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The (e,2e) spectrometer vacuum chamber (figure 1) is manufactured from 304 grade stainless steel, the construction being carried out by Vacuum Generators using a design of Jones, Read and Cvejanovic, full details of which may be found in T. 3mm thick mu-metal magnetic shielding is placed both internally and externally to decrease extraneous external magnetic fields to less than 5mG at the interaction region.
All inputs and outputs to the spectrometer pass through the vacuum chamber via feedthroughs on the top flange. The top flange of the vacuum system is a 25mm stainless steel flange with 11 conflat CF70 flanges arranged around a central 200mm flange designed for locating a hydrogen source. Two flanges are used for the high voltage channeltron and photomultiplier tube inputs and pulse outputs. Three rotary motion feedthroughs allow the analysers and electron gun angles to be changed externally via stepper motors for both analysers and for the electron gun. The ionisation gauge is placed at right angles to the main flange to prevent light from the thoriated iridium filament entering the vacuum system, which would increase noise on the photomultiplier tube.
The ionisation gauge feedthrough is sooted, and a loose cover is placed over the entrance in the chamber flange to prevent stray light from the ion gauge entering the vacuum system. The target gas enters the system via a CF70 flange which has two 6mm stainless steel tubes welded into the flange. The other gas feed is connected to a hypodermic needle set at 90A° to the incident beam for measurements in other geometries. The gas jet which is not currently used in the interaction region can be used to non-locally fill the chamber with gas for background counting measurements. The final CF70 flange is used when the photomultiplier tube is cooled, usually during baking of the system following exposure to atmosphere. The tube is cooled by passing vapour from a liquid nitrogen source through expansion coils wrapped around the photomultiplier tube housing. The electron gun is a two stage non-selected gun designed by Woolf (Ph.D thesis, University of Manchester) (see figure 3). The gun consists of a filament, grid and anode source which is focussed using a triple aperture lens through two 1mm defining apertures in the centre of the gun. The apertures define the electron beam pencil and beam angles, and are focussed onto the interaction region using a second triple aperture lens located between the apertures and interaction region. The electron gun is shielded using advance (constantin) sheet which is grounded to the body of the gun mounting.
The gun is mounted onto an x-y translation table to allow placement of the electron beam as accurately as possible to the centre of rotation of the analysers and electron gun rotation axis.
The gas jet is mounted from the electron gun using a narrow support of 310 stainless steel, and the Faraday cup can be attached to this support.
The original unselected electron gun built by Woolf was completely rewired and a custom built PTFE 25 way inline wire-wrap plug and socket was placed between the 52 way vacuum electrical feedthrough and the electron gun. The wiring from the 52 way feedthrough to the inline plug is shielded colour coded 30 awg PTFE coated advance wire, and the inline plug and socket connectors are non-magnetic gold plated wire wrap pins. The electron analysers are constructed from molybdenum and are mounted from two rotating gear plates controlled externally via the rotation feedthroughs on the top flange (see figure 2).
The input electrostatic lenses for each analyser are three element cylindrical lenses as shown in figure 4. The interaction region is focussed onto the entrance aperture of the hemispherical energy analysers with a beam angle of around zero degrees, the image of the analyser entrance aperture being larger than the size of the interaction region. Field gradients around the input and exit apertures are corrected using standard Jost correctors.
The energy selected electrons passing through the exit apertures are finally collected by Mullard X919BL channel electron multipliers which amplify the single electron to produce a current pulse at their output. A principle source of noise on the signal pulses arises from the turbo-molecular pump motor. The noise from the turbo pump is reduced to ~ 5mV using these techniques, and as the average pulse height from the saturated multiplier is around 20-40 mV, this allows adequate discrimination following amplification. The pre-amplifiers located external to the vacuum system are commercial 100X preamplifiers with 400pS rise-times, manufactured by Phillips Scientific. The NIM signals produced by the discriminators drive two ORTEC 437 Time to Amplitude Converter's via appropriate delay cables and an ORTEC pulse delay unit.
The main TAC output feeds an ORTEC Multichannel Analyser card located in the PC controlling the experiment, whereas the second monitor TAC drives an external MCA, allowing convenient monitoring of the coincidence signal as the experiment progresses. The high voltage supplies to the electron multipliers are two Brandenberg 5kV supplies controlled by the main PC via serial DAC's. Principally the filter is used to reduce pickup noise from entering via the HT cable onto the multiplier output. Secondly, the filter has a slow response time for high voltage spikes, and reduces the probability of damage to the pulse electronics should the EHT supply suddenly turn on or off.
