Proton Core Research Articles

Polarization effects and application to E2 transitions in even carbon isotopes

The electric quadrupole polarization effects of core neutrons/protons excitation in even carbon isotopes are investigated with the microscopic particle-vibration approach. The experimental B(E2,21+→0g.s.+) values are much better reproduced with microscopic polarization effects than with empirical polarization charges. The polarization effects of core neutrons are very different from that of core protons in carbon isotopes. It turns out the polarization effects both of core protons and neutrons are crucial to reproduce the experimental ratios of neutron/proton matrix element Mn/Mp for the transitions in neutron/proton rich carbon isotopes.

SHELL STRUCTURE EVOLUTION IN NUCLEI: NEW PARADIGM

Shell structure evolution in nuclei situated at the extremes of neutron and proton excess are investigated using in-beam gamma spectroscopy techniques with radioactive beams at GANIL. A selection of results obtained very recently is presented: i) The reduced transition probabilities [Formula: see text] of the neutron-rich 74 Zn and 70 Ni nuclei have been measured using Coulomb excitation at intermediate energy. An unexpected large proton core polarization has been found in 70 Ni and interpreted as being due to the monopole interaction between the neutron g 9/2 and protons f 7/2 and f 5/2 spin-orbit partner orbitals. ii) Two proton knock-out reactions has been performed in order to study the most neutron-rich nuclei at the N =28 shell closure. Gamma rays spectra and momentum distribution have been obtained for 42 Si and neighboring nuclei. Evidences has been found for a deformed structure for 42 Si and for the disappearance of the spherical N =28 shell effect. iii) The in-beam gamma spectroscopy of 36 Ca performed using neutron knock-out reactions revealed that N =16 is as large sub-shell closure as large as Z =16 in 36 S . The uniquely large excitation energy difference of the first 2+ state in these mirror nuclei turns out to be a consequence of the relatively pure neutron (in 36 Ca ) or proton (in 36 S ) 1 p ( d 3/2)-1 h ( s 1/2) nature.

Preferential Heating and Acceleration ofαParticles by Alfvén-Cyclotron Waves

Preferential heating and acceleration of heavy ions in the solar wind and corona represent a long-standing theoretical problem in space physics, and are distinct experimental signatures of kinetic processes occurring in collisionless plasmas. We show that fast and slow ion-acoustic waves (IAW) and transverse waves, driven by Alfvén-cyclotron wave parametric instabilities can selectively destroy the coherent fluid motion of different ion species and, in this way lead to their differential heating and acceleration. Trapping of the more abundant protons by the fast IAW generates a proton beam with drift speed of about the Alfvén speed. Because of their larger mass, alpha particles do not become significantly trapped and start, by conservation of total ion momentum, drifting relative to the receding bulk protons. Thus the resulting core protons and the alpha particles are differentially heated via pitch-angle scattering.

Two-dimensional hybrid simulations of the oblique electromagnetic alpha/proton instability in the solar wind

In the solar wind, alpha particles are observed to flow faster than the core protons. In this paper, two-dimensional hybrid simulations are performed to investigate the nonlinear evolution of oblique Alfvén waves excited by an alpha/proton beam instability in a low beta plasma. The propagation angles of the excited waves are within a finite range suggesting the generation of oblique Alfvén waves. During the nonlinear evolution, both the wave numbers and frequencies of the waves drift to smaller values, and the propagation angles decrease. At the same time, the propagation angle of the dominant mode also changes. Eventually the plasma system reaches a marginally stable state according to linear theory.

