Putting Tagged Atoms to Work JOHN A. T I M M , Simmons College, Boston, Massachusetts
T
RANSMUTATION, or the process of changing one element into another, was little more than an alchemist's dream twenty-five years ago. It was known to occur spontaneously in the disintegration of naturally radioactive elements but this process is beyond the control of man. A radioactive element disintegrates a t a constant rate which is determined by the nature of the nuclei of its atoms. These nuclei, protected from the shocks of collisions with other atoms by their planetaq-electron sheaths, remain unaffected by high temperatures. To effect transmutation of a stable nucleus some means had to be discovered by which the planetary-electron sheath could be penetrated and a blow struck with sufficient energy. For this purpose none but subatomic particles were small enough and none save the alpha-particles, emitted a t high velocities from naturally radioactive nuclei, had sufficientkinetic energy. TRANSMUTATION
In 1919 Lord Rutherford announced the first successful transmutations. By bombarding the atoms of some of the elements of low atomic weight with alphaparticles he succeeded in knocking protons from their nuclei. Since the number of protons in the nucleus of an atom determines the element to which the atom belongs, the loss of a proton indicates that a transmutation has been effected. Alpha-particles are nuclei of helium atoms and are composed of two protons and two neutrons. These are ejected from the nuclei of certain naturally radioactive atoms with velocities of the order of one-fifteenth of the velocity of light, fast enough to cover the distance of the earth's circumference thirty times in a minute. To give these doubly charged particles such a velocity would require a difference in potential of some 2,500,000 volts, far beyond attainment a t the time Rutherford announced his first successful transmutations. The use of alpha-particles from radioactive nuclei to accomplish transmutation was subject to severe limitations. These particles are ejected in all directions from a radioactive source. The radioactive atom cannot be persuaded to take careful aim a t some target atom before letting the alpha-particle fly. Hence the efficiencyof the process is extremely low, since atomic nuclei are very small, compared with the space between the planetary electrons and the space between atoms. Less than one in every 50,000 atoms found their mark in Rutherford's experiments. Further, since both the alpha-particles and the nuclei of the target atoms are positively charged, the forces of repulsion must be conquered before transmutation may take place. Since protons and deuterons, nuclei of ordinary and
heavy hydrogen atoms, respectively, bear single positive charges, their use as agents of transmutation is more efficient. Unfortunately, these nuclei are not among the products of radioactive disintegration. Hence, they must be given sufficient kinetic en6rgy by acceleration through a sufficiently large difference in potential. The production of differences in potential as large as several million volts with which to accelerate subatomic particles presented interesting problems to experimental physicists. Several snccessful machines are in operation which produce such voltages directly. Of this type, the electrostatic generators, designed by Van der G r a d ( I ) of the Massachusetts Institute of Technology, produce potential differences of from three to five million volts. One of the most ingenious, efficient, and successful machines is the cyclotron of E. 0. Lawrence (2) of the University of California a t Berkeley. Instead of producing high voltages directly, protons, deuterons, or helium nuclei are accelerated a hundred or more successive times each through a difference in potential of the order of 100,000 volts. A stream of hydrogen gas is ionized into protons (or deuterons) and electrons through contact with an electrically heated iilament. This occurs a t the center of an evacuated chamber, cylindrical in shape and between the poles of a powerful electromagnet. Here they move in circular paths in the magnetic field and in the space within two hollow D-shaped electrodes. These look as if they had been formed from a gigantic, old-fashioned pill-box cut in half vertically. The electrodes are connected to a source of current which alternates their polarity a t intervals equal to the time required by a proton to describe a half-circle within a "Dee." As each proton enters the gap between the "Dee's," it is attracted to the electrode of opposite polarity. Within this electrode and in