Laboratory Angles Research Articles

Near-zero energy proton production mechanisms from the three-body dissociation of H3+

The laboratory energy distribution of H+ from the dissociation of H3+ in low-keV H3+-He collisions is measured at a 0\ifmmode^\circ\else\textdegree\fi{} laboratory angle. An inelastic energy loss of Q=26\ifmmode\pm\else\textpm\fi{}2 eV has been determined for the excited states of H3+ that produce low-energy protons upon dissociation. An approximate projectile-frame energy distribution of H+ is also presented. Our results suggest that the near-zero projectile-frame energy protons are produced by either an electronic singlet excitation of the ground state of H3+ to a distribution of 2 E1\ensuremath{'}, 1 E1\ensuremath{''}, and 2 A12\ensuremath{''}, or as an alternative mechanism, a triplet excitation of H3+ to 1 E3\ensuremath{'} accompanied by a simultaneous triplet excitation of the He target. In either case, the near-zero energy protons are produced by a three-body dissociation of the excited (H3+)*. In the case of the triplet excitation, there are no long-range correlations for the motion of the neutral H atoms. For the singlet excitation, the neutral H atoms' motion is correlated and the angle between them is near 180\ifmmode^\circ\else\textdegree\fi{}. The effects of the long-range nature of the total potential between the fragments is discussed in terms of the hyperspherical coordinates.

Higher excitation levels of B11 via the B10(n,n)B10 and B10(n,n')B10* (0.72, 1.74, 2.15, 3.59, 4.77 MeV) reactions.

Differential cross sections for elastic and inelastic scattering of neutrons from isotopically enriched {sup 10}B samples have been measured for incident neutron energies from 3.02 to 6.45 MeV in 250 keV increments and from 7.02 to 12.01 MeV in 500 keV increments. Inelastic angular distributions for scattering to the states in parentheses in {sup 10}B have been measured from the indicated energy up to 12.01 MeV: (0.718) from 3.02 MeV; (1.74) from 3.27 MeV; (2.15) from 3.77 MeV; (3.59) from 5.52 MeV; (4.77) from 7.02 MeV. The measurements at 3.02, 3.51, 4.02, and 4.51 MeV were done at nine laboratory angles from 20{degree} to 158{degree} in 17.5{degree} increments with a sample that is isotopically 95.86% {sup 10}B. All other distributions measured scattering at 11 laboratory angles from 18{degree} to 158{degree} in 15{degree} increments from a sample that is isotopically 99.49% {sup 10}B. A multiple scattering code provided a simulation of the experimental scattering process allowing accurate corrections to the small measured inelastic cross sections. An eight-channel, multilevel {ital R}-matrix analysis was performed on the data. Level energies, spins, and parities were deduced for twelve levels above 13 MeV excitation in {sup 11}B. Only two definite and three tentative assignments formore » {ital T}=1/2 levels had been made previously above 13 MeV. The two definite levels were confirmed. Good agreement between the data and the {ital R}-matrix calculation in all analyzed channels was obtained for the proposed structure.« less

Measurements of polarization effects: A pseudorandom polarization switching time-of-flight spectrometer

A new method is presented to measure the polarization dependence of double-differential cross sections for the scattering of laser-excited atoms. This allows us to study these polarization effects separated for different scattering processes, which contribute to the total differential cross section at one laboratory angle. A first application demonstrates that the new technique allows us to study the polarization dependence of the reactive and the nonreactive cross sections for collisions of Na(32P3/2) with HF molecules even though, in this example, the polarization dependence of the total differential cross section is only 0.5%.

Behaviour of the 12C(4He, 4He)12C elastic scattering cross section at Eα = 0.4–1.8 MeV

Abstract The energy dependence of the 12C(4He, 4He)12C elastic scattering cross section has been studied between 0.4 and 1.8 MeV incident energy at a laboratory angle of 150°. It is found that, within a precision of 2%, the measured energy dependence is of the screened Rutherford form. Assuming the 12C(4He, 4He)12C elastic scattering cross section is screened Rutherford at low energies, Eα ≈ 0.5 MeV, we conclude that it is screened Rutherford up to at least 1.8 MeV. This conclusion is inconsistent with an absolute cross section value derived from a comparison with the 12C(d, p)13C nuclear reaction yield.

