Alpha-particle unstable states in12C

Alpha particle unstable states in¹²C have been investigated using the three reactions¹⁴N(d, α′)¹²C,¹⁴N(d, α′α)⁸Be and¹²C(α, α′)¹²C. Excitation cross sections and angular distributions have been measured for the reactions¹⁴N(d, α′) at 52 MeV and¹²C(α, α′) at 90 MeV. For a few angle pairs the α-decay of excited states in¹²C have been observed in a¹⁴N(d, α′α) correlation measurement. The reactions selectively excite onlyT=0 states. A previously undetected level with a large α-decay width (Γ=1.2 MeV) has been observed at 15.62 MeV excitation. This level shows up clearly in both reactions and is further distinguished from the nearby 14.08 state in the correlation measurement because of the distinctly different energy distributions of the decay products. On the basis of a particle angular distributions the 15.62 MeV level was assigned spin and parity 4⁺ and the level at 14.08 was assigned 3^?. The latter differs from the value suggest by earlier work. Comparison with DWBA calculations indicates that angular distributions of all other prominent levels are in agreement with their earlier assignments. Two levels at 19.20 and 20.30 MeV (both Γ?0.4 MeV) and three further levels at 21.81, 22.7 and 24.24 MeV also decay predominantly by α-emission.     

Messung von Wirkungsquerschnitten für die Reaktionen 27Al(N,P) 27Mg, 27Al(N, α)24Na und 27Al(N, α)24MNa im Energiebereich zwischen 6,3 MeV und 8,3 MeV

Measurements of Cross Sections for the Reactions ²⁷Al(N, P) ²⁷Mg, ²⁷Al(N, α)²⁴Na and ²⁷Al(N, α)^24mNa in the Energy Region between 6,3 MeV and 8,3 MeV Using the activation method cross-sections for the reactions ²⁷Al(n, α₀)²⁴Na, ²⁷Al(n, α₁)^24mNa and ²⁷Al(n, p)²⁷Mg have been measured in the energy range 6.3 MeV to 8.3 MeV. Fast neutrons were produced at the Stuttgart University 4 MV Dynamitron via the reaction ⁹Be(α, n)¹²C irradiating 0.52 μm beryllium layers on high pressure water-cooled copper backings. The energy of the He-ions was varied between 1.86 MeV and 2.76 MeV, while the average current was 200 μAmp. The neutron flux has been measured with a 2″ × 2″ NE 213 liquid scintillator cell.     

Cluster states in the neutron excess nucleus 22Ne

High-spin states of the ²²Ne nucleus in the excitation energy range of 15–30 MeV have been studied. The angular correlation method has been used to determine the spins of excited states. A number of new states with high angular momenta—20.0 MeV (9^?), 20.7 MeV (11^?), 21.6 MeV (9^?), 22.2 MeV (12⁺), and 25.0 MeV (9^?)—have been revealed. They are intensely populated in the reaction ¹⁴C(¹²C, α₁)²²Ne* → α₂ + ¹⁸O and correspond to the rotational bands of various structures.     

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.     

态布居测热核的核温度及其与碰撞参数的关系

测量了２５ＭｅＶ／ｕ^４０Ａｒ＋¹⁹⁷Ａｕ反应中两裂片符合下的小角α-α关联．由α-α关联函数提取了⁸Ｂｅ核的相对态布居和热核的核温度．实验测得发射温度随碰撞参数而稍有变化，从中心碰撞的４ＭｅＶ变化至周边碰撞的３.８ＭｅＶ．在周边碰撞中，观察到发射温度随着粒子能量的增加而升高     

The study of the (α, α'f) reaction at 120 MeV on 232Th and 238U: (I). Fission probabilities and angular distributions in the region of the giant quadrupole resonances

The fission decay channel of ²³²Th and ²³⁸U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for ²³⁸U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a (⁶Li, ⁶Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations.     

The width of the 5.11 MeV state in 10B and isovector parity mixing

We review estimates for effects of parity mixing on the α-capture of vector polarized ⁶Li to the 5.16 MeV, 2⁺, T = 1 state in ¹⁰B. The cross section depends on the ⁶Li polarization direction because of isovector parity mixing with the 5.11 MeV, 2^?, T = 0 state. The effect is enhanced due to isospin conservation. The α-width of the 5.11 MeV state, an important parameter for calculating the enhancement, has been measured to be 0.98 ± 0.07 keV. The consequences of parity mixing are reevaluated using best available values for the relevant parameters.     

