Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

Measurement of the ββ-decay rate of 100Mo to the first excited 0+ state in 100Ru

A new and independent confirmation of ββ(2ν) decay of ¹⁰⁰Mo to the first 0⁺ excited state in ¹⁰⁰Ru has been made. This was accomplished using a two-photon coincidence counting technique in which two HPGe detectors were used to observe the two emitted γrays (E _γ1 = 590.8 keV and E _γ2 = 539.6 keV) from the daughter nucleus as it deexcites to the ground state (0⁺ → 2⁺ → 0⁺). The half-life of this decay has been estimated as ～(5–8)×10²⁰ yr.     

Mössbauer study of the electromagnetic properties of the 69.6-keV level in Os 189

The 69.6-keV level with spin 5/2^? in¹⁸⁹Os was investigated by the recoilless resonance absorption technique. Its magnetic moment was found to beμ (5/2)=(0.977±0.014)μ _N. From the measured ratio of the excited and ground-state electric quadrupole moments,Q(5/2)/Q(3/2)=?0.79±0.09, the excited-state quadrupole momentQ(5/2)=?(0.72±0.11)b could be derived. TheE2/M1 mixing parameter of the 69.6-keV ground-state transition was found to beδ=+0.69±0.10. The results are interpreted in terms of two low-energy rotational bands with strong Coriolis mixing.     

Half-lives of levels in99Nb and99Mo

The half-lives of the 469-keV level in⁹⁹Nb and the 236-keV state in⁹⁹Mo have been determined to (0.21±0.06) ns and (0.87±0.15) ns, respectively, fromβ,γ-delayed coincidences. Values ofB(E2)=(89±28)e ² fm⁴ andB(M1)=(0.017±0.003) μ _n ² are deduced for the 469-keV transition in⁹⁹Nb and the 138-keV transition in99Mo. The result for theE2-transition probability seems to be in accordance with the particle-core coupling interpretation of the low-lying levels in⁹⁸Nb if⁹⁸Zr is used as the core. TheM1 probability for⁹⁹Mo agrees with the systematics for “l-forbidden” 1g_7/2→2d _5/2 neutron transitions.     

Anomalies of neutron and γ-ray emission in light charged particle accompanied fission of 252Cf

Correlations between the neutron and γ-ray emission and the kinetic energy of light charged particles (LCPs), such as protons, deuterons, tritons and alphas, from the fission of ²⁵²Cf were studied in a four parameter experiment. A ⊘ 16 cm × 5 cm liquid scintillator with n-γ discrimination and a ⊘ 10 cm × 10 cm NaI(Tl) crystal were employed to detect the neutrons and the γ rays, respectively, while a thin CsI(Tl) crystal, which had the ability to separate the LCPs, was used to determine the LCP energy. The experimental results show that for the α particle accompanied fission, the average total number of neutrons emitted per fission increases in the case of varying the alpha particle energy E_α from 7 to 11 MeV and then falls off linearly in a first approximation with increasing E_α, and the average total energy and the average total number of γ-rays per fission as well as the average γ-ray energy as functions of E_α show similar correlation features. For the hydrogen-ion accompanied fission, the correlations of the neutron and γ-ray emission with hydrogen ion (t, d and p) energy are very similar to those for the alpha-particle accompanied fission. The correlations of the neutron and γ-ray emission in the low LCP energy region, in contrast to that observed in the high LCP energy range, are called anomalies. On the basis of the results obtained by the three-point charge model and the liquid drop model calculation with shell and pairing correction, the anomalous behavior of the neutron and γ-ray emission is explained tentatively.     

Excitation of ground and gamma bands in the 24Mg(α, α')24Mg reaction at 120 MeV

The cross sections for excitation of the members of the ground state (g.s.) band 0^?(g.s.), 2⁺ (1.37 MeV) and 4⁺ (4.12 MeV) and the γ-band 2⁺_γ (4.24 MeV), 3_γ⁺(5.24 MeV) and 4_γ⁺(6.01 MeV) in ²⁴Mg have been measured in inelastic α-scattering at E_α = 120 MeV. The excitation of these states are found to be well described by a coupled-channel calculation (CCBA) performed in the framework of the asymmetric rotational model. Two sets of parameters are found to give excellent fits to the data, but in both a direct coupling between the ground state and the 4⁺ state is found necessary.     

