Use Of Monochromators Research Articles

Lossless Monochromator in an Ultrafast Electron Microscope Using Near-Field THz Radiation.

The ability to form monoenergetic electron beams is vital for high-resolution electron spectroscopy and imaging. Such capabilities are commonly achieved using an electron monochromator, which energy filters a dispersed electron beam, thus reducing the electron flux to yield down to meV energy resolution. This reduction in flux hinders the use of monochromators in many applications, such as ultrafast transmission electron microscopes (UTEMs). Here, we develop and demonstrate a mechanism for electron energy monochromation that does not reduce the flux-a lossless monochromator. The mechanism is based on the interaction of free-electron pulses with single-cycle THz near fields, created by nonlinear conversion of an optical laser pulse near the electron beam path inside a UTEM. Our experiment reduces the electron energy spread by a factor of up to 2.9 without compromising the beam flux. Moreover, as the electron-THz interaction takes place over an extended region of many tens of microns in free space, the realized technique is highly robust-granting uniform monochromation over a wide area, larger than the electron beam diameter. We further demonstrate the wide tunability of our method by monochromating the electron beam at multiple primary electron energies from 60 to 200keV, studying the effect of various electron and THz parameters on its performance. Our findings have direct applications in the fast-growing field of ultrafast electron microscopy, allowing time- and energy-resolved studies of exciton physics, phononic vibrational resonances, charge transport effects, and optical excitations in the mid IR to the far IR.

Population reference intervals for minimal erythemal doses in monochromator phototesting

Monochromator phototesting, to measure the minimal erythemal dose (MED), is useful in investigating patients with abnormal photosensitivity at different wavelengths. It relies on access to reliable, up-to-date data on the MED in normal individuals. The purpose of this study was to determine MED in normal subjects at different wavebands and compare these with historical controls. The study group consisted of 415 normal individuals (349 males) of skin types I-III living in Scotland. Age range was 18-83 years (median 31 years). Phototesting was performed using a monochromator at prescribed wavelengths from 295 to 430 nm. All calibrations were traceable to the National Physical Laboratory. Quality systems were maintained to ISO 9001 and ISO 17025 international standards and ultraviolet (UV) measurements accredited by the United Kingdom Accreditation Service (UKAS). The 95% reference interval (99% confidence interval for this) ranged from 6.8 to 27 mJ/cm(2) at 295 nm to >82,000 mJ/cm(2) at 430 nm. Results of the current investigation are broadly in agreement with values published 25 years ago by this centre. This validates the phototesting process based on the use of monochromators with attention to careful control of conditions during UV exposure and MED reading, supported by dosimetric calibration.

Evaluating experimental methods and techniques in X-ray diffraction using 280 000 data sets in the Powder Diffraction File

Release 2003 of the Powder Diffraction File (PDF) contains ∼280 000 unique entries organized in a series of tables in a relational database format. The PDF is available in two products, PDF-4/Full File and PDF-4/Organics, which allow users to access and query over 150 million filled entry fields. An editorial database is used to generate the commercial products that contain tables of experimental details and statistical evaluation criteria used by the editors to evaluate quality and determine quality marks for each entry in the PDF. This editorial database has nearly doubled the searchable entry fields. This database was mined to evaluate experimental methods in X-ray diffraction. Both experimental powder diffraction data and data calculated from predominantly single crystal X-ray structural analyses were assigned statistical quality criteria. For experimental powder data, the average delta two theta values for all d-spacings in the entry set were used. Calculated data were evaluated using R factor values as the primary quality criteria. A Quality Index, which measures the errors in refined unit cell parameters divided by the magnitude of the cell parameter, can be used to compare all types of data. Experimental variables were then analyzed versus these criteria. Variables include optic configuration, (i.e., Seeman–Bohlin, Guinier, Debye–Scherrer), use of internal and external standards, use of monochromators, wavelength divergence, wavelength selection, equipment radius, specimen transparency, and specimen absorption. This study significantly differs from prior round robin analyses in that the use of the database allows us to study very large population sets for every variable analyzed.

