N_TOF fission data of interest for ADS. Nuclear data needs. Simulation and design of Gen-IV and ADS systems require accurate nuclear data Current data libraries present important discrepancies and lacks. Fission Cross Sections. 1eV. 1MeV. Neutron Capture. decay. Thorium fuel. - PowerPoint PPT Presentation
TRANSCRIPT
N_TOF fission data of interest for ADS
Nuclear data needs
• Simulation and design of Gen-IV and ADS systems require accurate nuclear data
• Current data libraries present important discrepancies and lacks.
Fission Cross Sections
1eV 1MeV
234U
230Th 231Th 232Th 233Th 234Th
231Pa 232Pa 233Pa 234Pa 235Pa
232U 233U 235U 236U 237U 238U
Neutron Capture
decay
Thorium fuel
• 232Th + n 233Th 233Pa 233U (fissile)
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Hablar mas de Thorium fuel cycleNowadays, because of the actual problems in the energetic scenario, the alternative use of thorium as nuclear fuel is taking relevance, leading to the development of new concepts such as the Accelerator Driven Systems (ADS).The thorium-uranium fuel cycle is based on the production in of uranium-233 which is fissile like uranium-235 (the one used in current nuclear reactors).The process is the following: The Throrium-232 is converted into U-233 after a neutron capture followed by two beta decays In the breeding process a significant amount of uranium-234 is produced by neutron capture in the uranium-233 or in the praseodinium-233 plus beta decay, so that an accurate knowledge of its neutron cross sections must be achieved to simulate how it behaves inside a reactor. U-234 fission cross section is the main topic of this work.
Experimental Campaign
• A set of measurements of neutron-induced fission in actinides was launched within the FP5.
• Fission measurements in the framework of the n_TOF Collaboration and construction of the n_TOF facility.
n_TOF facility
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At the nTOF facility the neutrons are produced by spallation reactions in a lead target and their energies are determined by time-of-fligh techniques.
Fission Chambers @ n_TOF
PPAC chamberFIC chamber
Ionization chamber
Gas used: Ar (90 %) CF4 (10 %).
Gas pressure: 720 mbarElectric field: 600 V/cmGap pitch: 5 mmDeposit diameter: 5 cmDeposit thickness: 125 µg/cm Support thickness: 100 µm (Al)Deposits on both sides.Electrode diameter: 12 cmElectrode thick.: 15 µm (Al) Windows diameter: 12 cm (KAPTON 125 µm)
• Less than 1 % of flux attenuation in the full setup.
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Taking advantage of the very thin targets and detectors used, in the nTOF PPAC setup it is possible to include up to nine targets sandwiched by 10 detectors. This allow to study several nuclei simultaneously, increasing the statistic available, and using targets with known XS as references U-235 and U-238 in our case.Nevertheless, This makes the recognition of the fission events more complicate because the targets share the detectors and also because it is possible ...
•Minor actinide : 237Np
•Thorium cycle : 233U, 234U, 232Th
•Spallation target : 209Bi, natPb
•Reference isotopes : 235U, 238U
Measured isotopes
• Fissile target in a thin backing sandwiched by two detectors Detection of both fission fragments in coincidence.
• Fission event reconstruction: target position and emission angle. Efficiency limited by the cut at large angles.
Fission Detection Setup
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The idea for the fission detection is to put the target in-between two detectors, so that both fission fragments emitted in a fission reaction are detected in coincidence, In addition if the we can measure the position of the FF in the detectors, we can reconstruct their trajectories obtaining the origin in the target and the emission angle.This detection setup consists basicly of target and detectors.
Discrimination with coincidences
U-234: coincidencesU-234: singles
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paradela06/10/2005The use of the coincidence method allows us to improve the rejection of background produced by alpha particles and highly energetic reactions in detectors.This can be shown by comparing the signals measured by only one detector (T) against the results obtained demanding coincidence with the second detector. In this amplitude-energy plot, the signals due to alpha particles and high energy reactions that are detected...... are very suppressed when looking for coincidences and only fission events remain.
Cathode Positioning (I)
• Positioning by using stripped cathodes and delay line readout.
• The cathode signal is split in the delay line and transmitted to both ends
Delay Line
Stripped Cathode
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As explained before, PPAC detectors also provide the position of the crossing FF by using the two stripped cathodes, each one connected to a delay line. The signal produced in a few strips by the particle is transmitted through the delay line two the preamplifiers in both ends.
Cathode positioning (II)
Diagonal condition:
(Tch1-Tanode)+(Tch2-Tanode)=DLT DLT: Total delay line length (~320 ns)
The time difference between both cathode ends provides the position of the signal.
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Both cathode signals fulfil the condition that we call diagonal condition: that the sum of the travelling times of the split signals up to both cathode ends must be equal to the total length of the Delay Line (around 320 ns)The time difference between these signals tell us what is the position of the signal in the direction perpendicular to the strips. The two cathodes perpendicularly oriented provides the two-dimensional position of the FF in the PPAC.
Uranium target80 mm Ø
300 µg/cm2
2 µm Al backing
Epoxy frame
Targets (I)
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Very thin targets have been used with a samples of 8cm diameter of and an approximate thickness of300 micrograms per square centimeter which are deposited in a 2 microns aluminum backing.Let's point here that the backing is traversed by one of the two Fission fragments.
• Measurement of thickness and homogeneity by alpha counting and/or proton scattering.
• High purity samples (> 99 % for U-234).
