The lunar halo search was conducted because halos in terrestrial minerals serve as pointers to localized radioactivity, and make possible analytical studies on the problems of isotopic dating and mode of crystallization of the host mineral. Ancillary studies were conducted on terrestrial halos and on certain samples of special origin such as tektites and meteorites

Gentry, R. V.

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https://ntrs.nasa.gov/search.jsp?R=19760011937 2020-03-22T17:12:23+00:00Z
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FINAL REPORT TO NASA
It
Search for and Analysis of RadloacLive
Halos in Lunar Material
Robert V. Gentry
NASA-MSC Work Order No. T-1213A
Interagency Agreement No. 40-223-70
(NASA-CE-147462) SEARCH FOR AND ANALYSIS OF 	 E76-19025
RADIOACTIVE HI LOS IN ITIWAR MATERIAL Final
Report (Oak Ridge National Lab.) 	 6 p
HC $3.50	 CSCL 03B	 Unclas
G3/91 14269
, 	 E1
61AR 1976
RECEIVED
NASA Sll FACILITY	 Y
INPUT BRANCH I	 =i
7
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RADIOHALOS, TEKTITES AND UMAR RADIOACTIVE CHRONOIAOY
The rationale for the lunar halo search is that halos in terrestrial minerals
serve as pointers to localized radioactAvity r..nd thereby make possible analytical
9
studies that bear importantly on the general problems of isotopic dating and mode
of crystallization of the host mineral. For example, several minerals such as
mica, fluorite and cordierite often contain unusual halos originating with fairly
short half-life alpha radioactivity, e.g., 218 P0(tl/2 = 3 min), and it is not clear
how the presence of these halos can be reconciled with a hydrothermal, magmatic, or
metamorphic origin of the host minerals (1). Therefore, a further aim was to pursue
such ancillary studies on terrestrial halos and on certain samples of special origin
(e.g., tektites, meteorites).
With respect to the first aim, the MSC thin section collection from the
Apollo 11-15 suite of rocks has been personally scanned for halos on several occa-
sions with completely negative results. It is difficult to estimate the probability
that this means a real absence of halos because the only basis for such an estimate,
their terrestrial occurrence, is exceedingly spotty. Further, terrestrial halos are
most numerous in rock types that are generally conspicuously absent from the lunar
samples thus far available for study. The absence of halo,,
 is not due to a lack of
radioactive inclusions in lunar sample, since such have been well identified in
rocks such as 12013. Perhaps the high radiation environment indigenous to the lunar
surface effectively anneals the halos as they are being formed.
In parallel with the search for halos in lunar material, certain types of
terrestrial halos have been examined for information on the general field of radio-
active chronology. In particular, we have used the ion-microprobe mass spectromete^
to determine lead isotope ratios in the microscopic inclusions within certain ter-
restrial radiohalos that were expected to possess unusual Pb isotope abundances M.
^	 I
- 2 -
Specifically, one inclusion showed Pb/U and Pb /Th ratios > 5000 with 206 //204Fb
> 1000, i.e., no U, Th or 2o4 Pbdetected. In another case, the 
2o6Pb/207 Pb ratio
was p 60, an impossibly high value if the Pb isotopes had been derived from uranium
decay. Clearly, any attempt to date this sapple in the usual way would be impos-
sible, and it appears desirable to establish the extent of such anomalies in lunar
and other terrestrial samples.
While their exact origin and history is uncertain, iron meteorites are
important in radioactive chronology as reference material for defining primordial
Pb isotope ratios. Over the last decade, several analyses have shown modern radio-
genic lead	 addition to the primordial mixture in the troilite phase of a number
of iron meteorites ( 2). These anomalous results have usually been attributed to
terrestrial contamination (3), but in order to settle the question unambiguously,
we have initiated ion-probe Pb isotope studies of this class of samples. To evaluate
the potential of the ion -probe to perform accurate Pb isotope ratios on the meteor-
ites, NBS common lead was utilized as a standard in test runs. A comparison of ion-
probe ( IP) runs at Oak Ridge with the NBS reference data for their sample yielded
the following results (4); 2o4 Ph/ 2o6Fb = 0 . 0591 t 0 . 0074 ( IP), 0.059042 t 0.000037
(NBS); 207pb/206pb = 0.909 t 0 . 025 (IP), 0.91464 t 0.00033 (NBS); 208Fb/2o6Pb
2.174 t 0.035 (IP), 2.1681 t 0.0008 (NBs).
Another part of the study was to have been a comparison of lunar and terres-
trial U and Th halo sizes in relation to the fundamental question treated more than
30 years ago by Dirac ( 5) and more recently by Hoyle and Narlikar (6), i.e., that
of the possible lime variation of the fundamental physical constants and in particu-
lar of the radioactive transformation rate over geological time. Due to the absence
of lunar specimens comparative studies were not possible, yet terrestrial halo
investigations have yielded some data that may bear considerably on lunar radioactive
chrcnology. As background information I note that in the context of the Dirac
hypothesis, Oerling and Ovchinnokova ( 7) have recently reported differences in rock
ages as measured by various age-dating techniques, a situa`.ion which they attribute
i
to a variation in the alpha-beta branching ratio in the Precambrian era. A few
years ago Gamow (8) proposed using the r a ng structure of uranium halos to check
branching ratios in the Precambrian, not realizing that Wilkinson (9) had earlier