Quasifree analyzing powers using the (p

The (p\ensuremath{\rightarrow},n) reaction has been measured for $^{12}\mathrm{C}$ and $^{54}\mathrm{Fe}$ targets at 290 MeV for a lab angle of 20.4\ifmmode^\circ\else\textdegree\fi{} and at 420 MeV for a lab angle of 24.0\ifmmode^\circ\else\textdegree\fi{}. An additional angle of 27.0\ifmmode^\circ\else\textdegree\fi{} was measured for $^{12}\mathrm{C}$ at 290 MeV. Both cross sections and analyzing powers were obtained up to excitation energies close to the quasifree peak. The (p\ensuremath{\rightarrow},n) analyzing powers are suppressed relative to the free nucleon-nucleon values for the $^{12}\mathrm{C}$ data at 290 and 420 MeV, and for the $^{54}\mathrm{Fe}$ data at 290 MeV. An enhanced analyzing power is observed for the $^{54}\mathrm{Fe}$ data at 420 MeV. The data are compared with both relativistic and nonrelativistic model calculations.

High energy photon production in low energy 12C

Photons with energies of 5--40 MeV have been measured in the $^{12}$C${+}^{112}$,124Sn reactions at incident energies of 7.5 and 10.5 MeV/nucleon and at laboratory angles of 60\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 120\ifmmode^\circ\else\textdegree\fi{}. Statistical model calculations which describe the photon yields in the 5--20 MeV photon energy range cannot account for the relatively large yields measured at ${E}_{\ensuremath{\gamma}}$g20 MeV. For these energies, the angular distribution suggests photon emission from a source moving at the nucleon-nucleon center-of-mass velocity. The magnitude of the nonstatistical photon yields is roughly consistent with a scaling relationship based on nucleon-nucleon bremsstrahlung. However, this does not account for the observation that the $^{124}\mathrm{Sn}$ yield is considerably larger than the $^{112}\mathrm{Sn}$ yield and suggests differences in the neutron-proton dynamics play a role in these low energy collisions.

Neutron pickup strength in 87Sr from the (p

Differential cross section and analyzing power angular distributions have been obtained for the $^{87}(\mathit{p}\ensuremath{\rightarrow}$,d${)}^{86}$Sr reaction in two measurements using different magnetic spectrometers at incident proton energies of 94.2 and 91.8 MeV. Typical energy resolutions of 70 and 40 keV, respectively, were achieved. Optical-model parameters were obtained by fitting the elastic scattering data measured to laboratory angles of 90\ifmmode^\circ\else\textdegree\fi{} for protons at 94.2 MeV and to 120\ifmmode^\circ\else\textdegree\fi{} for deuterons at 88.0 MeV. Standard exact finite range distorted-wave Born approximation calculations using these optical-model parameters failed to describe the experimental data, while the adiabatic approximation improved the description to some extent. Orbital and total angular momenta, l and j, of the picked-up neutron were extracted from the angular distributions of the differential cross section and analyzing power for all the residual states observed. The rms radius of the 1${g}_{9/2}$ neutron orbital, obtained from magnetic electron scattering, was used in the extraction of strength for this orbital, while rms radii from Hartree-Fock calculations were used for the 2${p}_{1/2}$, 2${p}_{3/2}$, 1${f}_{5/2}$, and 1${f}_{7/2}$ orbitals. The observed 1${g}_{9/2}$ strength, spread over 20 states up to 5.4-MeV excitation, indicates a depletion of about 43% compared to simple shell-model expectations. Similarly, the observed 1${f}_{5/2}$ strength, spread over 17 states, indicates a depletion of about 32%. The weighted summed spectroscopic strength for the 1${g}_{9/2}$ orbital, deduced from the present study, is lower than that obtained from magnetic electron scattering.

Coincidence studies of quasimolecular electron emission in 700-keV Ar2+-Kr collisions.

Continuum electron emission following 700-keV Ar{sup 2+}-Kr collisions has been investigated by detecting electrons in coincidence with projectile ions scattered through laboratory angles between 5{degree} and 20{degree}. From the analysis of the experimental data in terms of quasimolecular Auger-electron emission, molecular-orbital binding energies at small internuclear distances as well as collision-broadening parameters have been inferred. The results indicate that the electron emission is dominated by the decay of 4{ital s}{sigma} and 4{ital p}{pi} vacancies in the highly charged Ar-Kr quasimolecule. The analysis also reveals the presence of an exponentially decreasing high-energy tail of the spectra, which cannot be ascribed to quasimolecular Auger electrons and which has not been taken into account in previous analyses.

Neutron inclusive measurements in 14N+Ag reactions at 35 MeV/nucleon.

Neutron inclusive (singles) spectra were measured by the time-of-flight method at laboratory angles from 15\ifmmode^\circ\else\textdegree\fi{} to 160\ifmmode^\circ\else\textdegree\fi{}. When parametrized in the thermal model, a targetlike source was sufficient to fit the data at the largest angle, but in addition a half-beam-velocity source was required at the other angles, and at the smallest angles a projectilelike source was also needed. The model parameters are in general agreement with values obtained previously in fitting proton inclusive data. Comparison is also made with neutron coincidence data. Less phenomenological models---Fermi jet, Boltzmann-Uehling-Uhlenbeck, and Boltzmann master equation---are also able to account for the main features of the data.

