Measuring the Neutron Lifetime with Magnetically Trapped Neutrons • F.H. DuBose, R. Golub, E. Korobkina , C.M. O’Shaughnessy, • G.R. Palmquist, Pil-Neyo Seo, P.R. Huffman (NC State Univ.); • L. Yang, J.M. Doyle (Harvard Univ.); • K.J. Coakley, H.P. Mumm, A.K. Thompson (NIST); • G.L. Yang, (NIST/U of Maryland); • S.K. Lamoreaux (LANL) Particle And Nuclei International Conference, Santa Fe, Oct. 28, 2005
p D 4He n D What Can We Learn from Neutron Beta Decay? • The Standard Model • Big-Bang Nucleosynthesis
Status of Test of Electroweak SM
Neutron Lifetime Data PDG Data + new Data P.E. Spivak, Zh. Eksp. Teor. Fiz. 94, 1 (1988): 891+- 9 (s) W. Mampe et al., PRL 63, 593 (1989): 887.6 +- 3.0 W. Paul et al., Z. Phys. 45, 25 (1989): 877 +- 10 V.V. Nezvizhevskii et al., JETP 75, 405 (1992): 888.4 +-3 .1 +- 1.1 W. Mampe et al., JETP Lett., 57, 82 (1993): 882.6 +- 2.7 J. Byrne et al., Europhys. Lett. 33, 187 (1996): 889.2 +- 3.0 +- 3.8 S. Arzumanov et al., Phys. Lett. B483, 15 (2000): 885.4 +- 0.9 +- 0.4 M.S. Dewey et al., PRL 91, 152302 (2003): 886.8 +- 1.2 +- 3.2 A.Z. Andreev et al., ILL Annual Report, 92-93 (2003): 882 +- 16 A. Serebrov et al., Phys. Lett. B605, 72 (2005): 878.5+-0.7+-0.3 world average
PMT Neutron Lifetime with Magnetically Trapped UCNs • Produce UCN using the “superthermal” process in a trap region • Adiabatic condition, • UCN moves classically in potential. Low-field seeking spin-state is trapped. • Detect decays using scintillation technique Etot = Ekin + m · B < |m| · Btrap Q-pole Trap
Bucking coils Pinch coil Q-pole n Mark II magnet Superconducting Q-pole Magnet • Trap depth=1.1 T (66neV), 4000 UCNs trapped in 1.5 liter volume) • 4-racetrack-shaped quadrupole coils to create a radial field gradient • Two solenoids to close the trap axially
Production of UCN in Superfluid He R. Golub and J.M. Pendlebury, Rep. Progr. Phys., 42 (1979) 439 • 0.89 nm (12 K or 0.95 meV) neutrons can scatter in liquid helium to near rest by emission of a single phonon. • Upscattering (by absorption of a 12 K phonon) ~ Population of 12 K phonons ~ e–12 K/Tbath , Tbath~250mK Phonon Excitations in Liquid Helium
Monochromator (Potassium Intercalated Stage-2 Graphite) 2cm x 5cm x 2mm The individual small monochromator pieces have high purity and mosaics ranging from 1-2 degrees →neutron-inducedbackground reduction and 85% reflectivity
0.89-nm Beam Performance Incident, transmitted, and reflected beam from the monochromater Reflected beam has l/2 and l/3 peaks filtered from the beam using Pyrolytic graphite and polycrystalline Bi (monochromatic beam: f=5x106ncm-2s-1)
Decay Detection • Superfluid He is used to produce UCNs and to detect decays ! • Recoil electron creates an ionization track in the helium. • Helium ions form excited He2* molecules (ns time scale) in both singlet and triplet states. • He2* singlet molecules decay, producing a large prompt (< 20 ns) emission of extreme ultraviolet (EUV) light. • EUV light (80 nm) converted to blue using the organic TPB.
