Determination of Deuterium in Gases by Neutron Time-of-FlightSpectrometry Max Peisach and Ren6 Pretorius Southern Unioersities Nuclear Institute, P.O. Box 17, Faure, C.P., South Africa
A new method for the determination of deuterium in gases uses time-of-flight spectrometry of neutrons emitted in (d,n) reactions. A pulsed deuteron beam of 2.0, 2.4, or 3.0 mev was used. Deuterium was determined in hydrogen, nitrogen, oxygen, and carbon dioxide at partial pressures ranging from 1to over 100 mm. The average time taken for an analysis was about 10 minutes. The relative standard deviation was *3.4% and the sensitivity, 2.5 X 10” grams per sq cm beam area, was sufficient to detect deuterium in natural hydrogen.
WITHTHE INCREASINGUSE of deuterium as a tracer in chemical systems, the need has arisen for a method of determining deuterium rapidly and, where possible, without disturbing the system by sampling. Nuclear methods have already proved successful for determining the deuterium content in water ( I ) and hydrogenous organic liquids (2) using reactions induced by recoil deuterons in neutron-irradiated samples. However, the use of “knock-on’’ deuterons depends on the presence of suitable nuclides in the sample which undergo deuteron reactions yielding measurable radioactivities. Furthermore, the method is not readily applicable to gases where difficulties arise either from the low activity produced in the irradiated sample or from the techniques for irradiating large amounts of gases under pressure in reactors. A novel method for the determination of deuterium in the surface layers of Zircaloy was described by Butler (3), who used accelerated deuterons with energies up to 3 mev to induce the nuclear reaction *H(dp~)~He and counted the neutrons produced. This method (3) breaks down when the sample contains other nuclides capable of yielding neutrons by (d,n) reactions a t the energy of the bombarding deuteron beam. In fact, a t deuteron energies above 1.85 mev, Butler observed interference from oxygen when the threshold for the reaction IsO(d,n)’7F was exceeded. The sensitivity of his method was further reduced by neutrons produced from (d,n)reactions on nitrogen-14 in the residual air in the vacuum system and from deuterons striking the aluminum beam tube. The energy of a neutron, E,,, emitted from a nuclear reaction is determined by the Q-value of the reaction, the energy of the deuteron incident on the target nucleus, &, and the angle Or. at which it is emitted. From the kinematics of a nuclear reaction, the energy of the neutron is given (4) by
vfE=o f
(1)
where
and md, m,, and M refer to the masses of the deuteron, neutron, and product nucleus, respectively. For a specific nuclear reaction, the neutrons will have energies related to the different Q-values pertaining to the corresponding excited states in which the product nucleus is left. I t may then be possible to utilize some selected neutron energy for determining the concentration of a specific nuclide in the target sample nondestructively, and any experimental method by which neutron energies may be determined could be suitable for analysis. A convenient and accurate method for determining neutron energies is by the time-of-flight technique where the time, t , taken by a neutron to cover a fixed distance, d, is measured. The relationship between t and E,, is given, nonrelativistically, by
72.3 X d (4) where the constant includes the mass of the neutron and conversion units, while E,, is given in mev, t in nanoseconds, and d in meters. The energy resolution attainable will thus depend on the precision in measuring t. Recent develop ments of fast electronics have made it possible to measure nanosecond time intervals. The two signals marking the start and end of the neutron flight are obtained from some electronic device coupled with the incident pulsed beam and a neutron detector. Clearly the duration of the pulse represents an error in the time measurement and constitutes a iimiting factor in the technique. The application of neutron time-of-flight spectroscopy to analysis was first discussed by Peisach and Pretorius (5,6). When solids or liquids are irradiated with charged particle beams, the entire energy of the beam is deposited within a relatively short distance in the sample, thus generating high temperatures which are frequently sufficient to destroy the sample. For this reason charged particle irradiations usually require refractory targets or targets with very good cooling. This problem does not arise in the case of gases, because the density of a gas is so low that only a small fraction of the beam energy is transferred to the gas per unit path length. For this reason charged particle irradiation can readily be carried out on gases, provided the container has a sufficientlythin window through which the beam may enter. Analyses based on such irradiations would then be nondestructive and the gas under investigation couid readily be part of a closed system which need not be disturbed by sampling.
