The Dow Chemical Company
Rocky Flats Division Golden, Colorado
I
Y G I I I I ~ Y
The B hck Sheep in Radioactive Families
T h e naturally occurring radioactive materials have acquired a reputation for radioactive hazard which is applied unjustly. The dangerous activity is not a property of the long-live elements themselves, but rather of their short-lived decay products. Often trained and experienced workers in the industry make errors in judgment with regard to handling such materials because they neglect the inevitable presence of radioactive daughter elements. The oldest and most respectable members of the radiation community are the thorium and uranium families ( 1 ) . Less well-known is the actinium family and a comparative newcomer is the neptunium family. A large part of the respectability of these family names is due to the long life and venerability of the parent. Usually when the family escutcheon is defiled it is by the hehavior of one of the short-lived progeny of these patriarchs. Even the relat.ively ~ophisticat~edscientist is frequently remiss in not appreciating that a long-lived alpha-emitting parent may become socially obnoxious through growth to equilibrium of a short-lived gamm* emitting daughter. He may not realize that some of the repulsive qualities ascribed to radium are not a radiation property of radium but are due to the children of this isotope. Almost every worker in the field must discover sometime for himself that the radioactivity of 238U,the hereditary head of the 4n 2 line of descent (2) is not in practice the 4.5 X lo8year alpha activity alone as described in the tables of radiation con~t~ants.Sometimes this discovery takes the form of high levels of penetrating radiation in a sidestream accompanying a chemical operation, frequently casting. The alpha daughter of 238Uis 234Th,originally identified as UX, which is a 24-day beta-emitter. When uranium is melted the thorium segregates in the crucible-melt interface. It has also been reported to concentrate on the crucible cover or to be revealed as local spots by autoradiography on a section of the casting. The beta daughter of 234This 234Pawhose 1.2-minute half-life prevents an accidental isolated appearance but which is very much in evidence associated with 234Thbecause of the 2.3-Mev beta decay accompanied by some gamma activity. On the other hand if the alpha activity of aged 238Uis examined critically, it is found that only half of the total activity is the 4.18-Mev alpha of this isotope; the other half is the 4.76-Mev alpha of Z34Uwhich has a half-life of 2.5 X lo5years. The equality in activity is evident in secular equilibrium where one 234Ualpha must appear
+
Work performed under AEC Contract AT(29-lb1106.
for each 238Uatom which decays. The 2aoThdaughter of 234U,ionium, also has a fairly respectable half-life of 80,000 years. I t decays, however, to 2Z6Rawhich has a half-life of only 1620 years. This half-life which is sufficiently long in terms of human life to constitute a permanent source of radiation in the body is stiU short enough to discharge a dangerous amount of energy within a measurable period of time as is demonstrated in the classic deaths from radium poisoning. The immediate daughter of radium is a noble gas, radon. The half-life of radon is sufficiently short that it does not occur naturally in significant concentrations except in mines. It was in the mines of Joachimsthal that physiological evidence of radiation damage to man was observed before the discovery of radiation was announced. The resulting cases of lung cancer are now ascribed not t,o radon but to the daughters, particularly Radium A or 218Poand Radium C t or 2 1 4 Pwhich ~ are solids and are retained in the lung while the radon is expelled. The bet,a and gamma rays of Radium B and Radium C are of negligible physiological importance (3). It is Radinm B, n4Pb with a half-life of 27 minutes, which is principally responsible for natural air-borne activity. Radon diffusing from the soil where radium exists on the average of 1 g per sq-mile-ft decays through short-lived A t,o Radium B which exists for a reasonable period and supports the shorter-lived C and C'. 232Th resembles uranium in being a long-lived alpha-emitter and head of the 4n clan (Fig. 1). Since it is an alphaemitter of 1.4 X 101° year half-life, many people are startled to learn that it is more active with respect to radiation than 28aUwhich has a shorter half-lie by an order of magnitude. It is easy to forget that the penetrating radiation is a property of the daughter of each and not the parent and that the concentration of the daughter is also a function of the daughter's half-lie instead of that of the parent. As a practical matter, a wood container of sufficient thickness to retain the weight of a piece of uranium also serves as an adequate shield to reduce the external radiation to the specification of the ICC regulations. But this rule of thumb does not hold for aged thorium. The exposure problem associated with thorium arises partially from the fact that there are no long-lived daughters to break the decay chain. If thorium is separated from its daughters, they are again essentially in secular equilibrium after about one month. 228Th is in equilibrium with la2Thin natural thorium in the same manner that 234Uis in equilibrium with 23sUin natural uranium. One atom of 22RThdecays for each which is the longestatom of 232Th. Thorium X (224Ra) lived daughter of 228Th(radiothorium) has a half-life Volume 43, Number 1 7, November 1966
/
589
Figure 1.
