Daniel Cano-Ott CIEMAT - Madrid danielno@ciemat.es

1. Measurement of the neutron capture cross sections of 233U, 237Np, 240,242Pu, 241,243Am and 245Cm with a Total Absorption Calorimeter at n_TOF The n_TOF collaboration Spokesperson: E. González-Romero (CIEMAT) GLIMOS: P. Cennini (CERN) Daniel Cano-Ott CIEMAT - Madrid daniel.cano@ciemat.es

2. Introduction Nuclear waste is one the main problems for the public perception of the nuclear energy production and for the sustainability of this energy source. Although a deep underground repository seems to be a scientifically proven and technologically viable solution for the nuclear waste for the first thousands of years, this option presents difficulties for social acceptability. Nuclear waste transmutation has been proposed as a way to reduce substantially (in a factor of 1/100 or more) the inventory of the long lived component of the nuclear waste, mainly the trans-uranium actinides. Actinide transmutation is proposed to take place by fission in nuclear systems like critical fast reactors (FR) or subcritical Accelerator Driven Systems (ADS). In most of the scenarios, the use of fast neutron energy spectra and specific fuel compositions, highly enriched in high mass trans-uranium actinides, are proposed. The actual knowledge on the neutron cross sections of actinides is mainly related to the exploitation of the U-Pu cycle in nuclear reactors with a thermal neutron spectrum and the design and operation of experimental fast U-Pu nuclear reactors. The detailed engineering designs, safety evaluations and the detailed performance assessment of dedicated transmutation ADS and critical reactors (i.e. with fuels highly enriched in transuranic isotopes) require more accurate and complete basic nuclear data.

3. Preliminary neutron sensitivity analysis evidence the need of determining the neutron capture cross sections of trans-uranium actinides with an average accuracy better or equal than 5%. Moreover, the physical quantities describing the behavior of a transmuter depend on a large number of reaction channels. The overall uncertainties in such quantities can be largely reduced by performing accurate and targeted measurements on specific isotopes and reaction channels. • Data with 5% energy resolution in the unresolved resonance region can improve the accuracy of calculations on some aspects of the behavior of an ADS core (i.e. changes of the neutron energy spectrum in the reflector). • Evaluators are forced quite often to question and re-investigate the accuracy of the existing data sets in order to reach a consistent result of their evaluation. Accurate data are also necessary to constrain the physics models (level densities, optical potentials) used during the evaluation process. • Of crucial importance is the new composition of the fuels proposed for transmutation devices, with large concentrations of minor actinides and high mass plutonium isotopes. These isotopes, with little relevance for the operation of present reactors, will play an important role in the neutronics of the transmuters.

4. Fuel composition in PWR and Transmutation Devices

5. Fraction of Neutron Captures in PWR and Transmutation Devices

6. The new fuel compositions modify severely the role of the different isotopes in the global operation of a transmuter (FR or ADS). Several key isotopes showing significant capture fractions in the transmutation scenarios can be identified: 237Np, 238,239,240,241,242Pu, 241,243Am and 244,245Cm Selection criteria based on the impact on the transmuter neutronics and availability of capture experimental data allows to set lower priority to the 239,241Pu measurements. The 238Pu (T1/2=87.74a) and 244Cm(T1/2=18.1a) capture cross section measurements are extremely difficult (also at n_TOF under the present conditions). The remaining isotopes are the ones listed in the proposal: 237Np, 240,242Pu, 241,243Am and 245Cm As a particular scenario, the Th fuel cycle has been proposed as an alternative to the nuclear energy production based on the U/Pu fuel cycle. 232Th + n  233Th (b-, T1/2=22 min)  233Pa (b-, T1/2=27 d)  233U The neutron capture in 232Th, after a fast series of radioactive decays, breeds 233U which fissions and provides the necessary neutron multiplication. The knowledge on the capture cross section of the fissile element 233U in the Th fuel cycle needs to be improved as well (related to the already accepted proposal n_TOF-07).

7. Not only the isotope list but also the energy range in which the capture cross section has to be measured needs to be extended. In a FR or and ADS core, the neutron captures of fast neutrons play an important role. For a typical ADS neutron energy spectrum, energies up to 1 MeV have to be considered in order to take into account up to 99% of the neutron captures inside the ADS core.

8. For these reasons, the measurement of neutron capture cross section of 233U, 237Np, 240,242Pu, 241,243Am and 245Cm at n_TOF is proposed: • Energy range from 0.1 eV to 1 MeV • Average accuracy of 5% • Energy resolution of 1/1000 in the resolved resonance region and better than 5% in the unresolved resonance region • Such measurements are part of the scientific program of the contract FIKW-CT-2000-00107 between the European Commission (5th Framework Program) and the n_TOF collaboration participating institutes.

9. Experimental Data Available • An exhaustive bibliographic research of the available experimental data has been performed: CINDA and EXFOR/CSISRS • 233U • Several data sets in the resolved resonance region (RRR) from capture, fission and total cross section measurements. Scarce data in the unresolved resonance region (URR). Experimental difficulties associated to the capture measurements due to the large fission over capture ratio.

10. 237Np Several data sets in both RRR and URR. Large or huge discrepancies between the two largest data sets. Highly radioactive samples were used.

11. 240Pu • Large uncertainties in the unresolved resonance region. • 242Pu • Small amount of data in the unresolved resonance region. No data above 100 keV.

12. 241Am • High intrinsic radioactivity. Large differences above 100 keV between the two largest data sets.

