Showing papers on "Time of flight published in 1991"

Digital time-of-flight measurement for ultrasonic sensors

Abstract: Ultrasonic sensor measurements are mostly based on the determination of the time of flight (TOF). The authors present the development of a digital algorithm for pulse-echo measurement applications, based on the use of a cross-correlation function to determine the TOF. Some experimental results are presented, and the possibility of realizing a low-cost real-time measurement system is considered. >

Large ion volume time‐of‐flight mass spectrometer with position‐ and velocity‐sensitive detection capabilities for cluster beams

Abstract: We have developed and tested a high‐performance time‐of‐flight mass spectrometer for clusters in a molecular beam. Several important improvements over conventional designs have been implemented that include a large ionization volume, a perfected ion steering system and a large area efficient ion detector. Cluster ions that are produced within the 200‐cm3 ion volume can be simultaneously detected over a size range of from 1 to about 10 000 amu with a resolution of about 1000. The mass spectrometer can also be operated in a position‐sensitive mode for cluster beam deflection measurements, and in a velocity‐sensitive mode to determine the cluster velocities. The performance of the spectrometer is demonstrated with Al and Fe clusters. We present mass spectra of cluster beams and also velocity, and magnetic and electric deflection measurements of collimated iron and aluminum cluster beams.

Time-of-flight analyzer and method

Abstract: A time-of-flight mass spectrometry and method of operating a TOF mass spectrometer are disclosed. The mass spectrometer includes one or more electrically charged accelerating plates for accelerating ions, a reflector, a first ion drift region upstream from the reflector, a second ion drift region downstream from the deflector, and an ion detector. The ion reflector includes a primary reflecting field for decelerating ions and reflecting low energy ions, and a second reflecting field for reflecting high energy ions and for establishing a substantially uniform ion flight time through the one or more accelerating fields and reflecting fields. According to the method of the present invention, the length of the ion drift regions may be adjusted such that ion travel time through these regions is equal to the ion travel time through the accelerating and reflecting fields. The second reflecting field downstream from the primary electrical field is adjusted such that high energy ions spend additional time in the second reflecting field compared to low energy ions to compensate for the shorter time high energy ions spend in the accelerating field and drift-free regions. The concepts of the present invention may be used with various techniques for producing ions, and ions may be formed in pulses by selectively activating a laser source, or formed in an ion beam pulsed toward the reflector by selectively activating the one or more of the accelerated fields.

Contraband detection system using direct imaging pulsed fast neutrons.

Abstract: A system and method for detecting contraband generates a highly collimated beam (13) of pulsed fast neutrons, having a pulse width on the order of a nanosecond, and an energy of 5 to 9 MeV. An object (14) under investigation is scanned with the collimated pulsed beam. The neutrons in the beam cause (n, η) reactions in a limited object volume or voxel, defined by the intersection of the collimated beam and the scanned object. The highly penetrating fast neutrons produce nuclear reactions with the atomic elements present within the object, causing gamma rays to be emitted. The gamma rays are detected in a gamma ray detector (20). The approximate time of flight of the neutron pulse is measured, allowing a determination to be made as to the particular voxel from which the gamma ray originated. The energy of the detected gamma rays identifies the particular elements from which the gamma rays originated. Thus, e.g., the carbon, nitrogen, oxygen, and chlorine content, which elements are commonly found in contraband, of a particular voxel can be determined directly and precisely. The voxel content information may be combined from a substantial sample of the voxels that make up the object to provide a direct indication of the elemental content of the object, and hence whether contraband is present or absent within the object.

