THE LAURITSEN QUARTZ-FIBER ELECTROSCOPE CLIFFORD S. GARNER University of California, Los Angeles, California
Tm availability of radioisotoped produced in the Oak
In use a charging voltage V is initially applied to the Ridge slow-neutron pile and allocated by the U. S. quartz fiber (covered with a thin coat of gold to make it Atomic Energy Commission (1) has greatly increased a conductor) and its metallic supporting arm, both of the use of such isotopes, not only in fundamental and which are insulated by a quartz, amber, sulfur, or polyapplied research, but also in the classroom for instmc- styrene insulator from the chamber wall. The mutual tional purposes. The relatively low cost, both of these electrostatic repulsion deflects the fiber away from the radioisotopes and of the electroscope wit11 which they stationary arm. In the presence of ionizing radiations may be detected and studied, affords even the small the gas inside the chamber is ionized and ions of the university or college an opportunity for the introduc- opposite sign are collected by the charged fiber system. tion of interesting, basic concepts of radioactivity and The reduced charge on the fiber system results in a nuclear chemistry into undergraduate curriculum. displacement of the fiber toward its uncharged position, The Lauritsen quartz-fiber electroscope is especially the restoring force being the elasticity of the bent suitable for equipping the small research or instructional fiber, and the rate of motion of the fiber is a measure of laboratory in which radioactivity measurements are the intensity of the radiations being measured. Inasto be made, inasmuch as this instrument is economical much as the capacitance C of the fiber system is nearly in cost and free from the maintenance problems com- constant, the decrease in voltage, AV, for a collected monly associated with nearly all other types of nuclear charge, q, is AV = q/C. In the typical Lauritsen detectors. Accordingly, i t is the purpose of this paper electroscope C is about 0.5 cm., or 0.5 X 10-'=farad. t o describe the principle, operation, and calibration of (0.5 ppf.), so the voltage sensitivity is very high and this type of electroscope, and to indicate a convehient only a small charge transfer is needed to cause considerarrangement for its application to experiments with able deflection. For example, about 2000 radioactive radioactive substances. disintegrations per minute (33 microrutherfords4) of a 2-Mev. beta emitter, located so 50 per cent of the PRINCIPLE beta particles enter the ionization chamber, will proThe Lauritsen electroscope (2) functions on the same duce a deflection comparable with the natural hackprinciple as the familiar gold-leaf electroscope, but the ground: assuming each beta particle travels an average former is superior in sensitivity, stability, ruggedness, distance of 5 cm. in the air inside the chamber, producand scale-linearity. As indicated in Figure 1, the ing an average 50 ion pairs per em., and that the voltage Lauritsen electroscopein its commerciallymanufactured is high enough to give a saturation current (no ion reform1 consists of an air-filled ionization chamber equip- combination), the charge collected by the fiber system ped with a quartz-fiber system, which acts as a sensitive per second is 2000 betas/m. X '1 m./BO s. X 0.50 voltmeter of low capacitance, and a grpund-glass win- X 50 ion pairs/cm:beta X 5 cm. X 1.6 X 10-l8 dow to admit light for observation of the fiber, together coulomb/ion = 7 X lo-'= coulomb/s.; the rate with a low power (ca. 50 X) microscope for viewing the of voltage decrease is then 7 X 10-'%oulomh set.-'/ displacement of the fiber2 against a graduated scale in 0.5 X 10-l2 farad. = 0.0014 volt/s., and for a voltage the eyepiece. An external temporary source of about sensitivity of 1 division per volt this corresponds to 200 volts d. c., such as a battery or vacuum tube rec- 0.0014 div./s., or about equal to normal background. tifier, is needed for charging the electroscope. Figure (It may he noted that the same sample counted on a 2 gives a circuit diagram foraninexpensive a.-c. operated Geiger-Mueller counter a t 50 per cent geometry would rectifier ~ h a r g e r including ,~ a white pilot light which give a counting rate of about 1000 counts per minute, serves to illuminate the fiber. 