Search Results (23 total)

The force concept inventory and a 10-question context-modified test were given to 647 students enrolled in introductory physics classes at the University of Arkansas. Context changes had an effect ranging from −3% to 10% on the individual questions. The average student score on the ten transformed questions was 3% higher than the average student score on the corresponding 10 force concept inventory questions. Therefore, the effect of contextual changes on the total of the 10 questions is not sufficient to affect normal use of the force concept inventory as a diagnostic instrument.

The cross section for α-particle emission in the ⁶He+²⁰⁹Bi reaction at energies near the Coulomb barrier is remarkably large. Possible reactions that may produce the observed α particles include two-neutron transfer, one-neutron transfer, and direct projectile breakup. Each of these mechanisms results in a distinctive angular correlation between the α particle and the outgoing neutron(s). A neutron-α-particle coincidence experiment was performed to study two-neutron transfer to unbound states of ²¹¹Bi. It is shown that approximately 55% of the observed α-particle yield at and beyond the grazing angle is because of this process. This is more than 2.5 times the fraction attributable to single-neutron transfer. The corresponding 2n-transfer cross section is 0.4 ± 0.1 b.

Neutron-rich ^147,149,151Pr nuclei, produced in the spontaneous fission of ²⁵²Cf, were studied using the Gammasphere array. Possible parity doublets in ¹⁴⁷Pr with N=88 and πh_11/2 bands in ^149,151Pr are proposed. These new data on the level structures of odd Pr isotopes suggest that octupole correlations may also be present in the neutron-rich ₅₉¹⁴⁷Pr₈₈ nucleus such as those observed in ₅₈¹⁴⁶Ce₈₈, and also that the h_11/2 bands in the ^149,151Pr track in energy the yrast bands in ^148,150Ce. The backbending related to the breaking of the neutron i_13/2 pair is observed at ħω≈0.27 (MeV) for the proton h_11/2 band of ¹⁴⁹Pr.

High spin states in neutron-rich odd-Z ^143,145Ba nuclei have been investigated from the study of prompt γ rays in the spontaneous fission of ²⁵²Cf by using γ-γ- and γ-γ-γ- coincidence techniques. Alternating parity bands are identified for the first time in ¹⁴⁵Ba and extended in ¹⁴³Ba. A new side band, with equal, constant dynamic, and kinetic moments of inertia equal to the rigid body value, as found in superdeformed bands, is discovered in ¹⁴⁵Ba. Enhanced E1 transitions between the negative- and positive-parity bands in these nuclei give evidence for strong octupole deformation in ¹⁴³Ba and in ¹⁴⁵Ba. These collective bands show competition and coexistence between symmetric and asymmetric shapes in ¹⁴⁵Ba. Evidence is found for crossing M1 and E1 transitions between the s=+i and s=-i doublets in ¹⁴³Ba.

This report details a greatly improved measurement of the four-neutron evaporation cross section following the fusion of ⁶He+²⁰⁹Bi for center-of-mass energies between 23.5 and 30.7 MeV. The results, for energies above the Coulomb barrier, are interpreted within the context of the standard statistical model.

Several γ transitions in ^101–103Nb, ^98,100Y, ¹⁴⁶La, and ¹⁴⁹Pr are identified from spontaneous fission studies employing a ²⁵²Cf source and the Gammasphere detector array. In particular, the pairing-free bands in ¹⁰⁰Y and ¹⁰²Nb are extended, two rather highly deformed negative-parity bands feeding the 4.3-s β-decaying isomer of ¹⁰²Nb are identified, and their possible bandhead configurations are discussed. In the case of ^101,103Nb, we have extended previously reported level schemes by over 20 transitions in each nuclide. For the fission partners ¹⁴⁶La and ¹⁴⁹Pr we have elucidated their previously unreported yrast level schemes.

The phenomenon of cold (neutronless) alpha ternary fission in spontaneous fission of ²⁵²Cf was experimentally observed by triple gamma coincidence technique with Gammasphere with 72 gamma-ray detectors. Correlated pairs of ₃₆Kr-₆₀Nd, ₃₈Sr-₅₈Ce, ₄₀Zr-₅₆Ba, ₄₂Mo-₅₄Xe, ₄₄Ru-₅₂Te, and ₄₆Pd-₅₀Sn were observed to be associated with α ternary fission of ²⁵²Cf. Yields of cold α ternary fission were extracted.

