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The $\ensuremath{\beta}$ decay of $^{125m,125}\mathrm{Ag}$ into levels in $^{125}\mathrm{Cd}$ was investigated at the Holifield Radioactive Ion Beam Facility (HRIBF). Uranium-238 targets were bombarded with 50-MeV protons with an intensity of $15\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}$A, and the induced fission products were mass separated and deposited on a moving tape in the center of the VANDLE array consisting of $\ensuremath{\gamma}$ detectors and plastic scintillators. A partial decay scheme has been assigned for both $\ensuremath{\beta}$ decay of the (9/${2}^{+}$) ground state of $^{125}\mathrm{Ag}$ and its low-lying (1/${2}^{\ensuremath{-}}$) isomer, with the energy of the low-lying (11/${2}^{\ensuremath{-}}$) isomeric state in $^{125}\mathrm{Cd}$ assigned as 188.5 keV. In addition, $\ensuremath{\beta}$-delayed neutron emission probabilities were also determined to be 1.2(2)% for the (9/${2}^{+}$) $^{125}\mathrm{Ag}$ ground state and 4.6(10)% for the (1/${2}^{\ensuremath{-}}$) isomer, which are substantially lower than the previously reported value.

The structure of Po-208 populated through the EC/beta(+) decay of At-208 is investigated using gamma-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed. rays which have been (re)assigned a position. The level scheme is compared to shell model calculations. Through this analysis approximately half of the beta-decay strength of At-208 is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations. This mass region provides an excellent testing ground for the competition between allowed and first-forbidden beta-decay calculations, important for the detailed understanding of the nucleosynthesis of heavy elements.

Background: Neutron-rich nuclei in the mass region around A=160 have been and will continue to be of interest for the study of nuclear structure because of the rapid onset of deformation between 88 and 90 neutrons. The observation of detailed changes in nuclear structures within this mass region has provided and will continue to provide insight into the nuclear force. Purpose: Investigations of γ rays emitted following Eu163 β-decay to Gd163 have been performed for evaluation of the nuclear structure of Gd163. Method: Data were collected at the LeRIBSS station of the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory with an array of four Clover HPGe detectors for γ-rays and two plastic scintillators for β detection. The γ rays were identified as belonging to Gd163 via mass selection and γ-γ-β, x-ray-γ, or γ-γ coincidences. Results: In total 107 new γ-ray transitions were observed in Gd163 from 53 newly identified levels. Conclusions: The structure of Gd163 has been identified for the first time. This structure has been evaluated in comparison to projected shell model, and potential energy surface calculations with good agreement.

Aquesta es una copia de la versio author’s final draft d'un article publicat a la revista [Physical Review C ]. URL d'aquest document a UPCommons E-prints: http://hdl.handle.net/2117/174873

The structure of $^{34}$Si was studied through $\gamma$ spectroscopy separately in the $\beta^-$ decays of $^{34}$Mg and $^{34}$Al at the ISOLDE facility of CERN. Different configurations in $^{34}$Si were populated independently from the two recently identified $\beta$-decaying states in $^{34}$Al having spin-parity assignments $J^\pi = 4^-$ dominated by the normal configuration $\pi (d_{5/2})^{-1} \otimes \nu (f_{7/2})$ and $J^\pi = 1^+$ by the intruder configuration $\pi (d_{5/2})^{-1} \otimes \nu (d_{3/2})^{-1}(f_{7/2})^{2}$. The paper reports on spectroscopic properties of $^{34}$Si such as an extended level scheme, spin and parity assignments based on log($ft$) values and $\gamma$-ray branching ratios, absolute $\beta$ feeding intensities and neutron emission probabilities. A total of 11 newly identified levels and 26 transitions were added to the previously known level scheme of $^{34}$Si. Large scale shell-model calculations using the {\sc sdpf-u-mix} interaction, able to treat higher order intruder configurations, are compared with the new results and conclusions are drawn concerning the predictive power of {\sc sdpf-u-mix}, the $N=20$ shell gap, the level of mixing between normal and intruder configurations for the 0$_1^+$, 0$_2^+$ and 2$_1^+$ states and the absence of triaxial deformation in $^{34}$Si.

Decay of $^{85}\mathrm{Ga}$ was studied by means of $\ensuremath{\beta}$-neutron-$\ensuremath{\gamma}$ spectroscopy. A pure beam of $^{85}\mathrm{Ga}$ was produced at the Holifield Radioactive Ion Beam Facility using a resonance ionization laser ion source and a high-resolution electromagnetic separator. The $\ensuremath{\beta}$-delayed neutron emission probability was measured for the first time, yielding 70(5)%. An upper limit of 0.1% for $\ensuremath{\beta}$-delayed two-neutron emission was also experimentally established for the first time. A detailed decay scheme including absolute $\ensuremath{\gamma}$-ray intensities was obtained. Results are compared with theoretical $\ensuremath{\beta}$-delayed emission models.

