Radiations of Zr⁹⁷ and Nb⁹⁷

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Title:
Radiations of Zr⁹⁷ and Nb⁹⁷
Physical Description:
14 p. : ill. ; 27 cm.
Language:
English
Creator:
Burgus, W. H
Knight, J. D
Prestwood, R. J
U.S. Atomic Energy Commission
Los Alamos Scientific Laboratory
Publisher:
U.S. Atomic Energy Commission, Technical Information Division
Place of Publication:
Oak Ridge, Tenn
Publication Date:

Subjects

Subjects / Keywords:
Zirconium   ( lcsh )
Niobium   ( lcsh )
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federal government publication   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )

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Bibliography:
Includes bibliographical references.
Statement of Responsibility:
by W.H. Burgus, J.D. Knight, R.J. Prestwood.
General Note:
Cover title.
General Note:
"AECU-726."
General Note:
"(LADC-765)."
General Note:
Work performed at Los Alamos Scientific Laboratory.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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UNITED STATES ATOMIC ENERGY COMMISSION

AECU 726
(LADC 765)


RADIATIONS OF Zr97 AND Nb97


By
W. H. Burgus
J. D. Knight
R. J. Prestwood


I-,,:


Los Alamos Scientific Laboratory


IL Technical Information Division, ORE, Oak Ridge, Tennessee

































Reproduced direct from copy
as submitted to this office.






PRINTED IN U.S.A.
PRICE 10 CENTS








-1 -


97 97
RADIATIONS OF Zr AND Nb



ABSTRACT

97 97
The radiations of Zr97 and Nb97 have been examined by beta-ray
97
spectrometry ani coincidence counting. The 17.0-hour Zr9 has been
97
found to decay to a 60-seccnd isomer of Nb the latter -ndergoing
97
isomeric transition to the 75-minute Nb97 ground state. The disintegration
97 97m
energies are; Zr E = 1.91+ .02 Mev; Nb 9 E = 0.747 + .005 Mev;
97 -
Nb E = 1.267 + .0Z Mev; E = 0.665 + .005 Mev.
P- y -


I. INTRODUCTION

97
Previous studies of the radiations of the 17. 0-hour Zr -75-minute
97
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
(1948).



measurements on the mixture in transient equilibrium anl on the niobium

fraction separated from the mixture, the following beta-and gamma-ray
97
energies have been reported: Zr : E = 2 Mev, E 0. 8 Mev;
97 P
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
97 97
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.


96 98
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
Commission.










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


95
(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
97 97
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
97
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









-3-


2
with 3.5 mg/cm mica end window,
96
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
2
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
2
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.



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
137m
on Ba E = 0. 626 Mev. (5). The upward curvature in the series of
e-


Figure 2



(5) J. Townsend, M. Cleland, and A. L. Hughes, Phys. Rev. 74, 499
(1948).


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
95
from Zr







-4-


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
Ne-
a conversion ratio e- 0.015 + .002 for the transition. The 2.5%
NP
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
Ne-
rough estimate gives -.0015.
NP-


III. GAMMA SPECTRA
97 97
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


Figure 3


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.
137 137m
A Cs -Ba source, with E = 0.663 Mev, was used as a gamma

standard.


IV. COINCIDENCE COUNTING

The harder of the two beta rays has already been identified with the
97
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









-5-


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
97 97
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


Figure 4



decrease in ILZ ratio with increasing absorber, extrapolating to zero at

approximatAly 500 mg/cm Al; this range corresponds to -1.2 Mev, the
97
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.
P-e-
Measurements of the beta-beta coincidence rate gave a net ratio of
-5P
- 6 x 10 This ratio, as predicted from the effective geometry of the

counting arrangement and the known internal conversion coefficient, should
-4
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







-6-


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
97
suspicion was confirmed by the discovery of an isomer of Nb 97. Rapid
97 97
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



V. CONCLUSIONS


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
97
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
97 97
occurred in about equal intensity in the Zr -Nb97 equilibrium mixture, it
97 97
is concluded that the 17. 0-hour Zr97 decays to the 75-minute Nb via the
97
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 (%)
(Mev)

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


_ _








-7-


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



Figure 7


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
95
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








-8-


TABLE 2


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
4
magnetic 2 or both, and therefore that the order of the transition is probably

5.






ACKNOWLEDGMENTS

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.









-9-


APPENDIX I

Procedure for Isolation of Fission Products Zr


97
Zr97 samples were isolated from fission product mixtures by the

following procedure. The irradiated uranium metal was dissolved in a
+4
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

acid mixture.

The barium was removed by addition of a few drops of concentrated

sulfuric acid and centrifugation of the resultant barium sulfate precipitate.







10 -



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


/2
{j)


W= (mc2)

Fig. 2-Fermi plot of electron spectrum of Zr97 in transient equilibrium with Nb97.








- 12


AMPERES
Fig. 3-Photoelectrons ejected from a 1-mil U radiator by gamma rays from Zr97 -Nb97.


10 i

9

8
x 7-




0.

w 4-
w 2
z
w










-3




-4
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.











50,000 1
40,000- ,75MIN. Nb97BKGD.

30,000- ___- -- ---- -- ---o--
0o

20,000-
"- OBSERVED COUNTING RATE



uJ 10,000
t--
D 8,000
S6,000
^ 5,000-
ST, = 60 18 SEC.
n 4,000

S3,000
0
2poo
0
0

1,000- GROWTH OF Nb97m
800-
600
0 60 120 180 240 300 360 420
TIME AFTER SEPARATION (SECONDS)

Fig. 5-Growth of Nb97m into freshly separated Zr97.


10,000
8 000-
000 ----OBSERVED COUNTING RATE
6,000 o o
5, 000 ----- -o- --.--_---o--
4,000- 75 MIN. Nb97 BKGD




S2,000- 0
I-



S1,000-

z 800 .,--T/2 =60 +8 SEC.
0
o 600
500 -
400




200- 0
DECAY OF Nb97m


100
0 60 120 180 240 300 360 420
TIME AFTER SEPARATION (SECONDS)

Fig. 6-Decay of Nb fraction separated from Zr97 -Nb97 equilibrium mixture.

















Nb97


17.0 h


1.267


7 0.665
innnn))nilY


Fig. 7-Proposed disintegration scheme for Zr97 -Nb97 chain.






END OF DOCUMENT


AEC. Oak Ridge, Tenn., 5-26-50-675-A19197


Zr97


Mo97







UNIVERSITY OF FLORIDA
3 126II 2 08917 1119IllII IIII I IIIIIII
3 1262 08917 1119


























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