Measurement of the $2^+\rightarrow 0^+$ ground-state transition in the $\beta$ decay of $^{20}$F

We report the first detection of the second-forbidden, non-unique, $2^+\rightarrow 0^+$, ground-state transition in the $\beta$ decay of $^{20}$F. A low-energy, mass-separated $^{20}\rm{F}^+$ beam produced at the IGISOL facility in Jyv\"askyl\"a, Finland, was implanted in a thin carbon foil and the $\beta$ spectrum measured using a magnetic transporter and a plastic-scintillator detector. The $\beta$-decay branching ratio inferred from the measurement is $b_{\beta} = [ 0.41\pm 0.08\textrm{(stat)}\pm 0.07\textrm{(sys)}] \times 10^{-5}$ corresponding to $\log ft = 10.89(11)$, making this one of the strongest second-forbidden, non-unique $\beta$ transitions ever measured. The experimental resu…

Spectral attenuation of solar ultraviolet radiation in humic lakes in Central Finland

The attenuation of solar ultraviolet (UV) radiation in five lakes in Central Finland was evaluated through field measurements and/or by determining the optical properties of the lake water during summer 1999. Spectral UV irradiance in the air and at several depths underwater was measured in three lakes (Lake Palosjärvi, Konnevesi, and Jyväsjärvi) with dissolved organic carbon (DOC) ranging from 4.9 to 8.7 mg l(-1) and chlorophyll a ranging from 1.6 to 16 g l(-1). According to the field measurements, 99% of the UV-B radiation was attenuated in approximately a half meter water column in the clearest lake. In the UV-A region at 380 nm, the corresponding attenuation occurred in the upper one me…

Experimental study of $^{100}$Tc $\beta$ decay with total absorption $\gamma$-ray spectroscopy

International audience; The β decay of Tc100 has been studied by using the total absorption γ-ray spectroscopy technique at the Ion Guide Isotope Separator On-Line facility in Jyväskylä. In this work the new Decay Total Absorption γ-ray Spectrometer in coincidence with a cylindrical plastic β detector has been employed. The β intensity to the ground state obtained from the analysis is in good agreement with previous high-resolution measurements. However, differences in the feeding to the first-excited state as well as weak feeding to a new level at high excitation energy have been deduced from this experiment. Theoretical calculations performed in the quasiparticle random-phase approximatio…

Electron-capture branch of Tc-100 and tests of nuclear wave functions for double-beta decays

We present a measurement of the electron-capture branch of 100Tc. Our value, B(EC)=(2.6±0.4)×10−5, implies that the 100Mo neutrino absorption cross section to the ground state of 100Tc is roughly one third larger than previously thought. Compared to previous measurements, our value of B(EC) prevents a smaller disagreement with QRPA calculations relevant to double-β decay matrix elements.

Measurement of the 2+--0+ ground-state transition in the ß decay of 20F

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Measurement of the 2+→0+ ground-state transition in the β decay of 20F

We report the first detection of the second-forbidden, nonunique, 2+→0+, ground-state transition in the β decay of 20F. A low-energy, mass-separated 20F+ beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the β spectrum measured using a magnetic transporter and a plastic-scintillator detector. The β-decay branching ratio inferred from the measurement is bβ=[0.41±0.08(stat)±0.07(sys)]×10−5 corresponding to logft=10.89(11), making this one of the strongest second-forbidden, nonunique β transitions ever measured. The experimental result is supported by shell-model calculations and has significant implications for the final evolution of stars tha…

β - and γ -spectroscopy study of Pd119 and Ag119

Total absorption γ-ray spectroscopy of the β decays of 96gs,mY

The β decays of the ground state (gs) and isomeric state (m) of 96Y have been studied with the total absorption γ-ray spectroscopy technique at the Ion Guide Isotope Separator On-Line facility. The separation of the 8+ isomeric state from the 0− ground state was achieved thanks to the purification capabilities of the JYFLTRAP double Penning trap system. The β-intensity distributions of both decays have been independently determined. In the analyses the deexcitation of the 1581.6 keV level in 96Zr, in which conversion electron emission competes with pair production, has been carefully considered and found to have significant impact on the β-detector efficiency, influencing the β-intensity di…

