Conversion Electron Spectroscopy Research Articles

COeCO: A new [formula omitted]-delayed conversion-electron spectroscopy setup for low-energy ISOL beams at the ALTO facility in Orsay

A new β-decay station, COnversion electrons Chasing at Orsay (COeCO), has been developed at ALTO to perform conversion electron spectroscopy studies of neutron-rich nuclei produced by photo-fission of a uranium carbide target. It is based on the collection of a low-energy ISOL beam on a mylar tape, and the transportation of the electrons emitted by the produced radioactive source through a magnetic field induced by two copper coils, towards a cooled Si(Li) detector. In this article, a detailed description of the new decay station and its components is given. The magnetic field induced by the coils was measured and compared to simulations performed with the COMSOL® software. The efficiency of the detection setup was estimated using a 207Bi and a 152Eu source as an off-line commissioning. Finally, the results obtained with a 96Rb radioactive beam for the on-line commissioning are presented.

Experimental determinations of the energy of the 9.4 keV (M1+E2) nuclear transition in 83Kr and the Kr electron binding energies in different matrices by ICES method

An energy value of 9406.3(5) eV was determined for the 9.4 keV (M1+E2) nuclear transition in 83Kr by the internal conversion electron spectrometry method. This value agrees within 1σ with the most precise values determined previously for which their weighted mean amounts to 9405.9(2) eV. A value of 22745.3(2) eV was obtained directly from our experimental data for the energy difference between the 32.1 keV and 9.4 keV transitions in 83Kr. Electron binding energies (related to the Fermi level) on the K, L, and M1,2,3 subshells of Kr implanted into a polycrystalline Pt matrix were also derived. They were found to be lower by weighted mean values of 11.6(4) and 1.7(6) eV compared to those for free Kr atoms and for Kr in an evaporated 83Rb layer on Al backing, respectively, and higher by a weighted mean value of 2.9(2) eV than the binding energies in Kr in an evaporated layer on polycrystalline Pt backing.

Transformation of ICEBall to fIREBall for conversion electron spectroscopy

Conversion electron spectroscopy is a valuable tool in nuclear structure studies, but the complex spectra resulting from the high density of levels in the spectra of heavy nuclei make it difficult to extract the most beneficial information from singles measurements. This work reports on the transformation of ICEBall, which was a mini-orange spectrometer that was used in coincidence with various Germanium detector arrays, to the improved fInternal conveRsion Electron Ball (fIREBall) spectrometer for the improved measurements of E0 transitions and conversion electron coefficients. A combination of FreeCAD, COMSOL, and Geant4 simulations were implemented to optimize the geometries for the magnet filters used in the spectrometer to significantly improve the efficiency of electron collection in the energy range of interest 200 keV-1 MeV. The array of six mini-orange Si(Li) detectors are to be replaced with six new, thicker Si(Li) detectors. Experimental tests reported here show a peak improvement in absolute efficiency from 2.8% to 5.3% for the 482 keV K electron line in 207Bi for a single magnet filter and detector pair.

Nuclear Structure and Decay Data for A=231 Isobars

The present evaluation of currently known nine nuclides of A=231: (231Rn, 233Fr, 231Ra, 231Ac, 231Th, 231Pa, 231U, 231Np and 231Pu) represents an update and revision of previous A=231 evaluation by 2013Br04. All the known decay and reaction data are evaluated, with recommended properties given in the Adopted Levels, Gammas datasets, for gamma-ray energies, photon branching ratios, level half-lives, spins, parities, transition probabilities, and configuration assignments. For 231Rn, only an identification of the nuclide has been made without an experimental determination of its half-life. For 231Fr, no excited states are known. About 30 levels in 231Ra up to 1774 keV excitation are known only from β− decay of 231Fr with level half-lives determined for five excited states. For 231Ac, excited states up to 3122 keV are known from β− decay, (t,α), and fragmentary information for high-spin levels from two heavy-ion reaction studies, with lifetimes of excited states measured for five levels. Extensive structure and rotational band information for 231Th is available for a large number of levels up to 1714 keV from β− and α decays, (n,γ) thermal and resonance, single-particle transfer reactions (d,p), (d,t) and (3He,α), with limited data for high-spin levels from one secondary reference, and half-lives available for five excited states. Very detailed structure data are available for 231Pa up to 2139 keV from β−, ε and α decays, (α,t), (d,d′) and high-spin data from heavy-ion Coulomb excitation and (p,2nγ) reaction, with level half-lives known for 34 excited states, mostly deduced by evaluators from transition probabilities determined in Coulomb excitation. Only eight excited states are known in 231U up to 1268 keV from ε and α decays, with no data for half-lives of excited states. For 231Np and 231Pu, only the ground-state is known from α decays, with a tentative level at 324 keV in 231Pu. Measurements of nuclear rms charge radius and isotope shifts for 231Ra have been made by 2018Ly01, and that for rms charge radius for 231Fr by 2014Bu06.235U is a well known nuclide for nuclear structure and reactor applications. There have been a large number of studies of its alpha decay to levels in 231Th, including some recent ones such as 2018Ma03 and 2017Le03, yet evaluators' analysis of the decay scheme suggests that, while α-particle transitions are well established, but several issues still remain about intricacies of gamma-ray and conversion electron spectroscopy which need to be resolved through dedicated experiments for a complete understanding of the decay scheme characteristics, although, this sort of research project would seem a challenging spectroscopic problem. As commented in individual datasets for other decays, important spectral information is lacking for most of the decay schemes, with almost no information available for some of these, resulting in many incomplete decay schemes in A=231 mass chain.

