Alpha-recoil damage: Relation to isotopic disequilibrium and leaching of radionuclides

CO16-7037/88/$3.00 + 00

Gemhimica et Cosmochimica Acfa Vol. 52, pp. 1459-1466 Pergamon Press pk. Printedin U.S.A.

Copyright Q 1988

Alpha-recoil damage: Relation to isotopic disequilibrium and leaching of radionuclides* ROBERT L. FLEISCHER General Electric Research and Development Center, Schenectady, NY 1230I, U.S.A (Received August 12, 1987;Accepted in revisedform February 25, 1988) Abstract-The observation by RAABE et al.(1973) of large differences between the solubilities of isotopically different plutonium dioxides, has led to the recognition of preferential etching of recoil damage as a widespread phenomenon for alpha-active radionuclides. The associated preferential solubility of the products of alpha decay, along with direct recoil ejection, are the two specific microscopic mechanisms that are documented as causes of isotopic disequilibrium in the U and Th decay series. Similarly, leaching plays a significant role in releasing 222Rnfrom natural substances, 222Rnbeing the alpha-decay product of 226Ra.The average annealing time in nature of the damage sites can be inferred from the extent of isotopic disequilibrium for different isotopic pairs in the Th and U decay chains. INTRODUCIION

One mechanism of 234U loss into water-direct ejection of a recoil atom into pore water-was conceived and demonstrated by KIGOSHI (1971) and confirmed by FLEISCHER and RAABE (1978a,b). Kigoshi immersed zircon grains in water and observed the build-up of 23?h, each atom of which receives about 72 keV from the alpha decay of 23*U. The second mechanism for which there is clear evidence is of recoil leaching (FLEISCHER and RAABE, 1978a,b; FLEISCHER, 1980, 1982a,b). The evidence was obtained using the nuclear track technique (FLEISCHER et al., 1975), and the start of the work came not from geochemistry but from an isotopic effect observed in an inhalation-toxicology study. This discussion is confined to microscopic mechanisms. The reader is reminded of major chemical sources of largescale disequilibrium such as is produced, for example, between 234U and 23@fhin the oceans, arising because of the negligible solubility of thorium. This sort of process is step 2 in separation events in which step 1 is one of the microscopic processes that were just listed. In short, here we are interested in release of isotopes from mineral grains, not in subsequent larger-scale chemical or physical separations.

THE PURPOSE OF this article is to describe concisely evidence

as to how three very different processes occur: How natural isotopic disequilibrium begins within heavy-element decay chains, how the rare gas 222Rn is released from the Earth, and how differential solubility occurs between the nuclear materials 239Pu02 and 23*PuO~. The mechanisms are common, but the fields of application range from isotope geochemistry, to mineral exploration, seismology, and health physics. A discovery in one field led to analysis and experimentation that contributed to other fields. Such connections, which are inherent in science, provide a special stimulus to those who encounter them. For geochemists the interest here will be in the release of products of alpha decay from rocks and soils. This common phenomenon is evidenced by emanation of 222Rn from the ground and by the remarkable variability of 234U/238Uratios on Earth. A brief review by SZABO( ! 969) shows activity ratios that range from 0.4 to 8.0. More recently KRONFELD(1974) and SHENG and KURODA ( 1984) measured activity ratios as high as 12 and 24, respectively. The role of water is critical in these processes. Although other evidence will be noted, the single most convincing result is the finding that 234U/238Uactivity ratios are unity on the moon (ROSHOLT and TATSUMOTO, 1970, 197 l), a place where there is no water to effect disequilibrium. On Earth the usual situation is that water which has permeated soil or rock becomes enriched in 234Uactivity relative to 238U, and the solid is partially depleted in 234U. Among the mechanisms that have been proposed, there are two unproved and two established ones. There is no clear evidence for these two mechanism: First, ROSHOLT et al. (1963) suggested that recoiling 234U would come to rest in unusual structural positions, some of which would favor a +6 valence state rather than the usual +4. They did not suggest how the presumably more soluble U6 would reach surfaces to be dissolved. Second, KIGOSHI ( 197 1) proposed that atoms which recoil into adjacent grains might have an easy diffusional path to the nearby surfaces.

DISSOLUTION A startling

isotopic

OF F’uOl AEROSOLS

effect was observed

in the dissolution

of PuO2 (RAABE et al., 1973), an experiment in which dissolution was compared for 239Pu02 and 238Pu02. It was found that in water and other aqueous solutions the oxide containing 238Pudissolved about 200 times more rapidly than that with 239Pu Since kinetic isotopic effects vary with the square root of the mass ratio, a 0.21% effect was expected (m - 1); and the discrepancy of - 100,000X needs a qualitatively different explanation. The discrepancy is resolved if it is noted that the ratio of the alpha decay rates, when corrected for 239Pu impurity in the 238Pu is also about 200 to 1, (FLEISCHER, 1975). Hence the dissolution is in some way connected with radiation damage that is caused by alpha decay. The definition of dis-

solution in the experiments is the removal by water (or water with EDTA, a chelating agent, ethylenediaminetetraacetic acid) of material held between two filters, as sketched in Fig. 1. Table 1 summarizes the results and the predictions of the kinetic and the radiation-damage models.