the field of the magnet i t describes a circular path. When it reaches the gap the polarity of the "Dee's" changes and the proton gains in kinetic energy as i t travels across the potential difference of the gap toward the other electrode which now is negatively charged. Since its velocity is greater when it enters this electrode, it moves in a circular path of greater radius. However, the time it takes to complete this greater distance is the same because of its increased velocity. Hence, it arrives a t the gap a t the instant the polarity again changes. After a hundred or more accelerations across the gap its velocity is so great that its path follows the circumference of the "Dee's." At this point a negatively charged plate
deflects the proton-stream through a thin foil window into the target of the element in which transmutation is to be induced. There are now nearly 20 cyclotrons in operation in this country. By far the largest will be the one under construction a t Berkeley. This instrument will use a chamber 20 feet in diameter with 1,000,000 volts potential diierence across the "Dee's." I t will accelerate protons to a kinetic energy of 100,000,000 electroc-volts. Neutrons have proved to he very effective agents of transmutation (3). Since they are without charge, they cannot he accelerated in a cyclotron. However, neutrons are easily generated by bombarding beryllium powder with alpha-particles from radon. Their &ectiveness in transmutation is due to the lack of any force of repulsion between them and the target nuclei. Oddly enough, neutrons, moving a t low velocities, are, in general, more effective than faster ones. TRANSMUTATION
Transmutation reactions may be summarized by equations. The nuclei involved are designated by the symbols of the corresponding elements. The atomic number, or the charge on the particle, is placed below and to the left of each symbol; and its mass number, or isotopic mass to the nearest whole number, appears above and to the right. Thus, when Rutherford bombarded nitrogen with alpha-particles, or helium nuclei. the isotope of oxygen of mass numher 17 and protons were formed: ,Nu
-
+ sHe4
sixteenth, etc. The full-life of any radioactive element is always infinite and, therefore, has no significance. The shorter its half-life period, however, the greater its radioactivity. The discovery of the Curie-Joliots of artificial radioactivity was followed by the identification of many radioactive isotopes, until today, a t least one such is known for each of the elements in the periodic table (4). A given kind of radioactive atom may he prepared by several different transmutation reactions., Thus, radiophosphorus may be prepared as follows:'
Radioactive sodium results from several transmutations: ,dlw
+ on1 +
--
,,Naa
+ rHe4 + ,HL
I ~ M ~ "a'
11Na'*
ttNaa3
1tNa4'
+ ,HP
+ ,H1
However prepared, radiosodium disintegrates as follows : ,,Naa+
,,,MgP+ -,eo
The usefulnessof a given radioactive isotope is in large measure determined by its half-life period. If this he too short, the isotope is too unstable to be useful. In Table I, the properties of radioactive isotopes of the more common elements are listed. Under "Source" in this table are listed the symbols of the target elements followed by the bombarding particles in parentheses.
+ ,HL
,OL'
In such equations the atomic numbers and the mass numbers balance.
THE PROPERTIES OR RADIOACTIVB ISOTOPES OR COMMON ELEMENTS*
ARTIPICIAL RADIOACTIVITY
In 1934, the Curie-Joliots discovered that the products of certain transmutation reactions are radioactive. When aluminum is bombarded with alpha-particles, neutrons are emitted and a radioactive isotope of phosphorus is formed: ,sAl2'
+ rHe+
+ on'
,sP80
These phosphorus atoms enter into a radioactive disintegration by emitting positions, or positive electrons: Thus, for the first time radioactive isotopes of the elements of low atomic numbers were produced. As in the cases of naturally radioactive elements, the rate a t which artificially radioactive atoms disintegrate is proportional to their number and to their inherent tendency to disintegrate. Each type of radioactive atom is characterized by a half-life period, the time taken for half of the atoms, present a t a given time, to disintegrate. Thus, the half-life period of radiophosphorus, ,sP30,is 2.5 minutes. At the end of five minutes, one-fourth of the original numher will remain; after 7.5 minutes, one-eighth; after 10 minutes, one-
,zMg" llSiJL P a ..A" . ,CIS4 ,9K4a soCa46 %sFeSg ~
e -,en ,en
.-
-.eO
+,en
-tea -leu -,en
10.2 min. 170 min. 14.5 da. 80 da. -~~~~ 33 min. 12.2 hr. 180 da. 40 da.
