INSTRUMENTATION by Ralph H. Müller
Graph Paper to Locate Center of X-Ray Peaks, Photoelectric Spectrometer, Miniature Photocells for Nonvisuai Readout of Meters, Infrared Polarizer, and Fluorescence Decay Instrument Described TNVESTIGATORS who have occasion to
analyze the shapes of recorded peaks are often driven to tedious plots and calculations to find the center of the peak and maximum peak height. We have been plagued with this chore in examining x-ray peaks consisting of unresolved Ka components. It is doubly exasperating if some mathematically gifted colleague announces loftily that the distribution has to be Gaussian and therefore quite simple. If you tell him that the observed distribution is nearly, but not exactly Gaussian, he merely feels sorry for you engaging in such a messy occupation as precise measurement. Some time ago we mentioned a paper by I. F. Boekelheide [Rev. Sci. Instr. 31, 9 (I960)], in which a Gaussian distribution or slight deviation therefrom can be readily analyzed on probability paper. More recently Peter Onno of the Radiation Laboratory of McGill University has described a new graph paper for the analysis of Gaussian distributions [Rev. Sci. Inst. 32, 1253 (1961)]. Although methods to accomplish the fitting have included the use of parabolic templates to fit the curves on a semilogarithmic plot as well as linear fittings of a normalized integral plot on probability paper as described above, Onno has devised a method combining the obvious advantages of a linear presentation with the improved definition and convenience of a differential plot. A graph paper for this purpose has been produced. By machine calculations, the proper ruling locations were computed from the equation y = ± c(log 100/R) 1 / 2 where y is the ordinate ruling location, c is a constant determining the scale, and R is the magnitude of the function, expressed as a percentage of the maximum value. The abscissa scale is linear, whereas that on the ordinate axis is symmetrical about the 100% line in order to facilitate
asymmetry analysis. In plotting a Gaussian function R = 100 exp [ - (x-io)V202] a straight line results with a slope = c/σ. Onno's method is illustrated by the analysis of the pulse height spectrum of the 662-kev. gamma-ray from Cs 137 . The Gaussian graph paper is obtainable from Canadian Charts and Supplies Ltd., Oakville, Ontario, as No. G8-10055. Photoelectric Spectrometer
A photodetecting instrument with flat wavelength response has been de scribed [McPherson, P. M., Sclar, N., Linden, B. P., Brower, W., Stair, A. T., Jr., J. Opt. Soc. Am. 51, No. 7 (1961)]. This is a photoelectric spectrometer weighing 2 pounds, which measures 1.5 by 1.5 inches. A different instrument is used to cover each of four spectral regions: 2000 to 2500, 2500 to 4000, 4000 to 5000, and 5000 to 10,000 A. The instrument is basically a prismtype spectrometer with a phototube detector which features a mask in the focal plane to provide sharp wavelength cutoffs, and to compensate for varia tions in electrical response with wave length. The dispersion unit is a stand ard spectrometer in design, consisting of a slit, collimating lens, prism, focus ing lens, and phototube detector. A unique singlepiece prism, suggested by Mertz, disperses the spectrum sym metrically about the optic axis. A second major modification contains a solid mask in the focal plane contain ing two 70 A. slits for isolating the two wavelengths at approximately 3050 and 3250 A. for monitoring ozone in the atmosphere. Another modification has been the design of a scanning slit in the focal plane providing a spectral scan once per second. These instru ments were designed primarily for measuring the radiation from nuclear
detonations from 2000 A. through the infrared. Miniature
Photocell Controls
For many years all sorts of schemes have been used for the nonvisuai read out of meters and galvanometers for signaling or for control purposes. Pho toelectric means for doing so usually involved drilling holes through the meter case. I t is not unexpected that the advent of extremely small photo cells such as the Si and CdS types have greatly simplified the problem. According to Product Engineering [32, No. 43, 60 (1961)], a photoelectric control unit that takes its instructions from the pointer of a meter has been designed by Sealy Engineering Co., Ltd., 33 Avery Hill Road, New Eltham, London, S.E. 9, England. The unit is a tiny black box, 2 5 / 3 2 X V 2 X V i e inches which can be fastened to the meter glass with adhesive. The box contains a lens, photocell, and light source. When the beam is eclipsed by the meter pointer, the photocurrent is cut off and an appropriate control signal results. The new device is readily adaptable to pen recorders, dial-type thermometers, and flying spot galvanometers. Several detector heads can be used for on-off controls. A suitable amplifier and power supply are available from the manufacturer. Infrared Polarizers
An ingenious design by Shin-ichi Takahashi [J. Opt. Soc. Am. 51, No. 4, 441 (1961)] provides infrared polar izers of the reflection type yielding 99.9% polarization compared with the theoretical value of 99.98%. A large number of glass plates coated with selenium are placed in parallel and tilted at the appropriate angle in a sort of Venetian-blind array. A double reflection occurs at each selenium sur face, but the emergent polarized beam is shifted laterally from the optic axis. VOL. 34, NO. 3, MARCH 1962 ·
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INSTRUMENTATION
New...