ββ(2ν) decay of 100Mo to the first excited 0+ state of 100Ru

The rate of the double-beta ββ(2ν) decay of ¹⁰⁰Mo to the first 0⁺ excited state in ¹⁰⁰Ru is measured by a γγ-coincidence technique in which two HPGe detectors are used to detect the two γ rays (E _γ1=590.76 keV and E _γ2=539.53 keV) from the daughter nucleus ¹⁰⁰Ru as it deexcites to the ground state via the transition sequence 0⁺ → 2⁺ → 0⁺. In contrast to all previous ββ-decay experiments, this technique provides data that are essentially background-free. By using a 1.05-kg isotopically enriched (98.4%) disk of ¹⁰⁰Mo, 22 coincidence events (with the background estimated at 2.5 events) are detected in 440 d of measuring time, which translates into a half-life time of [5.9 _?1.1 ^+1.7 (stat.)±0.6(syst.)]×10²⁰ yr.     

Conversion coefficients and E0 transitions in102, 104, 106Pd

In in-beam (p, p′) experiments, electron and γ-spectra were measured in the electron energy range of 500-1840 keV for¹⁰²Pd and¹⁰⁴Pd, and 600–1580 keV for¹⁰⁶Pd. The conversion coefficients of all transitions in this range were obtained with accuracies of about 20%, in some favourable cases 10%. Special attention was given to 0⁺′-0⁺ transitions from the two-phonon triplets to the ground states with the following results for the branching ratios 0⁺′-0⁺ (ground state) to 0⁺′-2⁺ (one-phonon state):¹⁰²Pd:T _k (E0)/T _γ (E2)<(2.1±3.6)·10^{?7 104}Pd:T _k (E0)/T _γ (E2)=(6.0±1.4)·10^{?5 106}Pd:T _k (E0)/T _γ (E2)=(6.0±2.0)·10^?4     

Angular distributions of prompt γ-rays in the binary and the light charged particle accompanied fission modes of252Cf

Double-differential emission probabilitiesW _γ(Θ _{γ, LF} E _γ) in angleΘ _{γ, LF} and energyE _γ for promptγ-rays have been measured in the spontaneous fission of²⁵²Cf. The detector system allowed theγ-deexcitation in both the binary and the light charged particle accompanied fission modes to be studied under equal conditions. Previous results for the binary fission mode have been confirmed. For the first time anisotropicγ-angular distributions were proved for the light charged particle accompanied fission mode. Theγ-anisotropies are rather similar for both fission modes but the shapes of the angular distributions differ considerably. Theγ-deexcitation of the fission fragments in the yrast region was described on the basis of the VMI model whereas reasonable assumptions were made for statisticalγ-emission. A simulation procedure starts from this conception and it was possible to reproduceγ-ray spectra and the correspondingγ-angular distributions. The observed energy-dependent shift of the maximum in the light charged particle accompanied fission mode may be understood as an influence of the equatorial alpha particles on the alignment of angular momentum produced in bending modes.     

g-Faktoren und Multipol-Mischungsverhältnisse in156Gd

By integral perturbedγ-γ-angular correlation the gyromagnetic ratios of a 2-quasi-particle state at 1,513 keV and the 4⁺ state of the ground state rotational band in¹⁵⁶Gd were measured, using sources prepared by¹⁵⁹Tb(γ, 3n) and¹⁵⁶Gd(d, 2n) reactions:g _{1;513 keV} ⁴⁺ =0.78±0.05;g _R ⁴⁺ =0.37±0.05. In addition we obtain the multipole mixing ratiosδ _{535 keV}(M2/E1)=?0.09±0.01;δ _{1,225 keV}(E2/M1)=+1.83±0.10 for the 535 keV and 1,225 keVγ-rays. The classification of the 1,513 keV state is discussed. The internal field of Gd in Tb metal at 77 °K was found to beH=(?192±16)kOe.     