Decay modes of the 9/2− isomeric state in Tl183

The internal transition decay and α decay of the T1/2=53.3ms, 9/2− isomeric state in the neutron-deficient isotope, Tl183, have been studied using the Resonance Ionization Laser Ion Source and the Windmill detection setup at the ISOLDE facility at CERN. Clean samples of Tl183m were produced by selective laser ionization and subsequent mass separation. An internal transition cascade of 356.5–272.2 keV γ rays has been identified. Multipolarities of these transitions have been determined by means of simultaneous γ-ray and conversion-electron spectroscopy. Improved data for the fine structure of the Tl183m→Au179 decay were deduced. This involves a 6058 keV α-decay transition, which populates a previously unknown state in Au179, that is tentatively assigned as a (9/2−). It is interpreted as a coupling of the 1h9/2 proton-intruder with the excited 0+ state in the Pt178 core.1 MoreReceived 11 January 2022Accepted 3 March 2022DOI:https://doi.org/10.1103/PhysRevC.105.034338Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayCollective modelsElectromagnetic transitionsEnergy levels & level densitiesIsomer decaysNuclear decayNuclear spin & parityNuclear structure & decaysRare & new isotopesShell modelProperties150 ≤ A ≤ 189Nuclear Physics

Clarifying the structure of low-lying states in Br72

The spins and parities of low-lying states in $^{72}\mathrm{Br}$ populated in the $\ensuremath{\beta}$ decay of $^{72}\mathrm{Kr}$ have been studied via conversion electron spectroscopy. The measurements were carried out at ISOLDE using a miniorange spectrometer with Si(Li) and HPGe detectors for electrons and $\ensuremath{\gamma}$ ray detection. Results of the conversion coefficients corresponding to transitions deexciting 12 levels in $^{72}\mathrm{Br}$ are reported. The multipolarities of the transitions are deduced and the spins and parities of the levels involved are discussed. From the multipolarities of the most intense transitions to the ground state, the spin and parity of the $^{72}\mathrm{Br}$ ground state have been definitely established as ${1}^{+}$. The spin of the 101.2-keV isomeric state is determined to be ${3}^{\ensuremath{-}}$. The level scheme is compared with mean-field and shell-model calculations and oblate deformation for the $^{72}\mathrm{Br}$ ground state is deduced. No $E0$ transitions have been found in $^{72}\mathrm{Br}$. $E0$ transitions in the neighboring isobaric nuclei, $^{72}\mathrm{Se}$ and $^{72}\mathrm{Ge}$, have also been studied.

The first experimental evidence for the (M1 + E2) mixed character of the 9.2 keV transition in 227Th

The 9.2 keV nuclear transition in 227Th was studied in the β−-decay of 227Ac by means of the internal conversion electron spectroscopy to clarify the spin-parity assignment of the ground state and the two lowest excited states of 227Th. The transition multipolarity was proved to be of mixed character M1 + E2 and the spectroscopic admixture parameter δ2(E2/M1)=0.695±0.248 (|δ(E2/M1)|=0.834±0.149) was determined. Nonzero value of δ(E2/M1) questioned the present theoretical interpretation of low-lying levels of 227Th. Calculations performed prefer the 1/2+, 3/2+, and 3/2+ sequence instead of the adopted 1/2+, 5/2+ and 3/2+ one for the 0.0, 9.2, and 24.3 keV levels, respectively.