(1) S. Amiel and M. Peisach, ANAL.CHEM., 34,1305 (1962). (2) E. Fabbri, E. Lazzarini, and V. Sanguist, Intern. J . Appl. Radiation Isotopes, 15,437 (1964). (3) J. P. Butler, “Radiochemical Methods of Analysis,” Vol. I, p. 393, Proceedings of Symposium, Salzburg, October 1964,
intern. At. Energy Agency, Vienna, 1965. (4) R . D. Evans, “The Atomic Nucleus,” p. 412, IMcGraw-Hill, New York, 1955.
( 5 ) M. Peisach, Chem. Communications, 632 (1966). (6) M. Peisach and R. Pretorius, S. African Knd. Chem.. 20,s (1966).
Figure 1. Gas-handling apparatus and irradiation cell A. B.
C. D. E.
F. G. H.
J.
K. Nikelrpindow L. platimrmbckofcell M. Conaedioatovaantmpun~p
-
N . Conoeetiontogasbdmg a p w 0. Detectorshield
P. Scintillationdetector
EXPERIMENTAL
Apparatus. A diagrammatic sketch of the apparatus is shown in Figure 1. Gas samples could either be inserted from a gas-sampling bottle (shown) or fed directly into the irradiation cell at the point where the gas sample bottle was inserted. The dead space was made as small as possible to reduce losses when handling gases not condensable with liquid nitrogen. The nickel windaw through which the irradiation beam passed was mounted with Araldite on the window support in such a way that the gas pressure helped to keep the window in position. Window supports of different lengths could be inserted, thus changing the path length of the beam through the gas. A beam piith length of 3 cm was suitable for most purposes, but when the gases had a relatively high density, or when relatively high pressures were used, better energy resolution could be r2ttained with shorter path lengths. The entire gas cell assembly was insulated, so that the total current could be measured with a current integrator (7). Electronic Equipment. A block diagram of the electronic equipment (8) is shown in Figure 2. Neutrons generated by the pulsed beam at the target were detected in a NE 213 liquid scintillation aetector placed at a distance d and an angle Or. from the target. The time of arrival of the pulsed irradiating beam on the target was given by a signal from a pickup probe placed near the target in the beam tube. The difference in time between the signal from the detector and ~
~
that caused by the beam pulse was converted by the time converter to a pulse, with amplitude proportional to time, and recorded by the multichannel analyzer. Because the neutron detector was sensitive to y-rays as well as neutrons, signals caused by y-rays from whatsoever source were rejected by pulse shape discrimination (8) and low-level electronic noise, by the energy discriminator. The current integrator (not shown in Figure 2) was set to accumulate a predetermined total current and automatically switched offthe measuring system when this value was reached. Irradiation and Measurement. Samples of gas at measured pressures were irradiated with a pulsed deuteron beam of energy between 2 and 3 mev obtained from the 5.5-mev van de Graaff accelerator at the Southern Universities Nuclear Institute. Pulses were 5 nsec long and 400 nsec apart. Low average beam currents of between 0.2 and 0.5 pa were used to prevent damage to the thin (0.0001-inch; nickel window which could not be cooled. The platinum back of the cell where most of the energy of the beam was dissipated was cooled externally with a jet of compressed air. Measurements were usually made at = 30" to reduce. the background of neutrons caused by the beam striking the beam tube, collimator holders, and other material in its path. From Equation 4 it is clear that better energy resolution is attained with longer fight paths. However, the count rate decreases with 8, so that a compromise h s to be reached between resolution and the duration of the analysis. At
_ _
(7) E. Blignaut and J. J. Kritzinger, N u l . Znstr. Methods, 36,176 (1965).
