Henry W. Newson

s- AND p-WAVE NEUTRON SPECTROSCOPY. PART VII. WIDTHS OF NEUTRON RESONANCES

Abstract Neutron reduced widths Γn0 and Γn1 are reported for about 200 resonances observed in neutron total cross sections of Ca40, 44, Ti48, Cr50, 52, 54, Fe54, 56, Ni58, 60, Sr88, Y89, Sn124, Te130, Ba136, 138, and Pb206, 207, 208, in the energy region 1 to 200 kev. Average parameters Γ n 0 , Γ n 0 D , and Γ n (1) D have been derived and the Wigner distribution for local spacings and the Porter-Thomas distribution for reduced widths are verified for the resonances in the even-even nuclei Ca40, Fe56, Ni58, and Ni60. A simple method of area analysis which is less tedious and time consuming than the method reported before in Part III is also described.

s- and p-wave neutron spectroscopy: Part VIII. Subshell effect on nuclear level spacing near A = 50☆☆☆

Abstract The neutron total cross sections of separated isotopes of several even A targets near A = 50 have been measured with high resolution using the Li7(p, n)Be7 reaction as the neutron source for a 20° collimator. The measurements on Ca40, Cr52, Fe54, and Fe56 are characterized by a number of narrow l > 0 resonances superimposed on much wider but easily distinguishable s-wave resonances. The Ti46, Ti48, and Ti50 nuclei exhibit a large enough proportion of these l > 0 resonances to complicate identification of s-wave resonances. Since the usefulness of area analysis is limited by strong resonance-resonance and resonance-potential interference, a multilevel formula obtained from the R-matrix formalism has been used to fit the wide s-wave resonances. After correcting the resulting 1 2 + level spacings to zero angular momentum and for variations in excitation energy, two other important influences on level spacing were observed; the neutron excess effect observed earlier by Newson and the subshell effects predicted by Rosenzweig. The former phenomenon has been verified beyond reasonable doubt and the latter which results from the influence of ground-state subshell occupation on the level spacing at higher excitation energy is consistent with Rosenzweigs predictions. The moment of inertia of some of these nuclei when excited to the neutron separation energy have been determined. They apparently fluctuate widely around the rigid body value but on the average are slightly higher. We have also found the distributions of widths and spacings to be in reasonable good agreement with the respective Porter-Thomas and Wigner predictions.

Fine structure of analogue states in 59Cu, 61Cu, 63Cu and 65Cu

Abstract Differential cross sections were measured at lab angles of 160°, 135°, 120° and 90° for proton elastic scattering on 59 Ni, 60 Ni, 62 Ni and 64 Ni at energies ranging between 1.8 and 3.3 MeV. The 64 Ni(p, n) 64 Cu total yield was measured from threshold (≈ 2.50 MeV) to 3.3 MeV. All data were taken using the TUNL 3 MV Van de Graaff accelerator and high-resolution electrostatic analyser-homogenizer system. A total resolution of 300–450 eV was realized using thin solid targets of the enriched nickel isotopes. There were nine l = 1 analogue states identified in the elastic scattering data. In almost all cases, the analogue state was fragmented into individual fine structure resonances. Spins, parities, total and partial widths were extracted from the data. Spectroscopic factors were obtained using the resonances parameters from the data and calculated single-particle widths. Coulomb energy differences were also extracted for the nine l = 1 analogue states.

NEUTRON CAPTURE CROSS SECTIONS IN THE Kev REGION. PART II. SPIN-ORBIT COUPLING AND THE OPTICAL MODEL

Abstract An intensive study of average neutron capture cross sections of nuclei with 75 ≦ A ≦ 130 has been carried out in the neutron energy region 3 to 200 kev, using the activation method. Assuming s-wave parameters within the limits of error specified by low-energy total cross sections determinations, the data has been analyzed for corresponding p-wave parameters: the neutron strength function Γ n (1) D and the γ-ray strength function Γ γ D 0 . It is found that the s- and p-wave γ-ray strength functions are the same within the rather large limits of experimental error for the odd A targets: Rh103, Ag107, Ag109, In115, I127, and As75. The even-even isotopes of Pd, and Mo are more complicated in this respect. The p-wave neutron strength functions Γ n (1) D derived from the analysis of these capture cross sections are found to be in agreement with Γ n (1) D as obtained from the analysis of neutron total cross sections. These strength functions exhibit a broad, double-peaked, distribution around A = 100 instead of the single peak predicted by the simple optical model. It is shown that the experimental results may be explained by the addition of a small spin-orbit part to the optical potential. The parameters of the square well optical potential V ± = V 0 [1 + iξ + δ 2 {−1 ± (2l + 1)}] which are found to fit the p-wave strength functions best are V0 = 41 Mev, ξ = 0.02, δ = 0.085, and R = 1.45 A 1 3 fermis.

Method of Canceling Energy Fluctuations of a Van de Graaff Ion Beam

A varying correction voltage is applied to the target which cancels beam energy fluctuations. This correction voltage is a known, constant multiple of a small, variable voltage which must be applied to the outer plate of an electrostatic analyzer to keep the beam centered at the analyzer image slits. The beam current at the analyzer image slits provides the signal which, when amplified by an electronic device called the Homogenizer, drives both the outer analyzer plate and the target. The energy spread is reduced to 250±120 ev and is probably due only to the ion source. If the HH+ beam is put through the analyzer, the unlimited H+ beam also has an energy spread of 250 ev. The 1.76‐Mev C13(p,γ) and 0.992‐Mev Al27(p,γ) resonances were used to test performance. Our data are consistent with Γ(C13) = 75 ev and Γ(Al27) = 100 ev. The upper limit for the C13 width is 150 ev.

