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UNITED STATES ATOMIC ENERGY COMMISSION
THE ISOTOPIC CONSTITUTION OF LANTHANUM AND CERIUM
Mark G. Inghram
R. ]. Hayden
D. C. Hess, Jr.
Argonne National Laboratory r
UNIV OF FL LiB
U S DEPOSITORY
This document consists of 6 pages.
Date of Manuscript: April 21, 1947
Date Declassified: July 7, 1947
This document is issued for official use.
Its issuance does not constitute authority
to declassify copies or versions of the
same or similar content and title
and by the same authorss.
Technical Information Division, Oak Ridge Directed Operations
Oak Ridge, Tennessee
THE ISOTOPIC CONSTITUTION OF LANTHANUM AND CERIUM
-~.. : .. By Mark G. Inghram, R. J. Hayden, and D. C. Hess, Jr.
.rt : ,
.Tb ,.iSotopic abundances of lanthanum and cerium have been determined by means of a mass
spectrometer. In addition to the known lanthanum isotope of mass 139, a new isotope of mass 138
and, undance 0.089% b*s been discovered. Since this isotope is isobaric with two neighboring
stable, isotoges.it ,hould.be radioactive. This activity was observed and the half-life found to be
approximately 2 x 10'0 years. Upper limits for the non-existence of other lanthanum isotopes were
determined. The abundance of cerium isotopes of masses 136, 138, 140, and 142 were shown to be
0.08%, 8.250%, 88.48%, and 11.07%, respectively. Upper limits for the non-existence of other
ceriuma isotopes were determined.
In view of the fact that all previous measurements of rare earth isotopic constitution have been
made plotometrically, it was decided to repeat these measurements, using electrometric methods.
Th eterminattion of isotopic abundances by measuring positive ion currents eliminates several of
the difficulties inherent in obtaining these abundances from the relative blackening of a photographic
plate. The usual photometric method is to relate isotopic abundances to optical densities of mass
spec a-linesby means of a density curve.
,7hen retols of printing standard intensities are liable to introduce errors if they are made by
X rays, light, different ions from those examined, or by ions of different velocity. Even though these
intensity marks agree with a H D curve, the slope is dependent on the method used in printing the
standard intensities. This leads to discrimination dependent upon isotopic abundance. In addition,
errors my a tise due to the fact 'that differentisotopes traverse different paths in a spectrograph.
Meaduiemlents'bf he' isotdpic constitution of lanthanum and cerium, together with upper limits for
lWmopo-exiAtencL of possible neighboring isotopes of these elements, are given in this paper.
The mass spectrometer used in this investigation was a Nier-type 60" six-inch radium of
curvature, single focusing mass spectrometer.' Ions of the elements studied were obtaiiied by
heating their oxides on a tungsten filament. The samples were placed on the filament in nitric
acid solution and upon heating, these reverted to the oxide. No organic binder was used. Ion emis-
sion began at approximately 1000"C. The ions formed were collimated into an ion beam of 2006
valtd fnelsgydby the cditimatalgg plates Of the'source and directed normally into a wedge-shaped
magndoi'field. This nigiaetic'field resolved the ton beam into its various mass components Wnd
refocutI abdain of onetmass through the collector slit to a collector plate. The collectorlplate
was's*#bountd d by Pn electrontrepelllng field'of 22.5 volts, so that secondary electrons formed at
the collkitor plate by positive-ion bombardment could not leave that plate. Ion currents were
measiCed by a vibrating reed electiometer.2 The electrometer 'output was recorded on a Bro*n
Electronik Strip Chatt Recorder so that permanent records of peak shapes and intensities were
dbtalind. Succeeding ISotopes were focused on the collector by an automatic, continuous variation
of the analyzer magnetic field. '
MDDC 1084 1
The pumping system consisted of a two stage, glass mercury diffusion pump backed by a Welch
Duo-Seal fore pump. A liquid air trap was used between the diffusion pump and the mass spectrometer
tube to trap out vapors. The operating pressure In the tube was about 3 x 10-' mm Hg as measured
by an ionization gauge.
DISCUSSION OF OBSERVATIONS
In discussing corrections which must be applied to the observed abundances, possible preferential
emission of the lighter mass ions due to their higher thermal velocities must be considered. Theoret-
ically, evaporation from the liquid phase gives a change in the abundance ratio of two isotopes propor-
tional to the square root of the ratio of the masses. This separation is attained only if there is perfea .
mixing at the surface of the liquid. In the solid phase, there is little mixing, and so there should be
little separation. Since the material on the filament was not in the molten state, no mass discAimliation
was expected. To test this hypothesis, isotope ratios were taken from the time emission began itll "'
the sample was completely evaporated from the filament. If any such mass discrimination touo pt e,;'
the last determinations would have given large ratios of heavier masses than did the first. No such: "
effect was noted and hence it was concluded that no vapor pressure discrimination took plate.' """
Another error would be introduced if the ratio of ions to atoms emitted from the filament were;d*-.
pendent on the mass of the isotope. However, since the efficiency of ionization in this type of soepue,-.
depends on the ionization potential and not on the mass, it is concluded that this effect is negligible.
