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Register a new userCharacterization of an INVS Model IV Neutron Counter for High Precision ($gamma,n$) Cross-Section Measurements
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A neutron counter designed for assay of radioactive materials has been adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum efficiency make it well suited for ($gamma,n$) cross-section measurements near the neutron emission threshold. A high precision characterization of the counter has been made using neutrons from several sources. Using a combination of measurements and simulations, the absolute detection efficiency of the neutron counter was determined to an accuracy of $pm$ 3% in the neutron energy range between 0.1 and 1 MeV. It is shown that this efficiency characterization is generally valid for a wide range of targets.
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We measured the 7Be(p,gamma)8B cross section from E_cm = 186 to 1200 keV, with a statistical-plus-systematic precision per point of better than +- 5%. All important systematic errors were measured including 8B backscattering losses. We obtain S_17(0) = 22.3 +- 0.7(expt) +- 0.5(theor) eV-b from our data at E_cm <= 300 keV and the theory of Descouvemont and Baye.
Nuclear level densities (NLDs) and $gamma$-ray strength functions ($gamma$SFs) have been extracted from particle-$gamma$ coincidences of the $^{92}$Zr($p,p gamma$)$^{92}$Zr and $^{92}$Zr($p,d gamma$)$^{91}$Zr reactions using the Oslo method. The new $^{91,92}$Zr $gamma$SF data, combined with photonuclear cross sections, cover the whole energy range from $E_{gamma} approx 1.5$~MeV up to the giant dipole resonance at $E_{gamma} approx 17$~MeV. The wide-range $gamma$SF data display structures at $E_{gamma} approx 9.5$~MeV, compatible with a superposition of the spin-flip $M1$ resonance and a pygmy $E1$ resonance. Furthermore, the $gamma$SF shows a minimum at $E_{gamma} approx 2-3$~MeV and an increase at lower $gamma$-ray energies. The experimentally constrained NLDs and $gamma$SFs are shown to reproduce known ($n, gamma$) and Maxwellian-averaged cross sections for $^{91,92}$Zr using the {sf TALYS} reaction code, thus serving as a benchmark for this indirect method of estimating ($n, gamma$) cross sections for Zr isotopes.
The cross section of the $^{23}$Na($n, gamma$)$^{24}$Na reaction has been measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at $kT=5.1$ and 25 keV produced via the $^{18}$O($p, n$)$^{18}$F and $^{7}$Li($p, n$)$^{7}$Be reactions, respectively. The derived capture cross sections $langlesigmarangle_{rm kT=5 keV}=9.1pm0.3$ mb and $langlesigmarangle_{rm kT=25 keV}=2.03 pm 0.05$ mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first $^{23}$Na resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of $s$-process nucleosynthesis are discussed.
The 62Ni(n,gamma)63Ni(t_1/2=100+-2 yrs) reaction plays an important role in the control of the flow path of the slow neutron-capture (s-) nucleosynthesis process. We have measured for the first time the total cross section of this reaction for a quasi-Maxwellian (kT = 25 keV) neutron flux. The measurement was performed by fast-neutron activation, combined with accelerator mass spectrometry to detect directly the 63Ni product nuclei. The experimental value of 28.4+-2.8 mb, fairly consistent with a recent theoretical estimate, affects the calculated net yield of 62Ni itself and the whole distribution of nuclei with 62<A <90 produced by the weak s-process in massive stars.
Study of the elastic scattering can produce a rich information on the dynamics of the strong interaction. The EPECUR collaboration is aimed at the research of baryon resonances in the second resonance region via pion-proton elastic scattering and kaon-lambda production. The experiment features high statistics and better than 1 MeV resolution in the invariant mass thus allowing searches for narrow resonances with the coupling to the pi p channel as low as 5%. The experiment is of formation type, i.e. the resonances are produced in s-channel and the scan over the invariant mass is done by the variation of the incident pion momentum which is measured with the accuracy of 0.1% with a set of 1 mm pitch proportional chambers located in the first focus of the beam line. The reaction is identified by a magnetless spectrometer based on wire drift chambers with a hexagonal structure. Background suppression in this case depends on the angular resolution, so the amount of matter in the chambers and the setup was minimized to reduce multiple scattering. The measurements started in 2009 with the setup optimized for elastic pion-proton scattering. With 3 billions of triggers already recorded the differential cross section of the elastic pi p-scattering on a liquid hydrogen target in the region of the diffraction minimum is measured with statistical accuracy about 1% in 1 MeV steps in terms of the invariant mass. The paper covers the experimental setup, current status and some preliminary results.
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