* Prepared for the Conference on “Isotope Tracers in Geochemistry and Geophysics” in honor of Professor Gerald J. Wasserburg’s Sixtieth Birthday, March 23-25, 1987, Pasadena, California. 1459

R. L. Fleischer

1460 WATER

Pu02 PARTIC

ILTERS

FIG. 1. Dissolution of Pu02 particles was measured by RAABE ef al. (1973)from Pu in the downstream water using this configuration. The filter hole size of 100 nm defines the maximum size of the “dissolved” Pu02 .

In trying to quantify the radiation-damage model the author pictured the process using the sketch in Fig. 2. Only alpha effects in the surface region of a particle (with a recoil range 6 of the surface) can contact the liquid. This geometric consideration leads to the observed variation of dissolution rate, which is roughly (radius)-‘. The primary mechanism proposed was that the recoiling ion from alpha decay propelled a spray of atoms and groups of atoms into the liquid. As an after-thought the alternative was noted of radiation damage followed by chemical etching, such as has been documented in micas (HUANG, and WALKER, 1967); in the RAABE et al. work the preferential chemical removal of portions of the damage is done by water and by EDTA solutions in water. As we shall see, the afterthought provides the better description, and what was originally thought to be the more likely model is wrong. The reason for suggesting the spray of debris was the hnding that the solvent contained not just isolated atoms of Pu but an assortment of clusters that contained up to lo4 Pu atoms (FLEISCHERand RAABE, 1977). Figure 3 shows evidence of a large cluster. By evaporating small droplets of the solvent, the “dissolved’ Pu can be deposited on clean quartz crystals and then caused to fission by neutrons, using doses of up to 5 - 10t9/cm2. The sensitivity which results from the fluence is such that a pair of tracks from a common point locates what is typically a cluster of -60 atoms of 239Pu, and the background of fission from natural 235Uin the quartz is neg-

FIG. 2. Recoil ejection of atoms. Alpha decay of “‘Pu mrght eject the recoil ?J nucleus if it was generated within its range 6 of the surface. A portion of the atoms along the track are envisioned as ejected in the process (FLEISCHER, 1975). Later work (FLE~SCHER and RAABE, 1978a) shows that the excavation of the damage region occurs from the later action of water rather than by direct recoil ejection.

ligible because the uranium content is so low (- 3 wt parts/ lo’*). Figure 4 shows the inferred size distribution of Pu02 particles in the solvent. A pertinent question is whether the observed clusters such as the one shown in Fig. 3 could be artifacts of coagulation in water either prior to or during the droplet-evaporation process. Experimental evidence contradicts this hypothesis.

Table 1. Relativedissolutionlifetimesof 239PuO2 and 238Pu02

CUZ

'239/'238 "edians of UntreatedParticles*

140(f90)

l4edians of EDTA-Treated Particles*

255(t190)

From Chemical KilletiCS

1.0021

OESERVED

(et 37°C) (from Raabe c 1973

al.,

FXPECTED From Radiation Damage

235(?10)+

* Lifetimeproportionalto particleradius assumed. + Ratio of decay rates. corrected7% for greaterenergy of 238 Pu decay, and 20% for known isotopicpurity of 238 Pu02 particles. (Fleischer,1975)

FIG. 3. Plutonium cluster found in the residue from evaporating a drop of water. 239Pufission fragment tracks were etched in quartz that was irradiated with 5 X lOI thermal neutrons/cm’. The star with -400 prongs represents a particle with about 1O4*” Pu atoms. The tracks are about 10 rrn in length (FLEISCHER and RA.&BE.19771.

1461

Radionuclide alpha-recoil damage NUMBER OF PRONGS PER PARTICLE AT 5 X lOI nvt @ ! Iplcpwxx,

