Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

The structure of25Mg

Gamma-decay modes and spin(-parity) assignments of levels in²⁵Mg have been systematically investigated up to 10 MeV excitation energy by particle-γ-ray angularcorrelation measurements with the²⁴Mg(d, pγ) reaction at 6.5 MeV bombarding energy and with the²²Ne(α,nγ) reaction at 11.8, 12.5, 14.4 and 15.5 MeV bombarding energy. A level scheme has been established which is comprehensive up to 8.3 MeV excitation energy forI≦9/2 and up to 10 MeV for 9/2O d _5/2 — 1s _1/2-O d _3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent. The first intruder states are located near 6.8 MeV excitation energy. The collective properties of²⁵Mg beyond the well established rotational bands are investigated using both the new experimental information and theB(E2)'s obtained from the shell model. The spectrum of²⁵Mg is completely rotational for the first five to six MeV above the yrast line. Shell modelB (M 1)'s reflect the Nilsson model structure of²⁵Mg in great detail. The prospectiveI ^π=9/2^?, 13/2^?, and 15/2^? members of the established negative-parity,K=1/2 band are found in levels atE _x=7801, 9410, and 8896 keV.     

DSA lifetime measurements in21Ne at high recoil velocity

States in²¹Ne up to 5 MeV excitation energy have been populated using the inverted reaction²H(²⁰Ne,pγ). The Doppler shift attenuation (DSA) analysis of thepγ coincidence spectra taken in a Ge(Li) detector at 45° and 135° and an annular silicon surface barrier detector near 0° yielded the lifetimes of 8 states in²¹Ne. Due to the large recoil ofπ _i/c～ 4% three new lifetimes were determined for the short lived levels at 2.80, 4.68 and 4.73 MeV, namely 10±4 fs, 16±4 fs and 10±4 fs, respectively. The results are compared with rotational and shell model calculations.     

Lifetime measurements in56Co with the58Ni(d, αγ)56Co reaction

The two-particle pick-up reaction⁵⁸Ni(d, αγ)⁵⁶Co atE _d =7.0 MeV was used to determine mean lifetimes of levels in⁵⁶Co up to 2.73 MeV excitation energy by the Doppler shift attenuation method,γ-rays were observed in a 43 cm³ Ge(Li)-detector at 90° to the beam axis in coincidence withα-particles detected in two 200 μm-silicon surface barrier detectors atΘ _α =±55°. Mean lifetimes for 10 levels in⁵⁶Co and two lifetime limits have been obtained usingF(τ)-curves calculated with the Winterbon theory. The experimental results are compared with the predictions of recent shell model calculations.     

Optical model and DWBA analysis of the7Li(d,d)- and7Li(d,p 0)-reactions in the energy region 1.0 to 2.6 MeV

Absolute differential cross sections for the elastic scattering of deuterons by⁷Li have been determined in the energy range of 1.0 to 2.6 MeV. Two excitation curves measured at laboratory angles of 90° and 160° vary smoothly with energy. At higher energies an extremely strong enhancement of the cross section relative to the Rutherford value was observed at large angles. It was, however, possible to find realistic optical potentials that describe these distributions fairly well over the whole energy region. For two of these potentials good fits could be obtained with DWBA calculations on the⁷Li (d, p ₀)-reaction, one of them yielding a spectroscopic factor in close agreement with shell model predictions.     

Untersuchung von40Ca mit der Reaktion39K(p, γ0)40Ca

Excitation functions of the reaction³⁹K(p, γ ₀)⁴⁰Ca at 0° and 90° have been taken in the proton energy range 2.5–6.0 MeV corresponding to an excitation of⁴⁰Ca between 10.8 and 14.1 MeV. Positions and strengths of several resonances appearing in the excitation functions are given. The structure of⁴⁰Ca is compared with predictions from 1 particle — 1 hole calculations.     