Capacitor C2 decouples the 1kW load resistor to earth for high frequency pulses from the CEM's, and the voltage appearing across this load is connected via the capacitor C3 to the input of the 6954 preamplifier. Since the input impedance of the preamplifier is 50W and the CEM is effectively an ideal constant current supply, most of the current passes through to the preamplifier. Finally, the 10M resistor prevents the output to the preamp from rising above ground when the preamp is disconnected from the circuit and is therefore a safety device for the preamplifier when initially connected.
The original design placed the atomic beam nozzle and Faraday cup on the same support structure fixed to the body of the electron gun, with the atomic beam direction being in the detection plane when the perpendicular plane geometry is chosen (see figure 2). A second configuration is adopted for exclusively perpendicular plane coincidence experiments, the atomic beam nozzle being oriented at an angle of 45A° to the detection plane so that the gas effusing from the nozzle does not fill the analysers when they oppose the nozzle (see figure 6). In this configuration the support strut for the atomic beam nozzle is terminated lower than the detection plane, as is the nozzle itself. For non-perpendicular plane measurements the Faraday cup is once mounted from the support strut and the gas effuses perpendicular to the electron beam direction. One difficulty when aligning an (e,2e) coincidence experiment lies in the focussing the incident electron beam and analysers onto the interaction region. The throughput of electron current as detected by the Faraday cup is insensitive to both position and focusing of the electron beam, since the cup opening is usually made quite large (in this case 6 - 10mm) and the bias voltage applied to the cup to prevent electrons returning to the interaction region allows all electron trajectories within this opening to be detected.
Conventionally, following maximisation of the Faraday cup current, the beam is steered and focussed onto the gas jet as detected by counts in the analysers. This has the disadvantage that the overlap volume is maximised for both atomic and electron beam density, which tends to place the interaction volume as close as possible to the output of the atomic beam nozzle, which is almost certainly not in the centre of the detection plane.
To eliminate the need for time consuming iterative techniques and the associated errors due to detection geometry, a separate method for tuning the electron gun is adopted that does not depend upon the analysers. The interaction between the target and the electron beam not only produces ionisation, but the valence states of the target also are excited with various probabilities depending upon the incident energy of the electron beam. These photons are detected by the photomultiplier tube, by placing a 450nm A± 50nm optical filter in front of the photocathode.
The interaction region is therefore defined where an aperture in front of the photocathode is focused by the lens. The plano-convex focussing glass lens is covered by a grounded fine tungsten mesh to eliminate surface charging.
The photomultiplier tube may be cooled by circulating nitrogen gas boiled off LN2 through copper coils encircling the photomultiplier tube housing. The photomultiplier tube used in the experiment is a 9789QB EMI photomultiplier tube with a bi-alkali photocathode peaking in the blue region of the optical spectrum. As with the electron multiplier circuitry, it is important to optimise the load of the photomultiplier tube so that reflections along the feeder line to the counting electronics are minimised. The 1nF 6kV capacitors across the dynodes close to the output stage are used as charge storage pumps that quickly refurbish the surface charge on the dynodes, allowing increased counting rates. The dynode resistors are 1% tolerance metal film resistors as these are more stable with temperature. The EHT supply to the photomultiplier tube is a Brandenberg 2kV supply operating at 1500V for this tube. The negative going output pulses from the tube measure typically 200-500mV in height, and therefore are able to directly drive an ORTEC 473A constant fraction discriminator located in one of the NIM crates.
All resistors and capacitors for the dynode chain are soldered to the photomultiplier tube base internal to the vacuum system, using standard 5-core solder.
Typical pulse heights from the electron multipliers in saturation mode are 25-40mV, whereas the typical height of the photomultiplier tube pulses is of the order of 250mV. The electron multiplier pulses are obtained from the multiplier supply circuitry as explained above. The amplified pulses from the 6954 amplifiers pass along doubly shielded RG-58 cable to the input of the ORTEC 473A discriminators.
The two analyser discriminators are located in separate NIM crates to increase isolation and reduce common mode pickup.
