Table of Contents
- Radium Measurements
- Approximate Method for Solids
- Gamma-Ray Measurement of Radium
- Radium Determination by Emanation Method
- Procedure when Radium is Soluble or in Solution
- Treatment for Solution Containing Barium in Excess over Radium
- Treatment for Solutions Containing Little or no Barium
- Treatment for Solution without Barium & Excess Barium Precipitant
- Procedure for Liquids Containing Excess of Sulphate or Carbonate
- Fusion Method for Radium Determination
- Direct Fusion Method
- Boiling off Emanation from Solutions
- Methods of Radium Determination Applicable to Various Substances
- Carnotite Residues and Tailings
- Nitric Acid Filtrate from Carnotite Ore
- Barium (Radium) Sulphates or Sulphides
- Sulphate or Carbonate Filtrate
- Barium (Radium) Chloride or Bromide Liquors or Crystals
- Construction and Use of Interchangeable Electroscope
- Details of Construction
- Use of Electroscope in Emanation Method
- Calibration of Electroscope
- Accessories for Electroscope
One of the most essential factors in the successful production and concentration of radium consists in following the material being concentrated, by means of careful quantitative determinations, through all the various operations from the original ore to the final product. This involves the radioactive analysis of a large number of products differing widely in chemical and physical properties and also varying in radium content through more than a billion fold.
As is well known, elements possessing radioactivity can be detected and even quantitatively measured in quantities far below the limits of any methods based on other properties. This fortunate property more than any other makes possible the concentration of an element from an ore containing about 1 part in 200 millions, by weight, to a product of any desired purity, up to 100 per cent, with a total loss not exceeding 15 per cent.
In the course of the work described in this bulletin, the principles of existing methods of radioactive measurement have been employed, but both instruments and methods have been modified to meet the practical requirements of plant control. One of the first principles recognized is that there is no universal method of radium determination at present and that each product requires study and individual treatment suited to its own peculiar chemical and physical characteristics. Nevertheless effort has been made as far as possible to unify and standardize the methods employed. Simplicity of procedure as far as consistent with accuracy has been the object sought, and the methods herein described are the result of large numbers of determinations made during more than a year of plant operation.
Three general methods of radium measurement have been employed: An alpha-ray method suitable for solids of low radium content, where no great degree of accuracy is desired; a gamma-ray method suitable for solids with a comparatively high radium content, where accuracy is desired; and an emanation method, suitable to any substance from which the radium emanation can be quantitatively liberated, and in the use of which an accuracy of 1 to 2 per cent is obtainable.
Approximate Method for Solids
A simple method of obtaining an approximate idea of the radium content of a solid susbtance not too high in activity consists in comparing its surface radiation with that of a standard substance.
The activity thus measured is essentially that of the alpha or non-penetrating rays. Although the result obtained may be accurate so far as the surface radiation is concerned, there is evidently no certainty that this indication closely approximates the radium content of the solid. For some ores, especially those ores of closely similar origin and general character, the comparison furnishes satisfactory results, but with other ores the deviation becomes great, on account of the differences in the nature of the gangue material, the radium distribution, the “emanating power,” and other variable factors.
Consequently it is evident that the method, although possessing the advantage of simplicity, has no great degree of accuracy.
Actually it has been used in the present work in two instances only—for control of the ore sorting at the mines and for the examination of the residue after nitric-acid extraction of the ore in the plant. As mentioned subsequently, the tailings, on account of their low radium and high silica content, present great difficulty of treatment by the emanation method. For this reason, and also because the low percentage of radium in the tailings allows a considerable relative deviation without invalidating the absolute results, it has been found in most cases convenient to use the solid method in testing the tailings for radium. The determinations have, however, been checked occasionally against those obtained by the emanation method.
The procedure consists simply in comparing the activity of the tailings, after they have been allowed to dry for a few days, with the activity of the original ore, both tailings and ore being spread over the same area on a plate introduced directly into the soild electroscope.
Gamma-Ray Measurement of Radium
The most accurate method for the determination of radium in salts containing an absolute quantity of not less than 0.1 milligram of radium, and having a concentration of at least 0.05 per cent is the gamma-ray method. This method involves comparing the rate of electroscopic discharge produced by the gamma radiation from a standard salt containing a known amount of radium with the gamma radiation of an unknown salt, the conditions of measurement being, of course, identical.
A standard tube should have its radium content determined by a careful comparison with the international standard or one of the subsidiary standards. The Federal Bureau of Standards possesses a
standard tube containing 20.28 milligrams of radium chloride, or 15.44 milligrams of radium element, by comparison with which the standard tube of the National Radium Institute measured 10.56 milligrams of radium element.
