S. Brandenburg

Decay of the isoscalar giant monopole resonance in 208Pb

Abstract The decay of the isoscalar giant monopole resonance (GMR) in 208 Pb has been studied with the 208 Pb(α, αn) 207 Pb reaction at θ α ′ = 0°. Comparison of the experimental data with detailed statistical model calculations shows a good overall agreement for the decay of both the GMR and the underlying continuum. Interpreting small discrepancies between the calculations and data for the GMR as indications for the presence of pre-equilibrium decay we find a direct decay branch of the GMR ⩽ 10% and a pre-equilibrium decay branch ⩽ 30%.

The effective 3he-nucleon force in a microscopic DWBA approach to the (3He, t) charge-exchange reaction

Abstract The ( 3 He, t) reaction has been studied on the targets 12 C, 16 O, 24 Mg, 26 Mg, 28 Si, 32 S, 42 Ca, 48 Ca, 54 Fe, 58 Ni and 90 Zr at incident energies 66–90 MeV. From angular distributions of transitions to selected states the parameters V ρ , V σρ , V Tρ and V LSρ of an effective projectile-nucleon force parametrized with single Yukawa potentials have been obtained for this energy domain. Differences from the force proposed earlier by Schaeffer for a lower energy are found, notably in the tensor component.

Experimental determination of monopole strength in 40Ca between 10 and 20 MeV excitation energy

The distribution of isoscalar monopole strength in 40Ca was determined from singles 40Ca(α, α′) and from coincident 40Ca(α,α′α0) data at Eα =120 MeV and scattering angles from 0° to 3°. Between 10.5 and 15.7 MeV excitation energy (23±5)% of the EO EWSR was located from the singles and from the coincidence measurements independently, while (30 ±6)% of the EO EWSR was found in the interval 10.5–20 MeV from the singles data only.

Particle decay of the giant resonance region in 90Zr and especially of the giant monopole resonance

Abstract The decay of the isoscalar giant monopole and quadrupole resonances in 90 Zr has been studied with the 90 Zr(α, αn) 89 Zr and the 90 Zr(α, αp) 89 reactions. For the giant monopole resonance (GMR), a branching ratio for (neutron : proton : alpha) decay of (85(20) : 10(2) : ( case9 2 + ground state of 90 Zr. The proton decay spectrum of the GMR is also in overall agreement with statistical model calculations. The proton as well as neutron decay from the region of the giant quadrupole resonance (GQR) indicate that in the singles (α, α) cross section the L >2 excitation amounts to 25–50% of the L = 2 cross section.

SPREADING WIDTH OF THE IAS IN EVEN PM ISOTOPES MEASURED VIA THE AND(HE-3,T)APM AND AND(HE-3,TPBARA-1ND REACTIONS

Abstract Total and escape widths of IAS in even- A Pm isotopes have been measured via the A Nd( 3 He, t) A Pm and A Nd( 3 He, t p ) A−1 Nd reactions. Values for the spreading widths have been deduced and found to increase strongly with the excitation energy of the IAS through the chain of Pm isotopes. This behaviour, in contrast with the much smoother global increase with mass number, is attributed to a dependence on level density.

Fission probability of the IAR in the actinide region studied via the (3He, tf) reaction

The widths and total fission probabilities, including first-, second- and third-chance fission of the IAR in 232Pa and 238Np Have been measured via the reactions (3He, t) and (3He, tf). The results are: Γ=306 ± 20 keV, Pf = 0.19 ± 0.01 for 232Pa and Γ=380 ±40 keV, Pf = 0.30 ± 0.04 for 238Np. Values for the spreading widths, Γ↓ are deduced. Global systematics of Γ↓ with mass number is indicated.

Study of the fission decay of the isoscalar monopole resonance via the 238U(α, α′) reaction at 0°

Abstract The fission decay of 238 U has been investigated using inelastic scattering of 120 MeV ga-particles to excite the 238 U nucleus. Angular correlations of the fission fragments have been measured for excitation energies between 5.7 and 15.7 MeV in coincidence with inelastically scattered α-particles between 0 and 3°. The difference in yield for fission in coincidence with inelastically scattered α-particles between 0–1.35° and 1.35–3° was used to deduce the fission decay of the giant monopole resonance. It was found that in the fission decay channel (22 ± 5)% of the E0 EWSR strength is located between 8 and 15 MeV excitation energy. The distribution of the deduced monopole strength is in agreement with recent theoretical calculations predicting splitting of the giant monopole resonance in deformed nuclei.

The 24Mg(3He, t)24Al reaction and L=1 giant resonance systematics using an effective 3He-n interaction at 27 MeV/nucleon☆

Differential cross sections from the Mg-24(He-3,t)Al-24 reaction have been measured from 0 to 32-degrees in the laboratory system. DWBA calculations using an effective projectile-nucleon interaction are compared with the data to verify the applicability of such an approach as well as to deduce spectroscopic information. By employing DWBA in conjunction with transition amplitudes obtained from realistic microscopic wave functions, J-pi assignments could be made. Strength normalizations for all observed transitions to positive-parity T = 1 final states of Al-24 are given. Localized DELTA-L = 1 strength around 10 MeV in excitation energy contains most of the expected isovector giant dipole resonance (GDR) and the magnetic quadrupole resonance (MQR) strengths with systematics that are consistent with those for most light, self-conjugate nuclei. A comparison with Coulomb-energy shifted (gamma, n) data assists in locating the GDR analog. The isovector 0- strength is not directly observed, but the total expected DELTA-L = 1, DELTA-S = 1 cross section is found in the broad collective bump at about 10 MeV and in a number of lower-lying states of negative parity.

Neutron decay of the excitation-energy region up to 60 MeV, excited by heavy ion scattering. (I) 208Pb

Abstract The neutron decay of the continuum in 208 Pb, excited by small-angle inelastic scattering of 84 MeV/nucleon 17 O ions in the range from 1.5° to 4.5°, has been measured. Statistical decay was found to dominate the excitation-energy interval studied, up to 60 MeV. In the excitation-energy interval of about 10 to 30 MeV a distinct direct-decay component to the hole states in 207 Pb and a decay pattern resembling pre-equilibrium decay was observed. The angular correlation of the hole-state population suggests that in addition to the direct decay to the hole states in 207 Pb, there is another, unidentified process populating the same states.

Neutron decay of the giant monopole resonance in 124Sn

Abstract The neutron decay of the isoscalar giant monopole resonance (GMR) and of the underlying continuum in 124 Sn was studied. The GMR decay mechanism is shown to be a function of the excitation energy. The non-statistical decay component decreases with excitation from (20±5)% in the lower part to (10±5)% in the higher part. For the lower part there is evidence for pre-equilibrium decay. The decay of the continuum also shows a non-statistical decay component.