N scale track alignment tool 93874,model trains shop uk sale,ho track plans software update,honda lionel train set - Good Point

11.02.2016
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Experimental results are shown for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. Ionization of atomic and molecular targets is one of the most complex and fundamental collision processes that can be studied in detail. To fully characterize these interactions, it is necessary to perform sophisticated measurements where an incident electron of well controlled momentum scatters and ionizes the target. In these (e,2e) experiments, information about the collision is obtained by measuring the outgoing momenta of the collision products. For molecular targets, it is difficult to know the orientation and alignment of the molecule prior to the interaction occurring, unless this can be defined prior to the collision or can be measured following the collision. The interaction of low to intermediate energy electrons with molecular targets has been much less studied compared with high energy electron collisions. Experiments on molecular targets often employ effusive beams produced at room temperature, and so the targets may have internal rotational and vibrational energy.
Results are presented here for an H2O target at significantly lower energies than has been studied before. Figure 1.1 Representation of the 1b1 state of H2O, showing an incident electron ionizing the molecule. The experiments described here were carried out in the fully computer controlled and computer optimized (e,2e) spectrometer that operates in Manchester.
For the coplanar symmetric experiments described here, the electron detectors were moved so that their angles with respect to the incident beam direction were equal to each other throughout measurement (see figure 1.2). For coplanar asymmetric experiments, analyzer 1 was fixed at a scattering angle thetaa = 22A°, while analyzer 2 was swept around the scattering plane. The fixed scattering angle thetaa = 22A° was chosen as this was the smallest angle that could be used without the incident electron beam from the gun striking the side of the analyzer (and so producing unacceptable levels of noise on the coincidence signal). The energy resolution of the spectrometer is set by the resolution of the electron gun and the pass energy of the analyzers. Production of a high quality molecular beam of H2O required careful monitoring of the experiment as it progressed. The de-gassed water was then transferred to a 150mm long tubular stainless steel flask which was connected to the main vacuum system using a swagelok fitting.
An AD590 sensor was secured to the aluminium block surrounding the flask, so that the temperature of the insulated assembly could be monitored and controlled. To monitor the molecular beam inside the vacuum chamber produced from the water, a quadrupole mass spectrometer was installed onto the spectrometer. Figure 1.3 shows a typical coincidence energy spectrum obtained from the experiment during operation.
The experiments were carried out over a period of several months, using electron beam currents ~200nA for all data runs. As the incident energy increases, the ratio of forward to backward scattering cross section increases, as has been seen for most atomic targets in this geometry. The measured coplanar asymmetric data is shown in figure 4, again normalized to unity at thetab = 45A°. As the incident energy is increased to 55eV above threshold, the forward lobe splits into two clear peaks of approximately equal magnitude, as seen in figure 1.5(b). Results from the 1b1 state of water have been presented for coplanar symmetric and asymmetric energies. These results have been compared to a DWBA theory in the paper J Phys B, however the averaging used in this theory was not a good approximation, and so is not expected to yield accurate results for this state.
Experimental results are presented for electron impact ionization of water in the energy regime from near threshold to intermediate energies.
Results of a sophisticated model which approximates the random orientation of the target using a spherical averaging of the wave-function prior to the collision, using sophisticated distorted wave Born calculations that include post-collisional interactions in first order and to all orders of perturbation theory are also shown for comparison (see J. The calculations predict the data most accurately at the lowest energy studied (4eV above threshold) in a coplanar symmetric geometry, whereas the comparison between theory and experiment is generally poor for higher energies and for non-coplanar geometries. Energy deposition and angular distributions resulting from electron collisions with water are used in charged particle track structure analyses to model radiation damage in biological samples. These low energy electrons have an effect over a much wider volume than the targeted cancer site. At incident energies less than ~200eV, the collision dynamics are strongly influenced by effects including post-collision interactions, target polarization, distortions in the wave-functions for the participating electrons and multiple collisions.
Current models now yield reasonable agreement with experimental data for a range of atoms, implying a good understanding of the collision dynamics under the conditions used in the experiments, and these models are now being extended to low energy (e,2e) collisions from molecules.
