Search Results (24 total)

The Spallation Neutron Source ring injection dump beam line has been suffering high beam losses since its commissioning. In order to understand the mechanisms of the beam losses, we have performed 3D simulation studies of the beam line. The 3D models consist of three injection chicane dipoles and one injection dump septum magnet. The 3D particle trajectories in the models are computed. We then extend particle optics calculations to the injection dump. Our studies have clearly shown some design and operation problems, which cause beam losses in the injection dump beam line. These include incorrect chicane dipole settings, incorrect position of a chicane dipole, too small aperture of injection dump septum, and inadequate focusing downstream. This paper reports our findings and the remedies to the injection beam loss problems.

Transverse beam profiles are observed to broaden with increasing intensity in the Proton Storage Ring at the Los Alamos Neutron Scattering Center. Measured profiles are simulated with an H^- injection model that includes a 2D particle-in-cell space charge calculation. Inclusion of space charge effects in the simulation improves the agreement between the experimentally observed profiles and the calculated profiles. The comparisons are made for a range of injected intensities.

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the ferrite. Results show that the minimum rf voltage necessary to stabilize the beam against e-p instability may be reduced over that previously measured. The injected bunch length was observed to be longer when the ferrite was heavily biased so that its effect was reduced.

The interaction of 800-MeV H^- ions with a thin foil produces protons, the ground state and excited states of neutral H⁰ atoms, and unstripped H^- ions. We investigated the distributions of individual H⁰ Stark states within the n=3 and 4 levels produced by C and Al₂O₃ foil stripping of H^- ions. Foils of various thicknesses were placed upstream of a magnet with a linearly increasing transverse field along the beam direction producing a motional electric field strong enough to ionize H⁰ states with n>~3. We consider three questions: (i) What are the populations of individual H⁰ Stark states produced in the interaction of 800-MeV H^- ions with thin C and Al₂O₃ foils, (ii) how do the relative population distributions change with foil thickness, and (iii) how is the population distribution produced in an Al₂O₃ foil modified when the foil is placed in a magnetic field? A simple qualitative model is presented to explain the major trends.

Measurements of H^- stripping and H⁰ excited-state production for a wide range of foil thicknesses and experimental conditions are reported. An 800-MeV H^- beam was passed through carbon or aluminum oxide foils of thicknesses ranging from 10 to 550 μg/cm² and the excited states produced were analyzed by field stripping in a special magnet downstream of the foil. The foil thicknesses were independently determined. The H⁰ atoms emerging in excited states with n>2 can be stripped to protons in fields of up to 1.3 T. The yield of excited states as a function of foil thickness and the cross sections for the various interactions are presented. The cross-section ratio of double to single ionization of H^- in carbon is found to be (1.8±0.9)%.

The lifetime of 800-MeV H^- ions against electron detachment in a static electric field was measured over a range of eight orders of magnitude in experiments at the High Resolution Atomic Beam Facility of the Los Alamos Meson Physics Facility. The ions traversed a linear gradient magnetic field of 1.3-T peak strength resulting in a 6-MV/cm peak rest-frame electric field capable of stripping a large fraction of H^- ions. The unstripped H^- ions, neutral H⁰ atoms, and protons were detected 5.5 m from the magnet. This spectrum was analyzed to determine the lifetime of the H^- ion versus electric-field strength and the results were compared with previous studies. Three parametrizations of the lifetime formula based on an existing theory were used to calculate the stripping probability. The data were fit to the lifetime formula and good agreement with theoretical predictions was found. Finally, a possible experiment for observing excited states of H^- is briefly discussed.

Elastic and inelastic π^± scattering by ²⁰⁶Pb has been studied to measure the isospin character of transitions to bound states. The data have been interpreted using both distorted wave impulse approximation and optical model potentials. The data for the collective states at 2.647 MeV (3^-) and 4.111 MeV (2⁺) are well reproduced with δ_l⁺=δ_l^-=δ_l^p, i.e., assuming that these transitions are isoscalar. For the 0.803-MeV, 2⁺ level we deduce M_n/M_p=2.6±0.3 which is in excellent agreement with a value obtained from inelastic heavy-ion scattering.

