Some New Aspects of Nuclear Instrumentation in Industrial Electronics

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<ul><li><p>1958 Anton and Youdin: New Aspects of Nuclear Instrumentation 51</p><p>increasingly important rule in this march towards theCONCLUSION automatic plant. There is no foreseeable technical limit toDuring the past ten years, the investment in process- the industrial measurement and control problems that can</p><p>control instruments in the chemical industry has risen be solved with the combination of electronics and otherfrom less than 2 per cent to 7 to 10 per cent of total plant techniques of applied physics. The primary factor regulat-investment. This trend will continue as long as plant ing the growth of electronics in industry is reliability. Withinvestment and operating costs can be reduced by using the advent of transistors, greatly improved reliabilitysmaller process equipment operating at higher speeds appears possible in the immediate future. The opportunitiesthat are in turn made possible by faster and more accurate and rewards to the electronic engineer in development,control. Electronic instruments, through their speed of design, and application within the chemical industry areresponse and almost unlimited sensitivity, are playing an surprisingly large and will grow at an accelerated rate.</p><p>Some New Aspects of Nuclear Instrumentationin Industrial Electronics*</p><p>N. ANTONt AND M. YOUDINt</p><p>T is often said that there is nothing new under the cost savings from radioisotopes now lie betweensun and, in a way, we are inclined to agree. However, $295,000,000 and $485,000,000 per year. Dr. Willard F.we must at the same time bear in mind that what is Libby, a member of the Atomic Energy Commission,</p><p>new to one person may be somewhat "old hat" to someone predicts that within a few years industrial savings fromelse. The term "new" applies equally to something that the use of radioisotopes will rise to $1,000,000,000 annually.has never been known until recently or to that which The application of isotopes to industrial use has alwaysonly recently has been made or manufactured. The been overshadowed by the explosive atom or the politicalengineer who has spent a few years of his life trying to dispute over how best to harness atomic power. By thebring to completion a prototype equipment whose principle end of last year, some 3663 organizations in this countrywas conceived in the laboratory some forgotten number had been licensed by the Government to possess and useof years back is perfectly aware of the several connotations radioisotopes. Of these, 1493 were industrial organizations.which are associated with the term "new." The petroleum industry to date has been the biggestWe deliberately have chosen to introduce this paper industrial benefactor of the radioisotope with savings</p><p>with these remarks because we are going to try to concern ranging from $140,000,000 to $215,000,000 annually.ourselves with nuclear instrumentation techniques in When one stops to think about it, it is very easy andindustry, which are new in the sense that they have stimulating to speculate about the applications of nuclearbecome feasible and possible as a result of recent circuit instrumentation to industrial electronics, because most ofor component development. They are not new from a these applications can be formulated in terms of threeconcept point of view. Everything which has the label elementary "black boxes." The first "black box" consists"nuclear" has received such a spectacular treatment of a radioactive source, which emits some form of wavefrom the press that it would be difficult indeed to find an or corpuscular radiation; the second "black box" is aapplication which some physicist has not already envi- detector for these radiations; and the third "black box"sioned and described in a speculative manner. * is a computer, which will resolve the data coming fromThe Atomic Energy Commission recently made a spot the detector. Using the three black boxes it is not difficult</p><p>check of the cost saving resulting from the use of radio- to illustrate how radioactive isotopes can be applied toisotopes in industry. This check indicated that industrial almost every phase of our daily existence.</p><p>The contributions which nuclear science will bring to* Presented at the Annual Conference on Electronics in Industry, industrial electronics wvill augment the prior art, rather</p><p>Chicago, Ill., April 9-10, 1957. than revolutionize it as in the field of power. The questiont President and Vice-President, respectively, Anton Electric</p><p>Labs., Inc., a subsidiary of United States Hoffman Machinery now very often asked by many electronics people is:Corp., Brooklyn, N.Y.. "Do we all have to become physicists?" The answer is,</p></li><li><p>52 IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS April</p><p>"No." The industrial applications engineer can learn all Neutrons, therefore, are very useful for gauging plasticthat he has to know to apply this new tool in the same laminates containing reinforcing glass