* For a more complete list see Mod. Phyr.. 9 , 359 (1937).
Mg(d) Mg(d). Alb). %(n) Si(d), P(n), S i b ) P(d), S(d), P(n), S(n), Cl(n) Clfnl ~ ( a )~, ( d ) cl(n) : K ( d . Cab), K(d), S c b ) Ca(n), Ca(d), Sc(n)
.
LIVINGSTON AND
BETHE,Rev.
DETECTION
Since the products of a transmutation, including those of radioactive disintegration, are moving in general with high velocities, they are able to strip
electrons from the atoms of any gas in their paths. a more laborious process than the detection of radioCharged gas ions are produced by these encounters and activity. Yet i t must be resorted to when radioactive the gas becomes a conductor of electricity. If such isotopes of long enough half-life periods are lacking. ionization occurs in a dust-free atmosphere, supersaturated with water vapor, liquid water-droplets con- APPLICATIONS OF THE PRODUCTS OF TRANSMUTATION dense on each gas ion. The track of the product of Some idea of the many problems which have been disintegration becomes visible and can be photographed. studied by the use of tracer isotopes may be obtained This, in brief, is the principle of the Wilson cloud- from the chart on the following page. chamber. CHEMISTRY More frequently used, however, is the Geiger-Miiller counter. This instrument consists of a cylindrical In the field of chemistry, tracer isotopes 'were first cathode and a wire anode placed a t the axis of the used by. Paneth and Hevesy as early as 191Z (7). cylinder. Both electrodes are sealed in a glass tube Smce a t that time induced radioactivity and even filled with a dry gas a t a pressure of about 0.1 atm. artificial transmutation were unknown, this pioneer The product of transmutation enters the counter work was limited in its scope by the few naturally through a thin window and creates ions by stripping radioactive isotopes of stable elements available. Thus electrons from the gas-atoms in the tube. These ions radium D, radium E, and actinium C", isotopes of are accelerated sufficiently toward the electrode of lead, bismuth, and thallium, respectively, were the opposite charge to ionize additional gas-atoms. The only tracer isotopes that could be used. Nevertheless, increased conductivity of the gas owing to this ion- many studies were made of the mechanism by which formation causes a momentary discharge in the tube atoms are exchanged in chemical reactions, on the rate which can be magnified in the external circuit by a t which lead atoms diffuse in lead crystals, on oxidavacuum-tube amplification. The amplified current tion-reduction couples and on the metabolism of is sufficient to produce a "click in a loud speaker, a animals and plants. "count" in a counting device, or to move a stylus on a In the field of analytical organic chemistry the recording drum. Electrons, both negative and positive, problem of the quantitative separation of organic comemitted by the nuclei of radioactive atoms, can be pounds from complicated mixtures in biological mareadily detected by this device. terial is extremely d i c u l t . In the analysis of a protein the quantitative isolation of the amino-acidPROPERTIES OF TRACER ISOTOPES hydrolysis products presents a problem which is It must be kept in mind that isotopes are atoms of practically insoluble. It is possible to isolate all of a the same element which differ only in the number of given acid but have the product impure, or to prepare neutrons in their nuclei. The charge of the nucleus some of the pure acid. Here the use of tracer isotopes of each isotopic atom is the same, and so also is the comes to the rescue. If x grams of glycine, which connumber of planetary electrons., Hence, in their tains an a per cent excess of NIS, are added to the chemical properties, isotopes are identical. Most of mixture of amino acids obtained by the hydrolysis of a the elements are mixtures of isotopes. However, the protein, the added material cannot be separated from relative amounts of each isotope are the same in all the glyciue in the hydrolysis mixture. If a pure sample samples of the element. The fact that these propor- of glycine is then extracted from the mixture which tions have remained constant and that no concentration contains a b per cent excess of NI5, the amount of of one isotope a t the expense of the others has occurred glycine in the original mixture (y) is given by the (5) is excellent evidence that the isotopes of an element equation : have the same