Beam Chopper Research Articles

Two-dimensional potential profile and density measurements by use of an improved gold neutral beam probe

An improved gold neutral beam probe system has been developed in order to measure both the potential profile and the electron density profile simultaneously at the central cell of the tandem mirror GAMMA10. A gold film detector was also developed in order to measure the plasma density profile. The yield curves of secondary electrons were measured as a function of the incident angle and electron energy using both the gold neutral beam and ultraviolet ray injection. Time evolution of the plasma line density was measured by adjusting the incident angle and adding a beam chopping method. The result is in good agreement in comparison with the line density measured using a microwave interferometer. It was found that this film detector is very useful for simultaneous measurements of the electrostatic potential and plasma density.

Atomic fraction of hydrogen plasma in a negative hydrogen ion source

Hydrogen atoms effusing out from an extraction hole of a 6 cm diam and 6 cm long ion source was measured using a quadrupole mass spectrometer coupled to a neutral beam chopping system. The system background for atomic signal detection was negligible. As the discharge current was increased, the signal of atomic hydrogen increased almost linearly with the discharge current. Meanwhile the signal of molecular hydrogen slightly decreased with increasing discharge current. The signal to noise ratio of the present system is as low as 1/80 for typical operation of the ion source. The status of the system modifications to reduce residual hydrogen gas is reported.

Beam diagnostics using a chopped beam

A new type of beam chopper has been developed and installed at HIMAC (Heavy Ion Medical Accelerator in Chiba) in NIRS (National Institute of Radiological Sciences). The chopper has only 11.6 cm length and has been set between the ion sources and the RFQ. It is similar to a beam transformer, and modulates the beam kinetic energy by 10%. The beam is successfully chopped because the RFQ has a rather small energy acceptance. A chopped beam was injected into the synchrotron and a beam study has been performed. A very short bunch was formed and accelerated to the top energy. The behavior of the short bunch shows that a dipole oscillation was excited because of an unexpected frequency error of the RF system. It was caused by noise in the B-clock (Gauss Clock) circuit. The dipole oscillation caused by the frequency error was significantly reduced and the acceleration efficiency was improved by replacing the B-clock module.

A beam chopping experiment to help students develop research skills

We describe a project involving the beam chopping process that develops student skills in modeling, computation, experimentation, and data acquisition and can be completed by introductory physics students.

New perspectives for very short-lived neutron activation analysis

Recent attempts to improve the performance of very short-lived neutron activation analysis are reviewed. It is shown that the combination of an intense cold neutron beam from a research reactor with a beam chopper offers higher signal-to-background ratio, more accurate timing and much simpler sample handling than conventional cyclic activation analysis. Application of a digital spectrum analyzer in data list mode allows for easy determination of the half-life. Hence, time-resolved activation analysis utilizing energy and time information becomes practical.

A dispersion-free high-speed beam chopper for ultrafast-pulsed-laser applications

A compact high-speed beam chopper for ultrafast-pulsed-laser applications isdesigned and tested. To achieve high-speed operation, the chopper blade ispositioned on the focal plane of a pair of confocal parabolic mirrors. When thepulse train of an ultrafast Ti:sapphire laser is focused and re-collimated by thesemirrors, it becomes modulated by the rotating chopper blade. By using 360 slotson the chopper blade and a precision-balanced high-speed DC electric motor,optical modulation rates up to 100 kHz are achieved. The application of thissystem to noise reduction in ultrafast-laser experiments is investigatedthrough interferometric autocorrelation measurements, where it is found thatoptical modulation rates above 60 kHz provide the lowest noise level.

Non-destructive interrogation of uranium using PGAA

Prompt gamma neutron activation analysis is proposed as an instant, non-destructive method for the assay of uranium and also for the determination of 235U-enrichment. Measurements were performed in the thermal and the cold neutron beams at the Budapest Research Reactor. A beam chopper was used to collect the delayed decay gamma radiation from short-lived nuclides separately. Partial gamma ray production cross-sections have been determined for a set of capture prompt, fission prompt and decay gamma lines and compared to those from the literature. The proposed method is also compared to NAA and the passive gamma spectroscopy of natural uranium.

