MiniMini--DD22First First experimentsexperiments at at the the

TRIGA TRIGA reactorreactor in Mainzin Mainz

5th International Workshop on

Ultra Cold & Cold NeutronsSt.Petersburg

13th to 18th, July 2005

SpeakerWolfgang Schmid

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I. Altarev, P. Eger, A. Frei, A. Gschrey, E. Gutsmiedl , F.J. Hartmann, S. Paul , W. Schmid, D. Tortorella, O.Zimmer1], W. Heil2], Y. Sobolev3], Y. Pokotilovski4],

K.Eberhardt, N Trautmann, N. Wiehl 5]

1]Technical University of Munich, Physics Department E18, Munich, Germany

2]University of Mainz, Physics Section, Mainz‚ Germany

3]Petersburg Nuclear Physics Institute, Gatchina, Russia

4]Joint Institute of Nuclear Research, Dubna, Russia

5]University of Mainz, Nuclear Chemistry Institute, Mainz, Germany

OverviewOverview

• Basic idea and concept of Mini-D2

• First experiments at a pulsed reactor

• Theory and reality

• Conclusion and outlook

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Basic Basic idea idea and and concept concept of Miniof Mini--DD22

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UCN

How areHow are UCN UCN producedproduced??

By downscattering of higher-energy neutrons

fission thermal

cold

We need a converter material with

• Large inelastic scattering cross section

• Small absorption cross section

• Very low temperature

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2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2010-5

10-4

10-3

10-2

10-1

100

Cross sections for solid D2

up-scattering σup

5 K

σup T )+σabs

σ(b

)

T (K)Deuterium temperature

σabs

liquid D (T = 19 K)2

and absorption (v = 2200 m/s)

σabs

n

Solid Solid orthoortho--DD22 as as aa converterconverter

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• Good σdown

• Rather low σabs

• In solid D2 σup becomes negligible compared to σabsbelow ~ 5K

(„super-thermal source“)

• High UCN conversion yield

• Ortho-D2 energetic ground state

� Solid ortho-D2 is a goodchoice as convertermaterial

MiniMini--DD22 inside the beaminside the beam tube SR 4tube SR 4

cold source

beam tube SR4

In-pile cryostat

UCN-storage tube

UCN storage tube equilibrium :• absorption and up-scatteringin the converter

• absorption and up-scatteringin the tube during wall collisions

• decay• escape through holes in the tube

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Converter zoneConverter zone of Miniof Mini--DD22

Solid D2converter

„In-pile cryostat“

300 K cooling channels(He gas)

UCN-storage tube

5 K cooling channels(He liquid)

25 K cooling channels(He gas)

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First First experiments experiments at at a a pulsed reactorpulsed reactor

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The The TRIGA TRIGA reactor reactor in Mainzin Mainz

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TheThe TRIGA Mainz TRIGA Mainz setupsetup

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pulsedneutrons

TestTest cryostatcryostat in in realityreality

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Monolayer neutron detectorMonolayer neutron detector

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Silicon PIN-diode with one layer of 6LiF

• Neutron detection via 6Li + n � 4He + 3H + 4.8MeV• 6LiF-layer with thickness of 0.8µm � tunneling probability < 1%• Surface potential 120neV• UCN are produced at U(D2) ~ 103neV and gain EG ~ 30neV gravitational energy � nearly all UCN can pass surface potential of detector, countrate NOT reduced• Successfully tested for UCN at the ILL/Grenoble• Efficiency ~ 50%

Theory Theory and and realityreality

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ProductionProduction raterateThe UCN production rate P is given by:

fifii

i

E

atom dEdEEEdEdE

dEdP

g

),()(0 0

σ∫ ∫

∆ ∞ Φ=

atomDD PNP ⋅=22

2

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and

For a flux of Φ ~ 1010 n/cm2s and a pulse length (FWHM) of 65 ms we expect a total number of ~ 100,000 UCNs per pulse

to leave the D2 converter!