chemical properties. b If radioactive atoms are mixed with non-radioactive y a - b atoms of the same element they will thereafter be inseparable, regardless of the number of chemical changes Since N13 is not radioactive, the relative proportion of through which the element passes. But throughout this isotope mixed with the more abundant N14 must the course of these changes the activities of the radio- be determined in a mass-spectrometer. active isotope, and therefore of the non-radioactive The synthesis of organic compounds containing atoms of the element, may be followed by Geiger- tracer isotopes has presented many new problems. If Muller counters or other suitable means. Hence the the tracer atoms are non-radioactive or relatively lougname, "tracer-atoms," has been given to radioactive lived, the problem may be solved by the traditional atoms used in this way to trace the course of an element methods of synthetic organic chemistry. Radiothrough changes in all ordinary chemical, physical, phosphorus with a half-life period of 14.5 days belongs and physiological systems. to this type. On the other hand, to incorporate radioIf a process is known by which the stable isotopes of carbon (half-life = 20.5 minutes) atoms in organic an element may be separated (6),i t is possible to en- molecules of any complexity requires that new faster rich a sample of an element with one of its isotopes. methods of synthesis be developed. Cramer and Thereafter, the presence of this enriched sample may be Kistiakowsky (8) have been able to synthesize lactic detected by means of a mass-spectrometer. This is acid in less than two hours from elementary carbon. "=_
+
0
PRODUCTION
MATERIALS
-
Cyclotron
-
Electrostatic Generator
I Isotope Separator
Natural Radio-Substances
Counters
A r a c i a l Radio-Substances
Electroscopes
Stahle Isotopes
Electrometers
Neutron Beams
Photographic Films
Photon Beams
Mass Spectrometers
Ian Beams
Standards
Electron Beams
F CHEMISTRY
ANIMALAND Hu-
Reaction Rates
MAN
Surface Studies
Metabolism Endocrine Physiology Bone and Tooth Formation
Metabolism
I f
Photosynthesis
INSECTS Metabolism
L
Disease Transmission
Turbine Flow
C
Internal Heat of Earth
Diffusion Phenomena
Lub. Film Thick-
Order-Disorder Studies
Analysis
I-
Small Solubilities
Low Vapor Pres-
Respiration Fluid Transport
t
Enamel Thickness
Ages of Rocks and Minerals
-
Sedimentation Pracesses
I. Well Loggmg
Equilibrium Measurements Completion of Peri-
L
Mineral Identification
GENEKAL RADIATION THERAPY Radium and Radon High Voltage XRays Cathode Rays
-
Fast Neutrons
SELECTIVERADIATTON THBRAPY
-
Slow Neutrons
PROTECTION
RADIOGXAPHY Gamma-Ray X-Ray Super-Voltage XRay
-
By far the most important elements in organic chemistry are carbon, hydrogen, oxygen, and nitrogen. We have mentioned the relatively shortlived C". The other radio-carbon, C14, has a far longer half-life of about 1000 years and can be produced by carbondeuteron or nitrogen-neutron transmutations (9). Deuterium, or heavy hydrogen, is available commercially for tracer work but is not radioactive. Tritrium, I H ~is, radioactive but its feeble radioactivity makes i t a poor tracer-isotope. No isotopes of sufficiently long life are available for oxygen and nitrogen. The stable isotope, N16,will be available soon commercially. In gravimetric analysis the radio-elements are so easily detected that i t bas been possible to test the completeness of a precipitation, the efficiency with which a precipitate may be washed, the aging of a precipitate, and the error in an analytical procedure. SYNTHESIS OF ATOMS OF UNKNOWN ELEMENTS
In 1934, four elements of atomic numbers 43, 61, 85, and 87 were unknown. The last of the alkali metals, 87. was detected by Mlle. Perey (10) as a member of the actinium series of radioactive elements. Element 43 in Group VIIA with manganese was prepared as a product of the transmutation of molybdenum (42) under neutron or deuteron bombardment (11). The halogen of atomic number 85 was prepared by bombarding bismuth with alpha-particles (12). Finally element 61. a rare earth, was formed by the transmutation of neodymium by deuterons (13);but because of the similarity in the chemical reactions of the rareearths, its properties could not .be determined. The properties of elements 43 and 85 have been determined although only about grams have been produced. Since these isotopes are radioactive, the part that they play in various chemical reactions can be followed. With this knowledge it is possible to predict in what types of minerals they may be found and the steps to be taken in their extraction. METALLURGY
