Delayed-neutron emission from short-lived Br and I isotopes

Rapid radiochemical separation techniques have been used to investigate the delayed-neutron emission from bromine and iodine isotopes produced by thermal-neutron induced fission of ²³⁵U. The nuclides 0.63 sec ⁹¹Br, 0.26 sec ⁹²Br and 0.41 sec ¹⁴¹I have been identified as delayed-neutron precursors. Relative neutron abundances of these and of the known precursors ^87–90Br and ^137–140I have been measured and absolute neutron abundances and neutron emission probabilities have been deduced. The P_n values are discussed within a simple semiempirical picture.     

The local structure of Cs(I) and Cs(II) in a Cs2ZnCl4 single crystal studied by nuclear magnetic resonance

The rotation patterns of the ¹³³Cs (I=7/2) nuclear magnetic resonance (NMR) in a Cs₂ZnCl₄ single crystal grown by using the slow evaporation method were measured in two mutually perpendicular crystal planes. Two different groups of Cs resonances were recorded; this result points to the existence of two types of crystallographically inequivalent Cs(I) and Cs(II). The angular dependences of the NMR spectra led to different values for the quadrupole coupling constants and asymmetry parameters: e²qQ/h=148 kHz and η=0.11 for the Cs(I) ion, and e²qQ/h=274 kHz and η=0.66 for the Cs(II) ion. The EFG tensors of Cs(I) and Cs(II) are asymmetric, and the orientations of the principal axes of the EFG tensors do not coincide. Only, the principal Y-axes of the EFG tensors coincide for the Cs(I) and Cs(II) sites. The Cs(I) ion is surrounded by 11 chlorine ions, making it rather free and high in symmetry. The Cs(II) ion has only nine neighbors and seems to be more tight than the Cs(I) ion.     

Nuclear magnetic resonance and transferred hyperfine interaction study of the crystallographically inequivalent Cs(I) and Cs(II) in a Cs2CuCl4 single crystal

¹³³Cs (I=7/2) nuclear magnetic resonance in a Cs₂CuCl₄ single crystal grown by using the slow evaporation method was measured in its three mutually perpendicular crystal planes. The ¹³³Cs resonances of two different groups with two crystallographically inequivalent cesium nuclei, Cs(I) and Cs(II), in the unit cell were recorded. The transferred hyperfine fields for Cs(I) and Cs(II) calculated from the paramagnetic shift and the molecular susceptibility measurements could be expressed by the linear equation H_hf=AT+B. The angular dependence of the ¹³³Cs nuclear magnetic resonance spectra showed that the Cs(I) and the Cs(II) nuclei had different values for the quadrupole coupling constant. The electric field gradient tensors of Cs(I) and Cs(II) were symmetric, and the orientations of their principal axes did not coincide. The Cs(I) ion surrounded by 11 chlorine ions had a small quadrupole parameter, a smaller charge distribution, and a small value for the transferred hyperfine field. However, the Cs(II) ion surrounded by nine chlorine ions had a larger quadrupole parameter, a larger charge distribution, and a larger value for the transferred hyperfine field.     

Delayed neutron emission probabilities of the precursors89,90,91Br and139,140,141I

The delayed neutron emission probabilities of^89,90,91Br and^139,140,141I have been measured. The ions AlBr⁺ and AlI⁺ are formed in the OSIRIS integrated target-ion source giving improved conditions for study of the most neutron-rich isotopes of iodine and bromine. The half-life determinations of⁹⁰Br,¹³⁹I and¹⁴¹I have been improved.     

Dissociative multiple photoionization of Br2, IBr,and I2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)

Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90∼978 eV for Br₂, 60∼133 eV for IBr, and 86∼998 eV for I₂. Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d¹⁰)→Br(3d⁹ϵf), I(4d¹⁰)→I(4d⁹ϵf), and I(3d¹⁰)→I(3d⁹ϵf) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p_3/2) (189.9 eV) and I(4p_3/2) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X³⁺–X²⁺ (X=Br, I). From the I(3d_5/2) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I³⁺–I³⁺ or I⁴⁺–I²⁺. A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60<E<133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption.     

