERKEO III und der Zerfall des freien Neutrons

Das Standardmodell der Teilchenphysik beschreibt den Zerfall des freien Neutrons durch nur drei Parameter. Dem gegenüber stehen eine Vielzahl experimenteller Beobachtungsgrößen, die es erlauben, durch Präzisionsmessungen das Standardmodell zu überprüfen. Das im Rahmen dieser Arbeit entwickelte Spektrometer PERKEO III zur Untersuchung des Neutronenzerfalls wird seinen Vorgänger PERKEO II ersetzen, dessen statistische und systematische Grenzen in den letzen Jahren erreicht wurden. Mit dem neuen Instrument sind Messungen mit einem kontinuierlichen oder gepulsten Neutronenstrahl möglich. Damit kann entweder die Statistik gegenüber dem Vorgänger um bis zu zwei Größenordnungen erhöht werden, oder aber die beiden größten, instrumentspezischen systematischen Einflüsse vermieden werden. Die nun zur Verfügung stehende Ereignisrate wird in einer ersten Messung mit PERKEO III im Winter 2006/2007 für eine Bestimmung des schwachen Magnetismus aus der Elektronen-Asymmetrie A genutzt werden. Diese Größe war bisher im Neutronenzerfall statistisch nicht zugänglich. Die Systematik dieses Experiments wird mit Hilfe unserer Messung mit PERKEO II analysiert. Für diese Messung wird PERKEO III am Leiter H113 des Instituts Laue-Langevin, Grenoble, installiert werden. Der Neutronenstrahl aus diesem Leiter wird charakterisiert und eine Parametrisierung der Strahleigenschaften ermöglicht eine schnelle Berechnung von Strahlprofilen und absoluten Ereignisraten. Eine Bestimmung der Reflektionseigenschaften zweier unmagnetischer Neutronenspiegel dient der Vorbereitung eines zukünftigen Experiments zum Neutronenzerfall

Determination of the Weak Axial Vector Coupling from a Measurement of the Beta-Asymmetry Parameter A in Neutron Beta Decay

We report on a new measurement of the neutron beta-asymmetry parameter $A$ with the instrument \perkeo. Main advancements are the high neutron polarization of $P = 99.7(1)$ from a novel arrangement of super mirror polarizers and reduced background from improvements in beam line and shielding. Leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. From the result $A_0 = -0.11996(58)$, we derive the ratio of the axial-vector to the vector coupling constant $\lambda = g_\mathrm{A}/g_\mathrm{V} = -1.2767(16)$Comment: 5 pages, 4 figure

Measurement of the Neutrino Asymmetry Parameter B in Neutron Decay

A new measurement of the neutrino asymmetry parameter B in neutron decay, the angular correlation between neutron spin and anti-neutrino momentum, is presented. The result, B=0.9802(50), agrees with the Standard Model expectation and earlier measurements, and permits improved tests on ``new physics'' in neutron decay.Comment: 4 pages, 2 figures; v2: revised PRL versio

A non-depolarizing CuTi neutron supermirror guide for PERC

Neutron guides are used to transport slow neutrons from sources to experiments. Conventional neutron supermirror guides use alternating thin layers based on nickel and titanium. Due to the magnetic properties of nickel, their neutron reflection properties are spin-dependent, in particular when exposed to high magnetic fields. Motivated by the requirements of precision experiments on neutron beta decay, we present novel supermirrors based on copper and titanium, which preserve the neutron beam polarization. These show excellent reflectivity and prove to be very stable even when exposed to high temperatures

ANNI - A pulsed cold neutron beam facility for particle physics at the ESS

Pulsed beams have tremendous advantages for precision experiments with cold neutrons. In order to minimise and measure systematic effects, they are used at continuous sources in spite of the related substantial decrease in intensity. At the European Spallation Source ESS these experiments will profit from the pulse structure of the source and its 50 times higher peak brightness compared to the most intense reactor facilities, making novel concepts feasible. Therefore, the cold neutron beam facility for particle physics ANNI was proposed as part of the ESS instrument suite. The proposed design has been re-optimised to take into account the present ESS cold moderator layout. We present design considerations, the optimised instrument parameters and performance, and expected gain factors for several reference experiments.Comment: 8 pages, 4 figures; submitted to the Proceedings of the International Workshop on Particle Physics at Neutron Sources PPNS 2018, Grenoble, France, May 24-26, 201