Conversion Electron Measurements Research Articles

The high-spin structure of 202Po and 204Po and systematical features of the even polonium isotopes

The high-spin structure of 202,204Po was studied using the 196,198Pt(12C, 6n) and 204Pb(α, 4n) reactions at a beam energy of 100 and 52-54 MeV, respectively. In-beam conversion electron and γ-ray spectroscopy measurements including γγ(t) coincidence, γ(t) and angular distributions were performed. The level scheme of 202Po was extended to spin 21 h and the excitation energy 5.5 MeV while levels in 204Po were observed at 5.9 MeV with a spin of 20 h. The previously observed isomeres in 202Po and 204Po were confirmed and in addition to these a Jπ = 13− level with the half-life 9(3) ns was observed in 204Po at 3387.3 keV. The expected configuration of this state is πh9/22 ⊗ vf5/2-1 i13/2-1. A shell-model description of the observed excited states is given.

Conversion electron and yrast state measurements in 73Br.

Gamma-ray and conversion electron measurements have been performed in the reactions {sup 58}Ni({sup 24}Mg,2{alpha}{ital p}){sup 73}Br at 110 MeV and {sup 40}Ca({sup 36}Ar,3{ital p}){sup 73}Br at 130 MeV, respectively. From {gamma}{gamma} coincidence data the previously known level scheme was confirmed and extended up to probable spin {ital I}=45/2. In the conversion electron spectra, the decay of the previously postulated 27 keV state was observed for the first time. From the measurement of internal conversion coefficients, multipolarities of seven transitions in the bandhead region of {sup 73}Br were determined and allowed spin-parity assignments to be established. High spin properties and quasiparticle alignment effects are discussed in comparison with {sup 75}Br.

Experimental evidence for missing low-energy transition in the 160Er to 160Ho decay

Conversion electron and photon measurements with the electrostatic and HPGe spectrometers, respectively, proved the existence of the 7.133+or-0.010 keV transition in 160Ho. The conversion intensity ratios for ten outer subshells agree with predictions for the E1 multipolarity and exclude all other multipolarities with L<or=4. The M2 admixture is less than 6*10-6 at the 99% confidence level. The transition intensity is roughly equal to the number of the 160Er decays. The proposed 1+(p7/2-(523)+n5/2-(523)) level in 160Ho is now confirmed.

Excited structure of the 56Ni-plus-three-nucleon isobars 59Ni and 59Cu

The reaction channels ( 3He, 2p) and ( 3He, pn) were used to study the excited states of 59Ni and 59Cu in the bombardment of the 58Ni with 23.1 MeV 3He ions. The de-excitation was studied using in-beam spectroscopic methods including γ-ray singles, yield functions,γ-ray angulardistribution, γγ-coincidence and conversion-electron measurements, from which 38 new transitions were assigned to 59Ni and 17 to 59Cu. Eleven new levels with spins up to J = ( 21 2 )kh and excitation energy E exc = 7949 keV were located in 59Ni, and six up to J = 17 2 kh and E exc = 5721 keV in 59Cu The neutron and proton 1 g 9 2 orbitals with J π = 9+ 2 were substantiated in both nuclides, as were the beginnings of their corresponding bands. Careful multipole-mixing analysis allowed a study of relative B( λ) values for λ = E1, M1, E2. Using these and relative branching ratios, absolute transition rates could be obtained for levels with previously reported mean lives. Band structures were elucidated from transition strengths and from comparison to the neighbouring even-even nuclides. A Hartree-Fock calculation was performed to determine the ground-state shapes and the single-particle structures of both 59Ni and 59Cu, and also of the even-even nuclides 58,60,62Ni. The possibility of shape coexistence in these nuclei is discussed.

