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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.

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Nuclear data needs
Nuclear data needs

  • Simulation and design of Gen-IV and ADS systems require accurate nuclear data

  • Current data libraries present important discrepancies and lacks.

Thorium fuel



 decay

Thorium fuel

  • 232Th + n  233Th  233Pa  233U (fissile)


















Experimental campaign
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.

Fission chambers @ n tof
Fission Chambers @ n_TOF

PPAC chamber

FIC chamber

… n_TOF

Fission Ionization Chamber Assembly

Ionization chamber

Gas used: Ar (90 %) CF4 (10 %).

Gas pressure: 720 mbar

Electric field: 600 V/cm

Gap pitch: 5 mm

Deposit diameter: 5 cm

Deposit thickness: 125 µg/cm

Support thickness: 100 µm (Al)

Deposits on both sides.

Electrode diameter: 12 cm

Electrode thick.: 15 µm (Al)

Windows diameter: 12 cm (KAPTON 125 µm)

List of targets
List of Targets


235U - 47mg (3 targets)

238U - 110mg (3 targets)

232Th - 80mg (2 targets)

236U - 20mg (2 targets)

237Np - 10mg (1 target)

234U - 34mg (6 targets)


235U - 30mg (1 targets)

238U - 90mg (2 targets)

241Am - 2mg (4 targets)

243Am - 10mg (4 targets)

245Cm - 2mg (2 target)

233U - 20mg (2 targets)

Fission chambers @ n tof1
Fission Chambers @ n_TOF

PPAC chamber

FIC chamber

Ppacs @ n tof

  • 10 detectors 9 targets

  • U-234(2) and Th-232(5)

  • Two reference targets: U-235 y U-238

  • Less than 1 % of flux attenuation in the full setup.

Measured isotopes
Measured isotopes

  • Minor actinide : 237Np

  • Thorium cycle : 233U, 234U, 232Th

  • Spallation target : 209Bi, natPb

  • Reference isotopes : 235U, 238U

Fission Detection Setup

  • 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.

Discrimination with coincidences

U-234: singles

U-234: coincidences

Discrimination with coincidences

Cathode positioning i
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

Stripped Cathode

Delay Line

Cathode positioning ii
Cathode positioning (II)

Diagonal condition:


DLT: Total delay line length (~320 ns)

The time difference

between both cathode

ends provides

the position of the signal.

Targets (I)

Epoxy frame

Uranium target

80 mm Ø

300 µg/cm2

2 µm Al backing

Targets ii
Targets (II)

  • Measurement of thickness and homogeneity by alpha counting and/or proton scattering.

  • High purity samples (> 99 % for U-234).


Y (mm)

X (mm)

 activity

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
PPAC signal analysis

  • Negative loop should be first

  • Xpos-Xneg < constant

  • const1 < Hneg/Hpos < const2



Light vs. Heavy Fission Fragments

Target 0 Energy < 10 MeV (Assymetric fission)

Light vs. Heavy Fission Fragments

Cathode signals for En around 1 MeV

Cross section analysis
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%)

Angular acceptance



Angular acceptance



Fission fragment angular distribution

Log E =6.0

Log E =5.8

Log E =5.9

Cos ()

Cos ()

Cos ()

Log E =5.4

Log E =5.6

Log E =5.5

Fission Fragment Angular Distribution

U-234 FFAD for neutron energies near the fission threshold

N tof beam at fission campaign

Y (mm)

X (mm)

n_TOF beam at fission campaign

  • Neutron spectrum in the whole energy range.

  • Beam profile


U234 resonances


Previous data

(IRMM report)

Better energy resolution achieved at nTOF


Np237 resonances


  • 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
Fission Chamber @ n_TOF

PPAC chamber

FIC chamber



Fic design
FIC Design

  • Gas – Ar 90%+CF4 10%

  • Pressure – 600 mbar

  • Distance – 5 mm

  • HV – 300 V

  • 17 targets in the beam

  • “noise” electrode