WILLARD F. LIBBY United States Atomic Energy Commission, Washington 25, D. C.
ALL matter consists of atoms, and a t o m consist of spheres of electronic gaseous material, a t the center of each of which is a tiny extremely dense nucleus. The discovery that this is true is one of the great scientific advances of all time. Sir Ernest Rutherford, following such men as Dalton, finally established the model. As one reads the earlier literature prior to the Rutherford discovery of atomic structure, one is impressed by the varieties of models for the atom which were suggested and how few people ever guessed anything like the real answer. All sorts of structures were suggested, but the actual structure was so far from the expected as to be resisted when evidence for it first appeared. Sir Ernest Rutherford showed beyond doubt, however, in his beautiful experiments on the scattering of alpha particles that at the very heart of an atom is a particle of diameter something like one ten-thousandth of the diameter of the whole atom in which resides nearly all of the atomic mass. Lord Rutherford's alpha particles passed through millions of atoms in a straight path without being appreciably deflected and then in one case in a million or more of these atomic penetrations bounced as though they had struck a body carrying the whole atomic mass. There was no other way to explain the results than the way in which Lord Rutherford interpreted them-bizarre and difficult as they were, they were accepted and the atomic structure established. We learned then that the properties of nuclear matter are very different from the properties of ordinary matter, and that atoms of different nuclear properties can have the same electronic structure. Since most chemical properties, and most physical properties with the exception of mass, are derived entirely from the electronic structure, the chemical and physical behavior of isotopic materials-that is, materials of different nuclear characteristics-are essentially identical. Atoms with the same electron structure are isotopic. Now it is important to teach this as a point of principle in college chemistry. Nothing is so striking a demonstration of the reality of this consequence of atomic structure than the examination of heavy and light water t o see how closely similar they are in their behavior, even though the hydrogen atoms in the heavy water have twice the mass of those in ordinary light water. Another striking example is the chemical behavior of radioactive materials as compared to non-radioactive materials, and the observation of how impossible it is in an ordinary laboratory experiment t o in any way separate the non-radioactive from the radioactive atoms Based on a paper presented before the Fourth Chemistry Institute for College Chemistry Teachers, The University of North Carolina, Chapel Hill, June 13, 1957.
of a given element. Radioactive sugar tastes and acts like ordinary sugar, radioactive copper like ordinary copper, and so on. While everyone knows that these facts are taught in college science courses, there is a possibility that they are not being sufficiently emphasized. The whole new atomic age turns on nuclear properties in one way or another. The electronic properties of matter continue to be as important as ever, but all the new things of the atom, to use the popular parlance, are nuclear in nature. The energy locked inside the atomic nucleus is the source of atomic power, and the awful magnitude of atomic explosions is a nuclear phenomenon. A bar of metal which looks like a piece of iron can, when laid near another similar bar, become white hot and vaporize itself and in fact explode. Nothing in our normal experience with ordinary matter prepares us or equips us for this fact. Nothing in the libraries prepares us for this. All human experience throughout the ages until the atomic age was born has been restricted to the outer electronic portion of the atom. Therefore, college teachers and particularly teachers of college chenjstry have an enormous task of acquainting their students with the facts of nature, especially those facts which are not encountered in ordinary experience. It is similar to the task the teacher in a school for the blind faces-to explain in understandable terms a whole world of reality of vital and immediate concern t o the individual. Therefore, isotopic tracers ought t o be more widely used as teaching tools in college chemistry, radioactive isotopes ought to be an everyday experience for college science students, and we should have radioactive reagents on the shelves in the ordinary chemistry laboratories so the students can be free to use these radioactive reagents for their regular experimental work in all the chemistry courses. Such a practice would increase the interest of the students in science and certainly would increase their general knowledge of the nuclear properties of matter such as radioactivity-a point that has become vital in many aspects of our everyday lives. USE OF GEIGER COUNTERS FOR ABSOLUTE ASSAY OF RADIOACTIVITY Most people know that radioactivity can be detected and measured with a Geiger counter. What is not so generally known, however, is that the absolute number of radioactive atoms can be determined in this way. It is true that the requirements for this absolute assay are harder of fulfillment than those for the mere qualitative detection of radioactivity, but they can be met and used by beginning students. In this way the power and range of the isotopic tracer technique is extended and the possible applications significantly increased. JOURNAL OF CHEMICAL EDUCATION
