Measuring high-energy γ-ray emission accompanying the spontaneous fission of <Superscript>252</Superscript>Cf nuclei

The probability of high-energy γ-ray emission accompanying the spontaneous fission of ²⁵²Cf nuclei in the energy range of 5–60 MeV was measured experimentally. The γ rays were detected by a BGO detector with a size of ∅7.6 × 7.6 cm in coincidence with neutrons detected by an organic polystyrene-based scintillator with a size of ∅6.0 × 2.0 cm. To reject events associated with the cosmic background, we propose an original method that combines rapid digitizing of the pulse shape and the time-of-flight method.     

Shift in the angular distributions of γ quanta accompanying <Superscript>235</Superscript>U fission by polarized thermal neutrons

The angular dependence of the γ-ray asymmetry relative to the plane formed by the directions of fission-fragment separation and longitudinal polarization of the thermal neutrons inducing ²³⁵U(n, f) fission was investigated. The results obtained confirm the existence of γ-ray emission asymmetry and the dependence of its coefficient on the angle between the axes of the fission-fragment and γ-ray detectors, revealed for the first time by the ITEP group at the FRM-II reactor in Munich. The observed T-odd effect of around ∼2 × 10⁻⁴ can be explained by the angular anisotropy of the γ-ray emission from fission fragments with large angular momenta oriented relative to the fission axis.     

Cross sections for the photofission of <Superscript>243</Superscript>Cm, <Superscript>245</Superscript>Cm, <Superscript>249</Superscript>Bk,and <Superscript>249</Superscript>Cf isotopes in the energy range from the threshold to 10–12 MeV

The relative method was used to measure the photofission cross sections for ²⁴³Cm and ²⁴⁹Cf isotopes in the energy range from 6 to 12 MeV, for ²⁴⁵Cm in the energy range from 5 to 10 MeV, and for ²⁴⁹Bk in the energy range from 5.5 to 10 MeV. The measurements were performed with an energy step of 50 to 200 keV by using the microtron installed at the Institute of Physics and Power Engineering (Obninsk). The cross section for ²³⁸U photofission was used as a reference in these measurements. Data on the cross sections for ²⁴³Cm, ²⁴⁵Cm, and ²⁴⁹Bk photofission were obtained for the first time, while data on the cross section for ²⁴⁹Cf photofission were obtained for the first time only in the energyregion E<10 MeV. The data on the ²⁴⁵Cm nucleus suggest that, in the energy region around 6 MeV, the cross section for its photofission has a maximum, which is likely to be due to the low-energy resonance structure of the dipole-photoabsorption cross section. For ²⁴⁹Cf, an anomalously large value of the photo fission cross section is observed in the region of the first maximum of the giant dipole resonance (E≈11 MeV). By comparing the energy dependences obtained for the fissilities of the ²⁴³Cm and ²⁴⁹Bk isotopes from photofission data with the fissilities from direct-reaction data, it is found that the observed fission thresholds agree and that there is a plateau-like dependence at energies above 7.5 MeV. For the ²⁴⁵Cm and ²⁴⁹Cf nuclei, there are no similar data for performing such a comparison. Data on the fissilities as obtained from the present series of relative measurements that employ the microtron bremsstrahlung spectrum are analyzed.     

Angular distributions and anisotropy of the fragments from neutron-induced fission of <Superscript>233</Superscript>U and <Superscript>209</Superscript>Bi in the intermediate energy range of 1–200 MeV

New results of the neutron-induced fission experiments carried out at the neutron time-of-flight spectrometer GNEIS of the PNPI are given. Angular distributions of fission fragments from the neutron-induced fission of ²³³U and ²⁰⁹Bi nuclei have been measured in the energy range 1–200 MeV using position sensitive multiwire proportional counters as fission fragment detector. The recent improvements of the measurement and data processing procedures are described. The data on anisotropy of fission fragments deduced from the measured angular distributions are presented in comparison with the experimental data of other authors.     

Fragment dependence of high energy γ-ray emission in the spontaneous fission of 252Cf

The high energy γ-ray emission accompanying the spontaneous fission of ²⁵²Cf has been measured in coincidence with individual fission fragments selected by discrete γ-ray transitions. The enhancement of the γ-ray emission probability in the energy range E_γ= 3–8 MeV has been observed for the fission fragments in the region of nearly symmetric mass splitting, confirming results reported in previous investigations. The γ-γ coincidence technique employed in the present work clearly demonstrate that the major contribution to this enhancement is caused by the fission channels where one fragment is near to the N= 82 or Z= 50 shell closures. The high energy γ-ray emission probability does not show any significant dependence on the number of neutrons emitted in the fission process, supporting the hypothesis that high energy γ-rays are mainly emitted from the fragments after the neutron evaporation. Received: 22 December 1998     

Cold valleys in the radioactive decay of <Superscript>248–254</Superscript>Cf isotopes

