can he used to make fine adjustments. Some typical circuits for this purpose are shown in Figure 2. Since no attempt is made to supply temperature compensation, the pH valves in Figure 2 are only approximate (see Nernst coefficientin Table 11). LIMITATION
Figure 2.
Scale shifting circuits
important to remember that the pH meter expands potential measurements around zero, which corresponds to pH7onthemeter. Theasymmetrycontrol allows the zero potential reading to heshiftedtoeitherendofthemeterscde. Thus, if the scale is expanded to 1.4 pH units full scale and the pH meter asymmetry control is adjusted so that the meter needle is a t 7 with the electrodes immersed in pH 7 buffer, the pH meter scale actually corresponds to pH values of 6.3 to 7.7. If the meter needle is adjusted to 0 with pH 7 buffer, the scale corresponds to pH 7.0 to 8.4. If the meter needle is adjusted to 14 with pH 7 buffer, the scale corresponds
to pH 5.6 to 7.0. Because of the limited range of the asymmetry control, the Zeromatic pH meter with scale expanded to 1.4 pH units full scale is useful in the range of about 5.6 to 8.4. To use this expanded scale at pH values outside this range, it is necessary to introduce a scale shifting potential into the reference lead. For example, if it is desired to have pH 4 at the center of the meter scale at 30" C., one must apply a potential of 3 X 60.1 or 180 mv. to the reference electrode. Simple voltage dividers, using a flashlight cell or mercury cell, can he made to apply approximately the necessary voltage and the pH meter asymmetry control
There are limitations to which the sensitivity can he increased. Since the Zeromatic pH meter is a line-operated instrument and there is sure to be a certain amount of line "noise," 140 mv. full scale is as far as the sensitivity should he increased with standard operation. It is possible to increase the expansion to 35 mv. full scale by using a constant voltage transformer to stabilize the line voltage. By using the transformer and increasing the pulse speed of the relay in the Zeromatic, an expansion to 7 my. full scale may be used. To increase the pulse speed, changes must he made in the circuit: Replace R28 with a %megohm resistor and R29 with a 68,M)O-ohm resistor. Common 5% 0.5-watt carbon resistors can he used. This modification increases the relay pulse rate to approximately 5 pulses per second.
Adapter for Alpha Counting with Conventional l o w level Beta Counter M. 1. Gonshor, J. E. Green, and R. E. Wood, Kennecott Research Center, Kennecott Copper Corp., Salt Lake City, Utah
WING the publication of
U. S. Geological Survey Bulletins No. 1097 A and 1097 B, eonsiderahle interest has developed in mining exploration circles in the age dating of rocks by the lead-alpha method [Larsen, E. S., Jr., Keevil, N. B., Harrison, H. C., Bull. Geol. SOC.Am. 63, 1045-52 (1952)l. An improvement in technique for alpha radioactivity measurements is described in the present paper. Since the appropriate accessory minerds for this determination, zircon in our case, and monazite or xenotime, are difficult and expensive to separate, only a minimum is collected. For t,his
r
Disassembled components
for alpho counting
reason also, the same sample must be used for the alpha count and for the lead determination. This means that a sample presentation system for alpha counting is required such that the entire sample can he recovered and arced in the spectrographic measurement for lead. This being the case, the use of mixed sample and scintillator or 4 pi alpha counters is ruled out. A standard Model 314 Packard Instruments Tri-Carb low level liquid sriutillation spectrometer was adapted for the detection and counting of alpha radiation. A prerequisite was that any alterations in the equipment be such that a conversion from beta to alpha detection or the reverse he accomplished with a minimum of effort, time, and equipment. Figure 1 shows the components of the final unit, disassembled to show construction details. Figure 2 is a photograph of the assembled unit. The assembled unit is shown mounted on a tripod which has one vertical leg and two slanted legs. This construction permits placement of the entire device in the corner of the liquid scintillation counter freezer chest. To change the Packard unit from low level beta radiation measurement to alpha detection, it is
only necessary t o remove a preamplifier from its standard position in the freezer and place it On the
~~~~i hop^^^^^^ adjustment of the multiplier
high voltnge to t,hat required for the particular tube in use are all that is required on the control panel. The cables connected to the unused preamplifier of the l'acksrd 314 unit are
.