Characteristics of d + α Bound and Resonant States from Analytic Continuation of the Effective-Range Expansion

In the present work one- and two-channel approaches using the expansion of the effective-range function K(E) in powers of energy E are applied to obtain the asymptotic normalization coefficients (ANCs) and vertex constants (VCs) for the d + α system. The coefficients of the K(E) expansion are found by fitting the d α phase shifts. By analytic continuation of K(E) thus obtained to the corresponding poles the ANCs and VCs for D wave d α resonances with J ^π = 1⁺, 2⁺, 3⁺ are calculated for the first time. The 1⁺-resonance and the bound state of ⁶Li (J ^π = 1⁺) are considered jointly in the two-channel (S + D) effective-range approach and the ANCs and VCs for the ground ⁶Li state are found.     

Kernphotoeffekt an Aluminium und Fluor mit der Lithium-Resonanz-Strahlung

Fragments from thermal fission of U²³⁵ have been slowed down in purified Ar and N₂. Ionization by reactor radiation produced sufficient free electrons in the gas to permit formation of negative fission product ions by electron attachment. These negative ions were collected in an ionization chamber. Theγ-radiation of the activated electrode showed that mainly J and Te had been collected. By analysis of theγ-spectra the independent fission yieldsη(J¹³⁵)=(4,6±1,1)% andη(Te¹³⁵)=(1,5±1,1)% were found. In the derivation of this result an estimated valueη(Te¹³⁴)=(6±0,7)% was used.     

Systematic study of the product((E(2_2~+)/E(2_1~+)) * B(E2)↑) through the asymmetric rotor model

A systematic study of the product((E(2_2~+)/E(2_1~+))*B(E2)↑) is carried out in the major shell space Z=50-82,N=82-126 within the framework of the asymmetric rotor model where the asymmetry parameter γ0reflects change in the nuclear structure.A systematic study of the product((E(2+)/E(2+))*B(E2)↑) with neutron number N is also discussed.The product((E(2_2~+)/E(2_1~+))*B(E2)↑) provides a direct correlation with the asymmetry parameter γ0.The effect of subshells is visible in Ba-Gd nuclei with N 82,but not in Hf-Pt nuclei with N 104.We study,for the first time,the dependency of the product((E(2_2~+)/E(2_1~+))*B(E2)↑) on the asymmetry parameterγ0.     

Spectroscopy of excited states in212Po,210Pb,and213At employing18O induced few-nucleon transfer reactions

γ-ray spectroscopic techniques have been applied to measure properties of excited states (E ^*<2MeV) in²¹⁰Pb,²¹²Po, and²¹³At after populating these neutron rich (N=128) nuclei via¹⁸O induced few-nucleon transfer reactions on²⁰⁸Pb and²⁰⁹Bi targets. In²¹²Po an isomeric state is located atE ^*=1,477 keV with a halflife oft _1/2=14.7±0.3 ns. This state is interpreted to be the 8⁺ yrast level which decays to the ground state via the measured $$8^ + \left( {121.1 keV} \right)6^ + \left( {223.3 keV} \right)4^ + \left( {405.1 keV} \right)2^ + \left( {727.8 keV} \right)0^ + $$ γ-cascade.α-decay (E _α≈10.2 MeV) of the isomer is also observed. In²¹⁰Pb the time spectra for members of the 4⁺ (297.8keV) 2⁺ (799.4keV)0⁺ γ-cascade show a delayed component with a halflife oft _1/2=152±13 ns which is attributed to the known 8⁺ yrast state atE ^*=1.27 MeV. For²¹³At first results on theγ-decay of excited states are presented.     

Odd neutron nuclei near A=100: Rotational bands in103Mo and105Mo populated in theβ − decays of103Nb and105Nb

Theβ ^? decays of¹⁰³Nb and¹⁰⁵Nb have been studied at the fission product separators JOSEF and LOHENGRIN. Half-lives of (1.5±0.2) s and (2.95±0.06) s, respectively, have been determined for these decays. Fromγ singles andγ-γ coincidence measurements extended level schemes for¹⁰³Mo and¹⁰⁵Mo have been established for the first time. The lowest energy levels of these nuclei are consistent with the interpretation as members of rotational bands built on a 3/2⁺ [411] Nilsson state. Evidence is presented for the location of the 9/2⁺ [404] configuration.     

