Knockout Mechanism Research Articles

Mechanism of the reaction 4He(e,e'd)2H.

The cross section for the $^{4}\mathrm{He}$(e,e'd${)}^{2}$H reaction has been measured as a function of the four-momentum transfer q at a missing momentum of 125 MeV/c. The data show that this reaction cannot be described as quasielastic knockout of a deuteron. In a microscopic description of the (e,e'd) cross section, assuming a direct knockout mechanism, the q dependence reflects the p-n relative wave function in $^{4}\mathrm{He}$. However, even a calculation in this framework using a relativistic $^{4}\mathrm{He}$ wave function does not describe the data, indicating that other reaction mechanisms play a non-negligible role.

Modelling of epitaxial thin-film growth on fcc(100) substrates at low temperatures

Long-lived diffracted intensity oscillations have been observed experimentally during thin-film deposition in several systems at temperatures below which thermally-activated diffusion is thought operative. Several models are presented for growth on fcc(100) substrates which can account for these observations. The requirement of a four-fold hollow, rather than atop, adsorption site dramatically influences film structure. Distinct, but short-lived, oscillations are found here even for random immobile adsorption. The prescription of deposition dynamics is also fundamental. Introducing a ‘downward funneling’ propensity, very short-range ‘transient mobility’ after deposition, or ‘knockout mechanisms’ greatly enhances the quasi-steady nature of film growth. The latter is reflected in a slowly-increasing or quasi-periodic film interface width. Long-lived intensity oscillations are also produced, despite the absence of any long-range mobility.

The 58Ni(p, α) reaction mechanism through a study of the analyzing power

Analyzing powers of the 58Ni(p, α) reaction at E p = 22 and 72 MeV have been calculated for transitions to the ground state of 55Co and to excited states in the continuum. DWBA calculations have been performed with both the α knock-out and triton pick-up models. The analyzing power at 72 MeV in the continuum was calculated with the statistical multistep direct theory. It is found that while the g.s. transition is dominated by the triton pick-up process, the analyzing power in the continuum can only be reproduced by an α knock-out mechanism. Some general considerations on the interplay between these two mechanisms and on the consistency with results obtained in the past with semiclassical preequilibrium models are made.

Continuum yields from 12C(p,p′) at incident proton energies of 90 and 200 MeV

The inclusive reaction 12C(p,p') at incident proton energies of 90 MeV and 200 MeV is investigated to determine whether the continuum spectra can be described in terms of a simple quasifree knockout model. Reasonable agreement is obtained at 200 MeV, irrespective of our choice of approximation to the half-shell proton-nucleon interaction. At 90 MeV, however, only the initial-energy prescription gives acceptable theoretical results. Realistic spectroscopic factors are extracted from the data, which further support the assumption that the quasifree knockout mechanism is an important component of the reaction leading to protons scattered into the continuum.

Coincidence 16O(e

Coincidence $^{16}(\mathrm{e}\ensuremath{\rightarrow}$,e'x) reactions with polarized electrons have been investigated in the one-photon-exchange formalism. Electromagnetic transition amplitudes are decomposed into multipoles. The nuclear energy continuum is obtained in the framework of a self-consistent random-phase approximation theory with a Skyrme III interaction. We focus our attention on the electron kinematics ${\ensuremath{\epsilon}}_{i}$=130 MeV, \ensuremath{\theta}=50\ifmmode^\circ\else\textdegree\fi{}, and on the nuclear excitation energies \ensuremath{\omega}=21 and 35 MeV. The polarization structure functions ${W}_{\mathrm{LT}\mathcal{'}}$ are discussed in parallel with angular distributions and asymmetries for the two hole states: 1${p}_{1/2}^{\mathrm{\ensuremath{-}}1}$ and 1${p}_{3/2}^{\mathrm{\ensuremath{-}}1}$. At 21 MeV we explore the properties of giant multipole resonances, whereas at 35 MeV, we show theoretical predictions for the competing knockout and semidirect reaction mechanisms. The neutron yield predicted in $^{16}(\mathrm{e}$,e'n) reactions is connected with the presence of random-phase approximation correlations in the nuclear energy continuum. Comparison is made between random-phase-approximation--Skyrme-III results and Novosibirsk experimental data.

40Ca( gamma,p0)39K reaction for E gamma =100-300 MeV.