The slow rise-time positive outputs from the 473A discriminators pass to three ORTEC 441 rate-meters which allow counting and monitoring of the pulses. The fast NIM pulses from the analyser 473A CFD's pass through appropriate delay lines to two 437 TAC's. This second multichannel analyser allows the operator to monitor the total accumulated coincidence signal during operation.
The lens supplies that control the voltages impressed onto the lens elements and deflectors in the experiment were developed over a number of years, and therefore take different forms.
Three of these supplies feed the deflectors in the electron gun, the other two supply the input deflectors in the analysers. In a second 19" crate is located the gun lens high voltage supplies together with the high voltage supplies for the Faraday cup. In a third 19" crate is located the analyser voltage supplies together with a set of serial loading supplies that control the electron multiplier EHT supplies. A further supply controls the current boost circuitry located in the filament constant current supply unit (see below). The outputs of each computer controlled supply that drives an individual lens element or deflector are connected to a 70 relay switching board. The Digital voltmeter is in turn accessed via an IEEE-488 interface which is addressed by an 8086 slave PC which converses with the central PC which controls the experiment. The outputs from the relay board connect to the spectrometer via shielded multicore cables which take the gun voltages to the 52 way feedthrough and the analyser voltages to the two 19 way feedthroughs located on the vacuum flange. The gun supply multicore also carries the filament supply current to the 52 way feedthrough.
The Faraday cup current is measured by the Keithley Digital voltmeter when under computer control but this can also be measured by an external pico-ammeter. In addition to the analyser electrostatic lens and deflector voltage connections, the 19 way CF70 feedthroughs are used to control signals from internal sensing opto-isolators which control the stepper motor interlocks.
The stepper motors that drive the analysers and electron gun around the detection plane are five phase motors with integral 10X reduction gearboxes driving the rotary motion feedthroughs. The intelligent stepper motor drivers are addressed via a serial port located on the controlling PC bus. An additional interference problem was traced to the RS232 serial line from the controlling PC. The PC bus was found to be poorly earthed due to very little copper being used for the bus earth.
This was cured by decoupling the 0V RS232 line from the main 0V line for the stepper motors, as is shown in figure 10. The analyser stepper motor supplies have a facility for halting the motors using limit switches which are optically coupled to the internal 68000 logic circuits driving the supplies.
The SD5443-3 phototransistor normally illuminated by the TEMT88PD photodiode during operation ensured a fail-safe mechanism should any of these components fail (this automatically prevent the motors from running). Should the analysers move into a position as defined by the four criteria (a) - (d) above, a shutter located on the analyser turntables moves between the appropriate photodiode and phototransistor, shutting off the light and thereby shutting down the stepper motor current. Only motion in the opposite direction is allowed by controlling logic, allowing the analyser to move away from the point of imminent collision but no closer. The six pairs of optocouplers that prevent the analysers moving into the Faraday cup, electron gun and gas jet support are located on the mounting struts supporting the main body of the spectrometer.
Two optocouplers are located on the side of analyser 2, and two shutters are located on the sides of analyser 1 to prevent the analysers colliding with each other.
Finally a ninth optocoupler is located over the electron gun counterweight which monitors when the gun is at an angle less than 65A°, where the sweep angle around the detection plane is reduced.
The current boost circuit operates from a serial driven supply card as detailed previously. The main filament drive is through six power diodes that feed the current from the constant current supply to the filament. The Darlington transistor T1 feeds additional current to the filament via three parallel 10 turn 100W trim resistors through a 1N4007 diode. As the constant current supply acts as an infinite resistance source, the boost circuit supplies additional current only when the voltage at the anode of the feed 1N4007 exceeds the output voltage of the constant current supply.
Finally, the filament bias voltage (and hence energy of the electrons) is set to the midpoint of the filament by two 100W balancing resistors located across the terminals. In addition to the ADC buffer card, three independent isolated regulated supplies drive REF01 voltage reference sources for measurement of the positions of 10 turn potentiometers located on the analyser and electron gun rotary drive shafts. The angular position of the electron gun and analysers is calibrated when the system is opened using visible laser diodes that define these angles to an accuracy of A±0.2A°. Figure 13 shows an overall block diagram detailing the computer control and high speed pulse interfacing to the spectrometer. 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. One of these is internally connected to a gas jet which can be set at 45A° to the incident electron beam and is used in the perpendicular plane experiments. These stainless steel feeder tubes are connected to the nozzle using grounded shielded PTFE hose.
