The neutron_Time Of Flight Facility at CERN First Commissioning Results

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The neutron_Time Of Flight Facility at CERN
First Commissioning Results
Catalin Borcea CERN - Switzerland
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Outline

• What is n_TOF ?
• General Purpose
• General layout of the Facility at CERN
• Main Features from Monte-Carlo Simulation
• Commissioning of n_TOF
• Objectives
• Detectors and measurements
• Results
• Conclusions and Outlook

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General Purpose

• The n_TOF is a high flux (7?105 n/cm2/7?1012 p)
  spallation neutron source followed by a 200 m
  time of flight basis.
• The aim of the n_TOF facility at CERN is the
  measurement of cross-sections needed for the
  design of innovative ADS applications like
• incineration of nuclear waste 1
• energy production 2
• radio-isotope production for medical applications
  3
• and many other basic science subjects in
  particular astrophysics 4.
• The unique features of this new spallation
  neutron source at the CERN-PS allows one to study
  systematically and with excellent resolution
  (2?10-4 at 1 keV), neutron cross-section of
  almost any element using targets of very modest
  mass, necessary for unstable or otherwise very
  expensive materials, in the interval from 1 eV to
  250 MeV.

1 C. Rubbia et al., A Realistic Plutonium
Elimination Scheme with Fast Energy Amplifiers
and Thorium-Plutonium Fuel, CERN/AT/95-53
(ET) 2 C. Rubbia et al., Conceptual Design of
a Fast Operated High Power Energy Amplifier,
CERN/AT/95-44 (ET) 3 C. Rubbia, Resonance
Enhancd Neutron Captures for Element Activation
and Waste Transmutation, CERN//LHC/97-04
(EET) 4 Proposal for a Neutron Time Of Flight
Facility, CERN/SPSC 99-8, SPSC/P 310, 17 March
1999.
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General Layout (1/4) n_TOF at CERN
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General Layout (2/4) the n_TOF line
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General Layout (3/4) PS beam
Proton pulse width Supercycle
structure (1 to 4 pulses per 16.8 s)

• Momentum 20 GeV/c Width 0.1413 GeV/c
• RMS bunch length 7 ns
• FWHM in V 2.355 ? 0.56 cm
• FWHM in H 2.355 ? 0.78 cm
• Neutron emission at an angle of 10o with respect
  to the proton beam direction.

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General Layout (4/4) the lead target

• 4 tons pure Lead (TARC)
• dimensions 80 ? 80 ? 60 cm3
• Proton entrance niche
• dimensions 50 ? 30 ? 20 cm3
• Aluminum container 140 l
• 3 cm of water around the target (cooling)
• 5 cm of water in front TOF surface (moderation)
• Aluminum window mounted onto the water tank
• e 1.6 mm
• ø 80 cm

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The n_TOF neutron source featuresfrom MC
simulation (1/4)Knowledge from TARC
n_TOF Optimized Lead target Flight Path L
TARC Large lead block (ø3.3 m, l3 m)

• The velocity of the neutron v is derived
• from the effective neutron path l in lead,
• v(lL)/T, DvDl/T
• Uncertainty on the energy DE/E2Dl/(lL)

• A gtgt ? Energy-Time relation (EAMC)
• 1 eV lt En lt 104 eV
• EK/(tt0)2 , l 5.74 ? 0.33 m

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The n_TOF neutron source featuresfrom MC
simulation (2/4) Optimization of flux and
resolution
Pb 2?2?1.6 m3
Dl (cm)
Pb 2?2?1.6 m3 5 cm H2O
Pb R40 cm h40 cm, 5 cm H2O
Dl for 10 ns pulse width
Neutron Energy log10(E) (eV)
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The n_TOF neutron source featuresfrom MC
simulation (3/4) Resolution Dl
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The n_TOF neutron source featuresfrom MC
simulation (4/4) Neutron spectral function

• Total fluence
• (no collimators)
• 7?105 n/cm2/7?1012 p
• Isolethargic flux
• 1.5?104(dn/dlnE)/cm2/7?1012 p
• Gravitational cut-off
• The very slow neutrons are
  cleared by the geometry of
• the pipe En lt 10-2 eV

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Commissioning of n_TOF

• Target behavior
• Temperature
• Safety
• Physics Detectors and Results
• Overview
• Activation of gold foils (absolute flux _at_ 4.9 eV)
• BC702
• Fission chambers (235U and 238U)
• Conclusions

