1231
V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8
RSass Spectra of Organic Compounds (Continued) Mass
No. 49.
Peak Ratio
Source, Baker. 88 87
Purity, C.P. 31. 1 1.66
io
0 95 29.4 2.46 59.7
62 610 60
2.43 100.0 6.14
59 56b
0.81 2.43
46 45 44 43 42 41
2.72 104. 19.9 680. 35.1 3.06
35 31 30
0.85 8.54 4.25
29 28 27 26 25 19 18 17 16 15 14 13 12 ~~
107. 38.3 49.6 12.7 0.81 2.41 3.29 0.46 1.31 54.6 13.3 2.37 0.36
No. 50. Carbon Bisulfide Source, Baker.
Peak Ratio
Purity, C.P. 2.51 1.97 100.0 29.7 1120.
66 64
1.00 11.4
59 46 45 44
2.08 4 . 65 1.94 106.
Source, Baker. 39 38.5 38 36 34 33 32
9.97 0.33 0.64
No. 51.
.
Thiophene
Source, Socony-Vacuum.
Source,
Mass
Socony-Vacuum.
Purity,
C.P.
39 38 37 36 35 34 33 32
19.5 4.96 3.56 2.28 0.19 0.27 0.36 1.60
28 27 26 25 24
0.58 0.76 2.07 0.68 0.17
Source, Sharples. Purity, redistilled commercial 123 10.9 122 1.29 121a 100.0 120 3.98 306. 119 118 4.03 117 314.
86
Purity,
85
84a 83 82 81 80
0.16 4.47 5.47 100.0 6.33 2.54 3.81 0.56
78 76 71
io
0.16 1.76 0.32 0.32
69 68 67 60
6.63 0.53 0.53 2.76
59 58 57 56 52 50
2.26 60.6 9.91 1.10 0.14 4.15
49 48 47 46 45 44 43 42.5 42 41.5 41 40.5 40
1.88 0.46 1.83 1.02 41.8 1.57 0.22 0.22 3.86 0.22 1.83 1.18 1.83
No. 52.
Pyridine
Source, Eastman. Purity, Eastman grade 79Q 100.0 78 12.7 77 0.51 1.35 ,n 1.70 .. 74 0.33
76 64 63 62 61
0.28 0.13 0.13 0.08
54 53 52 51 50
0.21 6.92 57.6 24.2 15.1
49 48 40
3.21 0.37 0.45
39.5 39 38.5 38 37 36 34.23
0.59 6.16 0.88 2.51 1.62 0.46 0.44
28 27 26 25 24
1.66 1.64 5.01 0.53 0.07
88
85 82
i.29 1.40 43.3 2.06 66.6
76 71
1.87 l.5i
69
60.5 59.5 58.5
2.60 3.59 10.6 10.8
49 48 47 44 43 42 41
16.4 0.75 50.8 2.79 0.44 1.12 1.93
38 37 36 35
4.37 9.84 11.9 28.8
28
21.8
88 .. X.5 -.
C.P.
ai
Peak Ratio
No. 53. Carbon TetrachlorideC
No. 51. (Contd.)
Purity, C.P. 6.72 1.93 76.2 0.53 3.94 0.75 77.9
28 27 25
Peak Ratio
Mass
No. 50. (Contd.)
74 73 71
80 79 780 77 76
Mass
Ethyl Acetate
Q
Reference peak.
b Metastable (1) C
N o ionization a t parent mass.
Radioactive Electronic Detector As Employed in Seederer-Kohlbusch Microbalance IRVING FEUER, Seederer-Kohlbusch, Inc., Englewood, N. J . Application of the radiation from radioactive substances has led to the construction of a new type of balance in the microchemical field. This balance possesses a sensitivity of at least 1 microgram per m m . deflection on a spotlight galvanometer, has a load capacity of 25 to 50 grams per pan, and exhibits little change of sensitivity with varying load.
