Knockout Mechanism Research Articles

Influence of Nuclear Cluster Structure in Proton-Induced Pre-Equilibrium Composite Particle Emission

Recent statistical multistep analyses of the pre-equilibrium reaction93Nb(p, α) at incident energies of 65 and 100 MeV are reviewed. It is shown that the α-cluster structure of the target nucleus is prominently displayed at the lower incident energy, whereas at the higher value its presence is obscured by the dominance of three-nucleon pickup. This result appears to be simply associated with a difference in the details of the effective momentum mismatch experienced by a cluster knockout mechanism, as opposed to a pickup process.

MFPR Model Parameters of the Athermal Irradiation-Induced Transport in Nuclear Fuels

Atomistic simulations of radiation impact due to collision cascades in oxide and nitride nuclear fuels are performed in this work using combination of Monte Carlo and molecular dynamics techniques. The key parameters of MFPR code models for the athermal self-diffusivity and irradiation-assisted fission product release from fuel are evaluated. The general solution of Olander's integro-differential equation for the knockout mechanism is developed, which allowed extension of the earlier approaches for the long-lived and stable nuclides.

Rapid Mobilization Reveals a Highly Engraftable Hematopoietic Stem Cell

Hematopoietic stem cells (HSCs) are at the apex of lifelong blood cell production. Recent clonal analysis studies suggest that HSCs are heterogeneous in function and those that contribute to homeostatic production may be distinct from those that engraft during transplant.We developed a rapid mobilization regimen utilizing a unique CXCR2 agonist (an N-terminal truncated MIP-2a) and the CXCR4 antagonist AMD3100. A single subcutaneous injection of both agents together resulted in rapid mobilization in mice with a peak progenitor cell content in blood reached within 15 minutes. This mobilization was equivalent to a 5-day regimen of G-CSF. This rapid mobilization is the result of synergistic signaling, and was blocked in CXCR4 or CXCR2 knockout mice, confirming receptor and mechanism specificity. Mobilization is caused by synergistic release of MMP-9 from neutrophils and mobilization was blocked in MMP-9 knockout mice, mice treated with an anti-MMP-9 antibody, TIMP-1 transgenic mice, or mice where neutrophils were depleted in vivo using anti-GR-1 antibody. In vivo confocal imaging of mice demonstrated that the mobilization regimen causes a rapid and transient increase in bone marrow vascular permeability, "opening the doorway" for hematopoietic egress to the peripheral blood.Transplantation of 2x106 peripheral blood mononuclear cells (PBMC) from the rapid regimen resulted in a 4 or 6 day quicker recovery of neutrophils and platelets, respectively, compared to a G-CSF mobilized graft (n=12 mice per group, P<0.01). In limiting dilution competitive transplants, the rapid regimen demonstrated a greater than 2-fold enhancement in competitiveness (n=30 mice/treatment group, 2 individual experiments, P<0.001). Additionally, in secondarily transplanted mice, competitiveness of the rapidly mobilized graft increased as measured by contribution to chimerism, while G-CSF mobilized grafts remained static (n=16 mice/group, P<0.01). Surprisingly, despite robust enhancement in both short and long-term engraftment by the rapidly mobilized graft, phenotypic analysis of the blood of mobilized mice for CD150+ CD48- Sca-1+ c-kit+ Lineage neg (SLAM SKL) cells, a highly purified HSC population, showed lower numbers of phenotypically defined HSCs than in the G-CSF group.These data suggested that a unique subset of "highly engraftable" HSCs (heHSCs) are mobilized by the rapid regimen compared to G-CSF. However, as our earlier studies were performed using grafts that contained the total PBMC fraction (similar to the clinical apheresis product) we could not rule out the potential contribution of accessory cells to the enhanced engrafting ability of the heHSCs. Therefore, in 3 independent experiments, we mobilized large cohorts of mice with the rapid regimen or G-CSF and sorted SLAM SKL cells from the PBMC fraction and competitively transplanted equal numbers of SLAM SKL cells from either the rapid regimen or G-CSF and tracked contribution to chimerism over 36 weeks. Remarkably, the heHSCs from the rapid regimen demonstrated a 2-fold enhancement in competitiveness compared to SLAM SKL cells from the G-CSF group (n=17 mice/group, P<0.0004).While appreciation for HSC heterogeneity has grown, methods are lacking for prospectively isolating differing HSC populations with known biologic function, to study molecular heterogeneity. Like panning for gold, we sought to use the differential mobilization properties of our rapid regimen and G-CSF as a "biologic sieve" to isolate the heterogeneous HSC populations from the blood. We again flow sorted SLAM SKL cells from mice mobilized with the rapid regimen or G-CSF and performed RNA-Seq analysis of the purified populations. The heHSCs mobilized by the rapid regimen had a unique transcriptomic signature compared to G-CSF mobilized or random HSCs acquired from bone marrow (P<0.000001). Strikingly, gene set enrichment analysis (GSEA) demonstrated that the heHSCs had a gene signature highly significantly clustered to that of fetal liver HSCs, further demonstrating the selective harvesting of a subset of highly engraftable stem cells.Our results mechanistically define a new mobilization strategy, that in a single day can mobilize a graft with superior engraftment properties compared to G-CSF, and selectively mobilize a novel population of heHSCs with an immature molecular phenotype capable of robust long-term engraftment. DisclosuresHoggatt:Magenta Therapeutics: Consultancy, Equity Ownership, Research Funding. Scadden:Magenta Therapeutics: Consultancy, Equity Ownership; GlaxoSmithKline: Research Funding; Harvard University: Patents & Royalties. Pelus:GlaxoSmithKline: Consultancy.

