Nuclear Mean Square Research Articles

Nuclear moments of strongly deformed strontium isotopes

Nuclear spins, moments and mean square charge radii of78–100Sr have been obtained by fast ion-beam collinear laserspectroscopy. The experiments performed at ISOLDE have been extended to include99Sr, measured by a non-optical detection scheme with a two-step optical pumping sequence. The results for the strongly deformed isotopes are discussed in the frame of the particle-plus-deformed core model.

Hyperfine structure and isotope shift of the neutron-rich barium isotopes139?146Ba and148Ba

The hyperfme structure and isotope shift in the 6s 2 S 1/2−6p 2P3/2 line of Ba II (455.4 nm) have been measured by collinear fast-beam laser spectroscopy for the neutron-rich isotopes139–146Ba and148Ba. Nuclear moments and mean square charge radii of these isotopes have been recalculated. The isotope shift of the isotope148Ba (T1/2=0.64 s) could be studied for the first time, yieldingδ〈r2〉138,148=1.245(3) fm2.

Fluorescent atom coincidence spectroscopy of extremely neutron-deficient barium isotopes

Fluorescent atom coincidence spectroscopy (FACS) has been used to measure the nuclear mean square radii and moments of the extremely neutron-deficient isotopes 120–124Ba. At N=65 an abrupt change in nuclear mean square charge radii is observed which can be understood in terms of the occupation of the spin-orbit partner g 7 2 5 2 [413] neutron and g 9 2 9 2 [404] proton orbitals and the consequent enhancement of the n-p interaction.

Mean square charge radii of radium isotopes and octupole deformation in the 220–228Ra region

The first investigation of isotope shifts in both the atomic and ionic resonance lines of radium has been carried out using the technique of on-line collinear fast-beam laser spectroscopy at the ISOLDE facility at CERN. The measurements cover 19 isotopes in the mass range 208⩽A⩽232 with half-lives between 23 ms and 1600 y. The differences in the nuclear mean square charge radii δ〈r2〉 have been evaluated and - together with earlier published spins and moments from the hyperfine structure - related to nuclear deformation. In particular the inversion of the odd-even staggering effect for the isotopes 221Ra, 223Ra and 225Ra can be interpreted by the presence of octupole instability and adds weight to the concept of near-stable octupole deformation in the odd-A isotopes which already explained their spins and magnetic moments.

Isotope shift of182Hg and an update of nuclear moments and charge radii in the isotope range181Hg-206Hg

The technique of collinear fast-beam laser spectroscopy has been used to measure the isotope shifts of the even-even isotopes of Hg (Z=80) in the mass range 182≤A≤198 at the on-line mass separator ISOLDE at CERN. The atomic transition studied (6s 6p 3 P 2- 6s7s 3 S 1,λ=546.1 nm) starts from a metastable state, which is populated in a quasi resonant charge transfer process. The resulting changes in nuclear mean square charge radii show clearly that182Hg follows the trend of the heavier, even, weakly oblate isotopes. Correspondingly the huge odd-even shape staggering in the light Hg isotopes continues and the nuclear shape staggering and shape coexistence persists down to the last isotope investigated,181Hg. An update of isotope shift and hyperfine structure data for181–206Hg is given, with a revised evaluation of the differences in nuclear mean square charge radii and of spectroscopic quadrupole moments.

Measurement of the optical isotope shift of 82Sr.

The optical isotope shift in the 6${s}^{2}$-6s6p (\ensuremath{\lambda}=460 nm) atomic resonance transition of $^{82}\mathrm{Sr}$ (${t}_{1/2}$=24 d) has been measured to be 418(2.5) MHz. The weak atomic beam of $^{82}\mathrm{Sr}$ was detected using the photon burst technique. The extracted value for the change in nuclear mean square radius is compared to predictions using the droplet model.

Collinear laser spectroscopy on unstable isotopes—A tool of nuclear physics

Atomic hyperfine structures and isotope shifts yield basic information about nuclear ground-state spins, moments and mean square charge radii. Recently, the collinear-beam laser experiments at ISOLDE have considerably enlarged the range of elements for which these studies can be extended into regions far from β-stability. The essential features of these experiments are outlined, and examples of the results on the rare-earth and radium isotopic chains are given. Finally, further improvements in sensitivity using non-optical detection are discussed.

Radio-frequency and laser spectroscopy studies of low-lying states in SrI

Precision hyperfine structure (hfs) measurements in the 5s4d1D2 metastable state of87Sr were performed using the laser radio-frequency double resonance method. The magnetic dipole and electric quadrupole interaction constants were found to bea=−5.5734(4) MHz andb=55.421(6) MHz. In addition high-resolution laser spectroscopy experiments were performed in the 5s4d1D2 ↔ 5s6p1P1 and 5s4d1D2 ↔ 5p4d1D2 transitions in SrI. The isotope shifts in the transitions and the hfs in the upper states were determined. Comparison of the experimental results with corresponding relativistic self-consistent-field (SCF) calculations shows that the states involved are very strongly perturbed. Therefore no reliable nuclear quadrupole moment for87Sr can be deduced from the data presented here without more detailed calculations. A relativistic SCF calculation of the electron density at the nucleus was used in combination with muonic values of the nuclear mean square radii to evaluate the field shifts. From the experimental isotope shifts the specific mass shifts in the two transitions can then be deduced. This shift was found to be particularly large in the transition 5s4d1D2 ↔ 5s6p1P1.