!ikrew-onclamp
Beam tube
Irradiation beam Pickul, probe Irradirition cell Cawampling bottle Sample connection point Tantalum collimator and window support Vacuum O-ring
(8) W. R. McMurray, P. van der Merwe, and I. J. van Herden, N w l . Phys., ,492,401 (1967). VOL 39, NO. 6, M A Y 1967 e
651
target followers
amplifier
I
I I
sync h r o n i s i s discriminator
H
La discriminator
discriminator
44
1
shaper
amplifier
SI ow
time conver tar
coincidence
I
I I
I
pulsed I I beam
multichannel analyser
gate
1
trigger pulse st rotc her
-
Figure 2. Block diPgram of ekdramc eqdPmeat(@
about 3 meters the resolution was adequate and sut3icient counts could be accumulated in 8 to 10 minutes to complete the analysis. To improve the precision, especially for gas samples in which the deuterium partial pressure was low, longer irradiations were carried out to increase the number of aeutron counts and thereby decrease the relative statistical
errors. RESULTS AND DISCUSSION
Backgrouod. Measurements camed out on the empty gas cell showed that the background spectrum consisted of a peak corresponding to 2.6mev neutrons from lT due to a carbon deposit which was gradually built up during irradiation, from the decomposition of oil vapors from the pumping system on the hot nickel window at the point of incidence of the irradiation beam. In addition there was a low continuum covering the entire energy range, due to y-rays not completely elimhated by pulse shape discrimination, and neutrons scattered into the detector. The contribution from scattered neutrons increased with the number of neutrons generated in the gas ceil and accordingly was a function of the pressure and composition of the gas. When deuterium was determined in a sample, the neutrons from the carbon deposit did not interfere, because their energy
was much lower than neutrons from deuterium. However, the background continuum did add to the deuterium peak in the neutron energy spectrum and had to be subtracted. This was cam4 out for all analyses and in all spectra shown in this paper. The error incurred in using data from an empty cell to correct for this background was negligible when the deuterium content was relatively high. For samples containing relatively small amounts of deuterium, this error could become appreciable; it was then advisable to determine the background by irradiation of a sample with approximately the same composition and pressure in which deuterium was absent. Neutron SpecIra. Table I lists the neutron energies expected from the irradiation of lY=, 14N,W, and IH with deuterons, as calculated from Equation 1 for & = 3.0 mev and OL = 30"; nf refers to the ith excited state in which the product nucleus is left after neutron emission. Typical neutron time-of-flight spectra obtained from samples of deuterium (at partial pressures, p d ) in hydrogen, oxygen, carbon dioxide, and nitrogen (at total pressures, P) are shown in Figure 3. From this figure it is obvious that neutrons emitted from deuterium can readily be distinguished from those emitted from the other major components in the mixtures. Accordingly, the number of counts under the
VOL 39, No. 6, MAY 1967 a
a
deuterium peak can be integrated and used as a measure of the deuterium content. When low concentrations of deuterium are determined in nitrogen, the tail of the 14N(no)neutron peak may overlap a portion of the deuterium peak. This interference may be overcome bychanging OL. The effect of such a change calculated for Ed = 3.0 mev is shown in Figure 4,where the energy calibration of the channel number is the same as that
in Figure 3 for easy comparison. The position of the deuterium peak in the neutron spectrum can thus be shifted to a position between the (no)and ( n l ) peaks of I4N by measuring at an angle of 70”. However, the differential cross sectior. for neutron emission from the reaction 2H(d,n)3He is angledependent and drops by a factor of about 2 (10)between 30° and70”. The energy of each neutron group in Figure 3 is about 200 kev lower than the values in Table I. This is due to the energy lost by the deuteron beam passing through the nickel window. Neutrons with energies below about 700 kev could not be distinguished from y-rays by pulse shape discriminatior,. Accordingly, neutron groups below 700 kev, that might have been expected from Table I, were not observed. When the deuteron beam passes through the gas in the irradiation cell its energy is degraded by an amount depending on the pressure and Composition of the gas mixture. For example, a cell 3 cm long, containing C 0 2at 100-mm pressure, will cause a loss of 128 kev from an incident beam of 3-mev deuterons. This loss will appear to broaden low energy neutron peaks in the energy spectrum much more than peaks occurring at higher energies, because the energy interval per channel increases with increasing energy. Accordingly, t b z shape of the high energy peak due to deuterium is little dfected by the variation of total pressure in the irradiation ce!:
(9) F. Everling, L. A. Koenig, J. H. E. Mattauch, and A. H. Wapstra, “Nuclear Data Tables,” Part I, National Academy of Sciences, Washington, 1961.
(10) J. E. Brolley and J. L. Fowler, in “Fast Neutron Physics,” Part I, J. B. Marion and J. L. Fowler, eds., p. 80, Interscience, New York, 1960.
Table I.