One follows the anode, another corrects the beam direction between the apertures and a third steers the beam onto the interaction region.
This shield is coated with aerodag colloidal graphite to reduce patch fields on the surface of the metal, and is made as small as possible to allow the analysers to approach the gun as close as is feasible when non-perpendicular plane geometries are selected (see figure 2).
The XY translation table is mounted from the rotating yoke, to which is attached the photomultiplier tube. This allows the gun to be dismounted from the system without the need for rewiring of the feedthrough when the need arises (filament changes, cleaning etc). The wiring between the gun and inline socket is also shielded colour coded PTFE coated advance wire facilitating spot welding to the electrostatic lens elements of the gun. The analysers are mounted onto x-z translation tables that allow them to view the interaction region accurately over the full range of angles accessible. These lenses have a capability for energy zooming of around 10 : 1, and have a maximum entrance acceptance angle of A±3A° as defined by the entrance apertures. This ensures that the overlap volume accepted by each analyser encompasses the interaction volume for all detection angles as the analysers move in the detection plane.
These devices use an activated lead glass internal surface which has a high coefficient of secondary electron emission when a high voltage (around 3000V) is applied across the multiplier. This noise may be picked up by the channel electron multiplier and the high voltage feed to the multiplier, and is generated internally in the vacuum system. The CFD's are operated in NaI mode as this has a 2ms dead time following the initial pulse, thus reducing the possibility of false triggering due to any mismatching impedance reflections along the delay cable. These EHT supplies are connected via VERO 200 high voltage couplings to high voltage cable connecting to the pickoff circuitry located on the high voltage feedthroughs. This allowed the analysers to roam the detection plane over a complete 360A° angular range. The configuration of the target source hypodermic when perpendicular plane measurements are conducted. The common procedure is to initially maximise the current from the gun onto the Faraday cup, then onto analyser counts as the electron beam is moved through the gas jet.
Maximising the Faraday cup current is therefore only a crude preliminary alignment of the incident electron gun beam. This therefore assumes that the analysers are initially focused onto the interaction region, and a slow iterative technique must be adopted where the analysers and electron gun are tuned so as to maximise the signal from both analysers.
In this case a true differential cross section is not obtained, since the scattered and ejected electron trajectories lie in a different plane than defined in the initial experimental alignment. An EMI 9789QB reduced photocathode photomultiplier tube is installed in the vacuum system together with optical focussing and defining elements (figure 2). A group of these excited states decay back to lower valence states with the emission of visible photons at wavelengths around 450nm for both helium and argon targets. Tuning of the electron gun to PMT counts therefore guarantees that the electron gun is steered and focussed to the point at the centre of the detection plane independent of the analysers or gas beam density. This is mainly used when the vacuum system is baked at temperatures above 55A°C, since the photocathode is rapidly damaged by temperatures exceeding this. The tube is a venetian blind dynode construction, and has a reduced photocathode to decrease the noise on the output of the tube to less than 20Hz. The photomultiplier tube base was thoroughly cleaned in an ultrasound following location of these components, to displace any flux residue from the solder joints. It is therefore necessary to amplify the channeltron pulses prior to transmission to the discriminators, whereas the photomultiplier tube pulses are sufficiently large to not require further amplification.
These pulses are amplified by Phillips Scientific 6954 100X amplifiers located external to the EHT feedthroughs connected by 10cm lengths of RG-58 cable.
These crates are further decoupled from the AC mains supply using filter circuitry on each supply line.
Internal to these rate-meters buffer 7400 TTL chips have been added to send to 32 bit counters located in the main PC via RG-58 cable. These supplies are fully floating, their mid-point voltage being equal to the lens element voltage in which the deflectors are housed. All these supplies are fully computer controlled isolated units with low output impedance to reduce noise and eliminate current loops. This relay switching unit allows either the voltage or the current on each lens element to be measured by a Keithley Digital voltmeter. This drive is further reduced internally by 10X reduction gears, resulting in a reduction ratio of 100:1.