One of the conditions for routine absolute assay is that the sample being measured should be placed cylindrically around the wall of the Geiger counter, for under these particular geometrical conditions the interposition of absorbing foils between the sample and counter results in an exponential decrease of the count rate, and this makes an absolute determination possible. I n other words, a plot of the logarithm of the count rate due to the sample against the thickness of the absorber gives a straight line. This is true for essentially all known beta radiations to a fair degree of approximation. The reason is that the cylindrical geometry controls the very large effects of beta ray scattering. The beta rays, of course, are electrons themselves just like those making up the bulk of atomic matter and as a consequence there is a large tendency for them to he scattered because of the equality of mass. When an alpha particle which weighs 7400 times as much as an electron passes through ordinary matter, the only collisions which can deflect it are those with the atomic nuclei, but in the case of the beta radiation the situation is different. For this reason the most popular types of Geiger counters in use at the present do not give exponential absorption unless very special controls are used, and these controls are too complicated for heginning classes. However, Geiger counters are easy to use with a cylindrical placement of the sample so they can give absolute assays in a simple and routine way. The reason that the exponential nature of the absorption is necessary is that it allows a good correction to he made for the self-absorption in the sample itself. The exponential character of beta radiation absorption curves under certain conditions has long been known, and it was shown some years ago that if the absorption of the radiation is exponential, the total self-ahsorption in a solid or liquid sample can be calculated irnrnediately. If the absorption coefficient for the radiation in the material of the sample itself be 1 / A , then the effective thickness of the sample considered to have no absorption is just A, and it has been shown that the value of A is determined by the energy of the beta radiation, the formula for A for aluminum being:
where E is the upper energy limit of the beta spectrum in Mev. Therefore, this relation can he used to calculate A, from the table of isotopes given in a standard chart or reference hook, from which the values of E can he obtained. Thus one is relieved of the necessity of measuring the absorption curves in each case. For materials other than aluminum, A is to he calculated by taking the X for the new material to he inversely proportional t o 1 plus the mean atomic weight of the material divided by 100 (see Fig. 1). For copper, for example,
F i g " . .
llbaorption cume. in A1
A1 Hdf-thickness Substrate Pb
Cu A1 CaC08
(mp./cm.9 4.9 4.8 5.0 4.8
sample and for the scattering outward of radiation initially directed into the body of the sample rather than towards the counter. The ratio of 47 to the mean solid angle we define as G (see equation (3) below and Figs. 2 and 3), and with this the count rate expected will he:
1. A,
1, >,u
1. As
v
VOLUME 34, NO. 12, DECEMBER, 1957
Cad5
Erna.. = 255 kev.
A G
With this methodology, the effective absorption of the air between the sample and the counter wall and of the counter wall itself can he taken into account and the only remaining corrections necessaly are for the geometry, that is, the mean solid angle subtended by the sensitive volume of the counter at the position of the
1.
thickness of the air between the sample and the counter wall = reciprocal of the absorption coelficient for the radiation in the air = thickness of the counter wall = reciprocal of the absorption coefficient of the radiation in the counter wall material = thickness of the sample itself = reciprocal of the absorption coefficient of the radiation in the material of the sample = the sample area. in cm.' = geometrical constant = absolute specific activity in disintegrations per min. per =
mp.
R is given m counts per min.
Actually, the G values as used in this formula have in them not only the geometry factor but also an averaged correction for the back-scattering effect, so they cannot he calculated solely from the dimensions but must he
C W T I C T TO INNER COLD SUIIFACS OF MYLAR
1 LUCllE MEAD
I
W L D COATED MYLAR PLASTIC 1 2 - ~ l n ? )
ME*"" *,Re TO WHICH 1 MIL. COUNTER WIRE IS SPOT WELDED
Figure 2.