If the thickness of glass of the containing tube differs from that of the standard, a correction of about 1 per cent per 1 millimeter difference of thickness should be applied. In accurate work a correction for the length of tube should also be made, which will vary with the distance from the electroscope. This correction may be determined in various ways. It has been found convenient for the purposes of the Denver laboratory to approximate the correction by shifting the standard from its normal position until one end coincides with the position that the end of the unknown would take. The percentage decrease in activity owing to slightly increased distance of the ends from the instrument thus determined, may be applied as a positive correction. Or, if preferred, the correction may be determined once for all for tubes of various lengths containing the same quantity of radium at several distances from the instrument.
The precautions to be used in sealing the salts in glass tubes have already been described (pp. 82-83). Only after a given tube has been sealed for one month or more is its gamma radiation proportional to its radium content. In making measurements of tubes sealed for a shorter time, account must be taken of the percentage rate of accumulation of emanation (and consequently the gamma radiation) according to the expression It = 1 — e -λt, in which It is the percentage accumulated at any time, t, e is the base of the Naperian logarithms, λ is the decay constant of radium emanation = 0.0075 (hour)-1. The function e-λt for various time intervals is solved in the Kolowrat Table A.
If the gamma-ray measurement must be made within a few days after the tube has been sealed, the importance of a sharply defined starting point for the accumulation can be appreciated (see p. 82). If the assumed starting point is correct, two separate measurements at different time intervals will be in accord; if not, a new theoretical starting point must be determined as follows: The percentage increase in activity between the first and second measurements is determined. Clearly there is only one period during which this increase can take place in the given interval of time, which can be found by consulting the Kolowrat table. This figure establishes the the corrected zero point, from which the two time intervals are reckoned anew and applied to the radium measurement. A third measurement at yet another interval will serve to test the correctness of the new zero.
Measurements made in this way, after only two days of accumulation, have agreed within 1 per cent with the measurements made by the Bureau of Standards after a much longer lapse of time. The final accuracy of the gamma-ray method is considered to be 0.3 per cent.
Almost any form of electroscope can be employed for the gamma-ray measurement by placing between the electroscope and the source of radiation a lead screen one-eighth to one-fourth inch thick. The screen should preferably be nearer to the electroscope than to the radium tube. The interchangeable electroscope (see p. 99) with an ordinary emanation chamber has been used satisfactorily.
However, for the sake of convenience, a special gamma-ray instrument has been designed by the authors for these measurements (see Pl XIV, A). It consists of a cylindrical discharge chamber, also containing the leaf system, mounted on a wooden base about 3½ feet long. The telescope is fixed directly into the ionization chamber, which is provided with a small window in the opposite side for the transmission of light. The cylinder is of brass with an interior lead lining one-eighth of an inch thick, besides which an additional lead plate (one-fourth of an inch thick) is provided outside the chamber, which may be removed if desired. The holder for the radium tube is mounted on a track running the full length of the 3-foot graduated extension, and along which it may be fixed at any point by means of a set screw. Idle tube holder itself is a grooved metal rod, in which the tube is held in a horizontal position opposite the middle of the discharge chamber by a single narrow spring clamp. The groove is graduated from its center in both directions to facilitate centering the tube and making length corrections.
The electroscopic procedure, with respect to charging before use, and determination of the natural leak, is identical with that described in detail for the emanation method.
In general, the gamma-ray method is simple and satisfactory. Its accuracy may be judged from the data contained in the table on page 83, in which all the radium data reported were obtained through its use. As carnotite is free from thorium, no complications arise from the presence of mesothorium in the radium salts.
Radium Determination by Emanation Method
The determination of radium by the emanation method involves separating radium emanation (as a gas) from its parent radium, and measuring its quantity in a gas-tight electroscope previously standardized with a known amount of radium emanation. Analyzed pitchblende has been employed to furnish known quantities of emanation for purposes of standardization.
Three general methods of procedure may be used as follows:
(1) Release and measure the emanation from a substance in which it is in equilibrium with the radium content. This condition will
usually not be fulfilled unless the substance has been retained for a month or more in a closed container. In exceptional instances, However, the radium might be contained in a solid of such compact structure, or with a glazed surface, so that no spontaneous loss of emanation could take place. But even with a dense mineral like pitchblende, the leak of emanation, called “emanating power,” amounts at ordinary temperature to several per cent. This circumstance suggests the second procedure.
(2) Liberate and measure the emanation retained in the solid and apply as correction the “emanating power,” which must be determined separately and preferably after the solid has been in a closed retainer for one month.
Both of the above procedures, applicable in general to solids only, involve in practice long delays, and, although they are adapted to scientific investigation, they are not suited to radium measurement for purposes of plant control when quick results are desired. The following procedure is shorter and probably preferable when its use is possible.
(3) Remove the emanation completely from a sample of the substance to be analyzed for radium, close it at once in a gas-tight vessel, and allow the emanation to accumulate for a convenient period (one to ten days). Then remove it and measure it, making a time correction to find the maximum amount that would have been formed on the attainment of equilibrium.
For removal of emanation the radium must be contained either in solution or in a state of fusion.
Some substances, like carnotite, can be deemanated merely by heating to a high temperature, but carnotite can not be deemanated a second time in this way, as the first heating changes its physical state so that a second heating does not produce complete deemanation; hence heating can be used only in the month-accumulation method.