Very-high-spin and core-coupled states in 213Rn. a test of the applicability of the deformed independent particle model

Excited states in 212,213Rn were populated in the bombardment of 204Hg by 14C ions of energies 80-94 MeV. The depopulation of the excited states was studied using in-beam spectroscopic methods including γ-ray excitation, γ-ray angular distribution, γ-γ-t coincidence, pulsed-beam-γ timing and conversion electron measurements. Spins up to Jπ = 30ℏ and a tentative J = (73/2)ℏ were observed, respectively. In 212Rn the previous level scheme was confirmed, with the addition of five new transitions of which one feeds the 30+ state. In 213Rn the previously reported level scheme was mainly confirmed and it was extended to (71/2) or (73/2)ℏ, which is the highest spin observed to-date in a lead-region nucleus. Several transition rates B(λ) were deduced from isomeric half-lives or branching ratios in both isotopes. Deformed Independent Particle Model (DIPM) calculations were performed to assign configurations, which in 213Rn usually consist of the odd neutrons vg9/2 and vj15/2 coupled to the four-proton states in 212Rn. The highest levels include neutron core-excitations, since the maximum valence-nucleon angular momentum in 213Rn is 55/2ℏ. The very topmost levels even need 2- particle-two-hole excitations. Shape changes in these two Rn isotopes are discussed, as are the transition rates.

Beta-decay of 27/2? isomers inN=83 nuclei

In theN=83 nucleus151Er we observed the isomericE3 transition in conversion electron measurements following on-line mass separation and located its 0.6 s 27/2− isomer at 2.586 MeV. Gamma-ray data establish a 4.7(3)% Gamow-Tellerβ-decay branch proceeding to high-spin states above 2 MeV excitation in151Ho. A weakβ-decay branch of 0.7(3)% was also observed for the 0.5 s 27/2− isomer in the149Dy isotone, and it was established that a similar 27/2− E3-isomer does not occur in153Yb. First results on the151Tm and151Er ground-stateβ-decays are briefly discussed.

High spin states in 98Tc via the 94Zr(7Li,3n)98Tc reaction.

The level scheme of $^{98}\mathrm{Tc}$ has been investigated via the $^{94}\mathrm{Zr}$${(}^{7}$Li,3n\ensuremath{\gamma}) reaction by in-beam \ensuremath{\gamma} spectroscopy and conversion-electron measurements. Gamma ray excitation functions, \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, and ${I}_{\ensuremath{\gamma}}$(0\ifmmode^\circ\else\textdegree\fi{})/${I}_{\ensuremath{\gamma}}$(90\ifmmode^\circ\else\textdegree\fi{}) anisotropies have been measured in the energy interval ${E}^{\mathrm{*}}$${(\mathrm{}}^{7}$Li)=23--31 MeV. High spin states up to I=14 and ${E}^{\mathrm{*}}$=3200 keV have been observed. A \ensuremath{\Delta}I=1 negative parity band based on the ${I}^{\ensuremath{\pi}}$${=8}^{\mathrm{\ensuremath{-}}}$, 1091 keV state has been identified. The level scheme has been compared with the predictions of the two quasiparticles plus rotor model. The negative parity band is associated with the \ensuremath{\pi}\ifmmode \tilde{}\else \~{}\fi{}${g}_{9/2}$\ensuremath{\bigotimes}\ensuremath{\nu}\ifmmode \tilde{}\else \~{}\fi{}${h}_{11/2}$ intrinsic configuration space.

E0 decays of 0+ states in 146Gd: Search for two-phonon octupole excitations.

Conversion-electron measurements following the $^{144}(\mathit{\ensuremath{\alpha}}$,2n) and $^{144}\mathrm{He}$,n) reactions reveal high-energy E0 transitions at 3020, 3485, and 3639 keV. The lowest of these deexcites the known two-neutron pairing vibrational state in $^{146}\mathrm{Gd}$. It is not clear whether either of the others can be associated with the ${0}^{+}$ member of the ${3}^{\mathrm{\ensuremath{-}}}$\ifmmode\times\else\texttimes\fi{}${3}^{\mathrm{\ensuremath{-}}}$ two-phonon octupole multiplet.

