Lifetime measurement of 22Ne(4+1) with the high-velocity coincidence DSA method

The mean life of ²²Ne(4⁺₁) has been measured by means of the high-velocity coincidence DSA method with the ⁴He(¹⁹F, pγ)²²Ne reaction. The ²²Ne recoils are slowed down in Ti, Cu, Ag and Au and the γ-ray Doppler patterns are observed with a large Ge(Li) detector at 0° in coincidence with protons. Interpretation of the patterns with recent experimental stopping powers yields $\text{τ}{}_{\mathrm{<mtext>m</mtext>}}\text{[}{}^{22}\text{Ne}\text{(3.36}\text{MeV}\text{)] = 328 ± 10}\text{fs}$. The present result serves as a calibration measurement and helps to reduce uncertainties in calculated stopping powers for low-velocity DSA measurements.     

Large-angle elastic scattering of α-particles from 20Ne and 22Ne

Resonances observed in the 177° (lab) excitation function of α-particles scattered elastically from ²⁰Ne are investigated in terms of a single Regge pole. The position of the pole in the complex angular momentum plane is determined from angular distributions measured at incident α-particle energies of 25.8 and 27.0 MeV, at which energy values prominent maxima were observed in the excitation function and the pole contribution to these angular distributions is therefore expected to be a maximum. The complete trajectory of the pole over the energy region investigated (24.6 to 31.7 MeV) is obtained by means of extrapolation. The pole trajectory across the surface absorption region of the target nucleus clearly shows the reason for the occurrence of only three large resonances in this energy region, the remaining resonances being attenuated considerably. An angular distribution obtained for ²²Ne(α, α)²²Ne at an incident energy of 27.0 MeV also indicates the presence of the Regge pole found for ²⁰Ne at the same energy, even though its strength is severely reduced due to the isotopic dependence effect.     

Recoil-distance measurements of the g-factor of 22Ne(21+)

Time-differential recoil-into-vacuum measurements with a plunger have been performed on the first-excited I^π = 2⁺ state of ²²Ne. The state was populated with the ⁴He(¹⁹F, p)²²Ne reaction on ⁴He-implanted foils. The measurements lead to a $\text{g-}\text{factor}\text{of}\text{¦g¦ = 0.326 ± 0.012}$ and to a mean life of τ_m = 5.2 ± 0.3 ps. The analysis also yields the population of excited two-electron ions and indicates that the population may depend on the thickness of the ¹²C contamination layer through which the ions recoil into vacuum.     

Yrast and high-spin states in 22Ne

High-spin states in ²²Ne have been investigated by the reactions ¹¹B(¹³C, d)²²Ne and ¹³(¹¹B, d)²²Ne up to $\text{E}{}^1\text{～- 19}\text{MeV}$. Yrast states were observed at 11.02 MeV (8⁺) and 15.46 MeV (10⁺) excitation energy. A backbending in ²²Ne is observed around spin 8⁺. The location of high-spin states I ≦ 10 is discussed in terms of the rotational band structure, Strutinsky-type calculations, and pure shell-model predictions.     

Determination of angular momenta in the incomplete-fusion channels of the reactions Ta+22Ne and Ir+12C

By using the γ-ray-multiplicity technique, experiments were carried out to determine the input angular momenta lⁱⁿ corresponding to the heavy-ion induced incomplete-fusion reactions involving the emission of fast charged particles. The α-particle and Li-nuclei emission channels were investigated in the reaction ¹⁸¹Ta + ²²Ne at the average Ne-ion energy of 155 MeV and for the α-particle emission channel in the reaction ^natIr+¹²C at E = 100 MeV. The separation of the reaction channels was carried out using an X-ray spectrometer. The angular distributions of the fission fragments were measured in the reaction ¹⁸¹Ta(²²Ne, αf). The data obtained indicate that input angular momenta are equal to about 60 ? and 50 ? for the channels of Li emission and α-emission, respectively, being practically independent of the particle energy. For the α-emission channel the lⁱⁿ values are the same for both reactions within experimental errors. The lifetime of the system of interacting target and projectile nuclei, prior to the emission of an α-particle, has been estimated to be equal to 10^?20s.     

The ground-state rotational band in 159Tb

162 MeV ⁴⁰Ca ions have been used to Coulomb excite the ground-state band of ¹⁵⁹Tb up to spin $\text{25}\text{2}$. Lifetimes for levels up to spin $\text{25}\text{2}$ have been determined with DSA and recoil distance methods. Multipole mixing ratios for several cascade transitions were extracted from an analysis of γ-ray angular correlation data. Reduced transition probabilities thus deduced in a model-independent way were found to be in agreement with the rotational model with Q₀ = 7.41 ± 0.06 e · b and g_K?g_R = 1.377 ± 0.010.     

