1. Fission setup based on PPACs using a coincidence technique L. Audouin, S. Isaev, L. Tassan-Got, C. Stephan IPN dOrsay I. Durán, C. Paradela, D. Tarrío Univ. Santiago de Compostela (on behalf of the n_TOF Collaboration)

2. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Motivation Fast reactors and ADS have renewed the interest on nuclear data, in particular those beyond 20 MeV. A new generation of facilities allows studying nuclear reactions at high neutron energies. For measuring fission, new devices are needed for discriminating fission from competing reactions ( spallation, multifragmentation,…)

3. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Parallel Plate Avalanche Counter (PPAC) Very thin detectors. High detection efficiency for heavy ions (FF) Fast timing with anode signal (0.5 ns resolution). Fragment position from cathode signals.

4. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Detection setup 10 PPACs and 9 targets ( 235 U and 238 U as references). Gas flow inside a reaction chamber Thin targets (300 µg/cm 2 ) and backings (550 µg/cm 2 ) Both fragments detected by the closest PPACs. Trajectory reconstruction.

5. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Coincidence technique U-234: singles U-234: coincidences Coincidence condition between anode signals rejects most of the background Coincidence technique

6. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Fission target identification Correlation between PPAC time differences allows the unambigous target identification T0T1T2 T1T2T0T1T2T0T1T2

7. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Cathode positioning 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.

8. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Cross Section: Energy Resolution Very low background: lower yield between resonances than expected from evaluations

9. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Cross Section: Energy Range Fission measured for neutron energies up to 1 GeV. Cross sections have been measured for different isotopes: 234,233 U, 237 Np, 232 Th, nat Pb, 209 Bi. [1] C. Paradela et al. Phys. Rev. C 82, 034601 (2010) [2] L. Audouin et al. Proc. of ND2007, p. 421 [3] D. Tarrío et al. Phys. Rev. C 82, 044620 (2011)

10. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent U-234 cosine distribution for neutron energies near the fission threshold Fission Fragment Angular Distribution 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

11. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Anisotropy extrapolation B (Anisotropy parameter) Angular Distribution of fission fragments is described by W( ) 1+Bcos 2 Results obtained fitting in our reduced angular range Despite the constraints, present results are quite in agreement with previous data from Leachman ( Phys. Rev. 137, B814 (1965) ) and Tutin et al. ( Nucl. Instrum. Meth. A 457 (2001) 646-652 ) Neutron energy (MeV) This work Leachman Tutin

12. C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent Conclusions A fission detection setup based on PPACs has been implemented for the CERN n_TOF facility. Both fission fragments are detected in coincidence and their trajectory is reconstructed. Cross sections and angular distributions can be measured up to 1 GeV at the CERN n_TOF facility.