P.H. Stelson

Excitation of collective states by the inelastic scattering of 14 MeV neutrons

Abstract Differential cross sections for the inelastic scattering of 14 MeV neutrons to low-lying collective states have been measured for the 13 elements Bi, Pb, Sb, Sn, Cd, Zn, Ni, Cr, S, P, Si, Al and Mg. The inelastically scattered neutrons were separated from the elastically scattered neutrons by a time-of-flight system which had an energy resolution of 450 keV for 14 MeV neutrons. The absolute cross sections were determined to an accuracy of 20 percent over the angular range 20° to 130°. The cross sections obtained for excitation of collective quadrupole and octupole states are compared to the distorted-wave theory for direct interactions. It is found that the experimental shapes of the differential cross sections are in reasonable agreement in most cases with those predicted by theory. The nuclear deformation parameters, βλ, are extracted and these are compared both to the available electromagnetic βλ values and to βλ values obtained from other direct interaction experiments.

COULOMB EXCITATION OF VIBRATIONAL TRIPLET STATES AND OCTUPOLE STATES IN THE EVEN CADMIUM NUCLEI

Abstract Oxygen-16 ions with energies of 42 to 49 MeV have been used to Coulomb excite higher states in the even cadmium isotopes. Double E2 excitation of the 0 + state at 1133 keV in 144 Cd has been observed. The E2 transition between the excited 0′ + state and the first 2 + state is strongly enhanced, i.e., B( E 2, 0′ → 2) B( E 2, 2 → 0) = 0.85±0.17 . The second 2 + state and the 4 + state in 116 Cd are degenerate to within 5 keV at 1222 keV excitation. The values obtained for the ratio B( E 2, 4 → 2) B( E 2, 2 → 0) are 1.42±0.19, 1.82±0.23, 1.80±0.23 and 1.63±0.36, respectively for 110 Cd, 112 Cd, 114 Cd and 116 Cd. States at 2051±16, 1968±16, 1945±16 and 1900±16 keV are excited in 110 Cd, 112 Cd, 114 Cd and 116 Cd, respectively. If these states are interpreted to be the result of direct E3 Coulomb excitation, the B (E3, 0 → 3) are 10.3, 10.6, 9.0, and 7.5×10 4 e 2 fm 6 with an accuracy of ±20% provided the branching ratio of cascades to crossover from the 3 − state is large compared to unity. These B (E3) values represent enhancement of 13 to 20. They are 3 to 6 times smaller than the results derived from yield measurements with 14–15 MeV α particles at Copenhagen.

Coulomb excitation of the even-A cadmium nuclei

Abstract Coulomb excitation of the first 2+ states of the even-A cadmium nuclei was produced by 2.7 and 3.0 MeV protons to obtain accurate B(E2, 0 → 2) values (absolute error ±4%). The B(E2, 0 → 2) obtained for 114Cd is (0.576±0.023)e2b2 and agrees well with the value of (0.567±0.018)e2 · b2 by Glenn and Saladin but disagrees with the recent value of (0.509±0.009) e2 · b2 by Simpson et al. obtained in the course of their measurement of the static quadrupole moment of the 2+ state. Ge(Li) detectors have been used to measure the angular distribution of γ-rays following Coulomb excitation with 11 MeV 4He ions in 106–116Cd. The angular distributions of the γ-rays from the 2′ → 2 transition by direct E2 Coulomb excitation have been obtained with an accuracy sufficient to establish the magnitude and sign of δ ≡ 〈2‖ M ( E 2)‖2′〉/ 〈2‖ M ( M 1)‖2′〉 . The values for δ are near −1.5 for 108,110,114,116Cd, −(8.4−4.7+100) or −(0.6±0.2) and −(3.5−0.8+0.9) or −(0.87±0.10) for 106Cd and 112Cd, respectively. The B(M1 2′ → 2) are large, being between (0.4 and 3) × 10 −2 (e h /2 Mc) 2 . In addition, the angular distributions of the γ-rays from the 2″ → 2 transitions in 112Cd and 114Cd were determined and are consistent with two positive values of δ. Improved values for the branching ratio of cascade/ crossover transitions for the decay of the 2′ and 2″ states have been obtained. With this information the ratios B(E2, 2′ → 2)/B(E2, 2 → 0) have been revised downward 10 to 40% from those given in a previous paper. The 2′ state in 106Cd and 108Cd is above the 4+ state in energy.