Thin Wall Flow Typ. Geigcr counter
determined by measurement of some standard such as ordinary KC1 whose natural radioactivity can be used for this purpose. Another equation which explicitly introduces the back-scattering term uses the true geometrical G. The G for equation (2) is about 10-15% smaller. Equation (2) works quite well, giving results for a wide variety of beta ray standards acrurate t o 5% or 10% under most conditions. For high energy beta rays where X is large as compared t o the size of the crystals constituting the sample, the G for a smooth sample surface is to be used. For soft'beta rays, however, it is necessary to use a factor which is larger, because the surface consists of a layer of randomly oriented crystals for which the penetrating power X of the soft beta rays is small compared to the size of the crystals. Of course for liquids or very finely divided crystals the hard beta or smooth surface G should he used even for soft beta rays. For normal crystalline samples as ordinarily prepared, however, G is too low and a larger value which applies to rough surfaces must be used. This number obtained by direct experimental measurement is 1.6 times larger than the smooth surface or hard beta G. In other words, we use a G determined by means of a hard beta standard-such as potassium, in a salt such as KC1, multiply it by 1.6 and thus obtain the G for soft beta radiation. A good dividing line between soft and hard betas is a t a X value of 10 mg./crn.% which corresponds to an energy according t o equation (1) or 0.32 Mev. The G for smooth sample surface (hard betas with A> 10 m g . / ~ m . ~and ) counter long relative to sample is calculated by equation (3).
ing by the meau life of the material (the meau life is the half-life divided by 0.693). For NaZ2,for example, with a 2.6 year half-life, the factor would be 2 million for d.p.m. In other words, it takes approximately 2 million NaZ2atoms to give 1 d.p.m., and if a precipitate of sodium carbonate showed an activity of one thousand d.p.m/mg. it would contain 2 billion NaZ2 atoms in each milligram of the solid. All of this can be done in a matter of a few minutes, and it will be observed that the actual physical measurement is very little different from the usual measurement made for determinations of relative values of radioactivity. The only special point is to control the distance between the sample and the counter wall quite accurately. This can be done (see Figs. 2 and 3) by using machined plastic cylinders which surround the counter, leaving some space between the counter wall and the cylinder. On this surface is placed the filter paper on which the sample rests. The area of the sample exposed can be determined by laying over the precipitate a rubber mask in which a hole of known area is cut, or alternatively the mask can be laid on the inner surface of the sample cylinder and the solid sample poured into the hole and then smoothed out with a spatula. It is usually best, if the X value is not too large, to use samples for which I,, the thickness of the sample, is several times A,. With this provision it is possible to make an absolute determination of the specific radioactivity of a solid in a matter of a very few minutes, which is good t o 5y0 or 10%. Abaomtion Data Reciprocal A." of ab-
Mazimum
from counter wire to inner counter wall from counter wire to inner sample surface (0.85 is the factor to correct for baek-scattering)
= distsnoe r = diatmee p
energy o f beta speeZso-
lope
tmm (Mev.)
C L P .155
With this methodology (the appropriate use of equations (2), (3), and (4)) it is possible to take a material of known chemical composition and calculate the count rate to be expected from a known absolute specific activity v , or to convert an observed count rate, R, immediately into an absolute specific activity to an accuracy of about 5% or 10%. Of course, the absolute content of radioactive atoms in the solid can be calculated from the absolute specific activity by multiply-
HalfLife
Absorbing material
Half- sorption thiekcoefiness ~i&t (mg.1 (mg.1 em.') em.')
5,600 yrs. A1
Mylt plastic
87 days A1
SS
0.167
CIS
0.716 320,000 yrs.
Mylar
AL
tiu Sn
Pb
164 days Al .25 X 10"l
yrs.
