The cryogenic neutron EDM experiment at ILL Technical challenges and solutions James Karamath...

The cryogenic neutron EDM experiment at ILL

Technical challenges and solutions

James Karamath

University of Sussex

2

In this talk…

(n)EDM motivation

Measurement principles and sensitivity

Brief (recent) nEDM history

The Cryo-EDM experiment Overview of apparatus Summary of my DPhil work

Summary/conclusions

James Karamath University of Sussex 08/05/23 03:10 PM

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(n)EDMs – so? I

P- and T-violating

CPV in SM not fully understood e.g. insufficient CPV for baryon asymmetry

Strong CP problem θCP < 10-10 rad. Axions?


n n

p

×

S+

-d

S

-

+

d

4

(n)EDMs – so? II

Estimated EDMs model dependent SM dn ~ 10-31 ecm Other models typically 105-6 times greater

e.g. SUSY: CP < 10-2

quark electric dipole moments: q q

gaugino

squark

5

nEDM measurement principle

B0 E<Sz> = + h/2

<Sz> = - h/2

h(0) = -2μ.B h()=2(-μ.B+dn.E)

h()=2(-μ.B-dn.E)

B0 B0 E

dn defined +ve

↑↑ - ↑↓= Δ = 4dn.E / h

Ramsey NMR performed on stored Ultra Cold Neutrons (UCN)

6

Ramsey’s method of separated fields

Start with spin polarised neutrons in uniform B-field (Bz)

Apply oscillating B-field pulse (Bxy) perpendicular to Bz. Precession axis rotates down to xy-plane

Apply large E-field and allow to precess freely for ~300s

Apply 2nd, phase coherent with the first, oscillating Bxy. Neutron precession axis rotates down to –z axis.

7

Ramsey’s method of separated fields

However if an EDM is present a phase difference builds up during the free precession

If 180 out of phase second pulse returns spin back to +z axis.

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Ramsey’s method of separated fields(2n-1)π out of phase

Experimental runs taken at approx π/2 off resonance. Here dN/dν is a maximum.

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nEDM statistical limit

Fundamental statistical limit

α = visibility [polarisation product]E = E-field strengthT = NMR coherence timeN = total # counted

NET

dn

2


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nEDM systematic limit

Main concern: changes in B-field accidentally correlated with E-field changes give false dn signal

h(ν↑↑–ν ↑↓) = 2|μn|(B↑↑–B↑↓) – 4dnE

True nEDM signal

False signal due to varying B

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nEDM experiments: history

Co-magnetometer era

Cryogenic UCN era

RT stored UCN era

NET

hdn

2

Beam eraΔB ≈ v x E / c2 limited

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RT nEDM experiment at ILL

Create UCN, can then be guided & stored

Polarise UCN UCN admitted into

cell with E and B-fields and stored…

Mercury polarised by Hg lamp and added to cell

N S

Storage cell

Magnet & polarizing foil / analysing foil

UCN

Approx scale 1 m

BE

Magnetic field coil

High voltage lead


Magnetic shielding

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RT nEDM experiment at ILL

Ramsey NMR performed

Released from cell Neutrons spin

analysed (# fn of precession)

Mercury spin analysed.

Repeat: E=↓or 0, B=↓

N S

Magnetic shielding

Storage cell

UCN detector

Approx scale 1 m

Magnetic field coil

B

High voltage lead

E

Magnet & polarizing foil / analysing foil


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Systematics I

Mercury fills cell uniformly, UCN sag under gravity, lower by ~3 mm.

Thus don’t sample EXACTLY the same B-field. Axial (z) gradients → problems…

Magnetometer problems

Hg nz


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Systematics II

Two conspiring effects v x E: motional particle in electric field

experiences B-field: ΔB ≈ v x E / c2

Axial field gradient dB/dz creates radial B-field (since .B=0) proportional to r, Br r

Let’s look at motion of a mercury atom across the storage cell

Geometric Phase Effect (GPE)

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Systematics III Geometric Phase Effect (GPE)

dB/dz → B r

B v x E Scales with E

like EDM!!!

