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UNITED STATES ATOMIC ENERGY COMMISSION
RADIATIONS OF Zr97 AND Nb97
W. H. Burgus
J. D. Knight
R. J. Prestwood
Los Alamos Scientific Laboratory
IL Technical Information Division, ORE, Oak Ridge, Tennessee
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RADIATIONS OF Zr AND Nb
The radiations of Zr97 and Nb97 have been examined by beta-ray
spectrometry ani coincidence counting. The 17.0-hour Zr9 has been
found to decay to a 60-seccnd isomer of Nb the latter -ndergoing
isomeric transition to the 75-minute Nb97 ground state. The disintegration
energies are; Zr E = 1.91+ .02 Mev; Nb 9 E = 0.747 + .005 Mev;
Nb E = 1.267 + .0Z Mev; E = 0.665 + .005 Mev.
P- y -
Previous studies of the radiations of the 17. 0-hour Zr -75-minute
Nb chain have been made by absorption methods (1), On the basis of
(1) S. Katcoff and B. Fin.lc, Plutonium Project Report CC-2310 (Jan. i945)
cited by G. T. Seaborg and I. Perlman, Rev. Mod. Phys. 20, 585
measurements on the mixture in transient equilibrium anl on the niobium
fraction separated from the mixture, the following beta-and gamma-ray
energies have been reported: Zr : E = 2 Mev, E 0. 8 Mev;
Nb E = 1.4 Mev, E = 0.78 Mev.
The present report describes the results of an examination of the
radiations and disintegration scheme of these nuclei by means of beta-ray
spectrometry and coincidence counting techniques. The Zr -Nb samples
used in this investigation were obtained by slow neutron irradiation of electro-
magnetically enriched Zr and by isolation from uranium fission products.
The enriched Zr and Mo98 used in this investigation were supplied by
Carbide and Carbon Chemicals Corporation, Y-12 Plant, Oak Ridge,
Tenn. on allocation from the Isotopes Division, U. S. Atomic Energy
Since in slew neutron uranium fission the 17, 0-hour Zr9 and the 65-day
Zr are produced with nearly equal yields (2), a certain amount of the
(2) C. D. Coryell, A. Y. Sakakura, and A. M. Ross, Bull. Am.Phys.
Soc. 24, No. 7, 23 (1949).
Zr95 activity was always present in Zr97 sources of fission origin.
However, by use of short neutron irradiations, with chemical separation
and measurement within the 24 hours following, it was possible to limit
the Zr95 contribution to a few percent in the low energy range and to a
negligible amount in the beta energy region above 0.4 Mev (3). To insure
(3) The beta spectrum of Zr95 is complex, with -98% of the disintegra-
tions having Emax = 0. 394 Mev and -2% Emax = 1 0 Mev. (V. A.
Nedzel and M. B. Sampson, Plutonium Project Report CC-2283
(Oct. 1944), cited by G. T. Seaborg and I. Perlman, loc. cit).
that the gamma-ray and conversion electron lines found belonged to the
Zr -Nb97 chain, the sources were left in the spectrometer for a few days
after measurement and the appropriate points were checked for 17-hour
decay. The average of a number of decay measurements on Zr from
96 97 98
Zr9(n,y) has given t = 17.0 + 0. 2 hours; for Nb produced by Mo
(y,p), t = 74 + 2 minutes.
II. BETA SPECTRA
The beta-ray spectrometer used in this investigation was of the single
coil magnetic lens type, and has been described in an earlier paper from
this laboratory (4). Resolution could be varied from 2.4 to 6.4 percent
(4) L. M. Langer, Phys. Rev. 77, 50 (1950).
by adjustment of a movable disk baffle. The detector was a G-M counter
with 3.5 mg/cm mica end window,
Beta spectra were taken with two kinds of sources, The Zr (n,y)
sources consisted of 13-14 mg of zirconium oxide spread over an area of
0.8 cm and sandwiched between two layers of rubber hydrochloride film
of 0.50 mg/cm2 thickness each. The fission product Zr sources were i. 6
mg of zirconium oxide spread over 0.6 cm ani similarly mounted. Pro-
vision for electrical leakage was made by spraying the sources with a light
coat of an Aquadag suspension. Since the beta sources as described were
thick enough to cause small distortions in the spectra, empirical energy
32 137 137 m
calibrations were carried out using carrier-free P and Cs -Ba
sources evaporated down in inactive zirconium oxide and mounted in the
same way as the active Zr-Nb sources.
