How far can we get with in-beam

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How far can we get with in-beam UCN production at the ESS?

Oliver Zimmer

Nordita workshop on Particle Physics with Neutrons at the ESS, Stockholm, 10-14 December

(2)

“In-beam” (in contra-distinction from “in-pile”) means:

UCN source is installed in a cold neutron beam far from a moderator Advantages of “in-beam” w.r.t. “in-pile”:

• Lower radiation level

 lower cooling power required

 low temperature (down to 0.5 K) attainable

 access for UCN reflectors for maximum UCN density

• Experiment can be close to source or even “in-situ”

 low UCN transport losses (see proposed nEDM searches)

• Low backgrounds

• Easy access to the source for troubleshooting Disadvantages:

Cold neutrons from limited solid angle used for conversion to UCNs

 lower UCN production rate

 lower UCN fluxes

 lower total UCN numbers in big vessels

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UCN production in superfluid He

„phonon-roton“

dispersion of superfluid ⁴He free neutron dispersion

q



7 nm^-1

(0.9 nm  UCN) 1 meV

(12 K)

T [K] _max [s]

1 100

0.8 310

0.7 510

0.5 820

0 880

 need T < 0.5 - 0.6 K and low-loss walls (e.g. magnetic reflector)

R. Golub, J.M. Pendlebury, PL 53A (1975) 133

converter

cold neutron beam



_UCN

= P 

^-1 = ^-1_decay+^-1upscattering +^-1_capture+^-1wall losses

_capture(⁴He) = 0

(4)

Moderator brilliance _{𝑑𝜆𝑑Ω}^𝑑Φ at 9 Å:

1.3 × 10¹³ s⁻¹cm⁻²sr⁻¹Å⁻¹ (peak at 5 MW) 5.2 × 10¹¹ s⁻¹cm⁻²sr⁻¹Å⁻¹ (average at 5 MW) Usable moderator surface:

3(vertical) × 8(horizontal) cm²

Facts about the planned ESS moderator (Ken Andersen):

Beam extraction with mirrors:

Mirror made of natural nickel (𝑚 = 1):

1.73 mrad/Å  15.6 mrad/9Å  Ω = 2.4 × 10⁻⁴ sr

UCN production rate density at ESS moderator surface:

𝜌 ≈ 6.2 s

⁻¹

cm

⁻³

× 𝑚

² 𝜌 ≈ 5 × 10⁻⁸ Åcm⁻¹𝑑Φ 𝑑𝜆 _9Å

(5)

For comparison, at ILL:

H172B monochromatic beam (SUN-2):

≈ 5 s⁻¹cm⁻³, 8 × 8 cm² H523 (SuperSUN):

≈ 15 s⁻¹cm⁻³,  7 cm

Challenges for ESS in-beam UCN source:

(a) Need high brilliance transfer from moderator to UCN source for 𝜌 to come close to 6.2 s⁻¹cm⁻³ × 𝑚²

(b) A larger and colder moderator would increase the total number of UCNs after accumulation (which is ∝ source volume); the moderated spectrum would best peak at 9 Å.

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(a) How to deliver the neutrons to the UCN source?

OK

(guide phase space filled)

not OK

??

CN source Neutron transport system

min. distance

UCN source

big moderator

(too) small Moderator ESS situation

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M M’

Is an elliptic guide an imaging device?

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y y’

M M’

Is an elliptic guide an imaging device?

(9)

M M’

y y’

Is an elliptic guide an imaging device?

(10)

M M’

y y’

Is an elliptic guide an imaging device?

(11)

M M’

y y’

Is an elliptic guide an imaging device?

(12)

M M’

y y’

Cussen, Nekrassov, Zendler and Lieutenant:

Multiple reflections in elliptic neutron guides, NIM A 705 (2013) 121

“Transport of neutrons by realistic elliptic guides usually involves many reflections, contrary to the usual expectations.”

Is an elliptic guide an imaging device?

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Cussen, Nekrassov, Zendler and Lieutenant:

Multiple reflections in elliptic neutron guides, NIM A 705 (2013) 121

“Transport of neutrons by realistic elliptic guides usually involves many reflections, contrary to the usual expectations.”

Is an elliptic guide an imaging device?

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y y’

M M’

Is an elliptic guide an imaging device?

In general NO,

but YES, if we limit the reflection area on the mirror

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Multi-mirror imaging optics for low-loss transport of divergent neutron beams and tailored wavelength spectra

M M’

Cylindrical system: radial component r Planar system: cartesian component y

arXiv:1611.07353

Fills large solid angle from small source

Single reflections with well-defined reflection angles

 no garland reflections

 beam divergence (q-resolution) adjustable with scrapers

 pure spectra adaptable to need of instrument

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M M’

0 1 2 3 4 5 6

0.0 0.5 1.0

reflectivity

m

0 5 10 15 20

0.0 2.5

Intenstity (arb. units)

neutron wavelength (A)

Broadband supermirrors

(polarising or non-polarising) - m-value tuning to a common short-wavelength cutoff:

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0 1 2 3 4 5 6 0.0

0.5 1.0

reflectivity

m

0 5 10 15 20

0.0 2.5

Intenstity (arb. units)

M M’

Broadband supermirrors

(polarising or non-polarising) - m-value tuning to a common short-wavelength cutoff:

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M M’

Bandpass supermirrors

(polarising or non-polarising) - monochromation to a common wavelength band:

0 1 2 3 4 5 6

0.0 0.5 1.0

reflectivity

m

0 5 10 15 20

0.0 2.5

intensity (arb. units)

(19)

Masahiro Hino:

(20)

1.25 1.30 1.35 0.0

0.2 0.4 0.6 0.8

1.0 NiTi bi-layers:

100 200 400

reflectivity

q = 2k₀ (nm^-1)

Sergei Masalovich, NIM A 705 (2013) 121:

Analysis and design of multilayer structures for neutron monochromators and supermirrors

Quarter-wave layers can be adapted to select the width of the plateau reflectivity

m = 6

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Imaging works!