A, =
Half-thickness 0.693
JOURNAL OF CHEMICAL EDUCATION
The table and figure show various features of this method. Figure 1 shows absorption curves for Ca"the very useful radioactive calcium isotope-under various conditions. Similar results are obtained with radiochlorine, ClS6,radiosulfur, F5, and radiocarbon, CT4,a11 extremely valuable conveniently long-lived isotopes in the chemical lahoratory. The table gives the data for the penetratiug power of these isotopes. LABORATORY EXPERIMENTS
The following five experiments illustrate the utility of the methodology just discussed. The radioactively labeled reagents required are: 6 N H&*04 (108 disintegrations per minute SU/ml.)
chloride was 50% complete in the case of the fresh AgC1, then would be;
6 N HCI* (106 d.p.m. CI'Yml.) Solid Na3C*Oa(loJ d.p.m. CL4/mg.) Analysis of a Solution for SO4-- Both Radiochemically a n d Gravimetricelly To 100 ml. of approximately 0.20 M Na~S04add accurately 0.10 ml. of 6 N HB*04,stir, heat, and add excess hot 1 M BaCL solution to precipitate BaSO,. Filter, dry, weigh, grind, and plare smoothly in lC-~rn.~ area in l-mm. thick rubber mask on inner surface of plastic cylinder which clips around thin wall (2 mg./cm.*) flow counter of 2 in. diameter with sample ~urface 4 mm. from wall. With these dimensions, the hard beta or smooth surface G = 2.20. Calculnte osao4, convert to a for SO1--, and calculate amount SO.-in unknown from dilution. ~. For example, if the count rate is 100 c.p.m. above background, then from equation (2):
If the filter paper had an area of 20 cm.' the count rate should be:
Therefore, the fraction exchange will be 0.50 R/1180. Caleulstions for the two precipitates will reveal the great decrease in exchange far the aged one, presumably due to crystal growth and consequent decrease in total surface area. Following Radiochemically t h e Precipitation of Basic Carbonates
Since the precipitate is thick with respect to 3.0 mg./cm.', the A value for SaS in BaSO,, the last term in equation (2) has been
omitted.
Therefore,
This indicates a 66.6-fold dilution (106/1.5 X 10').
To excess hot dilute CrtCl., AlCL, SnC4 solut,ione add in each 0.1 g. NseC'O. dissolved in hot water. Filter, dry, and count in 10-cm.' area hole, in rubber sheet inside plastic sample cylinder. For carbonates the count rates should he case
In 100
ml. of unknown there is then 65.1 times as muoh SO1-- as in 0.1 ml. of 6 N &SO4. This weight of SO,--is 1.89 g., ( 6 X 0.1 X 0.048 X 65.1). The corresponding weight of BaSO, precipitate should be 4.65 g. D e m o ~ t r a t i o nof Crystal Growth a n d Consequent Decrease i n Surface Area of Freshly Precipitated AgCl Precipitate approximately 1 g. of AgC1, decant the supernatant liquid, wash, decant and add immediately 10 ml. 6 N HCI* in 100 ml. of distilled water, shake five minutes, filter on Biichner filter, wash well, dry, weigh, and count by placing filter paper inside plastic cylinder around thin wall counter. Repeat after allowing AgCl to stand five minutes in its precipitating solution before adding HCI* solution ss described. If the exchange between the crystalline chloride and dissolved VOLUME 34, NO. 12, DECEMBER, 1957
Comparison with observed rates will show that the precipitatm are essentially CaCOa, Al(OH),, and Sn(0H)x.