In another arrangement, a twin stack of plates is used, one set sloped upward and the following set sloped downward, both at the same angle. Four reflec tions now occur and there is no lateral shift in the emergent beam. In most cases no extremely high degree of polarization is required and a single reflection on the selenium will suffice. The degree of polarization in this case amounts to 95%, and about 50% of the incident radiation is avail able. These polarizers have the ad vantage of a large aperture and hence an intense beam can be obtained easily. It is necessary to collimate the inci dent beam with a lens or mirror, but these have no effect on the polarization if they are properly used.
from Scientific Industries
Fluorescence Decay Instrument
MULTIPURPOSE ROTATOR First in a series of Micro-Diffusion determinations shown here set-up for ammonia by micro-diffusion . . .* The Multi-Purpose rotator has been redesigned with an adjustable tilt face that can be used in a vertical or horizontal position as desired. _. To increase the efficiency of the multi-purpose rotators, a NEW FEATURE HAS BEEN ADDED: A single head that incorporates the features of our previous eleven heads into one unit supplied with large and small clips for interchangeability. Adaptable for bottles, flasks, test tubes, syringes, etc New! Easily attachable head converts rotator to a PIPETTE SHAKER for uniform dispersion of cells or thorough mixing in 5 to 10 minutes... with no pipette leakage! Suitable for testing AMMONIA · UREA · BROMIDE · ACETONE · ALCOHOL · LACTIC ACID • CYANIDE · AMIDES.
*Now available ammonia reagent kit.
.. .at your laboratory supply dealer, or write to
SCIENTIFIC INDUSTRIES, INC. S DEPT.AC-362 . 220-05 97th AVENUE · QUEENS VILLAGE, L I . · NEW YORK Circle No. 93 on Readers' Service Card 116 A
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ANALYTICAL CHEMISTRY
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An instrument has been designed for the measurement of fluorescence decay times of Ι Ο 7 to Ι Ο 9 second [Birks, J. B., Dyson, D. J., J. Sci. Instr. 38, 282 (1961)]. It uses a 10 megacycle per second hydrogen discharge lamp, a fast photomultiplier, variable delay line, and detector circuit for phase and modulation analysis A phase fluorometer is an instrument designed primarily for the measure ment of fluorescence decay times. If a specimen is excited to fluorescence by a periodically-modulated light source and the phase of the excited light emis sion is compared with that of the excit ing light, the decay time, tF, can be cal culated. The phase φ = tan - 1 (wtF) where ω is the angular frequency of the modulation. In another manner, φ and therefore tF can be obtained. In this method, if ms is the degree of mod ulation of the light source, then that of the excited emission is mF, given by mF — ms cos ψ; hence there are alter native methods of measuring φ from which tF can be obtained. The authors describe the various methods which have been used in the past. In their design a 10 Mc per second modulated hydrogen discharge lamp is used, the reference signal be ing derived directly from the lamp current. A fast photomultiplier, a commercial variable-delay line, and a phase-detector circuit based on that of Venetta are employed for the phase and modulation measurements. This instrument has been used to measure the transit time variations in several photomultipliers and for pre liminary measurements of the fluores cence decay time of quinine sulfate solutions. These are of the order of 20 nanoseconds (n = 10~9 second). Extended work on the photofluores cence decay times of organic compounds is contemplated by the authors.