Inclusive and exclusive spectra of the He⁴(π,π′p) reaction were measured with π⁺ and π^- at T_π=140 MeV and θ_π=40°, and at 180 MeV and 30°, 40°, 60°, and 80° using the EPICS system at LAMPF in coincidence with eight plastic scintillators. The inclusive spectra yield cross section ratios R_π=σ(π⁺)/σ(π^-)=1.1±0.1 at both T_π and all θ_π. Exclusive spectra were obtained between He^4* excitation energies E_x=21.5 and 45.0 MeV. Angular correlation functions for (π,π′p) were extracted at T_π=180 MeV at the four pion angles for selected regions of E_x. The (π⁺,π^+′p) angular correlations show a distinct peak near the free proton knockout angle; however, the (π^-,π^-′p) data show either a flat distribution or a minimum near that angle. Calculations, using a distorted wave impulse approximation (DWIA) code which models the (π,π′p) reaction as a pion-included proton knockout, predict peaks in the angular correlation functions in the quasifree knockout direction and reproduce quite well the (π⁺,π^+′p) data above E_x=30 MeV. However, these calculations do not resemble the (π^-,π^-′p) data at any excitation energy. Above E_x=30 MeV, the ratios R_πp=σ(π⁺,π^+′p)/σ(π^-,π^-′p) were found to be unexpectedly large (up to R_πp=50) near the free proton knockout angle and very small (R_πp=0.3) in the opposite direction in contrast to DWIA predictions of 8 and 5, respectively. These discrepancies are evidence that strong interference occurs between the quasifree proton knockout and another process, especially in the (π^-,π^-′p) reaction. Near the He⁴→p+t breakup threshold, where the 2^-, T=0 state in He⁴ (E_x=21.8 MeV) is known to exist, the ratio R_πp was found to be between 1 and 2 at all pion angles which is in reasonable agreement with the expected value of 1 for the excitation and decay of a state of good isospin.

Differential cross sections were measured for pion elastic and inelastic scattering from ²⁰⁸Pb at T_π=120 and 250 MeV. Energy-dependent neutron- and proton-transition matrix elements for a range of excited states were extracted and tested for consistency, using several structure models.

Analyzing power A_y and spin-transfer observables D_ij have been measured for 500-MeV proton inelastic scattering from ²⁸Si. Measured values of the D_ij for the 9.70-MeV 5^-, T=0, the 11.58-MeV 6^-, T=0, and the 14.35-MeV 6^-, T=1 states are reported at 17° and 22°; values of A_y cover the range from about 10° to 26°. Nonrelativistic (NRIA) and relativistic (RIA) impulse-approximation calculations are compared with the data. The differences between the two types of calculations are generally small. The RIA yields excellent agreement with the D_ij data for the 5^- state, and both RIA and NRIA do well for D_ij data for the 6^-, T=0 state but poorly for the 6^-, T=1 state. For the A_y data, both types of calculation give fairly good predictions for the 5^- and 6^-, T=1 states, but not for the 6^-, T=0 state. Comparison between theory and experiment for the combinations of observables D_K, which are sensitive to individual terms in the nucleon-nucleon interaction, indicates a possible need for medium corrections in the T=1 tensor and spin-orbit forces.

We present cross sections for the reactions ⁴⁴Ca,⁵⁶Fe(π⁺,π^-) leading to the residual ground states and double-isobaric-analog states (DIAS) for incident pion energies from 100 to 300 MeV. Data include both angular distributions and 5° excitation functions. Ground-state cross sections were also obtained from the ¹²C and ¹⁶O contaminants in the ⁴⁴Ca target. The DIAS data are compared to known systematics and to theoretical predictions. Some features of the data are reproduced by the theoretical predictions, but major discrepancies exist.

