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Diamond-like Carbon for Ultra-cold Neutrons

Diamond-like Carbon for Ultra-cold Neutrons. Peter Fierlinger. E. W. Paul Scherrer Institut, Switzerland. neutron source SINQ. p-Accelerator. Synchrotron SLS. Contents. Ultra-cold neutrons (UCN) Motivation: Electric dipole moment of the neutron (nEDM)

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Diamond-like Carbon for Ultra-cold Neutrons

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  1. Diamond-like Carbon for Ultra-cold Neutrons Peter Fierlinger

  2. E W Paul Scherrer Institut, Switzerland neutron source SINQ p-Accelerator Synchrotron SLS

  3. Contents • Ultra-cold neutrons (UCN) • Motivation: • Electric dipole moment of the neutron (nEDM) • Life time of the free neutron • The new UCN source at the PSI accelerator • UCN related R&D: DLC • DLC test experiment @ ILL

  4. Ultra-cold neutrons E < 300 neV, T < 3 mK, v < 7 ms-1,  >50 nm - Gravity ~ 100 neV / m - Magnetic field ~ 60 neV / T - Strong interaction: „Fermi potential“ V┴ UCN can be stored in traps for ~ 1000 s

  5. Experimental Limit: ILL-Sussex-RAL (1999): ( -1.0 ± 3.6 ) ·10-26 e·cm Predicted: d ~ 10-26 - 10-28 e.cm (MSSM) d < 10-31 e.cm (SM) nEDM spin 1/2 Magnetic moment µ AXIAL VECTOR Electric dipole moment d POLAR VECTOR T transformation P transformation A nonzero particle EDM violates P, T and, assuming CPT conservation, also CP STATISTICAL LIMIT Purcell and Ramsey, PR78(1950)807, Lee and Yang, Landau

  6. n & CKM matrix PERKEO II (885.7±1 s) Universality: without PERKEO II STATISTICAL LIMIT

  7. p-accelerator @ PSI nEDM UCN Source Pulsed operation: 8 sec on 800 sec off Cockroft-Walton: 800keV, 40mA Injector II: 72MeV, 2mA Ring cyclotron: 600MeV, 2mA,

  8. UCN tank system (~6m high) UCN storage volume, 2m3 4000 UCN/cm3 Coated walls To experiments Shutter Cold sD2 moderator n-Guide Spallation target p beam D2O moderator

  9. Storage materials low loss probability per wall collision µ long storage time µ(E) ~  high Fermi potential more UCN low spin flip probability per wall collision  polarized UCN (e.g. in nEDM) typical UCN spectrum Intensity E

  10. "bad" 58Ni Cu Fe Ni 65Cu BeO Al C Be 300 K Pb Be 70 K DLC Diamond "good" Storage materials

  11. Diamond-like Carbon „sp2“ Production:e.g. pulsed laser deposition (PLD) DENSITY Target Laser Layer Substrate „sp3“

  12. Reflectometry V┴~ < 7 m/s ~ UCN Detector v┴ φ φ Ohter methods used: XPS, NEXAFS, Raman, LaWAVE

  13. No mechanical slits • Depolarization probability  • Loss probability µ measured simultaneously: Gravity: 1 m = 100 neV Magnetic field: 60 neV/T Adiabatic condition Most common storage material: Beryllium • μ(E,,T)~ 4.10-5 (at 70 K) • β~ 5.10-6 μ, β of DLC = ? • Monte Carlo program(E) • Experimental setup • Samples • Method I: µ(T,E) and (T,E) • Method II: (T,E) DLC test experiment Gravity Material polarized UCN 1.5 T Magnet

  14. Monte Carlo program Geant4: CERN particle tracking simulation toolkit Adapted for UCN: • Fermi potential, wall reflections • Wall losses & spin flips • Absorption, scattering • Gravity & magnetic fields (space-, time-dependent) • Spin tracking

  15. Setup: Cooling Sample (1m) Coils (300 A, 200 V) Switch n+3Het+p+780keV Detector Neutrons

  16. Samples DLC on PET Substrates: Al tubes Quartz tubes Al foils PET foils Coatings: DLC, laser arc, Dresden DLC, PLD, VT Be, sputtered, PNPI & TUM 70 mm Film thickness > 100 nm ( ~ 10 x penetration depth)

  17. Method I Detector count rate: 105 1 Sample B Magnet 100 % 0 Storage B "Cleaning" 0 100 200 300 400 time [s] UCN from ILL-turbine Detector

  18. spin- flipped Fall through magnet Losses from the storage volume Simulated ! Lost neutrons Count rate 60 100 B field 100 % simulated measured 90% Magnetic field Storagetime (s) "Cleaning" Method I: cleaning spin- flipped Losses from the storage volume Simulated ! Lost neutrons decay top magnet wall loss 60 100 time (s) 100 % 90% Magnetic field B field "Cleaning"