'The rutherford, rd., has been defined in this country (3) as The Lauritseu electroscope, Model 1, is manufsctured and sold by the Fred C. Honson Company, 3311 E. Colorado Street, that amount of any radioisotope which is undergoing one million Pasadena. Californirt. The orice is about $50. exclusive of the disinteerations oer second, and the use of this unit is raoidls charging device. For generai laboratory the sensitivity gaining recogniiion. The older unit, the "curie," c., origihall; should be specifiedin the range from 2-3 divisions per minute per defined as the amount of radon in radioactive equilibrium with one gram of radium, has been assigned the arbitrary value 3.7 X millicurie of rrtdium gamma. radiation at one meter. 1010 div./s. by most investigators in this country, although the 1 A second quarts fiber is fused a t right angles to the free end of themain fiber,and thiscross fiberisobserved in the microscope. actual disintegration rate of one gram of pure radium is now A similar vacuum tube charger is sold by the Henson Cam- known with greater precision and is slightly less than the above pany (see footnote 1) for about $25. Friction chargers are also number. International Committees have not yet been able to agree upon the names and values of radiosotivity units. obtainable from this source. 542
OCTOBER, 1949
ALUMINUM "CAN" 0.54mm (0.15q/cm2.) WALL
EYEPIECE SCALE
INSULATOR I
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OBJECTIVE L E N S
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GROUND (BLACK LEAD)
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\GOLD-PLATED QUARTZ FIBER (3-51 DIAM.) SUPPORTING ARM
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Cross Ssstion of Lauritsen Quartz-Fib.=
or some 10to 100 times the counter background, showing that the electroscope is roughly only one-tenth as sensitive to average beta radiations as is the G-M counter.) Because the electroscope can handle activities up to about 3 div./s., the range without change in sampledetector geometry is about 3/0.003 = 1000-fold. , Ordinarily the instrument is used only for relative measurements, but it may be used for the determination of absolute disintegration rates if calibrated with a proper standard sample under identical conditions of sample mounting, geometry, etc. SETTING UP AND ADJUSTING THE INSTRUMENT
The electroscope should be mounted firmly on a base of convenient height along with a sample holder and small light source. We have found the arrangement shown in Figures 3 and 4 very satisfactow, both for student use and for research. The charging unit is located inside the wooden-box base, A, to the back of which is attached a 6-volt pilot light, B, for illumination of the fiber. All controls are mounted on the front panel: master power toggle switch, C, light intensity potentiometer control, D, charging voltage potentiometer control, E, and charging button (on bakelite extension rod), 8'. A transparent shield, G, equipped with a side door for loading and unloading samples and absorbers, is constructed by cementing together '/,@-inch or L/s-inchlucite sheets (a less expensive shield may be fabricated by stretching colorless cellophane sheeting over a metal-rod frame); this shield serves to reduce the effect of drafts and to keep the instrument from gathering dust. A lucite (or metal) sample holder, H, with five sets of slots for diierent geometries, is screwed to the wooden base just under the ionization chamber. Unless the electroscope is to be used only for
Electroecope
gamma d e t e c t i ~ nthe , ~ can should be carefully removed and a semicylindrical hole about 6 cm. long by 5 cm. across cut in it with a jeweler's saw; after removing any burrs and cleaning out the can with hexane, coat the edges of the hole with an adhesive such as Duco household cement and roll the can onto a thin aluminum foil (1-2 m g . / ~ m thickness .~ is convenient and passes a11 but very ~veakbeta particles-a measured area of pinhole-free foil should be weighed on an analytical balance before cementing it onto, tkie can). Trim off any excess foil, behg sure there are no uncemented openings, then carefully reinstall the can with the foil mindo~v,I, downward. The chamber of the electroscope may readily be modified for special purposes, such as has been done, for example, by, Henriques and
' The "can" supplied commercially has a -wall thickness of 0.15 g./cm.l aluminum, which is sufficientto stop all beta radiai tions of msximum energy (E,) below 0.45 Mev. A spcoial thinwindow can is sold bv the Henson Com~anv. Because of the " correspondingly smaller specific ionization produoed by gammas, the electroscope will he very roughly 100 times less sensitive to gammas than to betas.