234U
activity
Targets (II)
X (mm)
Y
(mm
)
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The thickness of the deposited layer in the target and its inhomogeneities have been accurately measure by alpha counting. The presence of impurities have been also measured resulting high purity samples.
The n_TOF Data Acquisition System
•The n_TOF DAQ consists of 54 flash ADC channels with 8 bit amplitude resolution and sampling of 500 MSample/s.
•The full history of EVERY detector (BaF2 crystals and monitors) is digitised during a period of 16 ms (0.7 eV < En < 20 GeV) and recorded permanently on tape. Very useful feature since the raw data can be always re-investigated.
•The system has nearly zero dead time.
•7.5 TB disk space for temporary storage.
•Typical data rate of 2-3 TB/day on tape after compression.
•Pulse shape analysis is performed on the fly at the LXBATCH Linux Batch Farm at CERN (30 CPUs exclusively dedicated) and stored in highly compressed Data Summary Tapes.
•Quasi on-line analysis of the data with full statistics.
One of the big successes of n_TOF. Many TOF facilities are following the n_TOF example and moving to digital electronics!
PPAC signal analysis
1) Negative loop should be first
2) Xpos-Xneg < constant
3) const1 < Hneg/Hpos < const2
Target 0 Energy < 10 MeV (Assymetric fission)
Light vs. Heavy Fission Fragments
HFF LFF
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In order to estimate what are the efficiency reduction because of the hardware threshold cut, I have used the capability of the PPAC detectors to distinguish between the two types of the fission events: 50 % with the heavy fission fragment crossing the backing and 50 % in which the light fission fragment crosses the backing. The separation is achieved whatever is the emision angle.
Cathode signals for En around 1 MeV
Light vs. Heavy Fission Fragments
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Another effect, that I call HTC, that reduces our efficiency is the existance of cathode channels in the detector facing the backing with a too high threshold, so that fission event signals with small amplitude are not registered by the adquisition. I should note that this effect is not affecting high energy range (above MeV) because for short time-of-flights (around the first 10 microseconds) zero-suppression method is not active by the high counting rate,and DAQ register signals even below the set threshold.In order to correct such effect...
Cross Section Analysis
(E): fission cross section• n (x,y,E): fission rate obtained from raw data (x,y): surface density of the target (E): detection setup efficiency
a (x,y,E)/ b (x,y,E) ≈ 1 ± 0.01 (1%)
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We determine the fission cross section with respect to a reference cross section such as U-235.After fission event selection, we got the fission rates measured for each target. This ratio gives the different number of atoms in the targets that is independent of the neutron energy. It's included as a normalisation constant. Finally, efficiencies have a relevant impact in the XS calculation so that I have dedicated an important effort in order to characterise them for each target. The flux term is not appearing in this expression because the PPAC setup practically doesn't attenuate the flux and the ratio is asssumed as 1.
Angular acceptance
50º 50º
Simulations Measurements
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..but we found is that the cosine distribution falls from an angle around 50 degrees. This can be explained if we consider the energy losses of the fission fragments in the dead layers of the setup (backing and detectors). This implies that the heavier FFs begin to be stopped at angles around 58 degrees. Even more, if we demand that FF mush have a minimun residual energy to produce signal large enough, the full efficiency is missed from 50 degrees in agreement with data. Let's point here that most suppressed fission events are those in which the heaviest fission fragment crosses the backing.
U-234 FFAD for neutron energies near the fission threshold
Fission Fragment Angular Distribution
Cos ()Cos ()Cos ()
Log E =5.6 Log E =5.4
Log E =5.8
Log E =5.5
Log E =6.0 Log E =5.9
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The last factor studied was the FFAD. Up to now we have supossed that the fission fragment were emitted isotropically, so that a flat behaviour in the angle cosine was expected in the full efficiency angular range.But this is not the case for some energy regions where the fission fragment angular distribution is clearly anisotropic. And the anisotropy changes rapidly with the neutron energy from a peaked-forward distribution to a sideways distribution.As our fission setup has a reduced angular acceptance the variation of the FFAD with the energy has a direct effect in the detection efficiency.
Dependence of anisotropy with energy
238U
X (mm)
Y (
mm
)
n_TOF beam at fission campaign
• Neutron spectrum in the whole energy range.
• Beam profile
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The neutron beam of the nTOF facility is characterised by:-the energy-dependent neutron spectrum which is marked by the MeV neutron peak coming from spallation reactions and an isoletargic behaviour at lower energies due to the water moderator.-The energy resolution dominated at low energy by the different neutron paths in the target-moderator assembly and at high energies by the proton burst width of 7 ns.-the beam profile, here I show the X-profile from fission campaign.(- and the background in the Exp. Area, particularly the promtp photon flash which produces the first signal in the detectors correlated with the beam and is used for as T0 reference.)
Th-232 results
U234(n,f)
U234 resonances
234U
Previous data
(IRMM report)
Better energy resolution achieved at nTOF
Np237(n,f)
Np237 resonances
Conclusions
• The coincidence method has been used to obtain the fission results for the extensive n_TOF neutron energy range.
• Th233, U234, Np237 results have been already produced and U233 is in progress.
• Important discrepancies have been found in the Np237 resonance region.
Fission Chamber @ n_TOF
PPAC chamber FIC chamber
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Most of the collaboration activities are done the nTOF facility at CERN
FIC Design
• Gas – Ar 90%+CF4 10%
• Pressure – 600 mbar• Distance – 5 mm• HV – 300 V• 17 targets in the beam• “noise” electrode
