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RADIOISOTOPE EXPERIMENTS IN THE ORINS SUMMER INSTITUTE PROGRAMS RALPH T. OVERMAN, DAVID L. COFFEY, and LOWELL A. MUSE Special Training Division, Institute of Nuclear Studies, Oak Ridge, Tennessee
Dumw the summers from 1954 to 1957 the Oak Ridge Institute of Nuclear Studies has conducted five summer institutes for science teachers. Four of these institutes were for secondary school science teachers, with one being for college science teachers. These institutes have been supported by the National Science Foundation and the Atomic Energy Commission. The main emphasis has been to strengthen science teachers in subject matter fields by presenting lectures and demonstrations in fundamental chemistry and physics, the frontiers of science, teaching techniques, and related disciplines. While the institutes have not emphasized atomic energy programs, it was decided during the early planning that since a large amount of radiation detection equipment was on hand for the wide variety of other courses which ORINS presents, it would be well t o include some lecture and laboratory material on radioisotopes. The laboratory experiments were modifications of some of the experiments presented in the ORINS course for nrofessional research ~ersonnel. The experiments were conducted on six afternoons during the withthe 48 teachers divided into fOur-week groups of 16. The teachers rotated among the three physics experiments the first two weeks and among the chemistry experiments the last two weeks. It is apparent from the widespread interest in the ORINS Program that there is a demand for information about the set of experiments which are used to demon-
strate basic principles. Those described here should not he considered as more than suggestive of the type of procedure which can be followed rather simply but which has been found to be very well suited to provide experience in the essential techniques of radioisotope use. If the equipment is available, some of t.hese experiments can be performed by students a t either thesecondary school or early college level and will provide an insight into some of the problems and advantages of radioactive tracer methods. It has also been apparent that there is no substitute for the valuable motivation of doing experiments rather than mere reading about, a. field of interest. The Ouartz Fiber E1ectroscope An instrument of greater sensitivity than the common gold leaf electroscope is one which uses a quartz fiber. One such instrument is made by the Landsverk Electrometer Company. This inexpensive electroscope has a built-in charging system and provision for placing the sample inside the sensitive volume of the chamber. I t is desirable that an electroscope be equally sensitive to radiation at any position its scale. A measure of this canstsnt semitivity is the electroscope linearity. T o measure the linearity of the Lmdsverk eleetrosco~e nlace a weak (-0.05 PC.) Com . . source in the chamber. (One microcurie (we.) = 3.7 x 10' disintegrations per second.) Charge the electroscope until the fiber is just helaw zero. Start the timer as the fiber crosses zero, and record the time a t each interval of ten on the scale until the fiber orosses 100. Plot the scale divisions versus the time and determine the linear portion of the sede.
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The absorption of beta radiation within the sample, or self absorption, may he determined by measuring the activity of a set of C" or COWsamples of varying thicknesses, each containing the same total activity. The measured activity is proportional to the discharge rate of the electroscope and is given in divisions per minute. A plot of the measured activity versus the sample thickness shows the amount of radiation absorbed in the sample itself. The effect of beta absorption in external matter is determined by placing varying thicknesses of aluminum foil over a Co60 source. Measure the discharge rate corresponding to each thickness of aluminum and plot the measured activity versus the absorber thickness on semilogarithmie graph paper, with the m e a s ured activity on the log scale. (In most laboratory work, the density thickness, in units of mg./cm.l, is used since beta radi* tion absorption is essentially independent of the atomic number of the absorbing material.)
The Geiger Muller C o u n t e ~ An important characteristic of a G-M counter is the G-M plateau. This plateau represents a condition of the G-M tube in which the counting rate (caused by some source of radiation) is relatively independent of the voltage applied to the tube. Place an appropriate sample near the G-M tube and measure the eounting rate (in counts per minute) nt various settings of the high voltage. Be sure to begin with low voltages and move slowly (ahout 25 volt increments) to higher voltages. A plateau region will be found in which the voltage change causes relatively little change in the counting rate. The high voltage measurements must be made with care since too much voltage will ruin the tube. Plot the counting rate versus the voltage and from this graph select an operating point s t about 100 volts above the start of the plateau. A determination of the background reading a t this voltage should be taken and this counting rate subtracted from all subsequent measurements. The absorption of gamma rays may be studied by positioning a strong (-1 @c.) Co.0 source near the G-M tube. Place lead absorbers of varying thicknesses between the tube and source and measure the counting rates corresponding to each thickness. Plot the eounting rate versufi the absorber thickness on semilogarithmic p q e r with the counting rate being plotted as the ordinate.