The removal of emanation from a solution may be accomplished by aspiration or preferably by boiling. Only the latter has been used by the authors and is described subsequently. Removal from a fusion may be accomplished by passing air or some other gas over the fusion, but it is preferable to bubble air through the fused mass or to produce in it an evolution of gas to insure the complete removal of radium emanation.
PART 4: RADIUM Extraction & Recovery
Procedure when Radium is Soluble or in Solution
The determination of radium in a solution can generally be conveniently carried out directly if a few precautions are carefully observed. It has been repeatedly noted that radium solutions show some tendency to lose radium from a solution on standing, a phenomenon that manifests itself in a decrease of the successive quantities of emanation that can be obtained from the solution. This tendency of radium solutions has resulted in the almost complete abandonment of the practice of preserving the solutions over long periods of time for standardization purposes, a practice that produced serious errors in some earlier work. The loss is to be attributed to precipitation or adsorption in a form that will not readily give up its emanation. In general, the presence of precipitates or suspensions in the solution should be avoided, though this source of error has frequently been exaggerated.
Loss of radium by precipitation through small amounts of sulphate, originating either in the glass walls of the containers or in the reagents, should be guarded against. For analytical purposes a suitable preventive measure is adding a large excess of “protective barium.” On account of the chemical similarity of barium and radium any precipitant that affects the radium will be removed by the large excess of barium, or, rather, radium and barium will be precipitated in the same proportion in which they occur in solution, and therefore only a minimal quantity of radium is removed. Furthermore, it is desirable that the solution contain nitric acid up to the solubility limit of barium nitrate, which is rather low in the presence of nitric acid of 50 per cent strength. The object of the nitric acid is to prevent the removal of radium as basic salt, which may be formed in neutral chloride or bromide solutions by the action of the alpha particles; hot concentrated nitric acid also has the well-known property of rendering barium (radium) sulphate more soluble (see p. 28).
The two essentials in handling radium solutions for analytical purposes consist, then, in maintaining an excess of barium and a fairly high concentration of nitric acid. Following are described treatments for three kinds of solutions, under one of which any given solution will be included. The reasons for the prescribed treatment will be clear from the foregoing discussion.
Treatment for Solution Containing Barium in Excess over Radium
For a solution containing barium in large excess over radium the treatment is as follows:
Place a suitable portion of the solution—such as will contain about 1 x 10-8 gram of radium—in a small Jena flask, and add to it a suitable quantity of 1:1 nitric acid. Add a few glass beads, and boil 5 to 10 minutes to remove all emanation. Allow slight cooling and then close the flask tightly with a one-hole rubber stopper provided with a glass tube drawn out above to a capillary tip. Seal the tip while some steam is still in the flask, in order to provide a partial vacuum, which should be maintained until the flask is again opened, thus affording a proof that no outward leak of gas has taken place. Note the exact time and date of sealing.
Treatment for Solutions Containing Little or no Barium
The treatment for a solution containing little or no barium is to add a suitable portion to 1:1 nitric acid which is saturated with barium nitrate, and to proceed as in the treatment described above.
Treatment for Solution without Barium & Excess Barium Precipitant
A solution that contains no barium but an excess of barium precipitant, such as sulphate or carbonate is usually a filtrate from a radium-barium precipitation, and requires especially careful treatment; otherwise highly erroneous results will be obtained. If such a solution were boiled off and sealed directly, the results would usually be low, as much as tenfold, and the solution would continue to decrease in emanating power the longer it remained standing. This behavior has led the authors to the belief that in the precipitation of radium in low concentration, or at any rate, its removal, whatever the process, is a progressive time reaction. On the other hand, unduly high results may be obtained, especially in using the correct procedure, if the sampling has been incorrect, for example, when too much of the fine (frequently invisible) precipitate relatively rich in radium has been obtained in a given fraction of the liquor.
Such a relatively rich fraction might easily be obtained in siphoning the liquid from above a sulphate precipitate, if the sample of liquid were taken near the end of the process. It has been found necessary to take samples at intervals during the entire siphoning process and to make a composite solution.
Procedure for Liquids Containing Excess of Sulphate or Carbonate
The detailed procedure for treating a liquid containing an excess of sulphate or carbonate, but no barium, is as follows: An excess of barium salt is added to the liquid, and the precipitate is filtered off. The filtrate containing an excess of barium is made acid with nitric acid to the point of precipitation, and is given the treatment outlined for a solution containing barium in large excess over radium. The precipitate, if barium sulphate, is fused with four to five times its weight of a fusion mixture (1:1 Na2CO3 and K2CO3), and is treated as described later for fusions. If the precipitate is barium carbonate, it is dissolved in nitric acid containing sufficient sulphuric acid to precipitate an amount of barium sulphate convenient for fusion, which is filtered off. The filtrate that is obtained may be combined with the original filtrate, and given the treatment as described for a solution containing barium in large excess over radium. All radium is then contained either in the filtrate with excess of barium or in the fused precipitate. Both of these fractions are closed simultaneously (within 15 minutes), so that the time of accumulation will be the same for both lots of emanation, which can be later introduced into one electroscope to determine the total radium.