fibres, because oflength of time that he requires to study the details of any the selective absorption characteristics of the boronservosystem or complex electronic control. Remember glasses. Neutrons are utilized also in the detection andthat the third black box; i.e., the computer, is not a new analysis of hydrogeneous materials.device. It usually consists of either a pulse counter or a The most common type of neutron source is the Ra-a-Below-current integrator, together with the necessary source. This type of source can be produced by slurryingamplifiers, power supplies, and controls which are utilized finely ground berylium powder in an aqueous solution ofin conventional monitoring and control equipment. Let radium bromide, evaporating to dryness, and compressingus now examine the first two of these black boxes in the residue into a pellet. The pellet is then sealed in agreater detail and stress those points which are of special gas-tight container to confine the radon decay products.importance to the industrial electronics engineer. Before a radioactive source can be chosen for a given</p><p>application, it is necessary to give consideration to theTHE FIRST BLACK BOX following basic parameters. The order in which they are</p><p>The first black box will usually consist of a radioactive presented here is not necessarily the order in which they aresource which will be capable of emitting alpha, beta, and best considered for any given application, because of theirgamma radiation either singly or in combination. For mutual interdependence.some gauging applications, which will be discussed laterin this paper, a neutron-emitting radioactive source may Strengthbe necessary. The nature of each of these radiations can The optimum strength of source which should be usedbe described briefly as follows. for any given installation depends primarily on the requiredAlpha particles, which are positively charged helium accuracy of the result, the half-life of the source, the</p><p>nuclei, have ranges up to several centimeters in air. A energy of the radiation source, the geometry of the setup,common pure alpha emitter is polonium (also called the efficiency of the detector, and the type of instrumenta-radium F). Alpha particles are heavy and, when shot at tion which will be utilized.solid matter, have practically no penetrating ability. Each nuclear disintegration of the source is a completelyThey are used chiefly where surface phenomena, such as random and independent process. Such a random processstatic electricity, are involved. will obey the laws of statistics (in this case the Poisson</p><p>Beta particles, which are electrons, may travel through distribution), which predict that even though there is aseveral meters of air. Common beta emitters are carbon 14, definite average rate of disintegration the number actuallystrontium 90, radium D + E, and technetium 99. These counted in a given time will show deviations from thisare all pure beta emitters which have appreciable half- average. As a result of this randomness alone, the mostlives. Beta particles can pass through as much as 4 inch probable error in any given reading will be equal toof plastic, 6 inch of aluminum, or 3 inch of steel and 0.67 X the square root of the total number of particlestherefore are useful for gauging operations of light ma- counted. Of course, to this must be added the effects ofterials. dead time in the case of counter tube, the resolving timeGamma rays (and X rays) are electromagnetic in nature of the computer, and the effect of the decay in activity of</p><p>and are energetic enough to pass through many inches of the source itself, which is discussed later.steel-hence their usefulness for gauging heavy materials The energy of the radiation source determines ultimatelyor thick sections of light ones. Some of the gamma emitters the ability of the particles or waves to penetrate matter.which are most commonly used for industrial applications Unfortunately, most sources are not "monochromatic."are radium, cobalt 60, and cesium 134. When gamma rays For example, beta particles given off from radioactivepass through matter, a portion of the rays are scattered nuclei show a continuous energy distribution from sub-backward, making possible a thickness gauge which stantially zero to a very definite maximum. Thus, only aneeds access to only one side of the material to be gauged, specific fraction of the available beta particles are of useas compared with a standard absorption type of thickness to us in any given application. The geometry of thegauge, which requires the source to be on one side and system (often given as a percentage) is the fraction of thethe detector to be on the opposite side of the material total solid angle about the source which is subtended byto be gauged. the sensitive volume or window of the detector. TheThe neutron is an elementary uncharged particle only efficiency of the detector itself, which is also stated in a</p><p>slightly heavier than the proton. Its lack of charge gives it percentage, is the ratio of the number of rays or particlesa very