Based on the concept of cold valley in cold fission and fusion, we have investigated the cluster decay process in ^248–254Cf isotopes. In addition to alpha particle minima, other deep minima occur for S, Ar and Ca clusters. It is found that inclusion of proximity potential does not change the position of minima but minima become deeper. Taking Coulomb and proximity potential as interacting barrier for post-scission region, we computed half-lives and other characteristics for various clusters from these parents. Our study reveals that these parents are stable against light clusters and unstable against heavy clusters. Computed half-lives for alpha decay agree with experimental values within two orders of magnitude. The most probable clusters from these parents are predicted to be ⁴⁶Ar, ^48,50Ca which indicate the role of doubly or near doubly magic clusters in cluster radioactivity. Odd A clusters are found to be favorable for emission from odd A parents. Cluster decay model is extended to symmetric region and it is found that symmetric fission is also probable which stresses the role of doubly or near doubly magic ¹³²Sn nuclei. Geiger-Nuttal plots were studied for various clusters and are found to be linear with varying slopes and intercepts.      

The evaluation of the branching ratios of the decays φ(1020)→2π<Superscript>+</Superscript>2π<Superscript>−</Superscript>π<Superscript>0</Superscript>, π<Superscript>+</Superscript>π<Superscript>−</Superscript>3π<Superscript>0</Superscript>

Combining the Okubo-Zweig-Iizuka rule in the decay φ→ρπ→π⁺π^?π⁰ with the ρ→4π decay amplitudes, we calculate the φ→2π⁺2π^?π⁰ and φ→π⁺π^?3π⁰ ones. The partial widths of the above φ decays are evaluated, and the excitation curves in e⁺e^? annihilation are obtained, assuming reasonable particular relations among the parameters characterizing the anomalous terms of the HLS Lagrangian. The evaluated branching ratios B_φ→π⁺_π^?_3π⁰ ≈ 2 × 10^?7 and B_φ→2π⁺_2π^?_π⁰ ≈ 7 × 10^?7 are such that, with the luminosity L=500 pb^?1 attained at DAΦNE φ factory, one may already possess about 1685 events of the decays φ→5π.     

Emission spectroscopy of the A<Superscript>1</Superscript>Π–X<Superscript>1</Superscript>Σ<Superscript>+</Superscript> system of AlH

The high-resolution emission spectrum of the A¹ Π–X¹Σ⁺ transition of AlH was observed in the 18 000–25 000 cm^-1 spectral region using a conventional spectroscopic technique. The AlH molecules were excited in an Al hollow-cathode lamp filled with a mixture of Ne carried gas and a trace amount of NH₃. The emission from the discharge was observed with a plane grating spectrograph and recorded by a photomultiplier tube. In total 163 transition wave numbers belonging to six bands (0-0,1 and 1-0,1,2,3) were precisely measured and rotationally analysed. In the final fit the present data have been combined with available high-resolution measurements of the vibration-rotation bands by White et al. [J. Chem. Phys. 99, 8371 (1993)]. This procedure enabled extracting molecular constants for the A¹ Π and X¹ Σ⁺ states of AlH. A very slight local perturbation has been discovered in the v=1 vibration level of the A¹ Π state at J=5. This was probably caused by the interaction with the a³Π state.     

Experimental Determination of the Effective Resonance Absorption Integrals of <Superscript>238</Superscript>U and <Superscript>158</Superscript>Gd in Urania–Gadolinia Rods

The experimental procedure and the results of determination of the effective resonance neutron absorption integrals of ²³⁸U and ¹⁵⁸Gd for isolated UO₂–Gd₂O₃ rods are presented. Measurements were performed using the activation method. UO₂–Gd₂O₃ rods were irradiated in the neutron field with the Fermi spectrum formed at the center of the core of the F-1 research reactor at the National Research Centre Kurchatov Institute.     

A new determination of the γπ → ππ anomalous amplitude via πe<Superscript>-</Superscript> → π<Superscript>-</Superscript>e<Superscript>-</Superscript>π<Superscript>0</Superscript> data

We discuss the reaction π^-e^- → π^-e^-π⁰ with the purpose of obtaining information on the γπ → ππ anomalous amplitude _3π. We compare a full calculation at (p⁶) in chiral perturbation theory and various phenomenological predictions with the existing data of Amendolia et al. By integrating our theory results using Monte Carlo techniques, we obtain σ = 2.05 nb at (p⁶) and σ = 2.17 nb after including the dominant electromagnetic correction. Both results are in good agreement with the experimental cross-section of σ = (2.11±0.47) nb. On the basis of the ChPT results one would extract from the experimental cross-section as amplitudes _3π^(0)extr = (9.9±1.1) GeV^-3 and _3π^(0)extr = (9.6±1.1) GeV^-3, respectively, which have to be compared with the low-energy theorem _3π = e/(4π²F_π³) = 9.72GeV^-3. We emphasize the need for new data to allow for a comparison of experimental and theoretical distributions and to obtain _3π with smaller uncertainty.     