source, and then place thick pieces of duminum, copper, silver, and lead (atomic numbers 13, 29, 47, 82, respectively) behind the film. Plot the per cent increase in the counting rate versus the atomic number of the backing material. The principle of a beta ray thickness gauge may be demonstrated by a simple experiment. Measure the counting rate of a weak ( 4 . 0 5 uc.) P a P ssmnle. Then determine the countine versus the number of sheets as a calibration curve. Place an unknown number of cards in position and measure the counting rate. Determine the number of sheet8 by reference to the calibration curve.
G-M Survey Meters G-M survey meters give a direct reading of the counting rate of a G-M tube. Far certain isotopes the counting rate is a p proximately proportional to the ionieation rate, so the instruments are frequently calibrated in milliroentgen per hour (mr./ hr.\. ~~-~ The inverse square law of radiation may be studied by suspending a very "hot" (5-10 mc.) source of gamma radiation in the center of a large room, ahout four feet above the floor. (One millicurie (mc.) = 3.7 X 107 disintegrations per second.) M e a s ure the dose rate (in mr./hr.) at various distances (50 to 250 cm.) from the source. Plot the dose rate (minus background), R, versus 1/D2, where D is the source-detector distance. This plot should be linear if R i8 proportional to 1/D2 (and absorption and scatter effects are negligible). A rough assay of a. gamma ray source may be made by measuring the source-to-detector distance and the dose rate a t this distance. The measured dose rate is related to the source intensity by the expression
where R = dose rate in mr./hr. G = a constant for each isotone and is riven in mr./hr./mc. a t one cm. C = the number of millicu~ie~ in the source D = the source-detector distance in cm.
-
From equation (1)
C = RD2/G Some typical values for G are
Obviously if the survey meter is not calibrated correctly, the wror will reflect in the value determined for C. Conversely, a source of known intensity may be used to check and recalibrate the meter by equation (1).
Neutron Activation Stable elements or isotopes often become radioactive when bombarded with neutrons. If silver is bombarded for a short time, two radioactive isotooes are orodueed. The half-lives of the two mav be determined bv measurine the samnle activitv a t various times and plotting the measured activity versus the time on ssmilogarithmic graph paper. Place a radium-beryllium neutron source, oontaining 500 mg radium, in a tank of water or surround it with paraffin to slow the
;he next 15:econd interval record the count and reset the counter. Then beein another 15-second count. ete. After 10 minutes
mic scale versus the time. For greater accuracy, plot the time as that in the middle of the counting interval, or as the time the count started plus 7'/* seconds. The curve shows a definite break where the short-lived activity dies out and leaves the longlived activity. Since the activity decays exponentially, or linearly tts seen plotted on semilog graph paper, the long-lived component may he extrapolated linearly back to zero time. At each point the long-lived activity may be subtracted from the total (or measured) activity with the resulting curve representing the shortAived aetivity. This may he plotted on the same graph and the half-lives may be read directly as the time required far each sotivity to diminish by a factor of two.
Plant Experiment and Autoradiography This experiment allows students to observe the absorption of certain nutrient materials by plants. Both the rate of abaorption and the distribution of the nutrient in the plant can be observed. The nutrient used in this case is disodium phosphate tagged with Pas. Add approximately 80 pc. of Pa*to 100 ml. of water in a 2Wml. beaker. Next, add about 1 g. of disodium phosphate as a carrier for the Pas, and place a small tomato plant, its roots washed free of dirt. in the solution. Observe the count rate with
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VOLUME 35, NO. 6, JUNE, 1958
note the increase. After 2'/, hours, remove the plant from the solution. Since the solution an the roots will tend to smear activity while the plant is being prepared for radiography, the roots are cut off and thrown away. Place the leaves and stem of the plant on a. piece of Sman wrap, fold the Saran wrapover the plant, and place it in an X-ray exposure holder. In the darkroom, place an X-ray film directly under the sample and a flat sponge an top of the samplc. Close the folder and seal with photographio tape to make it light tight. After 6 hours take out the film and develop it.. The film shows distribution of phosphorous in the plant. Much smaller amounts of PSZwill suffice in this experiment if correspondingly longer uptake and exposure times are used.
