Measured velocity spectra and neutron densities of the PF2 ultracold-neutron beam ports at the Institut Laue–Langevin

https://doi.org/10.1016/j.nima.2019.163112 Get rights and content

Highlights

  • •
    The ultracold neutron spectrum of the Turbine at Institut Laue–Langevin was measured.
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    Measurement provides reference data for past and future experiments.
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    The spectrum was shown to have a Maxwell–Boltzmann shape.
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    Data allow to calculate neutron density for arbitrary velocity bands

Abstract

Ultracold neutrons (UCNs) are a useful tool for fundamental physics experiments. They can be used to probe the lifetime of free neutrons, search for new CP violating processes and exotic interactions beyond the Standard Model, perform Ramsey spectroscopy, and carry out neutron-optical interference experiments. All of these experiments require high neutron count rates for good statistics. For optimal exploitation of experimental beam time, these experiments need to be prepared and, at times, even simulated in advance. To this end, it is crucial to know the velocity-dependent UCN flux at each beam position. Knowing the absolute neutron flux also allows for an absolute calibration of previously gathered data. Using the same time-of-fight experimental setup, we have measured the differential neutron flux of three out of the four UCN beam ports at the PF2 instrument at Institut Laue–Langevin, Grenoble. These beam ports are commonly used for UCN flux experiments and proof-of-principle tests.

Introduction

Many fundamental physics experiments use ultracold neutrons (UCNs). These are neutrons with a kinetic energy low enough to be confined in material bottles or magnetic traps, typically ≲300 neV (v<7.6m∕s) [1], [2]. This property allows for long observation times and makes UCNs particularly interesting for three types of experiments: storage experiments determining the lifetime of the free neutron, the search for a non-zero electric dipole moment of the neutron (nEDM), and constraining dark matter candidates [3]. UCNs are also frequently used in transmission experiments [4] as well as low-energy experiments that probe gravitation on the micrometer scale [5], [6] and neutron-optical phenomena [7].
Various UCN converters are operational throughout the world. The oldest such converter, and by far the most often used for fundamental physics research until the present, is the UCN "Turbine" (instrument PF2) at the Institut Laue–Langevin [8], [9] with its four UCN beam ports. It slows very cold neutrons (VCN) down to UCN energies by reflecting them off receding polished metal blades exploiting the Doppler effect. The design of the Turbine favors experiments requiring a high UCN flux over those requiring a high UCN density.

Section snippets

Previous measurements

Steyerl et al. carried out UCN flux (also called current density) and density measurements after the Turbine had been installed [9], [10]. In these experiments, both flux and density inside the Turbine vessel were determined. However, no systematic measurement has yet been taken of the UCN flux available outside the vessel at the four PF2 beam ports (MAM, UCN, EDM, TEST), nor have these beam ports been compared with one another.
In 1999, the group led by A. V. Strelkov from JINR Dubna (Russia)

The experimental setup

For the comparative measurement of neutron spectra, the UCN ports of the Turbine were equipped with beam tube configurations that are often used by experimenters, see Fig. 1. The beam port PF2-MAM was permanently occupied by the long-term experiment "Gravitrap" [14] and thus its spectrum could not be measured.
For safety reasons, the vacuum in the Turbine’s beam guides is separated from the Turbine vacuum by a 100μm thick AlMg3 foil. The neutron guides between the safety foil of the Turbine and

Experimental results and calculations

The Turbine ports’ neutron spectra, which extend beyond the UCN range into the VCN energy range, are shown in Fig. 2. They were corrected for detector efficiency and chopper duty cycle, and extrapolated back to the position of the Turbine’s safety foil.
The measured differential neutron flux φ(v) (in units of cm−3) from Fig. 2 can be well approximated by a Maxwell–Boltzmann (MB) distribution fMB(v) for the neutron density N(v)=N0×ばつfMB(v), where φ(v)=N(v)×ばつv, and thus φ(v)=N04πmn2πkT3∕2v3×ばつexpmnv22kT

Conclusions

Using the same experimental equipment, we have measured and compared the differential neutron fluxes at three out of the four beam ports of the UCN Turbine at the Institut Laue–Langevin. From these data, we were able to calculate the integral neutron fluxes and extract the total neutron densities up to any arbitrary critical neutron velocity vcrit. Our measured values for the PF2-UCN and PF2-EDM beams indicate that both have a similar UCN density below the steel cut-off (6.0 m/s) of 10.4 cm−3

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We wish to thank the instrument scientists of PF2 for their advice and Dr. Tobias Rechberger for support during the experiment. The Ph.D. thesis of S. D. was done within a collaboration between the Institut Laue–Langevin (ILL), Grenoble, France and Technische Universität München, Munich, Germany. It received financial support from both the ILL and FRM II/ Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany. Furthermore, J. H. and S. D. acknowledge funding from Dr.-Ing.

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