Possible space charge discrimination near the filament are negligible because the emission;dl
the filament is temperature-limited rather than space-charge limited.. :
With this ion source, the electron focusing magnet usually used on the source of Nier-type ms
spectrometers was unnecessary and was omitted. Thus no magnetic isotope discrimination coul&l have
occurred in the source.3
The condition for no discrimination between light and heavy ions in their passage through a mass,i,-
spectrometer is that the ion paths for different ions be identical. This condition was fulfilled in these
measurements by bringing different masses to focus at a fixed point by varying the analyzsr ltigni*eic
field and keeping ion energy constant.
Inaccuracy in the current measuring system could have been caused by non linearity oLthe. ..r;.
2 x 10' .ohm input resistor of the vibrating reed electrometer or by non-linearity of response through -;
the electrometer and the recorder. However, measurement showed the input resistor to.be ionstantpoMi
better than 1% in the range of 1 millivolt to 10 volts, the range used in these experiments. MeasuRmment
of the linearity of the amplifying and recording system showed its response to be linear to within 0.2%.
Thus all systematic errors are less than 1%, and to this degree of accuracy the observed peak helghtsa,
are proportional to isotopic abundances.
EXPERIMENTAL RESULTS ''
Lanthanum Abundances '
La2O, heated on a tungsten filament gave rise to LaO* ions. A typical recorder curve.of.theas ikam,
is shown in Figure 1. Peaks of masses at 154, 155, 158, and 157 are present. The sensitivity ,e th 4crn
recorder at mass 155 is less by a factor of. 200 than the sensitivity over the rest of the curve. The. :.)r
peaks at 155, 156, and 151 are due to La'sOO", La"9 O", and Laae" 0" ions, respectively. Calculation:.
of abundance of these peaks gives values for the abundance of the oxygen isotopes of 99.752% 0" .i
0.041% O", and 0.207% O"8. These results agree with those of Murphey' within 2% so that within thi. n
limit these peaks are ascribed entirely to La"'0", La OQ7, and La'O". Thus, if there are any La"0.
and La"' isotopes they must be less than 2% of the height of the last two peaks. The ion at mass 15. we
ascribe to the oxide of a new isotope, Lase. ,
1'3916 ISOTOPES OF
x_ 10-n L-39-01
153 155 157 159
ATOMIC MASS UNITS
Figure 1. Recorder tracing of isotopes of
lanthanum as observed in the LaO* position.
The peak at mass 155 is recorded at one two-
hundredth of the sensitivity used for the rest
of the tracing.
The possibility that this peak is due to another element has been ruled out by the following con-
siderations: If the peak at mass 154 were due to an impurity of some other element in lanthanum,
this impurity would probably not be of the same intensity in various samples. We observed the
154 peak in two specially purified samples, a sample of spectrographically pure La,0, prepared
by Professor Rolla and obtained from Adam Hilger, Inc., as Laboratory Number 6781, and a second
sample of La(NO)),.6H0 obtained from Kahlbaum. Within 1% no difference could be detected in the
ratio of the 154 to 155 peaks in these two samples. Further evidence that the mass 154 is not due
to an element other than lanthanum is provided by the characteristics of surface ionization. For ex-
amplie if a mixture of lanthanum and cerium is placed on the filament, the ratio of the cerium to
the lanthanum increases strongly with time due to the fact that lanthanum ions are emitted at a
lower temperature than the cerium ions. This effect is present In varying degrees between lanthanum
and any other element which emits positive ions from a tungsten filament. No variation with time df
the 154 to 155 ratio could be detected. This indicates that the 154 peak belongs to lanthanum. A final
argument that the 154 peak is due to La'""O' is based on a consideration of the other elements which
could conceivably give rise to this peak. These are barium, cerium, samarium, and gadolinium;
since 0" is known to exist to less than one part in one hundred thousand, this could not be the cause
of this peak. A barium peak at mass 154 would have to be Ba'SEOL.6 Since barium does not emit as
BaGO, but rather as Ba* and because the Ba"'O" line was not observed at mass 153, this possibility
Is ruled out. Cerium is ruled out because, if the 154 peak were due to Ce"'O", the 156 peak due to
Ce'400"would have to be 700 times larger than was actually observed. The possibility of CeN is
ruled out by the fact that no charged nitrides are ever observed. Samarium and gadolinium are like-
wise ruled out because their other isotopes did not appear. The possibility of dissociation of LaOH
between the accelerating electric field and the magnet is ruled out by the fact that LaOH was not
present, and that no similar line appears with cerium. It must, therefore, be concluded that the
isotope of mass 138 exists in lanthanum as it occurs in nature.
