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European Workshop on Neutron Optics NOP '075 - 7 March 2007

Paul Scherrer Institut, Switzerland

Monte-Carlo simulationof phase space transformation

of ultra-cold neutrons

G. Zsigmond, S. Mayer, P. AllenspachPaul Scherrer Institut and Atominstitut der Österreichischen Universitäten

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Outline

Motivation of the project

Phase space transformation of ultra-cold neutrons• What is a PST ?• Monochromatic cold neutrons from UCN

UCN input for PST: • The UCN-Source at PSI• The PF2 beam-line at ILL

Most recent MC simulations on PST of UCN

Outlook

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Motivation of the PST of UCN project

Examine “moving” optics: phase space transformation

In particular: phase space transformation of UCN into monochromatic cold neutrons using the high phase space density of UCN.

The planned experiment is a „proof of principle“-experiment.

The experiment should deliver results about transformation efficiency (i.e. loss mechanisms) and beam properties as function of PST parameters.

The validation of the MC-simulations (VITESS) will be of high value for the planning of future PST.

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Phase space transformation of ultra-cold neutrons

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Q /2=π/d

Phase space transformer (PST)

Most recent: W. Henggeler et al, Journal of Neutron Research, 13 (2005) 251

Phase space transformation: d3k → d3k’

Doppler-Shifter; Magnetic or Gravity: ΔE[neV] = 60×ΔB[T] = 100×Δh[m]

Moving mosaic crystals in view of a gain in spectral density were first studied for neutron backscattering instruments - IN16B, HFBS, RSSM

Schelten et al. 1984

no acceleration

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Phase space transformer for UCN

Idea: Use with UCN?

Sharp energy distribution of the UCNs Doppler-shifted towards cold neutron energies gives highly monochromatic cold or very cold neutrons.

New generation superthermal UCN sources + PST

Recent: M. Boehm et al, Journal of Neutron Research, 13 (2005) 241-250

acceleration

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T2=50 K

T1=293.6 K

T3=25 K

Gain factors from PST (H. Rauch et al.)

80 200 400 600 800 1000 1200 1400 160010-2

10-1

100

101

102

103

104

105

106

222 /)( TUCN vvvUCN ev

vGain −−=

Gai

n

v (= 3956/λ[Å]) ≅ 2vPST [m/s]

Tmod.eff = 1 meV/k = 11.6 K Tmod.eff = 2 meV/k = 23.2 K Tmod.eff = 0.5 meV/k = 5.8 K

Gain factor of an ideal PST relative to the (very-)cold flux of the same SD2 moderator used for UCN (6 m/s) production

gain

Flux gain factors from PST (transport)

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Phase space transformer for UCN

previous fig. simplified to ki ≈ 0

kf = Q = 2π / d Analyzer

(PST crystal)

Fast rotating arm version (N=1)

vPST [m/s] ≈ 1978.0 / d-spacing [Å] / cosαworking condition

UCN guide

30ºDetector

Collimator

Vacuum chamber

Analyzer armsdepending on αof the PST

cold neutrons, VCNλ ≈ d-spacing

α

PST crystal

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UCN input for PST: UCN-PSI and PF2 at ILL

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MCUCN(VITESS)

MC simulation of a Ø70mmaperture after UCN guide

0 50 100 150 200 250 300 350 4000

1x106

2x106

3x106

4x106

5x106

6x106

coun

t-rat

e /

UC

N/s

exit time / s

Time average:

1.5×106 UCN/s

3.8×104 UCN/s/cm2

Input flux from the PSI-UCN

A new pulsed SD2 UCN source is under construction at PSIucn.web.psi.ch

UCN input for PST after end of 2008 (PSI-UCN starts operation)

Ni

DLC

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Input flux from the UCN guide PF2

End of 2007 a test experiment at ILL on the PF2 beamlineResearch Proposal submitted for cycle 149. (30/10/2007-19/12/2007)

Beam characteristics:

1600 UCN/s/cm2

for vz < 7 m/s

MC simulation of wavelength distribution

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Most recent results on PST of UCN

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Intercalated graphite crystals

Recent: C.E.H. Mattoni et al. Physica B 344 (2004) 343

proof of principle experiment

Spin tests follow soon

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Recent MC simulations of PST / 1

R = 30 cm R = 3000 cm

Check negative effect of the velocity gradient: v(r) = ω r☞ find ways do diminish it ,

R parameter as a measure of deviation from ideal (linear) PST

Positions of Bragg reflections

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Recent MC simulations of PST / 2

Check effect of the offset from backscattering (α)☞ we need to work at larger α

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Recent MC simulations of PST / 3

Check initial velocity distribution of the reflected neutrons☞For higher tilt angles the neutrons have to be faster in order to fulfill

the Bragg law.Bragg

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Outlook

• Learning about moving optics: The PST experiment will deliver results about transformation efficiency and beam properties as function of PST parameters and these can be used to validate the MC simulations.

• Work in progress:More sophisticated rotational systems will be simulated. The simulations of the simple system will be used as reference.

• Test „proof of principle“ experiment at ILL on the PF2 beamlineResearch Proposal submitted for cycle 149.