Excitation energy dependence of fragment characteristics for the photofission of 232Th

Independent and cumulative product yields were measured for the photofission of ²³²Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported ²³²Th-samples. The independent heavy fragment yields for the fission of the ²³²Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems ²³²Th, ²³⁴U and ²³⁶Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.     

Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in ^{110–114,116}Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited J ^π = 1^? states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2⁺ and 3^? states. These excitations are interpreted as the 1^? member of the quadrupole-octupole coupled quintuplet (2+?3^?). The fragmented strength observed in the odd isotopes ^111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.     

Yrast excitations in A = 126–131 Te nuclei from deep inelastic 130Te+64Ni reactions

Thick target γγ coincidence measurements for the system ¹³⁰Te + 275 MeV ⁶⁴Ni have been performed using the GASP Ge detector array at Legnaro. For the isotopic assignments of previously unknown γ-ray cascades, prompt γγ coincidences observed between Te and Ni partner products were of vital importance. The results yield much new information about excited states of moderate spins in A = 126–131 tellurium nuclei, especially about yrast excitations of the little studied odd-A isotopes ¹²⁷Te, ¹²⁹Te, and ¹³¹Te. Level systematics of tellurium nuclei are presented, and both single-particle and collective aspects of the level spectra are discussed.     

Excitation of 3−1, 5−1 and 7−1 states in Te isotopes A = 120, 122; 124, 126, 128 WITH (p,t) reactions

The reactions ^ATe(p, t)^A?2Te have been studied with even-A targets using 51.9 MeV protons. Three or more strongly excited triton peaks were observed in the spectra of the ^A-2Te nuclei at energies of ≈ 2–3 MeV excitation. Angular distributions are analyzed using DWBA theory. The lowest octupole (3^?₁) states of five Te isotopes are strongly excited. The lowest 3^? state in ¹²⁰Te is established at 2.09 ±0.02 MeV. The systematics of excitation energies and cross sections for the lowest 5^? and 7^? states are interpreted by a quasiparticle model.     

High-resolution investigation of the 112Sn(p,p')112Sn reaction

The ¹¹²Sn(p, p') reaction was studied at proton energies of 20.51 and 25.0 MeV. The outgoing protons were momentum analysed with an Enge split-pole spectrograph and recorded with position-sensitive solid-state detectors with a total resolution between 10 and 15 keV. Excitation energies and angular distributions for states below 5.5 MeV excitation energy were obtained. The angular distributions were compared with macroscopic DWBA calculations in order to extract L-values and deformation parameters. Coupled-channels calculations were performed to investigate two-phonon excitations and the mixing between one- and two-phonon states. The results of the present experiment are compared with previous experimental results and with number-projected BCS calculations. The results for some of the excited states were compared with similar results for the other even Sn isotopes.     

A theoretical description of energy spectra and two-neutron separation energies for neutron-rich zirconium isotopes

Very recently the atomic masses of neutron-rich Zr isotopes, from ⁹⁶Zr to ¹⁰⁴Zr, have been measured with high precision. Using a schematic Interacting Boson Model (IBM) Hamiltonian, the evolution from spherical to deformed shapes along the chain of Zr isotopes, describing at the same time the excitation energies as well as the two-neutron separation energies, can be rather well reproduced. The interplay between phase transitions and configuration mixing of intruder excitations in this mass region is succinctly addressed.     

Shell-model calculations for the properties of nuclei with A = 86–100 near the proton drip line

Extensive shell-model calculations in the mass range A = 86–100 on the neutron-deficient side of the nuclear chart are performed. The complete spectra of all isotopes are computed and compared to experimental excitation energies where available. Beta decay half lives are evaluated with both free and effective Gamow-Teller operators. The binding energies are obtained using a five parameter fit in addition to the shell-model energies. From the binding energies we deduce the proton drip line.     

