Self-Absorption of S" Radiation in Barium Sulfate FRANK C. LARSON, .4LFRED R. MAASS. C E L ~ R L E Sv. ROBINSON;, AND EDGAR s. GORDON Uriioersity of Wisconsin, Madison, Wis.
A technique has been developed for the collection of radioactive precipitates utilizing centrifuge tubes with removable flat bottoms. A study of the selfabsorption of S36 activity in barium sulfate has been reported. h value of 0.216 sq. cm. per mg. w-as found for the self-absorption coefficient of S35 i n barium sulfate on a Lusteroid backing.
A
FREQUENTLY used method for studying a given type of radiation is to determine an absorption curve by measuring the intensity of transmitted radiation as a function of absorber thickness. In the case of a disintegration which produces simple 8-spectra, such as that of P3*, C14, or S35,the intensity is nearly I= where d is the absorber thickness, and pis the absorption coefficient. Actually the semilogarithmic plot of the observed intensity us. the absorber thickness is generally not quite straight and ,u is the average value of the slope in a suitable region of the graph. For the ordinary absorption esperiment, foils of various thicknesses are interposed between a fixed radioactive sample and the detector. If, on the other hand, the absorber consists of a carrier material which is mixed with the radioisotope, self-absorption results. A self-absorption plot is obtained by measuring the apparent activity of a series of samples which contain equal amounts of the radioisotope but different amounts of carrier. This graph may be fitted by the curve Z
=
Io
PROC EDI: R E
Niquots of a standardized sample of #odium sulfate containing were precipitated as barium sulfate in the flat-bottomed centrifuge tubes illustrated in Figure 1. The Lusteroid planchet which fomred the bottoms of these tubes were molded in a glycerol bath heated to 200" C. Inert sodium sulfate was added in varying amounts to permit the precipitate to range from 1 t o 50 mg. per sq. cm. The aliquots containing inert carrier were diluted t o approximately 50 ml. and acidified to pH 3 with hydrochloric acid, and then 10 ml. of a 10% solution of barium chloride were added dropwise with stirring. After addition of the barium chloride, the precipitate was allowed to age and then centrifuged a t 2000 r.p.m. for 10 to 15 minutes. The supernatant was siphoned from the tubes, care being taken not to disturb the precipitate. The precipitate was resuspended in a small volume of distilled water, the sides of the tube were carefully washed, and the volume was made up to approsimately 50 ml. The precipitate was again centrifuged and the clear supernatant removed. After again resuspending the precipitate, sufficient ethanol was added t o the third wash to make a 5Oy0 solution. The samples nw.e then centrifuged and the clear supernatant removed. The $85
1
Present address, Biuplryvaical Laboratory, IIarvard Xediral School, Boa-
Nass.
The accuracy of the procedure as a method for gravimetric analysis for sulfate was demonstrated by a n a l y s e s of a s t a n d a r d sample of sodium sulfate f r o m which t h e o r y d e manded a yield of 127.2 mg. of b a r i u m s u l f a t e . The result of a series of seven analyses was 126.8 * 0.44 mg. vhich was 99.7%;, of the theoretical yield.
PYREX -BRASS
1 - e--ad by proper ad
choice of the self-absorption coefficient, LY. Generally, the fit is better than might be expected, for the derivation of the formula requires exact exponential absorption and parallel tracks for the @-rays,and back-scattering from the mounting is ignored. This effect of the backing appears as u deviation from the theoretical curve in the region of small thickness. A technique is described below for measuring S35 activity precipitated as barium sulfate using a thin mica window GeigerMuller counter. The data obtained by this method give a value of (Y = 0.216 sq. cm. per mg. for the self-ahsorption coefficient.
LOU.
Lusteroid planchet was removed and the small volume of alcohol which could not be siphoned from the tube without disturbing the surface of the precipitate was allowed to evaporate a t room temperature. Samples were finally dried in a vacuum desiccator over calcium sulfate and weighed.
---RUBBER -PLANCHET
RESU LI'S
-RUBBER
The sample of S36 active barium sulfate was mounted over an area of r Z j > BRASS 6.42 sq. cm. in the centei of a plastic film planchet Figure 1. Collection Appa3.8 cm. in diameter and ratiis for Barium Sulfate 0.05 cm, thick. The SamPrecipitate ple was placed 4 mm. below a thin (2.3 to 2.7 mg. per sq. cm.) mica window counter with a diameter of 27 mm. The counting rates have been corrected for resolution, backgi ound, and, when necessary, for decay. hpprouimately 5000 counts were recorded for each sample. The data are summarized in Figure 2. Each point represents the average of two or three obseivations of activity a t that sample thickness. The average value of each series of observed activity was plotted as the percentage of total activity and the curve has been drav n to fit the equation: -\
OLserved activity in per cent with
a =
=
100
__--
(l
0.216 sq. cm. per nig. DISCUSS103
The data are well fitted by a cuive of the type I
= IO (l _ C _Yd)
wheie Z is the observed activity, IO the activity without carrier, d the thickness of the sample in milligrams per square centimeter, and a the absorption coefficient. This equation has been used by 1206
V O L U M E 21, NO. 10, O C T O B E R 1949 Henriques (4),Libby (Y), Solomon (8), and Yankwich (9). Parameters IO and 01 were chosen in order to obtain a good fit to the data in the xvorking range of sample thickness from 1 to 50 mg. per sq. em. The value of 01 obtained was 0.216 sq. cm. per mg.
