Motivations

1. Motivations 233U is the main isotope of relevance for the Th/U fuel cycle (it plays the same role of 235U in the U/Pu cycle). The capture cross section of 233U is an important quantity for the development of reactors based on the Th/U fuel cycles. A request (since 2007) on the NEA-HPNDRL for data with <10% uncertainty, in particular in the unresolved resonance region. "Any effort done to confirm the neutron yields and the capture cross section or at least to confirm their uncertainty would be very much appreciated. The energy range of interest is below that of the current request.”

2. The first measurement A measurement of the capture cross section of 233U was proposed and performed at n_TOF in 2004, with the TAC. A high-purity U3O8 sample, 108 mg total mass, from Russia, was used. With TAC, both capture and fission events are detected, so that in principle one should be able to determine from the same data both capture and fission cross sections. Since the fission cross section is much bigger than the capture one (approx. a factor of 10), fission events may affect capture data. A 3% uncertainty in the subtraction of the fission contribution leads to 30% uncertainty in the capture yield.

3. The first measurement In order to discriminate capture from fission, a special analysis method was devised (C. Carrapico et al., “Neutron induced capture and fission discrimination using CalorimetricShapeDecomposition”, NIM A, in press). The method has been applied to determine the 233U capture cross sections (C. Carrapico, PhD Thesis, Un. Lisbon/ITN) The results obtained with this method at n_TOF is 25% different smaller than the only previous measurement (and evaluation). It is difficult to estimate the sytematic uncertainty on these results.

4. Problems (the past) and solutions (the future) • Due to radioprotection rules at CERN, the sample had to be encapsulated in ISO2919 certified capsule: • Al (same amount as U) • Ti (3 time more mass than U) • Consequence: it is not possible to extract from the first measurement capture cross section above ~1 keV(right whereitismostlyneeded). • Making a new measurement without Ti would greatly improve the background to signal ratio. The largefission “background” and the use of the Ti canningdidnotallowtoobtain the desiredaccuracy, up to 100 keV. To improve the accuracy on capture cross sections, it is mandatory to accurately determine the TAC's response to both capture and fission reactions. The fission tagging technique could improve greatly the knowledge on the TAC's response to fission and therefore to capture. The removal of the Ti canning may allow to extend the energy range.

5. The fission tagging technique Recently, a technique has been set-up and tested at n_TOF for measuring the capture/fission ratio (the so-called a-parameter): C. Guerrero et al., “Simultaneous measurement of neutron-induced capture and fission reactions at CERN”, Eur. Phys. Jour. A 48, 29 (2012) The technique has been used in a measurement (2012) of the 235U a-ratio (E. Mendoza). A stack of 10 samples where mounted in a MGAS, inside the TAC. Each sample had a mass of 4 mg (90% detection efficiency for fission fragments). The experiment was successful, except that Cu in the MGAS does not allow to go above a few keV(althoughthe use of the “Calorimetric Shape Decomposition” methodology may help). The main problem is the large capture cross section of Cu, which gave rise to a large capture background. We propose to make a similar “fission tagging” measurement for 233U, but with Al electrodes. The Al-MGAS was recently developed at CERN, specifically for n_TOF, and succesfully tested.

6. The proposal Measurement of the Capture Cross section of 233U withfissiontagging INFN – CEA – CIEMAT - CERN – ITN etc.. The measurement should be done in EA1, with the TAC+Al-FTMG. • We propose to use the same, high-purity sample of 233U used in the first measurement: • total mass of 108 mg (U3O8, corresponding to 90 mg of 233U) • available at CERN (right, Enrico?) Mgas Mgas The final configuration has not been decided yet. There are two options: “à la n_TOF”: 10 fission samples, each with mass of 4 mg “à la LANSCE”: only 2 fission samples (4 mg each) and the remaining material used as a capture sample (80 mg) . Option 1. results in more statistics on the fission/capture yield. However, the amount of material in the FTMGAS is larger, and so is the background. Option 2. gives less background problems, although the statistics on the “fission veto” events will be a factor of 5 less. Our proposal: OPTION 2 ( “fission tagging mode” only on the big resonances)

7. The proposal We propose to measure the 233U(n,g/f) cross sections with the TAC combined with the AL-FTMGAS In the first measurement 6x1017 protons were used, but the statistics was barely enough for resonances. We would need around a factor of 10, in particular for the URR. We propose 5x1018 protons. This number is similar to the 235Un,g/f) of 2012 (the capture cross section of 233U is typically lower than 235U, but the sample is twice as big). It will probably not be possible to determine all capture resonances with fission tagging. However, the measurement can provide confirmation (or normalization) of the results already obtained at n_TOFfor the biggest resonances (C. Carrapico et al.). Another issue to have in mind is the increase in sample activity from 2004 to 2005. A chemical separation may be of interest as activity accounts for a significant part of the count rate at lower energies.