the end of the evaporation

of the droplets. In short, the observed clusters are inferred to be a true description of the form in which 239Pu entered the solution. With the result shown in Fig. 4 in mind, the experiment described by Fig. 5 was done (FLEISCHER and RAABE, 1978a,b). Here PuOz was expected to eject PuOz particles with the size distribution measured in Fig. 4. Neutron irradiation of the detector that was used as a catcher should have shown fission from one 235Urecoil and N accompanying 239Pu atoms, the latter showing the size distribution that is given in Fig. 4. Using the observed number of events (F), the number of upward moving 235U recoils (n) from alpha decay of 239Pu, and the known fission cross sections ~235 and 0239for 235Uand 239Pu, the value of N is determined from F = (~235 + Nu239)@ and given in Table 2. Instead of N -400, a much lower number of ejected fissionable atoms was found in each of five experiments. As shown in the right hand portion of Fig. 5 and Table 2, the observed counts were those of solely the 235Urecoil nuclei that are ejected by alpha decay of 239Pu. Instead of one z3sU atom and 400 239Puatoms per recoil that crossed the surface of the PuOz, only the recoil appeared. The large percentage uncertainties in the right-hand column of Table 2 are there because the experiments were prepared with the larger number of atoms expected. But, as the diagram on the right in Fig. 5 shows, there is clear signal above the background. Calculation showed only 0.40 + 0.27 of an extra atom (above the expected 235Urecoil count), whereas about 300 times the total seen would be encountered had the recoiling *“U carried with it the amount of 239Puthat entered the solution in the earlier experiments. The conclusion is that the accelerated solution of 23*Pu02 is not from direct ejection but from subsequent etching of

-CALCULATED ‘QBSERVED

0.01 t

mcd

I IO I02 I03 IO NUMBER OF 23gPu ATOMS PER PARTICLE

FIG. 4. Size distribution of 23gPu-containing sub-particles in water in which Pu02 had been immersed. (FLEISCHER and RAABE, 1977). The dotted interpolation is justified by the fact that it would lead to a number of single tracks that agrees within 20% with the observed number, as shown.

The size distributions seen by FLELWHER and RAABE (1977) were unchanged with time, as compared with the predictions

of coagulation theory (SHELUDKO, 1966). Similarly for uranium present in solution no track sunbursts or stars were seen, but particulates could be recognized when present (FLEISCHER and DELANY, 1976). Therefore, no significant coagulation occurred from the time of sample collection until

SINGLE *=U

-

ATOMS

CATCHER,

RECOIL

FROM 239Pu

DETECTOR

PuO*

X-POSITION

(mm)

Y-POSITION

(mm)

THE CATCHER-DETECTOR IS NEUTRON-IRRADIATED AND THEN SCANNED TO COUNT THE RECOILING *=U ATOMS

00-

FIG. 5. Recoil ejection of atoms: Left: Schematic sketch of experiment to count ejected atoms. Upward moving atoms are caught on a high-purity quartz detector and later irradiated to produce fission-fragment tracks. The particles are 0.27 (kO.014) pm in dia. The detector is 0.06 + 0.005 cm from the substrate. Right: Fission track counts across the area that was adjacent to 0.85 &i of 239F’u02particles for 16 hr and was then exposed to 6 X lOI thermal neutrons per cm*. Only single 235Urecoils are observed (FLEISCHER and RAABE, 1978a).

1462

R. L. Fleiseher Table '2.

235 U recoils and ejected 239Pu atoms*

REC$ll;NTO Calculated

STOPPED IN PORES -0.10 + 0.16 0.94 f 0.39 3.30.107 3.27.107

-0.04 + 0.1: 0.72 t 0.34

*Fleiacher and Raabe(1978a) ETCHING OF TRACKS

the stored radiation damage from the exciting recoil atoms. The chemical attack must release the “rubble” with the size distribution given in Fig. 4, a distribution with a character that happens to be very similar to that produced by hypervelocity impacts on the Moon, but is distinct from that seen in ordinary comminution and that expected from coagulation (FLEISCHERand FL&ABE, 1978a).

FIG. 6. Mechanisms of recoil release from s&d grams. Top: direct recoil ejection: bottom: recoil into an adjacent grain, followed later by release by chemical dissolution of radiation damaged material containing the recoil atom or its prompt decay product. The prompt decay product is ‘wrh, half life 24 days (above); by the time an implanted “Th recoil in dry soil is later released by water it usually has decayed to 2’4U(below) (FLEISCHFRand RAAHE. I978a).

DEMONSTRATION OF CHEMICAL RELEASE OF RECOILS IMPLANTED IN MINERALS Sample

The implication of the results that were just discussed is that if recoil nuclei were implanted into a mineral, they would create damage trails, the trails would be chemically leachable, and some fraction of the implanted nuclei might be released by the leaching, The lower sketch in Fig. 6 shows the way in which 234U/238Udisequilibrium would be produced by such a process (and the upper sketch gives Kigoshi’s direct-recoil process). To test whether the implantation-leaching process occurs, one would like to implant a solid with nuclei that are readily traced, treat the solid with a reagent, and measure how many atoms are removed and how many are retained. The experiments with Pu02 provided a specially convenient system that used the decay 239Pu -+ 4He +

2351J.