Prototype Design (Mark II) n
NCNR reactor Beamline n
Neutron Lifetime Data from Mark II (after removing marginally trapped neutrons) Blue: The difference of the trapping and non-trapping data runs (~8 weeks) Green:Marginally trapped neutrons were removed using the field ramping (~2 weeks) Red: Natural helium, no trapped neutron expected
Backgrounds in Mark II Trapping Run Non-trapping Run Difference between Trapping and Non-trapping
Marginally Trapped Neutrons: UCNs in semi-stable orbit • There are neutron trajectories where • Etot = Ekin + m · B > |m| · Btrap • Ramping the field down to 0.3B0 eliminates UCNs on these orbits, but also reduces • the trapped UCNs by 50%.
Vwall UCN → Neutron Wall Interaction Even when the neutrons interact with the wall, they have a non-zero probability for reflection. V(He) = 20 neV Vwall(TPB) = 42 neV Vwall(c) = 180 neV Vwall(PTFE) = 123 neV Ef/Ei=(bf/bi)2/3 ≈ 0.44 Vwall(TPB) – V(He) ≈ 22 neV Kinetic energy at the trap edge (r=R): Eafter = Ef-bfR = 0.44 Ei – 0.3 ETrap ≈ 0.14ETrap 1 T deep trap -> Eafter ≈ 10 neV 3 T deep trap -> Eafter ≈ 30 neV > 22 neV
Toward Improving the Precision – Lager and Deeper magnet; KEK Q-pole • Mark II Mark III • (tn=844+53/-47 s) (x20 more neutrons @ NIST) • fMagnet= 10.5 cmfMagnet = 14 cm fTrap= 8.4 cmfTrap = 12 cm • L = 35 cm LTrap = 75 cm • BTrap= 1.1 T BTrap= 3.1 T
HTS Leads Test • Maximum test current: • 3410 A • LHe comsumption: • 48 l/day (1.45 W) • Vapor-cooled leads: • 260 l/day (8 W) • LN2 consumption: • 150 l/day • Change to double wall • transfer tube can cut • this consumption • below 100 l/day
Magnet Tests Q-pole: tested up to 3000 A (3.7 T) Solenoids: tested up to 250 A (4.0T) Assembly of Q-pole and Solenoids: Q: 90% of design current w/o quench S: 95% of design current w/o quench Q+S: quenching at 80% (1st) and at 90% (2nd) of design currents
Dewar Assembled Magnet HTS current leads
New Cryostat 1.6 m Solenoid Sideview (left tower) Sideview (right tower) KEK Q-pole Magnet Supporting Post
1 mm thick G-10 wall Al shrinking Fitting flanges Magnet Supporting Posts • Support 1000 kg of weight at 4K • Heat load to 4K, < 0.5W
Summary • Mark I and II demonstrated that measurement of the neutron lifetime with magnetically trapped UCNs from superthermal process appears feasible. • Deeper trap will increase trapped numbers (x20) and help to remove marginally bottled neutrons after the magnet ramping • Lower backgrounds: include a gamma shield in cryostat design and reduce acrylic mass • Mark III upgrade will improve accuracy to dt/t~±3 (s) in 2 years at NIST (06-07). • Then, we will move the experiment to the SNS 0.89-nm beamline to make 0.1% measurement.
Systematic Effects • Phonon up-scattering – negligible @150mK • Spin flip inside the trap - no zero magnetic field • - in non-adiabatic condition • 3He impurity – Mark II: < 10-13 (measured) <1s shift in neutron lifetime • Marginally trapped neutrons • Backgrounds
Previous Experiments Material Walled Bottle Beam
Backgrounds Background source Technique Proposed Improvements over Mark II Gammas high Z shielding 4p coverage reduce acrylic mass low background facilities Fast/epithermal Moderator/ 4p coverage neutrons absorber reduce acrylic mass low background facilities Cosmic rays vetoing better coverage Natural material selection radioactivity Neutron-induced material selection acrylic beamstop radioactivity monochromater external UCN production Neutron-induced material selection acrylic beamstop luminescence shielding better detection efficiency coincidence monochromator external UCN production Produce light even after beam off
Phonon Upscattering • Dominant process is two-phonon • upscattering; single phonon • process suppressed by • a Boltzman factor e-12K/T • Two phonon process scales • with temperature as; • 1/t2-phonon ≈ 10-2T7 • To achieve an upscattering • lifetime of 10-5tn requires • T < 150 mK T7 Data