Coulomb excitation of 106, 108, 110Pd

Abstract States in the two-phonon energy region of 106, 108, 110 Pd have been investigated via Coulomb excitation, which was effected with 9 and 10 MeV alpha particles and 42–49 MeV oxygen ions. A triplet of states of probable spin-parities 0 + , 2 + and 4 + was observed in each nucleus. Transition rates were extracted from the gamma-ray yields and angular distributions. Although the B (E2) values for transitions from these states to the first 2 + states are large compared to the single-particle estimate, they are consistently smaller than the prediction of the quadrupole-phonon model. The B (E2; 2′ → 0) values are about equal to the single-particle estimate; the B (M1; 2′ → 2) values scatter around 10 −3 × (eħ/2Mc) 2 . A third 2 + state was observed in 110 Pd at 1212.2 keV. The B (E2) value for its ground state transition is of the order of 1/10 the single-particle value. A state was also Coulomb excited in each nucleus at about 2 MeV. It is attributed to octupole vibrations of the nucleus. The values for ɛ B (E3; 3 → 0), where ɛ is the ratio for decay to the first 2 + state to the total decay, in single-particle units are 25, 19 and 17 for 106, 108, 110 Pd, respectively.

Coulomb excitation of states in the even-mass ruthenium nuclei with 16O and 4He ions

Abstract Ions of 16O with energies of 42 to 49 MeV and 10 MeV 4He ions have been used to Coulomb excited states in 98, 100, 102, 104Ru. The second 2′+ and 4+ states in 104Ru, 102Ru and 98Ru are nearly degenerate. The cascade γ-rays from the decay of the 2′+ and 4+ states to the first 2+ state have energies of 535.4 and 530.6 keV in 104Ru and 628.1 and 631.6 keV in 102Ru. The values obtained for the ratio B(E2, 4 → 2)/B(E2, 2 → 0) are 1.27±0.14, 1.37±0.27 and 1.17±0.22 for 100Ru, 102Ru and 104Ru, respectively. For 104Ru, the branching ratio of cascade to cross-over transitions from the decay of the 2′+ state is 1.7±0.2, and the ratio B(E2, 2′ → 2)/B(E2, 2 → 0) is 0.66±0.11. The angular distribution of the γ-rays from the 2′ → 2 transitionin 104Ru has been obtained with an accuracy sufficient to establish the magnitude and sign of δ. The value obtained for δ ≡ δ ≡ 〈2∥ M ( E 2)∥2′〉/〈2∥ M ( M 1)∥2′〉 is −(8.5−1.5+2.5). The B(M1, 2′ → 2) for 104Ru is (3.4−1.3+1.5) × 10−4 in units of ( e h /2Mc )2. A state at 983 keV in 104Ru is interpreted to be the result of double E2 excitation of a 0+ excited state. The E2 transition between the excited 0′+ state and the first 2+ state is 5.0±0.6 times smaller than the vibrational-model prediction.

Gamma-gamma angular correlations following Coulomb excitation

Abstract The triple correlation between two cascade gamma rays and a beam effecting Coulomb excitation has been measured for transitions in 107, 109 Ag, 110 Pd, 117 Sn and 190, 192 Os. These correlations were taken with the special geometry where one gamma-ray detector was placed at 0° relative to the incident beam. The results of this investigation coupled with previous information deduced from the angular distributions of the gamma rays following Coulomb excitation were used to establish E2/M1 admixtures and in the case of 117 Sn, spins of 5 2 for the states at 1020 and 1448 keV.

