Emission Spec t rogr a phic Dete rmina t io n of Americium in Plutonium CHARLES E. PlETRl and ALBERT W. WENZEL
U. S. Atomic Energy Commission, New Brunswick, N. J.
b Americium i s one o f the more important impurity ellements found in plutonium because 'of its growth (as Amz4') from radioactive decay o f Pu*II, a common plutonium isotopic constituent. A method i s described in which americium i s determined simultaneously with other impurities b y emission spectrography after separation from plutonium b y anion exchange in 8 N nitric acid. Europium i s used as an internal standard for americium while ccibalt i s used for the other impurity elements. The rate of burning of the sample during spectrographic exciiation i s controlled b y the use o f a gallium oxide matrix. The relative standard deviation o f the method for americium i s less than 6% with a lower limit o f detection o f 0.03 pg.
T
HI: SITRIC A C I D anion exchange ~ e ~ , a r a t i o n ~ e i i i i ~ :spectrograllhic iion method for the detrrniination of iniIiurities in lilutonium is especially valuabli, becauw of the many elements that can be tlotc~rniined ( 1 , 4 . 9.IO). I n thir nir,thod, Iilutonium, as the I'U(SO~)~ (>oni1il(-: -~ ( ? ) $ is adsorlwd onto a n anion cwhanpc rcsin in 8JH S 0 3 lvhile thrl unadsorbcd impurities arc w a s l i d into the i,fflurnt. .\lthough anicric.iiini ir on(' of the iiiorc~important iniliiirity elrmmt.; in plutonium which can be q a r a t c t l hy this ion exc>hangc method ( 2 , .5)> the wbscquc.nt qiectrogralihic rlt~te~xiinaticm of this elenicnt folloniny separation from plutonium hac not 1m.n re1iortc.d in detail in the literaturr. Th(>rt> is B distinct advantag(> in time,, c.onvc,niencw, and scheme of analy-i. t o the inclusion of an anicricium det t m i i na t ion K h(3n siniul t anro w l y detr,rniining other ini1)uritic.s in Iilutonium by eiiiisiion spectrography. I n addition, the ;availability of an a l t e i . n a t ~noiilntliocl-ieniic.al , mc~thodfor is of valiicl. amc~ric~iurii Illiilc thii I):i1wr rc,I)ort:: tlie dcve1ol)incnt of i w h a quiintitatiw einission s;iirlctropr:ilihic. nicthotl for thr, detc3rmin:ition of :init~iiciiin~ along with othor iitil)ui'ity cl(went* i n I)lutonium nictal ant3 ~ ) l i i t o i i i u nhiili'ate i tc>traliytiratc,the nietliotl alto :iI)1war+ :ilqilic.ablc~to other ~ ~ l i i t o n i umiiiImiin(1.; m and .;elutions.
EXPERIMENTAL
Apparatus and Reagents. Because of t h e toxirity of plutonium a n d americium, all operations with t h e w materials were performed in glove boxes. SPECTROGRAPHIC EQUIPUIA-T. 13aird-.\toniic Model GX-1 slicctrograllh with an Eagle mount, 3-meter concave grating having 15,000 lines Iier inch for a first order recilirocal di::i)ersion of 5.53 -4. was modified foi glove box operations (6). I'on.er was supplied by a 13-A Spectrosoutw, Model LIT-I, -4 ser-en-steii , converted to the nitrate form by washing with 8 5 H S O , , anti stored in water. Aiglass wool plug at the bottom of the ion exchange colunin was used to support the resin bed. The ion exchange column ( 6 ) (1-em. i.d.) \\-as loaded to a height of 6 (mi. with the resin and washed with 30 nil. of 8.Y H S 0 3 prior t o use. STAKDARD SOLUTIOXS. .\1i]irOXimately 5 nig. of AmOn (Oak Ridge National Laboratory. Oak Ridge, 'l'enn.1 was weighed to the neare>t 0.02 me. and dissolved in 20 nil. of lLY H S 0 3 . .\liquets of this solution were diluted with 1 S H S 0 3 containing 0.1 mp. of E u per nil. to make a w i e s of ,mliition> of the following concentrati~ns: 100. 50, 20! lo! 5 , 2 , 1, 0.5, 0.2 p g . of Ani Iier nil. [The anieririum ('ontent of these solution.; was verified radiochemically by alpha spectrometry ( 3 ) . ] S t a n d a d solutioni for other iniiiuritie. were 1iitpared in a hiniilar nianner (li, IO). IS,~I:RSAL STASDARD. High liurity Eii?03 (99.9+7,) and c'o metal (99.99cc) were disso1vt.d in 8.\- H S 0 3 and thiy >elution \\-as djlutrd to give a conceiiti,ation of 0.1 mg. of Eu per nil. arid 0.02 m g .of Co per l i d . in 1.Y H?;Os. -\ 500-pl. aliquot of thip solution war ured to dissolve the inil~urityresidue from the evaporated ion exchange efflwnt. GA&s ~IATRIS. Gallium oside, 99.999yc l)ui,ity (.\.I..\.G. ,1Ictall, Inc.. K c w Yoi,k, S.I-,), lvas used as a specwogralihic' matrix t u control t h e rate of hu~mingof the, wnillle during exc>itxtion in t h e arc. and t o Iiiuvide e ~ m >lit illuniinatioii, l'lic, nctd for t h i i technique liar liem r 1 i ~ c ~ i i i . in d detail 11reviou.ly is),