Observation of electrons from the 1P degrees resonance of D-

We have measured the electron energy spectra near 0\ifmmode^\circ\else\textdegree\fi{} produced in collisions of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ with Ar. Using a 400-keV ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ beam and with good experimental energy and angular resolution we have found structure in the ejected electron energy spectra which is due to the decay of the $^{1}\mathrm{P}$ \ifmmode^\circ\else\textdegree\fi{} shape resonance. The doubly differential cross sections (DDCS's) have been measured as a function of angle and it was found that this structure disappeared for laboratory angles greater than 1\ifmmode^\circ\else\textdegree\fi{} as expected. A resonance contribution to the DDCS's was extracted at ${\ensuremath{\theta}}_{L}$=0\ifmmode^\circ\else\textdegree\fi{}, transformed to the projectile frame, and fit with a Breit-Wigner shape. Our resonant energy is in reasonable agreement with other experiments. We also find a small asymmetry in the two resonant structures in the laboratory measurements at ${\ensuremath{\theta}}_{L}$=0\ifmmode^\circ\else\textdegree\fi{}.

Pion production by pions in the 16O(π +,π +π −) reaction at Tπ+ = 280 MeV

The results of an experiment of pion-induced pion production on 16O at a laboratory energy of 280 MeV are reported. The four-fold-differential cross section, d 4σ dΩ π +dΩ π+d E π+d E π, was measured, and the missing mass, M x , for the reaction 16O fs( π +, π + π −) at laboratory angles of 50°, 80° and 115° for the π −, and from 22° to 128° for the π + was deduced. The angular and energy distributions of the many-fold differential cross sections are compared with an A( π +, π + π −) model for the reaction. The observed cross sections are reproduced in magnitude and shape when the theoretical calculations consider a proper pion dispersion relation, \\ ̃ gw( q), inside the nuclear medium. The total cross section, deduced by integrating the four-fold differential cross sections, is compared with the available theoretical predictions for the 16O(π +, π +π −) reaction.

Coulomb Excitation of 142Ce and 144Nd

The N = 84 nuclei 142Ce and 144Nd have been Coulomb-excited using 4He, 12C and 160 projectiles. Scattered particles were detected at a mean laboratory angle of 170.6� with an annular silicon surface-barrier detector. The static quadrupole moment f the 2t state was determined for both nuclei. In addition, various electromagnetic transition probabilities involving the 2~, 2~, 1 and 4t states were measured. Taken in conjunction with previous information, the results show that the properties of 142Ce nd 144Nd are well described by the vibrational U(S) limit of IBM-2, and strongly support the proposition that the 2~ level in each nucleus is an essentially pure one d-boson mixed-symmetry state.

Momentum transfer and total differential cross sections in the impulsive Na+-N2 and Na+-CO collisions

Total differential cross sections (DCS) in Na+-N2 and Na+-CO collisions were measured over a wide range of laboratory angles 5 ⩽θ⩽ 90° at laboratory energies 100 ⩽Elab ⩽ 450 eV. In these impulsive collisions, the DCS for both collision systems decrease suddenly at certain large angles (cut-off angles). The DCS of Na+-N2 show one cut-off angle, while those for Na+-CO show two cut-off angles, and the DCS for these two systems display a characteristic difference around the cut-off angles. The angular dependences of the total DCS can be well reproduced by a classical trajectory calculation. The cut-off structure is due to a huge momentum transfer in the heavy particle collisions. The cut-off positions are close to those for binary Na+-C, Na+-N andNa+-O interactions, and are related to the rotational rainbows.

Spin-dependent observables for the 12C(p,p' gamma ) reaction at 400 MeV.

Protons scattered inelastically from /sup 12/C were detected at laboratory angles between 6/sup 0/ and 13/sup 0/ in coincidence with ..gamma.. rays from deexcitation of the 1/sup +/ T = 1 state in the recoil nucleus. Coincident cross sections for ..gamma.. rays emitted normal to the reaction plane and angular correlations for ..gamma.. rays emitted in the proton scattering plane at angles between 90/sup 0/ and 270/sup 0/ are presented and compared with predictions from both nonrelativistic and relativistic models of proton-nucleus scattering calculated in the impulse approximation. These data display a clear preference for the relativistic-model prediction.

