summarizes results. SiHC13 and GeHC13 have not been studied, but estimates for the t 4 e ) band have been made from published spectra (6, IO). Absorptivities vary somewhat with the spectrophotometer ( I ) , and for precise work each instrument should be calibrated. The constants given in the table will allow choice of the optimum cell length, however. Furthermore, for film deposition it is often more important to observe whether the reactant concentration remains constant than to know its exact value. Figure 2 shows an actual monitoring trace from an epitaxial silicon deposition using 0.33% Sic14 in H2. This was made by a Perkin-Elmer KBr Infracord with a slave recorder; the spectrophotometer scan clutch was disengaged so that a chosen wavelength could be observed continuously. The reference beam was attenuated with a screen to bring the transmittance to about 95% with only
hydrogen in the cell. The fast response to the introduction of Sic14 is evident. The instrument zero may be checked during operation by closing a shutter in the sample beam. Measurement a t 15 or 17 microns gives no signal, as shown. In ordinary operation the spectrophotometer remains set on the absorption maximum. ACKNOWLEDGMENT
The author thanks Fremont Reizman and J. L. Ashworth for aid in obtaining the infrared spectrograms. LITERATURE CITED
(1) Bauma:!
troscopy,
R. P., “Absorption Specpp. 164-5, Wiley, New
York, 1962. (2) Brewer, F. M., Dennis, L. M., J . Phys. Chem. 31, 1101 (1927). (3) Delwaulle, M. L., Frangois, M. F.,
Delhaye-Ruisset, M., J . Phys. Radium 15,206 (1954).
(4) The Eagle-Picher Co., , “Electronic
Metals and Compounds, Cincinnati, Ohio (1961).
(5) Hawkins, J. A., Polo, 9. R., Wilson, M. K., J . Chem. Phys. 21, 1122 (1953). (6) Jenkina, A. C., Chambers, G. F., Ind. Eng. Chem. 46,2367 (1954). (7) Kearby, K., J . Am. Chem. Soe. 58,
374 (1936). (8) Kelley, K. K., U. S. Bur. Mines Bull. 383 (1935). (9) Laubengayer, A. W., Tabern, D. L., J.Phys. Chem. 30,1047 (1926). (10) Lindeman, L. P., Wilson, M. K., Spectroehim. Acta 9, 47 (1957). (11) Nisel’son, L. A., Seryakov, G. V., Zh. Neorgan. Khim. 5, 1139 (1960). (12) Pohland, E., 2. Anorg. Allgem. Chem. 201,265 (1931). (13) Rand, M. J., ANAL.CHEM.35, 2126 (1963). (14) Smith, A. L., J.Chem. Phys. 21,1997 (1953). (15) Stull, D. R., Znd. Eng. Chem. 39, 517 (1947). (16) Theuerer, H. C., J . Electroehem. Soe. 108,649 (1961).
RECEIVED for review December 27, 1963. Accepted February 13, 1964. Presented before The Electrochemical Society, Xew York, September 30, 1963.
Activation Analysis of High Purity Materials W. J. ROSS Analytical Chemisfry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b Samples of high purity beryllium, aluminum, and iron have been analyzed for trace amounts of 6 2 possible elemental impurities by measuring the gamma activities of radionuclides produced by neutron activation. Ten elements are determined b y nondestructive techniques after bombarding a portion of the sample for 10 seconds and making a rapid measurement of the short-lived radionuclides that are induced. A second portion of the sample is activated for 2 0 minutes and analyzed for the gamma-emitting radionuclides of chlorine, bromine, and iodine. Forty-nine elements can b e determined in a third portion of the sample after a 16-hour irradiation. The induced radionuclides are divided into six groups b y a series of rapid chemical separations and each gamma activity is identified, counted, and compared with that of a standard radionuclide.