A study of the 18O(6Li,d)22Ne reaction at 32 MeV

Results from a study of the ¹⁸O(⁶Li, d)²²Ne reaction at a ⁶Li energy of 32 MeV are reported. The L-dependence of the shapes of the measured angular distributions provide a check on recent J^π assignments for some of the high-lying levels in ²²Ne. A finite range distorted wave analysis assuming a direct cluster transfer has been used to extract from the data α-particle spectroscopic strengths for most of the natural parity levels populated below 8 MeV of excitation. These strengths are compared with theoretical predictions for those few states for which a definite correspondence can be made between the calculated and experimental levels of ²²Ne. For transitions to the members of the ground-state band, the observed strengths disagree with the predictions. This disagreement has also been observed in the ¹⁶O(⁶Li, d) reaction and its cause is not understood. It is in marked contrast with the good agreement found for (⁶Li, d) reactions on targets of mass 20 ≦ A ≦ 24.     

Lifetime measurements in 29 Al

The lifetimes of the first six excited states of ²⁹Al have been measured by the DSA method with two different reactions,²⁶Mg(α, p)²⁹Al and²⁷Al(t, p)²⁹Al. The recoil distance method was also used to remeasure the mean lifetime of the first excited state, leading to a value τ_m = 6.5 ± 0.5 ps. Experimental results are compared with shell model calculations.     

The 22Ne(d,n) 23Na reaction

Absolute differential cross sections are determined for 32 states from the ²²Ne(d, n) ²³Na reaction by the neutron time-of-flight method. A gaseous ²²Ne target was bombarded with 5.5 MeV deuterons and angular distributions taken from 0° to 160°. In addition yield curves were taken at a fixed angle of 10° in 0.5 MeV steps from 2.5 to 5.5 MeV. The analysis of both types of data used computer programs for DWBA and compound-nucleus calculations. With two exceptions and three additions the l_p values determined in the present experiment agree with those of a recent (τ, d) experiment on the same target nucleus. The two previous (τ, d) experiments show considerable differences in proton transfer strengths to various states. The present experiment agrees well with the one which showed generally lower strengths for individual states, and hence with an assumption of greater spreading of the single-particle strength. The implications of those results on the Nilsson-model scheme for ²³Na are discussed.     

Lifetime measurements in 22Ne, 28Si and 31P relevant to the interpretation of the Z2 variation in low velocity DSAM results

Accurate lifetimes have been measured for low-lying levels in ²²Ne, ²⁸Si and ³¹P by bombarding ⁴He implanted targets with beams of ¹⁹F and ²⁸Si ions. Mean lifetimes determined by fitting Doppler-broadened γ-ray lineshapes were $\text{(E}{}_{\mathrm{<mtext>x</mtext>}}\text{in MeV}\text{, τ}\text{in ps}\text{):}{}^{22}\text{Ne}\text{(1.275, 5.15 ± 0.31; 3.357, 0.324 ± 0.009),}{}^{28}\text{Si}\text{(1.779, 0.667 ± 0.035),}{}^{31}\text{P}\text{(1.266, 0.70 ± 0.07; 2.234, 0.363 ± 0.024}$). The lifetime values for the 3.357 MeV level in ²²Ne and the 2.234 MeV level in ³¹P are used to calibrate low velocity DSAM lifetime data for these two levels and to obtain scaling factors to theoretical electronic stopping powers for Ne and P ions.     

The g-factor of the 6+ state in 50Ti

The rotation of the angular distribution pattern of the 524 keV γ-radiation from the 6⁺, 3200 keV state in ⁵⁰Ti was measured in an external magnetic field using the ⁴⁸Ca(α, 2n) reaction. The IPAD method was applied. From the experimental precession angle, the value of the g-factor, g = 1.57 ± 0.17, has been derived.     

Structure en bande du noyau 22Na: Le premier niveau Jπ = 6+

We investigate the two-body photodisintegration of ³He and its inverse, radiative p-d capture using bound-state functions corresponding to the N-N interaction being given by the Reid soft-core potential. For the two-body photodisintegration of ³He Coulomb effects and the final-state interactions between the proton and deuteron are not included. At low energy the shape of the angular distribution agrees well with experiment, but the 90° cross section exhibits an anomalous peak at 15 MeV due to electric dipole transitions connecting the deuteron and ³He D-states. The low-energy cross section is 25–40 % too small. The intermediate-energy angular distribution peaks too near the forward direction, and, contrary to experiment, has a minimum at 100°. At higher energy the 90° cross section in the center-of-momentum frame is at least an order of magnitude too small, but does display the correct energy dependence. This energy dependence is related to the properties of the bound-state wave functions and it is plausible that it will persist in an improved treatment (e.g. one which includes exchange currents) which properly accounts for the magnitude of the cross section. Contributions from the ³He S-states are negligible for photon energies between 100 and 150 MeV, but are dominant outside this energy region.     