A pulsed electron beam time of flight apparatus for measuring absolute electron impact ionization and dissociative ionization cross sections

Abstract: A new electron impact spectrometer has been constructed that utilizes a variable energy (4–500 eV) pulsed electron source with time‐of‐flight detection of electrons and ions. The apparatus can be used in a beam‐beam scattering mode or in a constant pressure mode suitable for absolute measurements. A newly designed data processing system is described that uses standard CAMAC modules (LeCroy model 4208 TDCs) and allows up to 32 separate detectors to be used simultaneously in a single hit mode or up to four separate detectors in a multihit mode with each detector capable of recording up to eight hits in the same experiment. The dead time between experiments is 9.2 μs which allows up to 100 000 experiments/s for experimental flight times not exceeding 1 μs at 1 ns timing resolution. Longer flight times, up to 8.3 ms in duration, can be accommodated but with reduced timing resolution. The determination of the partial ionization cross sections for Ar+, Ar2+, and Ar3+ from threshold up to 500 eV is used as an il...

Time-of-flight mass spectrometer

Abstract: A time-of-flight mass spectrometer, for example, a MALDI-TOF spectrometer, measures the characteristics of the charge to mass ratio of ionized particles by measuring the time taken for the particles to travel a pre-determined distance. The spectrometer comprises an accelerator (14) which accelerates the particles along at least two paths, which may be contained in a single beam of charged particles. Two detectors (26 and 30) mark the ends of the paths and are operable to detect the particles travelling therealong. The length of the path leading to the first detector (26) differs from that of the path leading to the second detector (30) to a sufficient extent to enable the difference in detection times of corresponding particles at the two detectors to be used to provide a measurement of said characteristics. Variations in initial velocities and/or ionization times of the particles will have similar effect on the outputs of both detectors so that one detector can, in effect, be used to calibrate or correct the output of the other detector.

Coincidence in time-of-flight aerosol spectrometers : phantom particle creation

Abstract: When using time-of-flight aerosol spectrometers, particle size measurement is based upon a particle's transit time between two laser beams. The particle's transit time is assumed to be the time difference between the two pulses of light that are produced as the particle passes through the two laser beams. Particle coincidence, which occurs when a second particle crosses the first laser beam before the first particle crosses the second laser beam, has a complex effect upon the measured size distribution. As a result of coincidence, time-of-flight aerosol spectrometers can replace real particles of one size with spurious, or phantom, particles of a different size in the measured distribution. When partial detection of a particle occurs, i.e., only one pulse from a particle is detected, another particle producing a pulse that occurs while the timer is open can cause the recording of a randomly sized phantom particle. The creation of these phantom particles, which we termed “open-timer” phantom particles, has...

An ion-storage time-of-flight mass spectrometer for analysis of electrospray ions

Abstract: A preliminary design and implementation of a novel approach to electrospray-mass spectrometry anr described. Based on a time-of flight mass analysis, the instrument provides several important advantages for on-line mass analysis: 1, simplicity, ease of use and low manufacturing cost; 2, rapid scan speed, yielding quasi-instantaneous full mass scans at repetition rates up to several kHz; 3, soft ionization and accurate mass determination of extremely large analyte molecules; 4, high sensitivity.

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

Abstract: A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH3 monomer from protonated clusters (NH3) n − 2NH 4 + . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.

Converting a reflectron time‐of‐flight mass spectrometer into a tandem instrument

Abstract: Adding a simple mass gate at the first time focus of a reflectron‐type time‐of‐flight mass spectrometer, one mass can be selected and its further decay or interaction studied. A new type of mass gate is described that does not have any grids. It is especially suitable for cluster studies. Three processes are identified that lead to an undesirable high background in a reflectron time‐of‐flight mass spectrometer.

Mass resolution of recoil fragment detector telescopes for 0.05–0.5 A MeV heavy recoiling fragments

Abstract: The factors governing the mass resolution for 0.05–0.5 A MeV recoil nuclei have been investigated for detector telescopes in which carbon-foil time zero detectors and ion-implanted silicon detectors are used to determine the time of flight and energy respectively. Experimentally determined second moments of the mass distribution have been compared with theoretical estimates based on literature data. The experimental mass resolution is in reasonably good absolute agreement with theoretical estimates. For low energy ( A MeV) particles the mass resolution is dominated by the contribution from the silicon detector and thus largely independent of timed flight length. In fact for detection of very low energy (0.1 A MeV) recoil nuclei timed flight lengths of less than 0.22 m are sufficient.