small interaction with electrons. It differs in this detected to the number incident.way from charged particles and from gamma photons.Interaction with nuclei is the only important flux-reducing Constancymechanism for neutrons passing through matter. Except If the application requires a quantitative result, thewhere the neutron-nuclear interaction is particularly source should, in general, havre as long a half-life as possible.great, matter is relatively transparent to neutrons. Half-life (sometimes called the period) of a source is the</p></li><li><p>195-8 Anton and Youdin: New Aspects of Nuclear Instrumentation 53</p><p>length of time in which one-half of the atoms in the source economic solution of the given problem. For example,will disintegrate. Of the common gamma sources, radium in the case of a liquid-level indicator or an oil-flow monitor,has the longest half-life ... 1620 years. Cobalt 60 has a the detector need only indicate the presence of radiation.half-life of only 5.3 years but is considerably less expensive In a thickness gauge, however, the presence of radiationthan radium. However, the initial cost of the source is not enough, since the quantity of radiation is a measureshould not be the only determining factor in making a of the parameter which is being monitored; namely,choice. The longer lasting radium may often obviate the thickness.need for providing an external calibration system and Detectors may take the form of a Geiger counter tube,will certainly eliminate the continuous need to compute an ionization chamber, a scintillation crystal-photomulti-the remaining radioactive output. The expense of replacing plier combination-and others too numerous to mention.cobalt 60 after it has decayed below the useful level must Great advances have been made of late in the developmentalso be considered. and production of rugged, dependable, and reproducibleCost detectors. Counter tubes for alpha, beta, and gammaradiations which can operate at temperatures up toThis item has already been considered under "Con- 1750C and which can withstand the most severe environ-</p><p>stancy." The important point to be emphasized is that the mental conditions and mechanical abuse now are readilytotal cost of the source "black box" should take into available. These tubes are electrically stable, have longaccount not only the initial cost of the radioactive source, shelf and active life, and are as interchangeable electricallybut also the additional costs which will be incurred in the as any good receiving and power tubes. Since the counterproduction and maintenance of the equipment as a result tube produces a number of pulses which is proportionalof the choice of a given radioactive source material. to the number of events which are impinging on it, it is</p><p>Health Hazard generally utilized with some form of ratemeter or pulsecounter. The scintillator also yields a pulse-type output</p><p>Certain sources are more hazardous to have around and is utilized instead of a counter tube because of itsthan others from a contamination point of view. For greater efficiency for gamma radiation.example, plated polonium sources for alpha radiation have It is very dangerous to treat each class of detector byalways been a source of concern because of the tendency generalization, as is often done. The applications engineerfor the plating to peel off. Radium sources which have is cautioned not to succomb to such categorical ratingbeen sealed without proper drying have been known to but to investigate the merit of each detector in terms ofexplode and spew out their contents. Cobalt corrodes the application. A typical example is in order. A con-unless it is plated and thus may flake. This is especially ventional Geiger counter tube is approximately onetrue for large slugs of cobalt which are loaded under water. per cent efficient for gamma radiation. By suitable process-New methods of producing radioactive sources have ing of the cathode, this efficiency can be increased by a</p><p>resulted in the development of foils which consist of a factor of two. A crystal photomultiplier combination,sandwich of gold sheet on each side of a radioactive source with proper choice of crystal, can yield a gamma detectionmatrix. This technique lends itself equally to radium, efficiency of approximately 40-50 per cent. Yet, becausestrontium, etc., and is useful for alpha, beta, and gamma of their relatively small size, ruggedness, ability to operatesources. The leakage which results when this foil is cut is at high temperature, and the simplicity of the associatednegligible and can be sealed-off easily if necessary. electronics, Geiger counters are nevertheless utilized in</p><p>It should be borne in mind that the source "black box" bundles of several hundred per probe for certain deep oilmay often be required to withstand more severe environ- well logging operations instead of the photomultiplier,mental requirements than are demanded of the detector which is larger in diameter, considerably more fragile,a...</p></li></ul>


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