Design and Development of a Mini-Orange Magnetic Spectrometer with Multichannel Facility for Conversion Electron Spectroscopy

Background: Conventional magnetic spectrometers used for conversion electron detection are very cumbersome, require strong magnetic fields and the spectra have to be scanned point by point and have very low transmission. A magnetic filter using permanent magnets and an Si(Li) detector would facilitate multichannel analysis with high transmission. The mini-orange is a new type of spectrometer for conversion electrons combining a solid state Si(Li) detector with a filter of permanent magnets around a central absorber of lead.Purpose: An indigenously developed magnetic spectrometer if optimized properly would be of great use in conversion electron spectroscopy for both online and offline experiments. Methods: A Mini-Orange magnetic spectrometer made of small permanent magnets has been designed and developed indigenously and optimized for its best performance condition. The transmission curves for different energy regions are plotted using the conversion electron spectra from the standard gamma transitions from 153Gd, 169Yb and 131Ba decays. The optimized spectrometer facilitates multichannel acquisition of conversion electron spectra for precision electron spectroscopy. The system also can be used in in-beam experiments with minor modifications of the vacuum chamber.Results: The optimized spectrometer was used for precision electron spectroscopy. Experimental transmission curves are then obtained by plotting Transmission (T) against the corresponding electron energy for low energy, medium energy and a broad energy range. Out of the several experiments done the optimum settings for f and g, that resulted in this curve, is identified at f = 7.5 cm and g = 4.5 cm. Conclusions: The optimized spectrometer facilitates multichannel acquisition of conversion electron spectra for precision electron spectroscopy. The system also can be used in in-beam experiments with minor modifications of the vacuum chamber.

γ -ray and conversion-electron spectroscopy of the high-spin isomer in Sm145

Background: High-spin isomers at {\approx}9-MeV excitation energies have been reported in several N = 83 isotones near Z = 64. Spin and parity assignments of J{^\pi} = 49/2+ remain tentative for a number of these states in the odd-A nuclei. Purpose: To study the decay of the (49/2 +) high-spin isomer in 145 Sm, make firm spin and parity assignments to the isomer and states populated in its decay, and investigate the structure of the nucleus. Methods: The 145Sm isomer was populated in the 124Sn(26Mg,5n) reaction. Gamma-ray and conversion-electron data were collected using the Solenogam array. Results: A revised lifetime of t1/2 = 3.52(16) mus was measured for the high-spin isomer. Several new states have been added to the level scheme, and a new state at 8815 keV is proposed as the isomer, based on decay-property systematics, transition strengths, and spin and parity assignments. Firm spin and parity assignments have been made to states up to and including the isomer and the new level scheme is interpreted using shell-model calculations performed with the KShell program. Conclusions: The interpretation of the 49/2+ isomer as a deformed excitation of the core neutrons remains unchanged, although there has been a significant revision of the level scheme below the isomer, and hence significant reinterpretations of the lower-lying states.

Gamma and conversion electron spectroscopy using GABRIELA

GABRIELA (Gamma Alpha Beta Recoil Investigations with the Electromagnetic Analyzer) is a detection system installed at the focal plane of the SHELS (Separator for Heavy Elements Spectroscopy) recoil separator for gamma and internal conversion electron spectroscopy of heavy and superheavy nuclei. GABRIELA has recently been upgraded. The characteristics of the new setup are presented using the Geant4 Monte Carlo simulation toolkit and validated against experimental results. The impact of summing on the gamma-ray and electron detection efficiencies is discussed.

In-beam γ -ray and electron spectroscopy of Md249,251

The odd-$Z$ $^{251}$Md nucleus was studied using combined $\gamma$-ray and conversion-electron in-beam spectroscopy. Besides the previously observed rotational band based on the $[521]1/2^-$ configuration, another rotational structure has been identified using $\gamma$-$\gamma$ coincidences. The use of electron spectroscopy allowed the rotational bands to be observed over a larger rotational frequency range. Using the transition intensities that depend on the gyromagnetic factor, a $[514]7/2^-$ single-particle configuration has been inferred for this band, i.e., the ground-state band. A physical background that dominates the electron spectrum with an intensity of $\simeq$ 60% was well reproduced by simulating a set of unresolved excited bands. Moreover, a detailed analysis of the intensity profile as a function of the angular momentum provided a method for deriving the orbital gyromagnetic factor, namely $g_K = 0.69^{+0.19}_{-0.16}$ for the ground-state band. The odd-$Z$ $^{249}$Md was studied using $\gamma$-ray in-beam spectroscopy. Evidence for octupole correlations resulting from the mixing of the $\Delta l = \Delta j = 3$ $[521]3/2^-$ and $[633]7/2^+$ Nilsson orbitals were found in both $^{249,251}$Md. A surprising similarity of the $^{251}$Md ground-state band transition energies with those of the excited band of $^{255}$Lr has been discussed in terms of identical bands. Skyrme-Hartree-Fock-Bogoliubov calculations were performed to investigate the origin of the similarities between these bands.

