Determination of Submicrogram Amounts of Boron Using the "B(n,a)'Li Reaction Mark Lelental Research Laboratories, Eastman Kodak Company, Rochester, N. Y . 14650
THEADDITION OF BORON to steel or its presence in certain alloys influences their properties seriously. This holds true particularly for materials used in nuclear reactor construction. The determination of boron contained in ocean water, where a single milliliter of water contains microgram amounts, and of boron content of tissues has become exceedingly important. Submicroanalytical methods are based upon utilization of nuclear properties of boron isotopes. The cross section for the lOB(n,a)7Li reaction is very high (3800 barns) in regard to thermal neutrons. This reaction proceeds through the unstable compound nucleus formed first which has a very short half-life and decays to an a-particle and a 7Li nucleus. Directly connected with this is a low saturation activity which disappears rapidly after stopping the neutron exposure. Thus, methods based upon the lOB(n,a)?Li reaction and measurement of y or a radioactivity after completion of activation have relatively low sensitivity. These difficulties can be avoided by registering radioactivity resulting from the ]OB(~,CU)~L~ reaction during the process of irradiation of samples in the neutron flux. The y radiation concomitant with this reaction is not measurable owing to the strong y field existing in the nuclear reactor channel. The only alternative is to measure the a particles which appear during the reaction. loB
+n
.--t
7Li
+a
(1)
Measurement of the alpha emission by a photographic method offers the greatest promise for boron analyses. Armijo and Rosenbaum ( I ) reviewed autoradiographic methods and showed that the sensitivity of the silver halide to neutrons, j3, and y radiation limits the amount of exposure that can be made in autoradiography. The sensitivity is in the parts per million range for the determination of boron in steel. For the determination of extremely low boron levels (ppb), it is necessary to have a material that is fully resistant to all types of radiation that exist in a nuclear reactor channel and one that can register alpha particles simultaneously. Fleischer, Price, and Walker (2, 3) found that certain plastics could register charged particles whose kinetic energy exceeded a certain minimum. A charged particle passing through a plastic material causes a local disturbance of its structure and, after chemical processing, a "track" can be observed microscopically. Fleischer et af., examined the suitability of various plastics for recording such tracks and concluded that cellulose esters were superior. This method has become the subject of a considerable amount of work (4-7). In subsequent work, this technique was applied to boron analysis (8-10). (1) J. S. Armijo and H. S . Rosenbaum, J . Appl. Phys., 38, 2064 (1967). (2) R. L. Fleischer and P. B. Price, Science, 140, 1221 (1963). (3) R . L. Fleischer, P. B. Price, and R . M. Walker, ibid., 149, 383 (1965). (4) K. Becker, Health Phys., 16, 113 (1969). ( 5 ) R. H. Boyett, D. R. Johnson, and K. Becker, Rudiut. Res., 42, l(1970). (6) E. V. Benton, USNRDL-TR-68-14 (1968). (7) D. R. Sears, ORNL 4355 (1969). 1270
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
Table I. Solution Formulas Koduk Etching Solution VX007 Metric Potassium hydroxide 60 grams Potassium thiocyanate 400 grams Water to make 1 liter Koduk Etching Solution VXOOS Potassium hydroxide 80 grams Ethylene glycol (Eustmun Organic Chemicals No. 133) 600 grams Water to make 1 liter
Avoirdupois, U.S. liquid 2 ounces 13 ounces 32 fl. ounces 2 . 5 ounces
20 ounces 32 fl. ounces
Thermal neutrons
Double coated tape 'B2O3
Cellulose nitrate film
Figure 1. Experimental arrangement for neutron irradiation of boron-containing sample
Cellulose acetate butyrate (CAB), cellulose triacetate (CTA), and cellulose nitrate (CN) films have shown the greatest promise, and these materials will be considered here. EXPERIMENTAL
All plastics used as detectors were first checked for their sensitivity to alpha particles by exposure to a zlOPosource, followed by etching. The plastics and etching conditions used were : cellulose nitrate (Kodak Experimental Special Cellulose Nitrate, CA 8015)-20 rnin at 75 OC in Kodak etching Solution VX007 followed by 8 min at 75 "C in Kodak Etching Solution VX008. (The formulations of the etching solutions are given in Table I); cellulose acetate butyrate (Eastman Organic Chemicals)--60 min in 28 KOH at 60 "C; cellulose triacetate (Eastman Organic Chemicals)-20 min in VX007 at 75 "C. All of these materials gave good etch patterns. They were next tested for applicability to boron determinations using the alpha emission described by Equation 1. Boron trioxide (B203)was vacuum-deposited under controlled conditions on double-sided Scotch Tape (3M, Type 605) which had been affixed to a microscope slide. The (8) C . P. Bean, R. L. Fleischer, P. S. Swartz, and H. R. Hart, J. Appl. Phys., 31, 2218 (1966). (9) B. A. Loveridge and C . A. J. McInnes, Microscope, 16 (2), 105 (1 968). (10) R. L. Fleischer and D. B. Lovett, Geochim. Cosrnochirn. Acta, 32, 1126 (1968).