A quadratic potential time‐of‐flight mass spectrometer for use with slow positron ionization sources and other large‐volume ion sources

Abstract: A time‐of‐flight spectrometer has been specially designed for measuring the masses of ions produced by low‐energy positrons interacting with organic molecules in a Penning trap. To make the flight times insensitive to the starting positions of the ions in the trap, acceleration was done using a potential that varied as the square of the distance of the ion from the detector. The containment of the positrons in the Penning trap for extended time periods effected very long collision paths between the positrons and the molecules. The entire length of the Penning trap source could be sampled. These features produced high sensitivity, enabling the measurement of ionization processes with small cross sections. The spectrometer is useful for mass spectrometry of ions produced by processes other than positron interaction. Both large and small volume sources can be accommodated.

Analysis of microchannel plate response in relation to pulsed laser time-of-flight photoemission spectroscopy

Abstract: Design considerations, construction details, performance evaluation, and experimental tests of a time‐of‐flight photoelectron spectrometer utilizing a pulsed laser source are presented. The new spectrometer has been designed for heat transport and carrier dynamics studies of solid surfaces using femtosecond laser excitation. Special attention is directed to analysis of the dynamic range and linearity of the detector in order to obtain quantitative information from measured electron yields.

Laser velocimetry technique for measuring the three dimensional velocity components of a particle in a fluid flow

Abstract: A laser velocimetry technique for the measurement of the three dimensional components of velocity of a particle in a fluid flow has been described. It is based on the use of a measurement region consisting of two optical probe volumes. The two optical probe volumes are physically separate and each consists of an optical fringe pattern formed by the interference of two coherent beams of the same wavelength. Two of the components of velocity are determined experimentally from the doppler signals arising from the frequency change of light scattered from a particle suspended in the fluid flow as it passes through probe volumes. The third component of velocity is derived from measuring the time of flight of a particle in the fluid flow as it passes from the center of one probe volume to another probe volume.

A double/triple time‐of‐flight mass spectrometer for the study of photoprocesses in clusters, or how to produce cluster ions with different temperatures

Abstract: An apparatus is described for the measurement of photo processes in mass‐selected cluster ions. A gridless reflectron type of time‐of‐flight (TOF) mass spectrometer is used to select an ion of known mass. The selected ion is photoexcited with a pulsed dye laser. The charged fragments are mass separated in a second linear TOF. A new combination of reflectron and linear TOF is described, which allows one to measure the complete photofragment mass distribution over an arbitrary large mass range for each laser shot, which was not possible earlier. It is discussed how clusters with different temperatures can be prepared. The first measurement of a temperature‐dependent effect for mass‐selected free cluster ions is presented.

Analysis of initial energies of fragments produced by 65-keV proton-molecule collisions using a time-of-flight mass spectrometer

Abstract: A method is described to evaluate initial-energy distributions of fragments produced by proton-molecule collisions. The fragments are analysed by means of a time-of-flight mass spectrometer and their initial energies are evaluated from their time-of-flight spectrum. These energies are presented for various fragments of H2, N2, O2, CO2, CH3Cl, CH3Br, CH3I, C2H6 and C3H8 produced with 65-keV protons. The results indicate the generation of several energetic fragments of the halomethanes, ethane and propane via dissociation of a doubly charged molecule followed by evaporation of one or more hydrogen atoms.

Time‐of‐flight measurement of the speed of light using an acousto–optic modulator

Abstract: An experiment is described in which the speed of light is measured by time of flight. The equipment in this experiment includes a laser and an acousto–optic modulator. A laser beam was rapidly switched on and off by an acousto–optic modulator and the time that was taken for a light pulse to go a known distance was measured. The group velocity of light was found to an accuracy of about 1.5% using a calibrated oscilloscope as the time reference. Further, by making a least‐squares fit to a set of measurements for different travel times, an uncertainty of 0.5% was obtained. This experiment illustrates acousto–optic modulators, optics, time‐of‐flight instrumentation, and data analysis.