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I IIZWATT, l MEG
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JOURNAL OF CHEMICAL EDUCATION
Figure 3.
Side View of Elact.oacope Arrangement
Margnetti (4) in adapting the instrument for the measurement of C14as carbon dioxide gas. Charge the electroscope and focus the.eyepiece, J, on the fiber. If a satisfactory focus cannot be thus achieved, loosen knob K and slowly'move the objective lens front or back to produce a sharp fwus (once each adjustment is made it need not be made again; the instrument is generally received with these adjustments already made). The pilot light illumination may be adjusted to suit each user's preference. A timer, L, good to second, such as a stop-watch or synchronous clock, is desirable for timing the rate of displacement of the fiber. CHARGING AND READING THE ELECTROSCOPE
After turning on the master power switch, turn on the charger and wait about 30 seconds for the vacuum tube to warm up. While depressing the charging button and while looking into the eyepiece, rotate the charger voltage control until the fiber is near zero on the scale, then release the charging button and turn off the charging unit. Immediately following charging, the fiber may be seen to move up the scale more rapidly than is
normal in the absence of a radioactive sample; this is caused by polarization of the insulator dielectric. Hence, accurate readings cannot be taken until the insulator becomes "saturated" or "equilibrated." This "soak-in" time may be determined by taking readings a t 0.5-minute intervals for about 40 minutes. If the electroscope has not previously been charged within about one hour, the equilibration time may be about 20 minutes in a typical case, whereas only about a 5-minute wait is required if the electroscope has been more recently charged. The waiting time depends upon the precision desired, and for the highest precision the electroscope should be charged a day in advance. The scale is marked with large graduations numbered 0; 1, 2, . . . 10, with each large graduation marked off into ten divisions not numbered. It is commonlv conventional among many nuclear chemists to call these large graduations 0, 10,20, . . . 100, so that each of the finest marked graduations represents one division, and this policy is followed in this article. With practice, the position of the fiber may be estimated to 0.1-0.2 division. I n making a reading one determines either the time taken for the fiber to be displaced between two given scale readings (most accurate procedure), or the number of divisions displacement in a given time. Customarily the result is expressed as so many divisions per second, e. g., 10.5 div./s. Generally, the precision of a measurement made on a Lauritsen electroscope is not better than 2 per cent standard deviation, although 0.5 per cent may be attained with practice, care, and the use of the same two division marks for a constant displacement. (Occasionally one may notice erratic motion of the fiber; this results from thermal convection currents inside the ionization. ch'amber arising from temperature graaients. Consequently, the instmment should be located in draft-free locations, away from hot or cold objects, and direct sunlight.) The readingisbest made after the radioactive sample has been placed in the holder and the fiber allowed to qove a few divisions Samples may be conveniently mounted on copper or aluminum planchets, or filter paper, or on 1-inch watch glasses, each affixed to the center of a sample card6and covered with a cellophane cover scotch cellulose-taped in place to prevent contamination spread. The card should bemarked with the sample number and with two arrows a t right angles40 each other; the card is always placed in the holder with the arrows pointing in the same two directions with respect to the holder, and gently pushed so as to contact the holder back and side to which the armws point, thus assuring reproducible geometry. CALIBRATION FOR NONLINEARITY OF SCALE
Usually the scale of a Lauritsen electroscope is linear to within 1 per cent over the range &50 divisions, be-
' No. 40 thickness) plain utility chip board, accurately back-cut to 2'/%inch X 3 ' / ~inch, is appropriate, and may be obtained from paper supply houses cut to size at about $15 per 10,000cards.
