Recent result from the A2 real photon facility at MAMI

The upgraded MAMI C accelerator is delivering electrons with an energy of 1558 MeV rou-tinely. The A2 collaboration is doing experiments with energy marked polarised real photons produced via "Bremsstrahlung". Recent results from the GDH-experiment using a longitudinal polarised frozen spin target in combination with the DAPHNE detector and from the Crystal Ball experiment at MAMI are presented.     

Recent result from the A2 real photon facility at MAMI

The upgraded MAMI C accelerator is delivering electrons with an energy of 1558 MeV routinely. The A2 collaboration is doing experiments with energy marked polarised real photons produced via ＂Bremsstrahlung＂. Recent results from the GDH-experiment using a longitudinal polarised frozen spin target in combination with the DAPHNE detector and from the Crystal Ball experiment at MAMI are presented.     

Excited states in 216Ra

Excited states in ²¹⁶Ra have been studied using in-beam spectroscopic techniques. The reactions used were ²⁰⁸Pb(¹²C, 4n)²¹⁶Ra and ²⁰⁸Pb(¹³C, 5n)²¹⁶Ra at 80 and 95 MeV, respectively. The level scheme of ²¹⁶Ra was extended up to the (28^?) state at 6266 keV. Two new isomers, whose mean lives are 8.7 ± 0.6 ns and 9.7 ± 0.9 ns, were found to be the 19^? state at 3763 keV and the (25^?) state at 5170 keV. Levels up to the (25 ^?) state in ²¹⁶Ra are interpreted as arising from the weak coupling of two neutrons to the excited states in ²¹⁴Ra.     

A study of the 26Mg(12C, 12B)26Al charge-exchange reaction

The reaction ²⁶Mg(¹²C, ¹²B)²⁶A1(5⁺, 3⁺) has been studied using a beam of 102 MeV of ¹²C. Shell-model, microscopic direct model and finite-range coupled reaction channel (CRC) calculations including recoil effects, have been performed, for comparison with the experimental data. DWBA calculations were performed for the intermediate states of interest in the ¹¹B + ²⁷Al and in the ¹³C + ²⁵Mg channels and these results were also compared with the experimental ones. The dominant reaction mechanism for ²⁶Mg(¹²C, ¹²B)²⁶Al(5⁺, 3⁺) appears to be the sequential mode.     

Level structure of 99Tc by inelastic scattering and proton stripping

The ⁹⁸Mo(³He, d)⁹⁹Tc and ⁹⁹Tc(d, d') reactions have been used to study the levels of of ⁹⁹Tc. Spin-parity assignments are made for nearly all states below 1.5 MeV excitation. Strong j-dependence is noted for l = 1 transitions in the proton stripping reaction, making possible several new spin assignments. The inelastic scattering data on ⁹⁹Tc are compared to similar data on ⁹⁸Mo, and are found to be in agreement with a coupling scheme based on a shell model with good seniority.     

Formation of 24Na in “fission-like” reactions

Formation of ²⁴Na in reactions induced by protons of energy 40–85 MeV on ⁴⁰Ca, ⁴⁵Sc, ⁴⁸Ti, and naturally occurring V was measured. Evidence is presented that ²⁴Na is formed in a binary break-up reaction. The results are compared with transition state (fission) calculations.     

The (d, α) reaction viewed as a two-step process

The ^{98, 97, 94}Mo(d, α)^{96, 95, 92}Nb and ⁹³Nb(d, α)⁹¹Zr reactions have been studied at 12 MeV bombarding energy. A DWBA analysis using one- and two-step processes has been carried out. The two-step process is found, in general, to dominate the one-step and together give rise to cross-section predictions within a factor of 10 of experiment. The angular distribution patterns are found to be best reproduced by DWBA predictions based on certain L-transfers (“parity violating”) associated with only the two-step process and which play but a minor role in their contribution to the overall cross section.     

Large angle cross section and analyzing power for proton-4he elastic scattering between 185 and 500 MeV

The differential cross section for the elastic scattering of protons from ⁴He has been measured for ten proton energies between 185 and 500 MeV in the angular range 144° to 168° in the lab system. The analyzing power has also been measured for seven energies in the same angular range. The differential cross section does not show the marked backward peaking that has been observed at both lower and higher energies. The possible structure in the 180° excitation function suggested for energies around 240 MeV is not observed. The analyzing power is large and negative, and shows strong dependence on both energy and angle.     