A planar elliptic multi-mirror already available as a McStas component thanks to Emmanuel Farhi

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k k+1

M M’

y

Geometrical neutron losses due to finite source size

no reflection

double reflection

These losses are  y/b_k and hence largest for the innermost mirrors b_k

(23)

Examplary system for 9 Å for a He-II UCN source

(MM’ = 30 m, mirror length = 2 m):

ESS moderator (y = 1.5 cm): losses < 10 % even for m < 1 mirrors

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Advantages of this type of optics (in fact of more general interest than only for a UCN source):

• Efficient brilliance transfer from small moderator

• Small-wavelength cut-off

• Low backgrounds of unwanted neutrons at instrument

• Monochromation of primary beam possible

• q-resolution (divergence) adaptable by scrapers

• Mirrors far away from source  small radiation damage

• Practical: easier exchange of beam tubes

• Options: stack several planar systems with different properties

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(b) Can we produce more very cold neutrons?

They would indeed be useful for everyone:

Neutron scattering community:

Particle physics community:

Counting statistics improvements e.g. for

• neutron-antineutron oscillation experiment

• beam neutron EDM experiment

• in-beam UCN source

From proceedings of workshop on application of a VCN source at Argonne, 2005

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Namiot’s proposal (1974):

“phononless cooling of neutrons to extremely low temperatures”

Cascaded neutron-deuteron spinflip scattering in a fully polarised medium in 30 T magnetic field

Energy transfer per nd spin flip collision:  0.1 eV/T

Zeeman energy of unpaired electron: 116 eV/T Usable for cooling?

Namiot, Sov. Phys. Dokl. 18, 481 (1974)

A suitable system should be paramagnetic because:

no dispersion  no kinematic restrictions  scattering cascadable!

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…look for weakly absorbing paramagnetic species:

0 = m +1

–1

Expect for Zeeman system:

0.12 meV T E_m

B

paramagnetic molecule ! 0.12 meV T

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…look for weakly absorbing paramagnetic species:

0 = m +1

–1

Expect for Zeeman system: but O₂ has triplet zero-field splitting:

0.12 meV T ≈ 0.4 meV = 𝑘_B × 𝟒. 𝟔 𝐊 !

E_m E_m

B …and it’s there without B-field !

m = 0 m = 1

paramagnetic molecule ! 0.12 meV T

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Solid oxygen is antiferromagnetic at low T (and dangerous)

O

₂

hydrate clathrate:

O₂ density ≈ 4.2 × 10²¹/ccm (90 % cage filling)

 stays paramagnetic at liq.He temperatures

 metastable (not explosive)

 neutron survival > 0.1 s if fully deuterated

Methane hydrate clathrate

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Inelastic scattering cross section for paramagnets?

to arrive at (for molecular oxygen with zero-field splitting):

Start from first-order time-dependent perturbation theory:

…

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0 1 2 3 4 5 6 0,00

0,05 0,10 0,15

neutron density (arb. units)

neutron energy (meV)

300 K 100 K 30 K

Neutron groups (j = 0, 1, 2, …):

E₁ E₂

Neutron moderation by the paramagnetic system

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For paramagnetic cascade cooling of neutrons

Solve rate equations for infinite medium:

source feeding depopulation absorption

and calculate stationary solutions…

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0 1 2 3 4 5 6 0,00

0,05 0,10 0,15

300 K 100 K 30 K

For low temperature of the medium, i.e., k_BT < E ^*  4.6 K:

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Stationary neutron group populations in O

₂

clathrate

0 2 4 6 8 10 12 14 16

1E-10 1E-9 1E-8 1E-7

medium temperature T :

30 K 8 K 2 K 1 K

neutron density n j (cm-3 neV-1 )

neutron energy E_j (meV)

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Moderator peculiarities:

Diffusion length (for inelastic magnetic processes):

Bragg-cutoff length:

whereas corresponds to

 O₂-hydrate is a flux trap for neutrons still to be converted to VCN!

for full moderation if there were only paramagnetic cooling

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Chazallon, Itoh, Koza, Kuhs, Schober, Chem. Phys. Phys. Chem. 4,

4809 (2002)

Short-circuiting of long paramagnetic cascade by Einstein modes at 4.8 meV?

0 1 2 3 4 5 6

0,00 0,05 0,10 0,15

300 K 100 K 30 K

 moderator could become much smaller

(41)

First experiments done at D20, IN4, IN6 and D7

Goal: determine absolute cross sections

A. Falenty, T. Hansen, M. Koza, W. Kuhs, O. Zimmer

Energy transfer (meV)

Scattering angle

shows dispersion-free excitation at 0.4 meV, magnetic form factor

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First experiments done at D20, IN4, IN6 and D7

Goal: determine absolute cross sections

A. Falenty, T. Hansen, M. Koza, W. Kuhs, O. Zimmer

Magnetic intensity seems in agreement with theoretical prediction

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For more details, please have a look at my paper:

“Neutron conversion and cascaded cooling in paramagnetic systems for a high-flux source of very cold neutrons”

Phys. Rev. C 93, 035503 (2016)

How far can we get with in-beam