Background: An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the $\ensuremath{\beta}$ decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called pandemonium effect and their $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ decay heat should be restudied using the total absorption technique. The fission products $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring $\ensuremath{\beta}$-delayed neutron emission probabilities is challenging and any new technique for measuring the $\ensuremath{\beta}$-neutron spectrum and the $\ensuremath{\beta}$-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques.Purpose: To obtain the complete $\ensuremath{\beta}$-decay pattern of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ and determine their consequences for reactor decay heat and ${\overline{\ensuremath{\nu}}}_{e}$ emission. Complete $\ensuremath{\beta}$-decay feeding includes ground state to ground state $\ensuremath{\beta}$ feeding with no associated $\ensuremath{\gamma}$ rays, ground state to excited states $\ensuremath{\beta}$ transitions followed by $\ensuremath{\gamma}$ transitions to the daughter nucleus ground state, and $\ensuremath{\beta}$-delayed neutron emission from the daughter nucleus in the case of $^{137}\mathrm{I}$.Method: We measured the complete $\ensuremath{\beta}$-decay intensities of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a technique for measuring the $\ensuremath{\beta}$-delayed neutron energy spectrum, which also provides a measurement of the $\ensuremath{\beta}$-neutron branching ratio, ${P}_{n}$.Results: We validate the current Evaluated Nuclear Structure Data File (ENSDF) evaluation of $^{137}\mathrm{Xe}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay. We find that major changes to the current ENSDF assessment of $^{137}\mathrm{I}\ensuremath{\beta}$-decay intensity are required. The average $\ensuremath{\gamma}$ energy per $\ensuremath{\beta}$ decay for $^{137}\mathrm{I}\ensuremath{\beta}$ decay ($\ensuremath{\gamma}$ decay heat) increases by 19%, from 1050--1250 keV, which increases the average $\ensuremath{\gamma}$ energy per $^{235}\mathrm{U}$ fission by $0.11%$. We measure a $\ensuremath{\beta}$-delayed neutron branching fraction for $^{137}\mathrm{I}\ensuremath{\beta}$ decay of $7.9\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{fit})\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{sys})%$ and we provide a $\ensuremath{\beta}$-neutron energy spectrum.Conclusions: The Modular Total Absorption Spectrometer measurements of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ demonstrate the importance of revisiting and remeasuring complex $\ensuremath{\beta}$-decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure $\ensuremath{\beta}$-delayed neutron energy spectra.

The $\ensuremath{\beta}$-decay properties of the very neutron-rich nucleus $^{85}\mathrm{Ge}$, produced in the proton-induced fission of $^{238}\mathrm{U}$, were studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The level scheme of $\phantom{\rule{0.16em}{0ex}}_{33}^{85}\mathrm{As}_{52}$ populated in $^{85}\mathrm{Ge}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}\ensuremath{\gamma}$ decay was reconstructed and compared to shell-model calculations. The investigation of the systematics of low-energy levels in $N=52$ isotones together with shell-model analysis allowed us to provide an estimate of the low-energy structure of the more exotic $N=52$ isotone $^{81}\mathrm{Cu}$.

$\ensuremath{\beta}$ decay of neutron-rich nuclide $^{94}\mathrm{Kr}$ was reinvestigated by means of a high resolution on-line mass separator and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ spectroscopy. In total 22 $\ensuremath{\gamma}$-ray transitions were assigned to the decay of $^{94}\mathrm{Kr}$, and a new isomeric state was identified. The new information allows us to build detailed levels systematics in a chain of odd-odd rubidium isotopes and draw conclusions on nuclear structure for some of the observed states. The discussed level structure affects the evolution of $\ensuremath{\beta}$-decay half-lives for neutron-rich selenium, krypton, and strontium isotopes.

