1. Searching for a nEDM at PSI
P. Schmidt-Wellenburg on behalf of the PSI-nEDM collaboration

2. The collaboration
6 countries
14 institutions
45 members

3. See talk by B. Lauss UCN source at PSI nEDM UCN Source
Proton Accelerator
590 MeV Cyclotron
2.2 mA beam current
See talk by B. Lauss
nEDM
UCN Source
2 experimental areas / 3 beamlines

4. Outline The apparatus Ongoing measurements and results
UCN - performance
Magnetometers and field control
High voltage and leakage currents
Ideas for the next generation experiment

5. Apparatus overview
This is a sketch of the apparatus. In the center you see both high voltage electrodes and the insulator ring. This makes up the precession volume for the neutrons. The upper electrode can be charge up to high voltage whereas the lower stays grounded. The entire setup is within an aluminum vacuum chamber. The main magnetic field coil, a cosine theta coil is wound onto the vacuum tank producing a field of 1muT. Further more whe have a total of 33 shrim coils to adjust and fine tune the field within the precession chamber. The vacuum tank is surrounded by four layers of Mu-Metal with to reduces the earth magnetic field. The neutrons coming from the source are polarised in a 5T superconducting magnet, than pass through a slit to be filled into the chamber. After the cycle the are detected inside a neutron detector. The spin analysis is done sequentially just above the detector. We use to types of magnetometers to monitor the field. Mercury gas is used to estimate the field within the chamber, Dominique Rebreyend will present this in his talk. Cesium magnetometers are used to measure the field locally around the main precession chamber. And can thus also be used as gradiometers.

6. The apparatus 2009 Transfer from ILL (France) to PSI (Switzerland)
Setup in thermally stabilized wooden house
Two independent air-conditionings
Six coils for surrounding field compensation (SFC)

7. Outline The apparatus Ongoing measurements and results
UCN - performance
Magnetometers and field control
High voltage and leakage currents
Ideas for the next generation experiment

8. UCN operation Empty Filling Monitor
Just before Christmas we got ready for UCN. The UCN source story has already been told today, thus I will only add a small supplement.
We prepared the apparatus for UCN, the SC magnet was flown in and installed at the end of the UCN source guide. we installed our own Nickel molybdenum coated glass guides, and the switch. In a rapid test at the BOA beam line here at PSI we could show, that the detector can digest high count rates. We identified the neutron signal and learned to discriminated it from the background. The detector was then finally mounted below our apparatus.
A. Serebrov et al., NIMA 545(2005)490

9. UCN detector 6Li doped glass scintillator stack3H 
6Li
6Li depleted
6Li enriched
110 µm µm
Time (ns)
Signal (mV)
6Li doped glass scintillator stack
9 independent channels (PMTs +DAQ)
High count rate capable > 10 M
We use a 6Li based scintillator in combination with a PMT that has been developed by the collaboration. It consist of 9 individual channels to accept high rates.
UCN are captured in the Li enriched region of the scintillator and the decay products of the reaction n+6Li -> 3H + He produce a signal in the PMT.
The reason for the stacked scintillators allow to collect all decay products. However, two stacks have been applied upside down as one can see in the signal to background separation.
30/11/2011
Integration gate: 9 –200 ns
Channel 1 & 9 scintillator upside down
Channel 8 has noise
G. Ban et al., NIMA 611 (2009) 280

10. UCN Detector Monitor mode ~70000 UCN Emptying ~30000 UCN
High UCN losses
High depolarization rate
G. Ban et al., NIMA 611 (2009) 280

11. UCN emptying curve
During emptying: high loss rate of stored spin component → wrong polarisation
τflip = 236 s τloss =16.5 s τ↓ = 163 s τ↑ = 16.9 s N = α0 = 0.999
Best fit to data

13. UCN storage time
We measured the storage time for neutrons in our system, as this directly influences our sensitivity (On the one hand a longer free precession time is preferable, on the other hand the number of neutrons counted decays with time du to slits, up scattering and neutron decay)
Two exponentials are fitted to accommodate for the fact, that the neutron spectrum softens with time.
The result was expected for setup and combination of materials.