T
HE general purpose of this paper is to describe a new system of neighing. Specifically it is a description of the principles involved in this system of weighing; the construction and a number of problems concerned with the construction of this microbalance; the operation of this balance; the characteristic developments which resulted because of the factors that determine the sensitivity, period, stability, and reproducibility of balances; the characteristics of the radioactive balance; the advantages and disadvantages of the radioactive microbalance; and the advantages of radioactive electronic instrumentation as applied to linear and angular displacements measurements which
in the microbalance field has resulted in a stable magnification (amplification) of minute differences hitherto not performed by this method. The radioactive balance was developed in the research laboratory of the Canadian Radium and Uranium Corporation, S e w York, S . Y. ( 4 ) , during 1946 and the early part of 1947. However it was not until the present date that a technically suitable radioactive microbalance was developed and the paper, published here, is the partial product of the research and development that have been accomplished in that direction by the author and J. E. Seederer.
ANALYTICAL CHEMISTRY
1232 Table I.
Group of Alpha-Emitters and Their Half-Life Values Element
Uranium I Radium Polonium Thorium Plutoniumaa8
Half-Life, Years 4 . 5 x 109 1622 140 days 1 . 3 7 X 10‘0 60
THE RADIOACTIVE BALAhCE
The term “radioactive balance” refers t o the complete system employed to translate n-eight into an easily measurable electric curient. I t is well known that alpha-particles emitted from a radioactive source produce ions in the atmosphere surrounding the source. The number of ions collected and measured by a detector depends upon the number of ions produced in a definite volume and time, which in turn depends upon the number of particles emitted, the range and energy of these particles, the geometrical distribution of the radioactive source in relation to the detecting device, the state of equilibrium of the source, the thickness of the protective absorbing material covering the source (stopping power of the absorber), the decay constant of the source, and numerous other factors ( 7 ) . I n general, the ionization current depends on the nature of the source and the type of detector system utilized. The ions produced may be collected by various means, such as alpha-ionization chambers, electrical alpha-counters, and scintillation counters. However, the method discussed here is a three-plate alpha-air-ionization chamber. The ionization current produced in the ionization chamber, being minute, is amplified in order to be practically utilized in the microbalance. A number of radioactive substances are found in nature. Some are alpha-emitters, some beta-emitters, and some both alpha- and beta-emitters, because of an accumulation of further beta-disintegrating by-products which are interspersed among the alpha-emitters and are separated with difficulty into their components. However, it is frequently desired to retain the subsequent interspersed alpha-emitters in a radioactive series in order to obtain a greater ionization current. The radium portion of the uranium-radium family is: Alpha-Emitters
Beta-Emitters
R% Rn RaA RaC RaC’ RaF
RaB RaC“ RaE
As a beta-particle produces about 0.01 the number of ions produced by an alpha-particle per centimeter, it would be advantageous to employ an alpha-radiating source of suitable intensity to produce the desired ionization current. A nuniber of alpha-emitters and their respective half-lives are listed in Table I. As it was necessary to have a constant (slo~v decay period), physically small, and yet a fairly 3 strong ionizing source, a salt of radium n-as \\ chosen as the alpha-emitter. The radium source (Cheniical Division, Canadian Radium &- Uranium Corporation) was prepared so as to be in equilibrium with its decay products, of Tvhich Rn, RaA, RaC, and RaC’ are the alpha-emitters in equilibrium with the parent radium. The half-life of radium being 1622 years, there is no need for the replacement of the radioactive source and no error in the calibration of the balance, as this is accomplished automatically by employing the procedure discussed below. The source is gold or rhodium plated to prevent loss of the radon gas or its subsequent decay products and also to prevent the physical renioval of any of the radioactive products through Figure 1.