S
are currently being placed on many materials used in the construction of nuclear reactors. These specifications have created a renewed demand for methods of determining trace amounts of essentially all naturally occurring elements. Although much of this demand can be met with currently available met’hods, many elements cannot be determined readily with the TRINGENT SPECIFICATIONS
1 1 14
ANALYTICAL CHEMISTRY
desired sensitivity by any method amenable to routine use. The techniques of activation analysis and gamma spectrometry have become very useful for the determination of micro or submicro amounts of most elements (2, 4). In this paper a method is described in which trace amounts of 62 elements in certain matrices can be determined by neutron activation analysis. The inherent sensitivity of neutron activation analysis is dependent on the accuracy with which the induced gammaemitting nuclides can be identified and measured. Many elements can be detected with extremely high sensitivitiesi.e ., 10-9 gram-after being subjected to a flux of 1012 n./sq.cm./sec. for several hours. Most elements, however, have sensitivities that do not differ by more than a factor of 100, -10-7 to gram. Consequently, the bulk of the gamma energies are easily masked when multiple activities are induced by neutron activation. This interference is especially serious when the major elemental constituentsof a sample are converted into gammaemitting nuclides. Partial resolution of gamma energies can be obtained by variation of the experimental conditions used, such as irradiation and decay times. This approach is not practical when a sample contains even trace amounts of such common elements as E a , Al, Mn, Cu, or Br because these
elements become highly radioactive during neutron bombardment and, subsequently, overshadow the activities of other nuclides. Several proposals (1, 3, 7, 8) have been reported wherein instrumental resolution of gamma energies has been simplified by postirradiation chemical treatment of the sample. The purpose of this paper is to extend these techniques and present a flexible method which can be used to determine 10 p.p.m. or less of essentially all of the naturally occurring elements that form gamma-emitting nuclides when bombarded by reactor neutrons. “Nondestructive” and “chemical” procedures have been combined to achieve this goal, and the resulting method is a compromise of possible conditions selected for maximum scope, sensitivity, simplicity, and accuracy. EXPERIMENTAL
Neutron Activation. Two reactor facilities were used for the activation of samples. An air-cooled facility with a flux of -8 X 1011 n./sq. cm./se.c. was used to irradiate 1-gram samples in polyethylene bottles for 20 minutes and 16 hours. A pneumatic facility with a flux of -6 X 1013 n./sq. cm./sec. was used to activate 1- to 100-mg. samples for 10 seconds. For these latter irradiations, the sampIes were sealed in high purity polyethylene tubing and placed in a polyethylene “rabbit.”
A?,
, i0sec
... 2 0 min
decoy 9.0rnin decay rnin decay
- 40 ---18.
6 x 10'3 n/cmt/sec Geometry. 0.5-in. absorber FIUX,
Table 1.
Sensitivity of the Method for Short Half-Life Isotopes
Radionuclide Identity Mgn A128 ~Al-41
f
Ti51
6
\T62
U
MnS6 Rhlo4 DY'~" EulS2
Half-life 9 . 5 5 min.
2 . 3 min.
1.83 h; 5.8 min. 3.77 min. 2.58 hr. 42 sec. 1 . 2 5 min.
9 . 3 hr.
54 min.
Approximate limit of quantitative measurePg-
7 0.1
1 20 0.005 0.006 0.02 0.04
0.007
0.004 4 10-See. irradiation in a flux of 6 X 1013 neutrons cm.+ sec.-l and a 120-sec.
decay. Peak channel contains 200 c./min. when sample is separated from a 3- X 3inch NaI(T1) crystal by a 0.5-inch polyethylene absorber. Figure 1,
Short half-life isotopes
Gamma Spectrometry. Identification and measurement of gamma activities were accomp1ir)hedwith a multichannel pulse height ,znalyzer coupled with a 3- X 3-inch NaI(T1). hfeasurements were made a t 10 k.e.v. per channel and with the source separated from the detector by a 0.5-inch polyethylene beta absorber. Standard Comparators. The identity and sensitivity of the radionuclides induced in 62 elements were established by irrao iating sufficient gram, of each amounts, 10-7 to element to yield 1000 c./min. in the peak channel of the primary gamma energy. The elementf listed in Table I were irradiated for 10 seconds in a flux of 6 X 1013 n./sq. cm./sec. and counted after 120 seconds decay. The halogens were determined as 37-min. C P , 18min. B P , and 25-min. 112*after being irradiated for 20 minites in a flux of 8 X 1011 n./sq. cm./sec. and allowed t o decay 20 minutes. The remainder of the elements under investigation were irradiated for 16 hours in a flux of 8 X 10*ln./sq. cm./sec. and counted after a decay period of 25 rrt 3 hours. The gamma spectra of the radionuclides