Exclusive ⁴He(π^±,π^±’p)³H spectra in the excitation energy range between 21.5 and 44 MeV in ⁴He were measured at T_π=180 MeV at several pion and proton angles. The cross section ratio R_πp=σ(π⁺,π⁺’p)/σ(π^-,π^-’p) was found to be between 1 and 2 near 22 MeV which is in the region of the J^π=2^-, T=0 state. Between 25 and 40 MeV in the continuum, which comprises the giant dipole resonance and broad 2^- and 2⁺ states, R_πp ranges from 0.22 to 45 in sharp contrast to the predicted values of ≊1 from a model assuming the entire yield is due to the sequential decay of a state of good isospin, and 6 to 9 from a distorted-wave impulse approximation calculation assuming quasifree proton knockout.

Analyzing powers have been measured for elastic and inelastic scattering of 500-MeV protons from ²⁸Si. These data for the first 0⁺, 2⁺, and 4⁺ states and the corresponding cross-section data have been analyzed with both Schrödinger and Dirac equation phenomenological coupled-channels methods. Good, qualitatively similar, results are achieved with the two methods.

Inelastic scattering of 162 MeV π^- and π⁺ was used to study the previously known J^π=8^- stretched transitions in ⁶⁰Ni. A consistent analysis of data from electron and pion scattering to stretched states using both harmonic oscillator and Woods-Saxon wave functions (unbound as necessary) was performed for ⁶⁰Ni and three other nuclei: ¹⁴C, ²⁸Si, and ⁵⁴Fe. The experimental pion scattering cross sections were reproduced by distorted-wave impulse approximation calculations using wave functions from (e,e’) with a normalization factor N that generally varied from 1 to 5. The ratio of the total isoscalar to the total isovector strength ranged from 0.2 to 1.2. The use of Woods-Saxon wave functions in the analysis did not significantly improve the fits to the angular distributions.

Inelastic electron scattering cross sections for ¹⁴C have been measured at a scattering angle of 180°, with incident beam energies ranging from 81.9 to 268.9 MeV. Transverse form factors were measured for transitions to low-lying natural-parity states, to unnatural-parity ‘‘stretched’’ J^π=4^- states, and to the J^π=2^- analog to the ¹⁴B ground state. Cross sections for 4^- states at 11.7 and 17.3 MeV are combined with pion scattering data to determine the isoscalar and isovector transition amplitudes. Form factors for other states are compared to shell-model calculations. From the excitation energy of the newly discovered J^πT=2^-2 state at 22.1 MeV, the ¹⁴-¹⁴B Coulomb energy difference is determined to be 2.25±0.10 MeV.

Giant resonances were observed in (π⁺,π^-) double charge exchange (at T_π=292 MeV) on ⁵⁶Fe, ⁸⁰Se, and ²⁰⁸Pb at excitation energies of 28.5, 34.6, and 43.6 MeV, respectively. An angular distribution for ⁵⁶Fe was measured and observed to have a dipole shape. The variation of excitation energy above the double isobaric analog state is consistent with an A^-1/3 dependence, as expected for giant dipole resonances built on the isobaric analog states. The ⁵⁶Fe angular distribution agrees well with a coupled-channels impulse-approximation calculation in which sequential single charge exchange through the isobaric analog to the giant dipole resonance is evaluated.

Form factors for transverse electron scattering from ¹²C and ¹³C have been determined for momentum transfers as high as q=4.6 fm^-1. Data are presented for the elastic (M1), 3.088 MeV (E1), and 9.50 MeV (M4) transitions in ¹³C, and for the 15.11 MeV (M1), 16.11 MeV (E2), and 16.58 MeV (M2) transitions in ¹²C. It is found that calculations in the lowest-order shell-model space fail to account for the measured q dependences of E1, M1, and E2 form factors beyond q=2 fm^-1. On the other hand, form factor shapes observed for M2 and M4 multipoles are satisfactorily described to q=4 fm^-1 within the minimal 1ħω configuration space.