The performance of a separated crystal X-ray monochromator during wiggler heating — Preliminary results

We report on the performance of a double crystal monochromator exposed to large heat loads from a 6 pole wiggler magnet running at 15.6 kG peak field and a 5.3 GeV electron beam. This source of synchrotron radiation can produce between 1.8 and 4.5 kW of X-ray power depending on beam conditions. Various single crystals of silicon have been tested in a nondispersive double crystal arrangement over a wide energy range along with several different cooling schemes. We report preliminary measurements of the performance of silicon crystals cooled by ambient helium gas or water cooled through conductive greases or through a 10 μm thick layer of liquid indium and gallium. A side cooled, thick crystal geometry shows good stability for photon energies from 20 to 45 keV. A thin, bottom cooled geometry shows promise at 7.4 keV. We have observed monochromator performance (rocking curve width, shift in edge, X-ray flux output vs electron beam current, harmonic rejection capabilities, etc.) under a variety of heating conditions. We summarize our suggestions for making effective use of monochromators under severe operating conditions.

Progress and prospects at the National Synchrotron Light Source (NSLS)

This report discusses the NSLS project from inception until the present. The commencement and motivation for the program are considered from the viewpoint of the needs of various experiments and the concept of the “basic source” is introduced. The basic source has high brightness and tunability within the ranges 0–0.3 keV and 3–20 keV. The inclusion of tunability implies the use of monochromators and the role of these is discussed. The basic source is important because it satisfies a very large proportion of users. The strategy of NSLS in providing a good basic source and special devices (monochromators and wigglers) to allow coverage of the energy ranges outside those of the basic source is described. The response of NSLS to various other general and special needs of experimental users is described. Turning to the free electron laser project, a brief description of that device is given followed by a progress report of the overall NSLS construction program. The broad conclusion is that the NSLS facility will be completed within 2–3 months of its scheduled timescale and within its budget. This is considered to be a highly successful conclusion to the construction program.

A reflectance spectrophotometer-surface fluorometer suitable for monitoring changes in hemoprotein spectra and fluorescence of flavins and nicotinamide nucleotides in intact tissues

A spectrophotometer-fluorometer was constructed for simultaneous measurement of reflectance spectrum changes and surface fluorescence of intact tissues and organs. The apparatus employs monochromators for dual-wavelength spectrophotometry and optical filters for fluorometry. Three light beams are time-shared by a novel type of modulator to hit the same area of the tissue to be measured. The analog circuitry was controlled by a digital logic circuitry clocked by an optically derived frequency based on the angular velocity of the rotating light modulator and produced a maximum of three sets of differences in the parameters measured. The optical design has the advantage of allowing the use of monochromators rather than filters in a multichannel apparatus. The electronics accept a wide range of light modulation frequencies without difficulties in the phasing of the detector gates, so that signal sampling can be optimized for the recording of fast and slow phenomena. The apparatus was used successfully for the simultaneous monitoring of the mitochondrial redox state and tissue oxygenation in isolated perfused rat hearts.

Synchrotron radiation as a new tool within photon-beam technology

Synchrotron radiation is now playing an increasingly important role in recent developments of new light sources. Synchrotron light emitted from a relativistic electron beam has a radiation pattern which makes it a unique source. The advantages with this type of radiation can be summarized as (a) continuous spectrum extending from the ir to the x-ray region, (b) strongly polarized, (c) highly collimated, (d) pulsed structure allowing time-resolution spectroscopy, and (e) high intensity making feasible the use of monochromators with narrow band pass. The Stanford Synchrotron Radiation Project has been in operation since May 1974 as a U.S. National Facility for uv and x-ray research in many disciplines using the radiation from the storage ring SPEAR at the Stanford Linear Accelerator Center. The radiation spectrum is characterized by the critical energy which varies as E3 (E=electron-beam energy) and is 11 keV for E=4 GeV. Useful flux is available out for approximately five times the critical energy. Five monochromators now share the beam run and cover the entire wavelength region from the visible to the hard x-ray region. Several experimental groups are involved in research programs including (a) uv and x-ray photoelectron spectroscopy, (b) optical reflectance and transmission studies, (c) soft x-ray absorption and extended x-ray absorption fine structure (EXAFS) studies, (d) low-angle scattering studies of certain biological materials, and (e) x-ray diffraction studies of protein crystals and other materials.