  19. Method I: storage Potential energy Wall collisions (E) 1000 800 600 400 200 0 1000 800 600 400 200 0 1 / (s.cm_height) [neV] |B| = 1.5 T

  20. Method I: spectrum Typical # of UCN stored ~ 600 measured simulated 120 s storage 320 s storage

  21. Method I: analysis log10  up to 450 s Detector count rate 1. 100 % 0 Magnetic field 2. 100 % 0

  22. 1 1 = + m n Compare to simulation ( E ) ( E ) t t st n Method I: loss probability Measurement: tot* with

  23. Method I: results Wall loss coefficient  [1 / wall collision] x 10-4 DLC is a good choice 

  24. Method I: analysis log10 Detector count rate 1. 1. 100 % 0 Magnetic field 2. 2. 100 % 0

  25. N + N sp bg Method I: depolarization <Nbg> ~ 1 in 200 s: Poisson Statistics

  26. Method II Detector Count rate: B 105 1 Sample Magnet 100 % 0 0 100 200 300 400 time [s] UCN Detector

  27. Method II: analysis Accumulating neutrons Wall collision distribution par Production Loss 1 / (s.cm_height) Height [mm] Energy [neV]

  28. Method I & II: results Spin flip probability  [1 / wall collision] …Method I …Method II

  29. NH ~ 0.3 NC Explains also spin flips Interpretation So-called „anomalous losses“: (0 K)~ 2.10-7 theor. but: ~ 10-5 exp. Hydrogen:  = C + H

  30. Conclusions • Monte Carlo package for UCN included in GEANT4 • Loss and depolarization measured simultaneously for the first time • Hydrogen is a good candidate for the explanation of the losses • DLC is top candidate for the UCN source at PSI

  31. BACKUP

  32. Motivation: nEDM Imagine the neutron were the size of the Earth... ILL-Sussex-RAL (1999): ( -1.0 ± 3.6 ) ·10-26 e·cm Theoretical predictions: SUSY : 10-25-10-28e·cm Dx  1mm

  33. nEDM measurement Polarized UCN in a trap B0 /2 Pulse + B0 B1 Free Precession 100 s B0 ±E + /2 Pulse B0 B1

  34. UCN Transmission EDM-UCN beam at ILL: • TOF • Foil coated with • Be (black) • DLC (red) Chopper Sample UCN Detector 2 m

  35. UCN Physics in Geant4 • Fermi potential, wall reflections • Wall losses & spin-flips • Absorption, scattering • gravitational & magnetic fields (space-, time-dependent) • Numerical solution of the Bloch equation components of P after /2 flip at |B| = 1g L/ from NIM A 457 (2001), 338-346

  36. Simulated spectrum shift (1 spin component) final initial Rel. Intensity 0 30 90 Energy [neV] Filling

  37. RK4

  38. Low field transitions B0 Bearth

  39. Spin tracking Coupled equations: Treated classically „Bloch“-equation

  40. Penetration depth Energy inside the barrier …. „Penetration depth“

  41. The Magnet

  42. Neutron life time CKM (quark mixing) matrix is unitary: (885.7±1 s) PDG 2004 STATISTICAL LIMIT Coupling for Leptons = Coupling for Quarks Vud (neutr) = 0.9725±0.0013 PDG 2004 Vud (nucl) = 0.9740±0.0005

  43. Superthermal converters • Superfluid He – zero absorption cross section but needs very low temperatures ( ~ 0.5 K) (NIST, ILL, SNS) • Solid D2 – absorption lifetime 150 ms, 2 orders of magnitude higher production rate as compared with He, temperature of ~ 8K sufficient (Munich, Los Alamos, PSI) • Solid CD4 – compared with D2 more low lying rotational states – investigations at the very beginning • Solid O2– phonons and magnons excitation but temperatures below 2K needed

  44. D2 nuclear spin : S = 0,2 (ortho) and S = 1(para) Ortho-D2 : J = 0,2,4 …(rotational quantum number) Para-D2 : J = 1,3,5… Energy of the lowest rotational state: Para-D2 J =1 E = 7.5 meV Ortho-D2 J = 0 E = 0 meV Importance of high ortho-D2 concentration Deuterium Additional up-scattering channel !

  45. Maxwell spectrum vc ~ 1 km/s vth ~ 2 km/s v vUCN < 7m/s

  46. 4He

  47. Maxwell Distribution Neutron density between v and v+dv at thermal equilibrium (average velocity)

  48. Raman spectra

  49. A oder  und B A mit den elektronen B mit neutrino

  50. Maxwell Distribution Neutron density between v and v+dv at thermal equilibrium (average velocity)

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