Use of Magnetic Resonance to Determine Radial Slices of Plants

The paper deals with the measurement of radial slices of plant stalks using magnetic resonance imaging. MR images (26£26mm, 256£256pixels, slice thickness 3mm) of radial slices weighted by spin density, relaxation time T2, and proton core density were measured. Current pulse sequences of spin echo (SE) and pulse-gradient-spin echo (PGSE) were used and the images measured were subsequently processed. The images being measured were compared with images obtained by the classical microscopic method of monitoring vascular bundles. The results show that the MR images characterize the location of vascular bundles mainly in younger plants and bring information on the distribution of protons throughout the stalk. An advantage of the MR method is that the plant does not get destructed and that the measurement is simpler. Magnetic resonance (MR) enables studying the motion of liquids in the stalks of plants via radial slices of plants. When classical methods are used, pictures are made of dyed thin radial slices of stalks and the structure of vascular bundles is analyzed. To produce thin slices of stalks is very demanding. Currently there are not many workplaces that publish results of studying via MR imaging the transport of ions through plants. Scheenen (1) describes the design of a microscope for the observation of vascular bundles in plants. MR is a non-destructive examination method but its drawback is lower resolution (100m) and higher image noise. Its advantage is the possibility of obtaining images weighted by spin density, relaxation times and difiusion. Making use of subsequent image processing, new information can be obtained on the plant structure and motion of liquids through the plant.

The extremely slow‐exchanging core and acid‐denatured state of green fluorescent protein

Green fluorescent protein (GFP) is a large protein with a complex eleven-stranded beta-barrel structure. Previous studies have shown that it has a complex energy landscape for folding on which there are several intermediate states and a denatured state with significant residual structure. Here, we use two different types of HD exchange measurement and nuclear magnetic resonance (NMR) techniques to probe the energy landscape for folding of GFP in further detail. HD exchange experiments were performed over a wide range of conditions including different concentrations of denaturant. Results show that the penetration model dominates the exchange mechanism, consistent with the known stability and slow unfolding kinetics of GFP. HD exchange experiments at high pH establish that there is an extremely slow-exchanging superstable core of amide protons in GFP that are clustered and located in beta-strands 1, 2, 4, 5, and 6. These residues form part of a mini-beta-sheet which we propose constitutes a folding nucleus. Using a pulsed-labeling strategy, the acid-denatured state has been investigated and the residual structure observed in earlier studies shown to locate to beta-strands 1 and 3. There is some evidence that this residual structure is stabilized by a localized hydrophobic collapse of the polypeptide chain.

Oblique proton fire hose instability in the expanding solar wind: Hybrid simulations

Oblique fire hose instability is investigated using hybrid simulations for proton betas of the order of one and for proton parallel temperatures sufficiently greater than the perpendicular ones. The simulations confirm previous simulation results showing that this instability has self‐destructing properties and efficiently reduces the proton temperature anisotropy. A parametric study using one‐dimensional standard hybrid simulations shows that stronger changes in the temperature anisotropy and stronger wave emissions appear for larger initial temperature anisotropies. An ideal, slow plasma expansion, modeled by a two‐dimensional hybrid expanding box simulation, leads to a generation of proton temperature anisotropy. The anisotropy leads first to destabilization of the dominant parallel fire hose, which interacts mainly with minor supra‐Alfvénic protons, whereas the evolution of core protons is determined by expansion. Consequently, the effective anisotropy is only slightly reduced and the system eventually becomes unstable with respect to the oblique fire hose instability. The oblique fire hose strongly scatters the protons and removes the anisotropy disrupting the parallel fire hose. An important portion of the fluctuating wave energy is dissipated to protons, and only long wavelength waves remain in the system. The system with low wave activity then develops again larger temperature anisotropies, and the evolution repeats itself. It is concluded that both parallel and oblique proton fire hose instabilities constrain the proton temperature anisotropy in the expanding solar wind, with the latter one constituting a final frontier for the anisotropy. These results give a possible explanation of some apparent discrepancies between observations and linear predictions. In addition, a simple bounded anisotropy model is developed to include some of the kinetic effects of the fire hose instabilities in fluid models.

Neutron decay spectroscopy of neutron-rich oxygen isotopes

Neutron decay spectroscopy of neutron-rich oxygen isotopes has been performed using the two-proton knock-out reaction 9Be( 26Ne, X) 24,23,22O. A combination of the knock-out of valence and core protons can explain the three observed spectra. These knock-out processes are selective and preferentially populate hole states. The observed narrow resonance state in 23O at an excitation energy of 2.8(1) MeV was assigned to the 5 / 2 + state.

