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