Figure 2. Assembled adapter unit proportion Fig. 1 VOL 33, NO. 9, AUGUST 1961
1293
removed to eliminate noise from that source from interfering with the alpha detector. In Figure l , A is the preamplifier taken from the standard Packard counter. 3 is the multiplier phototube; in this cme a Dumont No. 6292. C is a Mumetal shield with felt lining used to enclose the multiplier phototube both for protection and for the exclusion of light. D is a disk of vitreous quartz with a 90-micron thick CsI-TI activated scintillation crystal attached. E is the housing proper for the entire alpha detector unit. This piece was machined from a single block of high-purity aluminum 6l/* inches long by 3 3 / 4 inches in diameter. The housing is attached to a 1-inch thick Lucite collar for attachment to the tripod and for proper positioning of the preamplifier and multiplier phototube unit. F of Figure 1 is the bottom of the unit, which is also the sample holder. This piece is likewise made of aluminum, gold plated, to provide easier cleaning with less contamination from one sample to the next. The gold plating was used because gold can be expected to be more free of lead impurities than stock aluminum. There is a small depression in the center of F which is just large enough to accommodate the sample planchet, G. The sample planchet holds approximately 200 mg. of material and presents an area of exactly 1 sq.
cm. for counting. The sample planchet
fits into the depression of F and the brass ring, H, clamps the rubber ring of F against the base of E such that a
vacuum-tight seal is obtained and, a t the same time, the sample is held a t a highly reproducible separation from the scintillator crystal. This separation is approximately 2 mm. in this unit. The scintillator crystal is mounted with the quartz side toward the multiplier phototube, using a silicone oil of high viscosity. The CsI side of the scintillator crystal is sealed to the far side of the hole in E by rubber cement to form a vacuum-tight seal from above. Evacuation is made through a small hole in E connected to the vacuum nipple shown in the housing. Although the cesium iodide crystal has a lower output pulse than that of zinc sulfide, it was chosen because of its greater efficiency, due to its higher atomic number and density. The geometry of the counting chamber is such that essentially all alphas (2 pi) are detected by the scintillator crystal. This is true within the error imposed by counting statistics. A constant R = a / E of 0.496, given by Geological Survey Bulletin No. 1097 A, is used to convert counts per square centimeter per hour, E, from the thick
source emitter to alpha disintegrations per milligram per hour, a, for thin source emitters. This resulting a value is used with the lead concentrations in parts per million to obtain an age for the sample. For our work, calibration of the counter is carried out using U. S. Geological Survey analyzed samples with alpha activity of 100 to 1500 counts per hour per square centimeter. Our counts on these samples have agreed within counting error with those counts obtained by the Geological Survey. Samples of zircons separated from various mining exploration samples have ranged from 80 to 1000 counts per square centimeter per hour. A background of 40 to 60 counts per hour is customary with good stability from day to day. This background has recently been reduced to 20 to 30 counts per hour. Counts are ordinarily taken on samples and standards for 4 hours each, with 1hour background readings between samples to give a total of 4 hours on the background for each series of four to six samples. Samples encountered to date have had a minimum of 4 intensity to background ratio which provides counting accuracies of *IO% or less in 4hour counting periods.
Remote Operation of Single-Pan Balance for Weighing in Inert Atmospheres J.
E. Barney II, Spencer Chemical Co. Research Center, Merriam,
Kan.
years, increased attention Istruction, has been given to the design, conand operation of dry boxes N RECENT
for handling moisture- and oxygen-sensitive materials. Dry boxes constructed of plastic or stainless steel, in which the atmosphere can be maintained a t a dew point of -70" C. or lower and the oxygen content can be kept a t less than 5 p.p.m., may be purchased from a number of companies. Several very satisfactory dry boses have been described (1, ,%?)that can be constructed in a well-equipped shop. Comparatively little effort has been devoted, however, to the problem of weighing samples in the inert atmosphere available in dry boxes. Most balances, when left in dry boxes containing such moisture- and/or oxygensensitive compounds as TiC18, TiClr, and AlC13, will corrode rapidly, with subsequent loss in accuracy of weighing. In general, most workers (3) have used differential weighing techniques, wherein the actual weighings are made outside the dry box, or they have used inexpensive trip or torsion balances. For analytical weighings, however, the former technique is 'time-consuming, 1294
a
ANALYTICAL CHEMISTRY
Figure 1, Schematic diagram showing side view mounting of balance E. a/,-inch plywood base EA. Mettler H5 balance D. 1.
Manostat dry box 1 -inch angle iron supports L. 3/8-inch Plexiglas supports for motors M. Reversible synchronous motors S. '/,-inch l a g bolts SS. Snap action switch W. W a l l
while the latter is not accurate enough A novel apparatus that permits rapid, accurate analytical weighings in a dry box has been devised. A Mettler H5 analytical balance (Fisher Scientific Co., St. Louis, Mo.) is mounted above a dry box on a platform suspended from a wall. Holes cut in the platform and in the top of the dry box permit a weighing pan to be suspended inside the dry box. Small, reversible synchronous motors drive the pan-release knob and the vernier control knob of the balance through flexible couplings. Direction and angular distance of travel of these knobs are controlled by foot-operated momentary switches, and by snap-action switches activated by pins on the knobs. A side view of the apparatus is shown in Figure 1. Only one snap-action switch is shown; actually, four are required, two for each knob, in order to limit travel in both clockwise and counterclockwise directions. Also, the mirror used by the operator to view the optical scale of the balance is not shown. Any large truck mirror is satisfactory. It is conveniently mounted on one of the angle-iron braces.
