INSTRUMENTATION Scintillation counting is an outstanding example of ancient practices, now revived and invigorated by new techniques
O O M K time ago, we mentioned research ^ being done on the production of high vacua by means of ionic pumping. Recent contribution describes a very simple ionic pump. A. M. Gurewitsch and W. F. Westendorp [Rev. Sci. Instr., 25, 389 (1954)] give a brief description of the construction and use of such devices. The ionic pump involves the production of ions which oscillate in an electric and magnetic field and are then driven into suitable absorbing surfaces. Some sputtering occurs during the process. Carbon and titanium electrodes have been used as the adsorbing surfaces. Rare gases are pumped just as readily as other gases. The magnetic field is conveniently provided by a small permanent magnet. The ionic pump has been used at pressures from 0.1 to 5 X 10~7 mm. The pumping speed is high and as much as 27 cc. of nitrogen at N . T . P . has been absorbed without exhibiting saturation effects. I t is curious how very much these modern techniques are related to old and wellknown phenomena. T h e earliest x-ray tubes were gas tubes-—that is, an optimum air pressure was maintained in order to permit them to function. On continued use, these tubes became progressively "harder," until so little gas remained that excessive voltages were required to operate the tube. Such tubes were usually provided with an auxiliary anticathode consisting of a porous plug or shred of asbestos. Brief connection with this auxiliary electrode would restore the tube to the proper degree of "softness," because small amounts of gas could be liberated by electron bombardment. For a long time, the technique of degassing by electron bombardment has been a standard procedure. Under the proper conditions, the reverse VOLUME
2 6, N O . 5, M A Y
1954
can be accomplished and this is ionic pumping. Measurement of Lotv Pressure The measurement of low pressures is a precise art these days, and many excellent devices are commercially available. Despite this, older techniques are being revived and improved. Many years ago, the late A. H. Pfund at Johns Hopkins extended the range and precision of the McLeod gage by sealing a fine wire in the tip of its capillary. By using the fine wire as a Pirani gage and measuring its change in resistance with a simple bridge circuit, extremely small changes in pressure could be determined [Pfund, A. H., Phys. Rev., IS, 78 (1921)]. As might be expected, the thermistor has been drafted for this chore. As described by R. S. Bradley [J. Sci. Instr., 31, 129 (1954)], this modification of a McLeod gage is useful over the range of 1 to 10~ 7 mm. of mercury pressure. Directions are given for sealing the leads of a thermistor into a bubble at the top of the capillary. An auxiliary thermistor is mounted near the measuring thermistor for temperature compensation. A simple bridge circuit is used for balancing. The necessary compensating resistance varies linearly with the square root of the gas pressure. As it is essentially a Pirani gage, the readings are a function of gas composition as well. One advantage pointed out by the author is the ease and reliability with which the system can be calibrated in absolute terms. Two perennial instrumental problems are once more treated, this time in a single instrument. D . J. R. Laurence [J. Sci. Instr., 31, 137 (1954)] describes a densitometer for scanning paper electrophoresis patterns. The photometric problem is concerned once more with a photomulti-
by Ralph H. Müller
plier circuit with logarithmic response. The paper strips are mounted in a rectangular cassette and moved uniformly through the photometer by synchronous motor drive. For automatic recording of the photometer output, a Tinsley amplifier is used ahead of the recorder. Chromatographic scanners have now been described of almost every conceivable type: for the visible, ultraviolet, infrared, for measurement by transmission and reflectance, and for those labeled with radioactive tracers. I t would seem t h a t some enterprising manufacturer might now take the final stop and offer a completely automatic scanner for the purpose which, by suitable accessories, might have some degree of versatility. Scintillation Counting Last month, we, mentioned some of the methods and techniques of nuclear physics which command the attention of the analyst. The varieties of instrumentation are great and impressive. In the early days, Crookes's spinthariscope furnished the principal means of detecting heavy nuclear particles; indeed, Lord Rutherford's associates and disciples used the scintillation phenomenon to establish many of our most fundamental concepts. The visual scintillation counter went into enforced, but temporary, retirement with the advent of pulse ionization chambers and electronic amplifiers, and the subsequent development of the Geiger-Miiller counter made beta and gamma counting equally reliable. The scintillator was revived when it was shown that photomultiplier tubes could 33 A