Bond-rearrangement and ionization mechanisms in the photo-double-ionization of simple hydrocarbons (C2H4,C2H3F, and1,1−C2H2F2) near and above threshold

We investigate bond-rearrangement driven by photo-double-ionization (PDI) near and above the double-ionization threshold in a sequence of carbon-carbon double-bonded hydrocarbon molecules: ethylene, fluoroethylene, and 1,1-difluoroethylene. We employ the kinematically complete cold target recoil ion momentum spectroscopy method to resolve all photo-double-ionization events leading to two-ion fragments. We observe changes in the branching ratios of different dissociative ionization channels depending on the presence of no, one, or two fluorine atoms. The role of the fluorine atom in the bond-rearrangement channels is intriguing, as evident by the reordering of the threshold energies of the PDI in the fluorinated molecules. These effects offer a compelling argument that the electronegativity of the fluorine (or the polarity of the molecule) strongly influences the potential energy surfaces of the molecules and drives bond rearrangement during the dissociation process. The energy sharing and the relative angle between the three-dimensional momentum vectors of the two electrons enable us to distinguish between knockout and other ionization mechanisms of the PDI processes.

Single, double, and triple Auger decay probabilities ofC+(1s2s22p22D,2P)resonances

Single, double, and triple Auger decay rates of ${\mathrm{C}}^{+}(1s2{s}^{2}2{p}^{2}{\phantom{\rule{0.16em}{0ex}}}^{2}D,{\phantom{\rule{0.16em}{0ex}}}^{2}P)$ resonances were calculated in the framework of perturbation theory. The direct double Auger decay probabilities were calculated by using the approximate formulas according to the knockout and shakeoff mechanisms, in which the knockout mechanism was found to be dominant. Then the knockout mechanism was employed to investigate the complex triple Auger decay process, and the calculated rates have good agreement with the available experimental values.