Optical Isomer Shift for the Spontaneous-Fission IsomerAmm240

The optical isomer shift in the $^{8}S_{\frac{7}{2}}\ensuremath{\rightarrow}^{10}P_{\frac{7}{2}}$ atomic transition in neutral americium has been measured for the \ensuremath{\sim}1-ms spontaneously fissioning isomer, $^{240}\mathrm{Am}^{m}(\mathrm{S}\mathrm{F})$. With use of a laser-excited optical pumping technique, this transition is shifted by +2.6 \ifmmode\pm\else\textpm\fi{} 0.2 \AA{} to 6407.2\ifmmode\pm\else\textpm\fi{}0.2 \AA{} implying a difference in nuclear mean square radii, $\ensuremath{\delta}〈{r}^{2}〉$, of 5.1\ifmmode\pm\else\textpm\fi{}0.2 ${\mathrm{fm}}^{2}$ between the fission isomer and the normal ground state. These experiments provide the first direct experimental proof for the large deformations expected for fission isomers.

Changes in nuclear mean square charge radii of stable Krypton isotopes

The isotope shifts of stable even Kr isotopes (A=78 throughA=86) in the optical transitions at 432 nm and 557 nm were measured by means of polarization laser spectroscopy. The observed shifts are consistent with earlier results for other transitions. From the isotope shifts the changes in the nuclear mean square charge radiiδ〈r 2〉 were inferred using preliminary muonic isotope shift data. Starting from78Kr, a monotonic decrease of 〈r 2〉 with increasing mass number is found throughout theg 9/2 neutron shell. The effect onδ〈r 2〉 of nuclear deformations as well as possible contributions due to changes in the skin thickness of the nuclear charge distribution are discussed.

Pion exchange currents and nuclear charge form factors

The effect of the pion exchange current due to the Born term in the photoproduction amplitude on the charge form factors and distributions of 4He, 16O and 40Ca is calculated in the simple harmonic oscillator shell model. The exchange current contribution is most notable in the case of the α-particle while it is rather unimportant in 16O and 40Ca. The contribution to the nuclear mean square radius is small (≈ 0.03 fm 2) and roughly independent of the mass number.

Nuclear moments and optical isotope shift of radioactive 133Ba

Mass separated radioactive 133Ba has been investigated by means of optical spectroscopy. From hyperfine structure and isotope shift we obtain the nuclear magnetic moment μ I = −0.769(3) μ n and the change of the nuclear mean square charge radius δ〈r 2〉 ( 133 Ba − 132 Ba) = −0.017(3) fm 2 .

Hyperfine interactions and nuclear moments in even uranium isotopes from Mössbauer spectroscopy

Nuclear γ-resonance (Mössbauer effect) measurements have been performed using the 45 keV, 0 + → 2 + transition in 234U, 236U and 238U. Values are obtained for the ratio of quadrupole moments of the 2 + level in these isotopes, Q(234): Q(236): Q(238) = 1: (1.13 ± 0.09): (1.13 ± 0.10), and for the magnetic moments of the 2 + level, μ(234): μ(236): μ(238) = 1: (0.98 ± 0.06): (0.94 ± 0.09). No isomer shifts are detected, placing an upper limit on the change in nuclear mean square charge radius of | Δ〈 r 2〉| ≦ 2 × 10 −4 fm 2 for all three isotopes. Relative merits of the three uranium isotopes for nuclear γ-resonance spectroscopy are discussed, and some preliminary attempts at observing resonances in 233U and 235U are mentioned.

Nuclear Electric Field Gradient and Mean Square Displacement of the Iron Sites in Sodium Nitroprusside

Both the electric-field-gradient (EFG) and mean square displacement (MSD) tensor parameters at the Fe sites in sodium nitroprusside at 23\ifmmode^\circ\else\textdegree\fi{}C have been determined with a high precision by the M\"ossbauer technique. The EFG asymmetry parameter is $\ensuremath{\eta}=0.01\ifmmode\pm\else\textpm\fi{}0.01$. The principal axes of the two tensors do not coincide. Taking proper account of saturation effects was essential, even though the absorbers were only \ensuremath{\sim}0.2 mm thick. For the MSD, proper consideration of polarization effects was also important. A total of 34 runs on an absolutely calibrated mechanical drive gave for the quadrupole splitting $\ensuremath{\Delta}{E}_{Q}=1.7048\ifmmode\pm\else\textpm\fi{}0.0025$ mm/sec.