Some Neutron Energies from (d, n) Reactions E d = 3.0 mev, BL = 30” ‘ZC “N 160 ’H
Target Naturai abundance, % a d , no), mev ( 9 ) Neutron energy no
98.89
- 0.281 2.582
El n2
ns n4
n5
n6
nt
99.63 5,066
99.759 - 1.627
7.932 2.771 2.710 1.763 1.098 1.033 0.693 0.235
1.223 0.685
0.015 3.268 5.738
Table 11. Determinations of Deuterium
Gas containing deuterium KZ
Deuterium partial pressure, mm Hg, 20” C __ Known
Ea
2.G 2.4
H2
3.0
HZ
3.c
0 2
CG,
3.0
Nz
3.0
,4
107.3 41.6 5.3 97.3 38.1 24.2 10.9 i.60 78.9 55.8 46.7 42.9 13.85 5.15 1.15 87.4 46.5 21.4 9.63 46.7 4.33 2.01 47. i 18.07 5.36
Found B
107.1 42.7 5,? 97.6 37.7 23.5 10.9 1.66 78.46 55.75 46 64 43.53 14.30 5.18 1.03 88.23 46.21 21.02 9.75 46.22 4.43 2.08 41.47 17.20 5.15
Mean error = -0.02 mm Hg (mean value of B - Aj. Mean neutron count per m m pressure D2 = 419.2 at 3.0 mev. Relative standard deviation = i3.4%.
654
0
ANALYTICAL CHEMISTRY
Error
(B
- A)
-c.2 +1.1
-0.2
+o. 3
-0.4 -0.7 0.0 +O.# -0.44 -0.04 -0.06
+O. 63 +0.45
+o. 03
-0.12 +0.83 -0.29 -0 38 +o. 12 -0.48 +o. 10 t0.07 +o. 3’: -0.~7
-0.2:
Relative emor, % 100(B - A ) / A
-
0.2
+ 2.6 -3.7 + 0.3 - I.! - 2.9 0.0
+ 3.8 -
0.5
0.1 - 0.1 1.5
+ + 3.2
+ 0.6 -10.4
+ 0.5 - 0.6
- ‘.a
+ 1.2 - t.0 + 2.3 + 3.5 -+ 0.8 - 4.8
- 3.9
Neutron counts per 100 microcoulombs
Neutron counls per untr. pressilr,’
A‘
Nit! 445.7
47825 19088 2279 41242 15916 9945 4584 70 i
33072 2350~ 19659 18348 6028 2181 436 37191
19479 8859 4109 19481 1867 878 20009 724g 2169
458.8 430.2 423.9 417,’i 411.0 420.6 438.1 419.2 421.2 421 .G 427.1 435.3 423. f 379. .: 425.5 418.9 414.5 426.7 417.2 432.2 436.8 424.8 401.2 404.7
180
10 ..-
ENERGY (m.e.v.) 1.5 2.0 2.5 3 I
I
1
I
4 I
5 6
810
I
1
II
15
I
160 140 120 m aJ
100
aJ
& 80 Q,
9 a 60 J
40 20
0
100
150 200 CHANNEL NUMBER
Figure 4. Effect of measuring angle, ,,e, deuterium
250
300
on relative neutron energies from nitrogen and
Ed = 3.0 mev
ACCURACY AND PRECISION
The variation 01' neutron counts with partial pressure of deuterium was calibrated with deuterium gas enriched to 99.5 D at different deuteron bombarding energies over the pressure range up to 400 mm. The calibration curves remained linear over the entire pressure range. For E d = 2.0, 2.4, and 3.0 mev, the number of neutron counts obtained per millimeter pressure: of deuterium per 100 microcoulombs of beam current was Iespectively 4467,422.6, and 421.5. These values follow the trend expected from the variation of the reaction cross section with deuteron energy (see Figure 5). Because there is 1 ittle variation in reaction cross section between 2.4 and 3.0 mev, the calibration curve obtained at the latter energy was a.pplicable to all gas samples in which the cnergy loss of the bombarding beam was less than about 600 kev. Local heating of the gas in the path of the deuteron beam could decrease the density of the gas and thus decrease the rGte of neutron production. With the low average current s e d in this investigation, this eifect was not observed, in agreement with similar observations by Butler (3). The results of soine determinations of deuterium in different gases are shown i i Table II. For samples analyzed with 3.0-mev deuterons the mean error (given by the mean value uf B - A ) was -0.02-mm pressure, which was a measure of the accuracy of the method and showed that there was no bias. The mean value of the number of neutrons produced per 100 microcoulombs per mm pressure was 419.2, which is in agreement with the calibration value within the precision of the Inethod. The relative standard deviation was f 3.4z.