Fusion Method for Radium Determination
If the radium is contained in a substance not readily soluble, such as a radium-barium sulphate, fuse a suitable quantity in a small platinum or porcelain boat with four to five times the weight of sodium or potassium carbonate, and note the exact time of cooling. Close this boat in a glass tube as shown in figure 6. Allow the emanation to accumulate two or more days. Connect the glass tube at one end to a highly exhausted electroscope and at the other end to a stop-cock. Break the glass tips inside the rubber connection, and exhaust the air from the glass tube into the electroscope several times, leaving enough vacuum in the electroscope chamber to accommodate the gas to be introduced later. Break the glass tube, remove the boat and its contents, wrap in a filter paper, and place in the neck of a flask as shown in figure 7; it is then ready for treatment with 1:1 nitric acid after the flask has been connected with the gas burette, as shown in figure 8. In this treatment the flask is tipped until the acid comes in contact with the carbonate fusion, thus beginning a gas evolution. The stopcock is immediately opened to the gas burette above and the boat and contents are then thoroughly wet with acid and jarred down from the neck of the flask into the body of the acid. As regards larger fusions, the evolution of carbon dioxide may become rapid and care should be taken in handling them, but in small fusions not exceeding 1 gram the boat may be shaken directly down into the acid, which should be heated to boiling as soon as the gas evolution begins to slacken. All of the carbon dioxide is, of course, absorbed by the sodium hydroxide solution in the gas burette. The boiling off from this point on is performed as with solutions discussed below.
For small fusions of substances running about 1 part of radium per million, such as crude radium-barium sulphate and high-grade pitchblende, of which a sample of 20 to 40 milligrams would be taken, the authors have employed small handmade boats, each one being folded from a strip of platinum foil 1/1000 inch thick, ¾ inch broad, and 1½ inches long, the finished boat being about 1 inch long and about ¼ inch in cross section. Such boats have been found convenient, can be made at small expense, and have a reasonably long life if the material to be fused does not contain lead. For substances poorer in radium, necessitating larger samples, the authors have employed porcelain boats, flasks holding as much as a liter having been used for the solution in some instances. The gas evolution is so vigorous that a gas burette with an enlarged bulb at the top should be used to furnish an increased amount of sodium hydroxide solution.
Direct Fusion Method
If desired, one can use a fusion both before and after the accumulation of emanation instead of dissolving the fusion in acid. If this is done, as soon as the initial fusion cools, the thin platinum boat is unfolded, and the fusion is put into a Jena glass tube of the form shown in figure 6, and held in place at both ends by small glass-wool plugs, which react with the carbonate in the second fusion, giving an evolution of carbon dioxide, which assists in removing the emanation. Usually in this method the gas is not passed into a gas burette at all, but the exhausted electroscope is attached and allowed to pull a current of air directly through the hard-glass tube while it is being strongly heated with a Meker burner until the vacuum is exhausted. However, this practice would not be allowable except with substances free from thorium.
The tube is heated until it collapses completely, but collapse should not occur until a large volume of air has passed over the fusion. A small drying bulb is placed in front of and another is placed behind the tube. Between the hard-glass tube and the drying bulb next to the electroscope a small tube containing potash solution is introduced to prevent carbon dioxide from passing into the electroscope.
No gas except air should ever be introduced into the electroscope with the emanation because the specific ionization of different gases differs from that of air, and the difference may cause a large error in the comparison.
Boiling off Emanation from Solutions
For boiling off emanation from solutions, the procedure prescribed is as follows:
Set up apparatus as shown in figure 8, wiring the rubber connections at a and b to insure tightness. Put into the leveling bulb c a stick of sodium hydroxide 2 to 3 inches long, or more if a large quantity of carbon dioxide is to be absorbed; make sure that stopcock d is closed and stopcock e open; pour boiling distilled water into the leveling bulb and allow the alkali to go into solution. If the boiling is too violent, put a one-hole stopper lightly into the mouth of the leveling bulb. After the alkali has gone into solution raise the leveling bulb until the gas burette is filled to the stopcock e. If the quantity of air to be boiled off is small, some air may at first be left in the gas burette. Close stopcock e and lower bulb c to its original position. Break the glass tip f inside the rubber tubing at a, and slowly open d to ascertain whether there is vacuum in the flask g. If so, close d again and begin to heat flask g over wire gauze. Test the vacuum every few seconds and as soon as the pressure is outward open d, and cause the flask to boil vigorously. Continue boiling until live steam has heated to boiling all the liquid in the gas burette h. This boiling should never be less than 5 minutes, and sometimes 10 to 15 minutes’ boiling is desirable.
