prcseiice of either absorbed nitrogen or nitride salts leads t o high results because tlie entire volume of noncondensable gases is measured. If nitrogen is known to be present, a high-vacuum stopcock can be added as part of the confining volunie between the fiducial marks. ;Ifter the amount of gas present is calculated, the gas is transferred through tlie stopcock to an evacuated sanipling t d b and the ratio of oxj‘gen to nitrogen is determined mass spectrometric:illy or by the gas chromatograph. Dupraw and O’Seill (8)have shown that both oxygen and nitrogen can be determined by passing the gases through a copper furnace to remove osygen. The residual nitrogen is then measured manometrically. The method has been successfully applied to other halide salts, although
most of the experience reported herein has been with fluoride salts. Such compounds as holmium nitride and molybdenum disulfide have also been analyzed for oxygen impurity. The method is also useful for the determination of oxygen in metals. Obviously, it is not competitive with inert gas fusion or vacuum fusion methods. Hon-ever, in the case of metals or alloys that are excellent oxygen getters-for example, magnesium-where the fusion methods are not applicable, potassium bromotetrafluoride is particularly useful. This has been proved with yttrium-magnesium alloys in which the magnesium content is so high that it reacts with the liberated carbon monoxide in the fusion reaction in graphite to such a n extent t h a t no measurable gas is detected. Such alloys
are readily analyzed by the potassium bromotetrafluoride method. LITERATURE CITED
(1) Audrieth, L. F., “Acids, Bases, and
Non-Aqueous Systems,” p. 57, Pennsylvania State College, State College, Pa., 1949. ( 2 ) Dupraw, W.8., O’Keill, H. J., ANAL. CHEM.31, 1104 (1959). (3) EmelBus, H. J., Woolf, A. -k., J . Chern. Soc. 1950,164. (4)Hoekstra, H. R., Katz, J. M., . 4 s a ~ . CHEM.25, 1609 (1953). (5) Sheft, I., Martin, A. F., Iiatz, J. J., J. Am. Chem. Soc. 78, 1557 (1956). RECEIVED for review August 7, 1959. Accepted November 18, 1959. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 2 to 6, 1959. Work carried nut under Contract W-7405-eng-26 at Oak Ridge National Laboratory, operated by Union Carbide Corp. for the U. P. Atomic Energy Commission.
Instrumental Neutron Activation Analysis V. P. GUINN and
C. D. WAGNER
Shell Development Co., Emeryville, Culif.
A system of purely instrumental neutron activation analysis in which no chemical operations are employed i s described. It consists solely of neutron activation followed b y y - r a y spectrometry. Even with the relatively low thermal neutron flux available from the Van d e Graaff accelerator phatoneutron source ( 1 Os neutrons set.-' cmF2), useful analyses have been carried out for more than 25 elements, in many cases with sensitivities in the parts-per-million range. In general, the method i s much faster than chemical or spectroscopic methods of equal sensitivity and accuracy; it i s nondestructive and i s relatively free of serious interferences. No sample preparation i s involved. It i s useful not only for measurement of trace concentrations, but, because of its great speed, i s also chosen for analyses of many elements a t macro concentrations. Activation plus counting total only about 10 minutes per sample.