Proton Core Heating and Beam Formation via Parametrically Unstable Alfvén-Cyclotron Waves

Vlasov theory and one-dimensional hybrid simulations are used to study the effects that compressible fluctuations driven by parametric instabilities Alfvén-cyclotron waves have on proton velocity distributions. Field-aligned proton beams are generated during the saturation phase of the wave-particle interaction, with a drift speed which is slightly greater than the Alfvén speed and is maintained until the end of the simulation. The main part of the distribution becomes anisotropic due to phase mixing as is typically observed in the velocity distributions measured in the fast solar wind. We identify the key instabilities and also find that, even in the parameter regime where fluid theory appears to be appropriate, strong kinetic effects still prevail.

Core protonation of meso-tetraphenylporphyrin with tetrafluoroboric acid: unusual water-mediated hydrogen bonding of H 4tpp 2+ to the counterion

N-Protonation of meso-tetraphenylporphyrin (H 2tpp) with tetrafluoroboric acid gives a hydrated porphyrin dication of the formula [H 4tpp](BF 4) 2·2H 2O·CHCl 3, in which the saddled [H 4tpp] 2+ moiety is hydrogen bonded to the counterions through the two water molecules. Apparently, the formation of this unusual porphyrin diacid serves to optimize the hydrogen bonding. Rather than the tetrafluoroborate ion hydrogen bonding to the two N–H’s of [H 4tpp] 2+, the water molecules interact with [H 4tpp] 2+ and then with the BF 4 - counterions. The average pyrrole tilt angle with the N4 plane is 23.37° and an essentially saddled porphyrin core structure, with insignificant ruffling of the porphyrin core, is observed.

Coulomb Excitation of Neutron-Rich Zn Isotopes: First Observation of the21+State inZn80

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2(1)+ state in 78Zn could be firmly established and for the first time the 2+ --> 0(1)+ transition in 80Zn was observed at 1492(1) keV. B(E2,2(1)+ --> 0(1)+) values were extracted for (74,76,78,80)Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, 80Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus 78Ni.

Nonlinear ion‐acoustic waves supported by an ion beam

It is shown that in a plasma composed by electrons, a proton core, and a proton beam propagating along an external magnetic field, finite amplitude left‐hand polarized waves propagating backward relative to the beam direction can trigger nonlinear ion‐acoustic waves supported by the beam. A detailed study of such instabilities is carried out. The instability bounds are such that if there are large amplitude left‐hand polarized waves propagating toward the Sun, nonlinear ion‐acoustic waves supported by the beam should exist in high‐speed solar wind streams.

Experimental and numerical characterization of ion-cyclotron heated protons on the Alcator C-Mod tokamak

Energetic minority protons with ∼100 keV effective temperature are routinely created in Alcator C-Mod plasmas with the application of ion-cyclotron-range-of-frequency (ICRF) heating. A multichannel compact neutral particle analyzer (CNPA) is used to make measurements of these distributions in Alcator C-Mod's unique and reactor-relevant operating space via active and passive charge-exchange techniques. A radially injected 50 keV diagnostic hydrogen neutral beam is used for active analysis. Using a detailed model that accounts for beam, halo and impurity electron donors, core proton temperatures of ∼30–120 keV are directly measured for the first time in lower density (ne0 ∼ (0.8–1.5) × 1020 m−3) Alcator C-Mod plasmas with up to only ∼0.5 MW of ICRF power. The model found that the minority proton temperatures are peaked spatially away from r/a = 0, even for an on-axis resonance. Additionally, noticeable phase-space anisotropy is seen as expected for ICRF heating. The measured effective temperatures also scale approximately with the Stix parameter. The CNPA temperature measurements are compared with several leading simulation packages such as the TORIC/FPPRF and AORSA/CQL3D full-wave/Fokker–Planck (FW/FP) solvers. Preliminary comparisons with the AORSA/CQL3D code which include results from a new synthetic diagnostic show good agreement and demonstrate that accurate tracking of the minority distribution during iterations of the FW and FP solvers is required to simulate Alcator C-Mod's energetic minority populations with accuracy. Physically, poor wave focusing and preferential heating of trapped energetic protons are found to move the fast proton temperature profiles off-axis. These FW/FP analyses represent the first comparison between predictions of these detailed codes and core minority tail experimental measurements on Alcator C-Mod.