handling. .4t the present time the radium source is the most practicable and commercially obtainable in quantity, although other alpha-emitters (Po, Pa, Pu) may be employed to perform in like fashion. A similar system could also be made to operate with other types of particle radiators (beta, positron, neutron) ; however, the detecting and amplifying system would likewise be more complex and expensive. This discussion is therefore limited to an alpha-radiating source. Figure 1 is a schematic representation of the principle of the radioactive balance, wherein a microscope mounting replaces one of the arms of the balance. A radioactive foil mounted on a brass strip is attached t o a movable body tube on a microscope, 1, the base of which is fixed to a solid table (sensitivity of the micrometer adjustment, 1 division equals 3 microns). The radioactive foil irradiates a threeplate ionization chamber, of which 3 is an upper metal plate at a negative voltage, 4 is a loxver metal plate placed at a positive voltage equal to the voltage of the upper plate, and 5 is a center metal plate which is equidistant from plates 3 and 4 and is connected to one of the terminals of the galvanometer, G. The other terminal of G is connected to the center tap of the two voltages a t point 6. The foil, 2, mounted on the microscope is analogous to the indicating arm of a balance. A and B in Part I of the circuit are terminals which may be attached to a compensating system &s depicted in Part I1 (by similarly marked terminals). The current in the three-plate chamber is observed without any radiation from a radioactive foil by lowering the microscope out of the alpha-range of the chamber. If there is any current in the galvanometer, this may be compensated by throwing Part I1 into the circuit of Part I by a switch and then adjusting the 10,000-ohm potentiometer until the null point reading of the galvanometer is obtained. The foil on the microscope is now raised until a null reading is again obtained. At this point, for all practical purposes, equal portions of the foil irradiate equal halves of the three-plate ionization chamber. Now the chamber may be raised or lowered a definite number of divisions and the galvanometer calibrated in microns or microscope divisions in terms of current. Employing a sensitive galvanometer, without any electronic amplification (sensitivity 0.03 microampere per division), the author was able to obtain a reading of 1 micron, equal to 3 divisions on the galvanometer, a factor gain of 9 as compared with the microscope alone. The radioactive source employed here was polonium (strength in the order of 5 millicuries) and the above operation was performed in a series of preliminary experiments in order to test the linearity of the displacement. The current in the ionization chamber and the magnitude of the variation of the current Tyith linear displacement of the source are dependent on the following factors: nature of the radioactive source, concentration of the source, degree of voltage saturation between the plates, distance between the plates, electric field between the plates, thickness of the center plate, distance between the radioactive source and the plates, dimensional size of the source, and geometrical disposition of the radioactive area (source). The above factors were correlated and then adapted to the variables of the mechanical portion of the balance -4number of factors which were pertinent to the balance were: the mechanical sensitivity of the balance, w-here the mechanical
67-K I
4 67v.
7
I
I I
PRRT T
‘
I. 5 v. PRRT r
Schematic Diagram of Principle in Radioactive Balance
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8
1233 compensation voltages. Two 67-volt batteries provided the voltage for the ionization chambers. The meter was either a sensitive microammeter (0.04 microampere per division) or a spotlight galvanometer (0.03 microampere per division) and the resistor in the grid circuit was a T'ictoreen Hi-Meg resistor. All the other parts were conventional radio parts, tested for their insulation properties and found suitable. Figure 3 shows the circuit diagram of the amplifier of the balance depicted in Figure 2.
MlCR 06RAMS
+
R-2 Figure 2.
v
R-3
Radioactive Balance
I lhff I
r
1.5~; Figure 3.
45x
l.6~
1.5~:
Circuit for Radioactive Balance
sensitivity is defined as the deflection in millimeters per niilligram of weight a t the indicator point; the range of weight, rrhich could be translated linearly radioactively, mechanically, and electrically with good stabilit? ; the period of the balance (magnetically damped) ; the aim length; the restoring weight and its relationship to the tilt of the balance; the bending of the beam and compression of the knife-edges; the notching of the rider system and the errors involved therein; the pan and beam arrestment system and the errors involved therein; and temperature and humidity effects. I n the radioactive detector the author possessed an efficient method for translating mechanical compressions, expansions, distortions, and displacements in any direction into a visually observable electric current. As a result, he was able to determine may sources of error in the balance and to correct them where possible.