Assimilation of C"by Growing Plants Transplant about 20 g. of full-grown plants of some luxuriant variety such as barley to a small dish of nutrient solution. Place beside them a shallow dish containing 1 g. of Na&*03 in a pile beside which but not touching is a spot of concentrated H.S04. Put a bell jar over the assembly, move into the sunlight, shake the Narc01 powder into the acid or vice versa and allow to stand two or three hours. Remove the assembly outdoors, open, and bring the plants inside. Count and determine the amount of carbon taken in either by growth or photosynthetic exchange during the experiment. (Note: In order to count accurately, it would be well to freeze the plant with liquid nitrogen or dry ice and grind cold in a mortar and pestle. Then the powdered plant can br rounwd in an absolutr i . a y ns pn,vioosly &~cril,ed.) Ifrxcesr phnt mntrrid over 30 cm.' ares gavv 500 c.p.m., IIIW thcabrolure 3prrifir nctivir? mnkl i~ccnlculntrdfrom
from this o turns out to be 39 d.p.m./mg. which corresponds to 0.039 mg. Na&*O,/mg. or 0.0047 mg. C* assimilated per mg. of plant. The Degree of Uniformity of Carbon Assimilation in Thne-hour Growth of Plants Take the barley or other plant material from the previous experiment, extract with hat water, count residue, and compare with the count rate for the unextracted materid. A considerable decrease probably will be observed due to the slowness with which the large cellulose type molecules are synthesized. If a quantitative measure is desired, burn original plant and extracted material separately, collect the combustion gas in NHIOH and precipitate CsCOa for absolute counting. The rates should he easily mtasurable in both eases, for even the fibrous plant material will gather eansidersble carbon photosynthetically in three hours' exposure as described. CONCLUSION
The student and instructor both will see many ways of using the radioactive reagents to check and elucidate points in the standard experiments in general chemistry and qualitative and quantitative analysis. Their ready availability together with a rugged cylindrical thin wall counter make a tempting combination to the abler and more interested students even though they may be beginners in the subject. The ideas and principles involved are readily assimilated. These innovations may help interest the students in science so that more students will turn t o chemistry or physics or engineering. Even for those who do not do so, some of the essential facts about radioactivity and radiation may he rememhered-a very desirable end in itself as our whole society moves further into the atomic age. They should help bring about a general understanding of the new nuclear world and open up for our whole society the potentialities of the Peaceful Atom. Isotope uses alone will repay many times over the total costs for all military and peaceful atomic developments -for they are nearly doing so now. Isotope teaching will hasten the day of fulfillment and maximum benefits, so let us all put radioactive sulfuric and hydro-
chloric acids and radioactive sugar and radioactive ethyl alcohol on our reagent shelves tomorrow. Experiments such as the ones described require a special G e i ~ e rcounter, but these can be readily assembled as shown in Figures 2 and 3 (described in "Large Thin Wall Geiger Counter," T. T. SUGIHARA, R. L. WOLF GANG, and W. F. LIBBY,Rev. Sci.Inst., 24, 511(1953)). The counter operates on a mixture of helium and nbutane gas (1.3% butane by volume) a t atmospheric pressure, the gas flowing through slowly. The voltage required is about 1200 volts and any ordinary electronic equipment normally used with Geiger counters can be employed to supply the voltage and to amplify and record the pulses. The radioisotopes required can be obtained in the amounts needed (a few or a few hundred microcuries) and without the necessity of involved paper work through a General License. The Atomic Energy Commission has determined that small quantities of radioisotopes are not particularly hazardous t o use in normal laboratory and research operations provided, for example, they are not taken internally. At one time these quantities were referred to as "exempt quantities" in the sense that no formal approval was needed for their purchase. With the formalization of the entire system of byproduct licensing as one result of the Atomic Energy Act of 1954, the term ''exempt quantity" was dropped and a new term "General Licenee" was substituted. It is still not necessary for the purchaser to obtain formal approvdl from the Atomic Energy Commission to acquire, for example, ten microcuries of chlorine-36, or 100 microcuries of iodine-131 or 500 microcuries of sulfur-35 or 500 microcuries of carhon-14 or any combination thereof, provided, first, that he has no more than this quantity on hand a t any one time and, second, that he agrees not t o use them internally in any way. These restrictions are, as you can see, primarily for the purpose of satisfying certain health and safety requirements. I n return, the Atomic Energy Commission considers it not necessary t o inspect the facilities of the user. For those who are interested further, the details are presented in Atomic Energy Regulations on the Licensing of Byproduct Material (Title 10, Part 30, Section 30.21) copies of which are available. It is quite permissible t o order additional quantities of radioisotopes as needed either from the Atomic Energy Commission directly or authorized commercial suppliers with no more than a statement that the isotopes will be used under the General Licensing provisions. It is only in the larger quantities that the Commission feels it necessary to require an application for what is known as a "Specific License," for here the possibility of radiation hazards, for example, increases. Specific Licenses are granted for amounts up to tens of thousands of curies.
JOURNAL OF CHEMICAL EDUCATION
