Abstract
233 U plays the essential role of fissile nucleus in the Th-U fuel cycle. A particularity of 233 U is its small neutron capture cross-section which is about one order of magnitude lower than the fission cross-section on average. Therefore, the accuracy in the measurement of the 233 U capture cross-section essentially relies on efficient capture-fission discrimination thus a combined setup of fission and γ-detectors is needed. At CERN n_TOF the Total Absorption Calorimeter (TAC) coupled with compact fission detectors is used. Previously used MicroMegas (MGAS) detectors showed significant γ-background issues above 100 eV coming from the copper mesh. A new measurement campaign of the 233 U capture cross-section and alpha ratio is planned at the CERN n_TOF facility. For this measurement, a novel cylindrical multi ionization cell chamber was developed in order to provide a compact solution for 14 active targets read out by 8 anodes. Due to the high specific activity of 233 U fast timing properties are required and achieved with the use of customized electronics and the very fast ionizing gas CF4 together with a high electric field strength. This paper describes the new fission chamber and the results of the first tests with neutrons at GELINA proving that it is suitable for the 233 U measurement.
Highlights
233U plays the essential role of fissile nucleus in the ThU fuel cycle [1, 2], which has been proposed as an alternative to the U-Pu fuel cycle
The development of the fission chamber was focused on different points: compact design, fast and heavy ionizing gas and optimized electronics to ensure unambiguous alpha-fission discrimination for fission tagging; very low quantities of structure material to avoid neutron scattering background as well as parasitic reactions; and a reasonable amount of 233U to obtain the needed count rate for a measurement with sufficient statistics
One can expect about 1 MBq per anode depending on the actual mass of the deposits which requires fast and low-noise electronics in addition to a fast ionizing gas
Summary
233U plays the essential role of fissile nucleus in the ThU fuel cycle [1, 2], which has been proposed as an alternative to the U-Pu fuel cycle. Considering the scarce data available to assess the capture cross section, a first measurement [3] was proposed and successfully performed at the n TOF facility at CERN using the 4π Total Absorption Calorimeter (TAC) [4]. The measurement was extremely difficult due to the need to accurately distinguish between capture and fission gammarays without any additional discrimination tool and the measured capture cross section showed a significant disagreement in magnitude when compared with the ENDF/B-VII. library despite the agreement in shape. The development of the fission chamber was focused on different points: compact design, fast and heavy ionizing gas and optimized electronics to ensure unambiguous alpha-fission discrimination for fission tagging; very low quantities of structure material to avoid neutron scattering background as well as parasitic reactions; and a reasonable amount of 233U to obtain the needed count rate for a measurement with sufficient statistics
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