Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesP _n of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. TheP _n-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of²³⁵U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.     

States of167Ho from the decay of neutron rich nuclide167Dy

Theβ ^?-decay of ₆₆ ¹⁶⁷ Dy produced through the fast neutron reaction¹⁷⁰Er(n, α)¹⁶⁷Dy has been investigated by using several kinds of detectors and a high-capacity two-parameter recording system. The half-life andβ ^?-decay energy of¹⁶⁷Dy were determined to beT _1/2 = 6.20 ± 0.08min andQ _β-=2.35±0.06, respectively. The observed level scheme of ₆₇ ¹⁶⁷ Ho (completely unknown previously) contains 12 states, among them a 6.0±0.1 μsM2 isomer at 259.3 keV. On the basis of theoretical and systematic considerations combined with multipole determinations, the following Nilsson model assignments are proposed for the lowest states of¹⁶⁷Ho: 0 keV (7?/2 [523]), 259.3 keV (3+/2[411]), 319.8 keV (5/2 3+/2[411]), 392.5 keV (1+/2[411]), 410.0 keV (3/2 1+/2[411]), 569.7 keV (3?/2{7?/2[523], 2⁺}). Theβ-decay proceeds mainly to the proposed gamma-vibrational state at 569.7 keV with an anomalously low logft value 5.4, indicating similarity between the microscopic structures of this state and the famous ¦K ₀?2¦ gamma vibration of¹⁶⁵Ho.     

Fragmentation of the high spin discrete neutron and proton states of209Pb and209Bi

The distribution of the discrete 1h _9/2, 2f _7/2, 1i _13/2 and 2f _5/2 proton states of²⁰⁹Bi and 2g _9/2, 1i _11/2, 1j _15/2 and 2g_7/2 neutron states of²⁰⁹Pb have been obtained within the particle-vibration coupling model calculation and compared with the experimental datas baising on the most recent high resolution stripping reaction on²⁰⁸Pb using 480 MeV¹²C projectile. The optimised shell model energies arising from the core-polarisation effect have profound influence for both the study of the structures of the high spin continuum shell model states of²⁰⁸Pb and stability of superheavy nuclei.     

Calculation of resonance reduced neutron widths by semiclassical description

Peripheral interactions of¹²C nuclei with protons and carbon nuclei have been investigated in a 2m propane bubble chamber at 4.2 GeV/c per nucleon. Energetic and angular characteristics ofγ-quanta accompanying the production of multicharged fragments withZ _f =5 and 6 have been analyzed. It is shown that in the fragment rest system there is an excess of γ-quanta with ε_γ<40 MeV over the background from known sources (π ⁰→2γ decays, bremsstrahlung and nuclear excitation).     

Half-lives,neutron emission probabilities and fission yields of neutron-rich rubidium isotopes in the mass regionA=96 toA=100

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of²³⁵U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms¹⁰⁰Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP _n of⁹⁹Rb and of the fission yields of⁹⁷Rb,⁹⁸Rb, and⁹⁹Rb were performed.     

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of²³²Th. The fast neutrons (?14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using³H(²H,n)⁴He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ _f, through the relationT=knσ _føt wheren is the number of fissile atoms per cm² in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ _f of²³²Th, relative to the well measured fission cross section of²³⁸U, was found to be 0.36±0.04 barn.     

Specific heat of the superfluid neutron matter in neutron star crusts

By means of X-γ andγ-γ coincidence measurements of the³⁵Cl+⁵⁸Ni reaction products, 38γ lines have been identified to be in coincidence with KX(Tc)-rays and assigned to the decay of⁹⁰Ru. Its half-life of 11±3 s has been deduced from the 154.6 keVγ-decay. The result supports our previous identification of⁹⁰Ru produced in the same reactions.     

