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Register a new userOn the alpha activity of natural tungsten isotopes
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The indication for the alpha decay of 180-W with a half-life T1/2=1.1+0.8-0.4(stat)+-0.3(syst)x10^18 yr has been observed for the first time with the help of the super-low background 116-CdWO_4 crystal scintillators. In conservative approach the lower limit on half-life of 180-W has been established as T1/2>0.7x10^18 yr at 90% C.L. Besides, new T1/2 bounds were set for alpha decay of 182-W, 183-W, 184-W and 186-W at the level of 10^20 yr.
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The natural alpha decay of 180W has been unambiguously detected for the first time. The alpha peak is found in a (gamma,beta and neutron)-free background spectrum. This has been achieved by the simultaneous measurement of phonon and light signals with the CRESST cryogenic detectors. A half-life of T1/2 = (1.8 +- 0.2) x 10^18 y and an energy release of Q = (2516.4 +- 1.1 (stat.) +- 1.2 (sys.)) keV have been measured. New limits are also set on the half-lives of the other naturally occurring tungsten isotopes.
Total radiative thermal neutron-capture $gamma$-ray cross sections for the $^{182,183,184,186}$W isotopes were measured using guided neutron beams from the Budapest Research Reactor to induce prompt and delayed $gamma$ rays from elemental and isotopically-enriched tungsten targets. These cross sections were determined from the sum of measured $gamma$-ray cross sections feeding the ground state from low-lying levels below a cutoff energy, E$_{rm crit}$, where the level scheme is completely known, and continuum $gamma$ rays from levels above E$_{rm crit}$, calculated using the Monte Carlo statistical-decay code DICEBOX. The new cross sections determined in this work for the tungsten nuclides are: $sigma_{0}(^{182}{rm W}) = 20.5(14)$ b and $sigma_{11/2^{+}}(^{183}{rm W}^{m}, 5.2 {rm s}) = 0.177(18)$ b; $sigma_{0}(^{183}{rm W}) = 9.37(38)$ b and $sigma_{5^{-}}(^{184}{rm W}^{m}, 8.33 mu{rm s}) = 0.0247(55)$ b; $sigma_{0}(^{184}{rm W}) = 1.43(10)$ b and $sigma_{11/2^{+}}(^{185}{rm W}^{m}, 1.67 {rm min}) = 0.0062(16)$ b; and, $sigma_{0}(^{186}{rm W}) = 33.33(62)$ b and $sigma_{9/2^{+}}(^{187}{rm W}^{m}, 1.38 mu{rm s}) = 0.400(16)$ b. These results are consistent with earlier measurements in the literature. The $^{186}$W cross section was also independently confirmed from an activation measurement, following the decay of $^{187}$W, yielding values for $sigma_{0}(^{186}{rm W})$ that are consistent with our prompt $gamma$-ray measurement. The cross-section measurements were found to be insensitive to choice of level density or photon strength model, and only weakly dependent on E$_{rm crit}$. Total radiative-capture widths calculated with DICEBOX showed much greater model dependence, however, the recommended values could be reproduced with selected model choices. The decay schemes for all tungsten isotopes were improved in these analyses.
The aim of the present work is to measure the $^{121}$Sb($alpha,gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I, and $^{123}$Sb($alpha$,n)$^{126}$I reaction cross sections. The $alpha$-induced reactions on natural and enriched antimony targets were investigated using the activation technique. The ($alpha$,$gamma$) cross sections of $^{121}$Sb were measured and are reported for first time. To determine the cross section of the $^{121}$Sb($alpha$,$gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I, and $^{123}$Sb($alpha$,n)$^{126}$I reactions, the yields of $gamma$-rays following the $beta$-decay of the reaction products were measured. For the measurement of the lowest cross sections, the characteristic X-rays were counted with a LEPS (Low Energy Photon Spectrometer) detector. The cross section of the $^{121}$Sb($alpha$,$gamma$)$^{125}$I, $^{121}$Sb($alpha$,n)$^{124}$I and $^{123}$Sb($alpha$,n)$^{126}$I reactions were measured with high precision in an energy range between 9.74 MeV to 15.48 MeV, close to the astrophysically relevant energy window. The results are compared with the predictions of statistical model calculations. The ($alpha$,n) data show that the $alpha$ widths are predicted well for these reactions. The ($alpha$,$gamma$) results are overestimated by the calculations but this is due to the applied neutron- and $gamma$ widths. Relevant for the astrophysical reaction rate is the $alpha$ width used in the calculations.While for other reactions the $alpha$ widths seem to have been overestimated and their energy dependence was not described well in the measured energy range, this is not the case for the reactions studied here. The result is consistent with the proposal that additional reaction channels, such as Coulomb excitation, may have led to the discrepancies found in other reactions.
Natural silver targets have been irradiated by using a 50 MeV alpha-particle beam in order to measure the activation cross sections of radioisotopes in the 40-50 MeV energy range. Among the radio-products there are medically important isotopes such as $^{110m}$In and $^{111}$In. For optimizing the production of these radioisotopes and regarding their purity and specific activity the cross section data for every produced radioisotope are important. New data are measured in this energy range and the results of some previous measurements have been confirmed. Physical yield curves have been calculated by using the new cross section data completed with the results from the literature.
A new $alpha$-emitting isotope $^{214}$U, produced by fusion-evaporation reaction $^{182}$W($^{36}$Ar, 4n)$^{214}$U, was identified by employing the gas-filled recoil separator SHANS and recoil-$alpha$ correlation technique. More precise $alpha$-decay properties of even-even nuclei $^{216,218}$U were also measured in reactions of $^{40}$Ar, $^{40}$Ca with $^{180, 182, 184}$W targets. By combining the experimental data, improved $alpha$-decay reduced widths $delta^2$ for the even-even Po--Pu nuclei in the vicinity of magic neutron number $N=126$ were deduced. Their systematic trends are discussed in terms of $N_{p}N_{n}$ scheme in order to study the influence of proton-neutron interaction on $alpha$ decay in this region of nuclei. It is strikingly found that the reduced widths of $^{214,216}$U are significantly enhanced by a factor of two as compared with the $N_{p}N_{n}$ systematics for the $84 leq Z leq 90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the $pi 1f_{7/2}$ and $ u 1f_{5/2}$ spin-orbit partner orbits, which is supported by a large-scale shell model calculation.
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