Three-nucleon yrast states in145Sm

In-beamγ-ray and conversion electron measurements with (α, xn) reactions have established the¹⁴⁵Sm highspin states up toI ^π=25/2⁺ at 3.5 MeV excitation. A shell model analysis using empirical two- and one-body energies from neighbouring nuclei classifies the low-lying odd-parity levels as 3-quasiparticle states formed by the¹⁴⁴Sm two-proton-hole excitations and thef _7/2 valence neutron. The higher-lying positive-parity states involve particle-hole core excitations with one proton inh _11/2.     

Excitation of giant resonances in the Ca isotopes in (π±, π0) reactions

The excitation of the giant isovector dipole and monopole resonances in the even-A Ca isotopes in pion single-charge-exchange reactions is studied theoretically. Transition densities obtained from a sum-rule approach and from a microscopic charge-exchange RPA are employed in DWIA calculations. The relation of the (π^±, π⁰) cross sections to proton and neutron densities is discussed.     

Compound nucleus contribution and even-odd effect for (3He,p) reactions in the Ni region

The compound nucleus contributions to the proton spectra from 8 MeV and 10 MeV ³He induced (³He, p) reactions on even-A Ni isotopes were obtained. The relative cross sections for ⁵⁸Ni/⁶⁰Ni/⁶²Ni in the high excitation region are in fair agreement with predictions of statistical theory, but the absolute cross sections in the same region are smaller than the prediction by a factor of 3 to 8, and the shapes of the measured spectra for heavier isotopes do not agree with the prediction. These discrepancies between experiment and theory are in sharp contrast to the situation in (p, p′), (p, α), (α, p) and (α, α′), where good agreement was found.The proton spectra from (³He, p) reactions on nuclei in the A = 54–68 mass range have a systematic difference in slope between even-A targets and odd-A targets; it is similar to the systematic difference found previously in (p, p′) and (α, p) reactions, but none of these is readily explainable by theory.     

Proton induced fission of <Superscript>232</Superscript>Th at intermediate energies

The mass-energy distributions and cross sections of proton-induced fission of ²³²Th have been measured at the proton energies of 7, 10, 13, 20, 40, and 55 MeV. Experiments were carried out at the proton beam of the K-130 cyclotron of the JYFL Accelerator Laboratory of the University of Jyväskylä and U-150m cyclotron of the Institute of Nuclear Physics, Ministry of Energy of the Republic of Kazakhstan. The yields of fission fragments in the mass range A = 60–170 a.m.u. have been measured up to the level of 10?4%. The three humped shape of the mass distribution up has been observed at higher proton energies. The contribution of the symmetric component grows up with increasing proton incident energy; although even at 55 MeV of proton energy the shoulders in the mass energy distribution clearly indicate the asymmetric fission peaks. Evolution of shell structure was observed in the fission fragment mass distributions even at high excitation energy.     

Shell-model calculations for the zinc isotopes

Shell-model calculations for the zinc isotopes have been carried out with active particles distributed in the $\text{1}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{, 0}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{and}\text{1}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbits outside a closed “⁵⁶Ni” core. The effective Hamiltonian used was one obtained by Koops and Glaudemans from a fit to Ni and Cu level energies. An average absolute deviation of 0.19 MeV between the calculated and experimental ground-state binding energies is obtained for the A = 62?68 Zn isotopes. Good agreement is also found between most calculated and experimental excitation energies and spectroscopic factors for single-nucleon transfer for the low-lying levels in these nuclei. Experimentally known B(E2) values are generally well reproduced by the present model with effective charges of 1.0 ± 0.1 and 1.6 ± 0.2 for the neutron and proton, respectively. Magnetic dipole as well as Gamow-Teller transitions are not well accounted for by these calculations and seem to be sensitive to excitations of the ⁵⁶Ni core.     

High-spin states in 208Rn

The yrast decay scheme of ²⁰⁸Rn has been investigated up to spin $\text{≈ 20}\text{h}\text{?}$ and an excitation energy of ≈ 6 MeV. Several different γ-ray spectroscopic techniques were used to determine the properties of excited states and transitions in the nucleus. Significant changes to the previously established level scheme are proposed, based on the existence of an unobserved 3.1 keV transition. Simple empirical shell-model calculations of level energies aided in the assignment of shell-model configurations to excited states and the decay scheme is discussed in terms of these configurations. The energy level systematics for the even radon isotopes, from A = 206 to 212 are discussed, as are core polarization effects in the even radon isotopes (A = 204 to 210) and polonium isotopes (A = 202–208).     