The 2H( 3He, 3He p)n reaction with 33-MeV polarized 3He ions

Triple-differential cross sections and vector analyzing powers have been measured for the 2H( 3He, 3He p)n reaction at E He = 32.5 MeV using polarized 3He ions. Data for 3He lab angles in the range 15°–30° and proton lab angles in the range 20°–60° are presented. The results are compared to both PWIA and DWIA calculations for a quasi-free knockout reaction. Energy-sharing distributions obtained for several combinations of θ He and θ p where quasi-free scattering (QFS) predominates are fairly-well reproduced by both types of calculation, but the DWIA calculations are better able to reproduce both the shapes and magnitudes of the quasi-elastic scattering maxima. The vector analyzing powers are generally small (¦ A y¦ <0.3) near the QFS peaks, and are in agreement with the values predicted by both PWIA and DWIA calculations where QFS predominates. At angle pairs where quasi-free scattering is less pronounced, the PWIA calculations are still in fair agreement with the data in some cases, whereas the DWIA predictions reproduce neither the cross section nor the analyzing-power data. For θ He = 30°, θ p = 45°, the cross section is dominated by the np final-state-interaction peak corresponding to singlet deuterons. The relatively structureless analyzing power at these angles is consistent with a two-stage reaction process.

Production of pions and light fragments in 0.8A GeV La

The spectra of pions and light fragments at laboratory angles from 10/sup 0/ to 89/sup 0/ were measured in 0.8A GeV La+La collisions. Event multiplicity was also measured to select central collisions. For inclusive data, the slope of the pion spectrum at theta/sub c.m./ = 90/sup 0/ for La+La is as steep as that for Ar+KCl while the slope of the proton spectrum at theta/sub c.m./ = 90/sup 0/ becomes less steep with increasing mass number of the colliding nuclei. Furthermore, the ..pi../sup -/ yield is about half of the geometrical estimate while total nuclear charge yield agrees well. Since the yield ratio of ..pi../sup -/ to total nuclear charge at theta/sub c.m./ = 90/sup 0/ is significantly larger for high multiplicity events than for inclusive data, it appears that pion absorption by spectator nucleons is important. An azimuthal correlation of charged particle emission was also observed, which is consistent with collective flow of nuclear matter. The coalescence relation between protons and composite fragments holds for inclusive data in La+La collisions.

Reaction pi -p--> gamma n below the Delta resonance.

The reaction ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\gamma}n has been studied at ${T}_{\ensuremath{\pi}}$=45.6, 62.2, 76.4, 91.7, 106.8, and 121.9 MeV for nine laboratory angles between 30\ifmmode^\circ\else\textdegree\fi{} and 140\ifmmode^\circ\else\textdegree\fi{}. The differential cross section measurements have angle to angle errors of about 4% with an additional 3% normalization error. The results are typically a factor of 2 more accurate than previous radiative capture data and are more reliable than cross sections obtained from the reverse reaction \ensuremath{\gamma}d\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$${\mathrm{pp}}_{s}$. The data are compared with recent multipole analyses, various calculations, as well as previous data, and the best agreement is with the Tokyo multipole analysis.

Reaction pi -p--> pi 0n below the Delta resonance.

The reaction ..pi../sup -/p..--> pi../sup 0/n has been studied using a large NaI crystal to detect the decay ..gamma.. rays of the ..pi../sup 0/, at pion beam energies of 45.6, 62.2, 76.4, 91.7, 106.8, and 121.9 MeV. The NaI was set at nine different laboratory angles, but at almost every angle the angular distribution of the ..pi../sup 0/ can be derived from the energy spectrum of the ..gamma.. rays. The results are compared with recent phase-shift analyses and with various calculations. New values of the charge exchange scattering length are derived.

Studies of emissions of complex fragments and effective temperatures for collisions of 58Ni

Complex fragments of nuclear charge $Z$ from 4 to 13 were measured at five laboratory angles by use of surface-barrier-detector telescopes. Coincident $\ensuremath{\gamma}$ rays were measured in a $4\ensuremath{\pi}$ geometry using 64 NaI detectors. Kinematical analysis of the fragments revealed that those of $Z$ between 4 and 9 are consistent with the decay of a compound nucleus formed in a complete fusion reaction. $\ensuremath{\gamma}$ rays of excited states of $^{12}\mathrm{C}$, $^{15}\mathrm{N}$, and $^{16}\mathrm{O}$ were measured and effective temperatures of about 2 MeV were extracted. Hauser-Feshbach calculations reproduce the measured cross sections and the effective temperatures very well.

The hard neutron-proton bremsstrahlung at 76 MeV

The neutron-proton bremsstrahlung cross-section was measured at a neutron energy of 76 MeV. Outgoing protons and neutrons were detected at extreme forward laboratory angles. Measurements were compared with a soft-photon-approximation calculation. Non-coplanar events were also studied.