Beta-delayed-neutron studies of 135,136^{135,136}Sb and 140^{140}I performed with trapped ions

Beta-delayed-neutron ($\beta$n) spectroscopy was performed using the Beta-decay Paul Trap and an array of radiation detectors. The $\beta$n branching ratios and energy spectra for $^{135,136}$Sb and $^{140}$I were obtained by measuring the time of flight of recoil ions emerging from the trapped ion cloud. These nuclei are located at the edge of an isotopic region identified as having $\beta$n branching ratios that impact the r-process abundance pattern around the A~130 peak. For $^{135,136}$Sb and $^{140}$I, $\beta$n branching ratios of 14.6(11)%, 17.6(28)%, and 7.6(28)% were determined, respectively. The $\beta$n energy spectra obtained for $^{135}$Sb and $^{140}$I are compared with results from direct neutron measurements, and the $\beta$n energy spectrum for $^{136}$Sb has been measured for the first time

β-delayed neutron emission studies of i 137,138 and Cs 144,145 performed with trapped ions

A detailed study of the β-delayed neutron emission properties of I137,138 and Cs144,145 has been performed by confining ions in the Beta-decay Paul Trap. The daughter ions following β decay emerge from the trapped-ion cloud with negligible scattering allowing reconstruction of the recoil-ion energy from the time of flight. From this information, the neutron-emission branching ratios and neutron-energy spectra were deduced. The results for the I137 and Cs144,145 decays are in agreement with previous results performed using direct neutron-detection techniques. In the case of I138, a branching ratio of 6.18(50)% is obtained, yielding a value consistent with the more recent results, which are a factor of two larger than measurements made prior to 1978

Recoil ions from the β decay of Sb 134 confined in a Paul trap

The low-energy recoiling ions from the β decay of Sb134 were studied by using the Beta-decay Paul Trap. Using this apparatus, singly charged ions were suspended in vacuum at the center of a detector array used to detect emitted β particles, γ rays, and recoil ions in coincidence. The recoil ions emerge from the trap with negligible scattering, allowing β-decay properties and the charge-state distribution of the daughter ions to be determined from the β-ion coincidences. First-forbidden β-decay theory predicts a β-ν correlation coefficient of nearly unity for the 0- to 0+ transition from the ground state of Sb134 to the ground state of Te134. Although this transition was expected to have a nearly 100% branching ratio, an additional 17.2(52)% of the β-decay strength must populate high-lying excited states to obtain an angular correlation consistent with unity. The extracted charge-state distribution of the recoiling ions was compared with existing β-decay results and the average charge state was found to be consistent with the results from lighter nuclei

Recoil-ion detection efficiency for complex β decays studied using the Beta-decay Paul Trap

Beta-delayed neutron emission is being studied by detecting the β particles and recoiling ions emerging from the Beta-decay Paul Trap. For β decays to the ground state or γ-emitting states of the daughter nucleus, the fraction of recoiling ions which reach the ion detector in coincidence with a β particle has been determined for 134, 135Sb, 137, 138, 140I, and 144, 145Cs. This value is needed for the determination of the β-delayed neutron emission branching ratio solely from the recoil-ion time-of-flight (TOF) spectrum. The β-particle energy and recoil-ion TOF spectra were used to constrain a simple decay model, which can be used to determine the detection efficiency. The method is compared to simulations to estimate the uncertainty introduced by incomplete knowledge of the decay pattern. By fitting the simulation results to several β-ion coincidence properties measured during the experiment, the fraction of ions which reach the microchannel plate detector can be determined to within ±4%. This result opens the possibility of using the recoil-ion TOF spectra for high precision β-delayed neutron branching-ratio measurements

Gamma decay of pygmy states in 90,94Zr from inelastic scattering of light ions

We performed experiments to study the low-energy part of the E1 response (Pygmy Dipole Resonance) in 90,94Zr nuclei, by measuring the (p,p'γ) and (α,α'γ) inelastic scattering reactions at energies Ebeam,p = 80 MeV and Ebeam,α = 130 MeV respectively. The inelastically scattered particles were measured by employing the high-resolution spectrometer Grand Raiden. The gamma-rays emitted following the de-excitation of the Zr target nuclei were detected using both the clover type HPGe detectors of the CAGRA array and the large volume LaBr3:Ce scintillation detectors from the HECTOR+ array. Some preliminary results are presented here

Distinguishing fissions of 232Th, 237Np and 238U with beta-delayed gamma rays

Measurements of beta-delayed gamma-ray spectra following 14-MeV neutron-induced fissions of Th, U, and Np were conducted at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Spectra were collected for times ranging from 1 min to 14 h after irradiation. Intensity ratios of gamma-ray lines were extracted from the data that allow identification of the fissioning isotope.© 2013 Elsevier B.V. All rights reserved. 232 238 23

Reconstruction of β-delayed neutron energy spectra from recoil-ion spectroscopy of trapped ions

Beta-delayed one-neutron (βn) emission has been investigated by confining radioactive ions in an ion trap and detecting the β particles and recoiling nuclei that emerge following decay. In this approach, the βn energy spectrum and branching ratio can be deduced without needing to detect the neutrons, as the neutron emission is inferred from the observed time of flight of the recoiling ions. This paper details the dominant effects that influence the extraction of the neutron energy from the time-of-flight measurement and explores the impact they have on the energy calibration and resolution

Distinguishing fissions of 232Th, 237Np and 238U with beta-delayed gamma rays

Measurements of beta-delayed gamma-ray spectra following 14-MeV neutron-induced fissions of 232Th, 238U, and 237Np were conducted at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Spectra were collected for times ranging from 1 min to 14 h after irradiation. Intensity ratios of gamma-ray lines were extracted from the data that allow identification of the fissioning isotope.© 2013 Elsevier B.V. All rights reserved

Determining the [superscript 239]Np(n, f ) cross section using the surrogate ratio method

Publisher's version/PDFBackground: Neutron-induced fission cross-section data are needed in various fields of applied and basic nuclear science. However, cross sections of short-lived nuclei are difficult to measure directly due to experimental constraints. Purpose: The first experimental determination of the neutron-induced fission cross section of [superscript 239]Np at nonthermal energies was performed. This minor actinide is the waiting point to [superscript 240]Pu production in a nuclear reactor. Method: The surrogate ratio method was employed to indirectly deduce the [superscript 239]Np(n, f) cross section. The surrogate reactions used were [superscript 236]U([superscript 3]He, p) and [superscript 238]U([superscript 3]He, p) with the reference cross section given by the well-known [superscript 237]Np(n, f ) cross section. The ratio of observed fission reactions resulting from the two formed compound nuclei, [superscript 238]Np and [superscript 240]Np,was multiplied by the directly measured [superscript 237]Np(n, f ) cross section to determine the [superscript 239]Np(n, f ) cross section. Results: The [superscript 239]Np(n, f ) cross section was determined with an uncertainty ranging between 4% and 30% over the energy range of 0.5–20 MeV. The resulting cross section agrees closest with the JENDL-4.0 evaluation. Conclusions: The measured cross section falls in between the existing evaluations, but it does not match any evaluation exactly (with JENDL-4.0 being the closest match); hence reactor codes relying on existing evaluations may under- or overestimate the amount of [superscript 240]Pu produced during fuel burnup. The measurement helps constrain nuclear structure parameters used in the evaluations