Some nuclear and atomic properties of201Hg determined by conversion electron spectroscopy

Conversion electron measurements with an electrostatic spectrometer proved the existence of the 1,565±6 eV transition in201Hg. The conversion intensity ratios,N 1/N 2 =1.2±0.2,N 1/N 3=1.1±0.2,N 2/N 3=0.92±0.15,N 4/N 3=0.03± 0.02 andN 5/N 3=0.04 ±0.02 were determined. These values agree with our calculations for the M1±E2 multipolarity with theE2/M1 mixing ratioδ 2=(l.l±0.3)xl0−4 and exclude all pure multipolarities withL≦4. The total conversion coefficient for the aboveM1 +E2 mixture was evaluated to be (4.7±0.7)× 104. The reducedB(M1, 1/2→3/2) probability was derived to be (3.9 ±1.2) × 10−3 (eℏ/2Mc)2. The natural widths of theN-subshell conversion lines in mercury were found to beΓ(N 1)=8.3± 1.5,Γ(N 2) =5.8±1.5 and Γ(N 3) =6.5±1.0 eV. Monte Carlo calculations of electron scattering in matter yielded the conversion line shapes in qualitative agreement with the experiment.

Decay of 100Sr and a "pairing-free" K pi =1(+) rotational band in odd-odd 100Y.

The decay of $^{100}\mathrm{Sr}$ (193 ms) to the low-spin $^{100}\mathrm{Y}$ isomer (735 ms) was studied from mass-separated activity produced in thermal neutron fission of $^{235}\mathrm{U}$. \ensuremath{\gamma} singles and \ensuremath{\gamma}-\ensuremath{\gamma} coincidence measurements resulted in the placement of 67 \ensuremath{\gamma} transitions in a decay scheme with 26 levels below 2 Mev. The multipolarities of low-energy transitions were determined from internal conversion electron measurements. Logft values were deduced using an absolute \ensuremath{\gamma}-ray intensity determination for the A=100 decay chain. The ${1}^{+}$ levels in $^{100}\mathrm{Y}$ at 10.70 and 974.61 keV each receive \ensuremath{\sim}40% of the \ensuremath{\beta} feeding from $^{100}\mathrm{Sr}$. A ${K}^{\ensuremath{\pi}}$${=1}^{+}$ rotational band (with ${1}^{+}$, ${2}^{+}$, and ${3}^{+}$ levels at 10.70, 76.15, and 172.03 keV) is proposed which can be characterized as a nearly ``pairing-free'' band. Other levels in $^{100}\mathrm{Y}$ are discussed in terms of two-quasiparticle Nilsson orbitals.

Decay properties of139mSm and139Eu

The structure of139Sm is investigated through the decay properties of139mSm and139Eu. The decay of139mSm is measured to have a half-life of (10.7±0.6) s. Gamma-ray and internal conversion electron measurements lead to the construction of a decay pattern comprising seven transitions among five states. The decay of139Eu with a measured half-life of (17.9± 0.6) s is studied with beta decay spectroscopy techniques. A decay scheme consisting fourteen states in139Sm is proposed. Using deducedft values and transition multipolarities,J ρ assignments are made. Nuclear systematics of the odd-A, N=77 and 76 are discussed.

Direct measurement of the isomeric 12+to 10+and 33+/2 to 29+/2 transition energies in the isotopes194Pb to197Pb

The very low-energy transitions connecting the 12+ and 10+ states in the evenmass lead isotopes with A=196 and 194 and those joining the 33+/2-29+/2 levels for 197Pb and 195Pb have been detected through conversion electron measurements and their energies determined. The experimental reduced transition probabilities B(E2) are compared with the theoretical ones computed within two differential frameworks.

In-beam spectroscopic study of 107In

The nuclear level structure of 107In has been studied with the 106Cd(α, p2n) 107In and 106Cd( 3He, pn) 107In reactions. In addition to the ordinary in-beam measurements of γ-rays and conversion electrons, p-γ coincidence measurements were performed in order to identify the γ-transitions in 107In. The experimental data of positive parity states are compared with predictions of a hole-phonon coupling model, in which a g 9 2 proton hole couples to the quadrupole core vibration. This simple model appears to be inadequate to describe the observed level properties of 107In.