Evidence for shape coexistence in 52Cr through conversion-electron and pair-conversion spectroscopy

Electric monopole (E0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E0 transition strength (ρ2(E0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic 40Ca and 56Ni, E0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (56,58Fe). We have performed the first measurements of the E0 transitions in 52Cr via conversion-electron and pair-conversion spectroscopy using the Super-e spectrometer at the Australian National University Heavy Ion Accelerator Facility. We present the first spectra obtained for 52Cr, including the first observation of the E0 transition from the first-excited 0+ state in 52Cr, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E0 strength in the 1531keV 2+ → 2+ transition in 52Cr is ρ2(E0) × 103 = 470(190), and for the 1728-keV 23+ → 21+ transition, it is ρ2(E0) 103 = 1800(1200). The large E0 strengths observed are consistent with shape coexistence in this region. However, despite the relatively precise observation of the conversion-electron and electron-positron pair intensities, the E0 strengths have large uncertainties. More precise determinations of relevant spectroscopic quantities, such as the state lifetimes and transition mixing ratios for mixed M1 + E2 transitions, are needed to determine the E0 strength more precisely.

Solenogam: A new detector array for [formula omitted]-ray and conversion-electron spectroscopy of long-lived states in fusion-evaporation products

A new detector array, Solenogam, has been developed at the Australian National University Heavy Ion Accelerator Facility. Coupled initially to the SOLITAIRE 6.5 T, gas-filled, solenoidal separator, and later to an 8 T solenoid, the system enables the study of long-lived nuclear states through γ-ray and conversion-electron spectroscopy in a low-background environment. The detector system is described and results from the commissioning experiments are presented.

An experimental investigation of the 15.1 keV M1 + E2 nuclear transition in 227Th from the $\beta^{-}$ decay of 227Ac

Using the internal conversion electron spectroscopy, the energy of the 15.1 keV $ M1+E2$ nuclear transition in 227Th populated in the $ \beta^{-}$ decay of 227Ac was determined to be $ 15098.6 \pm 1.0$ eV. This value is more accurate than the present accepted one by a factor of 200. The present uncertainty in the transition multipolarity was removed and it was found to be $ M1+E2$ with the admixture $ \vert\delta (E2/M1)\vert = 0.035 \pm 0.006$.

Inner-shell ionization during α decay of superheavy isotopes from the tennessine Ts117293 and oganesson Og118294 chains

Presented here are calculations of probabilities of the $K$-, $L$-, and $M$-shell ionization during $\ensuremath{\alpha}$ decay of superheavy isotopes $_{117}^{293}\mathrm{Ts}$, $_{115}^{289}\mathrm{Mc}$, and $_{113}^{285}\mathrm{Nh}$ involved in the tennessine decay chain as well as $_{118}^{294}\mathrm{Og}$, $_{116}^{290}\mathrm{Lv}$, and $_{114}^{286}\mathrm{Fl}$ involved in the new oganesson decay chain. The ionization probabilities are of importance for handling data obtained by methods of the combined $\ensuremath{\alpha}$, $\ensuremath{\gamma}$, and conversion-electron spectroscopy used in the superheavy element synthesis analysis. Relativistic calculations are based on the quantum mechanical model. Electron wave functions are determined by the Dirac-Fock method. The $\ensuremath{\alpha}$-particle tunneling through the atomic Coulomb barrier is taken into account. Peculiarities of the $K$-, $L$-, and $M$-shell ionization are considered. Results demonstrate that the effect of tunneling through the Coulomb barrier for the $L$ and $M$ shells is of no significance as distinct from the $K$ shell, where the inclusion of the tunneling leads to a considerable decrease of the ionization probability. The probability of ionization from higher shells is larger than that from inner shells. However, the change from the $K$ shell to $L$ shell is much more significant than the change from the $L$ shell to $M$ shell. It has been found that only monopole and dipole terms of the radiative field $L=0,1$ make a contribution to the $K$- and ${L}_{1}$-shell ionization probabilities while contributions of all multipoles $L\ensuremath{\le}4$ may be important for the ${L}_{2}$, ${L}_{3}$, and particularly for ${M}_{1}\ensuremath{-}{M}_{5}$ subshells.