C
neutron activation analJsis, a‘ esempiified in the excellent studies by Leddicotte and Reynolds ( 7 ) , Meinke (9))and Jenkins ( S i , , YrnFIo>s activation, followed by addition of carrit’rs and holdback carrirrs, and the.:: hy regular wet chemical scparationt before (ounting. This method is unsurpassed for the detection of traces--1 e., parts per Tillion (p.p.m.) to parts per billion (p.p b.)-of certain elements in the prrwn of rt,latively OXVESTIOSAL
large concentrations of other elements that also become activated. However, because of the chemical operations required on each sample, the method is time-consuming Furthermore, the time delay precludes the utilization of many of the short-lived activities developed in the samples, activities with half lives on the order of minutes. The instrumental method, on the other hand, requires only the counting of the samples after activation. I n recent years increasing interest has been shown in a purely instrumental approach, and a number of studies involving no chemical separations are reported in the literature ( 1 , 3-6, 8, IO, 12). Some emplo? identification by beta energy and half life only, many involve y-ray spectrometry. Many activities with half lives on the order of minutes are employed to advantage. Such methods are classified as methods of instrumental neutron activation analysis. The method described here is based on y-ray spectrometry and half life, and utilizes predominantly activities with half lives in the range of minutes to hours. This method has been in operation for routine analysis for more than a year in these laboratories. THEORY AND CALCULATED SENSITIVITIES
The basic equation for the activity, Aa, in disintegrations per second (at zero decay time), generated per gram of element, by exposure to a thermal neutron
flux, f, neutrons sec.-l cm.-* for a period of time, ti, is: 0.693 t i
1 - e - T ) (1)
in which N is the number of target nuclei, per gram of the element, capable of forming the radioisotope in question, by the (n,y) reaction; u is the isotopic thermal neutron capture cross section, in square centimeters per nuclrus; and b.6 is the half life of the radioisotope formed. Equation 1 may also be written as:
(2)
in ,which a is the per cent abundance of the parent stable isotope, among the various stable isotopes of the element, U‘ is the isotopic cross section in barns, and M is the chemical atomic weight of the element. Equations 1 and 2 show that the activity produced is directly proportional t o the neutron flux, f, hence the minimum detectable amounts and concentrations of a n element are inversely proportional to the flux. Saturation (steady state) activity is achieved if ti >> t o 5, when the expression: 0.693 t i
(1 - e -
7)
approaches unity. The activity level produced is half of the saturation activity, As, when t , = t o 5 , 3 ‘( AS VOL. 32, NO. 3, MARCH 1960
317
10,000
9,000
m.e.v.
2.3-MIN.Al"
4
7,000
v,
ff
a
F t 0.51 m.e.v.
8,000
i
A
4,000
12.8-m. cu6' MIN. IRRADIATION MIN. DECAY
U
30 34
MIN.IFtRADIATION 7 MIN.DECAY 5
1,000
7 5 MIN.IRRMIATION 7 8 MIN-DECAY
t7
'""ir 1, *0°
400
30-MIN.Pl'" 37-MIN. C1" 0.72-
i B
0.79
zoo 0 0
10
20
30
40
50
60
70
f
0
10
20
30
CHANNEL NO.
Figure 1. Gamma-ray spectra of neutron-activated reforming catalyst
when t i = 2&, As when t i = 3to.s, and so on. Thus, if there is interest only in a short-lived activity, the sample need not be activated for more than one or two half lives of the activity being produced. This is an important advantage of the instrumental method, because it permits the approach of satu-
50
40
60
70
C H A N N E L NO.
Figure 3. Gamma-ray spectrum of neutron-activated copper at long decay
Figure 2. Representative gamma-ray spectra obtained with 7.6-cm. well crystal
b
ration activity levels with only very short irradiation times. The irradiation time is also a very useful parameter for the simplification of
/\
I-
a L
J
l
HIGHGAIN I
I
25-MlN. 1'"
I.
