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MICROCOPY RESOLUTION TEST CHART
NATIONAL BURE AU OF STANDARDS -1963
To be submitted to Physics Letters
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The Possible Radioactive Natura ou
túrdiotr
RELEASED FOR ANNOUNCEMINT
J. B. Ball
I IN NUCLEAR SCIENCE ABSTRACTS
Oak Ridge National Labor tortor
Oak Ridge, Tennessee

In the region of nuclei up to mass 209, there are presently
i known about eleven nuclei capable of beta decay but with half-lives
Un
NOT APPROVS FOR
1 mong enough to exist in measurable amounts in nature. These nucle}|
are listed along with their half lives and abundances'' in Table 1.
It is the purpose of this note to suggest that the heaviest naturally,
occuring isotope of zirconium also belongs to this group.
The results of binding energy calculations performed as part

of a shell model study of nuclei in the A ~ 90 region of the periodic
| This paper wus nobraitted for publication
in the opon literature at least months
prior to the instanca date of his Micro-
card. Since the U.S.A.L.C. hus no orto
dence that it has been published, the pa-
per is being distributed to Microard
form us a propriat.
table,-) exhibit the somewhat disturbing prediction that "zr is
unstable with respect to beta decay into Nb. The calculated decay
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--
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...-
..
energy is 0.39 MeV. The rms deviation between calculation and
experiment found in the above calculation is 115 keV. The assignment
w this as a probable error to each of the calculated masses yields an
96
uncertainty in the calculated decay energy of +0.16 MeV. Thus, a
stable ground state for Yºzr would be outside the limits of error for
the prediction.
It is interesting to note that the nuclear data sheets") show yozr
slightly above '°Nb, but list the isotope as stable. This is apparently
a reflection of the uncertainties in the mass values available at the time
Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
of compilation. This same reference lists two lower limits on the
half-life of "zr deduced from the essentially negative results obtained
in the course of a search for double beta decay.”, *The accurate mass
determinations of Reis, Damerow, and Johnson provide a new experi-
mental measure of the available decay energy. When their total nuclear
binding energies are used, it is found that Yºzr is in fact unstable with
respect to beta decay into *°Nb and the decay energy is 0.19 + 0.05 MeV.
Again, a stable ground state for *°Zr would be outside the limits of error
for the decay energy.
The apparent contradiction of finding Yºzr to be 2.8% abundant
in nature might be explained by the properties of the low-lying levels
in *°Nb. Unfortunately, almost nothing is known experimentally about
these levels. The low-lying levels predicted by the calculation of
ref. 2) are shown in Fig. 1 along with the decay energy deduced from
ref. 5). Within the limits of error on the decay energy and on the
calculated level positions, the only states likely to be available for beta
decay are a level with spin and parity 6+ and another of 5+. The decay
15
should thus be at least fourth forbidden and with the small energy avail.
able the half-life will be extremely long (104) years or greater).
It is interesting to conjecture that yozr is not stable, but in fact
a long-lived beta emitter, and that a careful experimental investigation
might reveal this radioactivity. The beta activity need not be looked for
directly as in ref. 3) and 4). The '°Nb daughter is also a beta emitter
and each beta decay is followed by a 0.77 MeV gamma. A possible
experimental approach is thus the repeated milking of a largo zirconium
sample and the examination of the niobium daughter for a 0.77 MeV
gamma with the characteristic 23 hour half-life.
of the eleven isotopes listed in Table 1, only ** Ca is an even
nucleus. Since there are no remaining adjacent isobars, both of which
possess a measured isotopic abundance, the only probable additions
to the table are other even nuclei such as "ºzr. An examination of other
even isotopic sequences such as Se, Sn, Xe, etc., does not reveal any
further likely candidates.
Table 1. Long-lived beta emitters with measurable isotopic abundances,
Isotope
Abundance
Decay modo
Half-life
1.3 x 10'y
jo690
1921
0.012%
Bº and K
0.18
>2 x 1016
23 Vio
0.24
Bº and K
~6 x 1014
37R by
27.85
4.7 x 1010
12.26
48 Cd13
>3 x 1015
ay kind of
95.72
6 x 1014
5276 423
0.87
>5 x 1013
57Latie
0.089
Band K
1.1x1011
7 Luos
2.59
Bº and K
3.6 x 1010
7372187
0.012
B' and K
>1 x 1013
187
75 Re 112
· 62.93
7 x 1010
Referoncos
Il Nuclear Data Sheets, compiled by K. Way ot al., (Prir sing and
Publishing Office, National Academy of Sciences - National
Research Council, Washington, D. C.)
2) K. H. Bhatt and J. B. Ball, Nuclear Phys. 63 (1965) 286.
3) J. A. McCarthy, Phys. Rev. 90 (1953) 853.
4) M. Awschalom, Phys. Rev. 101 (1956) 1041.
5) R. R. Rios, R. A. Damorow, and W, H. Johnson, Phys. Rev. 132
(1963) 1662.
ORNL-DWG 65 - 6253
0.22
0.19
0.16
402756
:
0.19 +0.05
4, No 96
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23h
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1. The predicted low-lying levels in *°Nb shown with respect to
the ground state of 962r. The decay energy is taken from
ref. 5).
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