Several γ transitions in ^105-109Tc nuclei were identified for the first time from spontaneous fission studies with a ²⁵²Cf source and the Gammasphere. New level schemes are proposed and related to the underlying nuclear structure. Positive parity bands with a large signature splitting observed in ^105,107,109Tc are evidently derived from g_9/2 orbitals and are similar to analogous bands in ¹⁰³Rh,¹⁰³Ag, and ⁹⁹Y. New γ transitions have also been identified in ^139-143Cs and used to construct level schemes for these isotopes. Correlated-pair fission yields extracted from the data show an appreciable field for the zero neutron ¹⁰⁹Tc/¹⁴³Cs pair.

From an experiment with Gammasphere and a ²⁵²Cf spontaneous fission source, a new negative-parity band in ¹⁵⁴Nd and new negative-parity levels in ¹⁵²Nd were identified and the yrast bands were extended to 18⁺ in ¹⁵⁴Nd and 20⁺ in ¹⁵²Nd in a triple gamma coincidence study. These new negative-parity bands are consistent with octupole vibrational mode. There is a constant difference as a function of spin between the J₁ values for the negative-parity band in ¹⁵²Nd and J₁ for the similar negative-parity band in ¹⁵⁴Nd, however, their J₂ values are essentially identical. These bands indicate a new kind of identical band.

Ten new transitions in ¹⁰⁷Mo have been observed and levels in ¹⁰⁹Mo are identified for the first time in a γ-γ-γ coincidence study from the spontaneous fission of ²⁵²Cf with 72 Compton suppressed Ge detectors in Gammasphere. Two sets of bands, each set intertwined by E1 transitions are observed in ¹⁰⁷Mo and one such set in ¹⁰⁹Mo. The observed level schemes are interpreted in terms of possible octupole deformation originating from the strong interaction of the h_11/2 and d_5/2 neutron shells.

High spin states of ⁸⁴Sr nucleus excited through the ⁵²Cr(³⁶S,2p2n) reaction at 130 MeV energy were studied utilizing the Oak Ridge Compton-Suppression Spectrometer System. The level scheme has been extended up to probably I^π=(24⁺) at 15 084 keV excitation energy for the positive parity band. The moment of inertia, for angular frequency higher than 0.58 MeV, presented an almost constant value.

Energy spectra and angular distributions of evaporated protons from the reaction ⁵²Cr(³⁴S, 2p2n)⁸²Sr at 130 MeV were measured in coincidence with discrete γ transitions. Large shifts and changes in the shape of the proton spectra were observed when high-spin states in different rotational bands are populated. They are interpreted as due to near-yrast stretched proton emission, which preferentially populates the yrast band by subbarrier protons.

The conversion of kinetic energy into target-like fragment excitation and orbital angular momentum into fragment spin has been studied in the reaction 333 MeV ²⁸Si with ¹⁸¹Ta. The light charged particles were detected in a small, highly segmented, 4π phoswich detector system placed in the spin spectrometer, a 4π NaI array which served as a neutron and gamma detector. Multiplicities of light charged particles and neutrons detected in coincidence with projectile-like fragments indicate that the excitation energy of the target-like fragment increases as the kinetic energy of the projectile-like fragment decreases through the quasielastic region and tends toward saturation as the kinetic energy of the projectile-like fragment approaches the kinetic energy corresponding to complete damping. Measurement of the γ-ray multiplicity in coincidence with the projectile-like fragments indicates that the angular momentum transferred to the target-like fragment increases with decreasing mass of the projectile-like fragment for the quasielastic energy region in contrast to the energy region corresponding to completely damped processes where the angular momentum of the target-like fragment decreases with increasing mass loss from the projectile. The influence of preequilibrium processes on both excitation energy and angular momentum transfer to the target-like fragment is discussed. These data present further evidence that l waves below the entrance-channel critical angular momentum for fusion must contribute to the nonfusing reaction channels.

The dependence of multifragment-emission processes upon linear momentum transfer to the target residue is determined for ³⁶induced reactions on ²³⁸U at E/A=35 MeV. Significantly more intermediate-mass fragments are emitted in central collisions. Multiplicity distributions for intermediate-mass fragments emitted behind the grazing angle resemble Poisson distributions and are consistent with a stochastic production process. Exit channels with up to five intermediate-mass fragments and two fission fragments were observed.