Decays of neutron-rich nuclei $^{82,83}\mathrm{Zn}$ and $^{82,83}\mathrm{Ga}$ produced in proton-induced fission of $^{238}\mathrm{U}$ were studied at the Holifield Radioactive Ion Beam Facility using on-line mass separation and $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy techniques. New $\ensuremath{\gamma}$-ray transitions were identified and level schemes, which include states at high excitation energies in the range between 3--7 MeV were constructed. These high-energy levels were identified to be populated through allowed Gamow-Teller $\ensuremath{\beta}$ transitions, and their structure was interpreted with new shell-model calculations. A $\ensuremath{\beta}$-delayed neutron branching ratio of $69\ifmmode\pm\else\textpm\fi{}7%$ was deduced for $^{82}\mathrm{Zn}$ and revised $\ensuremath{\beta}$-decay half-life values of $^{82}\mathrm{Zn}$ [155(17)(20) ms] and $^{83}\mathrm{Zn}$ [122(28) ms] were determined.

The $\ensuremath{\beta}$-decay properties of neutron-rich $^{87}\mathrm{As}$ produced in the proton-induced fission of $^{238}\mathrm{U}$ were studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The low-energy excited states in $N=53 ^{87}\mathrm{Se}$ and $N=52 ^{86}\mathrm{Se}$ were identified through $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\beta}$-delayed neutron-$\ensuremath{\gamma}$ decay of $^{87}\mathrm{As}$, respectively. The experimental systematics of low-energy levels of $N=53$ isotones, $Z=34 ^{87}\mathrm{Se},$ and $Z=32 ^{85}\mathrm{Ge}$, and along with an analysis of shell-model calculations, allow us to discuss the main features of excited states expected for the next $N=53$ isotone, $^{83}\mathrm{Zn}$.

The $\ensuremath{\beta}$-decay properties of very neutron rich nuclei $^{86}\mathrm{Ge}$ and $^{86}\mathrm{As}$ were measured at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Spectroscopic information on new excited states in $^{86}\mathrm{As}$ and in $^{86}\mathrm{Se}$ was obtained and is interpreted within an advanced shell model approach. These calculations, previously explaining well the structure of $^{84}\mathrm{Ge}$ and $^{85}\mathrm{Ge}$, are not able to reproduce all the experimentally determined features of the measured level schemes of $^{86}\mathrm{As}$ and $^{86}\mathrm{Se}$. The Gamow-Teller decay of $^{86}\mathrm{Ge}$ and $^{86}\mathrm{As}$ is also investigated in a shell-model framework. The fission yield for $^{86}\mathrm{Ge}$ is discussed.

Near shell closures, the presence of unexpected states at low energies provides a critical test of our understanding of the atomic nucleus. New measurements for the $N=42$ isotones $_{27}^{69}\mathrm{Co}$ and $_{29}^{71}\mathrm{Cu}$, along with recent data and calculations in the Ni isotopes, establish a full set of complementary, deformed, intruder states astride the closed-shell $_{28}\mathrm{Ni}$ isotopes. Nuclei with a one-proton hole or one-proton particle adjacent to $Z=28$ were populated in $\ensuremath{\beta}$-decay experiments and in multinucleon transfer reactions. A $\ensuremath{\beta}$-decaying isomer, with a 750(250)-ms half-life, has been identified in $_{27}^{69}\mathrm{Co}_{42}$. It likely has low spin and accompanies the previously established $7/{2}^{\ensuremath{-}}$ state. Complementary data for the levels of isotonic $_{29}^{71}\mathrm{Cu}_{42}$ support the presence of a deformed, $\mathrm{\ensuremath{\Delta}}J=1$ band built on the proton intruder $7/{2}^{\ensuremath{-}}$ level at 981 keV. These data, together with recent studies of lower-mass Co and Cu isotopes and extensive work near $^{68}\mathrm{Ni}$, support the view that intruder states based on particle-hole excitations accompany all closed proton shells with $Z\ensuremath{\ge}28$.

The excited states of odd-odd $^{82}\mathrm{As}$ are studied in the $\ensuremath{\beta}$ decay of $^{82}\mathrm{Ge}$. An isotopically pure beam of $^{83}\mathrm{Ga}$ was produced at the Holifield Radioactive Ion Beam Facility using a resonance ionization laser ion source and high-resolution electromagnetic separation. The atoms of $^{82}\mathrm{Ge}$ are created after $\ensuremath{\beta}$-delayed neutron emission in the decay of $^{83}\mathrm{Ga}$. The number of $^{82}\mathrm{Ge}$ atoms is found by normalization to the 1348-keV $\ensuremath{\gamma}$ ray. Detailed analysis of the decay scheme is compared with shell-model calculations with several commonly used $fpg$ shell interactions.