14. UCN Spin performance Rough adjustment of trim coils
UCN detection spin sequence not yet optimized

15. Ramsey curve taken with 250 s precession time
UCN Ramsey cycles
Ramsey curve taken with 250 s precession time

16. Sensitivity E=110/12 kV/cm N10=9838 N20=8042 T1=56.6 s T2=182.5 s
α0=0.79
Tα=556.6 s
Minimum:
σ(219s)=5.94×10-24 e·cm
→ σ(1d)=4×10-25 e·cm
Assuming 30 x more UCN

17. Outline The apparatus Ongoing measurements and results
UCN - performance
Magnetometers and field control
High voltage and leakage currents
Ideas for the next generation experiment

18. The measurement technique
Measure the difference of precession frequencies in parallel/anti-parallel fields:
For the main part of my talk I would like to concentrate on this little term there. Which makes life difficult when changing from parallel to anti parallel field configuration.
RAL-Sussex-ILL: dn < 2.9 x 10–26 e cm C.A.Baker et al., PRL 97 (2006)
for dn<10-26
ω < 60 nHz

19. Magnetic shield Four layer Mu-Metal
Shielding factors: x: 12000, y: 3000, z:8000

20. Surrounding field compensation
Surrounding field (~ 80μT)
Compensation and stabilization
Three coil pairs:
6m x 8m, d= 4m
9/18 windings
Six current supplies (10/20 A)
Ten 3-axis Fluxgates (FG)

21. Sensor positions Z Monitoring positions close to shield (~ 0.3…0.8 m)
nEDM
Coordinate
system
FG 9
FG 8
FG 3
FG 6
FG 1
Al frame Y
FG 5
FG 7
Thermo house
(first floor)
Magnetic shield
door
FG 0
FG 2 X 21

22. Results Feedback with inverted & regularized Matrix
Twelve sensors close to shield taken into account
(for x-direction shown below:
sensors 0x, 3x, 6x, and 8x are used)

23. Systematic effects Most important source of systematic effects
→Field mapping
→Online Cs-OPM measurement
→Dedicated B-drift runs (ramping E-field)
→Magnetic scanning at PTB, Berlin
× 10-27

24. Hg co-magnetometer
See talk by D. Rebreyend

25. Principle of Cs magnetometer

26. Monitoring of vertical magnetic gradients
Cesium magnetometers
Monitoring of vertical magnetic gradients
Two cesium magnetometer arrays
Stabilized laser
PID phase locked DAQ
±140kV 1 2 3 4 5 … 11 12
I have already told you that we used cesium magnetometer for mapping. The original reason was to have a insitu magnetic field gradiometer made of an array above the high voltage electrode and one array below the bottom ground electrode. This year we have setup a power stabilized laser. It provides the laser light to both cesium magnetometer arrays by a fiber beam splitter. The cesium are read out and driven by a dedicated data acquisition system. At the moment 9 are mounted further will be added this spring.

27. Homogenizer and
fiber bundle
Fiber bundle
Beam splitter mounted on
the enclosure support
31 vacuum feedthroughs
for optical fibers

28. Adjusting field gradients
Measure the response of all n=17 magnetometers to changes of each m=33 individual coil current
10nT
Known response allows to calculate ideal currents for given field setting (iterative process)
10pT

29. Gradients
STD from six gradiometer pairs

30. Uncompensated field drift
Magnetization through changing polarity
Coming now back to the table I have showed you in the beginning. One systematic effect is directly correlated with the change of the polarity of the electric field. It is called uncompensated field drift and is thought to be produced by a magnetization of something by charging currents. It is called uncompensated, as most of this changed field would be compensated by the change of the co magnetometer, however a gradient would not.
One can estimate the effect to be 1.8 time e cm per fT/cm of field gradient change.

31. Uncompensated field drift
+100
E [kV]
100 s
time
-100
For this purpose we have frequently ramped the electric field from +100 to minus 100 kV measuring at the same time the magnetic field with cesium magnetometers. This was done as HV did not work with cesium at vacuum.
Ramping speed = 1 kV/s
Charging current = 35 nA

32. Uncompensated field drift
No effect is observed at the level of 2.8 fT/cm
Translates into
What I show you here is a preliminary result of this autumns test run. You see for two cesium combinations that all data scatters around zero gradient. In fact, no effect was observed on the level of 2.8 fT/cm which translates into a systematic effect of 2.5 time 10 to the minus 27 e cm.
No effect is observed at the level of 2.8 fT/cm
Dedicated runs (daytime)

33. Outline The apparatus Ongoing measurements and results
UCN - performance
Magnetometers and field control
High voltage and leakage currents
Ideas for the next generation experiment