Spin-parity combinations in 16N from the 18O(d,α)16N reaction

The reaction ¹⁸O(d, α)¹⁶N has been studied using a tensor-polarized deuteron beam at incident energies of 8.50, 9.25, 10.00, 10.30, 10.75 and 11.30 MeV. From measurement of the tensor analysing power of the reaction when the α-particles are emitted near 0°, spin-parity combinations have been deduced for thirteen levels in ¹⁶N up to 6.2 MeV excitation. These results, together with previous work, lead to new and unambiguous spin and parity assignments for four of these levels.     

Isospin violation in α-induced reactions

A study has been made of the reactions ¹²C(α, α')¹²C and ¹⁰B(α, d)¹²C which lead to the 12.71 MeV T = 0 and the 15.11 MeV T = 1 levels in ¹²C. Significant structure is observed in the yield curves of the reactions ¹²C(α, α') ¹²C for both the isospin allowed and forbidden reactions up to the maximum available α-energy of 27 MeV; this structure is attributed to compound nucleus formation. No significant structure is observed in the reaction ¹⁰B(α, d)¹²C.     

The 24Mg(α, α')24Mg reaction; statistical analysis

The ²⁴Mg(α, α')²⁴Mg reaction to the 1.37 MeV state has been studied over an α-energy range of 9.625–13.825 MeV, in ≈40 keV steps, and over a 25°–160° angular interval. These cross sections have been analysed in terms of statistical theory and a number of deviations from its predictions are found. These deviations point to the importance of non-statistical processes, such as intermediate structure, to the α-scattering. The average compound nuclear width o₁ 62±7 keV is found for ²⁸Si over the 18–22 MeV excitation.     

Preequilibrium α-particle emission in heavy-ion reactions

The α-particle emission following ¹⁴N-induced reactions on various heavy targets at 85–115 MeV has been studied. Cross sections of heavy residual nuclei produced after α-emission were measured in the case of the ²⁰⁹Bi target and were found to be close to the angle-integrated cross sections of α-particles, indicating that the α-emission mainly takes place in a binary process. The measured angular distributions of α-particles are pronouncedly forward-peaked, while the energy spectra are always characterized by the Maxwellian distribution even at forward angles and reproduced excellently by the statistical evaporation formula when nuclear temperature is treated as a free parameter. The resultant value of the temperature is high (4–6 MeV) at forward angles and decreases monotonically with increasing emission angles.The energy and angular distributions of protons, deuterons and tritons were also measured in the ¹⁸¹Ta + ¹⁴N reaction at 115 MeV. The results are similar to those of α-particles. In particular, nuclear temperatures turned out nearly equal to each other, being consistent with the hot-spot interpretation for the relevant preequilibrium light-particle emission.     

Reactions of 40Ar with 159Tb, 142Nd and 144Sm; New α-activities from 189Bi, 173Pt and 177Au

Reactions of ⁴⁰Ar ions with targets of ¹⁵⁹Tb, ¹⁴²Nd, and ¹⁴⁴Sm have been studied at energies below 300 MeV with a helium gas-jet system. Excitation functions for (Ar, xn) reactions, where x = 5–10, were obtained for the radioactive products that decay by α-emission. Based on the characteristics of these excitation functions and on the systematics of α-decay, evidence is presented for the existence of the nuclides ¹⁸⁹Bi with α-particle energy E_α = 6.67±0.01 MeV and half-life < 1.5 sec, and ¹⁷³Pt with E_α = 6.19±0.01 MeV; and for the emission from ¹⁷⁷Au of an α-particle with E_α = 6.15±0.01 MeV.     

Response functions of 58Ni, 116Sn AND 208Pb to the excitation of intermediate-energy α-particles

Inelastic scattering of 340 MeV and 480 MeV α-particles has been measured on ⁵⁸Ni, ¹¹⁶Sn and ²⁰⁸Pb up to 60 MeV excitation energy. Consistent background subtraction and multipole analysis has provided the repartition of multipole strength for all three nuclei. The so-obtained response functions show the already known low-energy giant resonances in a detailed way, as well as new giant resonances at high energy.     