13. 243Am • The data is scarce. Systematic bias between the two data sets in the unresolved resonance region. • 245Cm • NO CAPTURE DATA AVAILABLE

14. The Experimental Conditions at n_TOF • Advantages of the n_TOF facility • High instantaneous neutron fluence and excellent energy resolution at a 185 m flight path. • Continuous neutron energy spectrum extending up to high energies which allows to perform differential cross section measurements ds(E)/dE in the region between 0.1 eV and 1 MeV (for capture). • Excellent characterisation of the facility: • Knowledge of the absolute neutron fluence distribution to the level of 5% to 10% (depending on the energy range) from 0.1 eV up to several hundred MeV. Remark: the measurements are performed relative to standard cross sections: 6Li(n,a), 197Au and others for capture. • Good knowledge of the background sources of the installation  accurate background corrections. • Knowledge of the resolution function of the neutron source  accurate resonance parameters. • Use of standard and well tested neutron cross section analysis tools (i.e. SAMMY, N. M. Larson) for corrections of the Doppler broadening, multiple scattering and extraction of resonance parameters. • -High accuracy and redundant monitoring of the neutron beam. • The systematic uncertainty associated to the normalisation of the relative measurements is below 1%.

15. Full Response of the PMT Pileup Saturated Pulse Reconstruction 636200 636400 636600 636800 637000 637200 time (ns) The Data Acquisition System (DAQ) at n_TOF The n_TOF DAQ is based on 64 channels of high performance flash ADCs. Each channel has 8 Mbytes memory and is operated at a sampling rate of 500 Msamples/s. Use of the high performance data storage CASTOR system and computing LXBATCH infrastructure at CERN. It allows a quasi on-line analysis of the data. It has been especially designed for having nearly zero dead time and good control of the detectors´ behaviour. The systematic uncertainties due to the detection system are well below 3%.

16. The Total Absorption Calorimeter at n_TOF Total Absorption Calorimetry has significant advantages over alternative techniques commonly employed in neutron capture cross sections measurements. Ex = En + Sn g ZA + n  g g ZA+1 • High efficiency of nearly 100% versus 0.1% for Moxon Rae detectors or 5% for total energy detectors (like C6D6): independence on the EM de-excitation pattern and optimal use of the neutron beam. Low systematic uncertainties associated to the counting efficiency at the level of 1%. • Good energy resolution which allows to identify structures in the energy deposition spectra. Direct background suppression mechanisms based on combined multiplicity and energy deposition analysis. • A Total Absorption Calorimeter is being built at n_TOF and will be fully operative in early Spring 2004. The TAC consists of 40 BaF2 crystals with hexagonal (30) and pentagonal (10) shapes with a thickness of 15 cm.

17. CAD design of the 4p TAC and one hexagonal module 4p TAC at FZK 1p assembly succesfully tested at n_TOF (CERN) in summer 2003

18. The sources of systematic uncertainties in the capture measurements with the TAC at n_TOF are mainly coming from the different types of background: • environmental background and beam related background. Extremely low except for the ultrarelativistic particle flash (completion of the shielding). • Radioactivity of the samples. The main contribution is due to soft g-rays and X-rays (Eg < 100 keV) and will be treated during the pulse shape analysis as a baseline correction. The contribution of higher energy g-rays is several orders of magnitude weaker and will be suppressed by energy discrimination + measured and subtracted from the data. • Sample related background: • In beam gamma rays. It will be largely suppressed by energy discrimination + measured (Pb g-ray scatterer) and subtracted from the data. • Inelastic reaction channels. Largely suppressed by energy discrimination. • Scattered Neutrons. Neutrons are scattered at the samples and sample canning materials. The neutron sensitivity of the BaF2 TAC ranges from 20% to 2% (depending on the neutron energy). Its magnitude will be highly suppressed by the use of a neutron absorber inside the TAC (of 6LiH). It will be measured (C neutron scatterer) and subtracted from the data. • Neutron induced fission g-rays and neutrons. It will be greatly suppressed by a combination of multiplicity and energy analysis. Furthermore, it is foreseen to be measured and subtracted from the data by using a fission tagging.

19. All sources of background have been carefully investigated by means of test measurements and exhaustive Monte Carlo simulations with different codes (GEANT3, GEANT4 and MCNPX)

20. 197Au(n,g) energy deposition spectrum without6LiH absorber. 197Au(n,g) energy deposition spectrum with6LiH absorber.

21. Fissile isotope 245Cm(n,g) energy deposition spectrum without6LiH absorber and with 6LiH 245Cm(n,g) multiplicity distributions without6LiH absorber and with 6LiH Measurements with and without absorber

22. Targets All targets(*) will be provided by IPPE-Obninsk and will be made of isotopically enriched material (>99.9%). The contamination in the capture measurements due to decay products has been calculated to be below 1%. The targets will consist of samples deposited on thin Al backings and encapsulated in cannings fulfilling the ISO 2919 norm (TIS requirement) for sealed containers. The activity of each actinide sample has been calculated for determining the magnitude of the g-ray background in the capture measurements. Each target will undergo a detailed spectroscopic characterization before being placed in the neutron beam. (*)Except 242Pu and 245Cm

23. Goals to be achieved • The beam time request for the measurements has been calculated for achieving the following goals: • Determine the neutron capture cross section of 233U, 237Np, 240,242Pu, 241,243Am and 245Cm in the energy range between 0.1 eV and 1 MeV with an overall accuracy of 5% dominated by the systematic uncertainties associated to the experimental technique. • In the resolved resonance region, the resonances will be measured with a 1% to 3% statistical uncertainty with an energy resolution of 10-3. • The capture cross section in the unresolved resonance region will be measured with a statistical uncertainty better than 2% and an energy resolution of 3%. • The reference cross section of 197Au will be determined with the same statistical uncertainty as the actinide cross sections. • The main sources of background necessary for correcting the data will be measured as well with an accuracy below 5%.

24. Beam Time Request (*) Assuming a PS supercylce of 16.4 s and an average number of 3.5 pulses per supercycle