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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
1
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
2
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
4
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
13
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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