After the glass tip f has been broken the liquid is likely to be carried upward by steam and in some instances has lodged in the stopcock d and caused serious explosions. As a precaution, a roll of thin platinum foil can be introduced into the glass tubing, as indicated at i, or the stopcock d may have a wide bore, which also obviates the danger mentioned.
After the boiling off has been completed, remove the flame, and as soon as the liquid begins to draw back through the stopcock d close the stopcock and remove the flask entirely. Evacuate the electroscope chamber to a suitable vacuum, either by means of an aspira¬tor or, more conveniently, a hand pump; connect the sulphuric-acid microdrying bulb l to the electroscope and to the gas burette, as indicated in figure 8. Be sure that stopcock j is closed; open first the cock of the electroscope for a moment and reclose it; then slowly open stopcock e to full width, and then gradually open the stopcock to the electroscope, allowing the gas to bubble through the microdrying bulb at a fairly rapid rate. When the liquid in the gas burette has risen exactly to the point k, close stopcock e and open stopcock j, allowing dry, dust-free air, which should preferably be taken from outside the laboratory, to sweep out the connections for a few minutes; then close the stopcock to the electroscope, reopen stopcock e, and allow the liquid in the gas burette to fall back 3 or 4 inches below the shoulder; close e, and then pour off all excess liquid out of c; close j and again open e to the electroscope, allowing air to bubble from the bottom of the gas burette h through its entire length to insure the removal of any emanation that may have remained dissolved in the liquid. This precaution is perhaps unnecessary, as the hot sodium hydroxide solution certainly does not take up much emanation, but nevertheless the precaution is in the direction of accuracy. Air should be allowed to bubble into the electroscope chamber until normal pressure has been almost restored.
The procedure just described for boiling off radium emanation is used for carbonate fusions introduced into acid, and also in handling any solids that are to be dissolved directly. For example, ground pitchblende and carnotite ore, which may be wrapped in filter paper in the way in which a fusion is wrapped (fig. 7) or sealed in small glass bulbs, which are opened by being crushed against the bottom of the flask by tapping on the glass stem projecting through a second hole in the rubber stopper. To economize time, two of the boiling operations may be carried out simultaneously by the same operator.
Methods of Radium Determination Applicable to Various Substances
The methods best adapted to determination of the radium in the various products that would usually present themselves for radium analysis in plant control are indicated below.
High-grade pitchblende is low in silica and readily soluble in hot 1:1 nitric acid. Hence, solution as well as fusion methods are applicable. As the radium-uranium ratio is normal, the radium content may also be calculated from a uranium analysis.
Carnotite is readily soluble in hot 1:1 nitric acid and one of the best methods for its radioactive analysis is solution from a sealed glass tube in which it has been inclosed for a month. Strong ignition (as with Meker burner) of the ore also removes the emanation initially, but second ignition does not do so; therefore, the ignition method is limited to carnotite that has been in a closed container for a month or more. The high silica content of carnotites imparts viscosity to the carbonate fusion, which renders removal of emanation by diffusion difficult. Higher temperature or direct bubbling of air through the fusion doubtless tends to obviate the difficulty, but the authors’ experience in general has not been favorable to the use of the fusion method for carnotite. The radium-uranium ratio is normal for large lots of well-sampled ore, and the radium may, hence, be calculated from the uranium content.
Carnotite Residues and Tailings
All the difficulties arising in the analytical treatment of carnotite are many fold multiplied in the treatment of extracted tailings, with the additional difficulty that as the radium content has already escaped solution, solution methods are not logically applicable. As already stated, the approximate alpha-ray method for solids gives results sufficiently accurate for most purposes (see p. —). In order to apply the emanation method it is necessary, first, to remove the silica from at least a 10-gram sample with hydrofluoric acid before proceeding with the fusion method.
Neither with carnotite nor tailings is it practicable to dissolve the carbonate fusion in acid, as the high silica content soon forms an impervious gel around the surface which prevents further attack.
Nitric Acid Filtrate from Carnotite Ore
Nitric acid filtrate from carnotite ore may be boiled and sealed directly with or without the addition of barium nitrate, as the original ore contains a large barium excess relative to the radium.
Barium (Radium) Sulphates or Sulphides
Barium (radium) sulphates or sulphides are fused with carbonate mixture in platinum or porcelain boats, described on page 95, sealed in glass tubes for accumulation, and either dropped into acid or fused directly.
Sulphate or Carbonate Filtrate
Sulphate or carbonate filtrate must be handled with all the precautions prescribed for liquids of this character on page 93.
Barium (Radium) Chloride or Bromide Liquors or Crystals
Barium (radium) chloride or bromide liquors or crystals may be treated according to the treatment prescribed for a solution containing barium in large excess over radium (p. 92), after suitable dilution for richer fractions from the crystallizing system. The dilution necessary becomes considerable, as much as one to a million in some instances. This dilution is carried out with pipettes and measuring flasks according to the usual methods of volumetric analysis, but involves unusual care in rinsing the vessels used for such large dilution.
