OCCURRENCE OP THE (4% t 1) SERIES m NATURE
Dec. 5, 1952
[CONTRIBUTION FROM THE
6081
ARGONNENATIONAL LABORATORY]
Occurrence of the (4n
+ 1) Series in Nature
BY D. F. PEPPARD, G. W. MASON, P. R.GRAYAND J. F. MECH RECEIVED JUNE 5,1952
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A small quantity of NpZ3?,the long-lived ancestor of the 4n 1 series, has been isolated from a natural source. The ThZz9 content of Th230obtained from Belgian Congo pitchblende concentrate and of ThZS2 obtained from Brazilian monazite has been determined indirectly through isolation of the Ac226 daughter. The respective values for the mass ratio ThZmt o t o Uza in the pitchblende conThZS2 in these sources is (4.0 f 0.6) X 10-11 and less than 3 X IO-". The mass ratio of UZaa and less than (4 f 2 ) X l o e l l , respectively. The maximum centrate and the monazite samples is set a t (1.3 f 0.2) X value for the mass ratio of Npza7t o U43*in the Belgian Congo pitchblende concentrate is thus calculated to be (1.8 f 0.4) X and the maximum value for the ratio of the respective neutron capture cross sections of Uz3* in the ore body for the (n,2n) and ( n , ~reactions ) is set at (1.3 f. 0.6) X 10-3. It is probable that the true values are much lower than these figures which are based on the assumption of zero contribution from the Tha3z(n,7)reaction. The relative contributions of the Uzm(n,2n)and the Thaaz(n,-y)reactions cannot be calculated, but it seems likely that the latter reaction makes the larger contribution.
other than Np237decay. However, i t is to be noted that ThZz9 will be formed if neutrons are captured by the Th2a2present in small quantities.' This correction is of unknown magnitude and may approach 100%. The ratio of Th229to Th230in a sample containing several grams of ThZao(ionium) which had been isolated, in this Laboratory, from an aqueous waste resulting from the processing of Belgian Congo pitchblende concentrate was measured by deterP mining the content of a 10.0 day a-active Th232(n,y)Th23a + U233 reaction. It has been pointed out by Seaborg3 t h a t ura- daughter of Th229. The requirements of the technique are stringent, nium-bearing ores should contain trace quantities a virtually weightless sample of Ac22bis resince P of Np237formed by the (n, 2n)U237+NpZa7 quired for determination of a-energies6; and actinreaction; and i t has been suggested by Garner, ium, a trivalent element, must be separated from Bonner and Seaborg4 that uranium-bearing ores divalent radium and lead, trivalent bismuth, and containing an appreciable relative quantity of tho- tetravalent thorium. In order to obviate the much rium should contain some long-lived U23aformed as more diacult problem of separating actinium from rare earths, the ThZ3Owas exhaustively purified B the result of the Th232(n,y)Th233 +U233 reaction. from rare earths (and and the Ac225allowed However, isolation of either Np237or U233from nat- to grow again. ural sources has not been reported, and except for Experimental the stable terminal member of the chain, BiBag,the Isolation of Np237 from a Sample of Th2m.-In the isolation family of radioactive heavy nuclides corresponding of approximately one microgram of naturally-occurring PuzaS t o the formula 4% 1 has not been found in nature.' from a uranium process waste,' the aqueous effluents from In the work presently reported, Np237was iso- the first thenoyltrifluoroacetone extraction cycle were comlated from an aqueous waste resulting from the proc- bined and set aside for further study. This composite aqueous phase was subsequently evaporated t o dryness. essing of Belgian Congo pitchblende concentrate. The residue was dissolved in 0.5 M " 0 3 , and the resulting The decay of Np237leads to the formation of solution made 0.1 M in K2Cr207. After a heating period of Th229(a-active, 7.34 x l o 3 years),' and the decay several hours the solution was saturated with magnesium of U238leads to the formation of ThZ3O(a-active, nitrate and contacted with eight equal-volume portions of ether (0.3 M in " 0 3 ) which were scrubbed suc8.0 X lo4 year^).^ Since the half-lives of these tho- diethyl cessively with twelve half-volume portions of a solution 3.5 rium isotopes are very long with respect to the time M in Mg( NOa), and 0.2 M in " 0 8 . The combined ether involved in processing an ore, but very short with extract was allowed to evaporate over water. This aqueous respect to the age of the ore deposit, it follows that phase was evaporated to dryness. The resulting residue was dissolved in 5 M HCl, and any the ratio of Th22gto Th2:j0,by activity, should be plutonium present reduced to the trivalent state (leaving by activity, any neptunium present in the tetravalent state). The identical to the ratio of Np337t o UZ3*, assuming that has been formed by no means neptunium was then extracted into four successive portions
Introduction
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The 4n, the 4n 2 and the 4n 3 radioactive series, whose respective long-lived parent nuclides are Th232,U238and U235,have long been known t o 1 series, of exist in nature. The "missing" 4n which NpZa7 may be considered the long-lived genitor, was reported by Hagemann, et al.', and English, et d 2 ,who prepared U233,whose mass number 1, by the conforms t o the general formula 4n
+
+
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(1) French Hagemann, I-. I . Katzin, M . H. Studier. G . T. Seaborg and A. Ghiorso, Phys. Rev., 79, 435 (19FjO). The decay scheme of the 1 series is given in this paper. The values for the half-lives, as 4n well as the characteristic a-energies, of the members were also taken from this reference. (2) A. C. English, T. E . Cranshaw, P. Demers, J. A. Harvey, E. P. Hincks, J. V. Jelley and A. N . May, i b i d . , 71, 253 (1947). (3) G . T. Seaborg, Chem. Eng. News, 25, 358 (1947). (4) C. S. Garner, N. A. Bonner and G . T . Seaborg, THIS JOURNAL, T O , 3453 (1948). (5) The values for certain half-lives have been taken from the Table of Isotopes of G. T . Seahorg and I. Perlman, Reus. Modern Phys., 90, 585 (1948).