con STRUCTION
O F RADIOACTIVE MICROBALANCE
I n order to produce a practical industrial microbalance it was necessary to amplify these minute radioactive ionization currents, as these currents vary from 2 X 10-8 to 1 X 10-12 ampere in intensity. iZ conventional electrometer tube circuit waq employed utilizing T'ictoreen VX-41 electrometer tubes ( 5 ) . Figure 2 depict. a front view of the entire balance. The power was supplied by means of batteries, the G type for the filament, and the A type for the accelerator grid, plate, and
The balance is essentially an ordinary balance in principle. P is three-plate ionization chamber mounted on a fine worm gear or rack-and-pinion verticallv adjusted base, B, the vertical adjustment of which is controlled by the wheel, T , to better than 0.5-micron linear displacement. This movable stage, B , which is similar to the mechaniB cal stage of a microscope, has a left to right distance-adjusting screw, L, in order to set the chambers a predetermined distance from the radioactive foil, RA, which is in turn mounted on the left-hand arm of the beam. The other arm serves as a rough visual indicator of the zero point and as a counterbalance for the left-hand extension. The three shielded wires, W , connect the ionization chamber to the amplifier portion of the circuit and the complete ionization chamber unit is solidly attached to the balance casing. The loiver, amplifier portion of the balance deR-3 picts the following from left to right. /o311 SA. The filament switch for the amplifier tube. S A four-position switch, of ivhich (81) is the off position, S2 the on position for the accelerator grid and plate circuits, S3 the on position for the compensation circuit, and S i the on position of the ionization input to the control grid circuit. (The selector system pictured here has been changed to a more stable system.) , R-1, R-2. Fine and coarse controls for the compensation circuit. -4sensitive microammeter or galvanometer with a 50-0-50 or a 100-0-100 scale (calibrated 1 microgram per division). Two potentiometers as shunts to control the sensitivitv of the balance euternally. M . A4 coarse meter (3.0 microamperes per division) which serves as a plate current indicator and as a rough null point meter. SB. A double-pole double-throw STT itch, which 3erves as an on-off sn-itch for the galvanometer. (This has also bcen changed to a more stable system.) S o t depicted in Figure 2 yet shown in Figure 3 are: the eleccontrols the trometer tube VS-11; a 25-ohm potentiometer 1%-hich filament current, set, a t 10 milliamperes; an accelerator grid potentioinetcr which sets the accelerator grid current between 200 and 300 microamperes; a plate potentiometer setting the current as desired; a fixed control grid resistor with resistance value between 50 and 1000 megohms as desired per degree of balance sensitivity; and the various battery supplies. OPERATION OF RADIOACTIVE hIICROBALANCE
T h e operation of the halnncc depictod in Figure 2 i
The filament switch, S A , is turned on (sequence important); the selector switch is turned to 82; the plate current which has been set a t a predetermined value (50 to 100 microamperes) is observed in meter M ; the switch is turned to position 83, and the plate current is compensated out by the potentiometers, R-1 and R-2. S B is opened and the plate current is finely compensated in the galvanometer circuit. The scale is now released from its arrested position and balanced by means of the adjusting screw, AS, until a rough zero point is obtained with the mechanical indicator a t the right portion of the balance (until now the balance has pot been connected with the electrical portion of t,he weighing system). SB is now closed, removing the galvanometer from circuit, S is t'urned to S4, and the ionization is observed on the coarse meter, M . Chamber P is then displaced by means of T until M reads approximately zero (0.5 to 1 division) (the same may be accomplished by continued adjustment of screw A S ) . S B is then opened and the fine galvanometer is placed in the circuit. P is displaced further until a zero reading is obtained on the gal-
e
1234
ANALYTICAL CHEMISTRY
vanometer operating a t a high sensitivity of the galvanometer. Now the switch, S B , is closed and later opened if the deflection on the rider scale is to be of great magnitude. The instrument is then calibrated by shifting a rider on the graduated beam a set number of divisions or by placing a known weight on one of the pans. In the case presented a weight shift of 50 micrograms was made by means of a rider and the galvanometer sensitivity was adjusted by means of the variable shunt, R3, to read 50 divisions for 50 micrograms, thereby obtaining an over-all balance sensitivity of 1 microgram equal to 1 division where 1 division is 1 mm. on the electrical meter. The calibrated balance is now ready for weighing. There are numerous variations of sensitivity calibration which may be employed with equal success; 50-, loo-, and 200-microgram weights or greater rider displacements may be calibrated electrically by means of precision shunt resistances or dial-calibrated precision potentiometers, which in turn would give the desired balance sensitivity. A typical Lyeighing proceeds as follows: The sensitivity calibrated balance is placed M ith S in position S3 and SB closed, removing the galvanometer from the circuit; the objectto be weighed and the weights are placed on the respective pans and the balance is brought to a rough zero adjustment with the appropriate ueights (observation of the right zero indicator plate). S is then shifted from S3 to S4 and the riders are adjusted until the meter reads zero. S B is opened, the galvanometer is placed in the circuit, and the deflection is observed; if the deflection is off scale the rider is shifted one notch in the proper direction and the reading is taken directly in microgram units. SB is then closed, the beam and pans are arrested, S is witched to position 83, and we are now ready for the next weighing. It is a good practice to check the electrical compensation point once per hour. The capacity of the above balance was 20 grams of load per pan. The double rider beam had a 0- to 1-mg. range in 50-microgram units and a 1- to 10-mg. range in milligram units. The rider system of this balance is discussed below.