Cold neutron PGAA facility at Budapest

The new cold neutron prompt gamma activation analysis (PGAA) facility provides improved capabilities for routine prompt gamma analysis and for the investigation of radiative neutron capture. The versatile beam chopper makes possible time-of-flight measurements and the acquisition of prompt and decay spectra simultaneously. The chopped beam PGAA technique combines the advantages of the in-beam measurement and the greater simplicity of decay gamma spectra.

Development of nanoprobe TOF-RBS system for semiconductor process

Medium energy (80–400 keV) Be and Si ion beams have been used to enhance scattering cross section in Rutherford backscattering spectroscopy. A multiscaling time-of-flight (TOF) detecting system with a large area micro-channel plate detector was installed to obtain high counting rate and enhanced mass resolution. A pulse width of less than 1 ns for beam chopping of TOF and chopping plates installed in the middle of an ExB mass filter resulted in the improvement of the time resolution. Obtained time resolutions of 2.14 and 3.56 ns for 100 keV Be + and Si +, respectively, provided adequate mass resolutions to identify almost all contaminated heavy metals on Si.

A gold film detector for radial profile measurement of plasma density by using a gold neutral beam probe

A gold film detector was developed in order to measure the plasma density profile by use of the gold neutral beam probe. The detector is a kind of neutral beam detector composed of a rectangular entrance collimator, two sets of grid plates, a gold thin film, a quartz backing, and a microcarbon coated copper plate. The secondary electrons emitted from the gold film were utilized for the neutral beam detection. The yield curves of the secondary electrons were measured as a function of the incident angle and electron energy in both cases of the gold neutral probing beam and ultraviolet ray injection. Time evolution of the plasma line density was measured by adjusting the incident angle and adding a beam chopping method. The result was in good agreement with the line density measured by a microwave interferometer. It was found that this film detector was quite useful in the viewpoint of simultaneous measurements of the electrostatic potential and plasma density profiles.

A synchronized rotating crystal x-ray beam chopper

A new x-ray beam chopper has been designed and tested at the Advanced Photon Source. This beam chopper uses the diffractive properties of a rotating crystal to produce transmission window times as short as about 10 ns from a single diffracting face for 10 keV radiation. Precision control of the crystal rotation allows phase locking with the orbital frequency of the synchrotron storage ring; hence, the transmission of short x-ray pulses, corresponding to the 50 ps full width at half maximum charge bunch length, is possible with any fill pattern presently used at the Advanced Photon Source storage ring.

Time-of-flight measurements in atomic beam devices using adiabatic high frequency transitions and sextupole magnets

Atomic beam devices are frequently equipped with sextupole magnets to focus the beam or to act as spin filters in combination with RF-transitions for manipulating the hyperfine population within the atomic beam. A useful tool for the analysis of sextupole systems, the application of time-of-flight (TOF) measurements is presented. TOF measurements are enabled without mechanical beam chopper by utilizing adiabatic radio frequency transitions to select atoms within a certain time interval. This method is especially interesting for the use in atomic beam devices that are already equipped with RF-transitions and sextupole magnets and where space limitations or the required quality of the vacuum do not allow the installation of a mechanical chopper. The measurements presented here were performed with the atomic beam polarimeter of the HERMES polarized deuterium target and the results have been used to optimize the sextupole system of the polarimeter.

A synchronized rotating crystal beam chopper (abstract)

A rotating crystal beam chopper that can be synchronized to the synchrotron storage ring frequency has been successfully fabricated and tested. The crystal chopper uses the storage ring frequency master clock as input and operates at 1/500 times its input frequency. It is phase locked to the input frequency. At the Advanced Photon Source, using the ring frequency clock (∼271 kHz), the crystal beam chopper rotates at about 542 Hz, with less than 3 ns jitter in the period. With its Si(111) reflecting surfaces, this corresponds to a transmission window of about 10 ns for 10 keV x rays, assuming the upstream monochromator is also Si(111). By dividing its input frequency, it is possible to operate the crystal chopper at 271 and 135.5 Hz for wider transmission windows. To date, this is the only device that has demonstrated a ns range transmission window and operates phase locked to the synchrotron storage ring frequency.