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of 0.4 M thenoyltrifluoroacetone in benzene from the diluted solution.* These organic phases were then scrubbed suc(6) The energies of the a-particles were determined by means of a multi-channel differential pulse analyzer: described by A. Ghiorso, A. H. Jaffey, H. P. Robinson and B. B. Weissbourd, Paper 16.8 in "The Transuranium Elements," National Nuclear Energy Series, Div. IV, Vol. 14B, McGraw-Hill Book Co., Inc., New York, N . Y., 1949. (7) D . F. Peppard, M . H. Studier, M. V. Gergel, G. W. hZason, J. C . Sullivan and J. F. Mech, THISJOURNAL, 73, 2529 (1951). (8) This procedure for separating neptunium from plutonium is reported by L. B. Magnusson, s. G. Thompson and G. T . Seaborg, Phys. &v., 18, a m (1950).
6082
D. F. PEPPARD, G . W.~ I A S OP. N ,K.
cessively with small portions of 1.0 M HCl, in order t o decrease the content of Th2"and trivalent Pu2", until the total a-activity of the composite organic phase (determined by measuring the a-disintegration rate of an aliquot evaporated on a 5-mil platinum disc) was reduced to 110 counts per minute. In order to simplify the interpretation of later a-pulse data the absence of Thzw was proved. The proof was furnished by the removal of added ThZ3'. Enough Tha4 tracer was added t o the organic phase to produce a total @-countingrate, under the counting conditions used, of 12,700 counts per minute. Following several hours of agitation t o permit thorium isotope exchange, the organic phase was scrubbed successively with six small portions of 1.0 M HCI in order t o strip the thorium from the organic phase. The entire organic phase was then evaporated on a 5-mil platinum disc. The total a-counting rate was 26 counts per minute, and the total @-countingrate was less than 50 counts per minute. Alpha pulse analysis showed 7 a-counts per minute due to Np23'. Most of the remaining activity was due to Puaa*. Isolation of AcZs from a Sample of Thzw, No Tracer Added.-A sample of thorium as the nitrate, containing 8.77 & 0.09 g. of Thzw(and 74 g. of T h 9 , isolated from an aqueous waste resulting from the processing of Belgian Congo pitchblende concentrate for uranium, was extracted into approximately 10 liters of 0.6 Jf solution of thenoyltrifluoroacetone@in benzene a t a pH of unity.I0 This solution was scrubbed with several small portions of 0.1 M HXOa t o remove traces of rare earths, yttrium and actinium. The solution was then allowed to age €or 80 days to permit the growth of AcZz6from the Th2* present. At the conclusion of the ageing period, the AcZz6was removed from the organic phase into a small volume of aqueous phase by scrubbing the organic phase with three successive 200-ml. portions of 0.1 hf HhT03," these aqueous phases being successively contacted with two 300-ml. portions of 0.0 ilf thenoyltrifluoroacetone in benzene followed by two 300-ml. portions of benzene. The benzene-thenoyltrifluoroacetone extracts containing the T h Z mwere combined, Ac22' tracer was added to a portion, and the extract allowed to age for the experiment described in the section,,, "Isolation of AC*=from a Sample of Th2", Tracer Added. The composite aqueous phase, containing radium, bismuth and lead daughters of Th2", of Th2S0and of ThZzD and relatively large quantities of ThZ3Oand Th*32in addition t o the desired AcZ25,was evaporated t o dryness. The residue \vas dissolved in a small quantity of water, and the solution c.otiverted to 10 ml. of saturated ammonium nitrate. 0.3 rll i n IISO1. This aqueous feed was then contacted successively with four 10-m1. portions of dibutoxytetraethyleneglycol" to extract thorium, followed by two 10-ml. portions of tributyl phosphate12 to extract actinium, leaving radium in the aqueous phase. (Each of these organic phases had previously been pre-equilibrated with a large volume of saturated ammonium nitrate, 0.3 AI in "0s.) The aqueous feed was then followed, in the extraction cycle, by one 10ml. portion of ammonium nitrate scrub (saturated ammonium nitrate, 0.3 >If in HSO8). The dibutoxytetraethyleneglycol phases were then discarded. The two tributyl phosphate phases, containing actinium and traces of radium, were contacted successively with two 5-ml. portions of ammonium nitrate scrub in order t o reduce the radium content and then contacted successively with three 10-ml. portions of 4 M HNOa in order t o transfer the actinium t o an aqueous phase. The aqueous reextract was scrubbed with carbon tetrachloride to remove traces of tributyl phosphateI3 and then evaporated t o dryness with a heat lamp, taking care not to heat beyond dryness. The residue was taken up in 2 nil. of saturated ammonium .. . . ...___
fP) Thenoyltrifluoroacetone was obtained from D o w Chemical Co.
(10) T h e extraction coefficients for actinium a n d thorium from a n acidic aqueous phase into a benzene solution of thenoyltrifiuoroacetone are given b y French Hagemann, THE JOURNAL, 71, 7138 (1950). (1 1) Dibutoxytetraethyleneglycol was obtained from Carbide and Carbon Chemical Corporation. (12) Tributyl phosphate, t h e w b u t y l orthoecter. \vab obtained f r o m Commercial Solvents Corporation. (13) This technique for removal of trace quantities of tributyl phosphatp is recummended h g J.