Recent experiments studying simple diatomic targets including H2 and N2 have provided benchmark data. The apparatus at Manchester can also access non-coplanar geometries, and so has provided additional data to further test these models. In a previous study the groups at Manchester and Missouri investigated the 1b1 (HOMO) state of H2O in coplanar kinematics. By contrast, the 3a1 orbital of interest here is involved in the O-H bonding and has a charge density distribution distorted from that of a symmetric atomic like orbital. The experimental triple differential cross sections (TDCS) presented here were measured in the (e,2e) apparatus at the University of Manchester. The spectrometer can be operated in a standard coplanar geometry where the momenta of all three electrons (the incident and two outgoing electrons) are within the same detection plane (psi = 0A°, figure 2.1). The power supplies for the electrostatic lenses in the electron gun and the electron analyzers are fully computer controlled and computer optimized, allowing for automated tuning at regular intervals, with the analyzers being re-optimized each time they move to a new angle. The energy of the spectrometer was re-calibrated at the start of each new kinematic arrangement, by measuring the coincidence binding energy spectrum.
Over the course of this study the binding energy spectra were recorded for various energies and geometries, and it is estimated that contamination from neighbouring orbitals was always less than 10%, and is more typically in the range of 0.5%. The distilled water sample used to provide the molecular target beam was contained within a 50mm diameter 100mm long stainless steel vessel sealed by a CF-70 flange to a 6.35mm swagelok fitting. The sample vessel and gas handling line were held at a constant temperature of 50A?C throughout data collection, so as to create sufficient driving pressure for the target beam. The purity of the target beam was verified with a Spectra VacScan mass spectrometer fitted to the scattering chamber. The background pressure within the scattering chamber was set to 1.1 x 10-5 torr during operation, and was found to remain constant throughout all data runs. During these experiments we observed an unusual behaviour of the tungsten hairpin filament used as the incident electron source.


To facilitate this, the current measured by the Faraday cup located on the opposite side of the interaction region to the electron gun was monitored by the computer control software, and the current through the filament adjusted to maintain a beam current of 300nA throughout data collection. In the present study we also measured the coincidence energy resolution from ionization of helium, and found this did not change, indicating that the temperature of emission remained approximately constant.
The data presented in figures 3-5 were then averaged over these angular sweeps, and the uncertainties in the measurements determined from the complete set of data for each scattering angle. The experimental data are not measured on an absolute scale and so experiment and theory are normalized to a maximum of unity at each energy, as shown in figure 2.2.
As the energy of the outgoing electrons is lowered, it would be expected that the Coulomb repulsion between the outgoing electrons should play an increasingly important role, driving the electrons apart. The best agreement between experimental data and theory is at the lowest energy, where the experimental data and MDW model are in excellent agreement for the binary peak. A key advantage of the spectrometer in Manchester is the ability to measure data for kinematics in non-coplanar geometries.
Non-coplanar measurements were hence taken here with both outgoing electrons having an energy of 10eV.
As shown in figure 1, the geometry adopted in this spectrometer provides a common normalization point (xi1 = xi2 = 90A°) for all gun angles psi, which allows ALL data at a given energy to be referenced to a common point. For the current measurements, the coplanar data have been normalized to a maximum of unity, as before.
The data in figure 2.3 show a clear trend indicating the binary and recoil peaks diminish in magnitude as the angle of the electron gun angle increases.
The data contrasts strongly with the theoretical models for the larger gun angles y, where the theories predict significantly more structure than is seen in the data. The final kinematic configuration used symmetric geometries and 20eV excess energy as above, however in this case the data are for unequal energy sharing between the outgoing electrons.
The data were only taken for coplanar and perpendicular plane geometries, so as to contrast differences in these two extremes.
The key differences that can be seen in these data for the coplanar geometry are that the binary peak moves to a smaller angle as the energy asymmetry increases, as might be expected from post collisional interactions. Agreement is mixed, and rather surprisingly gives best results at low energies, where it might be expected that the approximations are least accurate. The experimental results for both equal and non-equal energy sharing in the perpendicular plane show almost no structure, whereas the theoretical calculations predict that three clearly defined lobes should be seen.
This work was supported by the University of Manchester and the US National Science Foundation under Grant.
12.A A  Naja A, Staicu-Casagrande E M, Lahmam-Bennani A, Nekkab M, Mezdari F, Joulakian B, Chuluunbaatar O and Madison D H 2007 J.
Import charges previously quoted are subject to change if you increase you maximum bid amount. Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies. The experimental data show a wide variation in the cross section over this range of energies. These ionizing collisions are important in a wide range of different areas, ranging from plasma physics, astrophysics, atmospheric physics, medical and biological processes involving low energy electrons through to the study of high voltage discharges. The scattered electron is then detected in time correlated coincidence with either the electron ejected during ionization, or with the resulting ion. This is usually accomplished by determining a cross section that is five fold differential in angle and energy. In most cases, this means that an averaging over the orientation of the target must be included for theory to compare with experimental results. This energy region (from threshold to ~200eV) is important as it is here that the cross section for ionization is largest. The ionization potential from these ground ro-vibrational states is usually less defined than for atomic targets, due to the associated Franck-Condon overlap between the potential energy curves of the neutral molecules and the resulting ions. This apparatus is described in detail on other web pages on this fileserver, and has been designed to operate in the energy regime from ~10eV to ~300eV.