The results of this investigation, together with the previous results of Astonf, are summarized
in Table 1. The results are the average of forty separate determinations. The uncertainty tabulated
is the mean deviation of these determinations. Any other isotope of lanthanum is present to less than
Table 1. Isotopic constitution of lanthanum.
Reference Method 138 139
Aston' Photometric 100 per cent
Present work Electrometric 0.089 .001 99.911 .001
Calculation of the chemical atomic weight from these abundances using Dempster'se value of
-3.2 x 10-' for the packing fraction of lanthanum and the factor 1.000275 in converting from physical
to chemical atomic weight gives 138.92 which is in exact agreement with the international chemical
atomic weight of 138.92.
Lanthanum1s8 is an unusual isotope in that it occurs between two stable isobars, Ba"' and Ce"'.
This occurs at only two other places in the atomic table, namely in the case of K40 and LuI7. It is
similar to these isotopes also in that its nucleus contains an odd number of protons and an odd number
of neutrons. No naturally occurring odd-odd isotopes are stable heavier than N". Both KWO and Lu'"
are known to be radioactive with half-lives of 4 x 10' years and 7.3 x 10m years, respectively. In view
of the similarity between La"'s and these two isotopes, it was thought that the La1'8 would probably be
active, and a search for this activity was carried out, using a counter with a 3 mg/cm' window. An
activity of approximately 250 Key consisting of beta rays was found to be present. The specific activity
was also measured and assuming this activity to be due to the disintegration of Lan", the half-life of
this activity is approximately 2 x 10"oyears. :
Cerium was first studied by Aston and shown to consist mainly of isotopes 140 and 142. Later'
Dempster7 found two rare isotopes atmasses 136 and 138. A typical recorder curve for cerium,ob-'
talned with our apparatus is shown in Figure 2. The mass 156 was recorded at sensitivity 100 times
less, and mass 158 recorded at sensitivity 20 times less, than the sensitivity used over the rest of,the
curve. Only the masses 152, 154, 156, 157, 158, 159, and 160 were observed. These were due to thde
ions Ce"" O" CeS'"O", Ce'O", Ce"OL", Ce'420", Ce"O14 C e'201, and Ce"'O'", respectively. The
cerium isotopic abundances, corrected for the rare oxygen Isotopes are given in Table 2. These
results are the average of forty separate determinations. The uncertainty tabulated is the mean
deviation of these determinations.
No peaks other than those due to the oxides of these four cerium isotopes formed with Os', O",
and O" could be detected. The following upper limits for possible cerium isotopes of neighboring mass
were obtained: 134, 135, and 137, less than 0.004%; 139 and 141, less than 0.02%; 143, 144, 145, and
146, less than 0.004%. .*
[ 5 .
. IIDDC 1084
150 152 154 156 458 160
ATOMIC MASS UNITS
Figure 2. Recorder tracing of isotopes of cerium as observed in the CeO+
position. The peak at mass 156 is recorded at one-hundredth the sensitivity,
and the peak at 158 at one-twentieth the sensitivity, used at masses 152 and 154.
Table 2. Isotopic constitution of cerium.
Reference Method 136 138 140 142
Aston' Photometric 89.0 11.0
Dempster7 Photometric < 1% -
Cohen' Electron multiplier -0.18 0.22 -
Present work Electrometric 0.193 .005 0.250 .005 88.48 .10 11.07 .10
Calculation of the chemical atomic weight from these abundances, assuming a packing fraction of
-3.0 x 10", and the factor 1.000275 in converting from physical to chemical atomic weight, gives
140.10, which is in agreement with the international chemically determined atomic weight of 140.13.
We wish to express our appreciation to Prof. A. J. Dempster for many helpful discussions.
Sp140- 16 OF CERIUM
, t f .. ,___ ,
] IMDDC 1084
1. Nier, A.O.C., Rev. Sel. Iast. 11: 212 (1940).
2. Palevaky, H., R. K. Swank, and E. Sweuchik, Rev. 8el. Inst. 18: (1947).
3. Cogshell, E. F., N. D. Jordan, Journal of Applied Physics 13: 539 (1942).
4. Murphey, B., Phys. Rev. 59: 320 (1941).
5. Aston, F.W., Phil. Mag. 49: 1191 (1925).
6. Dempster, A.J., Phys. Rev. 53. 869 (1938).
7. Dempster, A. ., Phys. Rev. 49: 947 (1936).
8. Cohen, Arnold A., Phy. Rev. 63: 219 (1943).
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