Andreas Knecht 1ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

A Gravitational Spectrometer for Ultracold Neutrons

Andreas Knecht

Paul Scherrer Institut & Universität Zürich

Andreas Knecht 2ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Outline

Ultracold Neutrons

Neutron Electric Dipole Moment

UCN Detection & Standard Techniques for Determining UCN Velocities

Test of an Efficient Gravitational Spectrometerfor Ultracold Neutrons

Andreas Knecht 3ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Ultracold NeutronsNeutrons 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

Andreas Knecht 4ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

nEDMAn 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

Andreas Knecht 5ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

nEDM

Current best limit:

dn < 2.9×10-26 ecm

(Sussex-RAL-ILL)

-e

+e1μm

Andreas Knecht 6ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Systematics

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 controlVelocity dependence observable→ extrapolate to zero velocity

Andreas Knecht 7ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

UCN DetectionUCN 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 MeV6Li + n → 3H + α + 4.8 MeV3He + n → 3H + p + 0.8 MeV

Charged decay products are detected with standard scintillators or proportional gas counters.

Pulse height spectrum of 10B-reaction

7Li

noise

Andreas Knecht 8ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Standard TechniquesTime-of-flight spectrometry Absorbers at different heights

Transmission through→ foils with different Fermi potentials → absorbing gas at different pressures→ magnetic fields of different strengths

“inverted-U" shaped UCN guide UCN Measurement of the reach in the gravitational field

Andreas Knecht 9ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

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

Andreas Knecht 10ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Simulation

Andreas Knecht 11ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Setup at ILL1) PF2/TES beamline at ILL2) U-guide for calibration3) Gas cell for calibration4) Input guide into spectrometer5) Main spectrometer guide6)-9) UCN detectors 1-410) Vacuum equipment

Andreas Knecht 12ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Tests with U

• Step in count rates due to U with different lengths

• Problems with slits while turning the U

Andreas Knecht 13ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Tests with U

→ independent of slit and related effects→ change in spectrum clearly visible

Andreas Knecht 14ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

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) mbarπ2 = 3.422(3) mbarπ3 = 3.321(3) mbarπ4 = 3.258(3) mbar

change of average velocity in the different detectors

Andreas Knecht 15ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

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

Andreas Knecht 16ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

Backup

Andreas Knecht 17ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

TOF Measurements

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

Andreas Knecht 18ZH Doktorandenseminar 2009, 4. – 5. Juni 2009

nEDMMercury used to monitor B-field fluctuations via ω=γB.

Frequency visible as oscillating signal on PMT.

B1

+B0 ±E

B0 ±E

B0 ±E

L

B1

+B0 ±E