produced under these conditions were measured with the sources in essentially a point configuration--.e., 0.1- to 0.5-ml. solution, as well as in a 25-ml. volume of liquid contained in a 125ml. Erlenmeyer flesk. These spectra were combined nto a standard catalog and used for comparison with the gamma activities induced into samples under the f3ame conditions. The sensitivity, or limit of detection, was arbitrarily se1ec;ed to be that weight of an element that exhibits 200 c./min. in the peak channel (-1000 c./min. in a photopeak that spans 30 k.e.v. in the 0- t o 100-k.e.v. range, 50 k.e.v. in the 100- to 500-k.e.v. range, and 70 k.e.v. in higher (energyranges). Procedures. A. DETERMINATION OF CHLORIPI'E, BROMIVE, AND IODINE. One gram, or more, of a sample is
irradiated for 20 minutes in a flux of 8 X n./sq. cm./sec. After activation the sample is dissolved with HiS04 (and a t least 1 ml. each of HC1, HBr, and HI) and distilled to fumes of sot. The volatile acids are collected in cold NaOH solution diluted to 25-1111. volume before the gamma activities are counted. The decay time is normalized to 20 minutes, and the measured activities are compared with those of ClSq, BrS0,and 1'28 in the standard catalog. B. DETERMlN-4TION O F SHORT-LIVED RADIONUCLIDES. A small amount, 1 to 100 mg., of a sample is sealed in 0.5inch lengths of surgical grade polyethylene tubing (3/1&ch 0.d.) by melting and crimping the ends of the tubing. This tubing is then placed in a l/2- X 11/8-inch polyethylene rabbit, equipped with a screw-type cap and irradiated for 10 seconds in a pneumatic facility at a flux of 6 X 1013n./sq. em./ sec. After each irradiation, the polyethylene tubing that contains the sample is removed from the rabbit, partially severed, and squeezed several times with forceps to displace the entrapped radioactive gases (principally Ar49. The sample is then counted a t 0.5-inch geometry after minimum decay. The measured activities are normalized to a decay time of 120 seconds and compared with those of Mg27, A P , Ar41, Ti61, v 5 2 , Mn56, Rh104 Dyl65m EUlS and In"6 in the standarh cattalo;. C. SEQUENTIAL SEPARATION PROCEDURE. The remainder of the 62 elements are determined after irradiating 1 gram, or more, of a sample for 16 hours in a flux of 8 X 10l1 n./sq. cm./ see. and carrying the sample through the chemical procedure diagrammed in Figure 2. The sample is dissolved with minimum amounts of HC1 and HNO, and sufficient to retain the sample in solution when distilled to fumes of SO,. A second distillation from HBrHzSO4 solution is performed after "03 has been completely removed.
Both distillates are collected in cold NaOH solution and diluted to 25 ml. before the activities of the Group I1 nuclides are measured. The Group I elements are precipitated from the residual &So4 solution after the acid is diluted with H20, and then HC1 and 10 mg. each of Ag and W carriers are added. Precipitation of the Group I11 elements from the HC1-H2S04 solution is performed by adding -20 mg. of HgC12 and sufficient SnClz to reduce completely Hg+2 t o Hgo. A double precipitation of the Group IV and VI elements is performed to ensure complete removal of the soluble Group V elements from the bulky hydroxide precipitates obtained in the analysis of aluminum, beryllium, or iron samples. The final solutions of each of these three groups are diluted to 25 ml. before being counted a t 0.5-inch geometry. RESULTS
Scope and Sensitivity. Gammaemitting nuclides of 62 elements have been produced with three sets of irradiation conditions. Ten elements can be characterized only by the short-lived nuclides listed in Table I. Since the sensitivities of these elements differ by as much as lo*, the ideal resolution of gamma activities shown in Figure 1 is realized only when none of the elements is present in overshadowing abundance. This potential difficulty is the primary reason that nuclides such as 10.5-min. Co60m, 5.1min. Cus6, 2.8-hr. SrQm, 15-min. Molo1, and 50-min. Cd"7 are rejected as characterizing nuclides and longer lived nuclides are used instead. Through use of complementary chemical procedures, the remaining elements can be systemVOL. 36, NO. 6, MAY 1964
* 1 1 15
Sample Dissolve i n HCI, HN03, H2S04, HBr
I Residue ( G r w p I)
Distillate (Group II)
Solution
SrS (250 pg)
As76 (0.1 pg)
Au carrier Zn -t HCI
Agllom (12)
8 a l 3 I (100)
~e~~ (7)
Br8' (0.7)
or
Tal8' (1)
Ru97 (5)
HgCl .+SnC12 2
w187(0. I )
M O ~ ~ -( 1T. 0) C ~ Sn 1 1 7m (100) SbIz2 (0.1) . , Te13I _ , 131 (26)
Solution
Residue (Group Ill)
Re188 (0.02)
11) Pd109-Ag1wm (0.3 pg) 1,19' (0.005)
HgI9' (0.007)
Pt195 (2) Au198 (0.003)
I Solution (Group V )
Precipitate'
N~~~(0.4 pg)
Dirtolve in 8 M HCI Extract with TBP
K4' (22)
Co')
(6)
Ni58-Co58 (1400) Aqueous (Group VI)
Organic (Group IV) SC4
c?