34. High voltage Cesium work with HV HV did not work with Cs
Tests
HV works up to 200kV
Flashovers along fiber bundles
Reliable HV runs at ±150kV 3
Leakage 195 kV 2
Another crucial piece of the apparatus is the high voltage system. During investigations concerning the performance of the HV cesium we found out that the cesium do fine in high voltage whereas we can’t ramp the high voltage in vacuum.
We started detailed investigations taking out everything and showing, that we can operate the old apparatus up to 200kV, however, 195kV if we want leakage currents across the chamber below 1nA.
The we put back in the parts we need for measuring a magnetic field. The fiber bundles from the cesium provoked flashovers already at 100 kV, taking them apart in single fibers improved things dramatically. Another problem might be the polarizing optics in the gap between high voltage and ground. However, the system does not always give very clear answers. nA 1
15:06
15:14

35. Testing high voltage Configuration w bundle -110 kV vacuum
145 kV He/Ne
Configuration wo bundle
198 kV He/Ne
200 kV He/Ne

36. Leakage current Leakage currents are caused by the high voltage
and appear along the surface of the insulator ring.
Changing the polarity of the high voltage will change the direction of the leakage current, and hence the magnetic field produced by these currents
Most contribution of the leakage currents cancel out, not so jφ. jr jφ jφ jr jz jφ
A leakage current of 1 nA produces a false edm of 2 × e cm

37. Leakage current measurement
3.5600
FEMTO
3.5595
3.5590
3.5585
Current (nA)
3.5580
3.5575
3.5570
-80
-60
-40
-20 20 40 60 80
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
ILeak (nA)
UHV (kV)
ILeak ≤ 0.5 nA
σ ≤ 0.1 × 10-27e cm
0.75pA
3.5565
4 pA
3.5560
3.5555 2 4 6 8 10
Time (h)
Monitoring of leakage currents on ground electrode
Combination of protection circuit and highly sensitive current/voltage amplifier

38. Outline The apparatus Ongoing measurements and results
UCN - performance
Magnetometers
High voltage and leakage currents
Ideas for the next generation experiment

39. 3He see talk by A. Kraft n2EDM: General concept
Simultaneous measurement in 2 precession chambers
Laser based Hg co-magnetometer
3He magnetometers
Multiple Cs magnetometers for 3He readout and gradients
UCN chamber position at PSI UCN beam height E
3He see talk by A. Kraft

40. Thermohouse2
10×6×8 m3
EMC shield made of copper
Thermally stabilized

41. Conclusion & Outlook Apparatus is ready for data taking
Presently remeasuring UCN parameters
High quality adjustment of B-field gradients
Excellent performance of high voltage
nEDM data taking from Nov 2012
400 nights of data in 2013/2014 → σ < 5×10-27 e·cm
In parallel design of next generation experiment → σ < 5×10-28 e·cm

42. The Neutron EDM Collaboration
Physikalisch Technische Bundesanstalt, Berlin
Laboratoire de Physique Corpusculaire, Caen
Institute of Physics, Jagiellonian University, Cracow
Henryk Niedwodniczanski Inst. Of Nucl. Physics, Cracow
Joint Institute of Nuclear Reasearch, Dubna
Département de physique, Université de Fribourg, Fribourg
Laboratoire de Physique Subatomique et de Cosmologie, Grenoble
Biomagnetisches Zentrum, Jena
Katholieke Universiteit, Leuven
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Orsay
Inst. für Kernchemie, Johannes-Gutenberg-Universität, Mainz
Inst. für Physik, Johannes-Gutenberg-Universität, Mainz
Paul Scherrer Institut, Villigen
Eidgenössische Technische Hochschule, Zürich
M. Burghoff, A. Schnabel, J. Vogt
G. Ban, V. Helaine1, Th. Lefort, Y. Lemiere,
O. Naviliat-Cuncic, G. Quéméner
K. Bodek, G. Wyszynski3, J. Zejma
A. Kozela
N. Khomutov
Z. Grujic, M. Kasprzak, P. Knowles, H.C. Koch4, A. Weis
G. Pignol, D. Rebreyend
S. Afach, G. Lembke
N. Severijns, P. Pataguppi
S. Roccia
C. Plonka-Spehr, J. Zenner1
W. Heil, A. Kraft
G. Bison, Z. Chowdhuri, M. Daum, M. Fertl3 , B. Franke3,
B. Lauss, A. Mtchedlishvili, D. Ries3, PSW,
G. Zsigmond
K. Kirch1, J. Krempel, F. Piegsa
I would like to thank you all for your interest and all these colleagues for their excellent work!
Thank you very much.
also at: 1Paul Scherrer Institut, 2PNPI Gatchina, 3Eidgenössische Technische Hochschule, 4GUM Mainz