Study of the density of states of hydrogenated amorphous silicon from time-of-flight and modulated photocurrent experiments

Abstract: The densities of states (DOS) deduced from the modulated photocurrent (MPC) and the time of flight (TOF) techniques performed on two films of a-Si:H deposited in the same run are compared. For the MPC experiment the sample was built in the coplanar geometry whereas a Schottky diode was used for the TOF experiment. This comparison suggests that the MPC technique applied to a coplanar sample is sensitive to the electron trapping states. The influences on the MPC results of the flux and of the wavelength of the excitation light are studied. Probing the DOS closer to the illuminated interface by decreasing the wavelength, we observe a slight increase of the DOS, the increase on the a-Si:H/air side being more important than on the glass/a-Si:H side.

An electrostatic time‐of‐flight analyzer for simultaneous energy and mass determination of neutral particles

Abstract: A neutral particle energy analyzer has been developed to measure both the energy and the mass of the incoming particles with only one array of detectors. Energy analysis is performed by means of deflection in an electrostatic field. The mass of the particle is determined by measuring the flight time from a stripping foil to a channel electron multiplier in the focal plane of the analyzer. The application of a stripping foil instead of a gas cell increases the stripping efficiency and simplifies the vacuum system. Monte Carlo simulation studies and first calibration experiments show that the analyzer can be used even at energies down to 1 keV, as long as the stripping foil does not exceed a thickness of 2.0 μg/cm2. These simulations take into account the angular and energy straggling processes that take place in the foil. A triple‐coincidence technique can be applied to decrease the sensitivity of the analyzer to neutron and gamma background radiation considerably, as will be shown by calculations.

In-situ mass growth measurements of charged soot particles from low pressure flames

Abstract: In the present study, low-pressure premixed flat flames of acetylene and oxygen were burned under sooting conditions with C/O ratios ranging between 1.0 and 1.3. Soot particles were extracted from the flame zone at different heights by a molecular beam technique. The charged particles were deflected in an electric field of varying strength and tuned across two small “monochromator” slits in front of electrode plates connected to highly sensitive electrometric amplifiers. The measured current can be related to the particle kinetic energy distribution and is also dependent on the number of charges per particle. The particle velocity was simultaneously determined by a frequency-tuned repelling field used to measure the time of flight between two grids. The measurements of both the particle kinetic energy and the particle velocity allow the in-situ determination of the mass of charged particles of both polarities. Measurements of the most probable particle mass as a function of the height above the burner (HaB) confirm the experimentally observed particle surface growth law. In that case, the most probable diameter of singly charged particles were in the range 2 nm ≤dp ≤6 nm. Additional particles of both polarities with doubly charging were found to have a mean diameter of about 4 nm or more. The results also indicate the existence of large particles with multiple negative charges. At larger HaB an agglomeration mode of charged particles could directly be observed. Preliminary computer simulations confirm this experimental finding.

Energy distribution in aniline scattered from various low energy surfaces

Abstract: Aniline has been scattered from three organic substrates and a LiF single crystal surface. Applying multiphoton ionization and time of flight measurements the vibrational, rotational, and translational energy distributions were measured simultaneously. The NH2 ‘‘umbrella’’ like mode was found to be a very efficient accepting mode in the energy transfer process. The less rigid the surface, the greater the efficiency with which this mode is populated. The mode specificity does not exist for the rigid LiF surface. A model is presented, which explains all observations based on the collision time and kinematics.

An electron‐impact ionization time‐of‐flight mass spectrometer using a simple high‐voltage square pulse generator

Abstract: A versatile electron‐impact time‐of‐flight mass spectrometer (EI‐TOFMS) in a pulsed molecular beam has been constructed. The instrument contains a fast rise time, square pulse generator, biased at high voltage, which simplifies the implementation of EI‐TOFMS. Power field effect transistors are used as the active switching element, which afford compact size and simplified circuit construction. The effect of the shape of the repelling pulse on the flight time and space focusing condition is estimated by numerical calculation. Preliminary test results are provided for pulsed molecular beams of ammonia and hydrogen iodide.