This led to the ground line of the RS232 serial line moving around with respect to the 0V stepper motor reference as drive current to the motors returned along the ground line. The optocoupler logic circuit circuit that controls the stepper motors and avoids collisions between the analysers, the electron gun, the Faraday cup and the hypodermic support. Added to this filament supply is a computer controlled current boost circuit that can be used to increase the filament current during electron gun tuning. The additional current then returns to the 12V DC regulated supply via a second 1N4007 diode.
This therefore depends upon both the current driving the filament, as well as the resistance of the filament when hot (typically 4W - 6W).
This supply feeds current to the filament without changing the characteristics of the main constant current supply used while operating the spectrometer. These high stability reference sources ensure that the analyser and electron gun positions are accurately measured with time. A datalogger internal to the controlling PC establishes a cubic spline relationship between the measured angles and the corresponding ADC conversion from the potentiometers. This PC also controls the selection of the relay motherboard for measurement of the required lens element either in voltage or current mode when a request is sent from the controlling computer via an RS232 serial line. 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. The spectrometer is shown configured in the perpendicular plane, the electron gun being placed in the vertical position with respect to the analysers which span the horizontal detection plane.
The electron source is a heated tungsten hairpin filament heated by passing around 2A of DC current through the filament. The saturated gain of these devices yields around 1E8 electrons in the output current pulse for a single electron which enters the input cone.
This allows the analyser to roam over a complete 360 degrees of the detection plane when in this geometry (apart from the position where the analysers would clash wth each other). The analysers are then adjusted for maximum signal count and the procedure is iterated until a maximum is found. The dark noise count is also reduced when cooled, however since the reduced cathode PMT only produces dark counts of less than 20Hz at room temperature, this is not necessary during routine operations.
The 1.82M resistor was chosen to optimise the single photon counting efficiency of the tube. It is not found that these solder connections significantly affect the ultimate vacuum pressure in the system. Isolated DC supplies to these amplifiers is obtained from a standard +15V supply, the circuit diagram being given in figure 8.
The power supply to these TTL buffers is derived from the rate-meter supplies via 7805 5V regulators located internally. This supply boosts the current without changing the main supply, thus ensuring supply stability before and after the electron gun is tuned.
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 electron gun is a two stage non-energy selected gun that produces an incident electron beam current of up to 4 microamps at an incident energy from around 30eV to 300eV. The 470k resistors bias the dynodes to achieve saturation of the current pulse at the output of the tube, hence biasing the tube for single photon counting applications. This was found to be necessary to eliminate problems associated with current loops from the 20A peak motor current feeding the common point of the logic circuitry. 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. The analysers are identical, and consist of a 3-element electrostatic lens focussing the interaction region onto the entrance of a hemispherical electrostatic energy analyser.
The Anode deflector pair corrects for any misalignment of the filament emission point with respect to the axis of the electron gun. 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.
The Chamber is lined internally with 3mm thick mu-metal, and a mu-metal cylinder encloses the chamber externally as shown. The electrons selected in angle and energy are amplified using Mullard 919BL channel electron multipliers.
The defining apertures limit the pencil and beam angles of the electron beam following focussing by lens 2 onto the interaction region.
The load is set by the 50W resistor across the anode to the HT feed, the pulse being decoupled to ground via the 1nF capacitor. 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°. This reduces the magnetic field at the interaction region to less than 5mG, an acceptable level for the electron energies used in this experiment. The interaction region defined by the interaction of the gas target beam and the electron beam is focussed onto the photocathode of an EMI 9789QB photomultiplier tube via a lens, 450nmA±50nm optical filter and defining aperture. Placement of the apertures ensures that the beam angle is close to zero degrees at the interaction region over a wide range of energies, whereas the aperture opening reduces the pencil angle to around A±2 degrees over this energy range. Very little contamination is expected from neighbouring orbitals in the measured TDCS for the 3a1 state. The target gas effuses from a 0.6mm ID hypodermic needle constructed from platinum iridium alloy.
The 10MW safety resistor prevents the output charging up to a high voltage if the feed is disconnected. The deflectors between the apertures correct for space and surface charge beam variations inside the field free region, whereas the final deflectors allow the beam to be steered onto the interaction region defined by focussing a 1mm aperture in front of the photomultiplier tube onto the gas target beam.
