626 research outputs found

    New detection systems for an enhanced sensitivity in key stellar (n,Îł) measurements

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    Neutron capture cross-section measurements are fundamental in the study of astrophysical phenomena, such as the slow neutron capture (s-) process of nucleosynthesis operating in red-giant and massive stars. However, neutron capture measurements via the time-of-flight (TOF) technique on key s-process nuclei are often challenging. Difficulties arise from the limited mass (∌mg) available and the high sample-related background in the case of the unstable s-process branching points. Measurements on neutron magic nuclei, that act as s-process bottlenecks, are affected by low (n,Îł) cross sections and a dominant neutron scattering background. Overcoming these experimental challenges requires the combination of facilities with high instantaneous flux, such as n_TOFEAR2, with detection systems with an enhanced detection sensitivity and high counting rate capabilities. This contribution reviews some of the latest detector developments in detection systems for (n,Îł) measurements at n_TOF, such as i-TED, an innovative detection system which exploits the Compton imaging technique to reduce the dominant neutron scattering background and s-TED, a highly segmented total energy detector intended for high flux facilities. The discussion will be illustrated with results of the first measurement of key the s-process branching-point reaction 79Se(n,Îł).Title in Web of Science: New detection systems for an enhanced sensitivity in key stellar (n,gamma) measurements</p

    On the accuracy of cross-section measurements of neutron-induced reactions using the activation technique with natural targets: The case of Ge at E<math altimg="si5.svg" display="inline" id="d1e1043"><mrow><msub><mrow/><mrow><mi mathvariant="normal">n</mi></mrow></msub><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mn>17</mn><mo>.</mo><mn>9</mn></mrow></math> MeV

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    International audienceSeveral cross-section measurements of neutron-induced reactions on Ge found in literature, are performed utilizing natGe targets. The production of the same residual nucleus as the measured one might occur as a result of the unavoidable presence of neighboring isotopes in the same target, acting as a contamination. Corrections must be made based on theoretical calculations and models in order to resolve this problem. The accuracy and limits of a methodology for these “theoretical corrections” are investigated in this work using isotopically enriched targets, which can produce very accurate results without the need for such corrections. Experimental cross-section measurements have been made for the 76Ge(n,2n)75Ge, 72Ge(n,α)69mZn and 72Ge(n,p)72Ga reactions, via the activation technique, with the 27Al(n,α)24Na reaction used as reference, employing both a natGe and isotopically enriched Ge targets. The 3H(d,n)4He (D–T) reaction was used for producing the quasi-monoenergetic neutron beam in the 5.5 MV Tandem Accelerator Laboratory of the National Centre for Scientific Research “Demokritos” in Athens, Greece, at an incident deuteron beam energy of 2.9 MeV. Using HPGe detectors, Îł-ray spectroscopy was applied to determine the induced Îł-ray activity of the residual nuclei. ‱Cross section measurements with natural Ge targets need to be corrected for contaminations from neighboring isotopes‱A methodology is proposed for the correction of the contribution from parasitic channels‱The accuracy and limits of the methodology are investigated with two sets of measurements‱Enriched and natural Ge targets were employed for cross-section measurements via the activation techniqu

    Ion channeling implantation induced MgF2 crystal damage through the “eye” of photoluminescence spectroscopy

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    Magnesium fluoride (MgF2) single crystal has been widely used as a material for application in optics due to its excellent properties like birefringence, wide range of transparency and low refractive index. As such, MgF2 has been proposed for planar waveguide structures. Ion implantation method was frequently used for planar waveguide production due to its ability to modulate optical properties by introduction of impurities and defects in crystal lattice. In all optics fabrication processes, there are demands for a precise control of optical characteristics modulation and hence the need for precise distribution of implanted impurities and induced damage. In this study, 4 MeV C3+ ions with the fluence of 5×1015 ions/cm2 were implanted in (001) axial direction of MgF2 single crystal. In order to determine the damage depth distribution in the crystal sample, photoluminescence (PL) spectroscopy was proposed as a method of evaluation. PL spectroscopy was used as a convenient method for damage investigation of transparent and semi-transparent samples. The cross-section of the implanted zone was mapped with the step of 0.34 ”m and the variations in the spectra were investigated. It was shown that intensity evolution of two prominent wide bands with the intensity maximums at about 590 nm and 733 nm can be used for damage depth distribution estimation. Comparing the relative changes of derivatives of the band’s intensities, data related to the damage depth distribution were obtained. Obtained distribution was compared with the SRIM calculation of displacement damage. Considering the difference in implantation direction, good agreement with SRIM results was obtained. As a consequence of ion channeling, it was shown that damage distribution is extended deeper (for about 20%)

    High resolution

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    Neutron capture cross section measurements of isotopes close to s-process branching-points are of fundamental importance for the understanding of this nucleosynthesis mechanism through which about 50% of the elements heavier than iron are produced. We present in this contribution the results corresponding to the high resolution measurement, for first time ever, of the 80Se(n, Îł) cross section, in which 98 resonances never measured before have been reported. As a consequence, ten times more precise values for the MACS have been obtained compared to previous accepted value adopted in the astrophysical KADoNiS data base

    Measurement of the 77Se(n,Îł)^{77}Se ( n , Îł ) cross section up to 200 keV at the n_TOF facility at CERN

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    The 77Se(n,γ)^{77}Se ( n , γ ) reaction is of importance for 77Se^{77}Se abundance during the slow neutron capture process in massive stars. We have performed a new measurement of the 77Se^{77}Se radiative neutron capture cross section at the Neutron Time-of-Flight facility at CERN. Resonance capture kernels were derived up to 51 keV and cross sections up to 200 keV. Maxwellian-averaged cross sections were calculated for stellar temperatures between kT=5 keVkT=5 \space keV and kT=100 keVkT=100\space keV, with uncertainties between 4.2% and 5.7%. Our results lead to substantial decreases of 14% and 19% in 77Se^{77}Se abundances produced through the slow neutron capture process in selected stellar models of 15M⊙15M⊙ and 2M⊙2M⊙, respectively, compared to using previous recommendation of the cross section