Deterioration of large Ge(Li) diodes caused by fast neutrons

Abstract The large Ge(Li) gamma-ray detector has become a powerful tool for the investigation of nuclear reactions. Unfortunately these detectors are quite susceptible to fast neutron damage and this makes it difficult to decide whether or not to risk using a detector to study reactions at accelerators where fast neutrons are inevitably present. After ruining several detectors we decided to study the problem in a controlled way. A 30 cm 3 true coaxial diode was systematically irradiated by neutrons from a plutonium-beryllium source. An increase in the width of the 2.614 MeV gamma-ray from 208 Tl was first detected after an irradiation of 5 × 10 7 n/cm 2 . When the total irradiation had reached 6 × 10 8 n/cm 2 , the peak width had increased by more than 50 percent. The irradiated detector was then reprocessed to remove the damage. The diode was again subjected to neutron irradiation. The second curve of resolution deterioration as a function of neutron flux was quite similar to the first one. The procedure was repeated a third time with similar results. Thus, reprocessing a detector effectively removes the neutron damage at a cost of only about 10 to 15 percent of the original price. A method was also developed for using the gamma-ray spectrum to evaluate the amount of neutron flux incident on the detector. The number of counts in the 693 keV peak [from (n,n′) reaction with the 72 Ge in the detector] can be multiplied by 20 to get a rough measure of the neutron flux in n/cm 2 .

Elastic and inelastic proton scattering from 112, 114Cd

Abstract Differential cross sections for elastic and inelastic scattering of 13 MeV protons by 112, 114 Cd have been measured. Special effort was made to extract cross sections for the excitation of states which occur in the two-phonon region. The elastic scattering was fitted by searching for the best parameters of an optical model. The observed cross sections for excitation of the one-phonon 2 + and 3 − states were compared to the predictions of both the DWBA theory and the coupled-channel theory. Distortion parameters were extracted and compared to the electromagnetic values. The differential cross sections for excitation of the two-phonon states are very similar for the two cadmium isotopes, and they are also similar to those previously observed for the palladium isotopes. The coupled-channel theory is used to interpret the excitation of the two-phonon states. An appreciably better fit to the observed cross sections for excitation of the second 2 + states is obtained if a small admixture of the one-phonon state is assumed for the description of these states.

A position sensitive proportional detector for a magnetic spectrograph

Abstract A position sensitive detector has been constructed for use at the focal plane of an Enge split-pole spectrograph. The detector is a single-wire gas proportional detector of the Borkowski-Kopp type which derives the position information from the rise time of the pulses from the two ends of a high-resistance wire anode. A position resolution of 0.7 mm was obtained for the 20 cm long detector operated with the magnet. Spectra are presented which were obtained from the Coulomv excitation of transuranic elements by α-particles.

Coulomb excitation of 107, 109Ag

Abstract Coulomb excitation effected with 10 MeV alpha particles and with 45.5 MeV oxygen ions has been used to determine the properties of the low-lying 3 2 − and 5 2 − states in 107, 109Ag. The gamma-ray energies and branching ratios, and the B(E2), B(M1) and T 1 2 values were extracted from the gamma-ray yields, angular distributions, triple angular correlations and Doppler shift measurements. The results were compared to the predictions of the single-particle core weak coupling model. It was found that the 3 2 − 786.5 keV and 5 2 − 949.7 keV levels in 107Ag and the analogous states in 109Ag at 702.0 and 862.5 keV are consistent with the interpretation that they are due to coupling of the odd p 1 2 proton to the second 2+ core state. Another state of spin 3 2 was observed at 1464.7 keV in 107Ag and at 1324.2 keV in 109Ag. Still other states were populated via multiple Coulomb excitation, as effected with oxygen ions, at 973.5 and 1146.8 keV in 107Ag and at 912.3 and 1090.6 keV in 109ag.