Neutron capture cross sections in the kev region: Part I. Methods of measurement and analysis☆☆☆

Abstract The relative capture cross sections in the neutron energy range from three to two hundred kev for Au197, Pt198, and U238 have been measured with the beta activation technique. After normalization, the capture cross sections of Au197 and U238 were analyzed for the p-wave parameters, ( Γ γ D 0 ) p and Γ n (1) D . For Au197 it was found that the relation ( Γ γ D 0 ) p = ( Γ γ D 0 ) s was consistent with the experimental data. In the case of U238, ( Γ γ D 0 ) p = 2.6 ( Γ γ D 0 ) s . This relation can be explained qualitatively by considering parity effects in the shell model. The p-wave strength functions consistent with the experimental data were Γ n (1) D = 0.33 × 10 −4 and Γ n (1) D = 0.7 × 10 −4 for Au197 and U238, respectively. Techniques of measurement and analysis are described.

s- and p-wave neutron spectroscopy: Part XIa. Level spacings in even intermediate elements

Abstract High resolution neutron total cross sections of separated isotopes of Ca12, Ca44, Cr50, Cr54, Ni58, Ni60, Ni62, and Ni64 have been measured using the Li7(p, n)Be7 reaction as the source of neutrons. The data have been fitted with a multilevel formula derived from the R Matrix Theory. Widths have been obtained for all recognizable s-wave resonances and for most of the p-wave resonances. The distributions of the reduced widths and local spacings of the s-wave resonances were found to be in good agreement with the combined Porter-Thomas and Wigner distributions. The average s-wave level spacings have been corrected to an excitation energy of 6 MeV and combined with data for Part VIII in this region in order to compare the behavior of the average level spacings with the predictions of various shell models. We obtain reasonably good agreement with both the Rosenzweig model and the model of Newson and Duncan if we assume that, at the excitation energies involved, a major shell is closed at 16 nucleons instead of 20. The data indicate that a dependence of the average level spacing on neutron excess exists but that it probably is not a simple exponential. Attempts to find triple and quadruple coincidences of the resonances due to the Ni, Cr, and Ca isotopes were unsuccessful.

s- and p-wave neutron spectroscopy: Part II. Even-even nuclei

Abstract Total cross sections have been measured for Ni58, Ni60, Ce140, Ce142, Mg24, Mg25, Mg26, Si28, Zr90, Zr92, and Zr94 from 3 to 300 kev. Of the eight well separated levels found in Ni58, six are matched by levels in Ni60 at very nearly the same energies. This effect is very difficult to understand on the basis of conventional nuclear models. Strength functions ( Γ n 0 D ) of 3.4 × 10−4, 2.3 × 10−4, 1.0 × 10−4, and 1.2 × 10−4 and average level spacings of 23 kev, 21 kev, 3 kev, and 1 kev have been estimated for Ni58, Ni60, Ce140, and Ce142, respectively. The strength functions and level spacings of the Zr isotopes are difficult to interpret because of the importance of both s- and p-wave resonances; however, a preliminary average cross-section analysis of Zr and neighboring nuclei shows a very definite peak in the p-wave strength function at atomic weight 92. Resonance parameters have been obtained for resonances observed in the magnesium isotopes.

s- and p-wave neutron spectroscopy: Part IV. Experimental methods in the kev region☆

Abstract A new high-resolution collimation-detection system has been developed for neutron total cross-section measurements in the kev region. The instrument has a variable opening between conical polyethylene collimating surfaces and admits neutrons emitted from the Li(p,n) Be reaction at about 160° with respect to the direction of the protons. The neutrons are detected by two separate banks of boron trifluoride counters. Neutron energy spread is relatively constant over a wide range of neutron energies. The resolution is apparently limited at present by nonuniformity of the thin Li targets. The optimum resolution obtainable with a reasonable counting rate has been calculated. Measurements have been made of the yield at 160° of the small extraneous background scattered into the detectors by the target backing and other sources. The total cross section for Bi209 has been measured from 10 to 136 kev; prominent s-wave resonances occur at 12, 15, 32, 44.5, 68, 80, 84, 94, 101, 112, 116, and 134 kev in addition to five other less prominent levels and three known resonances below 10 kev. A resolution correction was applied to the first four resonances and the peak cross sections corrected in this manner are close to the theoretical values. The average level spacing is 10 ± 3 kev per s-wave channel (spin state). An estimate has been made of the neutron widths Γn by a new method (see previous paper). The s-wave strength function based on more than twenty resonances is (0.57 ± 0.17) × 10−4.

High resolution proton scattering from 42Ca

Abstract Differential cross sections were measured for 48Ti(p, p) and 48Ti(p, p1) at four angles between Ep = 1.8 and 3.1 MeV. The overall energy resolution was 250–350 eV. Spins, parities, total widths and partial widths were extracted for 301 resonances. Two analogue states were observed, and spectroscopic factors and Coulomb energies determined for these analogue states. A large positive correlation was observed between the elastic and inelastic widths for one analogue; no correlation was observed away from the analogue. The spacing distributions of the s 1 2 and p 1 2 resonances (after correction for the energy dependence of the average spacing) are in reasonable agreement with the Wigner distribution. The s 1 2 , p 1 2 and p 3 2 and p 3 2 proton strength functions were determined.