The momentum distribution of the electrons from fission product Zr97
in transient equilibrium with the 75-minute Nb97 is shown in Figure 1.
Spectrometer resolution was 2.5%. The Fermi plot of this spectrum, shown
in Figure 2, exhibits straight-line components corresponding to beta-rays
of energies 1. 91 +.02 and 1. 267 + .02 Mev respectively. The results of two
additional runs with somewhat heavier sources of fission and (n,y) origin
fell within the limits of error given above. Energy calibration was based
on Ba E = 0. 626 Mev. (5). The upward curvature in the series of
(5) J. Townsend, M. Cleland, and A. L. Hughes, Phys. Rev. 74, 499
points extending from about W = 1. 8 on backwards on the Fermi plot is due
presumably to source thickness distortions and to presence of beta-rays
In addition to the beta spectrum, there is a line of conversion
electrons at 0. 726 + .005 Mev. From a measurement of the relative areas
under the momentum distribution curve in Figure 1, with the assumption
that the conversion electron follows the 1. 91 Mev beta ray, there was obtained
a conversion ratio e- 0.015 + .002 for the transition. The 2.5%
resolution of the spectrometer did not permit a separation of the K and L
components of the conversion line. However, on the basis of a lat-r-run in
which the conversion peak was carefully mapped (Figure 1 inset) and its
energy compared with the gamma-ray energy for this transition, it is esti-
mated that at least 80% of the conversions occur in the K shell. There is
also evidence for a weak conversion peak at 0. 645 + .010 Mev, for which a
rough estimate gives -.0015.
III. GAMMA SPECTRA
Gamma spectra were measured on Zr -Nb from (, y) and fission
product sources, using both gold and uranium radiators. The photoelectron
spectrum for the uranium radiator and 2. 5% spectrometer resolution is
plotted in Figure 3. It is seen that there are two prominent gamma-rays of
about equal intensity, with E = 0. 749 + .005 Mev and 0. 665 + .005 Mev
respectively, plus possibly two weaker ones at 0.48 Mev and .-0.56 Mev.
A Cs -Ba source, with E = 0.663 Mev, was used as a gamma
IV. COINCIDENCE COUNTING
The harder of the two beta rays has already been identified with the
17.0-hour Zr97 on the basis of absorption studies. (1). While conventional
separation and counting techniques have also shown that one of the gammas
follows each beta, the small difference between the gamma-ray energies,
together with the rapid growth of Nb97 into freshly purified Zr97 samples,
has hitherto made it difficult to determine with any certainty which gamma
is associated with which beta.
To clarify the decay relationships a series of beta-gamma, gamma-
gamma, and beta-conversion electron measurements was made. In the
first, a Zr -Nb97 sample was mounted between two counter tubes faceto
face; one tube was shielded with sufficient aluminum to stop all betas, and
beta-camma coincidences were measured as a function of absorber in front
of the beta-counting tube. The results, plotted in Figure 4, show a gradual
decrease in ILZ ratio with increasing absorber, extrapolating to zero at
approximatAly 500 mg/cm Al; this range corresponds to -1.2 Mev, the
Nb97 beta energy. The sloping nature of the curve and the position of the
L- end-point show that there is no gamma in coincidence with the hard
beta. Measurements of beta-gamma coincidences as a function of gamma
absorber gave a -- ratio which decreased slowly with increasing absorber
thickness, indicating that the softer of the two gammas is the one which is
in coincidence with the beta. No gamma-gamma coincidences were found.
Measurements of the beta-beta coincidence rate gave a net ratio of
- 6 x 10 This ratio, as predicted from the effective geometry of the
counting arrangement and the known internal conversion coefficient, should
be > 4 x 104 if the 0. 726 Mev conversion electron were in coincidence with
one of the beta rays; in short, this conversion line and the harder of the two
gammas are not in coincidence with either of the beta transitions.