In this configuration, the energies selected for the scattered and ejected electrons were equal. The inner diameter of this flask was 9mm, and the outside diameter was 12.7mm, so that a standard swaged fitting could be used. This was accomplished using a Newport model 325 temperature controller, which supplied drive current to the TEC.
This proved invaluable, as it was then possible to continuously monitor the background gases in the chamber during operation. In this example, the analyzers were set to pass electrons of energy 20eV, and a symmetric geometry was chosen with thetaa = thetab = 45A°.
Over this period of time, the filament current driving the cathode had to be steadily decreased, probably since the water vapour within the vacuum chamber slowly reduced the diameter of the filament.
The data is normalized to unity at thetaa = thetab = theta = 45A°, since the results have not been placed on an absolute scale. Results were taken in symmetric coplanar and non-coplanar geometries, with both equal and non-equal outgoing electron energies. The human body, and other biological material comprise of ~80% water, which makes it an ideal test case to investigate processes occurring in the body. These models are an active area of research since the observation that high energy radiation that is used to treat cancers also liberates many low energy electrons, causing additional damage to cell DNA. Knowledge of the collision dynamics of low energy electrons with biological systems is hence needed, so as to develop robust models of these processes.
In such experiments the energy and momenta of the outgoing electrons are measured, giving a five fold differential cross section. In this regime these processes must be considered on an equal basis, and so the complexity of the interactions means that theoretical studies have mainly been limited to atomic targets. This contrasts to atoms which have a single nuclear scattering centre, and which can hence be described using a spherical basis.
A further challenge arises since the experiments cannot, at present, align the molecules prior to the collision, so the models must consider the random orientation of the targets for accurate comparison. The majority of data is recorded in a coplanar geometry, where the incident and two outgoing electrons are in the same plane, and were conducted at a higher incident energy than the studies presented here. The symmetry of the 2py oxygen atomic orbital, representing the lone pair of electrons on the oxygen atoms, prevents it from hybridizing with the H atomic orbital, leaving the molecular 1b1 HOMO orbital essentially atomic like, and therefore symmetric.
However, the orientation averaged molecular orbital used in the theoretical calculation is a bad approximation for that state given the cancellations due to the orbital symmetry. The molecular orbital used in the model therefore should not suffer from the same cancellation problem during the orientational averaging procedure. This apparatus is fully computer controlled and computer optimized, allowing it to operate continuously without user intervention. A coplanar geometry (psi = 0A?) is defined when all three electrons are in the detection plane. The angular resolution of the apparatus is estimated as A±3A°, based on geometric considerations of the electrostatic lenses at those energies.
Several freeze-pump-thaw cycles were performed using a salted ice slurry bath, to remove dissolved gas impurities from the water prior to admission into the scattering chamber.
Over time the emission current from the filament dramatically increased, when a constant current was delivered to the filament. The data were normalized to a collection time of 1000sec for each measurement, and up to 30 sweeps of the detection plane were used to produce statistically significant results. The overall shape of the TDCS measured at corresponding energies in a previous study of the 1b1 state are qualitatively similar, however the 1b1 state shows a second peak in the binary region emerging at higher energies that is not observed in the 3a1 state. PCI would also be expected to shift the recoil peak towards xi = 90A°, although this cannot be confirmed in the data.


This agreement diminishes as the energy increases, which is unexpected since the MDW model is usually more accurate at higher energies. The TDCS measured in the perpendicular plane is almost constant over all analyzer angles, which is very different to what is observed for atomic targets. The progression in both models shows a decrease in cross section from psi = 0A° to 45A°, after which the intensity once again increases.
There also appears to be a narrowing in the main binary peak as the asymmetry increases, with a new shoulder appearing around xi = 60A°.
This contrasts markedly with the calculations, which predict triple peaks in the perpendicular plane that change magnitude only marginally with the asymmetry.
This discrepancy is seen both for equal energy and for non-equal energy data, which have been taken in coplanar and perpendicular geometries. If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable. Theoretical approximations accurate at high impact energies can no longer be applied, and so there are stringent demands on theory which must be satisfied for accurate comparison with data. The (e,2e) coincidence signal for molecules may then have a correspondingly lower yield compared to an atomic target where the ionization potential is well defined. A pictorial representation of the water molecule in the 1b1 state is given in figure 1.1 below. The spectrometer allows measurements to be conducted from a coplanar geometry through to the perpendicular plane, although in the present measurements only a coplanar geometry was used.