Alpha-induced reactions on cobalt

Alpha-induced reactions on the target element cobalt were investigated in the energy region from 25 to 172.5 MeV using the stacked foil technique. Twenty excitation functions were measured for the production of ⁶¹Cu, ⁵⁷Ni, ^60m+gCo, ^58m+gCo, ⁵⁷Co, ⁵⁶Co, ⁵⁵Co, ⁵⁹Fe, ⁵²Fe, ⁵⁶Mn, ⁵⁴Mn, ^52m+gMn, ⁵¹Cr, ⁴⁸V, ⁴⁷Sc, ^46m+gSc, ^44mSc, ^44gSc, ⁴³K and ⁴²K. Only for ⁶¹Cu, ⁵⁸Co, ⁵⁷Co and ⁵⁶Mn were cross sections available earlier, which, however, were limited to α-energies ≦ 41 MeV. The experimental data are compared with calculations considering equilibrium as well as preequilibrium reactions according to the hybrid model of Blann. The excitation functions of half the product nuclides are dominated by preequilibrium effects over wide energy ranges. From the reaction ⁵⁹Co(α, 2n)⁶¹Cu an initial exciton number between n₀ = 4 (2n-2p-0h) and n₀ = 5 (2n-3p-0h) or (3n-2p-0h) is deduced. However, the theoretical analysis reveals several severe shortcomings, e.g. the underestimation of the excitation functions for ^60m+gCo, ^58m+gCo and ⁵⁹Fe for up to 2 orders of magnitude. For ^60m+gCo the discrepancy between theory and experiment can be attributed to break-up reactions of the α-particle, whereas for the two other product nuclides even more complex reaction mechanisms have to be assumed. Moreover, the strong contributions of preequilibrium emission of α-particles observed for (α,2pxn) and (α, 4pxn), which are not accounted for by the program OVERLAID ALICE, indicate that the initial reaction phase for α-induced reactions is more differentiated than is supposed by the present form of the hybrid model.     

A coupled-channels born-approximation analysis of rare-earth (d,t) transitions

A CCBA (coupled-channels Born approximation) analysis has been performed on a very large set of rare-earth (d, t) transitions. Calculations were performed for states in ¹⁵⁹Gd, ^{161, 163}Dy and ^{165, 167}Er which have been identified as members of rotational bands built on the Nilsson single- hole states $\text{11}\text{2}$^?[505], $\text{1}\text{2}$^?[521], $\text{3}\text{2}$^?[521], $\text{5}\text{2}$^?[523], $\text{5}\text{2}$⁺[642], $\text{5}\text{2}$⁺[633] and $\text{7}\text{2}$⁺[404]. In the cases where the DWBA works well, the CCBA gives results similar to those of DWBA. For the anomalous transitions the CCBA calculations tend to fit better than do the DWBA calculations — but the overall agreement is still rather poor.     

Neutron resonance parameters and radiative capture cross sections of 147Sm and 149Sm

Neutron capture and transmission measurements have been carried out on the separated isotopes of ¹⁴⁷Sm (98.34 %) and ¹⁴⁹Sm (97.72 %) at the 55 m time-of-flight station of the Japan Atomic Energy Research Institute electron linear accelerator. Resonance energies and neutron widths for a large number of resolved resonances were determined up to 2 keV for ¹⁴⁷Sm and 520 eV for ¹⁴⁹Sm. Radiation widths for 5 resonances in ¹⁴⁷Sm + n and 7 resonances in ¹⁴⁹Sm + n were derived. The s-wave strength functions, average level spacings and average radiation widths were obtained to be: $\text{10}{}^4\text{S}{}_0\text{= 4.8 ± 0.5,}\text{D}\text{= 5.7 ± 0.5}\text{eV}$ and $\text{Γ}{}_{\mathrm{\gamma }}\text{= 69 ± 2}\text{meV for}{}^{147}\text{Sm}$; a $\text{10}{}^4\text{S}{}_0\text{= 4.6 ± 0.6,}\text{D}\text{= 2.2 ± 0.2}\text{eV}$ and $\text{Γ}{}_{\mathrm{\gamma }}\text{= 62 ± 2}\text{meV for}{}^{149}\text{Sm}$. The average capture cr sections were deduced from 3.3 to 300 keV with an estimated accuracy of 5 to 15 %. The measured capture cross sections for ¹⁴⁹Sm are largely different from the evaluated data, which are obtained based on the statistical model calculation. Possible reasons for this disagreement are discussed.     

Evolution from deep inelastic transfer to quasi-fission

Recoil techniques have been used to study the evolution of the features of four-charge transfer reactions in ¹⁴⁸Sm targets when the mass and energy of the projectile are varied. Beams of ⁶³Cu and ⁵⁶Fe were used, and their energies were chosen in order to be equal to 1.2B and 1.5B for both projectiles, B being the interaction barrier. The residual nuclei ^149gTb, ¹⁵⁰Dy and ¹⁵¹Dy were identified by their radioactive properties. This study has shown a continuous evolution of the characteristics of the c.m. angular distributions, especially the position of the maxima, from typical deep inelastic transfer to quasi-fission features, when the value of $\text{E}\text{B}$ decreases. For equal values of this ratio, no difference was observed between Fe and Cu induced reactions. The c.m, energies corresponding to the maximum cross section increases when the incident energy is increased, for a given projectile. At low incident energies (igE = 1.2B), the relative motion appears to be completely damped, and the distance of the two nuclei after separation is equal to about 5 fm. At higher incident energies, the c.m. energy is significantly higher. This may mean that the relative motion is not completely damped. Part of the effect may be explained by the differences in the angular momenta involved in the reaction.     