545
OCTOBER, 1949
coming increasingly nonlinear beyond 50 divisions. For precise work a nonlinearity correction should be applied, unless one is always taking readings between the same two divisions, in which case the reciprocals of t,he times for discharge between these fixed divisions give directly the relative intensities or "activities." If a scale calibration is desired the following procedure may be employed. 1. Use a radioactive source with negligible decay during the time of calibration and of. such strength and location as to give an observed activity of about 0.05-0.1 div./s. (Weaker samples may he used but the time required for the calibration becomes too long.) 2. After charging electroscope allow time for the fiber system to equilibrate before taking readings. 3. Accurately determine the time for the fiber to trasverse the interval from 0.0 to 10.0, from 10.0 to 20.0, etc. Repeat at least once. The standard deviation for each interval is usually less thanl.5 per cent. 4. Calculate the sensitivity in div./s. for each scale interval, and plot as ordinate versus divisions as abscissa. An example of such a plot is shown in Figure 5. Mention should be made of the fact that the sensitivity also depends upon the sample strength. However, the sensitivitv is nracticallv inde~endentof intensitv for samples Gingreadings be~owabout 2 div./s.
If the fiber will not come into view upon charging, first be sure the eyepiece or objective lens has not accidentally been altered, then check with a d.-c. voltmeter to be sure the charger is giving enough voltage. If neither is found to be the cause, then it is probable that the fiber has become tangled about the supporting arm or even broken as a result of sudden moving or other rough handling of the instrument. Remove the can in a room as free as possible from dust and drafts (be careful not to breathe on the fiber system) and examine the quartz fiber with a magnifying lens (the fiber is barely visible to the unaided eye). If i t is tangled about the arm free it by gentle blowing with air from a rubber bulb; never try to untangle the fiber with a probe or other mechanical means. The can should be removed as seldom as possible, for there is always some chance of damage - to the gold - coat in this oDeration. Damaged foil windows may be replaced in the manner used for the original installation. Students should be cautioned, (a) to avoid denting or punching a hole in the foil window with the fingers, sample card, and absorbers, (b) not to change any adjustments other than light intensity, (e) to refrain
BACKGROUND CORRECTION
Essentially every radioactivity detection device, including the electroscope, gives a reading showing apparent radioactivity even in the absence of a radioactive sample. This "background" is due to cosmic radiation and to natural radioactive substances in the materials of construction (and often to contamination of the instrument by radioactive samples). In addition, the electroscope insulation system is not a perfect insulator, although this contributes negligibly to the background. Lauritsen electroscopes used without lead or other shielding generally have backgrounds of 0.001-0.005 div./s., reasonably constant over a given day and often much longer. Once'a day the background should be determined (be sure nwsamples are near the electroscope), and the value obtained should be subtracted from other readings made that day in order to get the discharge of the fiber arising from the radioactivity of the samples themselves. REPAIRING THE INSTRUMENT
Inasmuch as the instrument is fairly rugged and may be moved about without damage, maintenance is not a serious problem. However, certain repairs may occasionally be required. Erratic behavior of the fiber, or "creeping" a t an excessive rate in the absence of samples, may be caused by partial flaking off of the gold plate on the quartz fiber. Unless one is skilled a t such work, the repair is best made a t the factory. ~ometimesthe same effect is produced by lint or hairs left inside the cbamber; these can be removed by blowing air from a rubber bulb very gently on the fiber system and can.
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These may include determination of half-lives and absorption characteristics of radioisotopes, decay and growth relationships, radiochemical separations, studies of back-scattering and self-scattering effects, and tracer experiments. Some experiments which are especially suitable for the physical chemistry laboratory course will be described in a future paper. LITERATURE CITIED (1) "Isotopx," Catalog and Prim List No. 3, Isotopes Division, United States Atomic Energy Commission, P. 0. Box E,
from jarring the electroscope, and (d) to wash hands before using the instrument so as to avoid possible contamination by radioactive substances.
Oak Ridge, Tenn. (revision of July, 1949). (2) LAURITSEN, C. C., AND T. LAURITSEN, Rev. Sci.Instruments, 8. 438 (1937). (3) CONDON, E. U.,LND L. F. CWTI~S, Science,103, 712 (1946); the paper appeared also in Phys. Rev., 69,672 (1946). (4) F. C., JR..AND C. MARCNETPI. I d . En7. Chem.. . . HENRIQUES. ~ n a ~i d : 18,417 , (i946)