Determination of β-decay ground state feeding of nuclei of importance for reactor applications

In β-decay studies the determination of the decay probability to the ground state (g.s.) of the daughter nucleus often suffers from large systematic errors. The difficulty of the measurement is related to the absence of associated delayed γ-ray emission. In this work we revisit the 4πγ−β method proposed by Greenwood and collaborators in the 1990s, which has the potential to overcome some of the experimental difficulties. Our interest is driven by the need to determine accurately the β-intensity distributions of fission products that contribute significantly to the reactor decay heat and to the antineutrinos emitted by reactors. A number of such decays have large g.s. branches. The method is…

Precision mass measurements of $^{67}$Fe and $^{69,70}$Co : Nuclear structure toward N=40 and impact on r -process reaction rates

International audience; Accurate mass measurements of neutron-rich iron and cobalt isotopes Fe67 and Co69,70 have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the Co69,70 ground states and the 1/2− isomer in Co69 have been extracted for the first time. The measurements remove ambiguities in the previous mass values and yield a smoother trend on the mass surface, extending it beyond N=40. The moderate N=40 subshell gap has been found to weaken below Ni68, a region known for shape coexistence and increased collectivity. The excitation energy for the 1/2− intruder state in Co69 has been determined for the first tim…

New insights into triaxiality and shape coexistence from odd-mass $^{109}$Rh

International audience; Rapid shape evolutions near A=100 are now the focus of much attention in nuclear science. Much of the recent work has been centered on isotopes with Z≤40, where the shapes are observed to transition between near-spherical to highly deformed with only a single pair of neutrons added. At higher Z, the shape transitions become more gradual as triaxiality sets in, yet the coexistence of varying shapes continues to play an important role in the low-energy nuclear structure, particularly in the odd-Z isotopes. This work aims to characterize competing shapes in the triaxial region between Zr and Sn isotopes using ultrafast timing techniques to measure lifetimes of excited s…

Low-spin excitations in 89Br populated in β−decay of 89Se

Low-spin, excited states of the 89Br nucleus, populated in β− decay of 89Se have been studied for the first time. The 89Se nuclei were produced in proton-induced fission of natural thorium using the IGISOL facility and separated using a dipole magnet and the coupled JYFLTRAP Penning trap. Gamma radiation following the β− decay of 89Se was measured with an array of high-resolution germanium detectors. Levels scheme of 89Br was extended by 12 new levels and 31 new γ transitions. Spin-parity (3/2+) has been proposed for the ground state of the 89Se mother nucleus, replacing the (5/2+) assignment reported in data bases. The observed Gamow-Teller β− transition to the 1754.5-keV level indicates a…

Results of DTAS Campaign at IGISOL : Overview

The β decays of more than twenty fission fragments were measured in the first experiments with radioactive-ion beams employing the Decay Total Absorption γ-ray Spectrometer. In this work, we summarize the main results obtained so far from this experimental campaign carried out at the Ion Guide Isotope Separator On-Line facility. The advances introduced for these studies represent the state-of-the-art of our analysis methodology for segmented spectrometers. peerReviewed

Precision mass measurements of Fe 67 and Co 69 , 70 : Nuclear structure toward N = 40 and impact on r -process reaction rates

Study of the $\beta$-decay of $^{100}$Tc with Total Absorption $\gamma$-Ray Spectroscopy