High-spin neutron excitations in neutron-deficient even-mass Pb isotopes

High-spin states in 194, 196pb have been investigated using the 188Os( 12C, χn) 194, 196Pb and the 198Hg(α, 6n) 196Pb reactions. The experiments included γ-ray excitation function, γ-γ coincidence, γ-∑γ coincidence, γ-ray angular distribution, lifetime and conversion electron measurements. The level schemes of 194Pb and 196Pb have been extended up to states with J π = 15 − ( E = 3955 keV) and J π = 20 + ( E = 5444 keV), respectively. All levels except two new isomers which have most likely spin and parity J π = 10 − ( 194 Pb) and J π = 11 − ( 196 Pb) can be interpreted as neutron excitations. The excitation energies of levels with spin J ⩽ 12 are well accounted for by the one-broken-pair model including 1p1h excitations. The B(E2, 12 + −10 +) values reflect the systematic trend for a pure ( i 13 2 ) 2 configuration. Levels with spin 12 < J < 20 can be described as two-broken-pair model configurations. The excitation energies of these states can be reproduced quite well in a phenomenological approach in which two i 13 2 neutron holes are coupled to core excitations in the A + 2 nucleus and also by calculations performed within the two-broken-pair model. Within this framework the J π = 20 + state observed in 196Pb has a pure ( i 13 2 ) 4 configuration. The two new isomers have most likely a proton 2p2h structure. The excitation energy of the J π = 11 − isomer in 196Pb agrees with that predicted for a deformed 1 2 + [404] − 29 2 − [514] 13 2 + [606] 11 − state.

Near-Yrast spectroscopy of 164Yb and neighbouring nuclei: Systematics of octupole bands and bandcrossings

High-spin states in 164Yb have been populated in the 152Sm( 16O, 4n) and 150Sm( 18O, 4n) reactions. From studies of γγ coincidences, γ-ray angular distributions and conversion electron measurements the level scheme has been constructed. The g-band and the S-band have been established to spin and parity 22 + and 26 +', respectively, and the rotational sequences ( π, α) = (−, 1) 1, (−, 0) 1 and (−, 0) 2 to 23 −, 24 − and 18 −, respectively. Constructed two-quasineutron configurations and cranked shell model calculations are compared with the experimental results. Residual interactions between quasiparticles are extracted from data. The side-bands in 162, 164Er and 164, 166Yb are discussed, putting a special emphasis on the octupole bands and their interaction with the two-quasiparticle bands. Crossing frequencies and the gain in alignment are summarized for the Yb isotopes and the main features are discussed.

On the rotation-aligned coupling of the [formula omitted] proton-hole states in 191, 193Au

The rotational bands built on the excited 11 2 − levels in 191Au and 193Au were investigated in (α,4n) reactions with in-beam γ-ray and conversion electron spectroscopy. Half-lives of the rotation-aligned levels in these bands were determined from e −e − time spectra, which were measured with a new double-orange spectrometer. In addition, some information on the magnetic moments of the rotation-aligned levels was obtained from integral perturbed angular distribution measurements. Finally, the previously unknown low-energy 31 2 − → 27 2 − transition in 191Au was identified from conversion electron measurements. From these data it is concluded that the rotation-aligned Hg cores have predominantly νv i 13 2 2 structure, contrary to the earlier suggestion of a π h 11 2 2 core structure.