Hydrogen−Deuterium Exchange in Free and Prodomain-Complexed Subtilisin

Residue-specific exchange rates of 223 amide protons in free and prodomain-complexed subtilisin were determined in order to understand how the prodomain binding affects the energetics of subtilisin folding. In free subtilisin, amide protons can be categorized according to exchange rate: 74 fast exchangers (rates > or = 1 h(-1)); 52 medium exchangers (rates between 1 h(-1) and 1 day(-1)); 31 slow exchangers (rates between 1 day(-1) and 0.001 day(-1)). The remaining 66 amide proteins did not exchange detectibly over 9 months (k(obs) < year(-1)) and were denoted as core protons. Core residues occur throughout the main structural elements of subtilisin. Prodomain binding results in high protection factors (100-1000) in the central beta-sheet, particularly in the vicinity of beta-strands S5, S6, and S7 and the connecting loops between them. These connecting loops provide the ligands to the cation at metal site B. Overall, prodomain binding seems to facilitate the organization of the entire central beta-sheet and alpha-helix C in the left-handed crossover connection between beta-strands two and three. It also appears to facilitate the isomerization of multiple prolines late in folding, allowing the formation of metal site B. The gain of stability region around site B comes at the cost of stability in regions more distal to prodomain binding: the C-terminal alpha-helix H and the N-terminal alpha-helices A and B. The acceleration of exchange in these regions by prodomain binding reveals an antagonism between the folding intermediate and the full native structure. This antagonism helps to explain why the prodomain is needed to stabilize the folding intermediate as well as why the unfolding of free subtilisin seldom occurs via this intermediate.

Shell structure evolution far from stability: experimental results

Shell structure evolution in nuclei situated at the extremes of neutron and proton excess are investigated using in-beam gamma spectroscopy techniques with radioactive beams at GANIL. A selection of results obtained very recently is presented: i) The reduced transition probabilities B(E2;0+1 → 2+) of the neutron-rich 74Zn and 70Ni nuclei have been measured using Coulomb excitation at intermediate energy. An unexpected large proton core polarization has been found in 70Ni and interpreted as being due to the monopole interaction between the neutron g9/2 and protons f7/2 and f5/2 spin-orbit partner orbitals. ii) Two proton knock-out reactions has been performed in order to study the most neutron-rich nuclei at the N = 28 shell closure. Gamma rays spectra and momentum distribution have been obtained for 42Si and neighboring nuclei. Evidence has been found for a deformed structure at N = 28 for Silicon, despite a relatively large Z = 14 gap. iii) The in-beam gamma spectroscopy of 36Ca performed using neutron knock-out reactions revealed that N = 16 is as large sub-shell closure as Z = 16 in 36S. The uniquely large excitation energy difference of the first 2+ state in these mirror nuclei turns out to be a consequence of their relatively pure neutron or proton 1p(d3/2)-1h(s1/2) nature.

Limits on the core temperature anisotropy of solar wind protons

Abstract. We analyse the temperature anisotropy of the protons in the solar wind and thereby concentrate on plasma data obtained in the year 1976 of the Helios 1 and Helios 2 missions. We derive the core proton temperatures and , in the directions perpendicular and parallel to the magnetic field, as well as the core parallel plasma beta, . The data are separately analysed for two distance ranges, AU and AU, and divided into 24 bins for the plasma beta, in the range from to , and into 72 bins for the total temperature anisotropy, , which is here considered in the range from −0.9 to 0.9. The number of spectra in each bin is determined to obtain distributions. The statistical results are presented in two-dimensional histograms. For each column we define a critical upper and lower limit of the anisotropy. The resulting empirical points are compared with the known theoretical instability thresholds. The protons are found to come with increasing radial distance closer to the fire-hose instability threshold.