formation (14) obtained from these studies is the relatively high rates a t which complex reactions occur in the body. Esterificatiou and hydrolysis, hydrogenation and dehydrogenation occur continuously in fat metabolism. Fatty acid chains are shortened or lengthened. Proteins are hydrolyzed and new ones synthesized. Amino acids lose their alpha-amino groups and to other substances such groups are added. Using radioactive iron, Whipple, Hahn, and their colleagues (15) have investigated iron metabolism in normal and in anemic animals. Radioactive iodine (16) is being used to study the functioning of the healthy and diseased thyroid gland, which plays such an important role in the regulation of growth and body beat. Synthetic vitamin B1, containing radio-sulfur, has been used to study its storage, utilization, and excretion (17). It has been found that ten per cent of this vitamin is used up daily in the human body. PLANT METABOLISM
Kamen, Ruben, and their collaborators (18) have studied the mechanism of photosynthesis in plants using carbon dioxide in which was incorporated the short-lived radiocarbon, C". They propose a tentative mechanism for photosynthesis in which neither f o m aldehyde nor organic acids of low molecular weight function. The first step involves the addition of carbon dioxide to a compound of large molecular weight. RH
+ CO*
-
-
RCOOH
This is not a photochemical process. The second step RCOOH
+ H1O
RCHSOH
+ O1
occurs only in the sunlight. The upward path of minerals in plants (19) has beeu studied by feeding radio-tracers of potassium, sodium, phosphorus, and bromine to plants rooted in solution or sand media. The amounts of active material in different sections of bark and wood were determined. These experiments seem to show that a t least the greater proportion, if not all, of the minerals rise through the wood rather than the bark.
The diffusion of metal atoms withm crystals and across crystal boundaries is of the greatest importance to metallurgy. The annealing of ingots and forgings, the process of age hardening, quenchmg and precipiRADIATION THERAPY tation hardening, metallic diffusion coatings, oxidation, and the preparation of alloys by annealing of mixed Several elements in the diet find their way to and are metal powders are a few of the many phenomena in localized in certain tissues of the body; e . g., iodine in which diiusion plays an important role. By studying the thyroid gland and phosphorus in the skeletal the diiusion of tracer atoms through a homogeneous structure. It is possible, if these parts be diseased, to medium it is possible to determine the number of single subject them to irradiation by administering radiomoves made by an atom in a given time. This evi- active isotopes of these elements. Thus, J. H. Lawdence provides the means of testing any theoretical rence and his collaborators (20) have made use of the model for the mechanism of diffusion. fact that phosphorus is concentrated in the bone marrow in humans by treating cases of leukemia with ANIMAL METABOLISM radio-phosphorus. The results with chronic leukemias By feeding animals with synthetic fats and amino have beeu encouraging. Irradiation of certain types of cancer by fast neutrons acids in whose molecules tracer atoms are present, it has been possible to investigate the occurrence and the seems to offer certain advantages over X-radiation. rate of metabolic reactions. The most striking in- Since slow neutrons are readily captured by boron and
lithium and the resulting transmutations liberate energetic alpha-particles, attempts are being made by Kruger and others (21) to localize these elements in tissues to be irradiated. One cannot help being impressed by the rapidity with which discoveries in the field of nuclear physics have been put to practical use. Ten years ago, a cyclotron was a luxury which few universities could
afford. Its products are formed in such meager amounts that they cannot be weighed. Ten years ago such research was pure science indeed. Today the products of transmutations are used in the steel mill and in the hospital. Physicists, chemists, and physiologists have pooled their knowledge and resources and with such collaboration who can predict what will develop in the future?
