Velocity Filter Research Articles

Alpha decays of light uranium isotopes

With the use of a velocity filter, the \ensuremath{\alpha}-particle decays of $^{224}\mathrm{U}$ and $^{225}\mathrm{U}$ were studied in $^{19}\mathrm{F}$ bombardments of $^{209}\mathrm{Bi}$. The data obtained for these two isotopes are compared with those of previous investigators, and the \ensuremath{\alpha}-decay rates of $^{222,224,226}\mathrm{U}$ are discussed within the context of partial \ensuremath{\alpha} half-lives for even-even nuclei with Z\ensuremath{\ge}84.

Accelerator Mass Spectrometry at the Lund Pelletron Accelerator

Three years ago, funds were raised to equip the 3 MV Pelletron accelerator at the Department of Physics, Lund University for accelerator mass spectroscopy (AMS). We have modified the accelerator for mass spectroscopy by relocating focusing devices on both the low- and high-energy side of the accelerator and installing a Wien velocity filter and detectors for measuring the particle energy (E) and energy loss (ΔE). We have been working exclusively with 14C during the initial period. About 40 samples of elemental carbon have been produced, using Fe or Co as catalyst, during the last two years. The 12C− current from these samples is about 3–5 μA using an ANIS sputtering source. We are now planning 14C analyses in the fields of archaeology, Quaternary geology and radioecology.

Model of electron and ion distributions in the plasma sheet boundary layer

Electron and ion velocity space distributions in the plasma sheet boundary layer have distinct features and exhibit a characteristic evolution with depth in the boundary layer. Near the lobeward edge of the layer, enhanced earthward and tailward directed electron flux is observed. Somewhat deeper in the boundary layer, earthward and tailward directed ion beams are observed. The electron and ion beams have low‐speed cutoffs, and the earthward directed beams are consistently observed at lower speeds than the simultaneously observed tailward directed beams. The ion distributions evolve from magnetic‐field‐aligned beams, to “kidney bean” shaped distributions, to isotropic shells with increasing equatorward penetration into the boundary layer. A two‐dimensional model based on quasi‐steady reconnection occurring in the distant magnetotail is able to reproduce all of these observed features in the electron and ion distribution functions. The essential features of the model are the finite time‐of‐flight effect (velocity filter effect), conservation of energy, and conservation of magnetic moment as the particles stream from the low magnetic field region near the central plasma sheet to the higher field region in the boundary layer. The model can be used to estimate the central plasma sheet density, temperature, and bulk flow speed as functions of position earthward of a reconnection site from observed plasma sheet boundary layer distributions. These plasma distributions obtained from the model may be useful in determining the stability of the boundary layer plasma to various electrostatic and electromagnetic modes.

‘Self‐consistent’ production of ion conics on return current region auroral field lines: A time‐dependent, semi‐kinetic model

We describe initial results from a time‐dependent, semi‐kinetic model of plasma outflow incorporating wave‐particle interactions along current‐carrying auroral field lines. Electrostatic waves are generated by the current driven ion cyclotron instability (CDICI), causing perpendicular velocity diffusion of ions plus electron heating via anomalous resistivity when and where the relative drift between electrons and ions exceeds certain critical velocities. Using the local bulk parameters we calculate these critical velocities, and so are able to self‐consistently switch on and off the heating of the various particle species. Due to the dependence of these critical velocities on the bulk parameters of the species the heating effects exhibit quite complex spatial and temporal variations. A wide range of ion distribution functions are observed in these simulations, including conics with energies of a few electron volts and ‘ring’ distributions. The rings are seen to be a natural result of transverse heating and velocity filter effects and do not require coherent acceleration processes. We also observe the formation of a density depletion in hydrogen and enhanced oxygen densities at high altitudes.

A compact velocity filter for evaporation residue measurements

A velocity filter with crossed electric and magnetic fields has been built and used to measure evaporation residue angular distributions for a series of reactions using 16O, 19F and 28Si projectiles. The filter is compact, ∼ 20 cm long, and uses permanent magnets. The spatial separation between the residues and the intense elastic scattering is such that the latter can be physically blocked without affecting the observation of residues; this allows measurements to be made at angles as close as 0.5° to the beam direction. The measured efficiency of the filter is very close to 100%.

