Multichannel Spectra Research Articles

Practical issues concerning the family of multichannel MV spectra for recovery of point spectrum

The paper addresses some of the practical issues associated with the multichannel point spectrum identification scheme of Foias et al. (1990). These include a convergence test for detecting signal frequencies and identifying the associated spectral matrices. An in-depth example is presented. >

Portable Calibration Sources For Airborne Gamma-ray Spectrometers

Standard radioactive sources of known element abundances are essential for the calibration of airborne gamma-ray spectrometers. The conventional approach to calibration has been the determination of 3-channel stripping ratios and sensitivity constants from large airport calibration pads, and a well calibrated test strip, respectively. However, calibration pads are expensive to build and suitable test strips are difficult to find and expensive to calibrate. Also, both pads and test strips are subject to variations in radiation output due to variable moisture content and radon loss, and this can lead to considerable calibration errors. This paper describes an alternative approach to the calibration of airborne gamma-ray spectrometers. We have built a set of portable calibration sources, and have used these to simulate the response of an airborne spectrometer to infinite sources over a range of altitudes. The calibration has produced conventional 3-channel stripping ratios and sensitivity constants as a function of altitude, as well as good quality, pure multichannel spectra which are fundamental to any research into improved processing techniques. The sources are inexpensive, and since they are completely sealed, they do not suffer from variations in radiation output.

Multichannel Coherent Anti-Stokes Raman Spectroscopy in Liquids

A computer-controlled system has been developed for obtaining multichannel coherent anti-Stokes Raman (CARS) spectra of liquid samples. The technique utilizes a narrow (0.01 nm) pump laser beam and a partially broad-band (3–6 nm) probe laser beam. In addition to tuning the broad-band laser, a PDP 11/34 computer controls the crossing angle of the two beams, the collection angle of the multichannel output signal, and the tuning of a dispersive polychromator. Digital data are collected from a vidicon multichannel detector. CARS spectra of Rhodamine 6G (a highly fluorescent laser dye) and of perylene (a polyaromatic hydrocarbon of environmental interest) were obtained with the use of this system. The multichannel spectra are compared to those obtained from point-by-point scans.

Nondegenerate multichannel four-wave mixing spectra of nitric oxide: two-photon absorption resonance from low-lying excited states

We show how multichannel nondegenerate four-wave mixing spectroscopy can be applied for fast acquisition of moderate-resolution two-photon resonance spectra of gas-phase samples, specifically, nitric oxide. We present the first results reported for broadband spectra, which can be obtained with a single laser pulse from a number of low-lying vibroelectric states of this molecule, specifically, the A(2)Sigma(nu = 0), A(2)Sigma(3), D(2)Sigma(0), and C(2)II(0). The two-photon resonant susceptibility is normalized to the known nonresonant susceptibility of N(2), permitting comparison with previous data obtained by scanning techniques.

A lattice algorithm for factoring the spectrum of a moving average process

Triangular decomposition of the semi-infinite covariance matrix of a moving average process can be used as a spectral factorization technique. An efficient lattice algorithm is derived for performing the necessary computations. This technique is a special case of the fast Cholesky decomposition of stationary covariance matrices. The algorithm can be used to factor multichannel spectra to a desired degree of accuracy.

Relaxed and perturbed substrate conformations in enzyme active sites: evidence from multichannel resonance raman spectra.

A diode array based multichannel Raman spectrometer has made it possible to record complete, high quality, resonance Raman (RR) spectra of enzyme-substrate intermediates. The intermediates are dithioacylpapains in which the acyl group is either N-benzoylglycine or N-(beta-phenylpropionyl)glycine. RR data are reported for the unlabeled dithioacylpapains as well as for the intermediates labeled separately with ND, 15N, and 13C = S in the glycine residue. Comparison of the results for the dithioacylpapains with that of the corresponding labeled glycine ethyl dithioesters [Lee, H., Storer, A. C., & Carey, P. R. (1983) Biochemistry (preceding paper in this issue)] leads to the conclusion that for both substrates in the active site the dihedral angles in the glycine NH-C-C(= S) linkages assume an essentially relaxed type B conformation. Similarly, there is no evidence for distortion about the C(= O)-NH peptide bond which links the P1 and P2 sites on the substrate. However, for the N-benzoylglycine case there is evidence for some conformational distortion in the -S-C-C cysteine linkages. The present data favor a single homogeneous conformational population about the substrates' NH-C-C(= S) bonds in the native dithioacylpapains. However, below pH 3.0 the dithioacyl enzymes denature and the RR spectra of the 13C = S substituted species confirm that the conformational population reverts to the mixture of conformers A and B found for the corresponding ethyl dithioesters in solution.

Upper limit of peak area

We discuss here the determination of the upper limit of peak area in a multi-channel spectra, with a known significance level. This problem is especially important when the peak area is masked by the background statistical fluctuations. The problem is solved exactly and, thus, the results are valid in experiments with small numbers of events. The results are submitted to a Monte Carlo test and applied to the 92Nb beta decay.