The electrons passing through the interaction region which are not scattered are collected in a Faraday cup opposite the electron gun. Do you think big oil has trashed the beaches and economy of Alabama, Louisiana, Mississippi, Texas and California? The gun, photomultiplier tube, gas hypodermic and Faraday cup all rotate on a common yoke with an axis through the centre of the horizontal detection plane, allowing all geometries from the perpendicular plane to the coplanar geometry to be accessed. Those states seem to have some great beaches and oil industry too.A (2) By constitutional amendment, by statute or by a€?otherwisea€? to extend Floridaa€™s Gulf of Mexico seaward boundaries. Will you support an amendment, a statute or Governora€™s Executive Order to extend Floridaa€™s seaward boundaries? On July 4, at the Tallahassee Automobile Museum, citizens will sign the Declaration of Independence from Foreign Oil. It appears that the better argument is that our national security will be helped by oil drilling.
News reports have correctly quoted me, a€?Ita€™s time for energy independence.a€? Will you sign the Declaration?A (4) Demanding the federal government share profits from oil leases. Why havena€™t you demanded that Florida citizens share in the oil revenue?A (5) Florida should set up an Oil Revenue Trust Fund and make annual distributions to Florida Citizens like Alaska does.A (6) Benjamin Franklin believed elected officials in a republic should not be paid. Why dona€™t you follow the example of Benjamin Franklin and set up such a fund see pages 712-713 attached.A Ia€™m not big oil. Pace Allen, a spokesman for the group, says public interest in growing.a€?These polls have revealed that citizens understand, or are beginning to understand that technology has improved. If last summer is any indicator the support for offshore oil drilling will go up if the gas prices do.Governor Charlie Crist will be watching the polls in his bid for the US Senate.
Michelle Rehwinkel Vasilinda for your vote and article.A  Thank you DeVoe Moore for your article, "To rebuild, America must build something." Let's drill for Dakota Oil too!
Americans have the need for energy independence to secure the rights of life, liberty and the pursuit of happiness.Congress should end the tyrannical rule over our nation by hostile foreign nations by encouraging more production of American energy from all of America's bountiful resources. Seaward boundaries of StatesThe seaward boundary of each original coastal State is hereby approved and confirmed as a line three geographical miles distant from its coast line or, in the case of the Great Lakes, to the international boundary.
Any State admitted subsequent to the formation of the Union which has not already done so may extend its seaward boundaries to a line three geographical miles distant from its coast line, or to the international boundaries of the United States in the Great Lakes or any other body of water traversed by such boundaries. Any claim heretofore or hereafter asserted either by constitutional provision, statute, or otherwise, indicating the intent of a State so to extend its boundaries is hereby approved and confirmed, without prejudice to its claim, if any it has, that its boundaries extend beyond that line. The authorization contained in the preceding sentence of this section shall not be construed to be a limitation upon the authority conferred on the Secretary in other sections of thisAct. Payments made pursuant to such agreement, or pursuant to any stipulation between the United States and a State, shall be considered as compliance with section 6(a)(4) hereof [43 USC A§ 1335(a)(4)].
Upon the termination of such agreement or stipulation be reason of the final settlement or adjudication of such controversy, if the lands subject to any mineral lease are determined to be in whole or in part lands subject to the provisions of this Act, the lessee, if he has not already done so, shall comply with the requirementsof section 6(a) [43 USC A§ 1335(a)], and thereupon the provisions of section 6(b) [43 USC A§ 1335(b)] shall govern such lease. The notice concerning "Oil and Gas Operations in the Submerged Coastal Lands of the Gulf of Mexico" issued by the Secretary on December 11, 1950 (15 F. A§A§1301-1315 (2002).The Submerged Lands Act was enacted in response to litigation that effectively transferred ownership of the first three miles of a statea€™s coastal submerged lands to the federal government. California (1947), the United States successfully argued that the three nautical miles seaward of California belonged to the federal government, primarily finding that the federal governmenta€™s responsibility for the defense of the marginal seas and the conduction of foreign relations outweighed the interests of the individual states.A --more--In response, Congress adopted the SLA in 1953, granting title to the natural resources located within three miles of their coastline (three marine leagues for Texas and the Gulf coast of Florida). Texas) emphasizing that Congress could relinquish to the states the federal government's property rights over the submerged lands without interfering with U.S. Companies to drill and recover the oil in the Gulf of Mexico, in the 25 a€“ 125 mile range, for U.S. The second will require the state to permit and promote the building of at least one new refinery in the State of Florida.



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