Decay schemes and total decay energies of89Br and90Br

Spectroscopical investigations of⁸⁹Br and⁹⁰Br have been performed, utilizing the formation of the molecular ion AlBr⁺ in the ion-source of an isotope separator on-line a reactor. Detailed level schemes have been constructed for⁸⁹Kr and⁹⁰Kr, and theβ-strength function of⁸⁹Br and⁹⁰Br below the neutron binding energy is presented. The total decay energy of^89,90Br, as well as of⁸⁹Kr, have been measured by means ofβ γ-coincidences, and the population of excited states in⁸⁹Kr via⁹⁰Br delayed neutrons has been deduced.     

Study of the133Cs(γ,n)132Cs reaction

The photoneutron spectrum from the¹³³Cs(γ, n)¹³²Cs reaction induced by 9.720 MeV neutron capture gamma-rays has been studied with a³He-spectrometer. The energies and relative intensities of the emitted neutrons were determined. AQ-value of 8,986 +/?2 keV is suggested. The data reveal the existence of seventeen excited states in¹³²Cs in the region up to 551 keV.     

Does the simple statistical model describe delayed neutron emission?: The cases of 85As, 87Br, 135Sb and 137I

Delayed neutron spectra and branching ratios to excited states in final nuclei calculated with the statistical model are compared to experimental data for the decay of ⁸⁷Br, ¹³⁷I, ⁸⁵As and ¹³⁵Sb. For the first two precursors, the calculations support the experimental β-strength functions reported previously. For the latter two, it is shown that the statistical model cannot simultaneously reproduce both spectra and branching ratios for any choice of β-strength function when all levels populated by neutron emission are included in the calculations. The comparisons demonstrate that partial widths for neutron emission are not compatible with optical-model transmission coefficients. We conclude, as pointed out previously, that structure effects in the energy range probed by delayed neutron emission are not averaged out to the extent required by the statistical assumptions.     

Delayed neutron emission probabilities of gallium,bromine, rubidium,indium, antimony,iodine, and cesium precursors

The delayed neutron emission probability(P _n) has been measured for^79-83Ga,^87-89Br,^92-96Rb^127-132In^134,135Sb^137-139I and^{141 -145}Cs, by means of sources produced at the OSIRIS isotope separator at Studsvik. The half-life determinations of^{127, 129, 132}In have been improved.     

Ultraviolet photoelectron spectra of gas phase and condensed tin and lead dihalides (MX2 ; M = Sn,Pb; X = Cl,Br, I)

The photoelectron spectra and electronic structures of the methylene dihalides, CH₂X₂ (X = F, Cl, Br and I), have been calculated by the overtapping-spheres SCF-Xα-MS method. The results are in good agreement with experimental data. Calculated assignments of the spectra are also presented and interpreted by assuming interaction between lone-pair and bonding orbitals.     

Precise quantum defects of nS,nP and nD Levels in Cs I

Very accurate measurements of then ² S _1/2 (n=9–30),n ² P _1/2,3/2 (n=9–50) andn ² D _{3/2, 5/2} (n=5, 8–32) energy levels in Cs I have been performed in the wavelength-range 6,379 Å<λ<7,432 Å with a high-precision vacuum-wavemeter using either Doppler-free two-photon spectroscopy or Doppler-limited one-photon spectroscopy with a frequency-doubling crystal. Thermionic detectors were used throughout the experiments. By fitting the experimental energy values to extended Rydberg-Ritz formulas, accurate quantum defect data and series limits are given. The ionization energyE _i =31,406.4710(7) cm^?1 was adopted to be the best fit value, differing significantly from most recent evaluations of other authors, but agreeing well with former, less accurate measurements performed in our laboratory.     

Anomalous Cs I 5d-5⨍ lineshape in a non-debye plasma

Laser-induced cesium plasmas were diagnosed by emission spectroscopy, yielding electron densities in the range N_e = 10¹⁶?5 × 10¹⁷ cm^-3 and electron temperatures in the range T_e = 0.2-1 eV. The experimental lineshapes for T_e = 0.5 eV were found to be in good agreement with theory. For the more strongly coupled plasmas at N_e = 1-2 × 10¹⁶ cm^-3 and T_e = 0.2 eV, however, the Cs I 5d-5? lineshape was more asymmetric than predicted.     

