N. Colonna, R. Sarmentoand the n_TOF Collaboration

www.cern.ch/nTOF

Results on 236U(n,f) with FIC-0

n_TOF Collaboration MeetingDecember 16th 2010, CNA Seville

Motivation

Experimental setup

Data analysis

Results

Conclusions

Outline

Motivation

The search for energy alternatives to the fossil fuels lead to a ”renaissance” of the interest in nuclear energy production.

The current research on this area is directed towards the sustainability of nuclear reactor technology for energy production.

• Improved operational safety

• Economically competitive

• Reduced nuclear waste

Solutions:• ADS: Accelerator-Driven Systems• Generation-IV Nuclear Reactors

Th-232 fuel cycle: U-236 build-up

Objective:• To know the U-236 neutron induced fission cross-section with a relative uncertainty below 5%.

U-236 fission cross-section data from EXFOR (Experimental Nuclear Reaction Data)

Experimental data

1972 - Theobald et al., Geel(width-error corresponds to 2.5% of

the width)

1994 - Parker et al., Los Alamos

2008 - Wagemans et al., Geel*resonances observed but no

resonance parameters

ENERGY WIDTH WIDTH-ERR EV MILI-EV MILI-EV

5.4500E+00 2.9000E-01 5.4500E+00 1.3000E-03 1.0E-04 5.4500E+00 1.7000E-03 1.0E-04

2.9900E+01 1.6000E-01 3.4000E+01 1.8000E-01 4.3700E+01 4.3000E-01 7.1100E+01 2.9000E-01 8.6400E+01 3.0000E-01 1.2080E+02 3.4000E-01 1.2470E+02 2.1000E-01 1.9400E+02 5.0000E-01 2.1400E+02 3.2000E-01 2.7240E+02 4.0000E-01 2.8820E+02 4.8000E-01 3.0250E+02 4.6000E-01 3.7100E+02 4.2000E-01 3.7900E+02 3.0000E-01 4.1500E+02 5.9000E-01

1.2688E+03 8.2000E-01 3.0E-01 * 1.2817E+03 7.7000E+00 5.0E+00 * 1.2917E+03 9.3000E-01 1.1E-01 * 2.9589E+03 1.4000E+00 6.0E-01 6.3000E+03 1.0800E+01 6.0E+00 * 1.0400E+04 4.6000E+00 2.6E+00 *

Ressonance parameters: fission width

Experimental data

Scarce and ambiguous data

Experimental data

U-236/U-235 fission cross-section data from EXFOR (Experimental Nuclear Reaction Data)

U-236 fission cross-section data from ENDF (Evaluated Nuclear Data File)

Evaluated data

Shortcomings attributed to the U-236 fission cross-section evaluated data

1. The thermal cross-section - discrepancy by two orders-of-magnitude

2. The 5.45 eV resonance width - discrepancy by two orders-of-magnitude

3. The resonance region is filled with false U-236 resonances

4. Intermediate energy resonances are missing

5. No agreement on the absolute value up to few hundred keVs

Summary

Experimental setup

Experimental setup

U-236 samples informationIsotopic composition

(mass %)

U-235 0.05U-236 99.85U-238 0.1

Thisckness (atoms/barn)

sample 15.66 x 10-7 ± 1.4%

Diameter (mm) 80

Activity of alpha-radiation

at 04/02/04 (mCi)2.6 x 10-2

• 1.56 x 1018 protons from dedicated beam• 4.52 x 1017 protons from parasitic beam• no-beam runs

• fission colimator• FIC-0 detector• digitization by flash-ADCs working at a 250 MHz sampling rate.

• four U-236 samples• two U-235 samples

Experimental setup

FIC-0

One single fragment is detected per fission event - 2π detection efficiency. The other fission fragment is absorbed in the Al

electrode backing the sample to measure.

The ionization chamber operates in the ion saturation region - no avalanche multiplication occurs.