ELECTRODES. Sational Carbon Co. graphite electrodes were used : uiqier. cathode S o . L-1036: lower. ancde cap S o . L-4030 with pedestal to fit. The anode cap.; were sealed with two applications of 5 droll-: c w h of 27, Kcl-F greme in Cc'l,. The solvent \vas allovied t o evapoi'ate aftel, each allplication. =\cID$. Nitric and Ii~.drochloiica d s w r e prepared by di-as d e t e r m i n d by the Loh .\lamas Scientific Laboratoi,y. ~ v iai r c~d . This pliitoniii~n wntainrd aliout 200 p . ~ ) . i i i . of detected inilluritier in atldition to 126 1i.p.m. amei~icium a' of 1 I a y I I , 1961. Recommended Procedure. IY(3igIi a 100-mg. sample of plutonium mctal, or 200 m g . of plutoriiuni -ulfate tetrahydrate into a 50-nil. 'Teflon beaker. To lircpare the 1)lutonium metal for analy-i-. add ahout 2 nil. of d i s t i 1led con * t a n t - 11oi I in g H ('1 to t h e ljeaker and cover with a ' f r 4 o n watch glass;. After -aml)lc (lihsolution, add 2 nil. of concentrated IIXO, and rcmove the exce.;i HCI by e v a p orating t h e solution to a moist rv-itlue under heat lain1)*. c'ool :ind (lis-olve tlie residue in I O nil. of S.\- HSO,. For Iiliitoniuni iiilfat(x toti,ah>di~ate atid 10 nil. of K\.IISO, to the 1 ) r ~ A ~ r . cover. aim u i i t l t ~ i .1ic:it lanil)-. t o aid dissolution: anti cool. .\dd tlie 1)rcJliarid saiiil)lc~t o the ion exchange colunin anti collect t h v cloar rfflucnt in a 'I'eflon tliih at thcs i'atc of aljoiit 0 . 3 nil. 1)er m i n u t e . \ V n i h tlw xaniplc I~cakerivith 10 n i l . of S.Y I-ISOa, add t h e waihingr to tht, c,oluniii. :inti
*
relicat thi.; n-aslring
-i('~) n.itli
VOL. 36, NO. 10, SEPTEMBER 1964
30
1111,
1937
Table I.
Excitation and Photographic Conditions
Prearc Exposure Electrode gap Entrance slit Current Emulsion Spectral region
Sone 10 seconds 4 rnm 25 microns 2.5 d c amperes Eastiiian SA-1 2200-3450 A
of 8 5 "03 to wash the impurities from the column. Evaporate the effluent to incipient dryness under heat lamps. Cool the dish to room temperature and dissolve the impurity residues by adding 500 pl. of the europium-cobalt internal standard solution. Pipet 100pl, aliquots of the solution onto prepared graphite electrodes containing 2.5 mg. of GazO8and dry the electrodes under heat lamps. Determine residual impurities in reagents by following the above procedure without the sample. Excite the electrodes and 1)hotograph the spectrum under the conditions outlined in Table I. Densitometer the analytical lines for americium (2832.26 .I.)and europium (2531.95 -1.)on the processed photographic plate$, determine the relative log intensity ratio of this pair, read the americium value from the analytical curve, and calculate the americium content of the original sample. Other impurity elements are determined similarly but relative to the cobalt internal standard as reported elsewhere ( 9 ) . Prepare an analytical working curve for americium by pipetting loo-$. aliquots of the standard solutions directly on sealed graphite electrodes and following the recommended Ilrocedure. [The preparation of analytical curves for the other impurity elements has ibcd previoudy ( g ) ] , Plot the relative log inten4ty ratio of the arnericiuni-eui~ol,ium ])air aFainst the americium concentration (in micrograms). RESULTS
The americium contcnt was determined in thwe plutonium samplesHanford plutonium metal, National
Table II.
Sample Pu metal
DISCUSSION
The use of cobalt as a spectrographic internal standard is satisfactory for the determination of many impurity elements but preliminary work indicated that it was not adequate for use with americium. Several elements including some rare earths were evaluated as possible internal standards. Europium was selected as a n internal standard for americium since it was effective in maintaining a precision of within 6% (relative standard deviation) as compared to the usual 12 to 15y0 obtained using cobalt for the other impurity elements under the same spectrographic conditions. The present procedure employs europium as an internal standard for americium and cobalt for the other elements. The quantitative recovery of americium in the anion exchange process was substantiated by passing 10O-pl. aliquots of the standard solutions (0.02 to 1Opg. of Am) through the column and determining the americium content in the effluent both radiochemically and spectrograiihically. Complete recovery of americium was obtained within the experimcntal limits of the analytical methods ubed. .Xfter several years of storage, plutonium of even the highest original purity may contain alqireciable amounts (> 300 1i.p.m.) of americium because of
Table 111. Spectrographic Determination of Some Impurities in NBS No. 949 Plutonium Metal Standard Sample
Impurities, p.p.m. Element New Brunswick Los Alamos Mn .4Hd) ( S e n Brunswick Laboratory)
Bureau of Standards No. 949 plutonium metal, and New Brunsn ick Laboratory h(So4)2.4f&o- and is given in Table 11. The americium content determined by the anion exchange-spectrographic method was within 77, of the calculated and radiochemical values. The relative standard deviation of the method was determined to be less than 6%. The lower limit of detection for americium was 0.03 pg. .I spectrographic analysis for impuritieb in addition to americium was made on the NI3S No. 949 standard sample for the overall evaluation of the method. The results indicate general agreement and are shoirn in Table 111.
57
53
54
64
62
67
ACKNOWLEDGMENT
The authors thank L.C. Nelson, Jr. and H. 13ussell for their assistance in the radiochemical analyses. LITERATURE CITED
(1) Brody, J. K., Faris, J. P., Burhanan, R. F.,AXAL. CHEM.30, 1909 (lCI58). ( 2 ) Burhanan, I
