1. HOPE – a magnetic UCN trap to measure the neutron lifetime
Felix Rosenaua, Florian Martina, Sergey Ivanova, Kent Leungb, Martin Simsona, Oliver Zimmera
aInstitut Laue Langevin, bNorth Carolina State University
Motivation
Determination of
Uncertainty due to radiative corrections:
Accuracy needed to bring down the experimental error bar to the same level:
State of the art
In beam experiments
886.3 ± 1.2stat ± 3.2syst s
Nico et al. Phys. Rev. C 71 (2005)
Magnetic bottle experiments
Permanent magnet 20-pole bottle
Ezhov et al. to be published
He-II filled 4-pole trap:
–63 s
Material bottle experiments
888.4 ± 3.3 s t  12 s
Nesvizhevsky et al. JETP 75 (1992) 405
885.4 ± 0.9stat ± 0.4syst s t  100 s
Arzumanov et al. Phys. Lett. B 483 (2000) 15
878.5 ± 0.8 s t  5 s
Serebrov et al. Phys. Lett. B 605 (2005) 72
880.7 ± 1.8 s t  110 s
Pichlmaier et al. Phys. Lett. B 693 (2010) 221
881.6 ± 0.8stat ± 1.9syst s
Arzumanov et al. JETP Lett. 95 (2012) 224 R E A N L Y S I
Marciano & Sirlin PRL 96 (2006)
Dzhosyuk et al. J. Res. NIST 110 (2005) 339
General characteristics
Magneto-gravitational trapping
No wall interactions
Fast coil ramping
Allows “fill and empty” measurements
Full diameter trap openings
Efficient electron detection
Manipulation of UCN gas by moveable piston and absorber
Efficient filling and emptying
Reflecting walls
Monitoring of depolarisation and marginally traped UCNs
Value
Accuracy needed
0.34 s
310-4
Current values (PDG 2012)
880 ± 1.1 s (S=1.8)
± (S=1.9)
The magnet system
Superconducting solenoids
Top coil (5 T)
Possibility to focus protons onto a detector
Bias coil (0.5 T)
Avoid depolarisation
Decay particle extraction
Bottom coil (1.7 T)
Magnetic UCN plug
LHe bath
Charged particle detection
A charged particle detector can be installed on top of the trap
Bias field guides the particles to the detector
e--simulations gave a 60% probability of reaching the top of the trap
Absorber
Permanent magnet system
provides radial confinement
32 magnet slices form a Halbach type octupol
NdFeB permanent magnets, BR = 1.35 T
B(r) = BR(r/R)3
12 stacked modules: 1.2 m total length
Projections of a single electron track
inside the trap
Distribution of electrons at the top of the trap
UCN potential [neV]
Spectrum cleaning
PET absorber
A movable absorber comes from the top
The absorber is shaped to let pass the decay products
Teflon plug
A teflon plug comes from the bottom for symmetry breaking
Preliminary tests
permanent magnets have been tested on
PF2 using a material plug on the bottom
high storage times (>800s) have
achieved
Timeline
UCN guide
Plug
SUN2 on H172B: new UCN ILL, beam position for HOPE
Year
2013
2014
2015
2016
Quarter I II
III IV
Installation on SUN2
First systematic tests
Construction and tests of electron detector
“fill and empty” neutron lifetime experiment
Lifetime measurement with electron detection
Sketch and photo of the test setup at
Result: Long storage time,
limited by material plug