Ion Exchange Technique This experiment shows a method by which chemically similar ions may he separated in a simple and rapid manner. Prepare an ion exchange column by selecting a glass tube 9 cm. long by 4 mm. inside diameter, and placing in it finely ground "Dowex 1" resin, an ion exchange resin in the chloride form. Then treat the column by passing 10-15 ml. 8.0 N hydrochloric acid through it. After the column has been washed with acid, remove the excess aeid from the top of the column hy means of a pipet. Then add 20 microliters of Ni-Co" solution. This solution consists of about 1 mg. per ml. of each element, plus about 2 r c . of Cow per ml., in 8.0 N hydrochloric acid. In hydrochloric aeid of this concentration, a oomplex ion of cobalt chloride is formed having the probable composition (CoCL--). This complex is held on the positive surface of the anion exchange resin and may he observed as a green ring a t the top of the column. Nickel ions do not form a complex in 8.0 N hydrochloric acid. These remain as positively charged ions, and pass immediately down the column if eluted with 8.0 N hydrochloric acid. As each drop of liquid passes through the column, catoh it on a 1-inch watch glass and test for nickel by adding two drops of dimethylglyoxirne solution rtnd three drops of ammonium hydroxide. If nickel is present in the sample, a red color appears. The nickel is found to concentrate in a few of the samples, indicating a peak in the elution curve. Remove the excess acid from the top of the column, rtnd add water as the elutant. Again catch each drop on a watch glass, dry under s. heat lamp and count the residue in s G M counter. Plot a curve of counts per minute versus sample number. The cobalt appears, reaches a sharp peak and disappears, indicating that the negative complex was broken up by the water, allowing the cobalt ions to pass through the column.
Isotope Dilution Analysis A sample is taken fram a solution of known chemicalconoeutration and the specific activity (counts per minute per milligram) is determined. After diluting with an unknown amount of the same material. the soecific activitv is amin determined. Since
trifuge tube containing about 5 ml. of water. Precipitate the iron with ammonium hydroxide, centrifuge, wash with water, and recentrifuge. Transfer a portion of the precipitate to a weighed metal cup and dry under an infrared lamp. Weigh the sample and then oount in a G-M counter with an aluminum absorber of at least 200 m g . / ~ m .placed ~ above it. Record the weight (W,) and the count (A,). Next, take 1 ml. of the iron solution of unknown concentrstian and add to this 0.2 ml. (200 microliters) of the known solution. Mix the solutions thoroughly and treat the sample in the same manner as the previous one. Record the final weight and counting rate. W, = weight of iron recovered from known solution with recovery factor K , W2 = weight of iron in aliquot of known solution with reeoveFy factor IG W3 = weight of iron in 1 ml. of unknown solution with recovery factor K* A, = counts per minute recovered fram known solution with recovery factor Kt A* = counts per minute recovered fram unknown solution with reeoverv factor K,
K1
S1=K1=E S
- K2
K'A'
(Wz + Wa
=
specific activity of the tracer in known solution
A, = = W2
S,
-
S2
+ W3
specific activity of the tracer in unknown solution
- AIWI +
AtWs ASWI
Since the specific activity of the tracer is the same in both samples, Al/W, = A./We, substituting AlW2 for AIWL,then
s, = I + %w, SI
w,= @ - 1) w, Substitute the weight of iron added in the 0.2 ml. of known solution for Wa in the final equation. The weight of iron (Wa)in 1 ml. of the unknown solution can then he caloulated.
ACKNOWLEDGMENT is not necessary a t any time since it is only the ratio of weight to wlivity whirh l i usrd in t1.e v;rlr.ulutiou. I'rrparv .I 4 o r 1 o n of ftwir v1,loridr r i t l r a known ronrentr:rtion oi inm rmrniuinr- n cmall amount of 1%'' -". .\dd I ml. of t,his solution, containing approximately 15 mg. of iron, to a cen-
We would like to acknowledge the assistance of Donald R. Smith, L. K. Akers, Elizabeth Rona, H. K. Ezell, and other members of the Special Training Division staff.
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