The analyzer pass energy was then set to measure electrons so that the overall resolution of the coincidence signal (as measured using a helium atomic target) was 600meV. The pump removed background gas from the flask, and extracted dissolved gases held in the water. The flask was secured to the cold side of a 50mm x 50mm thermo-electric cooler (TEC) using a large aluminium block, and the assembly (flask + block) was insulated from the air using Styrofoam. The 1b1, 3a1 and 1b2 states are clearly resolved in this spectrum, which is fitted to three Gaussian peaks to ascertain the widths of the spectral profiles. These data were measured in a coplanar geometry with outgoing electron energies of 20eV detected at theta1 = theta2 = 55A°. Figure 1.5(a) shows the lowest energy results, with an incident energy 15eV above the ionization threshold.
Molecular wave-functions are generally not spherical, the nuclei within the molecule providing multiple scattering centres. This becomes a computationally intensive problem, and so approximations are usually made to allow these calculations to become tractable. In this study the analyzers were always kept in a symmetric configuration with xi1 = xi2 = xi. A needle valve at the entrance to the scattering chamber controlled the flow of target H2O vapour into the interaction region. This indicates that an H2O target molecular beam of high purity was created, as confirmed from binding energy spectral studies.
This increase in emission current was often more than a factor of 2 within a 24 hour period.
The solid line shows results from the Molecular Distorted Wave Born Approximation (MDW) while the dashed line was generated from the Molecular 3-body Distorted Wave Approximation (M3DW). This trend is much clearer in the present data compared to that from the 1b1 state measured at higher energies. For the theoretical model, the coplanar TDCS has also been normalized to unity for both MDW and M3DW models. These measurements were taken in a series of symmetric non-coplanar geometries with outgoing electron energies of 10eV. The M3DW model follows a trend that is closer to the experimental data, although neither model accurately predicts the results that have been obtained. The theoretical calculations are also shown, where once again the data and theory have been normalized to unity at the peak in the coplanar geometry. The minimum around 90A° in this geometry does not change substantially as the energy sharing changes. The angles that could be accessed in this configuration ranged from thetaa = thetab = 25A° through to thetaa = thetab = 135A°. The acceptance angle of the analyzer lenses was A±3A°, as set by input apertures at the entrance to the electrostatic lenses. This de-gassing procedure was carried out for ~2 hours before the water sample was used in the experiment, to ensure that the proportion of dissolved gases in the water was minimized. The hot side of the TEC was attached using thermal conducting paste to an aluminium heatsink to which a 12VDC boxer fan was secured, so that heat from the TEC could be dissipated efficiently. If this ratio reduced, the experiment was halted and the water changed to a newly de-gassed sample.
In this case, the 1b1 state was found to have a width of 620meV, which is comparable to that for an atomic target. The contrast between the maximum and minimum in the cross section increases as the energy increases. As the energy increases further to 95eV above threshold (figure 1.5(c)), the double peak in the forward direction is still visible but the magnitudes of the peaks are now different. The purity of this beam was monitored regularly using the mass spectrometer throughout this study.
To ensure constant incident electron beam current throughout the measurements as required, the filament current was hence also placed under computer control. The experimental and theoretical data has been independently normalised to unity at each energy. Dr Kate Nixon thanks the British Council for funding under the research exchange programme, and the Royal Society for a Newton International Fellowship.
For asymmetric geometries, thetaa is fixed at 22A° while analyzer 2 is swept around the scattering plane.
A glass flask was used for this procedure as this allowed the process to be visually monitored. This indicates that the energetic effects of ro-vibrational transitions from the ground state to the ion state are negligible for this state.
Further checks were made regularly on the energy of the incident electron beam by monitoring the coincidence energy spectrum (figure 1.3), and regular monitoring of the partial pressures of the gases in the vacuum chamber. A second peak occurs in the backscatter region, however the maximum of this peak appears to occur at thetab < 225A°.
For non-coplanar geometries the electron gun is lifted out of the plane, and is defined by the angle psi.
There is an offset of 1eV in the position of the peaks which is due to contact potentials in the spectrometer. The full line represents a three-Gaussian fit, whereas the dotted lines show the individual Gaussians from this fit, illustrating the degree of separation measured with the current energy resolution. The results in the backward direction also have structure, with a minimum occurring when thetab ~ 250A°.
Very little contamination is expected from neighbouring orbitals in the measured TDCS for the 3a1 state.



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