I
I
I 2H(d,n)3He
b'
%
1
I
i
_1
7 2 3 4 DEUTERON ENERGY (rn.c.v.) Figure 5. Variation of differential cross section (in laboratory coordinates) for reaction 2H(d,n)3Hewith deuteron energy at
eL
=
30"
Because the background count rate is a function of the content of the gas cell, the precision with which deuterium can be determined will vary with the gas composition. The minimum partial pressure of deuterium that could be meaVOL 39, NO. 6, MAY 1967 a
655
sured with a precision of about +3-4Z in a sample amtahhg either nitrogen or carbon dioxide at about Mo.mm pessure was about 0.8 mm for an irradiation lastingan hour with 2000-r(coulomb beam current- When a lower pecision of about was acceptable, 0.25-mm partial pressure
deuteriumcouldbemeasdwiththesamecurrent.
0
II
4 e
too
S
I00
N
m
With a fresh nickel window the background count over the energy region where the deuterium peak appears in the energy spectrum was about 250 counts per 100 microcoulombs for & = 2.0 mev and 0, = 0 " . Under these COD ditions the determination of I-mm pressure deuterium TP quired a n irradiation with a 500-microcoulomb beam (lasting between 15 and 30 minutes) for the relative standard deviation of the count to lie below that of tihe precision of the method. However, much shorter irradiations would have been suir6cient for qualitatively detecting the presence of deuteriun;. From the experience gained in the analyses it was considered that the method was sufficientlysensitive to detect the presence of a partial pressure of 0.1-mmdeuterium in the irradiation cell. As this amount would be present in"natural hydrogen" at pressures somewhat less than atmospheric, commercial hydrogen (the deuterium content varies appreciably from tk. 0.015 atom of natural hydrogen, but the value 0.010 atom represents a fair average) was used to check the sensitivity of the metbod. A sample of commercial hydrogen a t 50smm pressure and 20°C was analyzed with a 2-mev deuteron beam and an integrated current of 400 microcoulombs. Tk resulting spectrum, which is compared in Figure 6 with the background spectrum obtained immediately afterwards with an empty cell under the same irradiation conditions, ciearIj shows the presence of deuterium above the background. Th:. signal to noise ratio was 0.46. Considering the difficulty ii; evaluating the background counting rate, it is estimated that h signal of about half that observed here would be significant. Since the amount of deuterium in the 7-cc cell was 117 ng, and the deuterium traversed by the 4-nun diameter beam ic the 3-an long cell was 50.4 ng per sq cm, the sensitivity wouid thus be about 25 ng per sq cm. In terms of molecular cor!centration this limit is equivalent to about 50 pprn cf deuterium.
x
150
E
x
50 0
I " ( 7 E sA
0
m 20 C I
2
.
250 CHANNEL NUMBER
h l LDaATIONS
Interference in deuterium determinations may be expected when other components in the gas under investigation contain nuclides which could yield neutrons with energies sufhently near to those from deuterium. One such case, 14N,where the interference could be overcome by a change in the measuring angle, has already been discussed. An example UT interference where little could be mined from a change in measuring direction is the case where the gas contains a corn ponent enriched in carbon-13. This nuclide yields a datively large number of neutron groups on irradiation with deuterons. Inted-erence would be experienced from n:; h, ns, n4,or ns neutron groups, depending on the angle of measurement. The relative error introduced by such interference would &crease with an incin the partial pressure of deuterium being determined but would increase with iacreasing enrichment of carbon-13. Similar interferences could be expected from gases containing enriched nitr0gerkl.i and oxygen-17 andlor 18. A possible limitation in this method would be the presence in the gas sample under analysis, of a component which would ciecompose in contact with the heated spot on the nickel window or the rear piatinum wall of the gas cell, to yield nom vohtiie products. The decomposition products would be deposited in the path of the beam and could become Points where relatively intense neutron generation could OCCUT.
If this were the case, the background would become overwhelming. Furthermore, irradiation of such a sample would make the gas cell unusable for any subsequent sample. Even if the decomposition products did not cause excessive neutron generation, the deposition of material on the nickel window would increase the energy loss in the irradiation beam and would widen the energy spread of the bombarding particles, resulting in a distortion of the expected neutron energy spectrum.