In practice it has been found that changes of sensitivity with load may be virtually eliminated even with knife-edges out of lines (not radically in the vertical plane) by the utilization of a heavy centered low center of gravity beam operating a t a very low mechanical sensitivity (Figure 4, 8). The operation of a microbalance at extremely low mechanical sensitivities (1 nig. = 0.1 to 0.5 mm. Iyith a 15-cm., 6-inch, indicator fiom the center knife-edge) has appeared difficult with the present methods of observation. However, by means of radioactive motion translation the author was able to obtain large deflections for these minute displacements and thus to employ the heavy centered low center of gravity beam, which gives the following properties to the microbalance: Maintenance of a high weight sensitivity in conjunction with a simultaneous low sensitivity to changes in the dynamic displace ment of the center of gravity, due to load changes, compression of knife-edges, or flexure of the beam.
CHARACTERISTIC DEVELOPMENTS OF RADIOACTIVE MICROBALANCE
When the first radioactive balance was constructed, the characteristics sought were high sensitivity, linear electrical change of current with changing weight, electrical stability, and reproducibility. The balance had to be adapted mechanically to the electrical sensitivity and stability in order ultimately to attain a balance with high sensitivity, constancy under changing load as measured under high sensitivity, short weighing period, negligible rider displacement error, stability to tilt, vibration, ' and compression of the materials composing the balance, reproducibility of the beam and pan arrestment system, and temperature and humidity stability. Tests made under varied conditions resulted in a number of transformations in the conventional balance.
Figure 4.
T
Seko Microbalance Beam
Heavy Centered Beam and Sensitivity of Microbalance. If the center of gravity is lowered in the balance and none of the other mechanical features or characteristics is changed, the sensitivity of the balance decreases. .4s a result of the lowering of the center of gravity the response to load variations is decreased (9, 9). This lowering of the center of gravity may be accomplished practically by lowering the heavy center adjusting nut (the mass oi the beam remaining constant), by adding mass below the center turning point, or by a combination of both.
I. High center of gravity 11. Low center of gravity
Maintenance of a high weight sensitivity in conjunction with a simultaneous low sensitivity to mechanical vertical misalignment of the knives (minimization of the distance difference between the central knife-edge and the plane through the end knife-edges, static). A short damping period. Improved reproducibility, as there is improved stability to physical displacements with the heavy center. Increased sensitivity to tilt due to the high restoring moment of force because of the low center of gravity. This point is discussed below. The effect of the magnification or amplification factor, R, which may be radioactive, electronic, or optical, may be employed in an approximate geometrical scheme (Figure 5) and derivation, so that one may compare a normal high mechanical sensitivity beam with a heavy centered low center of gravity beam. Referring to Figure 5, I depicts the high center of gravity microbalance normally used and I1 depicts the heavy centered low center of gravity balance. CA and CIAl are the distances of the center of gravity from their respective turning points, d and d l . 10d = di. A B and AIBl are the respective deflectionsf andfl obtained by the placement of an identical weight on the appropriate pan,
f
= lOf1.
As the balances are assumed to be identical, on1 the necessary variables are chosen to illustrate the effect which a Eigh magnification factor, R, has on the over-all sensitivity. K Original sensitivity, balance I, f = d
K di
Original sensitivity, balance 11, f i = -
( 1)
In the above case arbitrarily chosen, mecha9cally j . = lOf1 where f and j 1 represent the respective mechanlcal sensitivities.
V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8
1235
30 -
I
i
Y w 0
1
I
40 1
20
I.