(0.2 pg)
(8 pg)
CuU (0.3) Zn69rn
(16)
Fe59 (13,000)
~a~~~(0.I )
Rba (60)
Go7' (0.5)
ceI4' (5)
Cd1I5 (2)
Z?7-Nb97 (42)
prI4' (3)
cs134(3)
H f l 8 I (1)
Nd147 ( 1 I )
IMOW1
Th233-Pa233 (1)
Sm153 (0.003) Gd159 (1.0)
U239-Np239 (0.1)
TbI6' (0.9)
Ho166 (0.02) Erl"
Yb175 (0.2)
FIUX: 8 x 10" n/cm2/sec Decay: 25 i 3 hr
Luln (0.004)
l
N
(0.6)
TmI7O (0.6)
Irradiation time: 16 hr
'
l
'
l
'
l
'
atically placed into one of six groups (excluding the halogens). I n the absence of overshadowing activities, the major photopeaks of these nuclides are observed in the relationships shown in the spectra of synthetic groups of nuclides in Figures 3 through 8. The sensitivities achieved under such ideal conditions are listed parenthetically in Figure 2 and can be enhanced, if the nuclide is isolated sufficiently, by extending the counting time beyond the standard I-minute period. The procedures were selected to yield quantitative separation of carrier-free amounts of each element with minimum contamination from other activities. iilthough alternate procedures were used whenever possible, several precipitation steps are included in the separation schemes and necessitate the addition of one or two carriers before each precipitation is performed. Otherwise, the separations have been performed without carriers to minimize the complexity of the sample solution. A sensitivity of 5 10 p.p.m. cannot be attained for Mg, Ar, Ti, and Sr by 10second irradiations unless samples in excess of 10 mg. can be activated. Longer irradiation in a flux of 10" yields short-lived nuclides of elements other than those in Table I and thereby increases the complexity of the gamma spectra. The sensitivities achieved for Fe, Xi, Rb, Zr, Sr, and Ba also exceed the desired limit unless samples in excess of 1 gram are irradiated for 16 hours. Molybdenum-99 is recovered in Group V while the Tc99 daughter of hI099 is distilled with Group 11. The 0.14 m.e.v. photopeak of Tcg9is measured to attain the sensitivity ascribed
to 310.
l
'
5
t
t 0.2
0.4
ENERGY, MeV
Figure 3.
11 16
Group
ANALYTICAL CHEMISTRY
I nuclides
Figure 4.
0.6 0.8 ENERGY, Mew
Group II nuclides
j.0
1.2
F II n
Although variations in the flux of the reactors used were insignificant, 0.1 to 0.5 ml. of standard solutions of .4u, Co, Mn, and Br are included with each sample as monitors. The precision with which individual gamma activities are measured is dependent on the size of the photopeaks and the magnitude of the underlying activity of higher energy gamma photons. A minimum peak of 200 counts in the maximum channel (-1000 counts in the peak) was selected as a limit of detection with a permissible underlying activity of 1000 counts per channel or less. Such a peak can be measured with a relative error of less than 10%. The degree to which each element is
LL 0.2 0.4 0.6
'OO
ENERGY, M e v
Figure 5.
Group 111 nuclides
Accuracy and Precision. The validity of results obtained by the nondestructive technj que is dependent on errors inherent i i the activation and measuring procedures. A probable source of error during activation is self-shielding by the matrix material and major components that exhibit high-thermal neutron cross sections. Reynolds and Mullins (6) have shown that t h i attenuation of thermal neutrons by aluminum is negligible. Attenuation by elements with smaller cross sections-i.e., Be and Fe, is considered, therefore, to be negligible also. Inasmuch as the materials analyzed by the described method must be relatively pure, possible attenuation of neutrons by Impurities has been disregarded.
isolated by the chemical separations also affects the precision and accuracy of the results. The procedures outlined in Figure 2 were developed with tracers produced by activating microgram amounts of all except the least sensitive elements. The separations were performed with individual elements as well as with composite solutions of each group and with synthetic solutions that contained all the elements listed in Figure 2. Quantitative separation and recovery of all except five elements is achieved by single-step separation procedures. The exceptions are (1) the separation of Sb and Re is 70 to 90% after a single distillation but is 90 to 95% complete when two distillations from H2S04-HBr are performed, (2) reduction and
I
I
I
,
'
--I
t
''0
L ' 012
'
A'
011
0.6
'
0!8 ' 4.;
'
1,;
1-
ENERGY, Mev
Figure 7.
Group V. nuclides
1.4
4.6
'I OO
0.4
Figure 8.
1.2 ENERGY, mev
0.0
1.6
2.0
Separation Group VI
VOL. 36 NO. 6 MAY 1964
1117
-i - g , 5 h r I r r a d i a t i o n ,
F--1-
I
Table II. Trace Elements in Beryllium
2s-hr Decay, IO-inch G e m e t r y
Element" Na A1
c1
sc
V Cr
Mn
CO CU
Ga
Concentration, p.p.m. SpectroActivation chemical 4
450 51 22 7 128 67 11 73
7