A 150 ms 10+ Isomer in146Dy

In the course of systematic studies of very proton rich nuclei in theN≈82 region with Ni induced compound reactions, an unusually long-lived high spin isomer has been found. After in-and off-beamγ and conversion electron measurements, asγ excitation functions,γ andeγ coincidences, pulsed beam techniques and multi spectrum analyses of the residual activities, this isomer has been tentatively assigned to be a 10⁺ state in¹⁴⁶Dy, which decays into two 7^? states byE3 transitions. The half-life of the isomer has been measured to be 150±20ms. The isomer has been found to follow the β decay of the previously unknown 3.9 s isotope¹⁴⁶Ho. The mechanism of the appearance of such an isomer is discussed.     

Electric monopole component in the 2+' → 2+ transition in 192Pt

Angular correlation measurements of K- and L-conversion electrons following the decay ¹⁹²Ir → ¹⁹²Pt have been made using a spectrometer with Ge(Li) and Si (Li) detectors. Absolute and relative internal conversion coefficients of transitions in ¹⁹²Pt were measured, using an ICC and prism β-spectrometers, to ≈ 2 % and values of δ_γ determined from the relative ICC. The experimental values measured in the study, A₂₂(K296γ316) = 0.138 ± 0.010, A₂₂(L296γ316) = 0.125 ± 0.013, α_K(296) = 0.0711 ± 0.0011, K/L_III = 9.65 ± 0.13. With δ_γ = + 5.4 ± 0.2, were employed for determining the E0/E2 amplitude ratios of conversion transitions, q(E0/E2), and the penetration parameter λ of the Ml component for the 2⁺' → 2⁺ (296 keV) transition in ¹⁹²Pt. The angular correlation measurements of L-conversion electrons enabled the elimination of one of two ranges of values of q and λ usually obtained. For the first time in our work, analysis of the e_Lγ angular correlation for the determination of the E0 component in the transition was carried out. As a result, q = +0.04 ± 0.05 with λ = ?4.5 ± 3.5 was obtained for the 296 keV transition. In this case, _ρ(E0) = 0.004 ± 0.005 agrees with _ρ(E0) = +0.006 determined theoretically by Kumar and Baranger for the 2⁺' → 2⁺ transition     

Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N⁺(E) has been separated into an ionization coefficient R⁺(E) and a total energy distribution, N(E), i.e. N⁺(E) = R⁺(E) N(E). Experimentally, the dependence of R⁺(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.     

Magnetic hyperfine interaction in muonic indium,osmium and mercury

The magnetic hyperfine interaction in several muonic atoms has been measured. The magnetic splittings of the 1s _1/2, 2s _1/2 and 2p _1/2 muon states in the 9/2⁺ ground state of¹¹⁵In, determined by measuring muonic X rays areΔE _mag(1s _1/2,¹¹⁵In 9/2⁺)=3580±70 eVΔE _mag(2s _1/2,¹¹⁵In 9/2⁺)=525±120 eVΔE _mag(2p _1/2,¹¹⁵In 9/2⁺)=850±180 eV. The magnetic splitting of nuclearγ rays in the presence of a muon in the 1s _1/2 state yields for the 1/2^? ground state in¹⁹⁹HgΔE _mag(1s _1/2,¹⁹⁹Hg 1/2^?)=468±115 eV and the following values for the first excited 2⁺ states in the nuclei^190,192Os and²⁰⁰Hg:ΔE _mag(1s _1/2,¹⁹⁰Os 2⁺)=665± ₄₀ ⁸⁰ eVΔE _mag(1s _1/2,¹⁹²Os 2⁺)=800±80 eVΔE _mag(1s _1/2,²⁰⁰Hg 2⁺)=655± ₇₅ ¹⁰⁵ eV. These data are compared with calculations using different nuclear models.     

Estimate of the probability of K+→μ+νμγ decay

About 100 000 K⁺-decays were observed in a magnetic spectrometer with streamer chambers. The estimation for the radiative decay K⁺→μ⁺ν_μγ, R^exp = [W(K⁺→μ⁺ν_μγ)]/[W(K⁺→all)]=(5.8±3.5) x 10^?3, at E_γ ? 9 MeV was obtained and the ratio R = [W(K⁺→π⁺π°)]/[W(K⁺→μ⁺ν_μ)] = 0.328 ± 0.005 was measured.