The \b{eta}-decay of 100 Tc has been studied using the Total Absorption {\gamma}-Ray Spectroscopy technique at IGISOL. In this work the new DTAS spectrometer in coincidence with a cylindrical plastic \b{eta} detector has been employed. The \b{eta}-intensity to the ground state obtained from the analysis is in good agreement with previous high-resolution measurements. However, differences in the feeding to the first excited state as well as weak feeding to a new level at high excitation energy have been deduced from this experiment. Theoretical calculations performed in the quasiparticle random- phase approximation (QRPA) framework are also reported. Comparison of these calculations with our…

β- and γ-spectroscopy study of 119Pd and 119Ag

Neutron-rich 119Pd nuclei were produced in fission of natural uranium, induced by 25-MeV protons. Fission fragments swiftly extracted with the Ion Guide Isotope Separation On-Line method were mass separated using a dipole magnet and a Penning trap, providing mono-isotopic samples of 119Pd. Their β− decay was measured with γγ- and βγ-spectroscopy methods using low-energy germanium detectors and a thin plastic scintillator. Two distinct nuclear-level structures were observed in 119Ag, based on the 1/2− and 7/2+ isomers reported previously. The β−-decay work was complemented by a prompt-γ study of levels in 119Ag populated in spontaneous fission of 252Cf, performed using the Gammasphere array …

Penning-trap-assisted study of excitations in $^{88}$Br populated in β decay of $^{88}$Se

Excited levels of 88Br populated in the β decay of 88Se have been studied by means of βγ and γ γ spectroscopy methods. Neutron-rich parent 88Se nuclei were produced with proton-induced fission of 238U using the Ion Guide Isotope Separator On-Line (IGISOL) method and separated from contaminants using a dipole magnet and the coupled JYFLTRAP Penning trap at the Accelerator Laboratory of the University of Jyvaskyl ¨ a. The level scheme ¨ of 88Br has been constructed and log f t values of levels were determined. The ground-state spin of 88Br is now firmly determined to be 1−. Low-energy levels in 88Br were interpreted as members of the πp3/2(νd5/2) 3, πp−1 3/2(νd5/2) 3, πf −1 5/2 (νd5/2) 3, and…

Discovery of an Exceptionally Strong β -Decay Transition of $^{20}$F and Implications for the Fate of Intermediate-Mass Stars

A significant fraction of stars between 7-11 solar masses are thought to become supernovae, but the explosion mechanism is unclear. The answer depends critically on the rate of electron capture on $^{20}$Ne in the degenerate oxygen-neon stellar core. However, due to the unknown strength of the transition between the ground states of $^{20}$Ne and $^{20}$F, it has not previously been possible to fully constrain the rate. By measuring the transition, we have established that its strength is exceptionally large and enhances the capture rate by several orders of magnitude. This has a decisive impact on the evolution of the core, increasing the likelihood that the star is (partially) disrupted b…

Total absorption γ-ray spectroscopy of the β-delayed neutron emitters 137I and 95Rb

The decays of the β-delayed neutron emitters 137I and 95Rb have been studied with the total absorption γ-ray spectroscopy technique. The purity of the beams provided by the JYFLTRAP Penning trap at the ion guide isotope separator on-line facility in Jyväskylä allowed us to carry out a campaign of isotopically pure measurements with the decay total absorption γ-ray spectrometer, a segmented detector composed of 18 NaI(Tl) modules. The contamination coming from the interaction of neutrons with the spectrometer has been carefully studied, and we have tested the use of time differences between prompt γ rays and delayed neutron interactions to eliminate this source of contamination. Due to the s…

Design of a High Intensity Neutron Source for Neutron-Induced Fission Yield Studies

The upgraded IGISOL facility with JYFLTRAP, at the accelerator laboratory of the University of Jyv\"askyl\"a, has been supplied with a new cyclotron which will provide protons of the order of 100 {\mu}A with up to 30 MeV energy, or deuterons with half the energy and intensity. This makes it an ideal place for measurements of neutron-induced fission products from various actinides, in view of proposed future nuclear fuel cycles. The groups at Uppsala University and University of Jyv\"askyl\"a are working on the design of a neutron converter that will be used as neutron source in fission yield studies. The design is based on simulations with Monte Carlo codes and a benchmark measurement that …