Absorption spectra changes caused by a laser flash of red light in etiolated maize leaves

Absorbance changes of etiolated maize leaves induced by a laser flash were examined at 2–4°C, using a custom-designed multichannel transient spectra analyzer with an 80 μs time resolution. The absorbance maximum at 650 nm observed immediately before the flash of red light decreased immediately after the flash and a new peak appeared at 678 nm within 100 μs after the flash. In the dark, this peak shifted to 680 nm within 60 s and to 682–683 nm within 300 s after the flash. Difference spectra measured 50 μs after a flash showed an absorbance increase at 678 nm, irrespective of flash fluences in the range between 0.3 and 28 mJ/pulse. The results indicate that in etiolated maize leaves chlorophyllide having a peak at 678 nm is induced in a single photoreaction from photochlorophyllide within 100 μs after a flash.

High resolution multichannel CARS spectra in combustion research

High resolution multichannel CARS spectra in combustion research

Position-Sensitive Detection of Extreme Ultraviolet Photons

Recent advances in photon detection techniques for the extreme ultraviolet region have been highlighted by the introduction of multichannel electron multipliers and image-sensing detector arrays. Current and potential applications for such position-sensitive detectors in vacuum-ultraviolet and extreme ultraviolet spectroscopy are discussed in this paper. Examples are presented of multichannel spectra of several highly-ionized atoms obtained using fast-ion spectroscopy.

Discrimination of counting fluctuations in multichannel spectra

The squared amplitude of the fine structures in a multichannel spectrum is compared locally to the estimated level of the counting fluctuations. Structures which are more likely to be due to fluctuations are discriminated, either according to a straightforward level criterion, or to a probability criterion. Discrimination is performed by condensing several contiguous channels to a single, broader channel. An example of application is presented. The FORTRAN subroutine package performing these tasks is briefly described; it makes a systematic use of the fast Walsh transform.

Application of wide-band multichannel picosecond absorption spectroscopy to the study of primary steps in bacterial photosynthesis

A description is given of the apparatus which can be used to record the absorption spectra in the 450–950 nm range of a sample excited by single picosecond laser pulses. The kinetics of disappearance of the spectra may be followed for 0–2 nsec. Wide-band multichannel absorption spectra of preparations of reaction centers from various strains of Rhodopseudomonas sphaeroides, recorded after excitation of a suspension by a single 532 nm picosecond pulse are reported.

Automatic analysis of gamma-ray spectra from germanium detectors

A review is given of techniques for the complete automatic analysis of multichannel gamma-ray spectra from high-resolution germanium detectors, and a FORTRAN computer program is described. Previously reported fitting methods, peak shape functions, and peak search techniques are discussed. A semi-empirical peak shape is presented which gives a good fit to the variety of peak shapes and intensities which may be found in a spectrum. Equivalence is shown between a peak search using the covariance of a general peak shape with the data and a search using a general zero-area transform on the data. A comparison is made of the sensitivities of four different transforms when used in a peak search.

Real-time Reduction of Nuclear Physics Data

Small scientific computers have been widely used in research laboratories for on-line data acquisition to improve the efficiency of data collection and to perform sophisticated data manipulation before storage. An equally productive use of such computers is for the subsequent reduction of these data in real time, permitting interaction between investigatar and computer and providing immediate interpretation of masses of data. This report describes the philosophy and techniques developed for the SCANS (Stanford Computers for the Analysis of Nuclear Structure) system for the reduction in real time of multichannel pulse-height spectra, which comprise the bulk of data in Nuclear Physics. The programming language for users is FORTRAN, to provide flexibility and ease in introducing and modifying sophisticated concepts such as nonlinear least-squares fitting. Software interface to specific real-time hardware devices such as oscilloscope display and light pen is accomplished via library subroutines which perform a variety of general purpose services. Several different applications of this approach to various types of data spectra are discussed to illustrate the degree of mutual interaction achieved between investigator and computer and the resultant optimization of reduction techniques to suit particular types of data.

A Rigorous Least-Squares Analysis of Complex Gamma-Ray Spectra with Partial Compensation for Instrumental Instability

The least-squares method for analyzing multichannel gamma-ray spectra is one of great power and versatility, but its full potential is difficult to realize due to instrumental instabilities. For sources having low or moderate activities and for which the counting times range from a few minutes to several hours, the most important instabilities are long term drifts in zero-energy channel-intercept and gain. A computer routine based on the use of a special reference source is described, which can detect changes in zero-intercept of as little as 0.05 channels (for a 512 channel analyzer), and in gain of as little as 0.1%. By prior transformation of the data to compensate for these effects, and by the use of appropriate weighting factors in the least-squares analysis, the goodness of fit is dramatically improved. Often, the goodness of fit indicator chi-squared over degrees of freedom is reduced from a value in excess of three, to a value which does not differ significantly from unity. The weighting procedure described takes into account the statistical errors in both the complex spectrum which is to be analyzed, and in the reference spectra which are its components. These computer methods have been applied to the analysis of gamma-spectra from biological materials containing low activities of the natural series of radionuclides, and of atomic fallout products.