ACKNOWLEDGMENT
The advice of Roy McMurray, Philip van der Merwe, and Izak van Heerden on the use of the time-of-flight spectrometer and their assistance during the course of the investigation sare gratefully acknowledged. Johan Kritzinger helped to design the irradiation cell which was made by Tobias Swart.
RECEIVED for review October 11, 1966. Accepted February 13,1967.
Analytical Behavior of Some Complex Chromium Nitrides, and Determination of Nitrogen in Chromium Alloys E. J. Lumley Aeronautical Research Laboratories, Department of Supply, Melbourne, Australia
Nitride-containing phases have been extracted from chromium and complex chromium alloys and examined for composition and solubility in the acids or acid combinations normally used in chemical nitrogen determinations. Some synthetically prepared nitrides of comparable compositions have been similarly tested. The chemical detetmination of nitrogen in chromium alioys has been e>:amined, and the effectiveness of various acids in th'e dissolution stage of the analysis assessed. The influence of chromous ions on the blank determination, and the effect of tartrate additions to the distillation mixture, have also been tested. THEP R o P E R T i E s OF METALS AND ALLOYS can be influenced considerably by dissolved and combined gases. This effect is very marked in chromium and chromium alloys, particuiariy with regard to the influence of nitrogen. Chromium and its alloys have bem examined extensively in these laboratories for potential use in high-temperature, high-stress components or gas turbines, and the nature and magnitude of the nitrogen effect have been reported ( I ) . The metal and its alioys were found to be susceptible to nitrogen pickup and genetratior. by atmospheric contamination during such operations as c.asting, melting, and heat treatment. Moreover, even slight increases in residual nitrogen contents could aftect properties, such as ductility, to a remarkable and generally adverse aegrct.. Nitrogen content w2.s thus a signincant factor in predicting metal or alloy propert es and service behavior. Accordingly, nitrogen determination constituted a fundamental phase of the general research progrdm. The experimental chromium-base alloys developed and examined varied widely in composition and complexity. A!ioying eiements Tat Nb, Ti, W, Mo, Zr, eo, B, Si,Cet and 5' were used either singly or in combinations up to quinary compositions. Usually these alloying eiements were present in fairly diiute amounts. varykg from about 0.5 to 8%. Nitrogen contents were also varied, and ranged from residua! amounts (0.001 %) to 4
_______
:z.
_ .
( 1j A. R . Edwards. J. 1. NisR, and H . L. Wain, Met. Rec., 4 (16), 403 (1959:.
It is clear that varying combinations of these alloying elements would unite with chromium and with any nitrogen present in the alloys and form complex nitrides, and that most of these nitrides would be chemically refractory. This was indicated in some early work, in which some nitriaes that were extracted from a series of Cr-Ta-N and Cr-Nb-N alloys were found difficult to dissolve in the acids normally used in chemical determinations of nitrogen. It is apparent that the acid-refractoriness of these nitrides and of nitrides occurring in other complex alloys would influence the effectiveness of the acid-solution stage of the chemical determination of nitrogen in such alloys. The actual method used in these laboratories for chemical nitrogen determination in chromium is a modification of the well-known acid solution-distillation procedure, discussed in a previous report ( 2 ) . The acid solution stage of the analysis consisted of initial decomposition of the sample in hydrochloric acid, followed by fuming with sulfuric acid. This was generally adequate for chromium, whether the metal was in the as-deposited (electrolytic) condition or in some heattreated form, but modifications were required in the analysis of complex alloys. Longer fuming times with increased quantities of sulfuric acid, or the addition of hydrofluoric acid, were found necessary for the effective decomposition ol' the refractory nitride phases present in these alloys. Even then the behavior of the nitrides during analysis was not easy to observe, because of the formation of dark-colored analytical solutions and the frequent precipitation of insoluble hydrolysis products. To observe more clearly and assess analytical behavior, various nitride and nitride-containing phases were extracted and collected from chromium and complex alloys, and their solubility in the analytical acids was examined. Nitrogen estimations were then made on a range of chromium and chromium alloys, including those used in extract solubility tests. The chromium samples used in these tests were either electrolytic or heat-treated. Some were nitrided to varying
~ _ _ _ _ _ (2) E. J. Lumley, Aero. Research labs., Met. Note, 7 (1957). VOL 39, NO. 6, M A Y 1967
657