The principles already described should suggest a suitable mode of procedure for any other substances that may present themselves for radioactive analysis in connection with the production of radium. But in dealing with any new substance, one should always try several methods for control before final selection.
Construction and Use of Interchangeable Electroscope
A modified form of the aluminum or gold-leaf electroscope of the C. T. R. Wilson type (Pl. XIV, B) has been found entirely satisfactory for all quantitative purposes. The chief modification consisted in making the upper part of the instrument carrying the telescope and leaf system separable from and interchangeable with a large number of gas-tight ionization chambers. This arrangement enables one to cany out a number of emanation determinations in a day without material additional expense, as the emanation chambers can be easily reproduced. The usual type of Wilson electroscope was further modified by building the telescope into an extension front from the leaf chamber, so that the relative positions of electroscope and leaf remain absolutely fixed. Too much stress can not be laid on the importance of this feature. During the year in which an instrument of this type has been in daily use the leaf has not been disturbed, nor has refocusing been necessary; an advantage of the utmost importance in maintaining the calibration constant. Effort has been made to design a simple instrument that could be constructed, with the exception of the telescope, by any mechanic.
Details of Construction
A description of the parts of the electroscope (Pl. XIV, B, and fig. 9) and of the emanation chamber follows. The emanation chamber is a gas-tight brass cylinder 4 inches high, and 3½ inches in diameter
with a volume of about ½ liter. The brass wall of the cylinder is about 1/16-inch thick, except the bottom plate, which is about 1/8 inch thick. The bottom plate projects ½ inch outside the cylinder and is screwed to a wooden base 6 by 6 inches. This projection also carries a binding post for grounding the instrument. The vertical cylinder projects into the base, into which it is carefully soldered so as to make a gas-tight joint.
On account of the difficulty of obtaining gas-tight brass stopcocks, glass ones have been used and are connected to the plain brass outlet tubes o (fig. 9) from the cylinder by means of heavy rubber tubing wired on and with the ends covered with piscein glue. These outlet tubes are ¼ inch in internal diameter and placed ½ inch from the top and bottom on opposite sides of the cylinder.
The electrode e is a brass cylinder ½ inch in diameter, projecting downward in the vertical axis of the cylinder to within ½ inch of the bottom and clearing the top by the same distance. The electrode is suspended by a small brass rod 1/8 inch in diameter, which screws into the top of the electrode, passes upward through the insulating material, d, and terminates in a small conical cap, c, serving to make metallic contact with the leaf system above.
The leaf system f is supported from the top of the cylinder where it is held in place by the sealing-wax insulation set in a milled-head cap, g, which screws into a vertical collar on the cylinder ¼ inch in height. The cap is hollowed out inside to contain the insulating, wax, from which a flat brass rod, f, ¼ inch broad, about 1/16 inch thick, and 2¼ inches long, projects downward, terminating below in a light brass spring, s, to make a slight contact with the conical top of the electrode of the ionization chamber. The spring should touch the electrode lightly or it will throw the leaf system out of position.
The aluminum leaf itself, about 2 inches in length, is attached to a small offset at the top of the brass rod by a moisture contact. The whole leaf system may be removed by unscrewing the cap without disturbing the rest of the instrument. If the cap does not screw down tightly into the desired position, a drop of solder may be placed across the joint between the screw head and the collar to prevent accidental displacement of the leaf.
The charging device k (fig. 9) consists of a brass rod threaded horizontally through a hard-rubber insulation n, in the side of the case. Inside the case the rod slopes upward at an angle of 45° and then
extends horizontally, so that contact can be made with the brass rod of the leaf system while being charged, or so that it may be turned and grounded against the wire grating of the outer case.
A collar ¾ inch long below the bottom of the cylinder makes a fairly snug contact fit over the collar on top of the discharge chamber and serves as support. A wooden frame is used to hold the upper half
of the instrument when it is detached from the base.
One of the best insulating materials for an instrument of this type is high-grade sealing wax, such as “bankers’ specie.” It has the advantage over sulphur of furnishing both gas-tight connection and good electrical insulation, and is, of course, much less expensive than amber. The sealing-wax insulation is bridged across the bottom of a cylindrical neck 3/8 inch in internal diameter and 1 inch high above
the top of the cylinder. It is desirable to have the minimum layer of wax that will give the necessary strength. A layer ¼ inch deep should be ample. The additional height of the neck merely furnishes a friction support for the upper part of the instrument. The electrode and insulation can be removed by unscrewing the whole collar, which is threaded into the upper brass plate of the cylinder 3/16 inch thick.
The collar screws down on a thin lead or rubber washer to insure gas tightness. The removal of the collar and electrode enables one to melt the wax into place with great ease and also to place the electrode in position without disturbing the soldered joint at the bottom of the emanation chamber.
A friction cap fitting snugly down over the neck and the projecting electrode stem protects the insulation from contamination when detached from the upper part of the instrument.
The leaf system and telescope are carried by the upper part of the instrument (Pl. XIV, B) and have the advantage over some instruments of being fixed in a perfectly rigid position with respect to each other.