TransportTransport

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• Calculation of UCN transport rates through guidance tube via Monte-Carlo simulation

• Monte-Carlo method simpler and more convenient than numerical solution of diffusion equation

• Used values:

With a estimated detection efficiency of 50% we expect

~ 5000 UCN counts per shot

- Wall loss probability µ = (1.2+0.1)*10-3

- Diffuse reflection probability f = 0.15

(both measured at ILL)

- UCN energy spectrum from 4 m/s (sD2 boundary velocity) to 6 m/s (Be boundary velocity)

Results Results of of experimentexperiment

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• Mean count rate of 2.6 UCN per shot over all shots(negative count rates result from background subtraction)

• Drop in UCN count rate while temperature of tube is decreased

• Count rate does not increase again afterwards

• Possible explanation: adsorption of thin D2 layer at the walls of the tube

General result:

Count rate 103 times lower than predicted!

Comparison with polyethyleneComparison with polyethylene• D2 converter replaced by thin polyethylene

plate (d ~ 10mm)• Measurement at ~ 300K, ~ 90K and ~ 5K• Calculations predict a production of

~ 500 UCN per pulse in the polyethylene• Monte-Carlo simulations predict a UCN count

rate of ~ 15 UCN per pulse• Actual count rates were 3.9 UCN per shot over

all shots

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� Setup itself contributes with a factor of ~ 4 to lowered count rate

� Most of the reduction in count rate is related with D2!

Possible reasons forPossible reasons for low low countcount raterate

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• D2 might be contaminated with strong neutron absorbers/up-scatterers

• Ortho/para ratio might have decreased as an effect of irradiation over long time

• Structure of the D2 crystal (cracks, snow) could have reduced UCNproduction rate

• Absorption might be increased on the tube walls due to deuteriumlayer

Conclusion Conclusion and and outlookoutlook

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What we have done What we have done and and what we what we have learnedhave learned......

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• D2 gas system is working

• Control electronics is working

• Detector and measuring electronics are working

• Cryogenic problems solved

• Background problems solved

• Debugging finished

• First UCNs counted

What we What we still still havehave to do...to do...

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Concerning deuterium:

• Extensive and detailed chemical analysis of D2 samples at the MPI for Chemistry in Mainz � was finished this week

• Analysis of ortho/para ratio with Raman spectrometer in Munich � will be finished next week

• Installation of a second Raman spectrometer at the University Mainz/Chemistry Section � will be finished this or next week

• Small desktop experiment will be installed at the FRM-II to investigate the structure of the deuterium crystal, crystal preparation procedures and effectsof long time irradiation � currently in preparation, duration ~ 2 - 3 month

Additionally:

• Al6061 guidance tube will be replaced by an electro polished stainless steel tube (ρ ~250 nm) � currently in preparation

• Possibility for a heating of the guidance tube will be installed � currently in preparation

Thank you Thank you for your attentionfor your attention!!

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We are testingWe are testing,, rebuildingrebuilding andand learninglearning..ForFor this purpose thethis purpose the testtest setupsetup waswas builtbuilt......

BibliographyBibliography(1) Altarev et al., Mini-D2 - A source for ultracold neutrons at the

FRM-II, internal report, Munich, 2000(2) Pokotilovski, Calculations of UCN production at the C channel of

TRIGA-Mainz reactor, private communication, Munich, 2005(3) Seidel, Development of a UCN-source for the beam tube SR4 at the

FRM-II, diploma thesis, Munich, 1999(4) Eberhardt, Aufbau und Nutzung des Forschungsreaktors TRIGA

Mainz, presentation, Mainz, 2005(5) Altarev et al., Evaluation of the properties of a beryllium coated

aluminium tube as possible UCN neutron guide, internal report, Munich, 2004

(6) Frei, Production of ultracold neutrons, presentation, Munich, 2005

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