The differential cross section for the /sup 40/Ca(..gamma..,p/sub 0/) /sup 39/K process has been measured in the energy range E/sub ..gamma../ = 100-300 MeV at laboratory angles of 45/sup 0/, 90/sup 0/, and 135/sup 0/. The cross section for the (..gamma..,p) reaction leaving /sup 39/K in its ground state was extracted from the tip region of the proton spectra measured at a series of bremsstrahlung endpoint energies. The data are compared with a distorted-wave impulse-approximation calculation based on a direct, single-particle knockout mechanism.

On the role of direct mechanisms in light-fragment production proceeding through the fast hadron-nucleus interaction

A consistent theoretical approach is used to evaluate the contribution from the quasi-elastic knock-out and “spectator” mechanisms to the invariant spectra of α-particles produced under large angles by interactions of protons, deuterons and α-particles with energies of a few GeV/nucleon with 1p-shell nuclei. Effects of rescattering on nucleons, constituting the knocked-out fragment, the residual nucleus and the incident particle, are investigated. It is shown that in the framework of the quasi-elastic knock-out mechanism one can get a good description of the experimental spectra of α-fragments produced under angles of about 90° with energies from 20 up to 60 MeV. As the energy of the observed particle is increasing, the contribution from the mechanism considered is diminished, but it is still appreciable (up to 10%) for energies up to 180 MeV.

4He fragmentation induced by 46–400 GeV Protons

Inclusivep, d and3He cross sections have been obtained using experimental data on p4He collisions over an incident proton energy range of 46-400 GeV. Considering the slopes of spectra, the effective nuclear temperature has been found to be T0=11 MeV, which is in good agreement with experimental results for lower energies and heavier targets. The shape of the3He inclusive spectrum suggests the existence of two different3He production mechanisms. The experimental3He cross section is compared with theoretical predictions assuming fragment formation as a result of the spectator and cluster knockout mechanisms. The4He→3He +n vertex constant has been found to be G2=10.9±0.2 fm.

16O( gamma,p)15N reaction for E gamma =100-400 MeV.

The differential cross section for the /sup 16/O(..gamma..,p)/sup 15/N process has been measured in the energy range E/sub ..gamma../ = 100--400 MeV at laboratory angles of 45/sup 0/, 90/sup 0/, and 135/sup 0/. The cross sections for the (..gamma..,p) reaction leaving /sup 15/N in its ground state, in its first and second excited states at E/sub x/roughly-equal5.3 MeV, and in its third excited state at E/sub x/ = 6.3 MeV were extracted from the tip region of the proton spectra measured at a series of bremsstrahlung end point energies. The data are compared with calculations based on a direct, single-particle knockout mechanism and on mechanisms involving two particles in an intermediate state.

The mechanism of the (p, ?) reaction: Pick-up or knock-out?

Several early studies of the (p, α) reaction to discrete states of the final nucleus indicated that it proceeds mainly by the pick-up mechanism, whereas more recent experiments provide qualitative arguments in favour of the knock-out mechanism. This paper reports calculations showing that the angular distributions and analysing powers of the90,92Zr(p, α)87,89Y and118Sn(p, α)115In reactions can be equally well fitted by distorted wave calculations using either mechanism.

(π±,π±p) reaction at 245 MeV

The inclusive (..pi../sup + -/,..pi../sup + -/p) reactions on C, Fe, and Bi were studied at 245 MeV in a broad kinematic range by means of coincidence measurement of the outgoing particles. The ..pi..-p angular correlations and proton-energy spectra show features consistent with those expected from quasifree scattering. It is observed that about 80% of the inclusive inelastic scattering cross section at backward pion angles may be attributed to nucleon knockout mechanisms. The results allow identification of the direct quasifree process, unperturbed by higher order effects, which accounts for 30, 20, and 15% of the C, Fe, and Bi inclusive (..pi../sup +/,..pi../sup +/) differential cross sections, respectively. The ratio of positive to negative pion cross sections for quasifree scattering, integrated over the proton energy and angle, are in agreement with the ratio for free ..pi..-p scattering. Such is not the case for various proton angles. The deviation of the positive to negative ratio at the peak of the proton angular correlation from the free scattering ratio is most pronounced for more forward pion angles.

Direct mechanism in medium energy (γ, p 0) reactions

Medium energy (γ, p 0) reactions are investigated with the purpose of finding how far a direct knockout mechanism holds. Final state interactions are included in a cross section where the hole spectral function is not factorized. The approximate factorized formula gives large differences (a priori out of control). The choice of different optical model potentials and bound state wave functions is discussed and results are compared with experimental data for 12C, 16O and 40Ca. The sensitivity to the optical potential is confirmed. The direct mechanism appears able to reproduce the experimental cross sections up to photon energies corresponding to a recoil momentum larger than 2 fm −1.