LITERATURE CITED
(1) VAN ATTA, NORTARUP, VAN ATTA, AND VAN DER GRAAPP, Phys. Rev.; 49, 761-76 (1936). (2) LAWRENCE AND LNINGSTON, i M . . 45, 60&12 (1934); AND COOKSEY, ibid.. 50, 113140 (1936); LAWRENCE LNINGSTON, Rm. Sci. Imtr., 7.5548 (1936). (3) FERMI, Proc. Roy. Soc., A146, 483 (1934). (4) SEABORG. Chem. Re%. 27, 199 (1940). (5) ScnoEmrMER AND RITTENBERG, J . B i d . Chem., 127, 285 (1939); DOLE,5. Am. Chem. Soc., 58, 580,2552 (1936). (6) UREY. "Separation and use of stable isotopes," J. A##. Phys., 12,270 (1941). (7) PANETH, "Radio-elements as indicators," McGraw-Hill Book Company, New York City, 1928. (8) CRMER AND KrSTIAKOWSKY. J. B i d . Chem., 137, 549 RICE, RUBEN, AND KAMEN. (1941); see also NAKINSKY, I. Am. Chem. Soc., 64, 2299 (1942). AND -EN, Phys. Rev., 57,549 (1940); QMEN AND (9) RUBEN RUBEN. ibid.. 58, 194 (1940). Comfit. rend., 208,97 (1939). (10) PBREY. 1. Chem. Phys., 5,712 (1937); Nature, (11) PERFSERAND SEGRB, 143,460 (1939). MACKENZIE, AND SEGRS, Phys. Rm., 57, 459 (12) CORSON, m dn) \-"--,.
(13) POOL AND QUILL,ibid., 53, 437 (1938).
SCHOENHEIMER AND RITTENBERG. Physiol. Rev.. 20. 218
(1940). WHIPPLE, HAHN,ET AL.,J. Exp. Med., 69, 739 (1939); 70, 443 (1939); 71, 731 (1940); J. B i d . Chem., 134, 585 nnnn, \'Vl",.
HERTZ, ROBERTS, AND EVANS, Proc. Sot. Exfi. B i d . Med., 38, 510 (1938); HERTZ, ROBERTS, MEANS AND EVANS, Am. J. Physiol., 128, 565 (1940); HAMILTON AND SOLEY. ibid., 127, 557 (1939); Proc. Nat. Acad. Sci.. 26, 483 ,I Odnl ,A"=",.
BORSOOK,BUCIEUN, HATCHER, YOST.AND
MCMILLAN,
Proc. Nat. Acad. Sci., 26,412 (1940). RUBEN, HASSID, AND KAMEN, J. Am. Chem. Soc., 61, 661 ICILMEN, HASSID, AND DEVATJLT. SCG (1939); RUBEN, e w e , 90, 570 (1939); RUBEN, KAMEN, AND Hnssm, J. Am. Chem. Soc., 62, 3443 (1940); RUBEN, KAKEN, AND PERRY, ibid., 62,3450 (1940); RUBEN AND KAMEN, ibid., 62, 3451 (1940); RUBEN,RANDALL, KAMEN, AND HYDE, ibid., 63,877 (1941). GUSTAPSON. I. Applied Phys.. 12, 327 (1941); STOUT AND HOAGLAND, Am. J. Botany. 27, 425 (1940); ARNON. STOUT, AND SIPOS, ibid., 27, 791 (1940). SCOTT, AND TTIITLE, "New international clinLAWRENCE. ics." Lippinmtt. 1939, pp. 33-58. KRUGER, Proc. Nut. Acad. Sci.. 26, 181 (1940); ZAHL, COPPER, AND DUNNING, ibid., 26, 589 (1940).