The horizontal cylinder b (fig. 9) containing the leaf system is 1¼ inches deep and 3¼ inches in diameter; the ends are closed by sheet mica held in place by steel-wire rings which fit in grooves in the edge of the cylinder in the same way that an automobile tire is held in place. This arrangement has proved most convenient and far preferable to the use of screws. Inside the mica plates and in close contact with them, fine iron-wire gauze serves to conduct off any stray electrical charge. Circular openings in the gauze 1½ inches in diameter furnish a clear field of vision opposite the leaf system.
Opposite the aluminum leaf is a vertical brass plate (not shown in diagram) parallel to the leaf, which may be pushed in so that it almost touches the leaf, and thus protects it from mechanical disturbance during transportation. Whenever the instrument is in use this protector should be withdrawn against the outer case. It may be turned crosswise, if necessary, to remove it as far as possible from the leaf.
Instead of suppporting the telescope on an upright fixed to the same wooden base as the rest of the electroscope, it appeared preferable to fasten it firmly to the case carrying the leaf system. Three arms, such as the one shown in Plate XIV, B, carrying a solid brass vertical plate, are firmly screwed onto the case of the leaf system. The telescope fits tightly into a heavy horizontal collar which is screwed into the front plate which is thickened by two small plates to increase the depth of the screw thread. The telescope may be fitted firmly into place and soldered after focusing, or the collar may be split and carry a tightening screw for readjustment.
The telescope used is a Bausch & Lomb type with a 32-mm. objective and a No. 5 eyepiece, carrying a micrometer scale serving to measure the rate of discharge of the leaf. The eyepiece fits firmly into its case, so that its rotation is difficult after the micrometer scale has been set parallel to the leaf.
By means of a charging battery a charge can be maintained for some time on the instrument. Otherwise one can charge with amber or hard rubber, with an ordinary rubber comb for example.
The glass tube (see Pl XIV, B) fixed in the wooden base of the instrument about 1½ inches from the cylinder serves to hold a small sealed tube of radium salt used in controlling the calibration of the instrument. A suitable quantity of radium (about 1 milligram of element, in a sealed tube) furnishes a constant source of penetrating radiation, which may be conveniently employed to control the calibration by measuring the rate of discharge when this tube is placed in the glass tube fixed into the base, and by making comparison with the discharge obtained in the same way at the time of calibration with emanation. Such a measurement can be made in a few minutes and saves a great deal of time in avoiding the repetition of the calibration with emanation, if no marked change is found to have taken place in the rate of discharge.
Use of Electroscope in Emanation Method
The principle underlying the use of the emanation electroscope is that, in a given discharge chamber containing at two different times different quantities of radium emanation, the ionization and consequent rate of discharge will be proportional to the quantity of emanation present. If in one case this quantity is known, the unknown quantity can be determined by a direct comparison of the two rates of discharge. The principle seems simple and with the observation of a few essential precautions is really so in application.
Owing to the rapid decomposition of radium emanation into the series of elements, radium A, B, and C, each of which deposits as a solid “active layer” on the walls of the chamber, and contributes materially to the activity, it is necessary to wait three hours after the introduction of the emanation for the active deposit to have reached a maximum. This maximum is maintained with little change between the third and fourth hours, and hence the measurement of the rate of discharge may be made during this period.
It is also to be noted, however, that these active decomposition products of radium emanation carry a positive electrical charge when formed, and hence the position at which they are deposited in the chamber will be somewhat dependent on the electrical field to which they are exposed during deposition, and in turn the ionization and rate of discharge will be influenced. It would seem simple to allow the deposition of the active layer always to take pace with no electrical field, but the difficulty here is that as soon as the field is made for measuring the rate of discharge, a shift in the position of newly formed RaA takes place so rapidly that even in the few minutes necessary for measurement, the rate of discharge may change considerably either increasing or decreasing according to whether the new position of RaA is more or less favorable to ionization. To overcome this difficulty one practice is to keep the electroscope charged during the entire three hours of activation, but this procedure may be inconvenient if different instruments are being used on the same charging line, and it has been found more satisfactory in the laboratory to charge for 15 minutes before the measurement. Thus any shift of RaA can be practically completed, and any shift of RaC (through RaB, an a-rayless product) will not have proceeded to any considerable degree.
The measurement itself consists in determining with a stop watch the time elapsing during the passage of the leaf over a certain part of the scale, reading being made always between the same scale divisions. Two or three closely agreeing measurements suffice, but if the deviations are greater than 1 per cent, an average of 10 measurements is taken. The discharge is then reckoned in terms of scale divisions per second. From the result is subtracted the “natural leak” of the instrument, which is determined before the introduction of emanation. Even with a double contact of wax insulation, the natural leak maintains a low value of about .0003 to 0.005 divisions per second.