interpreted the same data (somewhat erroneously) in favor of confirming presently
9
accepted values. Interestingly, the same data which for decades have been utilized
to establish decay rote invariance and branching ratio constancy are now being
interpreted to imply a variation in the radioactive decay rate (10). Clearly the
same data cannot support both conclusions!
From decay theory it has been argued previously that the near agreement be-
tween uranium and thorium halo ring radii and compu.ed alpha ranges in the same
material was proof of an invariant decay rate (11). To see this is not exactly
true wei use the a-decay theory notation given in (12) to compute the fractional
change in the decay constant (X) arising from the fractional change in ring radius
(r). In this case In T r-- 3Z1/2R1/2 - 4ZE 1/2 ; X = vT where T is the tra__..mission
probability, E the energy of the emitted a-particle, Z the atomic number of the
daughter nucleus, v is the frequency with which the a particle strikes the barrier,
and R is the nuclear radius. It then follows that in the appropriate units dX/X
1(3/2)Z1/2R1/2 - 11 (dR,/R) + (1/2 + 2Z/El/2 ]X (dE/E). A particle of mass ml and
charge z  has a range r (halo radius) given approximately by the expression r =
const. E2/Z12ml . Subsequent calculations lead to the expression dX/X ^ :6 43(dR/R)
+ 46(dr/r). Since the minimum uncertainty in measuring halo radii is about Ar
=,O.1 µm, then the resulting fractional uncertainty in X for 238U (halo radius 12.5 µm)
is dX/Xs4 46(0.2/12.5) 9:; 1/2 assuming dR = 0. since some uncertainties are involved
in the conversion of air to mineral ranges, Van de Graaff He ++ ion beams of varying
energy were utilized to induce coloration bands in the mineral containing the halos.
In examining scores of U and Th halos and in attempting to match their ring radii
with the corresponding ranges obtained from the experimentally determined range-
energy curve, I have found occasional rather than systematic differences in certain
F,
-4-
halo ring radii. Since the Van de Graaff induced ranges correlated closely with
halo radii at 4.2 Mev (13 '- 0.5 µm V.d.G. vs 12.7 t 0.2 um for halo radii) and
5.3 Mev (19 t 0.5 µm V.d.G vs 18.9 t 0.2 pm for halo radii), at present I do not
attribute such differences tr an actual change in Ea.
e
An unanswered question is whether X variations might occur without changes in
halo radius (dE = 0). Wilkinson (9) has suggested that the physical constants
might vary in some unknown fashion without affecting X. The contra-positive of
this suggestion would imply the correlation of measured a-ranges and halo radii is
not a test of X invariance. Variations in ?. might then be detected by noting age
differences in samples determined by radiometric, geologic, or archeologic methods.
As a case in point, there is a wide discrepancy between the radiometric (13, 14)
ages (0.7 my and 4 my) and the geologic and stratigraphic (15, 16) age (several
thousand years) of " = australites. That two cosmic events separated by several
million years would result in similar strerm-field tektite patterns is in itself
most unusual irrespective of whether the tektites are of lunar or terrestrial
origin (17). Rviden-e suggesting a common link between the 11/Na (4 my) australites
and the general australite population comes from recent Th/U determinations at
ORNL (18), i.e., Th/U = 5.9 (AN-87) and 8.3 (P-192) in the H/Na group compared to
Th/U values of 4.8 - 8.5 previously reported for the general group (19).
It has been remarked that the fission track age of the new H/Na group of
australites may be high for some reason (17). This is quite significant in that
it is universally agreed that fission tracks can form only after the tektites
have cooled. Unless the australite fossil fission tracks originated with a spon-
taneous fissioning nuclide besides 238U, the only remaining possibility for a "high"
fission track age is a hiatus in the decay rate due! to causes presently unknown.
While this explanation is unusual, I do not think it can be aefinitely ruled out by
simply appealing to the regularity of radiohalo data.
Acceptance of this hypothesis would not necessarily have implied significant
changes in the other fundamental constants but would affect age dating.
-5-
1. R. V. Gentry, Science 173, 727 (1971).
2. B. Doe, Pb Isotopes, Springer•Verlag, New York (1970).
3. V. M. Iversby, Geochim. at Cosmochim Acts, 34, 65 (1970).
4. S. S. Cristy, D. V. Ferree, J. F. McIaug^ilin, Oak Ridge Y-12 Operations.
5. P. A. M. Dirac, Nature 139, ;23 (197)•
6. F. Hoyle and J. V. Narlikar, Nature 233, 41 (1971).
7. E. K. Gerling and G. V. Ovchinnokova ) Geochemistry Int. 7, 717 (1970).
8. G. Gamow, Science 158, 766 (1967).
9. D. H. Wilkinson, Phil. Mag. 3, 5 82 (1958)•
10. R. M. Spector, Phys. Rev. A, Feb. 1972 (to be published).
U. G. H. Henderson and L. G. Turnbull, Proc. Roy. Soc. A145, 582 (1934).
12. R. B. Leighton, Principles of Modern Physics, McGraw-Hill.
13. R. L. Fleischer and P. B. Price, 'eochim. et Cosmochim Acts, 28, 755 (1964).
14. R. L. Fleischer, P. B. Price and R. T. Woods, EPSL 72 51 (1969).
15. George Baker, Ga. Min. Newsletter XV, 62 (1962).
16. E. D. Gill, Jour. Geophys. Res. 75, 996 (1970).
17. D. R. Chapman ar.d L. C. Scheiber, Jour. Geophys. Res. 74, 6737 (1969).
18. G. D. O'Kelley, J. S. Eldridge, K. J. Northcutt, Chem. Div. ORNL (1972).
19. R. D. Cherry and J. A. S. Adams, Geochim. et  Cosmochim. Acts, 27, 1089 (1963)•


Search for and analysis of radioactive halos in lunar material

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