A recoil separator for use in radioactive ion beam experiments

The effectiveness of a recoil separator for the measurement of radiative capture reactions at radioactive ion beam facilities has been examined. A system consisting of velocity, energy, and momentum filters was tested with the ^1H(^(12)C, ^(13)N)γ reaction at the E_(cm) = 422 keV resonance; excitation functions obtained with both the ^(13)N recoils and the capture gamma rays were compared. Projectile ^(12)C fluxes were measured directly with Faraday cups and indirectly with Rutherford backscattering into silicon detectors, while the ^(13)N recoil flux was measured directly with silicon detectors and indirectly by its residual radioactivity. A suppression of the ^(12)C beam particles by a factor of 10^(−10) was observed when the system was tuned for the recoil ^(13)N's. Possible improvements of the system are discussed.

Velocity filters for multiple interferences in two-dimensional geophysical data

A procedure for the velocity filtering of two-dimensional geophysical data is discussed. This procedure involves repeatedly applying multichannel filters to a small number of overlapping subarrays of the input data. The multichannel filters can be chosen so that the resulting velocity filter is capable of suppressing multiple interferences traveling at different velocities. The two-dimensional frequency-wavenumber expression of the velocity filter is derived. With this expression, it becomes possible to implement the filter using two-dimensional fast Fourier transform (FFT) techniques. Two examples which illustrate the performance of the derived filters are presented. >

Investigation of the velocity distribution of evaporation residues produced in20Ne+197Au reactions atE/A=5.7?11.4 MeV/u

The velocity distributions of evaporation — residues produced in complete fusion reactions of20Ne+197Au at bombarding energies ofE/A=5.7, 8.6 and 11.4MeV/u were measured with use of the velocity filter SHIP. It could be shown, that the production cross section for evaporation residues is influenced by emission of fast, light particles having an angular distribution, that is peaked in forward direction (preequilibrium particles). This contribution is, however too small to explain the enhancement of the evaporation residue production cross section by about two orders of magnitude compared to statistical model calculations. This latter effect is interpreted as mainly due to a delayed onset of compound fission.

Signatures of solar wind injection and transport in the dayside cusp: EXOS‐D observations

Structure of charged particle precipitation in the polar cusp is studied in comparison with the simultaneously observed electric field. The polar cusp is characterized by precipitation of intense low‐energy (∼100 eV) electrons and energetic (0.1∼a few keV/e) ions which often show an energy dispersion. The precipitation region is sometimes divided into two discrete zones which have different dispersion characteristics, one showing an ion energy dispersion in which the characteristic ion energy falls with increasing latitude, and the other showing either clear dispersion of different form or non‐dispersion. On the other hand, the electric field in the cusp reveals non‐uniform and duskward or dawnward convective flows with velocity shears and intense fluctuations. It is remarkable that, nevertheless, ion precipitation shows a clear energy dispersion. We discuss how various types of energy dispersion can be interpreted in terms of the modified velocity filter effect in spite of complexities in the electric field.

Aliasing in the tau-p transform and the removal of spatially aliased coherent noise

The tau-p transform is a discretized Radon transform. The choice of discretization parameters is a very important part of performing the transform. Insufficient sampling in the tau direction leads to aliasing problems equivalent to those encountered in any one‐dimensional time series. A simple graphical method illustrates that too coarse sampling in the p direction results in reconstructions containing data duplicated incorrectly at different spatial positions. The spacing of these duplications is dependent on the temporal frequency of the data. Insufficient spatial sampling of the original seismic data causes events to plot at multiple p values in tau-p domain, again dependent on temporal frequency. Therefore, to velocity filter spatially aliased noise efficiently, multiple p values must be filtered. The use of appropriate filters in the p-f domain can substantially improve the noise rejection capabilities of velocity filters on spatially aliased noise while having little effect on desired reflection signals.

Fusion cross sections for 46,50Ti+90Zr,93Nb and some systematics of heavy-ion fusion at barrier and subbarrier energies.