Scales with dB/dz

(GPEHg ~ 40GPEn)

Resultant

i.e. B0 field into page has gradient

Shifts resonance of particle

Using Mercury

introduces error

E and B0 into page

Rotating B field

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Final result

Room temperature experiment gave the result;

dn = (+0.61.5(stat) 0.8(syst)) x 10-26) ecm

i.e. |dn| < 3.0 x 10-26 ecm (90% CL).

New cryogenic experiment will eventually be x100 more sensitive…

www.neutronedm.org

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The Cryogenic nEDM experiment

Reminder: NET

dn

2

RT Cryo

N /day 6x106 ~6x108

T /s ~130 ~260 0.75 ~0.9E /kV/cm ~12 ~25(B0 /μT 1 5)

~10-28ecm

*

*with new beamline

x20 x5*

x2

x1.2

x2

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Improved production of UCN (↑N) I

Crosses at 0.89 nm for free (cold) n. Neutron loses all energy by phonon emission → UCN.

Reverse suppressed by Boltzmann factor, He-II is at 0.5K, no 12K phonons.

Dispersion curves for He-II and free neutrons


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Improved production of UCN (↑N) II

Idea by Pendlebury and Golub in 1970’s, experimentally verified in 2002 (detected in He-II) for cold neutron beam at ILL (~1 UCN/cm3/sec).

Also better guides – smoother & better neutron holding surfaces, Be / BeO / DLC coated → more neutrons guided/stored. Allows longer T too.


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Polarisation and detection (α) I

Polarisation by Si-Fe multi-layer polarizer, 95±6% initial polarisation.

Can lose polarisation in 2 ways: “Wall losses” magnetic impurities in walls,

generally not aligned with neutron spin Gradients in B-field, if not smooth and steady

have similar effect


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Polarisation and detection (α) II

Detector: solid state, works in 0.5K He-II.

n (6Li, α) 3H reaction - alpha or triton detected

Thin, polarised Fe layer - spin analysis


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Magnetic field issues I

Target – need ~ 100 fT stability (NMR)Need ~ 1 nT/m spatial homogeneity (GPE)Perturbations ~ 0.1 μT (cranes!)Need (axial) shielding factor ~ 106

Mu-metal shielding ~ 50 Superconducting shielding ~ 8x105

Active shielding (feedback coils) ~ 15

Shielding factors

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Magnetic field issues II

CRYOGENIC nEDM! Utilise superconducting shield and B0 solenoid. Major part of fluctuations across whole chamber

(common mode variations) Magnetometer (zero E-field) cell(s) see same Very stable B0(t) current

Holding field x5 to reduce GPE of the neutrons by factor of 25 (GPEn 1/B0

2)

Extra benefits


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Magnetic field issues III

~fT sensitivity 12 pickup loops will

sit behind grounded electrodes.

Will show temporal stability of B-field at this level.

Additional sensitivity from zero-field cell(s)

SQUIDS

26James Karamath University of Sussex 08/05/23 03:10 PM

Now have a 400 kV supply to connect to HV electrode.

Will sit in 3bar SF6. For 160 kV use N2:CO2 first.

Improving the E-field (↑E) I: The HV

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Improving the E-field (↑E) II: HV line 1

50 kV ~1 GOhm resistors

Superfluid containment vessel (SCV)

HV electrode Ground electrodes

400 kV bipolar stack

N.B. Diamond-like-carbon coated titanium electrodesBeO spacers

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Improving the E-field (↑E) II: HV line 2

Spellman +130 kV

Spellman -130 kV

Thick walled PTFE tube and thin-walled SS tube HV “cryo-cable”.

Standard 150 kV cable

HV connection

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The dielectric strength of LHe

Has been tested in the past, mostly at 4.2 K (760 torr), at small electrode gaps (sub-mm) and with small electrodes. Superfluid data is limited and generally at low voltages (sub-40 kV, often sub-20 kV).

Usually the breakdown strength as a function of gap is studied. We’d like to know the strength as the pressure/temperature falls – esp. in the superfluid state.James Karamath University of Sussex 08/05/23 03:10 PM

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The dielectric strength of LHe II Past literature

He-I data

4.2 < T(K) < 2.2Nope – put in the final versions from thesis

Past literature representative Vbd (d ) data, T = 4.2 K.