    Cross Section Measurements and Theoretical Study of the

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    Experimental cross section measurements for the 176Hf(n,2n)175Hf and 174Hf(n,2n)173Hf reactions were carried out, using the activation technique. The neutron beam energy in the range of 15.3-20.3 MeV was produced via the 3H(d,n)4He reaction at the 5.5 MeV Tandem Van de Graaf accelerator laboratory of NCSR “Demokritos”. A thin metallic foil of natural Hf was used, while for the determination of the neutron flux at the target position, reference foils of Al were placed at the front and back of the Hf target. The irradiations were continuous for ~24-48 hours, leading to a total neutron fluence of 1010-1011 n/cm2 and a BF3 detector was used for monitoring the neutron flux during the irradiations. After the end of each irradiation, the activity of the Hf target and the Al reference foils were measured off-line by two HPGe detectors. The 176Hf(n,2n)175Hf reaction has been corrected for the contribution of the 177Hf(n,3n)175Hf and 174Hf(n,γ)175Hf reactions. Statistical model calculations based on the Hauser-Feshbach theory have also been performed using the EMPIRE 3.2.3 code. The predictions have been compared with the data of the present work as well as with data from literature

    Measurement of the

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    The neutron capture cross section of 241Am is an important quantity for nuclear energy production and fuel cycle scenarios. Several measurements have been performed in recent years with the aim to reduce existing uncertainties in evaluated data. Two previous measurements, performed at the 185 m flight-path station EAR1 of the neutron time-of-flight facility n_TOF at CERN, have permitted to substantially extend the resolved resonance region, but suffered in the near-thermal energy range from the unfavorable signal-to-background ratio resulting from the combination of the high radioactivity of 241Am and the rather low thermal neutron flux. The here presented 241Am(n,Îł) measurement, performed with C6D6 liquid scintillator gamma detectors at the 20 m flight-path station EAR2 of the n_TOF facility, took advantage of the much higher neutron flux. The current status of the analysis of the data, focussed on the low-energy region, will be described here

    Compton imaging for enhanced sensitivity (n,Îł) cross section TOF experiments: Status and prospects

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    Radiative neutron-capture cross sections are of pivotal importance in many fields such as nucle-osynthesis studies or innovative reactor technologies. A large number of isotopes have been measured with high accuracy, but there are still a large number of relevant isotopes whose cross sections could not be experimentally determined yet, at least with sufficient accuracy and completeness, owing to limitations in detection techniques, sample production methods or in the facilities themselves. In the context of the HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions) project over the last six years we have developed a novel detection technique aimed at background suppression in radiative neutron-capture time-of-flight measurements. This new technique utilizes a complex detection set-up based on position-sensitive radiation-detectors deployed in a Compton-camera array configuration. The latter enables to implement gamma-ray imaging techniques, which help to disentangle true capture events arising from the sample under study and contaminant background events from the surroundings. A summary on the main developments is given in this contribution together with an update on recent experiments at CERN n_TOF and an outlook on future steps

    Measurement of the <math><mrow><mmultiscripts><mi>Se</mi><mprescripts/><none/><mn>77</mn></mmultiscripts><mo>(</mo><mi>n</mi><mo>,</mo><mi>γ</mi><mo>)</mo></mrow></math> cross section up to 200 keV at the n_TOF facility at CERN

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    International audienceThe Se77(n,γ) reaction is of importance for Se77 abundance during the slow neutron capture process in massive stars. We have performed a new measurement of the Se77 radiative neutron capture cross section at the Neutron Time-of-Flight facility at CERN. Resonance capture kernels were derived up to 51 keV and cross sections up to 200 keV. Maxwellian-averaged cross sections were calculated for stellar temperatures between kT=5keV and kT=100keV, with uncertainties between 4.2% and 5.7%. Our results lead to substantial decreases of 14% and 19% in Se77 abundances produced through the slow neutron capture process in selected stellar models of 15M⊙ and 2M⊙, respectively, compared to using previous recommendation of the cross section

    Results of the

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    Accurate neutron capture cross section data for minor actinides (MAs) are required to estimate the production and transmutation rates of MAs in light water reactors, critical fast reactors like Gen-IV systems, and other innovative reactor systems such as accelerator driven systems (ADS). In particular, 244Cm, 246Cm and 248Cm play a role in the transport, storage and transmutation of the nuclear waste of the current nuclear reactors, due to the contribution of these isotopes to the radiotoxicity, neutron emission, and decay heat in the spent nuclear fuel. Also, capture reactions in these Cm isotopes open the path for the formation of heavier elements. In this work, the results of the capture cross section measurement on 244Cm, 246Cm and 248Cm performed at the CERN n_TOF facility are presented. It is important to notice that the Cm samples used in the experiment at n_TOF have been used previously in an experiment at J-PARC, this experiment and the previous one done in the 70s with a nuclear explosion were the only previous capture experiments for these isotopes. At n_TOF, the capture cross section measurements of 244Cm, 246Cm and 248Cm were performed at the 20 m vertical flight path (EAR2) with three C6D6 total energy detectors. In addition, the cross section of 244Cm was measured at the 185 m flight path (EAR1) with a Total Absorption Calorimeter (TAC). The combination of measurements in EAR1 and EAR2 has contributed to controlling and reducing the systematic uncertainties in the results. The compatibility of the different measurements performed and the techniques to obtain the results are presented in this paper as well as the procedure to obtain the resonance parameters
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