Independent and cumulative yields of very neutron-rich nuclei in 20 MeVp- and 18–41 MeVd-induced fission ofU238

Independent and cumulative yields in nearly symmetric fission have been measured for the first time by bombarding $^{238}\mathrm{U}$ with 20-MeV protons and with 18-, 25-, and 41-MeV deuterons. Isobaric charge dispersion has been determined in the A=110, A=112, and A=114 mass chains and, from asymmetric fission, in the A=80 mass chain. The average isobaric charges in the p+U reaction were 44.01\ifmmode\pm\else\textpm\fi{}0.20 (A=110), 45.15\ifmmode\pm\else\textpm\fi{}0.15 (A=112), and 45.74\ifmmode\pm\else\textpm\fi{}0.15 (A=114). The widths (standard deviations) of the charge dispersion curves were 0.82\ifmmode\pm\else\textpm\fi{}0.08 (A=112) and 0.54\ifmmode\pm\else\textpm\fi{}0.08 (A=114…

Excited states in 87Br populated in β decay of 87Se

Excited levels in 87Br, populated in β decay of 87Se, have been studied by means of γ-ray spectroscopy using an array of broad energy Ge detectors. 87Se nuclei were produced by irradiating a natural Th target with 25-MeV protons. Fission products were extracted from the target chamber using the IGISOL technique, then separated on a dipole magnet and Penning trap (JYFLTRAP) setup. The scheme of excited levels of 87Br has been significantly extended. 114 new transitions and 51 new levels were established. β feedings and log(ft) values of levels were determined. The upper limit for β feeding to the ground state of 87Br was determined to be 23(5)%. Ground state spin and parity 5/2− was confirme…

Measurement of the 2+→0+ ground-state transition in the β decay of F 20

| openaire: EC/H2020/654002/EU//ENSAR2 We report the first detection of the second-forbidden, nonunique, 2(+) -> 0(+), ground-state transition in the beta decay of F-20. A low-energy, mass-separated F-20(+) beam produced at the IGISOL facility in Jyvaskyla, Finland, was implanted in a thin carbon foil and the beta spectrum measured using a magnetic transporter and a plastic-scintillator detector. The beta-decay branching ratio inferred from the measurement is b(beta) = [0.41 +/- 0.08(stat) +/- 0.07(sys)] x 10(-5) corresponding to log ft = 10.89(11), making this one of the strongest second-forbidden, nonunique beta transitions ever measured. The experimental result is supported by shell-mode…

Measurement of the 2 + → 0 + ground-state transition in the β decay of F 20

First trap-assisted decay spectroscopy of the $$^{81}$$Ge ground state

AbstractThe $$\beta $$ β -delayed $$\gamma $$ γ spectroscopy of $$^{81}$$ 81 As has been performed using a purified beam of $$^{81}$$ 81 Ge $$(9/2^+)$$ ( 9 / 2 + ) ground state at the Ion Guide Isotope Separator On-Line facility (IGISOL). The $$^{81}$$ 81 Ge$$^+$$ + ions were produced using proton-induced fission of $$^{232}$$ 232 Th and selected with the double Penning trap JYFLTRAP for the post-trap decay spectroscopy measurements. The low-spin $$(1/2^+)$$ ( 1 / 2 + ) isomeric-state ions $$^{81m}\hbox {Ge}^+$$ 81 m Ge + were not observed in the fission products. The intrinsic half-life of the $$^{81}$$ 81 Ge ground state has been determined as $$T_{1/2}=6.4(2)~\hbox {s}$$ T 1 / 2 = 6.4 ( …

Discovery of an Exceptionally Strong β -Decay Transition of F 20 and Implications for the Fate of Intermediate-Mass Stars