Results of decay spectroscopy on nuclei in the vicinity of the doubly magic $^{48}\mathrm{Ni}$ are presented. The measurements were performed with a time projection chamber with optical readout which records tracks of ions and protons in the gaseous volume. Six decays of $^{48}\mathrm{Ni}$, including four events of two-proton ground-state radioactivity, were recorded. An advanced reconstruction procedure yielded the $2p$ decay energy for $^{48}\mathrm{Ni}$ of ${Q}_{2p}=1.29(4)$ MeV. In addition, the energy spectra of $\ensuremath{\beta}$-delayed protons emitted in the decays of $^{44}\mathrm{Cr}$ and $^{46}\mathrm{Fe}$, as well as half-lives and branching ratios, were determined. The results were found to be consistent with the previous measurements made with Si detectors. A new proton line in the decay of $^{44}\mathrm{Cr}$ corresponding to the decay energy of 760 keV is reported. The first evidence for the $\ensuremath{\beta}2p$ decay of $^{46}\mathrm{Fe}$, based on one clear event, is shown.

The $\ensuremath{\beta}$-decay properties of neutron-rich ${}^{85}$Ga produced in proton-induced fission of ${}^{238}$U were studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Data consisted of $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences were collected at the Low-energy Radioactive Ion Beam Spectroscopy Station after high-resolution on-line mass separation. For the first time, the excited states in the $N=53$ isotone ${}^{85}$Ge were established from $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ decay of ${}^{85}$Ga. The level scheme of the $N=52$ isotone ${}^{84}$Ge was improved and includes now 0${}^{+}$, 2${}^{+}$, (2)${}^{+}$, 4${}^{+}$, and (0)${}^{+}$ states populated in the $\ensuremath{\beta}$-delayed-neutron-$\ensuremath{\gamma}$ decay of ${}^{85}$Ga. Advanced shell-model calculations were used to analyze experimental data on ${}^{85}$Ge and ${}^{84}$Ge.

Short lived neutron rich nuclei including 93 Br, 93 Kr and 94 Kr were produced in proton induced fission of 238 U at the HRIBF in Oak Ridge. Their beta decay was studied by means of a high resolution on line mass separator and beta gamma spectroscopy methods. The half life of 93Br T1/2 = 152(8) ms and delayed branching ratio of Pn = 53-8+11 may be compared to the previously reported values of T1/2 = 102(10) ms and Pn = 68(7)%. At the same time the half life of 94Kr T1/2 = 227(14) ms and B delayed branching ratio of Pn = 1.9+0.6 0.2 % of 93Kr are in very good agreement with literature values. The decay properties of 93Br include four new gamma transitions following beta delayed neutron emission.

The $\ensuremath{\beta}$-decay properties of several neutron-rich germanium and arsenic isotopes were measured at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The measurements of almost-pure radioactive sources were enabled by the combination of ion-source chemistry and two-stage mass separation. The half-life of ${}^{86}$Ge ($226\ifmmode\pm\else\textpm\fi{}21$ ms) was determined for the first time, while those of ${}^{84,85}$Ge and ${}^{84\ensuremath{-}87}$As were remeasured. The results are compared to theoretical predictions of the gross theory of $\ensuremath{\beta}$ decay, the finite-range droplet model, and the new calculations using the energy density functional DF3a with continuum quasiparticle random-phase approximation (CQRPA). We confirm the robustness and good predictive power of the latter model for nuclei near closed shells. These DF3a+CQRPA calculations were used recently to analyze $r$-process isobaric abundances.

Background: The nuclei in the region around ${}^{68}$Ni display an apparent rapid development of collectivity as protons are removed from the ${f}_{7/2}$ single-particle state along the $N=40$ isotonic chain. Proton and neutron excitations across the $Z=28$ and $N=40$ gaps are observed in odd-A ${}_{27}$Co and ${}_{26}$Fe isotopes. Little spectroscopic information beyond the excited 2${}^{+}$ and 4${}^{+}$ is available in the even-even ${}_{26}^{66,68}\mathrm{Fe}$ nuclei to compare with shell model calculations.Purpose: Our goal is to determine the low-energy level schemes of ${}^{66,68}$Fe and compare the observed excitations with shell model calculations to identify states wherein a contribution from excitations across $Z=28$ and $N=40$ are present.Method: The low-energy states of ${}^{66,68}$Fe were populated through the beta decay of ${}^{66,68}$Mn produced at the National Superconducting Cyclotron Laboratory. Beta-delayed gamma-ray transitions were detected and correlated to the respective parent isotope to construct a low-energy level scheme.Results: The low-energy level schemes of ${}^{66,68}$Fe were constructed from observed gamma-ray coincidences and absolute gamma-ray intensities. Tentative spin and parity assignments were assigned based on comparisons with shell model calculations and systematics. The two lowest 0${}^{+}$ and 2${}^{+}$ states were characterized in terms of the number of protons and neutrons excited across the respective shell gaps.Conclusion: The removal of two protons from ${}^{68}$Ni to ${}^{66}$Fe results in an inversion of the normal configuration and the one characterized by significant excitation across the $Z=28$ and $N=40$ gaps. Approximately, one proton and two neutrons are excited across their respective single-particle gaps in the ground state of ${}^{66}$Fe.

