1. Measurement of the neutron lifetime with a Magneto Gravitational trap
Gilles Ban
LPC-Caen, ENSICAEN–CNRS/IN2P3 Université de Caen Basse-Normandie
Caen, France

2. People and institutions
Present Russian-French collaboration
V.Ezhov1, A.Z.Andreev1, G.Ban2, B.A.Bazarov1, P.Geltenbort3, A.G.Glushkov1, M.G.Groshev1, V.A.Knyazkov1, N.A.Kovrizhnykh4, O.Naviliat-Cuncic2, V.L.Ryabov1
1 Petersburg Nuclear Physics Institute, Gatchina, Russia
2 LPC-Caen, University of Caen, ENSICAEN, Caen, France
3 Institut Laue-Lagnevin, Grenoble, France
4 Institute of Electro-physical Apparatus, St-Petersburg, Russia
Supported in part by
RFBR
ECO-NET
“ONELIX”

3. OUTLINE 1. Motivations for precision measurements of tn
2. Ultra cold neutrons, and source
3. Neutron lifetime experiment
4. Conclusion and outlook

4. Motivations 1) Value ? PDG tn = 885,7 ± 0,8 s 2) CKM Matrix, Vud
Strength of weak interaction u and d quarks
3) BBN input
Helium abundance directly related to tn

5. Present status of tn
PDG : (885.7±0.8) s
All recent storage experiments (including the most precise) used material storage of UCN (although not under the same conditions).
A.Serebrov et al.
PLB 605 (2005) 72
PRC 78 (2008)
878.5 ± 0.8 s
A. Pichlmaier et al
PLB 683 (2010)
880.7 ± 1.8 s

6. tn accepted value ? From the Particle Data Group
K Nakamura et al. JPG 38 (2010)
« It is up to workers in this field to resolve this issue. Until this major disagreement is understood our present average of ± 0.8 s must be suspect »
New experiments are underway, beam, material walls, magnetic trapping since 2005…

7. Vud, CKM and SM ) 3 1 ( sec 2 4908 l t + × ± = V
Neutron lifetime, axial/vector strength, up and down mixing are related
 Knowing 2 gives the other one ) 3 1 (
sec 2
4908 l t + × = ud V
(Marciano, Sirlin 2006)
l=Ga/Gv extract from A measurement
t experiments
Other way 0+0+ ,very high precison
0,97425 ± (Towner & Hardy PRC 79, 2009)

8. BBN Input
As the universe expands, the temperature drop results in a departure from equilibrium conditions (neutron-proton inter-conversion breaks). Neutrons are free to decay.
Nucleosynthesis begins effectively below the photo-dissociation threshold for deuterons (2.23 MeV).
Nearly all surviving neutrons end up in bound 4He nuclei. The experimental parameter most important in determining
Yp is the neutron lifetime, which normalizes (the inverse of) Γn↔p.
Courtesy A. Coc
Still even a few seconds difference in t won’t strongly affect the knowledge of Yp
Or constraint more
Astrophysical observations

9. Techniques Cold neutrons Beam ( p Penning trap),
Protons and/or beta detection
 Fiducial volume, detection efficiency
Materials or magnetic bottles Ultra cold neutrons
Protons/beta detection, surviving neutrons
 Losses, wall interaction or depolarisation

10. Ultra Cold Neutrons Ultra Cold Neutrons, UCNs
Fast neutrons En~ MeV up to ~1 GeV  ~ 10-7 eV= 100 neV
Very Cold Moderator (<30K)
Liquid or solid D, SF He
100 neV  1 meter in the gravitational field
 1 T, 60 eV/T
 few m/s (slower/faster than you ?)
 Collective interaction, containment, guiding
For example copper Ef=170 neV
Most Powerfull user source
Institut Laue Langevin , ILL , Grenoble

11. Institut Laue Langevin, HFR, UCN Source
ILL, Grenoble, France
the only multi-user UCN source in operation
high flux reactor(thermal power: 58.3 MW)
thermal n-flux: 1.5 ×1015n/s/cm2
cold neutron source: 20 l of LD2at 25K
vertical neutron guide 13m, 7×7 cm2, (58Ni coated)
UCN source: rotating turbine
UCN density at experiment: ρ= 20 /cm3

12. 3D magnetic trapping
For mn  neV/T, a 2T field generates a 120 neV barrier.
Force due to field gradient, F = -m (dB/dz), repels only one spin state.
Use permanent magnets.
Step 3: 3D confinement
Step 1: 1D confinement
- top (gravity)
- bottom (magnetic shutter)
1 – permanent magnets
2 – magnetic poles
Step 2: 2D confinement
Add magnetic shutter

13. …in real life 20 segments; 560 NdFeB magnets; FeCo poles
inner walls covered with Fomblin oil
(reflect “wrong” spin neurons during filling and depolarized neutrons during storage)
field gradient near wall: 2 T/cm
15 l total- (9 l storage-) volumes

14. Experimental setup Main elements:
lift, trap, solenoid, shutter, detector to
vacuum
pump
material
shutter
magnets
and poles
yoke
outer
solenoid
UCN detector
absorber
lift
cylinder
UCN
Lift: Fomblin coated Al cylinder + PE disk
Trap: Fomblin coated magnets and poles
[V.F. Ezhov et al., NIM A 611 (2009) 167]

15. Normalized time spectra
stop storage measurement
(start 100 s background measurement)
Storage time
1000 s
1300 s
1800 s
→ Continuous monitoring of “leaking” neutrons (which are forced depolarized in the trap, reflected by the Fomblin coated walls and detected)

16. Systematics
Depolarized neutrons not detected (Absorbed by walls) = missing : beta decay like
Forced depolarisation to monitor the lost neutrons which are not detected
Vaccuum : measurements at different pressure 1s precision = torr >>Work pressure= torr
Systematics studies limited by number of UCN
Need of strong source (>100 /cm3)

17. Result
tn = ± 2.0 ± ?.?(syst) [s]
Coming soon

18. Future plans Final Value
Construction of a new trap with a 90 l storage volume
60 segments disposed on a Ø80x40cm2 cone, Ø6cm guide
NdFeCo magnets and FeCoNi poles
Fill trap from top with adapted “lift”
Use forced depolarization to change trapping conditions (leaks)
Add spin analysis system with UCN detection
Project funded (France/Russia)
Runs in at ILL in 2011

19. Summary
The use of permanent magnets has provided so far the most sensitive value for the neutron lifetime with magnetically stored UCNs.
Key features of the trap for the measurement of the neutron lifetime are:
- filling of the trap from top with a slow lift
- continuous monitoring of leaking neutrons
- tuning leaks by depolarizing neutrons inside the trap
A final value of the neutron lifetime is expected to be issued soon from measurements performed with the prototype trap. Lower than PDG to what extent (??)
An improved setup is under construction which will enable to reach a statistical precision below 1s (and hopefully solve the existing puzzle).
Move to a stronger stronger source for systematics studies (??)

20. THANK YOU FOR YOUR ATTENTION !!

21. Trap filling sequence 1.Fill lift volume 3.Move lift down
4.Open lift and
move lift up
2.Close lift volume
vlift << vn
lift
cylinder
material
shutter
UCN
UCN detector
3 He

22. Vud, CKM and SM
To be competitive with this value t and l have to reach 10-4 precision ~10 times better precision for both
Using Vud and l values one gets (S. Paul NIM A )
880.1 ± 1.3 s < t < 887 ± 3.46 s