I
I
1 I 40 60 PAN LOAD, GRAMS
I
I
100
80
Figure 6. Sensibility Load Curves of Radioactive Balance
The balance is now adjusted electrically by variable shunts R and
RL,so that the following condition now exists:
The over-all deflection (electrical plus mechanical) of the two balances is now equal, and the over-all sensitivity of both balances a t zero pan load is the same. The balances are now loaded with equal loads and the change of sensitivity of the respective balances is determined. -4s the balances are identical, the change in the center of gravity is assumed to be identical for both. This is designated by Ad. The change of sensitivity under load may now be considered.
RK
+
S e w sensitivity, balance I kf = ___ d Ad iiew sensitivity, balance I1 k ~ f=~10dRi*K Ad
(3)
With K , R, R1, and d remaining constant, changes in Ad produce greater variations of sensitivity in balance I than in balance 11. It is apparent that changes of sensitivity produced by a dynamic change of the center of gravity or by the mechanical misalignment of the end knives with respect to the center knife are minimized by the utilization of the second type of balance. Figure 6 shows curves of sensibility plotted against load, obtained with and without a heavy centered beam. With the procedure employed no pan and beam arrestment errors were made, as the ionization chamber n-as always shifted to compensate for any mechanical errors; series of measurements were run with negligible rider errors ( 1 2 micrograms average error); and the sensibilities of the balance were adjusted mechanically a t zero pan load utilizing the electrical indicator. Figure 6 (upper) shows the operation of a normal beam (1) at a sensitivity of 1 mg. = 2 mm. and a heavy centered beam operating a t the same sensitivity. The percentage change of sensitivity of the two curves a t an added pan load of 40 grams is 40% for
curve 1 and 397, for curve 2 and a t 100 grams pan load it is 68% for curve 1 and 647, for curve 2. The difference in change of sensitivity a t 40 grams between the two curves is 1% and a t 100 grams 47,. The large decrease of sensibilities under increasing load was partially due to the imperfect alignment of the knifeedges and the flexure of the beam. Figure 6 (second from top) shows the same heavy centered beam operating a t a reduced sensitivity ( 5 mg. = 1 mm.). The change in sensitivity is 16% a t a 100-gram load. For a mechanical sensitivity factor change of 10 we have decreased the load sensitivity change by a factor of 4, and as we can maintain the high over-all sensitivity of the balance, it may again be seen that it is a gain to operate with the low center of gravity. Figure 6 (second from bottom) shoMs the operation of another heavy centered beam having a knife-edged alignment such that the sensibility increases with increasing load. The mechanical sensitivity for curve 1 is 1 mg. = 5.5 mm. and for curve 2, 1 mg. = 0.45 mm. The change of sensibility for curve 1 was 76Yc and for curve 2, 77,. The weight displacement for obtaining this data was 200 micrograms. The periods for the beam curves were 1.5 to 2 minutes for curve 1 and 15 to 20 seconds for curve 2. Figure 6 (bottom) shows the same beam with a weight displacement of 50 micrograms and a mechanical sensitivity of 1 mg. = 1 mm. The change of sensibility was 4Yc a t a 100-gram added pan load. The results with the 200-microgram weight displacement can be considered more reliable, as the rider error in the latter set of measurements n a s greater. The period of the beam was about 40 seconds. The utilization of the low center of gravity permits more rapid weighing because of the shorter period of swing. Experiments t o determine the maximum sensitivity attainable with a given radioactive foil Tvere made with the viewpoint of reducing the mechanical sensitivity in order to decrease the period and the sensitivity response to load change as much as possible.