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Proton delivered by PS

• 15 days in November 2000
• 1 bunch of 1011 to 4x1011 protons each
• Total number of protons on the Target
• SIp 2.6x1016

• 12 days in April 2001
• 4 bunches of 6x1012 protons each
• Total number of protons on the Target
• SIp 8.6x1017

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Dose rate from the target TIS Measurement after
2 months (not in contact)

• During the 1st period, the number of
• protons delivered by the PS was limited
• to be able to approach the target in case
• of problems.
• FLUKA simulations were performed to
• estimate the dose rate from the neutron
• exit face of the target
• Aluminum container 5 mSv/h
• 15 mSv/h max. due to screws (Inox)

50 mSv/h
300 mSv/h
p face 330 mSv/h
50 mSv/h
400 mSv/h
220 mSv/h
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Dose rate measured on the lead target wall (mSv/h)
Neutron exit face Hot spot _at_ 1.1 mSv/h
Proton entrance face Hot spot _at_ 2.15 mSv/h
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Temperature behavior of the lead
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Neutron Fluence Detectors
FC 238U
6Li(n,a) scintillation in ZnS
Fission chamber 235U
ZnS transparent plastic
Double foil activation method
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Flux _at_ 4.9 eV from Au foils

• Double foil Method
• Material with a large resonance and
• relatively low cross-section elsewhere
• Two foils of same material and same
• thickness activated under the same
• neutron beam
• The thickness of the foils chosen such
• as the first foil (that sees the neutrons
• first) depletes entirely the flux at the
• resonance energy, leaving the rest
• of the spectrum almost unchanged
• The difference of capture between
• the two foils is the contribution of
• the main resonance
• Neutron Flux
• We assume isolethargic flux FresC/Eres
• in the main resonance region
• Number of captures in a foil of thickness d,
• around the resonance energy

4.9 eV, sc27400 b
1st foil depletes the flux _at_ 4.9 eV
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Raw Data from BC-702

• BC-702 FEATURES
• Transparent Plastic (hydrogen)
• 6Li(n,a) scintillation in ZnS
• Rise time less than 30 ns
• Decay time of 250 ns
• Normal operation _at_ 800 V
• Blinded for 10 ms after the g flash remnant
  dim light (phosphorescence) for tens of ms
• Worked _at_ 720 V and low proton intensity (less
  than 1011 p/bunch)
• Calibrated _at_ PTB for En gt 24 keV
• Calibration _at_ GEEL for thermal energies

• BC-702 RAW DATA (1st period)
• 5000 events/bunch (1011 p/bunch)
• MC spectrum folded with rescaled MC efficiency

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Neutron fluence with and w/o collimators (Nov.)
1st period (_at_173 m) No collimator 3rd period (_at_
182.5 m) Two collimators ø 11 cm, l2 m ø 1.8
cm, l2.5 m Fluence calculated for 2.6 cm active
diameter
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Fission chambers from PTB()

• Parallel Plate Ionization Chambers 235U and 238U
• Fissile deposit ø 7.6 cm density 440 µg/cm2
• Neutron beam _at_182.5 m is ø 3.3 cm
• Total weight seen by the beam 38 mg
• Gas used in the counters Ar-CF4 (90/10 )
• Ionization by g-flash blinds for 2 µs (50 MeV)
• Total number of events for 235U 6.57 ? 107
• Total number of events for 238U 3.17 ? 105

()Physikalisch-Technische Bundesanstalt,
Braunschweig
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Fission Events from 235U
ENDF-B/VI
Data
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Average Neutron Fluence _at_182.5 m
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Integrated Neutron Fluence
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Effective flight path l calculation
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Beam Profile with BC-702
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Conclusion

• Safety
• All tests passed successfully
• Physics
• Good overall agreement for both spectral function
  and resolution
• Excellent quality data from fission chambers
• The three independent measurements are consistent
  with each other (Au, Bc702, FC)
• Facility operational for fission measurements
• Actual background level needs further reduction
  for capture measurements (more measurements are
  foreseen)

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Outlook

• Physics program approved
• P123 Determination of the neutron fluence, the
  beam characteristics and the backgrounds at the
  CERN-PS TOF Facility.
• P124 The importance of 22Ne(a,n)25Mg, as a
  s-Process Neutron Source and the s-Process
  Thermometer 151Sm
• P125 Re/Os cosmochronometer
• For the september INTC
• Capture and fission cross-section measurements
  related to ADS