Alpha decay of new neutron deficient gold,mercury, and thallium isotopes

In fusion evaporation reactions of a⁹²Mo beam with targets of neutron deficient Rb — Mo isotopes very neutron deficient isotopes of elements between Au and Po have been produced. The new isotopes^{173, 174}Au,^{175, 176}Hg, and¹⁷⁹Tl were identified by alpha spectroscopy. The mass excess value of¹⁷⁶Hg could be linked to known values of theN?Z=16 chain. The location of the new isotopes with respect to the proton drip line is discussed. A new high energyα transition of (7.20±0.02) MeV andT _1/2=(1.4±0.5) ms has been found in the reaction⁹²Mo+⁸⁹Y→¹⁸¹Tl^* at an excitation energy of 37 MeV. It is tentatively assigned to isotopes produced in 2-particle evaporation channels.     

Isobaric spin mixing in the analog states of 90Zr

A 90° photoproton energy spectrum has been obtained from the reaction ⁹⁰Zr(γ, p)⁸⁹Y using an isotopically enriched target foil. Previously unreported proton groups are observed at E_p = 6.95, 9.55, 10.68 and 11.03 MeV. A total photoneutron cross section and a low-energy neutron energy spectrum are also presented, and isospin mixing is demonstrated by comparison with the photoproton data. The possibility of T_> strength in the region 23–24 MeV excitation is noted.     

Statistical analysis of the (d,p) and (d,d) reactions on 40Ca

Excitation functions of the ⁴⁰Ca(d, p)⁴¹Ca and ⁴⁰Ca(d, d)⁴⁰Ca reactions have been measured at 45°, 90°, 135° and 170° from E_d = 4.50 to 5.43 MeV in 10 keV steps. Angular distributions of these reactions have been taken at E_d = 4.70, 5.00 and 5.30 MeV from 25° to 170° in 5° steps. Transitions were observed to the excited states for the range 0.0 ≦ E_x ≦ 3.74 MeV in ⁴¹Ca. Rapid fluctuations in the excitation functions and strong variations of the angular distributions with the incident energy were observed, suggesting that the contribution from compound nucleus processes is very large. Various quantities extracted from the experimental data were compared to the predictions of the statistical theories combined with the DWBA theory for the calculation of the direct reaction amplitudes. The results of the present analysis are consistent with the predictions of the standard statistical theories based on the neglect of the channel-channel correlation.     

The heavy-ion reaction channels of the system 16O + 16O

The reaction channels of the system ¹⁶O + ¹⁶O with outgoing heavy particles from lithium to magnesium have been measured using a ΔE-E telescope. Excitation functions from 49 to 65 MeV at θ_Lab = 30° and angular distributions from θ_Lab = 10° (20°) to 50° at E_Lab = 51.5 MeV are presented for the strong transitions. The excitation function of the ¹²C-²⁰Ne (4.25 MeV) channel shows a pronounced regular cross structure with peaks at 52 and 60 MeV. A selective excitation of certain states in the inelastic scattering and the ¹²C-²⁰Ne channel is observed; the yields of the other heavy-ion channels being weaker by at least one order of magnitude. An explanation of this phenomenon is given by considering the angular momentum matching between entrance and exit channels. Furthermore it is shown that no strong dependence of the cross sections on the transferred angular momentum or on the nuclear structure of the final states is observed. Possible implications of these results on the reaction mechanism are discussed.     

Investigation of the role of the projectile-target orientation angles on the evaporation residue production

The measured yield of evaporation residues in reactions with massive nuclei have been well reproduced by using the partial fusion and quasifission cross sections obtained in the dinuclear-system model. The influence of the orientation angles of the projectile- and target-nucleus symmetry axes relative to the beam direction on the production of the evaporation residues is investigated for the ⁴⁸Ca + ¹⁵⁴Sm reaction as a function of the beam energy. At the low beam energies only the orientation angles close to αP = 30° (projectile) and αP = 0°–15° (target) can contribute to the formation of evaporation residues. At large beam energies (about E _c.m. = 140–180 MeV) the collisions at all values of orientation angles αP and α _T of reactants can contribute to the evaporation residue cross section which ranges between 10–100 mb, while at E _c.m. > 185 MeV the evaporation residue cross section ranges between 0.1–1 mb because the fission barrier for the compound nucleus decreases by increasing its excitation energy and angular momentum.     