Analysis of (particle,xn) reactions on tantalum and gold: (II). 181Ta, 197Au(3He,xn) excitation functions and equilibrium statistical model analysis

Excitation functions for the reactions ${}^{197}\text{Au}\text{(}{}^3\text{He}\text{, x}\text{n}\text{)}{}^{\mathrm{200?x}}\text{Tl}\text{(x = 3, … 7)}$ and ${}^{181}\text{Ta}\text{(}{}^3\text{He}\text{, x}\text{n}\text{)}{}^{\mathrm{184?x}}\text{Re}\text{(x = 3, … 7)}$ were measured at bombarding energies up to 75 MeV. The data are compared to the results of an equilibrium statistical model calculation including angular momentum conservation and γ-ray competition. Satisfactory agreement was found except for the high energy tails.     

Resonance strength measurements in the 24Mg(p, γ) 25Al reaction

The strength ωγ of the E_p = 823 keV resonance in ²⁴Mg(p, γ) ²⁵Al was determined to be ωγ = 490 ± 70 me V, by comparison with the Rutherford cross section measured in the energy region of E_p = 600–800 keV. The strength of other low-lying resonances at E_p = 224, 419, 1483, 1623, 1654 and 2010 keV were determined relative to this new value.     

Gamma-gamma energy correlations and moment of inertia in light Xe isotopes

High-spin properties of even ^118–122Xe isotopes have been studied in experiments when a 118 MeV ¹²C beam impinged on ¹¹⁴Cd and ¹¹⁶Cd targets. The γ-rays emitted following the reactions were detected by six NaI(Tl) counters. The coincidence events from the counters were sorted into a two-dimensional matrices, from which the energy-correlation spectra were extracted. From the energy correlation spectra the quantities of the collective moment of inertia $\text{I}$⁽²⁾ are deduced and compared with data obtained for the same nuclei in previous experiments, as well as with results from theoretical studies.     

12C1 and 8Be production in 12C + 208Pb collisions

The mechanisms involved in the production of fast α-particles in ¹²C-induced reactions have been studied for the ¹²C + ²⁰⁸Pb system at the bombarding energies of E_12c = 132, 187 and 230 MeV. Absolute cross sections for the reactions ²⁰⁸Pb(¹²C. ¹²C¹→α + ⁸Be), ²⁰⁸Pb(¹²C, ⁸Be(g.s.)) and ²⁰⁸Pb(¹²C, ⁸Be(2.94 MeV)) have been determined by coincidence measurement of two or three correlated α-particles. Inclusive α-particle production cross sections were also measured at E_12c = 187 MeV. It is found that the inelastic process (¹²C, ¹²C¹→α + ⁸Be) does not contribute significantly to fast α-particle production but that the production of ⁸Be by projectile fragmentation is an important source of α-particles. At the highest bombarding energy (230 MeV) it appears that the ¹²C → 3α fragmentation reaction becomes more prominent at the expense of the ¹²C→α + ⁸Be fragmentation channel.     

Elastic and inelastic scattering of 15 MeV polarized deuterons from isotopes of Ca,Cu, Sr,Zr, and Mo

The elastic and inelastic scattering of 15 MeV polarized deuterons from ⁴⁸Ca, ⁶³Cu, ⁸⁸Sr, ⁹⁰Zr, ⁹²Zr, and ⁹²Mo has been investigated. Angular distributions of the cross section and vector analyzing power iT₁₁ have been measured for all these nuclei; the tensor analyzing powers T₂₀ and T₂₂ have been studied for ⁹²Zr. Cross sections and vector analyzing powers are generally well explained by the optical model for elastic scattering and by the DWBA with a macroscopic form factor for the inelastic scattering; this is consistent with previous work. Distributions for ⁴⁸Ca, however, are poorly fitted. Anomalous behavior of the N = 50 nuclei found in the inelastic scattering of polarized protons is not present for deuterons. Tensor analyzing powers are not well explained by standard procedures: use of approximate folding model optical parameters did not improve the fits. The distribution of iT₁₁ for the $\text{1}\text{2}$^? state in ⁶³Cu is significantly different from the distributions for the $\text{5}\text{2}$^? and $\text{7}\text{2}$^? states.     

Measurement of nuclear deformation parameters from elastic and inelastic scattering of α and 3He ions on Te isotopes

Cross sections for the elastic scattering of ⁴He by ^{122, 124, 126, 128}Te and ³He by ¹²⁸Te and for inelastic transitions to the lowest 2⁺ states of Te isotopes were measured for several scattering angles at bombarding energies in the vicinity of the Coulomb barrier. The data were analyzed in the framework of the optical model and the DWBA from which optical potential and deformation parameters were extracted. A detailed study of the fit procedure is presented with emphasis on the significance of the extracted parameters and the correlations among them.