We have developed a method of implanted ion counting in order to determine the absolute decay probability of the 776.5 keV $\ensuremath{\gamma}$-ray transition in the decay sequence of ${}^{82}$Sr${\ensuremath{\rightarrow}}^{82}$Rb${\ensuremath{\rightarrow}}^{82}$Kr. A 215 MeV beam of ${}^{82}$Sr was produced at the Holifield Radioactive Ion Beam Facility and passed through an ionization chamber that counted and identified the ions before they were implanted into thin aluminum foils. Subsequent offline measurements using a Ge detector deduced the probability per decay of ${}^{82}$Rb for the 776.5 keV $\ensuremath{\gamma}$ ray in ${}^{82}$Kr to be 0.1493(37), in agreement with the accepted average value of 0.1508(16). This new technique measures directly the number of decaying nuclei in a given sample and significantly reduces the dependence on knowledge of the complete decay level scheme.

The low-energy level structures for the neutron-rich Co isotopes at N = 39 and N = 41 are constructed following the beta decay of the respective even-even Fe isotopes. Spin and parity assignments of the lowest energy populated state in {sup 66}Co and {sup 68}Co are consistent with a 1{sup +} spin and parity assignment and attributed to the coupling of the deformed proton configurations identified in {sup 67}Co and deformed neutron configurations inferred from neighboring Fe isotones. Comparisons along the N = 39 and N = 41 isotonic chains reveal a similarity in the structures of the Co and Mn isotopes.

The low-energy level structures of 64 25Mn39 and 66 25Mn41 were investigated through both the decay of Mn metastable states and the population of levels following the decay of 64Cr and 66Cr. The deduced level schemes and tentatively assigned spins and parities suggest the coexistence of spherical and deformed configurations above and below N = 40 for the odd-odd Mn isotopes. The low-energy deformed configurations are attributed to the coupling between a proton in a K = 1/2 level with neutrons in either the K = 1/2 or K = 3/2+ levels originating from the p3/2, p1/2, and g9/2 single-particle states, respectively.

The decay of the extremely neutron-deficient $^{48}\mathrm{Ni}$ was studied by means of an imaging time-projection chamber, which allowed the recording of tracks of charged particles. The decays of six atoms were observed. Four of them clearly correspond to two-proton radioactivity, providing the first direct evidence for this decay mode in $^{48}\mathrm{Ni}$. Two decays represent $\ensuremath{\beta}$-delayed proton emission. The half-life of $^{48}\mathrm{Ni}$ is determined to be ${T}_{1/2}=2.{1}_{\ensuremath{-}0.4}^{+1.4}$ ms.

Accurate measurements of the B-8 neutrino spectrum are important for the interpretation of solar neutrino data. Experimentally, the B-8 neutrino spectrum can be obtained from the measurement of the beta-delayed alpha spectrum. We report on an alpha-alpha coincidence measurement performed at the IGISOL facility in Jyvaskyla, Finland. Our measurement allows extensive cross-checks to be performed and gives a more intense neutrino spectrum at high energies compared to the present standard. The deviation reaches 4% at the end point of the spectrum. Below 11 MeV, the deviation is less than 1%.

The $\ensuremath{\beta}$ decay of the $N=51$ nucleus $^{81}\mathrm{Zn}$ was studied by means of $\ensuremath{\beta}$-$\ensuremath{\gamma}$ spectroscopy and from isotopically pure beams produced at the Holifield Radioactive Ion Beam Facility (HRIBF). We observe several competing $\ensuremath{\beta}$ transitions populating $^{81}\mathrm{Ga}$ that are interpreted as allowed Gamow-Teller decays to positive parity, core excited states, and first-forbidden decays to negative parity states. The measured $\ensuremath{\beta}$-decay pattern suggests an assignment of ${I}^{\ensuremath{\pi}}=5/{2}^{+}$ for the $^{81}\mathrm{Zn}$ ground state. The systematics of core excited states in $N=50$ isotones indicate a strengthening of the $N=40$ subshell gap near $^{78}\mathrm{Ni}$.