Single-photon double and triple ionization of acetaldehyde (ethanal) studied by multi-electron coincidence spectroscopy

Single-photon multiple ionization processes of acetaldehyde (ethanal) have been experimentally investigated by utilizing a multi-particle coincidence technique based on the time-of-flight magnetic bottle principle, in combination with either a synchrotron radiation source or a pulsed helium discharge lamp. The processes investigated include double and triple ionization in the valence region as well as single and double Auger decay of core-ionized acetaldehyde. The latter are studied site-selectively for chemically different carbon core vacancies, scrutinizing early theoretical predictions specifically made for the case of acetaldehyde. Moreover, Auger processes in shake-up and core-valence ionized states are investigated. In the cases where the processes involve simultaneous emission of two electrons, the distributions of the energy sharing are presented, emphasizing either the knock-out or shake-off mechanism.

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

Different mechanisms of radiation damage of wide-gap metal oxides as well as a dual influence of impurity ions on the efficiency of radiation damage have been considered on the example of binary ionic MgO and complex ionic–covalent Lu3Al5O12 single crystals. Particular emphasis has been placed on irradiation with ∼2GeV heavy ions (197Au, 209Bi, 238U, fluence of 1012ions/cm2) providing extremely high density of electronic excitations within ion tracks. Besides knock-out mechanism for Frenkel pair formation, the additional mechanism through the collapse of mobile discrete breathers at certain lattice places (e.g., complex impurity centres) leads to the creation of complex defects that involve a large number of host atoms. The experimental manifestations of the radiation creation of intrinsic and impurity antisite defects (Lu|Al or Ce|Al – a heavy ion in a wrong cation site) have been detected in LuAG and LuAG:Ce3+ single crystals. Light doping of LuAG causes a small enhancement of radiation resistance, while pair impurity centres (for instance, Ce|Lu–Ce|Al or Cr3+–Cr3+ in MgO) are formed with a rise of impurity concentration. These complex impurity centres as well as radiation-induced intrinsic antisite defects (Lu|Al strongly interacting with Lu in a regular site) tentatively serve as the places for breathers collapse, thus decreasing the material resistance against dense irradiation.

Knockout reactions fromO14at 305 MeV/nucleon

The cross section for one-proton knockout from $^{14}\mathrm{O}$ on a carbon target at 305 MeV/nucleon was measured to be ${\ensuremath{\sigma}}_{\ensuremath{-}p}(^{14}\mathrm{O}\ensuremath{\rightarrow}^{13}\mathrm{N})=35(5)\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$. The one-proton one-neutron ($pn$) removal cross section ${\ensuremath{\sigma}}_{\ensuremath{-}p\ensuremath{-}n}(^{14}\mathrm{O}\ensuremath{\rightarrow}^{12}\mathrm{N})=30(6)\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$ and two-proton one-neutron ($2pn$) removal cross section ${\ensuremath{\sigma}}_{\ensuremath{-}2p\ensuremath{-}n}(^{14}\mathrm{O}\ensuremath{\rightarrow}^{11}\mathrm{C})=41(6)\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$ were also obtained. The deduced reduction factor, the ratio of the experimental and theoretical cross sections, is close to unity for one-proton knockout from $^{14}\mathrm{O}$, consistent with the systematics as well as the result obtained at a much lower beam energy. The indirect reaction process, which involves a one-neutron knockout plus proton(s) evaporation mechanism, is expected to contribute much to the $pn$ and $2pn$ removal.

Separated response function ratios in exclusive, forward π(±) electroproduction.

The study of exclusive π(±) electroproduction on the nucleon, including separation of the various structure functions, is of interest for a number of reasons. The ratio RL=σL(π-)/σL(π+) is sensitive to isoscalar contamination to the dominant isovector pion exchange amplitude, which is the basis for the determination of the charged pion form factor from electroproduction data. A change in the value of RT=σT(π-)/σT(π+) from unity at small -t, to 1/4 at large -t, would suggest a transition from coupling to a (virtual) pion to coupling to individual quarks. Furthermore, the mentioned ratios may show an earlier approach to perturbative QCD than the individual cross sections. We have performed the first complete separation of the four unpolarized electromagnetic structure functions above the dominant resonances in forward, exclusive π(±) electroproduction on the deuteron at central Q(2) values of 0.6, 1.0, 1.6 GeV(2) at W=1.95 GeV, and Q(2)=2.45 GeV(2) at W=2.22 GeV. Here, we present the L and T cross sections, with emphasis on RL and RT, and compare them with theoretical calculations. Results for the separated ratio RL indicate dominance of the pion-pole diagram at low -t, while results for RT are consistent with a transition between pion knockout and quark knockout mechanisms.

