1. A Gravitational Spectrometer for Ultracold Neutrons Andreas Knecht Paul Scherrer Institut & Universität Zürich

2. Outline • Ultracold Neutrons • Neutron Electric Dipole Moment • UCN Detection & Standard Techniques for Determining UCN Velocities • Test of an Efficient Gravitational Spectrometerfor Ultracold Neutrons

3. Ultracold Neutrons Gravity • Neutrons with properties: • Kinetic energy E < 300 neV • Velocity v < 7.5 m/s • Wavelenghts λ > 500 Ǻ • Temperature T < 3 mK • Interactions: • Gravitational: Vg = mgh = 100 neV/m • Magnetic: Vm = -μB = 60 neV/T • Strong: Fermi potential depending on material; VF up to 350 neV • Weak: n → p + e + ν Material polarized UCN 1.5 T Magnet

4. nEDM • An EDM couples to an electric field as a MDM couples to a magnetic field: • Measure EDM from the difference of precession frequencies for parallel/anti-parallel fields: • Non-zero EDM violates both parity P and time reversal T → violates also CP • understand mechanism of CP violation • understand baryon asymmetry

5. -e 1μm +e nEDM Current best limit: dn < 2.9×10-26ecm (Sussex-RAL-ILL)

6. Several systematic effects exist in nEDM measurements which depend on the velocity of UCN (e.g. systematics stemming from magnetic field gradient) Measure nEDM as a function of velocity Need an efficient velocity dependent UCN detection system in order to reach statistics Two scenarios: No velocity dependence observable in measured EDM→ false velocity-dependent EDM effects under control Velocity dependence observable→ extrapolate to zero velocity Systematics

7. UCN are neutral and have only tiny kinetic energies convert them into charged products information on UCN velocity is lost in this process Most widely used reactions: 10B + n → 7Li + α + 2.3/2.8 MeV 6Li + n → 3H + α + 4.8 MeV 3He + n → 3H + p + 0.8 MeV Charged decay products are detected with standard scintillators or proportional gas counters. UCN Detection noise  7Li  Pulse height spectrum of 10B-reaction

8. Standard Techniques • Time-of-flight spectrometry Absorbers at different heights Transmission through→ foils with different Fermi potentials → absorbing gas at different pressures→ magnetic fields of different strengths Measurement of the reach in the gravitational field “inverted-U" shaped UCN guide UCN

9. Efficient Gravitational Spectrometer Inclined UCN guide with 4 attached detectors at different heights Large diameter of main guide: 230 mm→ reduces back diffusion (~10%) Guides made from NiMo coated plexiglas Extract spectrum from distribution over the 4 detectors UCN

10. Simulation

11. Setup at ILL 1) PF2/TES beamline at ILL 2) U-guide for calibration 3) Gas cell for calibration 4) Input guide into spectrometer 5) Main spectrometer guide 6)-9) UCN detectors 1-4 10) Vacuum equipment

12. Tests with U • Step in count rates due to U with different lengths • Problems with slits while turning the U

13. Tests with U → independent of slit and related effects→ change in spectrum clearly visible

14. Tests with Gas Cell Gas type: 3He (also N2, O2, Ar) Exponential fit: exp(-p/πi) σabs~ 1/v → πi~ v π1 = 3.586(7) mbar π2 = 3.422(3) mbar π3 = 3.321(3) mbar π4 = 3.258(3) mbar change of average velocity in the different detectors

15. Conclusion & Outlook • Determination of UCN velocity is a way to control veloctiy dependent systematics in nEDM measurements • Successful test of an efficient velocity dependent UCN detection system • Analysis ongoing…→ Need to characterise system and tune simulation to the data

16. Backup

17. TOF Measurements Spectra after U-guide, gas cell and of the direct beam also measured using TOF technique.

18. w = w L + + B0 B0 B0 B0 ±E ±E ±E ±E B1 B1 nEDM • Mercury used to monitor B-field fluctuations via ω=γB. • Frequency visible as oscillating signal on PMT.