The angular momentum dependence of large fragment production in long lived reactions is studied by measurements of fragment cross sections from reactions with substantially different angular momentum distributions and the coincident γ-ray multiplicity distributions. The results indicate that the primary l-wave distributions move to larger mean values and decrease in width and skewness with increasing mass symmetry in the decay channel. The results also confirm that the partition of angular momentum in kinetic energy relaxed heavy-ion reactions is that expected for a rigidly rotating intermediate.

An in-beam spectroscopic study of ¹³⁵Pr was made using a 91 MeV ¹²⁰Sn(¹⁹F,4n) reaction. Two positive parity bands were observed based on the g_7/2 proton orbital with (π,α)=(+,±(1/2)). A strongly populated negative parity (-,-(1/2)) band was also seen (based on the h_11/2 proton orbital). The unfavored (-,+(1/2)) structure may also be present. In both the positive and negative parity bands the first backbend is attributed to the alignment of two h_11/2 protons on the basis of cranked shell model calculations. The crossing frequency was found to increase in the (-,-(1/2)) band because of blocking of the h_11/2 orbital. The alignment of protons in ¹³⁵Pr causes a shift of γ towards ∼+10° with nearly collective prolate deformation. Also, there is some evidence for a second backbend and an upbend in the case of positive and negative parity bands, respectively. They could result from the alignment of the h_11/2 neutrons.

The decay of ²⁰⁴Bi(6⁺, 11.3 h) is found to populate a delayed 7^- state (T_{1 / 2}=0.45 μs, E^*=2264 keV) in ²⁰⁴Pb. A transition from this state accounts for 60% of the feeding of the well-known 9^- isomer at 2185 keV.

[RADIOACTIVITY ²⁰⁴Bi; measured E_γ, I_γ, γγ(t) ²⁰⁴Pb; deduced levels, J, π, T_{1 / 2}.]

Pb K x rays and ∼ 1.08-MeV K-conversion electrons have been found among radiations emitted in the decay of 4.2-min ²⁰⁶Tl. These radiations imply the existence of weak β transitions (0.08%) to the 1.17-MeV 0⁺ state in ²⁰⁶Pb, which decays primarily by an E0 transition to the ground state.

The ¹⁴¹Pr(α,α^′) reaction at E_α=45.0 MeV and ΔE_α^′≈35 keV was studied to investigate the particle-vibration interaction in this N=82 nuclide and to elucidate the ¹⁴¹Pr level structure in the 0-4-MeV region. The α-particle angular distributions were measured in ≤2° steps over ranges of 7-80° for elastic scattering and 12-66° for inelastic scattering. A ¹⁴¹Pr(d,d^′) spectrum at E_d=28.7 MeV and with higher resolution (20 keV) was also recorded. These measurements, in combination with other high-resolution studies, establish the existence of approximately 50 levels with excitation energies of 1.1-3.0 MeV. Approximately 20 of these levels can be identified in the (α,α^′) spectra and are interpreted as states with significant collective components. Although most of these levels are in either a positive-parity group between 1.1 and 1.9 MeV or a negative-parity group between 2.0 and 2.5 MeV, it is found that the assumption of a weak particle-vibration interaction based on the 2⁺ and 3^- core states of ¹⁴⁰Ce and the d_{5 / 2} ground state of ¹⁴¹Pr is inadequate to explain the results. Qualitative agreement is found with the results of recent shell-model studies which assumed nine active protons outside an inert Z=50, N=82 core. In addition, some evidence is found for the approximate validity of the ΔB=0 selection rule for inelastic scattering transitions which was recently proposed by Hecht and Adler.

We have studied the decay of ¹⁰⁷Rh by γ-ray spectroscopy using Ge(Li) detectors and Ge(Li)-NaI(Tl)_γγ coincidence techniques. A decay scheme has been constructed incorporating all but six of the 39 transitions reported. This decay scheme involves excited states in ¹⁰⁷Pd at 115.6, 302.8, 312.2, 348.2, 381.8, 392.5, 471.2, 567.7, 670.0, 1102, and 1148 keV. A half-life of 850±100 nsec was found for the 115.6-keV level, and γγ delayed-coincidence spectroscopy was employed to determine the features of the decay scheme related to this level. β branching ratios and logft values have been derived from the relative γ-ray intensities and the β / γ ratio. We find that the β decay of ¹⁰⁷Rh tends to avoid populating ¹⁰⁷Pd levels that have strong single-particle character [that is, levels populated strongly in (d,p) and (d,t) reactions].