8He cluster structure studied by recoil proton tagged knockout reaction

Knockout reaction experiment for 8He at 82.3 MeV/u on Hydrogen target was carried out at the RIPS beam line in RIKEN. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The quasi-free knockout mechanism is identified through the polar angle correlation and checked by various kinematics conditions. The absolute differential cross sections for 6He core cluster are obtained and compared with the simple Glauber model calculations. The extracted spectroscopic factor is close to unity and a shrinking of the cluster size is evidenced.

Level-to-level and total probability for Auger decay including direct double processes of Ar2p−1hole states

Auger decay including direct double processes of Ar $2{p}^{\ensuremath{-}1}$ hole levels is investigated in the framework of perturbation theory implemented by distorted wave approximation with balanced large-scale configuration interaction among the successive ions being taken into account. The complex transition amplitude obtained from the second perturbation theory for the direct double Auger decay (DAD) is decomposed into approximate formulas according to two generally agreed mechanisms of shake-off and knock-out. Practical computations showed that the knock-out condition is fulfilled and thus justified such a decomposition treatment of knock-out mechanism. The contribution to the DAD probability from knock-out mechanism is larger than shake-off by an order of magnitude and therefore the former is dominant. The interference effect between the knock-out and shake-off mechanisms should be trivial and neglecting it we obtained a branching ratio of 12.0$%$ for the direct DAD into triply charged states of Ar${}^{3+}$. By including cascade double decay, a total branching ratio of 14.9$%$ is obtained. Our result explained recent experimental results on the branching ratio into triply charged ion, and should be useful for further detailed experimental investigations.

Recoiled Proton Tagged Knockout Reaction for 8He

An experiment for knockout reaction induced by 8He beam at 82.5 MeV/nucleon on CH2 and C targets was carried out. The 6He and 4He core fragments at forward angles and the recoiled protons at large angles were detected coincidently. From this exclusive measurement the valence nucleon knockout mechanism and the core knockout mechanism are separated, which can be applied to the exclusive spectroscopic study on the structure of exotic nuclei.

Knockout Reaction Mechanism for 6He+

A knockout reaction experiment was carried out by using the 6He beam at 82.5 MeV/nucleon impinging on CH2 and C targets. The α core fragments at forward angles were detected in coincidence with the recoiled protons at larger angles. From this exclusive measurement the valence nucleon knockout mechanism and the core knockout mechanism are separated. This study provides a basis for the exclusive spectroscopic investigation of the exotic nuclei.

Knockout reaction induced by 6He at 82.3 MeV/u

A knockout reaction experiment was carried out using a 6He beam at 82.3 MeV/u impinged on carbon and hydrogen targets. The recoil protons were detected in coincidence with the forward moving charged fragments and neutrons. It is demonstrated that through the correlation between the recoil protons and forward moving charged fragments, the core fragment knockout and valence nucleon knockout reaction mechanisms can be discriminated at energies around 100 MeV/u. The 5He ground-state resonance was reconstructed based on the selection of the valence nucleon knockout mechanism. The resonance parameters were obtained for both carbon and hydrogen targets, which are consistent with those previously reported in the literature. The importance of selecting appropriate reaction mechanisms is discussed based on reconstructions using events with larger transverse momentum transfer.