Enhanced Core Polarization inNi70andZn74

The reduced transition probabilities B(E2;0(+) --> 2(+)(1)) of the neutron-rich (74)Zn and (70)Ni nuclei have been measured by Coulomb excitation in a (208)Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d'Ions Lourds via interactions of a 60A MeV (76)Ge beam with a Be target. The B(E2) value for (70)Ni(42) is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to (74)Zn(44). The enhanced proton core polarization in (70)Ni is attributed to the monopole interaction between the neutron in the g(9/2) and protons in the f(7/2) and f(5/2) spin-orbit partner orbitals. This interaction could result in a weakening of magicity in (78)Ni(50).

Solar wind proton temperature anisotropy: Linear theory and WIND/SWE observations

We present a comparison between WIND/SWE observations (Kasper et al., 2006) of β∥p and T⊥p/T∥p (where β∥p is the proton parallel beta and T⊥p and T∥p are the perpendicular and parallel proton temperatures, respectively; here parallel and perpendicular indicate directions with respect to the ambient magnetic field) and predictions of the Vlasov linear theory. In the slow solar wind, the observed proton temperature anisotropy seems to be constrained by oblique instabilities, by the mirror one and the oblique fire hose, contrary to the results of the linear theory which predicts a dominance of the proton cyclotron instability and the parallel fire hose. The fast solar wind core protons exhibit an anticorrelation between β∥c and T⊥c/T∥c (where β∥c is the core proton parallel beta and T⊥c and T∥c are the perpendicular and parallel core proton temperatures, respectively) similar to that observed in the HELIOS data (Marsch et al., 2004).

Effect of nonlinear circularly polarized waves on linear instabilities triggered by an alpha particle beam

It is shown that nonlinear left‐hand polarized waves can either stabilize or destabilize linear right‐hand polarized instabilities triggered by an alpha particle beam in a magnetized electron proton plasma. For differential alpha particle speeds above the threshold for instability the destabilization depends upon the plasma βi = vth.i/vA, where vth.i is the thermal velocity of the i plasma component and vA is the Alfvén velocity. For any given finite‐amplitude wave frequency, there is always a βi value such that above this value the system is destabilized further. For drift alpha particle speeds well above the instability threshold the destabilization can occur for any βi. It is also shown that the presence of the large‐amplitude wave can trigger purely electrostatic ion‐acoustic instabilities. The unstable waves are supported by the proton core.

Influence of Nonlinear Circularly Polarized Waves on Linear Electromagnetic and Electrostatic Beam‐Plasma Waves

We study the influence of nonlinear circularly polarized waves, propagating backward relative to the external magnetic field, on the behavior of linear instabilities in a system composed of electrons, background protons, and a proton beam. We find that the presence of both nonlinear left- and right-handed polarized backward-propagating waves induces the stabilization of linear right-hand polarized forward-propagating instabilities when the amplitude wave is above a threshold value. It is shown that the threshold amplitude is lower for backward-propagating waves than for forward-propagating waves in the parameter space we surveyed. We also find that the presence of nonlinear backward-propagating waves can destabilize linear ion-acoustic modes. These ion-acoustic modes are supported either by the proton core or by the proton beam. The instabilities occur when the phase velocities of the forward- and backward-propagating ion-acoustic waves relative to the proton core or to the proton beam become equal. We study the dependence of the amplitude threshold on the large-amplitude wave frequency, the plasma-β of the various species, and the proton-beam velocity. These results have already been invoked in order to explain some features of the solar wind. We believe that they can also be important in the study of other stars and quite generally in any astrophysical system involving beam-plasma interactions.