However, in view of the fact that the photoelectron spectrum showed
the two gamma rays to be of about equal intensity, and one was known to be
associated with each beta, it appeared that the Zr97 beta decay must lead
to a metastable state of Nb97 with a lifetime appreciably greater than the
resolving time of the coincidence circuit (-0.5 x 10- seconds). This
suspicion was confirmed by the discovery of an isomer of Nb 97. Rapid
chemical separation of the Zr from samples of Zr -Nb97 mixtures and
counting of the two fractions brought to light a gamma emitter which grew
into the Zr fraction with a 60 + 8-second half-life and decayed in the Nb
fraction with the same period. Growth and decay curves are shown in
Figures 5 and 6.
Figures 5 and 6
On the basis of the findings described in the preceding sections, the
harder gamma-ray and its conversion electron are assigned to the 60-second
isomeric transition of Nb and the softer gamma-ray to the 75-minute
Nb97 decay. Also, since no evidence was found for beta-radiation harder
than the 1. 91 Mev component belonging to Zr97, and the two gamma-rays
occurred in about equal intensity in the Zr -Nb97 equilibrium mixture, it
is concluded that the 17. 0-hour Zr97 decays to the 75-minute Nb via the
60-second Nb97 isomer.
Results of the energy measurements are summarized in Table I:
TABLE I. SUMMARY OF DATA
Beta Conversion Gamma Averaged value Estimated
energy electron energy gamma energy internal
(Mev) energy (Mev) (Mev) conversion (%)
Zr97 1.91 + .02
Nb97m 0.726 0.749 0.747 + .005 1.5 + 0.2
N9767 + 0.645 0.665 0.665 + .005 0.15
Nb 1.267 + .02 0.645 0.665 0.665 + .005 0.15
The over-all results of this investigation are consistent with the disintegration
scheme shown in Figure 7. It is of interest to compare the ft values for Zr97
97 95 95 97
and Nb with those for Zr and Nb The 17. 0-hour Zr decay, with ft
7 97 5
~1.4 x 10 is probably first forbidden; the 75-minute Nb with ft N2.6 x 10 ,
should be allowed. The corresponding values for 65-day Zr (assuming all
decay via the 0. 394 Mev beta-ray) and 35-day Nb9 are 3.4 x 10 and x 10
respectively. The Nb values compare well, suggesting that the same type of
transition is involved in both cases. The ft for Zr though also probably in
the first forbidden category, is four-fold smaller than that of Zr ; it appears
that in this instance the two transitions are of different kinds.
Some idea as to the order of the isomeric transition of Nb 97may be
obtained from the data in Table 1. On the basis of half-life and energy for this
transition, one finds, following the approximation treatment described by Segre
and Helmholz (6), that the order of the transition lies between 4 and 5, nearer
(6) E. Segre and A. C. Helmholz, Rev. Mod. Phys. 21, 271 (1949).
the latter. Perhaps a better estimate may be made by a comparison of the
observed conversion coefficient with the K-shell internal conversion coefficients
recently calculated by M. E. Rose et al. (7). Coefficients for the most probable
(7) M. E. Rose, G. H. Goertzel, B. I. Spinrad, J. Harr and P. Strong,
Phys. Rev. 76, 1883 (1949). We wish to thank Dr. Rose for providing
us with a copy of these tables.
types of transition, obtained by interpolation from the tables of these authors,
are given in Table 2. The observed conversion coefficient for the 0. 747 Mev
K-shcll internal conversion coefficients for Z = 41, E = 0.747 Mev
4 5 3 4
Coefficient Electric 24 Electric 2 Magnetic 2 Magnetic 2
N .0071 .0138 .0082 .0187
transition is 0.015 + .002, of which >80% is estimated to be K-conversion.