Figure 3. Scanning electron micrograph. g B/cm2. Ma .tracks, 2.0 X angle 45"; 1em = 50pm
Alpha
amount of boron that adhered to the tape was analyzed by a benzoin-based fluorimetric method whose sensitivity was 1X gram of boron. The coverages of boron ranged to 3 x 10' gram B/cm2. from 2 x For neutron irradiation, samples of the tape (containing B203)were laminated t o a piece of the plastic and exposed to the neutron flux (2 x loLznjcm2,sec) in the reactor o p erated by the Western New York Nuclear Research Center at Buffalo, N.Y. The experimental configuration is shown in Figure 1. Initially, it was believed that doses of 2 x 10'' thermal neutrons/cma were needed and the samples were exposed in the reactor channel for 1M) sec. After the appropriate etching treatment, all samples were extremely turbid so the experiments were repeated using 5-sec exposures. These, too, were too turbid to allow etch-track densities to be obtained. It was thought that gamma radiation could be "fogging" the plastic sheeting, so CAB, CN, and CTA films ' Po source were exposed that had been irradiated by the ZO to gamma radiation from 6aCo. The exposure time to the cobalt source was chosen so that the total dose corresponded to the gamma radiation exposure it received in the reactor. The ability of all three materials to register aluha uarticles was unaffectedby the gamma exposure.
whose flux is equal to that of the thermal neutrons. Therefore, experiments were carried out in which the plastics were given the same dose to the fast neutrons produced by the neutron generator. This generator uses aH(d,n)4Hereaction. All three materials were fogged by this exposure, a finding recently observed elsewhere (11, 12). The reactions '2C(n,nf)3a and elastic scattering 14N(n,a)11B,'aO(n,n)laC, by 'ZC are possible sources of this effect. Carbon recoil nuclei have been postulated t o be the main cause of this response (4, 11). Therefore it was necessary to choose a location in the nuclear reactor with a high thermal-to-fast-neutron flux ratio as described by the "cadmium ratio." The conditions finally picked consisted of a 1.9 x 10' n/cm2 . sec flux for 290 sec (total dose of 5.5 X 10" njcm' in position in a channel having a cadmium ratio of 50 as determined using gold foil. Because of significant lessening of the total neutron dose, it was necessary to pick the most sensitive plastic detector, (11) H. Berger, Inf. J . App/. Radiaf. Isotopes, 21, 59 (1970). (12) M. Heinzelman, Ber. Kernforschungslange Julich, Juel-603-ST, 191 (1968). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1271
Alter exposure;me rape was srrippeo Irom me piasric ano, because of excessive radioactivity, it was disposed of at the Reactor Center. The cellulose nitrate showed some radioactivity initially, but after one week there was none; no etching was done, therefore, until after this period had elapsed. The cellulose nitrate was etched using the solutions VX007, 3.5 min, followed by VXOO8 for 2 min, all at 60 "C. The tracks had a diameter of 2 p, which made microscopic examination and counting very easy. The best pictures were obtained by the scanning electron microscope at SOOX. Figure 2 is a micrograph taken after etching of the cellulose nitrate surface exposed in the .configuration of Figure 1, but with no boron present. Figure 3 shows a boron-containing sample after exposure and etching. Figure 4 shows the tracks made by exposure of CN film with no boron present to the fast neutron flux which produced the difficulties described above. In all counting determinations, an area was chosen in the micrograph so that approximately 1000 tracks were counted for each boron determination. The statistical error of counting was thus kept below 4%. Figure 5 is the calibration curve obtained by correlating the boron analysis given by the fluorometric determination with that provided by the track density. It is evident that the dependence is linear at low boron concentration.
minimizing the fast neutron effect increases the sensitivity of this method considerably. Based on a 10% relative standard deviation in background determination, and using the formula for paired observations given by Currie (13),the limit of quantitative boron determination in this work is estimated to be 3.8 X lO-'O gram B/cm'. The very low level of background (Figure 2) makes this limit of sensitivity possible. Hence, this method is the most sensitive known for boron determinations. Since it is based upon a nuclear reaction, and thus does not sufferfrom the limitations imposed by spectrophotometric methods, it can be applied to all boron compounds. This method is singularly free from interference by other elements. 6Lithium(present to the extent of 7.6% in natural lithium) is the only element that will produce the tracks obtained with 'OB. The sampling limitation requires that the boron-containing material be in a layer of a thickness less than the OL particles range to preclude detection loss caused by selfabsorption. The procedure offers significant possibilities in the study of processes dealing with the adsorption of boron compounds or the spatial distribution of microgram amounts of boron.
RESULTS AND DISCUSSION The results as given in Figure 5 indicate that for a dose of 5.5 X 10" neutrons/cm2,one track corresponds to 3 X lo-" gram of boron. For this dose, the number of tracks resulting from the interaction of neutrons with the CN gives a background of 3.2 X 10' tracks/cmz. The comparison of this value with the 6.6 X lo5 tracks/cm2 produced by a boron amount of 5.5 X g/cm3with the same neutron dose shows that the value of the background is negligible when determining boron amount in the g/cm*range. 1272
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
ACKNOWLEDGMENT
I wish to thank E. Przyhylowicz and H. M. Cleare for many helpful comments and suggestions. RECEIVED for review October 19,1971. Accepted January 31, 1972. (13) L. A. Currie, ANAL.CHEM., 40,586(1968).