1

10

100

1000

0.001 0.01 0.1 1 10

Electrode Separation d /cm

Bre

akdo

wn

Vol

tage

V /k

V

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The dielectric strength of LHe III Past literature

He-II data

2.2 < T(K) < 1.4

Past literature representative Vbd (d ) data, T < 2.3 K.

1

10

100

1000

0.001 0.01 0.1 1 10Electrode Separation d /cm

Bre

akdo

wn

Vol

tage

V /k

V

A

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The dielectric strength of LHe IV

Test electrodes submerged in He-II in bath cryostat.

Studying Vbd and Ileak as function of d, T, dielectric spacers, purity… up to 130 kV. Also electrode damage.

E

±HV

cryostat

He-II (T, purity…)

gap (d, V, spacers)

Sussex HV tests

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The dielectric strength of LHe IV Sussex HV results

Vbd (P) at d = 0.50 cm (many data runs)

0102030405060708090

100110120

1 10 100 1000Pressure /torr

Bre

akdo

wn

Vol

tage

V /k

V

34

The dielectric strength of LHe V

Vbd (T) (past literature and present data)

0

20

40

60

80

100

120

140

160

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Temperature /K

Bre

akdo

wn

Vol

tage

V /k

V

Long d=5 cm scaled Long d=0.5 cmKaramath d=0.7 cm scaled Karamath d=0.5 cm rawWu d=0.127 cm scaled (1 bar) Blank d=0.1 cm scaledWu d=0.0254 cm scaled (1 bar) Mathes d=0.019 cm scaled

l

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The dielectric strength of LHe VI

Histogram of all 0.7 cm sub-1.8 K Vbd data

0

5

10

15

20

25

30

35

40

35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80

Breakdown Voltage V /kV

Freq

uenc

y

35 45 55 65 75

All sub-1.8 K d=0.7cm breakdown dataGaussian

Extremal Type I

Extremal Type II

Statistics

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The dielectric strength of LHe VII

Size effects: Weak but important dependence on electrode area or stressed fluid volume may decrease dielectric strength.

Leakage currents never found to be >0.1 nA (sensitivity limited) even immediately below breakdown.

~0.3 mm craters in electrodes when breakdown occurs at >80 kV. Bad news for DLC coated electrodes.

37

The dielectric strength of LHe VI

Breakdown strength reduced by insulating BeO spacers by a factor of ~1.4. Due to surface tracking along the BeO.

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The dielectric strength of LHe summary

At 0.7 cm gap the breakdown field strength was approx 80 ± 10 kV/cm. i.e. ~50 kV/cm for 1 in 1000 chance of breakdown. May have to half this if size effects indeed exist.

What controls breakdown – pressure or temperature?! May hold key to improving Vbd.

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And so, the CryoEDM experiment I

n guide tubes + spin analyser

E ~ 25kV/cm

E = 0kV/cmSpin flipper coil (measure other spin)

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And so, the CryoEDM experiment IIHV electrode

Ground electrodes

HV in

z

Carbon fibre

support

BeO spacers

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And so, the CryoEDM experiment IIIHV electrode

Ground electrodes

G10 Superfluid

containment vessel

HV in

z Neutrons in/out

Guides not shown

250l He-II 0.5K

**

* BeO spacers/guides

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And so, the CryoEDM experiment IV

1m

Dynamic shielding coils

Magnetic (mu-metal) shields

Superconducting shield and solenoid

The shielded region

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Schedule / Future

Finish construction THIS YEARStart data taking THIS YEARFirst results ~2009Upgrade neutron guide to ↑N ~2009 ?


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Summary(n)EDMs help study T-violation and are

constraining new physics.Final RT result: |dn| < 3.0 x 10-26 ecm.Aim to push well into 10-28 ecm.Further work needed to understand

dielectric properties of He-II. Only 20 kV/cm? (Paper in preparation.) Can pressure/purity/electrode material make a difference?

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Done!

Thanks for listening

The cryogenic neutron EDM experiment at ILL Technical challenges and solutions James Karamath...

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