An excitation function from T_π=50 to 120 MeV at theta_lab=35° and an angular distribution at 60 MeV have been measured for the pion double-charge-exchange reaction ¹²C(π⁺,π^-) ^12O(g.s.). The cross section is found to decrease with increasing energy. The ¹²C excitation function and the angular distribution are very similar in shape to those of the double-isobaric-analog-state transitions observed for ¹⁴C and ¹⁸O, but the magnitude is a factor of 6 smaller.

The ⁵⁹Co(α,t)⁶⁰Ni reaction was used to populate states known to be of spin 8^- by inelastic electron scattering measurements. Single-nucleon stripping spectroscopic factors obtained by comparison to exact-finite-range distorted-wave Born approximation calculations are compared to the fractions of M8 single-particle strength found from electron scattering for 10 transitions. Distributions of T=3 8^- strength were found to be rather similar in the two studies, with about 34% of the T=3 single-particle 8^- stripping strength located.

Transverse electron scattering form factors have been measured for elastic scattering and for the transition to the 2.313 MeV state in ¹⁴N. Existing structure models could not simultaneously describe the two M1 form factors. Consequently, new phenomenological wave functions were determined by fitting the electron scattering results and other observables. For the 2.313 MeV transition, the new wave functions give smaller L=2 transition amplitudes than the earlier models. This reduction and the deduced value for the L=0 transition amplitude appear to be supported by the measurements of the ¹⁴N(p,p’) ¹⁴N, ¹⁴C(p,n)¹⁴N, and ¹⁴(γ,π⁺) ¹⁴C reactions. Although the L=0 transition amplitude is found to be relatively small, it is not small enough to give quantitative agreement with the severely retarded ¹⁴C β-decay rate. The possibility that the vanishingly small β-decay matrix element is the result of destructive interference between the one-body matrix element and other terms is discussed.

Transverse quasielastic electron scattering cross sections have been measured for the deuteron at 180° for incident electron energies of 220, 270, and 320 MeV. At the quasielastic peak the four-momentum transfers squared varied from 3.4 to 6.3 fm^-2. The measured spectra include the region from the elastic peak through the entire quasielastic peak. Results are compared with recent calculations incorporating meson-exchange currents and isobar configurations. At large neutron-proton separation energies, the data support the need for the inclusion of large isobar configuration components in the cross section.

Cross sections for the 9.50, 16.08, and 21.47 MeV M4 transitions in ¹³C have been measured by inelastic electron scattering. Isoscalar and isovector transition amplitudes were deduced by comparison with (π,π’) and (p,p’) data. In particular, the transition amplitudes obtained for the 9.50 MeV excitation give a remarkably consistent description of all existing measurements. A comparison was made with M4 transitions in other carbon isotopes, ¹²C and ¹⁴C. In each case, detailed shell model calculations accurately predict the energies, relative strengths, and isospin character of the observed M4 transitions. However, the shell model gives (e,e’) M4 cross sections that exceed the data by a factor of 2.

The (α,t) stripping reaction on ²⁵Mg was used to measure the f_7/2 proton spectroscopic factors for the 4^-;T=0 (5.39 MeV), 6^-;0 (6.89 and 7.53 MeV), and 6^-;1 (9.26, 11.97, 12.40, and 12.55 MeV) states of ²⁶Al. The T=1 levels were identified with the aid of their analogs known in ²⁶Mg by inelastic electron scattering. Only 31% of the single particle strength is found for the two 6^-;0 levels and only 59% is found for five 6^-;1 levels. The lowest 6^-;1 state has more than twice the spectroscopic factor of any of the others. Further 6^- candidate states are also examined.

A simple Lane model is used to parametrize the energy systematics of the isospin splitting of high-spin magnetic states in non-self-conjugate nuclei. A strength parameter V₁=106±10 MeV is found.