Core polarization of the133Cs atom by the 7p electron

The hfs of the 7² P _1/2 state of Cs has been measured by optical double resonance yieldingA(7² P _1/2,¹³³Cs)=94.35 (4) MHz. The core polarization contribution to the hfs and the value 〈r ^?3〉 _j =2.54 · 10⁺²⁴ cm^?3 of the 7p electron of Cs has been calculated from the experimental data and was compared with current theories indicating still an appreciable uncertainty in the atomic wave functions of this one-electron atom.     

Core excitation in79Br and81Br

TheE2 andM1 reduced transition probabilities between the low lying levels, in the case of⁷⁹Br and⁸¹Br, have been calculated on the core-excitation model. To account for the observed M1 transition probabilities admixture among the like spin states is considered. In addition, the ground state static moments have been computed. For the same amount of admixture among the like spin states, a good agreement with the experimental results is obtained, for both⁷⁹Br and⁸¹Br. The results are compared with other existing theoretical calculations. From our analysis, we obtain, the quadrupole moment of the first excited 2⁺ state in the neighbouring even-even nuclei. It is in good agreement with the value calculated on the basis of the anharmonic vibrational model.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Electronic excitation of HgX (X = Cl,Br, I) due to electron and methyl mercury halide collisions

Electronic excitation of HgX (X = Cl, Br, I) radicals in the B-state has been observed as the result of collisions with low energy electrons and methyl mercury halide (CH₃HgX) molecules. The emission intensity has been observed to be much weaker than that observed for electron-HgX₂ collisions under similar experimental conditions. Using the strongest band head of the B-X band system, an attempt has been made to calculate the emission cross section due to electron CH₃HgX collisions at 10 eV electron kinetic energy. For HgCl, HgBr, and HgI radicals, these cross sections are 1 × 10^-18, 7 × 10^-17, and 2 × 10^-17 cm², respectively, with an estimated uncertainty of ±30%. Our measured threshold electron energy for excitation of CH₃HgX molecules and observation of the B-X emission band system and emission cross sections measured at 10 eV are greatly different from those measured by Allision and Zare [Chem. Phys. 35, 263 (1978)].     

Electronic states and transitions of the TeX (X = C1, Br,I) radicals

Ab initio multireference configuration interaction calculations including spin-orbit coupling have been carried out for the first time for valence electronic states of the TeX (X = Cl, Br, I) radicals and compared with the results for the isovalent TeF and IO systems obtained earlier at a similar level of theoretical treatment. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the X² II(σ²π⁴π?³) ground state bonding becomes consistently weaker down the TeX group (calc. D_e, = 25480cm^?1 for TeF, 12 100cm^?1 for Tel) due to the more covalent character of bonding in the heavier radicals. The first excited state, A ⁴Σ^? (π?→ σ?), is calculated to be bound in all systems. It is split into Ω 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground X²II state. For larger internuclear distances, the A₁ ⁴Σ^? _1/2 state undergoes an avoided crossing with X₂ ²II_1/2, which causes a shoulder in the X₂ potential curve and also leads to a crossing between the A₁, and A₂ curves and large distinctions in their vibrational frequencies. The π? → σ? B²Σ^?, C²δ, and 1 ²Σ⁺ states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 2²II(σ²π³ π?⁴) state has a repulsive potential curve. Electric dipole transition moments and radiative lifetimes have also been calculated for the low lying bound states in all systems. Most of them are found to be quite weak. The A^1,2 → X^1,2 spectra are dominated by parallel contributions, with the A₂ → X₁ being the strongest one. The T values for this transition are quite similar and lie in the 17–30 μs range. Radiative lifetime values for the B → X^1,2 transitions demonstrate very irregular behaviour for various, TeX radicals, due to strong admixture of A⁴Σ^? character to the X^1,2 states near the B²Σ^? potential minimum. The A^{1,2 4}Σ^? _1/2,3/2 and B²Σ^? _1/2 states of TeX (X = Cl, Br, I) still await their experimental observation.     

Nuclear spin of a new isomer in 119Cs and magnetic moment of 120Cs evidence for strongly deformed nuclear shapes of the light cesium isotopes

The nuclear spin, $\text{I =}\text{3}\text{2}$, of a new isomer in ¹¹⁹Cs and the magnetic moment, μ_I = 3.92(5) n.m., of ¹²⁰Cs have been determined in on-line ABMR experiments at the ISOLDE facility, CERN. The experimental data give evidence for strongly deformed nuclear shapes of the light cesium isotopes.