Comparison of the yields of high-energy gamma-rays withE γ≧10 MeV measured in the reactionsnatSn+40Ar and68Zn+64Zn

The spectra of theKX-rays of reaction products were observed in coincidence with high energy gamma-rays (E _γ≧10 MeV) in the^natSn+⁴⁰Ar (E _Ar=300 MeV) and⁶⁸Zn +⁶⁴Zn (E _Zn=290 MeV) reactions. Two groups ofKX-rays can be clearly identified in the spectra. The first one corresponds to the evaporation residue of the compound nucleus and the second one to the target-like products of the reaction. The spectra of gamma transitions gated by theKX-rays of the evaporation residues were measured in both reactions. The relative yields and the positions of the broad shoulders observed at an energyE _γ≧10 MeV in these spectra, differ drastically in the asymmetric and the symmetric systems. The ratio between the yields of gamma-rays emitted at angles of 0° and 90° to the beam direction was determined in the^natSn+⁴⁰Ar reaction. It was found to be greater than unity only atE _γ=(10–14) MeV. This experimental evidence may indicate the emission of high energy gamma-rays from the dinuclear system prior to the formation of the compound nucleus.     

Evidence of non-statistical structures in the elastic and inelastic scattering of58Ni+58Ni and58Ni+62Ni and intermediate dinuclear states

Excitation functions and angular distributions of⁵⁸Ni+⁵⁸Ni and⁵⁸Ni+⁶²Ni scattering at energies just above the Coulomb barrier have been measured aroundθ _cm=90° in energy stepsΔE _cm=0.25 MeV fromE _cm ? 110 MeV toE _cm ? 120 MeV for⁵⁸Ni+⁵⁸Ni and fromE _cm ? 110 MeV toE _cm ? 118 MeV for⁵⁸Ni+⁶²Ni. Evidence for structure of non-statistical character has been found in the angle-summed excitation functions; this evidence is corroborated by the analysis of the angular distributions. This is the first time that non-statistical structure in elastic and inelastic scattering is reported with high confidence level for this mass and excitation energy ranges. Attempts are presented to understand the nature of this structure, including the presence of intermediate dinuclear states and virtual states in a potential well.     

Depolarization in proton inclusive inelastic scattering and in the (d,p) reaction on medium-weight nuclei

The energy-averaged depolarization parameter K_y^y′ has been measured for the inelastic scattering of 18 MeV protons from ⁵⁴Fe, ⁶³Cu and ⁹²Mo at 45°, 90° and 135°, and for 14.35 MeV protons from ⁶³Cu at 45° and 135°. In all cases K_y^y′ varies from approximately unity for scattering with low energy loss to approximately zero for inelastic scattering to high excitation energies. The change from one of these values to the other occurs over a region ≈ 6 MeV wide centered at about 5 MeV excitation. A simple two-component model fits both the K_y^y′ and inelastic crosssection data. K_y^y′ has also been measured for the ⁵⁴Fe($\text{d}$, $\text{p}$)Fe reaction with 16 MeV deuterons incident. Here K_y^y′ changes from approximately the maximum possible value, $\text{2}\text{3}$, to about zero in a 6 MeV region centered at roughly 13 MeV excitation. The ($\text{d}$,$\text{p}$) data can be fitted by an extension of the model used for the proton scattering data.     

Spin and parity assignments for 35Cl levels from the 31P(α, p)34S reaction

³⁵Cl states at excitation energies between 9.9 and 11.8 MeV have been identified through sharp resonances in the ³¹P(α, p_o)³⁴S excitation functions at 25°, 105° and 155° for E_α = 3.25–5.50 MeV. Forty-eight on-resonance angular distributions, normalized to an absolute cross section scale, have been subjected to single-level and two-level analyses resulting in spin and parity assignments for each resonance. Approximately half the resonances were of the pure single-state type, having unique angular distribution shapes. Data from 12 resonances of an earlier experiment ¹) were analyzed with the same theory, extending the diagnostics down to a ³⁵Cl excitation energy of 9.1 MeV. A set of optical potentials consistent in all four reactions that this experimental program encompasses has been incorporated in the present analysis. Validity of the optical potential is demonstrated for α-particles elastically scattered by ³¹P.     

