Abstract
The present uncertainty about a sustainable energy supply is characterized by considerable concern. The greenhouse effect, the foreseen exhaustion of mineral fuels, the environmental impact of fission nuclear energy and the long - term fusion research, led to the consideration of many advanced strategies for nuclear fuels and the equivalent nuclear energy systems. These strategies include: a) the increase of the operation time of nuclear reactors, the better combustion of fuels, the recycling of plutonium and mainly the "incineration" of actinides and long lived products of fission, b) The Accelerator Driven Systems (ADS) or Energy Amplifier (EA), as were defined by C. Rubbia and the use of the thorium fuel cycle. The detailed study and evaluation of feasibility and safety of these strategies require very good knowledge of nuclear data and particularly the cross - sections for neutron induced reactions. Existing nuclear data cover specific energy regions and isotopes and frequently are ...
The present uncertainty about a sustainable energy supply is characterized by considerable concern. The greenhouse effect, the foreseen exhaustion of mineral fuels, the environmental impact of fission nuclear energy and the long - term fusion research, led to the consideration of many advanced strategies for nuclear fuels and the equivalent nuclear energy systems. These strategies include: a) the increase of the operation time of nuclear reactors, the better combustion of fuels, the recycling of plutonium and mainly the "incineration" of actinides and long lived products of fission, b) The Accelerator Driven Systems (ADS) or Energy Amplifier (EA), as were defined by C. Rubbia and the use of the thorium fuel cycle. The detailed study and evaluation of feasibility and safety of these strategies require very good knowledge of nuclear data and particularly the cross - sections for neutron induced reactions. Existing nuclear data cover specific energy regions and isotopes and frequently are incomplete and with many discrepancies among them. These differences are more pronounced in the case of actinides, products of fission and isotopes of the Th - U cycle, mainly due to their radioactivity. The European Union approved, in the frame of the 5th EURATOM program, the n_TOF-ND-ADS experiment that was proposed by the nTOF collaboration. The main advantage of this proposal is a complete set of high precision cross - section measurements, extending over eight orders of magnitude in the neutron energy, satisfying research and industrial requirements. The experiments were carried out in the nTOF installations at CERN in Geneva, Switzerland. A detailed description of its performances can be found elsewhere. At n TOF, neutrons are produced via spallation reactions induced by a pulsed, 6 ns wide, 20 GeV/c proton beam with up to 7 X 10¹² protons per pulse, impinging on a 80 X 80 X 60 cm³ lead target. The repetition period of the proton pulses of 2.4 s on average is low enough to prevent any overlapping of neutrons in subsequent cycles. A 5 cm water slab surrounding the lead target serves as both a coolant and a moderator of the initially fast neutron spectrum, providing a wide energy spectrum from 1 eV to about 1 GeV. An evacuated neutron beamline leads to the experimental area with the fission sample position at 185.2 m from the lead target. Two collimators are present in the neutron beam, one with a diameter of 13.5 cm placed at 135 m from the lead target and one at 175 m with a diameter of 8 cm for the fission measurements. This collimation results in a nearly symmetric Gaussian-shaped beam profile at the sample position, with an energy-dependent standard deviation, which is about 0.77 cm at low neutron energies. At a distance of 145 m, a 1.5 T magnet is placed in order to remove the residual charged particles going along the neutron beamline. The neutron beamline extends for an additional 12 m beyond the experimental area to minimize the background from backscattered neutrons. The detectors were placed at a distance roughly 180 m and with a light bent with respect of the proton beam axis. Aim of this work was the determination of the neutron induced fission cross - section of ²³⁴U and ²³²Th, at the energy region of 20 KeV - 1 GeV. For their measurement the Fission Induction Chamber (FIC) detector was used, which is a stack of several parallel -plate ionization chambers with 5 mm spacing between electrodes and operating with argon- tetrafluormethane (90%Ar+ I0%CF4) at 700 mbar pressure. High-voltage of 400 V was applied to the targets, while intermediate electrodes were connected to the ground. The charge from the fission fragments is detected with an efficiency 98% while 2% stands for the fragments that were absorbed inside the target (ff direction along the target surface). The “neutron time of flight" (nTOF) technique was used for the calculation of the neutron incident energy. As the proton beam enters the lead target, the neutron beam is created, along with relativistic particles, the so called “gamma-flash'’, which travels with the speed of light and enters the detector body. This is the “start” signal used for the time determination, while the “stop” signal corresponds to the detection of the fission fragment, caused by the neutron. This time interval denotes the time of flight of this neutron, traveling along the known flight path between the lead target and the FIC detector, thus defining its energy. A flash Analog to Digital Converter (fADC) was used for recording both of the ’’gamma -flash” and the fission fragment signal, which was processed with electronic units such as sensitive preamplifiers, fast linear amplifiers and twisted pair drivers. fADC's record the amplitude of the detector signal in time intervals of 25 nsec (sampling rate 40 MHz). In this respect, in the present work the fADC’s were used for recording the fission fragments and the “gamma - flash”, as a function of time, providing the neutron time of flight (fission fragment arrival - “gamma flash” arrival). Data Acquition (DAQ) computer software was developed to record the digitized signals from CAEN or Acqiris digitizers, on the hard disk of the computer. Following the recording of experimental data, the analysis required the development of the appropriate software to deal with the large amount of data in an automatic way. Functions of the software include the counting of fission events, after discrimination from background events, the calculation of the energy of the neutron which caused the fission and finally the calculation of neutron induced fission cross - section in the whole energy range. During the data analysis process, it was realized that the “gamma - flash” introduced high electronic noise in the detector signal. The intensity of that signal caused the oscillation and undershooting of the baseline. The pulse analysis software was developed in such a way, to face this deformity. The oscillation of the baseline had a specific pattern, so "average signals" could be created and subtracted from each signal. Thus the noise was removed in most of the cases, however, in the region from 10 - 100 MeV the oscillation of the baseline was more intense and the noise remained. Thus the fission cross - section in this energy region was not accurately determined and it only constitutes an approach of the real value. However, in the energies 20 keV - 10 MeV and 100 MeV - 0.5 GeV, the neutron induced fission cross - sections of the isotopes ²³⁴U and ²³²Th were determined for the first time in the bibliography, in a systematic and consistent way, along with the statistical and experimental errors. Due to the uncertainty in the precise number of neutrons of the beam, the ratio of the neutron induced fission cross - section of the isotopes ²³⁴U and ²³²Th was determined, with respect to that of ²³⁵U and ²³⁸U. For ²³⁵U and ²³⁸U the neutron induced fission cross - section is considered to be well known and is frequently used as a reference reaction.
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