Ion-source and low-energy beam-transport issues with the front-end systems for the spallation neutron source

The front-end systems (FES) of the spallation neutron source project are being built by Berkeley Lab and will deliver a pulsed 40 mA H− ion beam at 2.5 MeV energy to the subsequent drift-tube linac. The FES accelerator components comprise a rf driven, volume-production, cesium-enhanced, multicusp ion source; an electrostatic low-energy beam transport (LEBT) that includes provisions for transverse focusing, steering, and beam chopping; a radio-frequency quadrupole accelerator; and a medium-energy beam transport line. The challenges for ion source and LEBT design are the generation of a plasma suitable for creating the required high H− ion density, lifetime of the rf antenna at 6% duty factor, removal of the parasitic electron population from the extracted negative ions, and emittance conservation. The article discusses these issues in detail and highlights key experimental results obtained so far.

A simple positron lifetime spectrometer for a magnetically guided low-energy beam

We present a new, simple, and inexpensive positron lifetime spectrometer intended for the depth-resolved characterization of thin films and buried interfaces. The spectrometer operates on a conventional magnetically guided positron beam with energies ranging from 1 to ∼50 keV. Given is a detailed description of the performance of the apparatus, built on thorough experimental investigations and computer simulations. A timing resolution of 350±13 ps at full-width-at-half-maximum was obtained. The count rate for thin films (low positron energies) was of the order of 1000 s−1. A maximum peak-to-background ratio>105, aiding the measurements of long-living (10–100 ns) positronium in voids, was achieved by constant beam rate reduction and by beam chopping. Examples are presented for measured lifetimes in well-characterized systems.

Ion beam dose measurement in nuclear microprobe using a compact beam chopper

The accurate measurement of ion beam dose is a key point of quantitative analytical methods like PIGE, PIXE or RBS. To solve this problem for our Oxford-type scanning nuclear microprobe (SNM), a small-sized compact beam chopper has been developed. Ion beam dose is derived from the observed RBS yield from the surface of a wing after calibrating the yield versus collected charge. The wing is rotated by a stepper motor and is optionally covered with foils made of Al, Ni or Au. The beam chopper is capable to measure ion beam doses for different ions at a wide range of energy and current with a precision in the order of 1%.

A new scheme of direct beam chopping in a negative hydrogen ion source

Direct fast H− beam chopping in a surface-plasma-type negative hydrogen ion source has been studied at KEK. The H− beam is chopped at several MHz by modulating the converter bias voltage of the ion source. However, in this case, a slow rise time of the chopped H− beam is one of the most serious problems for synchrotron injection. In order to make the rise time faster, a negative voltage was applied to the anode electrode. In the new scheme, mesh inserts were installed inside the ion extraction hole of the anode. Applying a retarding voltage on the meshes, the fast beam chopping was tested and the results are reported and discussed in the article.

A new high-speed beam chopper for time-resolved X-ray studies

A new high-speed X-ray beam chopper, which can be phase locked to the temporal structure of the Advanced Photon Source (APS) storage ring, has been developed and tested. The open window time of the chopper is 2450 ns, which corresponds to approximately 67% of the revolution time of the APS storage ring. By phase locking the rotation of the beam chopper to the storage-ring orbital frequency, any portion of the storage-ring fill pattern can be positioned within the beam-chopper transmission-time window.

Single ion hitting to living samples

Single ion hit system for living biological samples has been designed. An external scanning microbeam system with about 1 μm spatial resolution, a fast beam chopping system with less than 5 ns and a thin plastic scintillator based transmission-type detection system are combined together at the JAERI light ion microbeam system. The first step to develop a single ion facility was completed in this work. The next steps to verify the reliability of the total system have been started.

Observation of Decay of 92Rh

By means of beam chopping and γ - γ concidence technique, the in-beam γ-rays and decay γ-rays of the 150 MeV 40Ca+58Ni reaction products have been measured. The four strongest transition γ-rays of 92Rh have been clearly observed. The decay γ-rays of 92Rh have been identified for the first time. The spin and parity of 6+ are tentatively assigned to the ground state of 92Rh. Part of the decay scheme of 92Rh is proposed.