Direct nucleon emission by polarized photons

The (γ,p) and (γ,n) reactions with polarized photons at intermediate energies are discussed under the hypothesis of a direct knockout mechanism with final state distortion.

Intermediate giant resonance excitation effects in direct reaction (p,2p) correlations

The transition amplitude for (p,2p) reactions initiated by protons with energies in excess of 100 MeV is cast into a form which facilitates identification of components associated with the standard (direct) knockout mechanism, and with semidirect processes in which excited states of the target, notably the giant resonances, mediate the transitions. Calculations of both direct and semidirect amplitudes are presented, for the /sup 12/C(p,2p) reaction, in an asymmetric energy-sharing geometry for which a half distorted-wave approximation is appropriate. Estimates of the semidirect effects, using coupling strengths deemed appropriate from analyses of /sup 12/C(p,p') data, indicate that under favorable kinematic conditions such effects should be discernable in correlations.

Direct nucleon emission by polarized photons

The (γ, p) and (γ, n) reactions with polarized photons at intermediate energies are discussed under the hypothesis of a direct knockout mechanism with final state distortion. As in this frame the behaviour of the cross sections is quite different, the experiment is proposed as a crucial test of the direct emission in photoreactions.

Quasi-free (α,2α) reaction induced by 140 MeV alpha particles onBe9,C12,O16, andNe20

Measurements of the ($\ensuremath{\alpha}$,$2\ensuremath{\alpha}$) reaction on $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{20}\mathrm{Ne}$ targets at ${E}_{\ensuremath{\alpha}}=140$ MeV were made at 20 angle pairs. The quasi-free knockout mechanism appears to dominate the reaction. The experimental data were analyzed with distorted-wave impulse approximation calculations. The factorization approximation employed in the calculations was tested explicitly and found to be valid. The shapes of the calculated energy sharing spectra are in satisfactory agreement with the data. The predicted absolute cross sections were found to be very sensitive to the cluster-core bound state radius parameter, and a bound state radius $R=2.52{{A}_{c}}^{\frac{1}{3}}$ fm is necessary to obtain absolute spectroscopic factors consistent with existing theoretical values. Several possible explanations for this excessive value are suggested. Comparisons are made with other alpha knockout and transfer reactions.NUCLEAR REACTIONS $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{20}\mathrm{Ne}$ ($\ensuremath{\alpha}$,$2\ensuremath{\alpha}$), ${E}_{\ensuremath{\alpha}}=140$ MeV; measured $\ensuremath{\sigma}({E}_{\ensuremath{\alpha}1},{E}_{\ensuremath{\alpha}2},{\ensuremath{\theta}}_{\ensuremath{\alpha}1},{\ensuremath{\theta}}_{\ensuremath{\alpha}2})$ DWIA analysis; deduced absolute spectroscopic factors.

Electron-nucleon interaction in quasi-free scattering: (I). Non-relativistic Hamiltonian through second order

In the frame of a program aiming at examining the approximations involved in the theoretical treatment of (e, e′p) reactions, the electron-nucleon interaction producing the knock-out mechanism is considered in the non-relativistic approach of McVoy and Van Hove and compared with the relativistic one based on the one-photon-exchange Feynman graph. In this paper the non-relativistic Hamiltonian through second order in the nucleon recoil velocity is adopted; in the next paper higher order terms are included. The two approaches appear equivalent as far as the usual DWIA cross section is concerned, where a factorization occurs between the electron-nucleon cross section and the nucleon momentum distribution. However the unfactorized DWIA cross section behaves differently. This is attributed to a different off-shell behaviour of the two scattering amplitudes.

The mass dependence of the nuclear photoeffect at energies above the giant dipole resonance

Experimental data on the (γ, p) reaction at E γ = 60−100 MeV for targets in the range A = 7−93 are compared with predictions based on a single-particle knock-out mechanism using shell model wavefunctions. The results show that this mechanism is more important than has generally been believed

On the knock-out mechanism for the12C(P,?)9B reaction

The mechanism of the reaction12C(P,α)9B is studied using zero range distorted wave Born approximation. The knock out mechanism is assumed to represent this reaction both in the forward and backward angles.

133Xe gas release during irradiation of uranium metal in the presence of air

Uranium metal samples of large surface to volume ratios were irradiated in the presence of 1 cm a of air. It was found that 1.708 +- 0.015% of ' 33 Xe was released during irradiation. It was proved that this amount of ~ 3 a Xe is due to the decay of ~ a 31 originally released. The release of 133 Xe was attributed to the knock-out mechanism.