Study of the level structure of239U using the thermal neutron capture reaction

For the investigation of vibrational states in odd A nuclei we have studied the level structure of²³⁹U by the²³⁸U (n _thermal,γ) reaction. Various complementary measurement techniques as curved-crystal-, anti-Compton-,γ-γ coincidence- and conversion electron spectroscopy have been applied. The resulting data have been used to establish the deexcitation scheme of²³⁹U up to ～1.3 MeV and to make spin assignments. Most of the levels are interpreted in terms of the Nilsson model. The data also indicate the presence of the Nilsson states ¦501?¦ and ¦750¦. Three members of theΒ-vibrational band built on the ¦631?¦ state and one member of theΒ-vibration built on the ¦622↑¦ ground state have been identified through transitions with strongE0 admixtures. The octupole vibrational state built on the ¦631?¦ band is proposed at 815 keV.     

The scission neutron spectrum of252CF (SF)

The scission neutron spectrum was obtained as a difference between the integral experimental fission neutron spectrum evaluated by Mannhart based on measurements in the laboratory reference system from several authors in the different neutron energy ranges, and the laboratory fission neutron spectrum stemming only from those neutrons which are evaporated from the fully accelerated fission fragments. The scission neutron spectrum was represented by a Weisskopf evaporation spectrump _s(? _l )=P ₀/(T _s)²·?_l·exp(??_l/T_s). A least squares fit gave for the fraction of scission neutrons with respect to all fission neutrons a value ofp ₀=(0.011±0.003) and for the pseudo scission neutron temperature a value ofT _s=(0.20±0.03) MeV. The low pseudo temperature is compatible with a cold nuclear matter of the prescission configuration. The low energy of the scission neutrons indicates that they come from a slowly moving source. The width of the distribution suggests that the source is small: radius of about 5 fm. This information conforms with the idea of satellite droplets which are formed when the neck snaps.     

Effect of quadrupole and octopole vibrations of206Pb on the 1j 15/2 neutron particle state of207Pb

The quadrupole as well as the octopole vibrational states of²⁰⁶Pb are coupled to the 2g _9/2, 1i _11/2 and 1j _15/2 neutron states to explain the fragmentation of 1j _15/2 state in²⁰⁷Pb as observed in the²⁰⁶Pb(d,p) reaction. The structures of weak fragmented 1j _15/2 states are deduced through the particle-core coupling scheme.     

Determination of the neutron binding energy of the delayed neutron emitter137Xe

The neutron binding energy of¹³⁷Xe has been deduced to be 4025.2±0.6 keV from a study of the¹³⁶Xe(n, γ)¹³⁷Xe reaction. The importance of a precise value for this quantity is due to the fact that an accurate determination of binding energies of delayed neutron emitters is possible only for⁸⁷Kr and¹³⁷Xe, neighbouring stable isotopes. Nuclear reaction.¹³⁶Xe(n,γ), enriched target; measurede89-01, deduced neutron binding energy, Ge(Li) detector.     

Levels in154Eu populated by thermal neutron capture

The level scheme of¹⁵⁴Eu has been investigated by means of thermal neutron captureγ-rays and conversion electrons. The high energyγ-ray spectrum from thermal neutron capture in enriched¹⁵³Eu has been studied in the energy range of 5700 to 6500 keV. Low energyγ radiation has been observed with Ge(Li) and Si(Li) detectors from 5 to 300 keV and conversion electrons have been measured from 15 to 300 keV. Low energy (n, γγ) coincidences and half lives of transitions have been measured. The data, combined with three very recent studies of the 8^? isomer decay in¹⁵⁴Eu has led to the construction of a level scheme with 12 excited levels. Nuclear Reaction¹⁵³Eu(n, γ),E _n =thermal, measuredE _γ ,I _γ ,E _ce ,I _ce ,γγ-coincidence, halflives,¹⁵⁴Eu deduced levels.     