100
5 80 W
u I I :
W
a 60 >-
c 2 tu
1207 data given are well fitted above 4.6 mg. per sq. em. by using 01 = 0.28 sq. cm. per mg. Below 4.6 mg. per sq. em. the experimental curve falls below the theoretical and the extrapolated activity for 0 thickness is 87% of the theoretical. Henriques et al. ( 4 ) employing benzidine sulfate precipitation onto filter paper obtained a value of a = 0.265 sq. cm. per mg., fitting the region from 1 to 11 mg. per sq. cm. Yankwich (IO)and Glendenin (8) have found that the theoretical absorption curve fits the experimental data for sample thicknesses larger than 20’3 of the range, while for thinner samples a departure is observed due to the difference in back-scattering from the planchet material as compared to the carrier material. On this basis one would expect better agreement with the values given by Hendricks et al. and the authors. The value obtained by Henriques for the 1 to 11 mg. per sq. em. range is definitely a function of the backing. The discrepancies noted here are not consistent with the theory in its present state. For accurate work, a self-absorption curve should be run by the experimenter himself. BIBLIOGRAPHY
40
n
w
> E W
cn
20
IO 20 30 40 50 RESIDUE MG. /SQ.CM. Figure 2. Geiger Counter Self-Absorption Curve for Sa6in Barium Sulfate
0
Hendricks et nl. (3) precipitated barium sulfate on brass disks and obtained a self-absorption curve. No theoretical curve was fitted originally but, using their value for the sample area, the
(1) Glendenin, L.E.,.VucZeonics, 2,12 (1948). (2) Glendenin, L. E., private communication. (3) Hendricks, R. H., Bryner, L. C., Thomas, M . D., and Ivie, J. C., J. Phys. Chen., 47,469 (1943). (4) Henriques, F. C.,Jr., Kistiakowsky, G. B., Margnetti, C., and Schneider, W. G., IXU. ENG.CHEM.,ANAL. ED.,18, 349 (1946). ( 5 ) Johnson, F., and Willard, J. E., Science, 109,11 (1949). (6) Leslie, W.B., U. S. Atomic Energy Commission, M.D.D.C., 674, (Nov. 15,1946). (7) Libby, W.F., A N ~ LCHEM., . 19,2 (1947). (8) Solomon, A. K., Gould, R. G., and Afinsen, C. B., Phys. Rev., 72, 1097 (1947). (9) Yankwich, P.E.,h’orris, T. €I., andIHuston, J., ANAL.CHEM., 19,439(1947). (10) Yankwich, P. E., and Weigl, J. W., Science, 107, 651 (1948). RECEIVEDFebruary 28, 1949. Work supported in part by the Research Coininittee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation. Supported in part from funds received from the UnitediStates Public Health Service.
Rapid Qualitative Method for Acrylonitrile C. E. BROCKWAY B. F. Goodrich Research Center, Brecksville, Ohio Acrylonitrile can be identified by conversion to 8-piperidinopropionitrile, the picrate of which is a suitable derivative. Water and organic solvents, except the stronger acids and bases, do not interfere. Acrylonitrile in concentrations as low as 1% can be detected and identified by this method. Methyl and ethyl acrylate can be similarly characterized by conversion to the picrates of the methyl and ethyl esters, respectively, of 4-piperidinopropionic acid.
I
N CONNECTION with its use in nitrile rubbers the need arose for a rapid and simple method of identifying acrylonitrile. No such method has been described in the literature. A great variety of crystalline derivatives obtained by the cyanoethylation reaction have been described, but the procedures involved do not in general lend themselves to adaptation as simple qualitative methods. Among the most rapid reactions of acrylonitrile are those with certain nonaromatic amines. The aminonitriles obtained are themselves liquids, but may be readily converted into solid derivatives. Such a method for characterizing acrylonitrile has the advantage of greater speed and convenience
than the usual methods for nitriles, and water and various other substances do not interfere. AMINO DERIVATIVES OF ACRYLONITRILE
Piperidine and morpholine (6),certain substituted piperidines (1), and some of the primary aliphatic amines (5)react vigorously with acrylonitrile in the absence of catalyst to give the corre sponding 8-aminopropionitriles. The present study was confined to the use of morpholine and piperidine, particularly the latter. The properties of the two aminonitriles and their derivatives are listed in Table I.
