E.G. Bilpuch

A high-resolution study of isobaric analogue resonances in 41K

Abstract Cross sections for 40Ar(p, p)40Ar and 40 Ar ( p , α) 37 Cl have been measured from 1.63 to 2.60 MeV at a lab angle of 135° and with an energy resolution of about 250 eV. The 40Ar(p, n)40K cross section has been measured from threshold (2.344 MeV) to 2.60 MeV. Almost all single elastic scattering resonances near 1.87 and 2.45 MeV can be interpreted as fine structure of the isobaric analogues of the fourth ( J π = 3 2 + ) and sixth ( J π = 1 2 + ) excited states of 41Ar. These two analogue resonances appear primarily as an enhancement of the normal (T = Tz) states in the compound nucleus. Two very narrow elastic scattering resonances are also observed near 1.1 MeV. These data are interpreted in the light of recent theoretical work on isobaric analogue resonances.

Fine structure of analogue states

Abstract This article reviews all the high-resolution data on fragmented analogue states taken at the Triangle Universities Nuclear Laboratory, over a period of ten years. There are fifty analogue states observed by proton scattering on targets of masses between 40 and 64, and mass 92. The number of fragments may be only two or three, or as many as fifty. Of the total, only 17 states have a sufficient fine structure pattern where an analysis is attempted. In a few cases, inelastic widths and photon and neutron widths are observed besides proton elastic ones. The article discusses the optimum method of analysis of the data with a view to extracting the physical parameters of the analogue: the energy, proton spectroscopic factor, spreading width, shift and asymetry parameter. The last three quantities are discussed from the viewpoint of the Robson model based on one-channel external mixing of the analogue by Coulomb forces. This model has qualitative success, particularly in describing the large asymmetry sometimes seen, but it is not quantitatively adequate. When the proton spectroscopic factors are compared to the neutron counterparts for the parent (as measured by (d,p) studies), after allowing for Coulomb effects, the proton ones are smaller by an amount which increases to 30% in the heavier nuclei (Ni,Mo). This is in line with the situation on 2 0 8 Pb. Thus it seems that the inadequacy of a Coulomb explanation of the analogue-parent energy shift (the Nolen-Schiffer anomaly) has a counterpart in the spectroscopic factors.

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.

Fine structure of analogue states in 55Co, 57Co and 59Co

Abstract Differential cross sections were measured at four angles for proton elastic scattering from 54, 56, 58Fe at energies ranging between 1.8 and 3.3 MeV. Using the TUNL 3 MV Van de Graaff accelerator and high-resolution analyser-homogenizer system, a total resolution of 300–400 eV was achieved for thin solid targets of enriched iron isotopes. Seven analogue states in 55Co and 57Co appeared as single resonances; three analogue states in 59Co showed well-developed fine-structure distributions. Spin, parities and widths were determined for approximately 200 resonances. Spectroscopic factors and Coulomb energy differences were extracted for the analogue states.

Fine structure of analogue states in 51Mn, 53Mn and 55Mn

Abstract Excitation functions were measured for proton elastic scattering from 50, 52, 54 Cr, inelastic scattering from 50 Cr and the (p, n) reaction on 54 Cr. Using the TUNL 3 MV Van de Graaff accelerator and high-resolution analyser-homogenizer system, a total resolution of 300–400 eV was achieved for thin solid targets of enriched chronium isotopes. Eleven analogue states were observed; six of these showed well-developed fine structure patterns. Spins, parities and partial widths were determined for approximately 330 resonances. Spectroscopic factors and Coulomb energy differences were extracted for the eleven analogue states.

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 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.