Two-neutrino positron double-beta decay of <Superscript>106</Superscript>Cd for the 0<Superscript>+</Superscript> → 0<Superscript>+</Superscript> transition

The two-neutrino positron double-beta decay of ¹⁰⁶Cd for the 0 ⁺ 0⁺ transition has been studied in the Hartree-Fock-Bogoliubov model in conjunction with the summation method. In the first step, the reliability of the intrinsic wave functions of ¹⁰⁶Cd and ¹⁰⁶Pd nuclei has been tested by comparing the theoretically calculated results for yrast spectra, reduced B(E2: 0⁺ 2⁺) transition probabilities, quadrupole moments Q(2⁺) and gyromagnetic factors g(2⁺) with the available experimental data. In the second step, the nuclear transition matrix element M₂ and the half-life T_1/2² for the 0 ⁺ 0⁺ transition have been calculated with these wave functions. Moreover, we have studied the effect of deformation on the nuclear transition matrix element M₂.     

True ternary fission of <Superscript>236</Superscript>U: A sequential mechanism?

A mechanism is proposed and calculations are performed to explain the true ternary fission of ²³⁶U nuclei experimentally observed via the missing mass method. The yield of the first excited fragment ⁷²Ni after the emission of neutrons corresponds to the most probable ⁶⁸Ni yield obtained in the experiment. The good agreement between the theoretical results and the experimental data means that the assumption of the sequential mechanism for true ternary fission is reasonable and that shell effects should be considered in the theoretical model.     

Perturbative analysis of the 2νββ decays of <Superscript>100</Superscript>Mo and <Superscript>116</Superscript>Cd

We have performed a theoretical analysis of the ground-state-to-ground-state transitions in ¹⁰⁰Mo and ¹¹⁶Cd, based on the quasiparticle random-phase approximation and on a straightforward perturbative scheme. The results show that the single-state dominance found in the realistic calculations of the nuclear matrix elements, which is consistent with data, can be viewed as a result of the interference between few two-quasiparticle configurations. Received: 12 August 2002 / Accepted: 23 October 2002 / Published online: 18 February 2003 RID="a" ID="a"e-mail: civitarese@fisica.unlp.edu.ar Communicated by V.V. Anisovich     

Experimental investigation of ρ<Superscript>0</Superscript> photoproduction on the pion in the H1 experiment

Experimental results on quasielastic photoproduction of the ρ⁰ meson in association with a neutron, obtained at the HERA collider, are presented. The total and differential cross sections of the γp → ρ⁰ nπ⁺ reaction at the positron–proton center-of-mass energy of √s =319 GeV are measured. The data collected with the H1 detector in 2006 and 2007 correspond to an integrated luminosity of 1.16 pb^?1. The kinematic region of the photon–proton cms energy of 20 < W _γp <100 GeV, photon virtuality of Q ² < 2 GeV², and the ρ⁰ transverse momentum below 1 GeV/c is analyzed. Secondary neutrons with energies x _L > 0.35 (in proton-energy units) and emission angles below 0.75 mrad are selected. The model of double peripheral exchange, in which the ρ⁰ is elastically produced via the photon interaction with the virtual pion from the proton–neutron vertex, is employed for interpreting the results. The cross section for the ρ⁰ elastic photoproduction on the pion, γπ⁺→ ρ⁰π⁺, is extracted in the one-pion-exchange approximation. The magnitude of the cross section suggests that the γp → ρ⁰ nπ⁺ reaction is significantly affected by absorption.     

On the energy of the 8<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> 1479-keV level in <Superscript>178</Superscript>Hf

The difference in the 332-and 326-keV transition energies from ¹⁷⁸Ta decay is measured with a magnetic β spectrometer and a γ spectrometer. The energy of the 332-keV transition (E(332) = 331 607 ± 4 eV) and the energy of the 8 ₂ ^? 1479-keV level in ¹⁷⁸Hf (E(1479) = 1479001 ± 6 eV) are determined with high accuracy.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

Accurate characterisation of the C(3)<Superscript>1</Superscript>Σ<Superscript>+</Superscript> state of the NaRb molecule

We present a first detailed experimental study of the C(3)¹Σ⁺ state of the NaRb molecule converging to the Na(3p) + Rb(5s) states of separated atoms. Two different high resolution spectroscopic methods have been applied: the Fourier transform spectroscopy of laser induced fluorescence and the V-type optical-optical double resonance polarization labeling spectroscopy. The entire data field for the C¹Σ⁺ state of Na⁸⁵Rb and Na⁸⁷Rb consists of rovibrational levels with v'=0–64 and J'=4–123. The data were incorporated into a direct fit of a single potential energy curve to the level energies using the Inverted Perturbation Approach method. As the experimental data extend to the “shelf" region of the potential at large internuclear separations of 8.5 ?, the C state's suitability for photoassociation yielding cold heteronuclear NaRb is discussed. Electronic supplementary material Online Material