E0 transition strength in stable Ni isotopes

Excited states in $^{58,60,62}$Ni were populated via inelastic proton scattering at the Australian National University as well as via inelastic neutron scattering at the University of Kentucky Accelerator Laboratory. The Super-e electron spectrometer and the CAESAR Compton-suppressed HPGe array were used in complementary experiments to measure conversion coefficients and $\delta(E2/M1)$ mixing ratios, respectively, for a number of $2^+ \rightarrow 2^+$ transitions. The data obtained were combined with lifetimes and branching ratios to determine $E0$, $M1$, and $E2$ transition strengths between $2^+$ states. The $E0$ transition strengths between $0^+$ states were measured using internal conversion electron spectroscopy and compare well to previous results from internal pair formation spectroscopy. The $E0$ transition strengths between the lowest-lying $2^+$ states were found to be consistently large for the isotopes studied.

In-beam internal conversion electron spectroscopy with the SPICE detector

The SPectrometer for Internal Conversion Electrons (SPICE) has been commissioned for use in conjunction with the TIGRESS γ-ray spectrometer at TRIUMF’s ISAC-II facility. SPICE features a permanent rare-earth magnetic lens to collect and direct internal conversion electrons emitted from nuclear reactions to a thick, highly segmented, lithium-drifted silicon detector. This arrangement, combined with TIGRESS, enables in-beam γ-ray and internal conversion electron spectroscopy to be performed with stable and radioactive ion beams. Technical aspects of the device, capabilities, and initial performance are presented.

Influence of octupole vibration on the low-lying structure of Fm251 and other heavy N=151 isotones

The structure of low-lying excited states in $^{251}\mathrm{Fm}$, populated by the $\ensuremath{\alpha}$ decay of $^{255}\mathrm{No}$, has been investigated by means of combined $\ensuremath{\gamma}$ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the $1/{2}^{+}\ensuremath{\rightarrow}5/{2}^{+}$ and $5/{2}^{+}\ensuremath{\rightarrow}9/{2}^{\ensuremath{-}}$ transitions have been measured. The determined $M2/E3$ mixing ratio and lifetime for the $5/{2}^{+}$ decay to the ground state allowed to determine the corresponding reduced transitions strengths of $B(E3)=18(6)$ W.u. and $B(M2)=3.0(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ W.u. These results, as well as the results of previous studies in $N=151$ isotopes, are compared to theoretical calculations beyond the mean-field approach, including the first QRPA calculations using the Gogny D1M parametrization for such heavy odd-$N$ nuclei. The comparison points to the importance of accounting for the octupole vibrations for a proper understanding of the low-lying nuclear structure of some of the heaviest elements.

Quasiparticle phonon model description of low-energy states in Pr152

Delayed $\ensuremath{\gamma}$-ray and conversion-electron spectroscopy is performed on $A=152$ fission fragments, at the Lohengrin spectrometer of the Institut Laue-Langevin, providing a new decay scheme for $^{152}\mathrm{Pr}$. The quasiparticle phonon model, combined with the particle-rotor model, which allows octupole correlations and Coriolis mixing to be taken into account, is applied to analyze its low-energy structure. The main configurations are found to be $(\ensuremath{\pi}3/2[422]\ensuremath{\bigotimes}\ensuremath{\nu}5/2[642]){1}^{+}$ for the isomer and $(\ensuremath{\pi}3/2[541]\ensuremath{\bigotimes}\ensuremath{\nu}3/2[521]){3}^{+}$ for the ground state.

K-shell ionization during the α-decay of superheavy nuclei

The probabilities of K-shell ionization in daughter atoms during the α-decay of extensively studied 84Po isotopes and superheavy 117 294 Ts, 113 286 Nh, 109 278 Mt, 105 270 Db, 102 253 No, 100 249 Fm, and 111 272 Rg nuclei are calculated. Monopole and dipole terms are considered. More accurate expressions for α-particle tunneling through the atomic Coulomb barrier are derived. The electron wave functions are calculated using the relativistic Dirac–Fock method. The results are essential for processing the spectra analyzed in combined α-, γ-, and conversion-electron spectroscopy.