The fission cross‐section is measured by detecting the fission fragments (FF) - electrons and ion pairs are produced in the gas by the FF and this charge is collected by applying a voltage between the electrodes.

The gas was chosen by its fast timing properties to avoid pile-up problems.

Experimental setup

Gas 90% Ar + 10% CF4

Gas pressure 720 mbar

Gap between the electrodes 20 mm

Electric field 550 V/cm

Electron drift velocity 12 cm/μs

FIC-0

Data analysis

1. Event reconstruction: time, amplitude and baseline

2. Energy calibration

3. Background subtraction: α-particles and impurities

4. Corrections: efficiency and dead-time

5. Extraction of the neutron flux

6. Extraction of the cross-section: ratio method

The signal reconstruction from the raw data was made by applying C++ routines developed

using the ROOT framework and based on the Advanced Spectra Processing Function class

TSpectrum.

Dedicated files are created storing the amplitude, baseline, time information and area

of the signal peaks, as well as the time and intensity of the proton bunch originating the

respective spallation neutrons.

Event reconstructionM. Calviani

Amplitude and time-of-flight spectra of FF

Event reconstruction

Fission fragment (FF) digitized signature

M. Calviani

• U-235 data

• Agreement found in all measured energy range

• The same parameters were used for the U-236 (exception of flight-path)

Normalized n_TOF U-235 counts + U-235 fission cross-

section from data libraries

Energy calibration

α-particles:

• Contribution up to 1 keV

• Activity from the samples

U-235 impurity in the U-236 samples:

• 0.05% in mass

• Ressonance structure from 10 eV up to 1 keV

Background subtraction

The intrinsic detection efficiency was calculated by performing

computational simulations with

FLUKA. The values were obtained for

setting a threshold on the energy deposited

by the fission fragments at 35 MeV.

Detection efficiency

M. Calviani

• Important above 100 keV

• High for the U-235, less than 3% for the U-236

Correction factor

cr - count-rate

δt : dead-time of the detector 220 ns

Dead-time correction

c - FF countsb - Background countsε - Detection efficiencyΔdt - Dead-time correction factorN - Sample thicknessσ : Evaluated cross-section

Flux from FIC-0 + Flux from other fission measurement made with FIC-1

Neutron flux

Neutron flux

• Above 400 keV - need for correction

• Change of sampling rate in the TOF-to-energy calibration

• Different fractions of signals from dedicated and parasitic beams were accounted

High energy correction

Cross-section

c - FF countsb - Background countsε - Detection efficiencyΔdt - Dead-time correction factorN - Sample thicknessσ : Evaluated cross-section

Obtained from the ratio between the U-236 and U-235 counts

Results

ENDF/B-VIIJENDL/AC-2008Alekseev et al.

n_TOF

5.45 eV resonance

Results: low energy E(eV)<10

• U-236 resonances observed in the evaluations: not detected• Originally from non-subtracted background in previous measurements

Results: 10<E(eV)<103

JENDL/AC-2008n_TOF

Results: intermediate energy 103<E(eV)<104

Resonance triplet at 1.25 keV measured with excelent energy resolution

Results: intermediate energy 103<E(eV)<104

Results: intermediate energy 103<E(eV)<104

Results: below threshold 104<E(eV)<105.5

Results: below threshold 104<E(eV)<105.5

Results at energies below the threshold: follow well the evaluation of JEFF-3.1

Results at and above the threshold energy of 1 MeV: good agreement with previous experimental data

Results: around the threshold 105.5<E(eV)

Conclusions

• Cross-section measured for the first time from 200 meV up to the threshold energy

• The low energy cross-section - up to the 5.45 eV resonance - in most evaluated data libraries is highly overestimated. No U-236 resonances were observed from 10 eV up to 1 keV - explanation to be found for low cross-section

• Confirmation of the intermediate energy structure with resonances measured above 1 keV

• Justification problem at the threshold energy

• Ongoing work on the resonance fitting and parameter determination