The galvanometer was placed a t maximum sensitivity where currents in the order of 1.9 X ampere could be measured (sensitivity depends greatly on amplifier tube used). When null point readings were taken, a high galvanometer sensitivity was also employed. The strength of the radioactive foil originally utilized was 200 micrograms of radium. In order to obtain a greater deflection per unit displacement of area, the radioactive foil, the area of which had been 25 X 5 sq. mm., was replaced by two foils of total area 50 X 5 sq. mm. and of double the radioactive strength; however, as only a small solid angle of the foil was radiating the small ionization chamber, the total effective radiation was on the order of 20 to 25 micrograms. The mechanical sensitivity of the balance was 5 mg. = 1 mm., a 0.2-micron displacement per microgram per chamber. When fine null point readings were being taken and the galvanometer operated at high sensitivity in order to measure these minute displacements (0.2 to 0.3 micron), numerous oscillations were observed which interfered with the stable measurement of these displacements and ~ h i c hhad not been apparent a t low galvanometer sensitivity. These are due to the fact that the number of particles arriving a t the detecting ionization chamber per sepond fluctuates with the time ( 7 ) . The entire number of particles (alpha and beta) noted during any second nil1 be more or less than the average value by an amount expressed as the fluctuation. The distribution of the number of particles around the average number can be represented by a Gaussian error curve when the average number is large. This result together with the uncertainty can be expressed by AVvv( 2 ) . The number of alpha-particles emitted in the ionization chamber was on the order of 3 X l o 6 particles per second, and the number of betaparticles on the order of 2 X 106 particles per second and the fluctuation on the order of 0.057,. The ionization current measured continuously was on the order of 5 X 10-9 to 2 X 10-8 ampere and the oscillating current which would be apparent a t high galvanometer sensitivity because of the particle fluctuations would be on the order of 2 X IO-'* to lo-" ampere. In order to
ANALYTICAL CHEMISTRY
1236 eliminate (mask) these fluctuations and to maintain the high galvanometer sensitivity, two 500-microfarad condensers were placed across the galvanometer terminals. The above change improved the stability and 0.2- to 0.3-micron displacement could be detected readily. (Improvements have been made here.)
LFNGTH / 2 0 # M e
-
fsSs3m RID€U
t?fDER WEIGHT: 2 f l t .
1 Figure 7.
WEiGHTr O.Sfi.9.
IT Rider Scale Systems
The sensitivity and the stability of thc balance mag be additionally increased by further increasing the strength of the radioactive foil. The ionization current may also be increased through the utilization of a larger ionization chamber; however, the physical dimensions of the balance served as a practicable limit to the size of the chamber. The balance which resulted from the above experiments possessed a high sensitivity, good stability, and a lovi period, the characteristics of which are given below. RIDER SYSTEM
When sensitivities on the order of 1 microgram per division were desired, the machining errors of all parts o i the balance were of prime importance, especially if a rider weight system were employed. Figure 7 shows the rider scale first used. The rider scale length was 30 mm.; there were 21 notches in order to render a total displacement of 1 mg. and a notch displacement of 50 micrograms. The distance from the center of one notch to t,he center of the following notch was 1.5 mm. If a misplacement of 0.1 to 0.2 mm. were made, the error in weighing would be from 3.3 to 6.6 micrograms. The above displacement error was reduced by employing a longer rider scale (120 mm.) having a 6-mm. distance between succeeding center points and a longer rider of lighter weight and smaller diameter (Figure 7 , 11).
ternal forces, such as the compression of one side of the balance table, tilt of the balance, and compression due to loads placed on the balance proper. Figure 8 depicts the variation of balance weight with an external load placed on one side of the balance. The balance operating with a low center of gravity requires a good solid foundation not subject to physical distortions. However, the advantage obtained when operating of gravity justifies a good foundation. witJhthe l o center ~ Material Stability. h number of experiments were run on some of the materials employed in order to determine those best suited for the balance. Similar tests were run on the various linear and angular displacements in the balance. Figure 9 depicts a general type of setup employed in these measurements, the results oi which will be available a t a later date. The balance may be so arranged that with multiple chambers one may determine t,he Keight and the various errors both in direction and magnitude in terms of weight. However, it is the purpose a t present to employ only a single ionization chamber for the radioactive microbalance (Figure lo), and to limit the mechanical errors as much as possible. PAS AND BEAM ARRESTMENT SYSTEM
The initial beam and pan arrestment system was a singlemotion straight fall-away type with a three-point beam arrestment and a two-point arrest'ment for each stirrup. Glass points were employed as bearings. The present beam and pan arrestment system employs a double-motion arrestment (the beam being out of phase with the pans and stirrups) with a front to back arc movement. Agate grooves and glass points were employed as bearings for t'he beam and metal points and disks \vere employed as pan arrests. Changes are being made in the present beam and pan arrestment system. All the measurements in the curves were minimally affected by the arrestment system ut,ilized, as the ionization chamber was ahvays brought to the electrical zero point and the chamber null point, (wheel T , Figure 2) before the load-sensitivity measurements were made, and the ionization chambcr plates
I
3 2 u
The machining of any rider system should be in the order of 0.0025 mm. (0.0001 inch) if accurate reproducible results are to be obtained when employing a rider system having a greater weight displacement greater than 1 to 2 mg., especially on a microbalance. Khen weighing masses on the order of micrograms, one is approaching the limit of mechanical tolerances.