Decoy xhome of the thorium family (4" Series).
of 3.64 days. Thus in about seven half-lives the daughters are present in equilibrium quantities and instead of the expected single alpha of 232Th,actually six alpha particles are counted. These relatively energetic alpha emissions are accompanied by beta and gamma activity. An unusual phenomena occurs in the thorium chain (4). The first decay product of thorium, mesothorium 1 or z2sRahas a half-life of 6.7 years which is the longest in the chain and controls the decay rate. Since the half-life of 218This only 1.9 years, the integrated activity of a sample of pure thorium will return to its equilibrium value in a month, decay to a minimum value in four years and return to equilibrium in 50 years as the 228Radaughter (which is the parent of 228Th) regains full activity. The peculiar behavior of thorium alpha activity with time is shown in Figure 2. Many of the early conclusions with respect to the biological effect of radium may be questioned because
of the possible presence of the radium isotope, mesothorium 1. This medium-lived daughter is a radium isotope and as such may be contaminant in radium preparations. It is even suspected that mesothorium may have been sold to innocent physicians as radium. Because of the 6.7-year half-life, the preparation decreased rapidly in effectiveness. The gamma energy of the daughter Thorium C" is almost two and one-half times that of the radium daughter Radium C although only one-third of the Thorium C decays by alpha emission to Thorium C". A thorium daughter is employed by dermatologists in the treatment of psoriasis. The sk'm is painted with a solution of Thorium X, 224Ra,an alpha-emitter of 3.64 days half-life. Because of the short half-life, the agent does not constitute a permanent radiation hazard. Thorium X decays through a series of shortr lived alpha-emitting daughters to Thorium B and Thorium C ' which furnish all intense source of beta rays to irradiate the skin. Because of the integrated energy of the alpha-emitting daughters of thorium, this element has been compared with plutonium in toxicity (5). The insolubility of thorium provides a parent of long biological halflife which contributes a chain of energetic progeny with no intervening long-lived daughter to block the release of energy in decay to a stable isotope. Until the advent of the nuclear chain reactor, the 4n 3 series had not distinguished itself in any way. The naturally-occurring parent 235U,which occurs with less than 1% of the abundance of 238U,decays through 23'Pa and 2 2 7 Awhich ~ are long enough in halflife to contribute slight radioactivity and short enough to prevent their growth to appreciable concentrations. The synthetic isotope 239P11 decays to 235U. Although 23sPucoritributes no black sheep progeny to the radioactive commuuity, it is associated with other troublesome daughter activity. As it is formed in the nuclear reactor, the 2 3 8 Pis~ allowed to remain in the 238Umatrix until a quantity sufficient to separate economically has accumulated. While the material remains in the neutron flux,it captur~sadditional neutrons to form Z 4 0 Pwhich ~ is inseparable from 2 3 9 P ~ and remains as a contaminant. 2 4 0 Pis~novel to classic radioactivity in that it undergoes spontaneous fission and contributes a nominal number of neutrons to the other particles emitted. The number of fission neutrons present are usually insignificant. In turn the 2 4 0 Pmay ~ also capture neutrons before the chemical processing of the fertile material with the result that high exposure plutonium will contain a certain amount of 2 4 1 P ~ This . isotope decays by both alpha and beta emission to 237Uand 241Amrespectively. Since the alpha decay of each of these daughters is accompanied by gamma activity, high exposure plutonium becomes difficult to handle without shielding. 1 decay 241Amand 287Uare members of the 4n series. This series is not known among the naturally radioactive materials because of the relatively short half-lives of all of the members of the family. A second synthetic isotope and third fissionable material, 233U,is a member of the series. As in the case of 239Pu, s33Usuffers isotopic contamination as it is formed in the nuclear reactor. Y J is produced concurrently by an n-2n reaction. 232Uis a member of the 4n series
+
+
Figure 2.
590
/
The behavior of thorium +ha
activity mr o function of time.
Journal of Chemicd Mucotion
common to =SZThwhich decays through the five alphaemitting daughters controlled by the 1.9-year half-life of radiothorium, 228Th,to the highly energetic gammaemitter, Thorium C". These daughters of the by-product isotopic impurity render 23aUa messy material to handle as compared t,o 2a5Uor even 2a9Pu. Literature Cited (1) RUTRERPORD, C. F., CHADWICK,E., JR., A N D ELUS, C. D., "Radiation from Radioactive Snbstanees," Univenity Press, Cambridge, 1951 : Chapter 1. LAPP,R. E.,AND
ANDEEWS, H. L., "Nuclear hdiation Physics," 3rd. edition, Prentiee-Hall, Ino., Englewood Cliffs, N. J., 1963, Chapter 4: FRIEDLANDER, G., KENNEDY. J. W., and MILLER,J . M., Nudeal Radiation, Edition 11, John Wiley & Sons, N. Y., 1964, Chapt,er 3. ( 2 ) The mass uumbem of the members of t,he series = 4n 2. (3) "Cont,ml of Raddon Daught,ers in Uranium Mineq and Calculation of Biological Effects," Public Health Service Publication, U. S. Depart,ment of Health, Education and Welfare, Public Health Servire. (4) FRESCO, J., JETTER, E., and HARLEY, J., A'ucleonics, 10, NO. 8, 60 (1952). (5) National Bureau of Standards Handbook No. 69, p. 83.
+
Volume 43, Number 1 1 , November 1966
/
591