Alpha-decay widths in 20Ne

The α-decay of several unbound levels in ²⁰Ne has been studied by ¹⁶O(α, α) elastic and inelastic scattering. A narrow resonance, Γ_c.m. = 13±4 eV, with J^π = 5^? was found at $\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{(}{}^{20}\text{Ne}\text{) = 8.451±0.005}\text{MeV}$ and is associated with the lowest K = 2^? quasirotational band. Several new, narrow resonances were found between E_x = 16.0?18.4 MeV. Reduced α-decay widths have been obtained for the lowest K = 0⁺, 2^? and 0^?bands. For states described predominantly by the (8, 2) representation of SU(3) we note a reduction of the reduced widths with increasing spin. Reduced widths of positive parity bands are reviewed.     

Recoil-distance measurements of g-factors for 24Mg(21+) and 20Ne(21+)

Time-differential recoil-into-vacuum measurements have been performed with a plunger on the first-excited I^π = 2⁺ states of ²⁴Mg and ²⁰Ne. The states were populated by the reactions ¹²C(¹⁶O, α)²⁴Mg and ¹²C(¹²C, α)²⁰Ne. The measured anisotropy of the α-γ angular correlation was greatly increased by means of a vertical slit on the annular particle detector. Values of $\text{¦g¦= 0.51 ± 0.02}\text{and}\text{0.54 ± 0.04}$ have been deduced for the ²⁴Mg and ²⁰Ne g-factors, respectively. The mean lives of these states have been determined as τ_m = 2.09 ± 0.13 ps and 0.8 ± 0.2 ps, respectively. Various theoretical calculations are discussed and compared with the measured g-factors.The analysis of the measurement also yields values for the populations of electronic states contributing to the hyperfine interaction. For ²⁰Ne the populations of the different electronic configurations are compared with the results of a separate time-integral measurement, in which the correlations were measured for each ionic state separately. Large fractions of two-electron excited states are found to contribute.     

Alpha-particle strengths from the 16O(6Li,d)20Ne reaction

The ¹⁶O(⁶Li, d)²⁰Ne reaction has been studied at bombarding energies of 20, 32 and 38 MeV. The α-particle spectroscopic strengths have been extracted for levels up to 12.15 MeV in excitation. Nondirect processes appear to contribute significantly to all levels at 20 MeV and to high spin levels (6⁺ and 8⁺) at 32 MeV. Strengths extracted for members of the ground state band assuming (sd)⁴ transfer are unequal at both 32 and 38 MeV, in marked contrast to theoretical predictions. To explain this, particle-hole correlations in ¹⁶O(g.s.), inelastic channel coupling in the reaction and perhaps other effects as well, have to be considered. Strengths extracted for members of excited bands and α-decay reduced widths compare poorly with each other and with simple SU(3) predictions.     

The electric quadrupole moment of the 6+ state of 112Sn

The spectroscopic quadrupole moment of the isomeric 6⁺' (2552 keV) state of ¹¹⁷Sn has been derived from TDPAD measurements in a Cd metal host target at 500 K as $\text{Q}{}_{\mathrm{<mtext>I</mtext>}}\text{(6}{}^{\mathrm{+}}\text{,}{}^{112}\text{Sn}\text{) = ± 0.29(6)}\text{b}$. The calibration of the effective field gradient for Sn in Cd at 500 K, given by Herrlander et al. has been used. A comparison of the results with shell model estimates makes a mixed configuration of $\text{|(}\text{g}\text{7}\text{2}\text{)}{}^{\mathrm{?2}}\text{6}{}^{\mathrm{+}}\text{〉)}\text{and}\text{(}\text{g}\text{7}\text{2}{}^{\mathrm{?1}}\text{d}\text{5}\text{2}{}^{\mathrm{?1}}\text{)6}{}^{\mathrm{+}}\text{〉)}$ for the 6⁺ state most likely.     

Formation and deexcitation of the 32S compound nucleus via the 20Ne+12C and 16O+16O entrance channels

Reaction-product cross sections following ²⁰Ne+¹²C and ¹⁶O + ¹⁶O collisions at several incident energies have been measured with a E-ΔE counter telescope. They are compared to statistical model predictions. Fair agreement is obtained for the high-Z evaporation residue cross sections, but a strong discrepancy is observed for the lower-Z reaction products. Possible explanations are discussed. It is shown that the compound nucleus formation does not depend on the structure of the colliding ions in the entrance channel and also that it is not limited by the ³²S yrast line.     

Three particle reactions observed from 10Ne + 12C at 160 MeV

Coincidences between light particles (Z ? 4) and heavy ions (A ? 9) have been measured for the ²⁰Ne + ¹²C reaction at $\text{E}{}_{\mathrm{<mtext>lab</mtext>}}\text{(}{}^{20}\text{Ne}\text{) = 160}\text{MeV}$. α, ¹⁶O events from the ¹²C(²⁰Ne, α¹⁶O)¹²C reaction and α, ²⁰Ne events from ¹²C(²⁰Ne, α²⁰Ne)⁸Be have been found. Energy distributions and angular correlations of these events are consistent with α-decay from the intermediate nuclei ²⁰Ne and ²⁴Mg formed by inelastic scattering and α-transfer in a first reaction step.