1. A new simultaneous spin analyser (USSA) for the PSI nEDM experiment
Challenges of the world-wide experimental search
for the EDM of the neutron
Ascona 2014
A new simultaneous spin analyser (USSA)
for the PSI nEDM experiment
V. Hélaine*, T. Lefort, G. Ban, D. Goupillière, Y. Lemière, G. Quéméner
on behalf on the PSI nEDM collaboration
* Shared PhD between PSI and LPC
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Laboratoire de Physique Corpusculaire - Caen

2. The reasons for the spin analysis
Neutron EDM extraction requires neutron frequency measurement
Neutron frequency extracted from the central Ramsey fringe fit
Higher fit sensitivity where the slope is the largest: 4 working points
N↑ and N↓ need to be analysed and counted in order to get :
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

3. The experimental set-up: OILL spectrometer
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

4. The sequential spin analyser
Spin analysis performed by transmission:
- Magnetised iron layer: V = VF(Fe) ± µ.Bfoil
- spin down component able to cross, spin up reflected
ASF use to flip the spin up component
Spin analysis is sequential: one spin is counted
while the other one is stored above the foil
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Main drawbacks
UCN losses and depolarisations during storage above the foil
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

5. The simultaneous spin analyser: principle*
SCOPE
Analyse both spin states at the same time in order to reduce the UCN losses and depolarisations
MEANS
Double the sequential analyser system  two arms
Each arm analyse one spin component
RESULTS
Storage time is reduced: N and α are increased
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Room for improvements:
- UCN reflections on the magnetised foil
- slits in the switch
[* I.S. Altarev et al, PLB 102 (1981) p 13]
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

6. The simultaneous spin analyser: design
For each spin states, two sets of UCN
½ are directly going into their dedicated arm (directly counted)
½ are going in the wrong arm and need to travel back
Directly counted UCN:
 specular reflection on the walls
 separation piece between both arms
Reflected UCN: get back UCN gone in the “wrong” arm
 define the upper part of the spin analyser
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

7. The simultaneous spin analyser: design*
U-shaped Simultaneous Spin Analyser (USSA)
 design based on GEANT4-UCN simulations
Directly counted UCN:
 low roughness (glass walls)
 separation piece between both arms (30 °)
Reflected UCN:
 define the USSA upper part geometry (45 °)
Make sure that spin analysis is symmetric
 Identical spin analysers (yoke, magnets, foils, ASF)
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
As compact (and light) as possible: L ~1 m, M ~150 kg
(Vacuum chamber not drawn)
(*Mechanical design performed by D. Goupilliere)
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

8. The simultaneous spin analyser: glass walls
USSA walls (15 assembled plates): made of float glass (roughness ~nm; non-magnetic)
NiMo (85/15) coating: non-magnetic, VFermi = 220 neV
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
(Coating PSI sputtering facility)
Assembled plates are not vacuum tight: surrounding vacuum chamber is used
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

9. The simultaneous spin analyser: spin analysis
Analysing foils: 400 nm of sputtered Fe on 25 µm Al
- field within the foil: 2 T
Magnetisation system: 40 magnets + soft iron yoke
- Magnetising field at the foil location : mT
- mounted outside the vacuum chamber
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
(Iron layer PSI)
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

10. The simultaneous spin analyser: spin analysis
Adiabatic spin flipper (ASF):
- axial RF field: coils wounded around each arm (BRF~ 0.2 mT, fres~ 20 kHz)
- transverse static field gradient: stray field of the mag. syst.* + 2 coils (dBtrans/dz~ 0.15 mT/cm)
Adiabaticity parameter k ~ 20 at the ASF location (Vn = 8 m/s)
Transverse field gradient define the ASF location:
We want to have the adiabatic SF as close as possible -> compact system
The field gradient has to rather low in order to increase the adiabaticity parameter.
During USSA operation: 1 SF ON, 1 SF OFF
- RF shielding is required to avoid crosstalk between arms
- 1 mm thick copper tube surrounding the RF coils
[* S.V. Grigoriev et al, NIM A 384 (1997) p 451]
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

11. The simultaneous spin analyser: UCN detectors
Two multi-channels detectors (NANOSC): based on 6Li doped glass fast scintillators ( ~70 ns)
n + 6Li  3H (2.73 MeV) +  (2.05 MeV)
Channel composition
- stack of scintillators: 6Li depleted + 6Li enriched
 edge events are suppressed
- molecular sticking between scinti.: optical coupling
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Main characteristics
- able to count up to 106 counts/s
- same efficiency as 3He gas detector
- first one is in operation since 2010
Light
pipe
Couple to the FASTER acquisition syst. (
PMT
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

12. USSA: first characterization @ ILL & PSI
First tests have been performed at ILL (04/2013)*
Further characterization performed at PSI (08/2013): WEST-2 beam line
NANOSC: same detection efficiency εA = εB ± 2 %
UCN detection within USSA (no polarisation, no spin analysis)
Transmission (no foil): ± 0.6 %
Asymmetry between arms w/o foils: 0.43 ± 0.07 %
Asymmetry between arms with foils : 0.40 ± 0.11 %
Spin analysis
ASF efficiencies: fA = 97.0 ± 1.2 % ; fB = 97.1 ± 0.9 %
Analysing power: PA = 91.0 ± 3.4 % ; PB = 89.7 ± 2.7 %
Crosstalk between arms: ΔN/N = 0.15 ± 0.62 %
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Symmetric UCN detection and spin analysis
*(thanks to Peter, Thomas and participating collaborators )
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

13. USSA: tests below the spectrometer
USSA has been tested during nEDM run and compared to sequential analyser
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
Sensitivity (dn) improvement: 18.2 ± 6.1 %
Further improvement using 58NiMo ?
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen

14. USSA: first characterization @ PSI
First beam polarisation P is measured:
(P1 removed)
(P1 removed)
(P2 removed)
(P1 removed)
(P removed)
Beam polarisation: P=70.6 ± 1.2 %
Then USSA characterization with P and SF1:
fA = 97.0 ± 1.2 %
fB = 97.1 ± 0.9 %
(PB removed)
(PA removed)
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
PA = 91.0 ± 3.4 %
PB = 89.7 ± 2.7 %
(PB removed)
(PA removed)
Cross-talk measurement:
- PA removed: NB counted with SFA ON or OFF
Challenges of the world-wide experimental search for the EDM of the neutron, Ascona 2014
Lefort Thomas – Interactions fondamentales – LPC Caen