I
10080-
0
s
O.IMG.-
I:6 6o '[0 -
g
0.006'
40-
c
eo
-
DAMPIKG SYSTEM
I
5
The present balance is magnetically damped, as a short weighing period is desired: however, the balance mag be employed successfully without any damping mechanism by thoac who use swing methods of weighing.
I POUNDS 10
I 15
Figure 8. Tilt Curve o r Radioactive Balance
KNIFE-EDGE SYSTEM
The knife-edges employed in the original experiem. (0.373 inch) for the si& knives.
Successful
bearings of the same dimensionq.
+---
CH~DAER R rWEIGHT-. TO
STABILITY
Balance Case.
v CENTER K N I F E EDGE
AflfLlFlER
f OF
.SENSIT1WSY
L
FLEXURE
BEWI
CHflMBFR C TO
AM PL I N E R
CHIINBE
With a low center of gravity i
V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8
1237
Table 11. Characteristics of Radioactive Microbalance Characteristics sensitivity Period Reproducibility
Stability Humidity Temwrature sensiti*ty change nith load,
.
Type A, Low Center of Gravity. 1 Mg. i Type B, High Center of G r a ~ t y 1 0.1 to 0.4 Mm. Mg. = 2 to 5 M m L Eleatrid Mechanical Eliotrieal Mechanical 2 x lo-l~arnpere 0 . 0 1 to 1 microgram 3 x 10-10hrnpere Imiorogr&mtoO.l mg. 2 to 4 seconds 10 to30 reconde 2 to 4 seconds 1 to 2 minutes Good (1 to 6 rnioropraml Good Chhngea with average error with present beam and pan arrestment system
Good No effect N o effect
...
Dbmping
...
Load owseity
...
Good
1 to 5%
... ...
Magnetio or none at SI1 50 gram per pa"
Good No effect No effeot
...
...
...
were parallel with the left t o right line of disolacement of the radioactive foil. CHARACTERISTICS O F RADIOACTIVE
:
the sensitivity changes greatly with increasing load. Improvements are being made in the ahove characteristics and additional data. are being compiled. It has been possible to determine the
mechanical errors and limitations of the balance through radioactive displacement measurements, as well as through the utilization of capacitance, inducchanges with load ... tance, optical systems, ete. (f,6). These 30 to 7;% have taken the form of measurement of linear displacements on the order of 0.1 M*.neti0 micron; rncasuremcnt of angular d i c 20 grams per pan placements 0.005' to 0.0001 '; linear, solid distortion, and compression measurcmentsc.g., measurement of flexure of beams; and vibration measurements. In the course of the research on the heavy centered radioactive microbalance a number of new testing and measuring instruments were developed because of thc necessity of performing extremely fine displacement measurements. These include an angle measuring 8. tilt and compression meter, a vibration meter, and air convection current meter. In radioactive instrumentation one possesses a tool that has virtually unlimited research and industrial possibilities and the radioactive balance is hut one example of the application of radioactive instrumentation to science and industry.
Table I1 gives characteristics of the ritdioarr,vr , , , , ~ , u u ~ , I I , , ~ l i , Types A and B. The radioactive mierohblance has the advantages of high sensitivity, external control of sensitivity, constsney of sensi. tivity under increasing load, stability ,f sensitivity, external method of calibrating fine nreights, extrelnely short nreighing periods possible (Type A), external electrical reading device ACKNOWLEDGMENT which can be read easily, large load capacity, and measurement of various balance errors instantaneously and directly. Control The author wishes to thank H. K. Alher, Arthur h. lnomas and weight limiting can he adapted radioactively for fast approxiCompany, Philadelphia, Pa.; A. H. Corwin, Department of
. ..