Alpha-particle D-state and configuration-mixing effects in the 89Y(d, α)87Sr reaction

Angular distributions of cross sections and vector and tensor analyzing powers have been measured for low-lying levels populated by ⁸⁹Y(d, α)⁸⁷Sr reaction at 9.0, 12.0, and 16.0 MeV. Full finite-range DWBA calculations that include the effects of an α-particle D-state component and (L, J) mixing have been performed. The ground-state transition indicates that the D-state parameter D₂ = −0.30 ± 0.1 fm². The angular distributions of the tensor analyzing powers are very sensitive to the magnitude and sign of the mixing amplitudes and can be used to determine the neutron-proton configurations transferred. The mixing amplitudes for the 0.388 MeV and 0.873 MeV states in ⁸⁷Sr are established from the analysis.     

Large-angle enhancements in the 16O(6Li, α)18F reaction at 48 MeV

The elastic scattering of ⁶Li + ¹⁶O at 48 MeV has been measured and fitted with an optical model calculation. Measurements have been made of the ¹⁶O(⁶Li, α)F reaction at 48 MeV populating the 1⁺ g.s., 3⁺ 0.927 MeV and 5⁺ 1.122 MeV states in ¹⁸F. The data exhibit cross sections at large angles comparable to those at forward angles, and have been compared with exact finite-range DWBA calculations. Exchange contributions were included for the 1⁺ g.s. and were unable to account for the large-angle data. Calculated statistical compound nucleus cross sections were approximately a factor of 100 below the data. The same conclusions are reached for previously published data at 34 MeV.     

A study of the second excited state of8Be by the10B(d,α2)8Be-reaction

The second excited state of⁸Be has been studied by analysis of the coincidence spectra of the reaction¹⁰B(d, α)2α forE _d =1.0-2.0 MeV. The excitation energy was found to be 12.0±0.4 MeV, and the reduced width 0.85±0.12 MeV. Differential crosssections of the (d, α ₂)-reaction were deduced.     

Studies of single neutron holes in the transitional nuclei N = 83–89:: The 150, 148Nd(d,t) and 150, 148, 146Nd(3He, α) pick-up reactions

The level structures of the ^{145, 147, 149}Nd nuclei up to about 5 MeV excitation energy have been investigated with the (³He, α) reaction at 24 MeV. Additional 17 MeV (d, t) data have been obtained for ^{147, 149}Nd. The angular distributions have been analyzed with standard DWBA calculations, and spectroscopic factors have been deduced. Two groups of states carrying h_{$\text{11}\text{2}$} single-particle strength may be associated with the $\text{9}\text{2}$^? [514] and $\text{11}\text{2}$^? [505] Nilsson orbitals. A considerable amount of high-l single-particle strength may be found in the continuum observed in the (³He, α) spectra above 3 MeV in all the nuclei.     

Energy levels of 56Co studied by the 58Ni(d, α)56Co reaction near 0°

Tensor analyzing powers, T₂₀, of outgoing α-particles in the ⁵⁸Ni($\text{d}$, α)⁵⁶Co reaction at detection angles near 0° have been measured for excited states in ⁵⁶Co for beam energies of 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. Thirty-seven spin-parity combinations for ⁵⁶Co excited states have been deduced. Previous J^π ambiguities for 11 of these states have been eliminated, and results in conflict with existing assignments for the levels at 3.235 and 3.378 MeV have been obtained. A search for 0⁺ states was carried out from angular distribution measurements at forward angles of the unpolarized (d, α) reaction. The combined results from this and previous experiments were found to be in reasonable agreement with calculated level schemes.     

The (He,p) and (d, α) reactions to states in Co

The ⁵⁶Fe(³He, p)⁵⁸Co and ⁶⁰Ni(d, α)⁵⁸Co reactions have been investigated at 12.0 and 7.0 MeV bombarding energies, respectively, using the MIT multiple-gap spectrograph. For the (³He, p) reaction, levels below 6.0 MeV of excitation were observed and analysed using the DWBA to obtain L-values. In the (d, α) reaction, levels up to 3.4 MeV of excitation were observed. A level scheme for ⁵⁸Co is proposed. The ⁵⁸Fe(0.0) analogue was observed, and candidates for the antianalogue level are identified.     

Inelastic electron scattering from 6Li near the α-d threshold

The form factor for the alpha-deuteron continuum in ⁶Li has been measured in the excitation region within 2 MeV of threshold, for momentum transfers q < 1.2 fm^?1. The results are interpreted in terms of an α-d cluster model. The monopole breakup appears to be predominant near threshold, but the influence of the J = 2⁺ (4.31 MeV) quadrupole resonance becomes important a few MeV above threshold. The form factor for the C2 transition to the 3⁺ (2.185 MeV) state is also presented.