The mass number of Rh¹⁰⁷ (21.7±0.4 min) has been confirmed by alpha bombardments of Ru¹⁰⁴ at selected energies, by fastneutron bombardment of Pd¹⁰⁸, and by chemical identification; and the mass number of Rh¹⁰⁸ (16.8±0.5 sec) has been established by the nature of its γ radiations. The mass numbers of the two precursors Ru¹⁰⁷ (4.2±0.3 min) and Ru¹⁰⁸ (4.5±0.2 min) have thereby also been established. The main radiations associated with these species are as follows (energies in keV): Ru¹⁰⁷ β groups of 2100±300 and 3150±300, γ rays of 195 (14%, coincident with the 2100 β), 370 (∼6%, coincident with the 195 γ), 480 (weak, seen only in coincidence with the 195 γ), 860 (7%, coincident with the 195 γ), 930 (4%), 1030 (4%), and 1290 (4%); Ru¹⁰⁸ β groups 1150±100 and 1320±100, a γ of 165±3 (28%, coincident with the 1150 β); Rh¹⁰⁷ β groups 840±40, 940±70, 1140±50, and 1200±50 keV, γ rays of 115 (0.5%), 285 (∼3%, seen only in coincidence with the 390 γ), 307 (73%, coincident with the 1200 β), 365 (∼2%, coincident with the 307 γ), 390 (11%, coincident with the 1140 and 840 β groups), 470 (1%), 570 (2%, coincident with the 940 β), and 675 (3%, coincident with the 840 β), and γ coincidence-sum lines at 680 (∼3%), 880 (very weak), and 1140 (≤0.5%); Rh¹⁰⁸ β of 4500±600 keV, γ rays of 430 (43%), 510 (10%, partly coincident with the 430 γ), 620 (22%, coincident with the 430 γ), 1520 (5%), and 2000 (≤3%). No 940 γ from the Pd¹⁰⁸ 940 level could be detected (<3%), and less than one-third of the 510 γ rays follow β transitions to the 940 level. All γ rays above 115 in the four species are in fast coincidence with β rays. There are other γ rays associated with these species, especially for Ru¹⁰⁷ and/or Rh¹⁰⁸ above 1500. No evidence was found for isomers of Pd¹⁰⁷, Rh¹⁰⁷, Pd¹⁰⁸, or Rh¹⁰⁸ from decay of 21.7-min Rh¹⁰⁷ or 16.8-sec Rh¹⁰⁸. No Rh¹⁰⁷ or Rh¹⁰⁸ isomers could be isolated. However a γ ray of 21 keV, presumed to be K x rays of rhodium, appears in the short-lived ruthenium spectrum, with an intensity 5 to 8% relative to Ru¹⁰⁷; this is attributed to a short-lived (<10 sec) Rh¹⁰⁷ isomer formed in >8% of the Ru¹⁰⁷ disintegrations. The following decay paths are proposed: decay of Ru¹⁰⁷ (Q^β=3200) proceeding to Rh¹⁰⁷ excited levels of 1290 (4%), 1030 (11%), 930 (4%), 675 or 1530 (<5%), 565 (∼6%), and 195 (0 to 5%), and to ground state (74%), with the reservation that it is likely that some of these levels should be referred to the supposed isomeric level rather than to the ground level; decay of Rh¹⁰⁷ (Q^β=1510) proceeding to Pd¹⁰⁷ levels of 1140 (∼0.2%), 675 (may be more than one level, 7%), 570 (2%), 470 (1%), 390 (8%), and 307 (71%), and to ground state (0 to 17%); decay of Ru¹⁰⁸ (presumably 0+, Q^β=1320) proceeding to Rh¹⁰⁸ levels of 165 (0+ or 1+, 28%) and ground state (1+, 72%); decay of Rh¹⁰⁸ (1+, Q^β=4500) proceeding to Pd¹⁰⁸ levels of 2000 or higher (≤3%), 1520 or higher (≥5%), 1050 (0+, 22%), 940 (2+, 0 to 5%), and 430 (2+, ∼17%), and to ground state (0+, 51%). Interpretations of these schemes are presented.