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L. Tassan-Got, L. Audouin, M. Mac Cormick, IN2P3/IPN Orsay-France

Extension to experiment INTC-P-209 Angular distributions in the neutron-induced fission of actinides. L. Tassan-Got, L. Audouin, M. Mac Cormick, IN2P3/IPN Orsay-France C. Paradela, I. Duran, D. Tarrio, USC-Spain V. Vlachoudis, CERN

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L. Tassan-Got, L. Audouin, M. Mac Cormick, IN2P3/IPN Orsay-France

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  1. Extension to experiment INTC-P-209Angular distributions in the neutron-induced fission of actinides L. Tassan-Got, L. Audouin, M. Mac Cormick, IN2P3/IPN Orsay-France C. Paradela, I. Duran, D. Tarrio, USC-Spain V. Vlachoudis, CERN and the n_TOF collaboration

  2. Motivations • Fission fragment angular distribution is an ingredient in the measurement of the fission cross section (detector efficiency) • Angular distribution gives information on the spin properties of fissionning nucleus • Information on angular momentum deposition in the spallation domain • The angular distributions are not well known above 10 MeV

  3. Poor knowledge above 10 MeV nTOF well suited for describing the quick variations with energy

  4. 232Th Spallation domain ? Tutin et al., NIM A457 (2001) 646 Need to confirm the high anisotropy of 232Th at high energy

  5. Experiment • Take advantage of the full efficiency up to 45° • Isotopes to be measured : 238U, 235U, 232Th • Requested amount of protons : 1.5 1018

  6. New situation : Working Sector A at n_TOF • Possible accomodation of more radioactive isotopes • Interesting isotopes for fission fragment angular distributions : 234U and 237Np

  7. Configuration modification • 9 targets interleaved with 10 PPAC • Original configuration : 7 x 232Th, 1 x 235U, 1 x 238U Beam time : 1.5 1018 protons • New configuration : 2 x 234U, 1x 237Np, 4 x 232Th, 1 x 235U, 1 x 238U Beam time : 2.0 1018 protons

  8. Counts for 232Th • 4000 counts per energy bin in region of interest • 70 bin/decade around 1.6 MeV 30 bin/decade around 6.8 MeV

  9. Counts for 234U • 4000 counts per energy bin in region of interest • 70 bin/decade around 0.45 MeV (oscillations in anisotropy)

  10. Angular distribution : combination of spins • Basically : • J combines incident spins and orbital momentum • K projection along fission axis • If K << J forward peaking • If K ~ J sideward peaking (even-even target nucleus) • The angular distribution may vary quickly with the energy according to (J,K) of the vibrational resonances

  11. Detection cell • Detection in coincidence of the 2 fragments (discrimination of fission) • Thin detectors, targets and backings • Limitation of the accepted solid angle (stopping of fragments in electrodes) • solid angle truncated but measured (trajectory reconstruction)

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