Spectrum of the high spin neutron hole states of205Pb

The high resolution (³He,α) reaction on²⁰⁶Pb shows the distribution of the 2f _7/2, 1h _9/2 and 1i _{1 3/2} neutron states of²⁰⁵Pb within the 6 MeV excitation energy of²⁰⁵Pb. The spectrum of these three-hole states is obtained within the hole-core vibrational coupling scheme. The shell model energies of the neutron hole states arising from the core-polarization effect are compared with the Bansal-French energy weighted sum rule. The possible implication of the present neutron hole energies has been discussed in the light of the deduced shell model wave functions of the collective states of²⁰⁶Pb.     

Cross sections and γ-rays from multiple neutron capture in151Eu

Primary capture gamma rays following multiple neutron capture in¹⁵¹Eu have been measured as a function of the irradiation time. Neutron capture cross sections of the¹⁵²Eu 3^? groundstate and of the¹⁵²Eu 0^? isomer (T _1/2=9.3h) were determined. These cross sections are relevant for the interpretation of inelastic scattering of neutrons at isomeric states with energy gain (neutron acceleration). The level scheme of¹⁵³Eu has been extended. Neutron binding energies of¹⁵²Eu,¹⁵³Eu,¹⁵³Gd and¹⁵⁴Gd are given.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

The continuum shell-model neutron states of209Pb

The neutron strength distributions of the three high-spin 1k _17/2, 2h _11/2 and 1j_13/2 states of²⁰⁹Pb have been obtained within the formalism of the core-polarisation effect where the effect of interaction of the neutron shell-model states of²⁰⁹Pb with the collective vibrational states (originating also from the giant resonances) have been taken into consideration. The theoretical results have been discussed in the light of works on 1k _17/2, 2h1_11/2 and 1j_13/2 neutron orbitals of²⁰⁹Pb. The shell-model energies of the neutron states have been obtained by Skyrme-Hartree-Fock method     

Intense mass-separated beams of halogens and beta-delayed neutron emission from heavy bromine isotopes

Improved production yields of short-lived halogens were obtained from a ThO₂ target, irradiated with 600 MeV protons, in combination with a negative surface ionization source. Mass-separated samples were studied by decay spectroscopy. Production yields of radioactive isotopes of chlorine, bromine, iodine and astatine are presented. Half-lives and relative neutron emission probabilities were measured for the heavy bromine isotopes^89?92Br. Normalizing to earlier publishedP _n values for⁸⁹Br, the results are:⁸⁹Br (4.30±0.14s,P _n =13.6±0.8%),⁹⁰Br (1.92±0.06s,P _n =24.8±1.5%),⁹¹Br (0.53 ±0.03 s,P _n =30.1 ±2.1%), and⁹²Br (0.31 ±0.02 s,P _n =34.6±2.5%). Energy spectra ofβ-delayed neutrons were measured.     

Disordering effects and property changes in fast neutron irradiated YIG

Damage region structure and property changes of YIG irradiated atD=10¹⁸?7.8×10¹⁹ n/cm² were studied. Damage regions at 300 K were found to consist of 1) a core of Fe³⁺ paramagnetic phase (PP) withgΔ=0.8 mm/s; 2) a shell of Fe³⁺ intermediate magnetic phase with heavily distorted bond geometry and <H _eff>≤300 kOe; 3) Fe³⁺ (a, d) surrounded by oxygen vacancies and interstitials. The dose dependence of PP concentration is given byC _PP=1-exp(?βD), yielding PP core radiusr _PP=12,5 Å. Magnetic ordering in PP was found to arise atT _tr=90 K. NGR probabilityf′ under irradiation was found to decrease linearly according to Δf′/f′=?C _PP(D). Net magnetization change was found, using the Gilleo model, to obey an analogous relationship ΔM(T)/M(T)=?C _PP(D).T _c dose dependence is given by ΔT _c/T _c=?0.5×C _PP(D) and can be related to lattice parameter change to yield Δa ₀/a ₀=(1.42±0.04)×10^?4×C _PP(D). External field experiments revealed a complex dependence ofK ₁ on PP concentration, elastic stress field magnitude and a with a minimum atD=10¹⁹ n/cm².