The 12C(13C, α)21Ne reaction: High-spin states,resonances and coherence widths

Excitation functions were measured for states of ²¹Ne populated by the ${}^{12}\text{C}\text{(}{}^{13}\text{C}\text{, α)}$ reaction over the bombarding energy range E_lab = 18.2–32.0 MeV (18.4–27.0 MeV) at θ_lab = 7°(25°) in in 200 keV steps, and average coherence widths of states and the moment of inertia of the compound nucleus ²⁵Mg were obtained from these excitation functions. A statistical analysis of these data was performed. Angular distributions for states in ²¹Ne to 10 MeV in excitation energy were measured at θ_lab = 7°, 18°, 28° and 43° at bombarding energies from 29.0 to 31.0 MeV in 400 keV steps. These data along with Hauser-Feshbach predictions allow us to suggest spins for some states as well as to suggest possible candidates for rotational bands in ²¹Ne.     

Spin and parity assignments for 23Na levels from the 19F(α, p)22Ne reaction

Resonances were found in the 120° and 160° excitation functions for the ¹⁹F(α, p_o)²²Ne reaction between E = 2.5 and 5.0 MeV corresponding to ²³Na levels at excitation energies between 12.56 and 14.51 MeV. Twenty-one on-resonance angular distributions were analysed with single-level and two-level theory to extract ²³Na spin and parity information. The results of an earlier experiment were analysed by the same procedure, extending the diagnostics down to a ²³Na excitation energy of 11.55 MeV. The analysis incorporated an optical potential for the α-particle consistent with previous channel-spin-$\text{1}\text{2}$ reaction studies and α-induced reaction data.     

Alpha decay of the new isotope172Au

The first observation of the extremely neutrondeficient nucleus¹⁷²Au is reported, produced using the fusion evaporation reaction⁷⁰Ge+¹⁰⁶Cd→¹⁷⁶Hg^* (E_x?64 MeV). Mass separated evaporation residues were implanted into a double-sided silicon strip detector, and the energy and time of subsequent decay events were recorded. The alpha decay of¹⁷²Au was measured with an energy =6860±10 KeV, corresponding to =7020±10 Kev, and a half-life of 4±1 ms. No evidence was seen for a proton decay branch, implying a limit ofb _p ≤ 2 %.     

Line shape and excitation strength of the first excited state in9Be

The line shape and the excitation strength of the very weak first excited J ^π =1/2⁺ state at E_x=1.684 MeV in Zeitschrift für Physik Zeitschrift für Physik⁹Be has been investigated with high-resolution inelastic electron scattering at E₀=45 and 49 MeV and scattering angles θ=105°, 117°, 129° and 165°, and with high-resolution inelastic proton scattering at E₀=13MeV and θ=15° and 18°. Due to lying just above the neutron threshold the level has a strongly asymmetric line shape which in both experiments can be described consistently with a Breit-Wigner expression modified on the low energy side by the threshold behaviour of the cross section. The resonance energy is E_R=1.684 ± 0.007 MeV and the width T=217± 10 keV in thec.m. system. A single particle potential model calculation reproduces the line shape and the resonance parameters fairly well. In addition, the inelastic electron scattering form factor has been measured. In the range of momentum transfersq =0.24-0.46 fm^?1 it is dominated by a 0p_3/2→ 1s_1/2 particle-hole transition. The transition is mainly longitudinal and of isoscalar nature with a strength of B (E1)↑ =0.027 + 0.002 e² fm², but a small M2 contribution ofB(M2)↑=8.8 ±1.5 μ _N ² fm² has also been detected.