arXiv subjects

B. van den Brandt

Publications and source records attributed to B. van den Brandt.

8 recordsLinked to original sources

arXiv2017

High-power closed-cycle $^4$He cryostat with top-loading sample exchange

We report on the development of a versatile cryogen-free laboratory cryostat based upon a commercial pulse tube cryocooler. It provides enough cooling power for continuous recondensation of circulating $^4$He gas at a condensation pressure of approximately 250~mbar. Moreover, the cryostat allows for exchange of different cryostat-inserts as well as fast and easy "wet" top-loading of samples directly into the 1 K pot with a turn-over time of less than 75~min. Starting from room temperature and using a $^4$He cryostat-insert, a base temperature of 1.0~K is reached within approximately seven hours and a cooling power of 250~mW is established at 1.24~K.

physics.ins-det↗

arXiv2012

Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins

We report on a polarized-neutron Laue diffraction experiment on a single crystal of neodynium doped lanthanum magnesium nitrate hydrate containing polarized proton spins. By using dynamic nuclear polarization to polarize the proton spins, we demonstrate that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.

cond-mat.mtrl-sci↗

arXiv2008

Accumulation and extraction of ultracold neutrons from a superfluid helium converter coated with fluorinated grease

We report experiments on the production of ultracold neutrons (UCN) in a converter of superfluid helium coated with fluorinated grease. We employed our technique of window-free extraction of accumulated UCN from the helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. The time constant for UCN passage through the same extraction aperture as in a previous experiment was a factor two shorter, despite a lower mean velocity of the accumulated UCN in the present experiments. A time-of-flight measurement of the cold neutron spectrum incident on the converter allowed us to estimate the multi-phonon contribution to the UCN production. The UCN production rate inferred from two methods agrees with the theoretical expectation.

nucl-ex↗

arXiv2007

Quantum Gravity Boundary Terms from Spectral Action of Noncommutative Space

We study the boundary terms of the spectral action of the noncommutative space, defined by the spectral triple dictated by the physical spectrum of the standard model, unifying gravity with all other fundamental interactions. We prove that the spectral action predicts uniquely the gravitational boundary term required for consistency of quantum gravity with the correct sign and coefficient. This is a remarkable result given the lack of freedom in the spectral action to tune this term.

hep-th↗

arXiv2004

Low Energy Analyzing Powers in Pion-Proton Elastic Scattering

Analyzing powers of pion-proton elastic scattering have been measured at PSI with the Low Energy Pion Spectrometer LEPS as well as a novel polarized scintillator target. Angular distributions between 40 and 120 deg (c.m.) were taken at 45.2, 51.2, 57.2, 68.5, 77.2, and 87.2 MeV incoming pion kinetic energy for pi+ p scattering, and at 67.3 and 87.2 MeV for pi- p scattering. These new measurements constitute a substantial extension of the polarization data base at low energies. Predictions from phase shift analyses are compared with the experimental results, and deviations are observed at low energies.

nucl-ex↗

arXiv2004

The spin-dependent nd scattering length - a proposed high-accuracy measurement

The understanding of few-nucleon systems at low energies is essential, e.g. for accurate predictions of element abundances in big-bang and stellar fusion. Novel effective field theories, taking only nucleons, or nucleons and pions as explicit degrees of freedom, provide a systematic approach, permitting an estimate of theoretical uncertainties. Basic constants parameterising the short range physics are derived from only a handful of experimental values. The doublet neutron scattering length a_2 of the deuteron is particularly sensitive to a three-nucleon contact interaction, but experimentally known with only 6% accuracy. It can be deduced from the two experimentally accessible parameters of the nd scattering length. We plan to measure the poorly known "incoherent" nd scattering length a_{i,d} with 10^{-3} accuracy, using a Ramsey apparatus for pseudomagnetic precession with a cold polarised neutron beam at PSI. A polarised target containing both deuterons and protons will permit a measurement relative to the incoherent np scattering length, which is know experimentally with an accuracy of 2.4\times 10^{-4}.

nucl-ex↗

arXiv1995

Search for the Hypothetical pi -> mu x Decay

The KARMEN collaboration has reported the possible observation of a hitherto unknown neutral and weakly interacting particle x, which is produced in the decay pi -> mu + x with a mass m(x) = 33.9 MeV. We have searched for this hypothetical decay branch by studying muons from pion decay in flight with the LEPS spectrometer at the piE3 channel at PSI and find branching ratios BR(pi- to mu- anti-x) < 4e-7 and BR(pi+ to mu+ x) < 7e-8 (95\% C.L.). Together with the limit BR > 2e-8 derived in a recent theoretical paper our result would leave only a narrow region for the existence of x if it is a heavy neutrino.

nucl-ex↗

arXiv1994

Detection of nuclear recoils in prototype dark matter detectors, made from Al, Sn and Zn Superheated Superconducting Granules

This work is part of an ongoing project to develop a Superheated Superconducting Granule (SSG) detector for cold dark matter and neutrinos. The response of SSG devices to nuclear recoils has been explored irradiating SSG detectors with a 70MeV neutron beam. The aim of the experiment was to test the sensitivity of Sn, Al and Zn SSG detectors to nuclear recoil energies down to a few keV. The detector consisted of a hollow teflon cylinder (0.1cm$^3$ inner volume) filled with tiny superconducting metastable granules embedded in a dielectric medium. The nuclear recoil energies deposited in the SSG were determined measuring the neutron scattering angles with a neutron hodoscope. Coincidences in time between the SSG and the hodoscope signals have been clearly established. In this paper the results of the neutron irradiation experiments at different SSG intrinsic thresholds are discussed and compared to Monte Carlo simulations. The results show that SSG are sensitive to recoil energies down to $\sim$1keV. The limited angular resolution of the neutron hodoscope prevented us from measuring the SSG sensitivity to even lower recoil energies.

cond-mat↗