RADON-222 EMANATION CHARACTERISTICS OF LUNAR FINES
J.A-So p dams, P.M.C. Barretto, R.B. Clark and G.E. Fryer,Depart-
ment of Geology, Rice university, Houston, Texas. 77001
Systematic studies of radon emanation from returned lunar
samples and their terrestrial analogues (obsidians, tektites,
and basalts) have been underway for only a few months in order
to investigate the problem of the radon mobility on the lunar
surface and its possible contribution to lead isotopic frac-
tionation and redistribution. A pilot study of 20 terrestrial
zircon and 15 sphene concentrates by Barretto(l1 for which the u
and Pb (plus some Th) isotopic data were available from others
workers indicated: a) positive correlation between ~ b - uage dis-
cordance and percent of current radon emanation; b) positive
correlation between natural alpha dosage and emanation. Even
with uncertainities regarding uranium microdistribution ("hot
spots" versus homogeneous distributions) and annealing of the
radiation damage by past thermal events these correlations were
observed in 85% of the samples. c) positive correlation between
weathering and radon escape (weathered granitic rocks 9-16%,
soils 15-60%,). Also a very discordant zircon(2) exhibited an
unusually high radon loss (12%) as well as lead loss, These
observations suggest that radon and lead diffusion loss mecha-
nisms are related. Furthermore, consequences similar to lead
diffusion or volatilization can be achieved by cumulative radon
migration, although on a smaller scale. Radiation damage is
considered to be a major factor in increasing both radon emana-
tion and lead diffusion losses, and the radon diffusion within
the crystal seems important as a means of direct transfer of the
pb isotope precursors and/or as a mean of moving the lead to
sites from which it could be easily volatilized or diffused.
Table I lists the emanation characteristics for 8 bulk
lunar fines samples, one small sample of KREEP glass slus-forcom-
parison purposes -
three terrestrial analogues. The samples
were de-emanated at room temperature and a pressure of one
atmosphere. Background and sample counts were for periods in
excess of 15 hours, As shown in Table I, uniformily low radon
escape rates and losses characterize the analysed lunar material.
From over 55 terrestrial rocks analysed for radon emanation, the
obsidian and tektite (bediasite) are the materials which best
approach the lunar material in emanation characteristics, in
glassy state, and in both uranium content and distribution,
0Lunar and Planetary Institute Provided by the NASA Astrophysics Data System
RADON-222 EMANATION
A d a s , J.A.S. e t al.
Analyses of d i f f e r e n t g r a i n s i z e s of t h e s e two t e r r e s t r i a l
samples r e v e a l e d t h e expected i n c r e a s e i n t h e percentage of
radon l o s s w i t h d e c r e a s e of p a r t i c l e s i z e ( i n c r e a s e i n s u r f a c e
a r e a ) , From t h e s e s t u d i e s it i s concluded t h a t t h e low radon
escape f o r t h e l u n a r f i n e s i s due t o t h e i n t e r a c t i o n of t h r e e
f a c t o r s : 1) a r e l a t i v e uniform d i s t r i b u t i o n of U and Th through-
o u t t h e g l a s s y m a t e r i a l , 2) low r a d i o a c t i v e c o n c e n t r a t i o n . These
two f a c t o r s combined w i l l g i v e very low r a d i a t i o n damage r a t e s
~~
( 0 . 6 1 ~ 1 0alpha/mg/year) t h u s never r e a c h i n g t h e damage s t a t e
observed f o r sphenes and z i r c o n s . 3 ) Absence of chemical weath-
e r i n g t o r e d i s t r i b u t e uranium and i t s daughters i n micro-
f i s s u r e s , g r a i n s u r f a c e o r o t h e r s i t e s from which radon can
e a s i l y escape, By c o n t r a s t some of t h e U-rich phases i n l u n a r
r o c k s a s r e p o r t e d by T h i e l 0 ) among o t h e r s would be expected t o
accumulate e x t e n s i v e r a d i a t i o n damage over l u n a r time spans and
a s a r e s u l t e x h i b i t h i g h e r radon emanation r a t e s .
I n conclusion, t h e data reported here suggest t h a t very
l i t t l e ~n~~~ i s escaping from t h e s o i l a t t h e l u n a r s u r f a c e and
t h e r e f o r e it h a s a s m a l l d i r e c t c o n t r i b u t i o n t o t h e excess of
i s o t o p i c l e a d . However, l o c a l m i c r o c o n c e n t r a t i o n s of h i g h l y
emanating m a t e r i a l might w e l l enhance t h e ~n~~~ y i e l d on t h e
s c a l e n e c e s s a r y t o e x p l a i n t h e Surveyor V a l p h a s p e c t r a , t h e
Apollo 15 command module po210 a 1 ha s p e c t r a , and t h e 12070
sample p r e v i o u s l y r e p o r t e d by u s ( )E . Emanation d a t a on b r e c c i a s
and r o c k s ( e s p e c i a l l y h i g h uranium m a t e r i a l l i k e rock 12013) a r e
needed a s w e l l a s experiments under c o n d i t i o n s approximating t h e
l u n a r da and n i g h t ( v a r i a t i o n s on t h e " s t i c k i n g power",
Heymann ( S be£o r e f u r t h e r c o n c l u s i o n s about t h e importance of
radon m o b i l i t y on t h e moon can be reached.
REFERENCES
(1) BARRETTO P.M.C. (1972) Ph.D. T h e s i s , ice U n i v e r s i t y .
( 2 ) STERN T.W., E EL
GOLDICH S.S., N W L M.R. (1966) E a r t h Plan.
-
S c i , Let. L, p, 369-371.
( 3 ) THIEL K., HERR W. and BECKER J. (1972) E a r t h Plan. S c i .
-
Let. I p. 31-44.
& ,
DM
( 4 ) A A S J,A,S., BARRETTO P.M.C., CLARK R.B. and D W A L J.S.
(1971) Nature 231, p. 174-175.
( 5 ) HEXMANN D. and YANIV A. (1971) Nature 233, p . 37-39.
0Lunar and Planetary Institute Provided by the NASA Astrophysics Data System
RADON-2 2 2 EMANATION
Adas, J.A.S. et al.
h
N C
h
V
h
N
h
N
h
3
-
I n
h
C U C
h
U
h
h
- n x
#a,k
..
a,
-E-lrd
w w V w u ~ u w u ~ V V
b * ~ l - rl m -lJ
m m m o m m rd rn
a P r l wb d a 3 3 N
ul
I n
I
a
d
r
r
n
l
r d
c
a
r
B
r l m r l d ' o , + m . r l
N m o a 3 ~ b o 3 c o o
O O L n O
m r J a > O
o O
o O
N O L on
O l
f T l r l
r l r l u r l d d r l r l u z g w
0Lunar and Planetary Institute Provided by the NASA Astrophysics Data System