Doublet structure of the first excited state of 71As

The possible multiplet structure of the first-excited state of ⁷¹As has been investigated by the high resolution ⁷⁰Ge(³He, dγ) reaction and the γ-rays emitted in the decay of ⁷¹Se. Both studies reveal a doublet at 143.2 keV and 147.3 keV excitation energies. The existence of this doublet in ⁷¹As brings the low-lying level structure in better resemblance to the heavier odd-A arsenic isotopes.     

Structure of 10C and 11C from elastic and inelastic scattering on a proton target

We measured elastic and inelastic scattering to the low-lying states of ¹⁰C and ¹¹C isotopes on a proton target with respective incident energies 45.3 and 40.6 A MeV. Data are analyzed with a microscopic complex potential. Elastic data are sensitive to the rms matter radius, which has been deduced for both isotopes. The moment of the neutron transition density was deduced for ¹⁰C from inelastic scattering.     

Dipole excitations in 122Te, 126Te and 130Te

Excited states of the nuclei ¹²²Te, ¹²⁶Te and ¹³⁰Te were populated via the (γ, γ') reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL-CLUSTER detector and a single detector. In all nuclei several dipole transitions were identified at energies around 3 MeV. The lowest corresponding J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict one 1^? state arising from the coupling of the first quadrupole and the first octupole phonon and one 1⁺ state arising from the coupling of the first and the isovector second quadrupole phonon at about 3 MeV. The calculated transition strengths are compatible with experimental ones.     

Measurement of the proton polarization in the 3He(d,p)4He Rection between 2.0 and 6.0 MeV

Angular distributions of the polarization of ³He(d, p)⁴He protons have been measured for unpolarized deuterons with laboratory energies of 1.99, 2.81, 3.94 and 6.00 MeV. The polarizations were determined from the left-right asymmetry of elastic scattering through 60° in ⁴He gas using a vaned polarimeter whose analyzing power was computed by trajectory tracing from the known p-α polarizations. A contour diagram of proton polarization for deuteron energies of 1 to 12 MeV is presented. A Legendre polynomial expansion of four or five terms has been fitted to the products p(θ)σ(θ). The coefficients of the expansions of these and other measurements show resonance-like behaviour at ⁵Li excitations of 20.0 and 20.9 MeV. The vector-polarized beam and polarized-target analyzing powers are compared with the proton polarization. The proton polarizations are in good agreement with the ³H(d, n)⁴He neutron polarizations when compared at the same entrance-channel energy but disagree when compared at equal exit-channel energies or compound-nucleus excitations.     

Level densities in the Pairing plus Quadrupole model for Erbium isotopes

The level densities, up to about 100 MeV of excitation energy, for even Erbium isotopes are computed using the Pairing plus Quadrupole model, in the framework of the Static Path Approximation (SPA). The resulting level densities are in reasonable agreement with the empiricalA/8 law below 40 MeV of excitation energy. At higher excitation energies (U?60 MeV) the level densities agree with the Fermi gas formula witha?A/10. The inclusion of small amplitude collective motion (RPA-SPA) does not improve the results over the SPA at high excitation energy, and gives small corrections to the ground state energy.     

The triaxial rotation vibration model in the XeBa region

Collective quadrupole degrees of freedom give rise to vibrations and rotations in nuclei. The axial Rotation Vibration Model (RVM) is here extended to describe also triaxial equilibrium shapes with β and γ vibrations allowing for the interaction between vibrations and rotations. This Triaxial Rotation Vibration Model (TRVM( is applied to Ba and Xe isotopes with A ≈ 120 to 130. This are has recently been pointed out to be a region for the O(6) limit of the Interacting Boson Approximation (IBA). The present work shows that the TRVM can equally well describe these nuclei concerning their excitation energies and E2 branching ratios.