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detect light flashes too feeble for visual detection and produce large pulses of very short duration. These early results by Curran and Baker and independently by Kallmann opened up the whole field of modern scintillation counting. It was soon found that successful scintillation counting was by no means limited to the old stand-bys such as zinc sulfideor calcium tungstate, but t h a t many organic solids, liquids, and solutions afforded interesting possibilities. Some years ago, we mentioned in this column that nuclear physicists would do well to enlist the aid of good organic chemists in this search. This must have occurred to hundreds of others, because, at present, the subject is in a high state of development and lias enlisted the efforts of organic chemists, students of solid state physics, and, of course, the full resources of modern electronics. Not only has the choice of substance been reinvestigated, but it has been produced in clear, single crystals. A good scintillator produces large pulses of short duration, is trans parent to its own light pulses, and emits in a spectral region to which the photomultiplier is most sensitive. The entire subject is now so well advanced t h a t special monographs have appeared and have filled the need of classifying and ordering our present knowledge. J. B. Berks ("Scin tillation Counters," 148 + viii pages, New York, McGraw-Hill Book Co., Lon don, Pergamon Press, Ltd., 1953), in eight concise chapters discusses the principles of the scintillation counter and the latest de velopments in photomultiplier tubes, many of which are concerned directly with this problem. Other questions dealt with are pulse height and time resolution. Separate chapters cover inorganic phos phors, organic crystalline phosphors, and organic plastic and solution phosphors. A final chapter deals with applications, many of which are distinctive, such as the spec trometry of gamma and x-radiations. In the measurement of decay times, or of in tervals between ionizing events, the scin tillation counter offers an improvement of more than 1000-fold in resolving times of gas counters and will discriminate to the order of 10 ~9 second. Another timely and informative mono graph is by S. C. Curran of the University of Glasgow ("Luminescence and the Scin tillation Counter," 219 + χ pages, London, Butterworths Scientific Publications, New York, Academic Press, 1953). Of the twelve chapters, the first three are concerned with the general principles of the scintillation counter, radiations, and ANALYTICAL
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DATA
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INSTRUMENTATION their interaction with matter, and, as a preliminary t o the discussion on electron multipliers, a separate chapter on second ary emission phenomena. A chapter deals with t h e characteristics and per formance data of commercial tubes. Both this and Birk's monograph deal with the R.C.A. photomultipliers a n d t h e un focused Venetian blind type (E.M.I.) tubes produced by Electrical and Musical Industries, Ltd. The more recent Farnsworth box dynode type made by DuMont was not available a t t h e time. T h e theoretical and experimental aspects of the luminescence of solids and t h e fluores cence of organic solids and liquids are treated in two other chapters. There is also considerable information on the prepara tion of scintillators, their properties, and extensive applications of t h e method. A separate chapter deals with special cir cuitry suitable for scintillation counters. T h e fact t h a t acceptable scintillators can be made from plastic has facilitated t h e construction of very efficient counter as semblies, because one of the problems is the efficient collection of light and bringing it eventually to the photocathode. When solution scintillators are em ployed, a number of phenomena are in volved. Many of these may be ignored in the practical process of counting, but, of themselves, are interesting phenomena. I t has been known for many years that the intensity of fluorescence in a solution is not a linear function of the concentration of the fluorescing solute. Self-quenching effects set in a n d t h e fluorescence a p proaches a limiting value or even de creases, f Then, too, other substances which hâve strong quenching ability can be added to the solution. There is no completely satisfactory theory of fluorescence quenching. This is unfortunate, and, in analytical applications, one may be driven to t h a t supreme abomination, "the working curve." I t is altogether possible that the unsatisfactory state of our knowledge concerning fluorescence in solutions may be lessened b y scintillation studies. This is a reasonable hope, because modern techniques provide much detailed information about the light-emission process, its amplitude, decay rate, etc., whereas the older techniques provided relatively meager information. Much of this is of no particular interest to the designer of a good counter, b u t is important for the physical chemist or t h e analyst. Instrumentwise, scintillation counting is a n outstanding example of ancient practices revived and invigorated by new techniques. ANALYTICAL
CHEMISTRY