Another source of error, which seems especially pronounced in using sealing wax as insulation, is in the so-called “electrical soak” of the insulator, meaning that a certain time is necessary for the insulator to become fully charged. Unless sufficient time is allowed (hot less than 15 minutes) for this process to complete itself, the rate of discharge is erratic.
The procedure in the use of the electroscope is then as follows:
- Set up electroscope as shown in Plate XIV, B, and charge for 15 minutes from a battery with just sufficient, voltage to hold the leaf on the part of the scale to be used later.
- Observe the natural leak during 15 or more minutes.
- Carry out the calibration control by means of penetrating rays if radium is available for this purpose.
- Detach the top and evacuate the lower chamber to the desired vacuum.
- Pass the emanation-air mixture through a sulphuric acid drying tube into the evacuated chamber and restore normal pressure.
- Allow the emanation to stand in the discharge chamber for three hours.
- Charge for 15 minutes as before.
- Take three readings if agreements are good, or ten if deviations are greater than 1 per cent.
- Clean out the emanation chamber by drawing dry, dust-free air through it for some time (over night if convenient).
- Calculate the discharge and subtract the natural leak, expressing both in divisions per second.
- Compare the corrected discharge with the calibration of the instrument to determine the quantity of radium under measurement, taking time corrections into consideration.
Calibration of Electroscope
The calibration of the electroscope is carried out in exactly the same way as in ordinary measurement, except that a known quantity of emanation is introduced. This known quantity may be obtained in two ways, as follows:
- From a standard solution of some radium salt by passing air through it until its emanation is all transferred into the electroscope. This practice has two disadvantages, the necessity of having and taking care of such a standard solution, and the uncertainty attaching to the quantity of radium emanation removed from it, owing to the great tendency of radium in such small quantity to be precipitated out in part or to be occluded in the walls of the vessel during prolonged standing. In short, the practice of employing standard radium solutions, though rather general, is not to be recommended, and has been pronounced unsatisfactory at the Radium Institute in Vienna.
- The preferable practice is to use high-grade analyzed pitchblende, a suitable quantity being dissolved for each standardization, and the quantity of radium being calculated from the uranium analysis. The quantity of radium emanation obtained on dissolving the pitchblende will not correspond exactly to the radium content because a small fraction (2 to 5 per cent) of the gas diffuses from the ore; this fraction, termed the “emanating power,” must be determined by sealing a quantity of the ore in a tube for a month or more, and drawing off the emanation into an electroscope by the passage of air. The emanating power thus determined in the standard sample is used as a subtractive correction. Convenient quantities of radium emanation are those that will produce a discharge of the order of 1 to 2 scale divisions per second.
Given a standard pitchblende containing 60 per cent uranium metal and having an emanating power of 3 per cent. If the Ra/U ratio is 3.33 x 10-7, 1 milligram of pitchblende contains 2 x 10-10, grams of radium, but as only 97 per cent of this radium can give off emanation, 1 milligram of pitchblende on being dissolved will furnish emanation equivalent to 1.94 x 10-10 grams of radium. For the electroscope herein described use 20 to 40 milligrams of high-grade pitchblende.
Contamination of the discharge chamber may come about through the gradual accumulation of active deposit on the inner walls, which results in the increase of the natural leak of the instrument. For this reason more emanation than is necessary for a measurement is never introduced. The removal of emanation from the chamber should take place at once after the completion of the measurement, and to avoid the introduction of any emanation that may be present in the laboratory air, air is drawn from the outside, being passed through a train of cotton batting to remove dust and through sulphuric acid to remove moisture. Should the discharge chamber become contaminated in spite of all precautions, the chamber is opened and the walls thoroughly washed with dilute (1 to 3) nitric acid, followed by washing with distilled water and drying. This operation is repeated until the natural leak is sufficiently reduced. Contamination of the insulation itself usually necessitates its complete removal.
In measurements of great accuracy it is desirable to calibrate each discharge chamber separately, but by taking greater precaution in the construction, and position of the electrode each chamber can be made to have the same electrical capacity and hence one calibration will serve for all. It seems practicable to reproduce chambers that shall have the same calibration value within 2 per cent.
The convertible electroscope may also be used with other forms of discharge chamber than that used for emanation. For example, it may be used in water analysis by attaching it to a water chamber of the fontactometer type, or may be attached to an open a-ray chamber such as is used for the cursory examination of ores, or to any other desired form of discharge chamber.
Accessories for Electroscope
Accessories for the interchangeble electroscope are listed below.
A wrench for removing the collar carrying the insulation. This is used only in renewing or remelting the insulation. This is done only when the gas leak or the electrical leak has become unduly high. In dry climate, sealing wax dries out rather rapidly, and it is found necessary to remelt the sealing wax about once in one or two months. The operation is simple, but necessitates a new standardization of the chamber.
Two brass dies, used in remelting or replacing the insulation to hold the wax below the electrode.
One brass cap with hole, used in centering the electrode during remelting the insulation.
One hard-rubber cap with brass binding screw head, used in charging one chamber while the regular top is being used on another chamber.