Measurements of the fusion cross sections in the barrier and subbarrier regions are presented for the four heavy-ion systems $^{46,50}\mathrm{Ti}$${+}^{90}$Zr${,}^{93}$Nb. The measured cross sections varied from 0.04 to 400 mb. The evaporation residues were detected by the use of a velocity filter. Procedures are described for extracting fusion cross sections from such measurements. The observed differences in the subbarrier fusion cross sections for the $^{90}\mathrm{Zr}$ and $^{93}\mathrm{Nb}$ nuclei, both of which have small collectivity, have led us to question the view that excursions of the strong nuclear force due to collective motions of the colliding nuclei are the primary enhancement mechanisms for the observed heavy-ion subbarrier fusion cross sections.A simple formula is presented, based on a flat distribution of barriers, which is applied to the near-barrier region (10--200 mb). This formula states that the quantity (\ensuremath{\sigma}E${)}^{1/2}$ vs E is linear in the near-barrier region with a zero intercept defining a threshold energy for fusion. These threshold values reflect variations in the binding energies of the valence neutrons. We also point out that the far-subbarrier region (10 mb) shows large variations in fusion cross sections and that these variations reflect differences in the collectivities of the colliding nuclei (especially noticeable differences are seen between permanently deformed nuclei and ``vibrational'' nuclei). These systematics of heavy-ion subbarier fusion suggest that neck formation is playing an important role. The barrier for neutron transfer vanishes at distances typically 1.5 fm beyond the typical barrier distance and this distance could vary with the binding energy of the valence neutrons. The presence of neutrons in the region between the nuclei could promote neck formation which provides a force strong enough to overcome the Coulomb force. The collective properties of the colliding nuclei are then interpreted as a modulation of the threshold for neck formation and thereby reflect the large observed differences in the far-subbarrier cross sections. Formulas are presented for ${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}$ which result from a flat distribution of barriers. These formulas predict a broad bell-shaped spin distribution and are compared to the measured distribution in the subbarrier region for the system $^{64}\mathrm{Ni}$${+}^{100}$Mo.

Trace Pair Filtering for Separation of Upgoing and Downgoing Waves in Vsp (Vertical Seismic Profile)

Most multichannel algorithms used for separation of upgoing and downgoing waves in VSPs assume depth-stationarity of the signal on all the traces used in the separation filter. However, if the depth-window of the filters becomes too large (100-200 m) signal stationarity cannot be assumed. On the other hand, stationarity is a physically reasonable assumption for two neighbouring probe locations (5-20 m). A comparison is made of three algorithms used for the separation of upgoing and downgoing waves, that require only two adjacent traces and their first arrival times. They are independent of in-depth trace-spacing, provided there are no geological discontinuities between adjacent traces. The first approach (near-theoretical solution) operates in the frequency domain. A system of two equations and two unknowns is solved for every frequency within the best coherencebandwidth. The second approach (simple solution) is a delay-and-sum, and subtraction filter based on the semblance of the signals. The third approach (Wiener solution) uses a Wiener filter to predict the strongest wave, which is generally the downgoing wave. The upgoing wave is then obtained by subtracting the estimated downgoing wave from the full wave record. A second application of the Wiener filter on the upgoing waves can enhance them. The near-theoretical solution and the simple solution are narrow-pass velocity filters and are applicable to zero-offset VSPs. The simple solution is suitable for noisy data. When the signal-to-noise ratio is high, the best results are obtained using the near-theoretical solution. The velocity-filter bandwidth can be increased in the Wiener solution, so that it can also be used in case of dipping reflectors or offset VSPs. The Wiener solution is suitable for noisy data, and its effectiveness can be increased by using a reference trace. The performance of these algorithms on synthetic and field data is evaluated in terms of signal-to-noise ratio, detection of upgoing waves and sensitivity to first arrival time estimates.

Two-phase velocity measurements in dense particle-laden jets

Mean velocities and velocity fluctuations are reported for both phases in two downward flowing, low-Reynolds-number, particle-laden jets. The Reynolds number based on the initial bulk air flow is 780. Initial solid-air volume flux ratios of 0.0027 and 0.0225 are studied for spherical particles having a number-average diameter of 163 μm. Both cases may be considerd as gravity driven flows, and in most regions of these jets there is a measurable slip velocity between phases. Gas-phase velocities are obtained using a combination of amplitude discrimination and velocity filtering. The velocity filtering is done on the computer as a postprocessing step. Comparison of the two jets shows that high particle loading decreases the slip velocity, decreases the velocity fluctuations of both phases, and decreases the spreading rate of the particles.