First β-decay scheme of 107Nb : New insight into the low-energy levels of 107Mo

Monoisotopic samples of 107Nb nuclei, produced in the proton-induced fission of 238U and separated using the IGISOL mass separator coupled to a Penning trap, were used to perform β- and γ-coincidence spectroscopy of 107Mo. Gamma transitions and excited levels in 107Mo were observed in β decay for the first time. Spin and parity 1/2+ for the ground state of 107Mo is proposed, to replace the previous 5/2+ assignment. The experimental β-decay half-life of 107Nb was estimated to be 0.27±0.02 s. peerReviewed

Target thickness dependence of the Be(p,xn) neutron energy spectrum

We report on the current status of the analysis of an experiment performed at The Svedberg Laboratory, with the aim of investigating the produced neutron field by Be(p,xn) converters of three different thicknesses with a 30 MeV proton beam. The neutron energy spectra were measured with the Time of Flight technique using a BC-501 liquid scintillator with good n-γ Pulse Shape Discrimination properties, while the detected events were recorded simultaneously by two Data AcQuisition systems. In this paper, we present the experimental setup, the analysis technique and some preliminary results.  AlFONS

β - and γ -spectroscopy study of Pd 119 and Ag 119

Precision mass measurements of 67Fe and 69,70Co: Nuclear structure toward N = 40 and impact on r-process reaction rates

Accurate mass measurements of neutron-rich iron and cobalt isotopes 67Fe and 69,70Co have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the 69,70Co ground states and the 1/2− isomer in 69Co have been extracted for the first time. The measurements remove ambiguities in the previous mass values and yield a smoother trend on the mass surface, extending it beyond N=40. The moderate N=40 subshell gap has been found to weaken below 68Ni, a region known for shape coexistence and increased collectivity. The excitation energy for the 1/2− intruder state in 69Co has been determined for the first time and is compared to lar…

Benchmark of a multi-physics Monte Carlo simulation of an ionguide for neutron-induced fission products

AbstractTo enhance the production of medium-heavy, neutron-rich nuclei, and to facilitate measurements of independent yields of neutron-induced fission, a proton-to-neutron converter and a dedicated ion guide for neutron-induced fission have been developed for the IGISOL facility at the University of Jyväskylä. The ion guide holds the fissionable targets, and the fission products emerging from the targets are collected in helium gas and transported to the downstream experimental stations. A computer model, based on a combination of MCNPX for modeling the neutron production, the fission code GEF, and GEANT4 for the transport of the fission products, was developed. The model will be used to i…

Mass measurements towards doubly magic Ni-78 : Hydrodynamics versus nuclear mass contribution in core-collapse supernovae

International audience; We report the first high-precision mass measurements of the neutron-rich nuclei 74,75Ni and the clearly identified ground state of 76Cu, along with a more precise mass-excess value of 78Cu, performed with the double Penning trap JYFLTRAP at the Ion Guide Isotope Separator On-Line (IGISOL) facility. These new results lead to a quantitative estimation of the quenching for the N=50 neutron shell gap. The impact of this shell quenching on core-collapse supernova dynamics is specifically tested using a dedicated statistical equilibrium approach that allows a variation of the mass model independent of the other microphysical inputs. We conclude that the impact of nuclear m…

Nuclear charge radii of molybdenum fission fragments

Abstract Radioisotopes of molybdenum have been studied using laser spectroscopy techniques at the IGISOL facility, University of Jyvaskyla. Differences in nuclear charge radii have been determined for neutron deficient isotopes 90,91Mo and neutron rich isotopes 102–106,108Mo (and all stable isotopes). A smooth transition in the mean square charge radii is observed as the neutron number increases with no sudden shape change observed in the region around N = 60 . As N increases, the nuclear deformation appears to go beyond a maximum and a fall off at N = 66 is observed. The magnetic moments of the odd isotopes 91,103,105Mo are also determined.