Low- and medium-spin states in theN=124 isotones208Po and209At

Excited levels in208Po and209At were populated in the reactions (p, 2n) and (3He, 3n), and208Po also in the EC-decay of the208At ground state, which was produced in the209Bi(3He, 4n) reaction. The experimental procedures comprised gamma-ray excitation functions,γ- γ coincidences, as well as gamma-ray angular distribution and conversionelectron measurements, both on- and off-line. The properties of about twenty-five levels in each nuclide were studied. In208Po the energy of the 8+→6+ transition could be determined to be 4.0±0.2keV and the 8+ state thus has an excitation energy of 1,528 keV. The effective quadrupole charge of theh9/2 proton can be derived aseeff(τ h9/2)=1.69(15)e. For the previously reported isomeric half-lives more accurate values are obtained, the systematics of electricL=1, 2 and 3 transition rates are discussed, and their microscopic structure is elucidated. Shell model considerations are used to support the configuration assignments, as are the logft values for the strongest208At(EC)208Po decay branches. The effect of core polarixation on the excitation energies of yrast states is studied in both208Po and209At. Many of the states populated in the208At EC decay are shown to be of core-excited character.

Decay of 185Au: Sign of shape coexistence in 185Pt

The decay of 185Au has been studied on-line with mass-separated sources from the ISOCELE facility. Precise conversion-electron measurements have been performed with a 180° magnetic spectrograph. The level scheme of 185Pt has been established, and the T 1 2 = 33 min isomeric state has been located at 103.2 keV with respect to the T 1 2 = 71 min ground state. Two highly converted transitions have been observed. The level scheme is discussed in the framework of an “axial-rotor + quasiparticle” approach: numerous states are interpreted assuming a prolate shape of the nucleus. Ten levels with low spin and negative parity ( I ⩽ 7 2 − ) decay mainly to the 1 2 −[521] band via strong M1 transitions and are not expected from the calculations performed with the prolate cores. The possibility of shape coexistence is discussed.

E0 transitions in 82Kr.

The first excited ${0}^{+}$ state of $^{82}\mathrm{Kr}$ appears to have a noncollective nature, in contrast to the ${0}_{3}^{+}$ and other low-spin states in this nucleus. This is corroborated by a conversion-electron measurement of the ${0}_{2}^{+}$\ensuremath{\rightarrow}${0}_{1}^{+}$ and the ${0}_{3}^{+}$\ensuremath{\rightarrow}${0}_{1}^{+}$ transitions in $^{82}\mathrm{Kr}$, following the decay of $^{82}\mathrm{Rb}^{\mathrm{g}}$. The values B(E${0;0}_{2}^{+}$\ensuremath{\rightarrow}${0}_{1}^{+}$)=3.1 (5) ${e}^{2}$${\mathrm{fm}}^{4}$ and ${X}_{311}$=B(E${0;0}_{3}^{+}$\ensuremath{\rightarrow}${0}_{1}^{+}$ ${2}_{1}^{+}$)=0 .097(24) have been deduced from the intensities of the conversion lines, observed with a mini-orange spectrometer. Previous eviewed.

Half-life measurements in doubly-odd 186, 188, 190Au nuclei and the 188Hg → Au decay

Abstract A level scheme has been established for 188Hg → Au decay which was studied with the on-line isotope separator ISOCELE. Precise conversion-electron measurements were performed with a semicircular magnetic spectrograph. The half-lives of the 16.0, 82.7 and 114.8 keV levels in 188Au were measured with a lens electron spectrometer and reduced transition rates were deduced. Similarly the half-lives of the 36.1, 113.9, 227.7, 251.5, 288.0 and 363.6 keV levels in 186Au and of the 28.9 and 171.5 keV levels in 190Au were measured. Comparison of the reduced e.m. transition rates shows that a 1 + and a 2 − state have respectively the same structure in 186, 188, 190, 192Au, the same conclusion holding for the 1 − g.s of 188, 190, 192Au. Additional measurements in 189, 191Hg and 185, 187Au together with data from the literature enable us to interpret them as a coupling of certain quasiparticle states of neighbouring odd- A nuclei corresponding to oblate shapes. Indications are given that some other negative- and positive-parity states in 188Au also belong to an oblate system. The nucleus 188Au appears as the last of a series of doubly-odd gold nuclei where no shape coexistence has, as yet, been observed.