EFFICIENT MULTICHANNEL FILTERING OF SEISMIC DATA1

AbstractMultichannel filters are used to eliminate coherent noise from surface seismic data, for wavefield separation from VSP stacks, and for signal enhancement. Their success generally depends on the choice of the filter parameters and the domain of application. Multichannel filters can be applied to shots (monitors), common‐receiver traces, CDP traces and stacked sections. Cascaded applications in these domains are currently performed in the seismic industry for better noise suppression and for signal enhancement. One‐step shot‐domain filtering is adequate for some applications. However, in practice, cascaded applications in shot‐and common‐receiver domains usually give better results when the S/N ratio is low. Multichannel filtering after stacking (especially after repeated applications in shot and/or receiver domains) may create undesirable results such as artificial continuations, or smearing and smoothing of small features such as small throw faults and fine stratigraphic details. Consequently, multichannel filtering after stacking must be undertaken with the utmost care and occasionally only as a last resort.Multichannel filters with fan‐shaped responses (linear moveout filters) should be applied after NMO correction. These are the filters commonly used in the seismic industry where they have such names as velocity filters, moveout filters, f‐k filters and coherency filters. Filtering before NMO correction may result in break‐up and flattening especially of those shallow reflection events with relatively higher curvatures and diffractions. NMO correction is needed prior to wavefield separation from VSP stacks for the same practical reasons outlined above whenever source‐receiver offsets are involved.Creation of artificial lineup and smearing at the outputs of multichannel filters is presently the common practical concern. Optimum multichannel filters with well‐defined pass, reject and transition bands overcome the latter problems when applied before stacking and after NMO correction. The trace dimension of these filters must be kept small to avoid such lineups and the smoothing of small structures. Good results can be obtained with only five traces, but seven traces seems to be a better compromise both in surface and well seismic applications. The so‐called f‐k filtering and τ‐p domain filtering are no exceptions to the above practical considerations.Residual static computations after multichannel filtering also need special consideration. Since multichannel filtering improves spatial continuity, residual static algorithms using local correlation, i.e. nonsurface‐consistent algorithms, may be impractical especially after multichannel filtering.

Velocity distribution of micron-size particles in thin film laser ablation deposition (LAD) of metals and oxide superconductors

Experiments have been carried out to investigate the velocity distribution of micron-size particles (undesirable in thin-film deposition) produced in addition to atomic and ionic species, by laser ablation of metallic and oxide superconductor targets. Time resolved deposits were obtained using a rotating substrate device. Optical and electron microscopy were subsequently used to examine the deposited material and to count the particle density as a function of velocity. Particle velocities were found to be of the order of a few hundred meters per second, an order of magnitude smaller than those of atomic and ionic species. This shows that particle elimination in LAD of thin films is possible by means of a high speed velocity filter.

Simultaneous velocity filtering of hyperbolic reflections and balancing of offset‐dependent wavelets

Hyperbolic reflections and convolutional wavelets are fundamental models for seismic data processing. Each sample of a “stacked” zero‐offset section can parameterize an impulsive hyperbolic reflection in a midpoint gather. Convolutional wavelets can model source waveforms and near‐surface filtering at the shot and geophone positions. An optimized inversion of the combined modeling equations for hyperbolic traveltimes and convolutional wavelets makes explicit any interdependence and nonuniqueness in these two sets of parameters. I first estimate stacked traces that best model the recorded data and then find nonimpulsive wavelets to improve the fit with the data. These wavelets are used for a new estimate of the stacked traces, and so on. Estimated stacked traces model short average wavelets with a superposition of approximately parallel hyperbolas; estimated wavelets adjust the phases and amplitudes of inconsistent traces, including static shifts. Deconvolution of land data with estimated wavelets makes wavelets consistent over offset; remaining static shifts are midpoint‐consistent. This phase balancing improves the resolution of stacked data and of velocity analyses. If precise velocity functions are not known, then many stacked traces can be inverted simultaneously, each with a different velocity function. However, the increased number of overlain hyperbolas can more easily model the effects of inconsistent wavelets. As a compromise, I limit velocity functions to reasonable regions selected from a stacking velocity analysis—a few functions cover velocities of primary and multiple reflections. Multiple reflections are modeled separately and then subtracted from marine data. The model can be extended to include more complicated amplitude changes in reflectivity. Migrated reflectivity functions would add an extra constraint on the continuity of reflections over midpoint. Including the effect of dip moveout in the model would make stacking and migration velocities equivalent.