By reference to Table 2, it appears that the transition is electric 25 or
magnetic 2 or both, and therefore that the order of the transition is probably
We are greatly indebted to Robert E. Carter for his advice and
assistance with the beta-ray spectrometer measurements. Our thanks are
also due E. O. Swickard and Jane Heydorn of the Los Alamos Fast Reactor
group for making the neutron irradiations and Martin Warren of the betatron
group for the gamma-ray irradiations.
Procedure for Isolation of Fission Products Zr
Zr97 samples were isolated from fission product mixtures by the
following procedure. The irradiated uranium metal was dissolved in a
minimum quantity of hot 16 f. nitric acid. After solution, 10 mg. of Zr4
carrier was added, the solution boiled to expel any excess nitric acid, and
a few drops of 5 f. hydroxylamine hydrochloride solution was added to
insure reduction of neptunium. The solution was then made 6-10 f. in nitric
acid, transferred to a Lusteroid centrifuge tube an.l made 2 f. in hydrofluoric
acid. All further operations, in which hydrofluoric acid was present, were
carried out in Lusteroid. Six successive lanthanum fluoride scavenging
precipitations were then made by addition of 5 mg. quantities of La+3 carrier
to the solution. Each of the precipitates was centrifuged out and discarded.
From the solution remaining after the lanthanum fluoride scavenging,
zirconium was then precipitated as barium fluozirconate by the addition of a
five-fold excess of Ba The precipitate was centrifuged out and the super-
natant solution discarded. The barium fluozirconate precipitate was dissolved
in several ml. of water, several drops of 16 f. nitric acid, and a slight excess
of saturated boric acid solution added to complex the fluoride. Barium fluo-
zirconate was then reprecipitated by addition of an excess of hydrofluoric acid
plus a small amount of Ba It was centrifuged out as before and the super-
natant liquid discarded. Resolution and reprecipitation of barium fluozirconate
were twice repeated and the final precipitate was dissolved in a hydrochloric
acid-boric acid mixture instead of the previously-employed nitric acid-boric
The barium was removed by addition of a few drops of concentrated
sulfuric acid and centrifugation of the resultant barium sulfate precipitate.
The zirconium-containing solution was diluted to about 25 ml. and a slight
excess of 6% cupferron solution was added dropwise to precipitate the
zirconium. The precipitate was filtered out on Whatman 42 filter paper
and was ignited to zirconium dioxide. This zirconium dioxide served as
the source material for some of the radioactivity measurements.
2 3 4 5 6 7 8
Hp ( GAUSS CM ) x 10
Fig. 1-The electron spectrum of Zr97 in transient equilibrium with Nb97.
Inset: Conversion peaks from Nb97m and Nb97.
E,= I 910 MEV
Fig. 2-Fermi plot of electron spectrum of Zr97 in transient equilibrium with Nb97.
Fig. 3-Photoelectrons ejected from a 1-mil U radiator by gamma rays from Zr97 -Nb97.
0 100 200 300 400 500 600 700
TOTAL BETA ABSORBER (mgAl/cm2)
Fig. 4-Beta-gamma coincidences as a function of beta absorber thickness, Zr97 -Nb97.
40,000- ,75MIN. Nb97BKGD.
30,000- ___- -- ---- -- ---o--
"- OBSERVED COUNTING RATE
ST, = 60 18 SEC.
1,000- GROWTH OF Nb97m
0 60 120 180 240 300 360 420
TIME AFTER SEPARATION (SECONDS)
Fig. 5-Growth of Nb97m into freshly separated Zr97.
000 ----OBSERVED COUNTING RATE
6,000 o o
5, 000 ----- -o- --.--_---o--
4,000- 75 MIN. Nb97 BKGD
z 800 .,--T/2 =60 +8 SEC.
DECAY OF Nb97m
0 60 120 180 240 300 360 420
TIME AFTER SEPARATION (SECONDS)
Fig. 6-Decay of Nb fraction separated from Zr97 -Nb97 equilibrium mixture.
Fig. 7-Proposed disintegration scheme for Zr97 -Nb97 chain.
END OF DOCUMENT
AEC. Oak Ridge, Tenn., 5-26-50-675-A19197
UNIVERSITY OF FLORIDA
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