Table
Theoretical and Experimental Detection Sensitivities for 2 7 Elements
(25-gram samples, 2-hour irradiation at 108 neutron flux) P.P.M. Detectable Radioisotope Element Formed Half Life Theorya Exptl.6 Na 15 h 5.2 6.4 9.5 m 80 82 R.1g A1 2.3 m 1.3 2.4 c1 37 m 6.8 6.8 ' K 1 2 . 5h 220 150 Ca 220 270 8.7m Ti 5.8m 72 40 V 3.8m 0.20 0.12 Cr 27 d 3,300 1,,300 Mn 2.6 h 0.11 0.12 co 0,450 10.5 m 0.036 Iii 2.6 h 180 320 7 4 cu 13 h, 5 . 1 m Zn 370 14 h 460 A8 27 h 7.6 5.2 Br 3.4 18 m, 36 h 1.4 M O 15 m, 15 m 28 14 32 38 Ru 4 . 5 h, 37 h 2.3m 1.2 3.8 Ag In 0.012 54 m 0.081d 40 m, 9 . 5 m 65 40 Sn Sb 3 . 5 I11 0.34 8.9c I 3.3 1. o 25 m Re 20 m, 17 h 0.044 0.5c 5.1 w 24 h 3.5 Pt 17 30 m, 3 . 1 d 80 Au 1.1 1.3 2.7 d Defined as concentration in 25-gram sample that will give 1000 gamma d.p.m. * Defined as concentration in 25-gram sample that will give 500 c.p.m. (equivalent to 1000 gamma d.p.m.). c Discrepancy due to high degree of internal conversion of very soft gammas. d Discrepancy due mostly to indium self-shielding. (1
318
0
ANALYTICAL CHEMISTRY
F!
~~
a. I
0
10
20
30
40
50
60
CHANNEL NO,
the y-ray spectrum produced by a complex sample. Thus, brief irradiation, followed by rapid counting, produces a spectrum in which the short-lived activities developed in the sample are accentuated. Similarly, a longer irradiation, followed by an appropriately long decay time before counting, produces a spectrum in which the longerlived activities developed in the sample predominate. This point is illustrated by the two y-ray spectra shown in Figure 1. Both spectra are of the same sample, a platinum-chlorine-on-alumina catalyst, but spectrum il was obtained in a 5-minute irradiation with only a 7-minute decay time prior t o counting. Spectrum 33 was obtained in a 'ibminute irradiation x-ith a 78-minute decay
10,000.
Figure 4. Gamma-ray spectra of neutron-activated tin at two decay times 30-mlnute irradiation
L
9,000
-
8,000
-
9.5-MIN. Sn’” 40 -MIN. Sn’”
0.33
~n”’ 0.15
&
7,000
-
6,000
-
0.
c9
0
1
iz:
5,000 4,000
u
m.e.v. ~ n ” ~
L(, -
0.15
~n”’
-
3,000 2,000 1,000
0
9,000
8,000
-
7,000
-
6,000
-
0.30 m.e.v.
Tc lo’
30 MIN.
IRRADIATION
i
5
16 MIN. DECAY
I 1
!
~n”’
/ ) &1
o
-
0.33
20
40
60
8010
20
40
60
80
the & Z S activity predominates (M28 has a haif life of 2.3 minutesand ?-rays of 1.78-1n.e.v. energy), whereas in spectrumB the pti99 and cis3 activites predominate [ p p and C138 have haif lives of 30 and 37 minutes, respectively, xiid ernit p r a y s of 0.32, 0.54, and 0.72 to 0.79 n1.e.v. (Pt) and 1.59 and 2.16
laboratories with a 3-m.e.v. Van de Graaff electron accelerator, but may readily be converted, as mentioned above, to sensitivities at other fluxes. The calculated sensitivities are based on the indicated literature values of isotopic abundances, isotopic cross sections, chemical atomic weights, half lives, and y-ray yields per disintegration. They assume an irradiation time of 2 hours’ maximum, a 25-gram sample, and negligible decay before counting. Use of such large samples is possible because of the low self-absorption of neutrons and y-rays, and greatly increases the sensitivity in terms of minimum detectable concentrations. This is important when employing a flux (108) so much lower than typical nuclear The reactor fluxes (loi1 to minimum detectable activity is arbitrarily defined as 1000 gamma disintegrations per minute.
per-million level. These theoretical sensitivities have been checked experimentally (Table 1). The experimental results on known samples were extrapolated to an arbitrarily defined 500 net counts Per minute (c.P.m.1 as the minimum accurately measurable counting rate (aPProximate1Y equivalent to 1000 gamma d.p.m.). Gamma-ray spectra
VOL. 32, NO. 3, MARCH 1960
319
20.2 MIN.DECAY
7.3 M I X D E C A Y 10,000
10,000
-
9,000
-
8,000
-
0.84 m.e.v.