Recoil proton tagged knockout reaction for 8He

We report for the first time the discrimination of the core fragment knockout and valence nucleon knockout reaction mechanisms at medium energy range, by the use of the recoil proton tagging technique. Intense 8He beams at 82.3 MeV/u were supplied by the RIPS beam line at RIKEN, and impinged on both hydrogen and carbon targets. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The core fragment knockout mechanism is identified through the polar angle correlation and checked by various kinematics relations. This mechanism may be used to extract the cluster structure information of unstable nuclei. On the other hand, with the selection of the tagged valence nucleon knockout mechanism, a narrower peak of 7He ground state is obtained. The extracted neutron spectroscopic factor Sn=0.512(18) is relatively smaller than the no-tagged one, and is in good agreement with the prediction of ab initio Greenʼs function Monte Carlo calculations.

Evidence of Single-Photon Two-Site Core Double Ionization ofC2H2Molecules

We observe the formation in a single-photon transition of two core holes, each at a different carbon atom of the C2H2 molecule. At a photon energy of 770.5 eV, the probability of this 2-site core double ionization amounts to 1.6 ± 0.4% of the 1-site core double ionization. A simple theoretical model based on the knockout mechanism gives reasonable agreement with experiment. Spectroscopy and Auger decays of the associated double core hole states are also investigated.

Knockout reaction induced by 6He at 61.2 MeV/u

A knockout reaction induced by 6He at 61.2 MeV/u was carried out at the HIRFL-RIBLL radioactive beam line. The α core fragments at forward angles were detected in coincidence with the recoiled protons at large angles. From this coincident measurement the valence nucleon knockout mechanism and the core knockout mechanism can be separated according to the polar angle correlation between the core fragments and the recoiled protons. It is demonstrated that, when reconstructing the resonant state of a weakly bound nucleus, the contamination resulting from the core knockout mechanism should be eliminated in order to obtain the correct structure information.

Knockout reaction mechanism studied by 6He projectile

Knockout reaction experiment was carried out by using the 6He beams at 61.2 MeV/u impinging on a CH2 target. The α core fragments at forward angles were detected in coincidence with the recoiled protons at larger angles. From this exclusive measurement the valence nucleon knockout mechanism and the core knockout mechanism can be distinguished by the relation between the polar angles of the core fragments and the recoiled protons, respectively. It is demonstrated that the core knockout mechanism may result in some strong contamination to the real invariant mass spectrum.

Inclusive charged-current neutrino-nucleus reactions

We present a model for weak charged-current induced nuclear reactions at energies of interest for current and future neutrino oscillation experiments. This model is a natural extension of the work in Refs. [1,2], where the quasielastic contribution to the inclusive electron and neutrino scattering on nuclei was analyzed. The model is based on a systematic many-body expansion of the gauge boson absorption modes that includes one, two, and even three-body mechanisms, as well as the excitation of $\ensuremath{\Delta}$ isobars. The whole scheme has no free parameters, besides those previously adjusted to the weak pion production off the nucleon cross sections in the deuteron, since all nuclear effects were set up in previous studies of photon, electron, and pion interactions with nuclei. We have discussed at length the recent charged-current quasielastic MiniBooNE cross section data, and showed that two-nucleon knockout mechanisms are essential to describing these measurements.

Quasi-elastic electroproduction of charged $ \rho$ -mesons on nucleons

The electroproduction of charged $ \rho$ -mesons on the nucleon at intermediate energy is discussed for quasi-elastic kinematics. It is shown that at these kinematics both the longitudinal $ \sigma_{{L}}^{}$ and transverse $ \sigma_{{T}}^{}$ cross-sections are dominated by the $ \rho$ -meson t -pole contribution, and thus the corresponding dσL(T)/dt data can give a valuable information on the $ \rho$ -meson component of the nucleon cloud. The differential cross-sections for the reaction p(e, e ′ $ \rho^{+}_{}$ )n at Q 2 = 2 , 3.5GeV^2 and at the invariant mass W = 3 and 4GeV are calculated on the basis of quasi-elastic knockout mechanism with form factors. Questions about the gauge invariance of the electroproduction amplitude are considered and it is noted an important difference between photo- and electroproduction amplitudes.