On-line measurements of target properties in experiments for the investigation of the heaviest known elements

The progress in investigations of the heaviest known elements in the recent years was only possible with an elaborate target technology. Large area thin and homogeneous targets of Pb and Bi, mounted on a rotating wheel to allow high projectile currents, were used in the experiments performed at the velocity filter SHIP at GSI Darmstadt. Various techniques for the on-line monitoring of the targets will be described and their importance for the experiments will be discussed. The experimental results from the investigations of the heaviest elements will be summarized briefly.

Application of synthetic-aperture processing to towed-array data

A synthetic-aperture processing technique has been applied to towed-array hydrophone data and has demonstrated that significant increase in signal gain, improved angular resolution, and improved peak-to-sidelobe level ratios can be achieved. The synthetic aperture was formed by the coherent sum of subaperture beams at successive time intervals. When the signal spatial coherence was greater than the synthetic-aperture length and the signal stationary over the synthetic-aperture processing time, 85% of the achievable coherent processing gain was realized. This gain was described by (x−1)10 log(NTF), with x the coherent gain factor and NTF the number of sequential time samples. The coherent gain factor was found at 175 Hz to be 1.7±0.3 for synthetic-aperture lengths between 192 and 962 m, formed from subaperture lengths between 5 and 80 m over coherent processing times of less than 9 min. Beamwidths were found to be comparable to 58°, divided by the effective aperture length in wavelengths. Synthetic apertures, formed from subaperture lengths of 160 and 640 m, showed improvement when a tow-ship phase reference was employed and yielded x=1.4, compared to a maximum value of 2. The synthetic processing technique was also shown to function as a velocity filter sensitive to the relative velocity of the source and receiver. These results demonstrate that the formation of midfrequency synthetic-aperture arrays in the deep-ocean environment is feasible.

The influence of IMF-Bz and/or AE on the polar cusp: An overview of observations from the AUREOL-3 satellite

Experimental evidence for direct or indirect effects of the polarity and amplitude of the IMF- Bz component and the activity index AE on particle precipitation in the low-altitude cusp have been searched for, using the particle data from the AUREOL-3 satellite. The observations show that the precipitation of H + and He ++ ions often display energy-latitude dispersions over 1 to 2° in latitude which reflect the signature of the direct injection of plasma from the magnetosheath and the subsequent effect of the velocity filter. The ion dispersions explained by sunward convection over the polar cap are preferentially observed when Bz>0, and over the summer hemisphere. In this case their localization implies that the poleward boundary of the cusp is directly connected to a region of dayside merging between the IMF and the tail lobe field lines. We also present a statistical study of the position of the cusp boundaries and their relation to Bz and AE. Cross-correlation analysis shows that the best correlation with both AE and Bz is obtained when the ion precipitation in the cusp is organized with ion dispersions. The boundary directly linked to the plasma entry/reconnection region, equatorial for Bz < 0, polar for Bz > 0, is most strongly correlated with Bz shifted 30–40 minutes in time. Under the same conditions, the correlation with the instantaneous AE index is also very strong. Finally we suggest that the cusp position is only indirectly controlled by Bz, with a delay of about 40 minutes necessary for establishing a new convection system and stable ionospheric currents inside the magnetosphere.

On suboptimal detection of 3-dimensional moving targets

The author designates matched filters that are completely characterized by the velocity of the target as assumed velocity filters (AVFs). Like most matched filtering techniques where the signal parameters range in a continuum, the AVF must be implemented suboptimally by partitioning the velocity space. The author investigates the possibility of using a signal-to-noise ratio (SNR) loss factor as the criterion for the partition. The loss factor is a measurement of the loss of SNR at the output of the matched filter due to mismatch of filter parameters. In the scenario of detecting a moving satellite from a ground-based sensor, because of the vast sky the sensor has to search, it is important to keep the number of filters minimal. The author shows that, with a fixed loss factor, the number of filters required for coverage increases linearly as the span of the two-dimensional velocity space increases quadratically. The rate of increase is further reduced when the loss factor is made proportional to expected target angular speed. >