9,000
Mg27
8,000
2 8 MIN.DECAY
0.30 m.e.v. Tcl"
-
1 0 2 MIN.DECAY
I 15-MIN. Mo'" 15-MIN.Tc Io' 24-HR. W'"
0.19 7,000
g
6,000
,BS
0
2
5,000
23
0.84
4,000
3,000 2,000 1,000 C
10
20
30
Mg"
1,000
I
0
4
10
C H A N N E L VO.
Figure 7. Gamma-ray spectra of neutron-activated cracking catalyst a t two decay times
'b
APPARATUS
Neutron Source. Keutron generation is accomplished with a 3-m.e.v. vertical Van de Graaff electron accelerator (High Voltage Engineering C w p . ) . At its maximum power of 3 kw., t h e 3-m.e.17. I-ma. electron beam is absorbed completely in a small, 0.3-cm.-thick gold target in which about lo$% of the beam energy is converted t o bremsstrahlung. T h e beam is scanned magnetically over a n 8-cm. length to avoid overheating of the watercooled gold target. A 15-em. cube of Tygon-coated beryllium metal is centered 5 em. below the gold target, the bremsstrahlung impinging upon the berylliuni generate neutrons by the 1 3 c - 9 (2,n) 2 He4 reaction, as in the work c:i lloses and Sddick (11). As this reaction is 1.67-m.e.v. endoergic, only the x-ray photons in the range of 1.67 to 3 m.e.v. are effective. The neutrons are generated throughout, the block and have initial energies ranging from 0 to 1.2 m.e.v. The beryllium is mounted in a 46-cm.-diameter aluminum tank filled with water. The *Ipper surface of the beryllium is 0.6 em. below the surface of the water. The water tank is mounted concentrically within a larger tank, 58 cm. in diameter, and the space between the two tanks is filled with a n aqueous mush of boric acid t c prevent neutrons from rscsping into the room. The top of the entire assembly is covered by a sheet of Rord, 0.6 cm. thick, t o minimize activation of the gold target by neutrons. At a distance of 0.3 em. from the beryllium block are eight ver-
320
8
ANALYTICAL CHEMISTRY
4 / o 10 C H A N N E L NO. 30
I
20
30
40
Figure 8. Gamma-ray spectra of neutron-activated tungsten-molybdenum catalyst a t two decay times 30-minute irradiation
1 0-minute irradiation
L I stal counter and 100-channel analyzer. The principal y-rays in these spectra are shown in Table 11.
20
tical polystyrene tubes, each capable of accomniodating three 30-ml. polyethylene screw-cap sample bottles. The use of large samples 125 grams) improves the sensitivity of detection considerably. Activation of a number of different clements in the ports shows that the thermal neutron flux ranges from about 5 X lo7 near the bottom of the block to about 8 X 10' approvimately twothirds of the way up the side of the block, then decreasing to about 7 X 107 near the top. Scintillation Counter. Most of t h e studies carried out to date in these laboratories have employed a 5.6-em. high sotiium iodide cr!,stal, 7.6 cm. in diameter, n ith a 3 2-cm.-diameter well, 5.7 cm. deep. This crystal will accomznodate a 30-ml. polyethylene sample bottle. The riystal is optically coupled t o a Du Mont 6363 multiplier phototube 7.6 em. in diameter. The counter is surrounded by 7.6 em. of lead shielding, which results in a background counting rate of 250 c.p.m. Representative y-ray 3pectra (a) obtained with 25-ml. samples in this counter are shown in Figure 2 . The spectra shown illustrate three simple spectra of three different energies (0.059 m.e.v. from 10.