Thus a layer of 239Pu02, will emit 235U recoil atoms at velocities which will implant them in an adjacent target mineral. The locations of 235Uatoms can then be sensitively located and counted by neutron-induced fission and subsequent scanning of fission tracks (FLEISCHER,1980). Figure 7 shows the four steps in the experiments, which allow measurement of both the 23’U in the leaching solution and that left in the leached mineral. Leached-out recoils will be found in the solutions; those that are not remain in the minerals. Both the solutions and the minerals are then analyzed by fission track induction. Using *‘?J as an implanted nuclide has another advantage beyond being easily traced by inducing fission tracks: It is similar to 234U in mass, chemical behavior, and the energy given it by alpha decay. It is therefore closely relevant to the 23“U/238Udisequilibrium that is one of the major interests here.

resulls on mineral3

Figures 8 and 9 give results for quartz and muscovite respectively as a function of the concentrations of solutions of HCI and NaOH, and for water of neutral pH. Consider Fig. 8 (FLEISCHER,1980). After a 24-hour soak. the 235Uin the implanted samples is given by the open circles. that in the solutions is given by the filled half-circles. One control sample is merely as-implanted, the second (the higher of the two controls) was rinsed for 5 s in water. and shows that the 235Uwas not easily removed and therefore was not superficially located. The solutions typically removed 40% of the implanted uranium, and most of the missing uranium appears in the leaching solutions. It is likely that the slight. unaccounted for loss of uranium is due to plate-out on the surfaces of containers. The results confirm that implanted alpha-recoil nuclei can be released by leaching; and, remarkably, they show that for quartz the results are insensitive to the composition of the solutions. This unexpected behavior is seen also for orthoclase and obsidian, but is not general, as the results for muscovite (Fig. 9) illustrate (FWSCHER. I982a). For Fig. 9 only the implanted samples were measured. In spite of the appreciable scatter, which defines the reproducibility of these experiments, it can be seen that the amount of “‘U retained decreases for the stronger solutions, either acidic or basic. The effect of time in increasing the “‘U loss is shown by the open squares, which correspond to 7-day leaches. More data on the effect of time are reported by FLEISCHER(1982a). Data for I I different time intervals given in Fig. I2 of that paper shows that loss of z”U continues for at least 100 days for muscovite in water; at that leaching time more than 70% of the implanted nuclei have been released. The statistical nature of damage by atomic collisions implies that some recoil nuclei will come to rest outside the contin-

1463

Radionuclide alpha-recoil damage

X CHEMICAL TREATMENT TO PRODUCE POSSIBLE TRACK LEACHING

In contrast, recoiling nuclei from alpha decay are low-velocity, lightly ionizing ions that are not expected to produce tracks through ioni~tion. Only the alternative of atomic collisions appears tenable as a mechanism for damage in all the materials for which experiments such as are reported in Figs. 8 and 9 have been done. The fact that the preferential leaching occurs in silicon (FLEISCBER, 1982b) is telling, since ionization-produced tracks are not expected nor observed there (FLEISCHERet al., 1965). This statement is supported by Fig. l-l 1 of FLEISCHERet al. (1975), which shows that atomic displacements produced by collisions become dominant at the low velocities of recoil nuclei, where ionization is negligible, and the reverse is true at higher velocities. The classic first estimate of the low velocity collisions is that of SEITZ (1949). The high velocity tracks were unknown at that time.

SAMPLES

Radiometric measurement of recoil-trackpermanence III

d

NEUTRON IRRADIATION TO INDUCE FISSION

fY

Ip

&

DRIED SOLUTION DROPLET

ETCHING AND READOUT OF INDUCED FISSION TRACKS

ETCHED DETECTOR

553 ETCHED MINERAL

ETCHEO DETECTOR zzzi UNETCHED SUBSTRATE

FIG. 7. Technique for implanting 235Uin minerals and for locating the nuclei after treatment of the mineral with a solution. The 235U nuclei that recoil from the alpha decay of 239Puare implanted (step I), and uranium nuclei are found at the end of recoil tracks (just as 234Uis located in nature). The implanted samples are separately exposed to different solutions (step II), which may release 23sU, The mineral is then (step III) placed next to a track detector, and the residue from a dried droplet of the solution is similarly mounted and both assemblies are exposed to thermal neutrons, which induce fission of 235Unuclei; encircled n, neutrons; ff, fission fragments. In step IV, the detectors are etched and the induced fission tracks counted to

measure how much 23sUwas retained in the mineral and how much went into the solution. Test-tub-shard sites that are open are where recoilsare reIeased;shaded sites represent unleached siteswhere recoils are retained {WEIGHER, 1980). uous damage that connects with a surface; hence less than 100% of the recoils will be preferentially leachable.

The observed extent of isotopic disequilibrium in the 238U and 232Th decay series can be used to infer how long alpharecoil damage is retained in certain minerals. Eyal and coworkers have observed 22BTh/232Thand 234U/238Uratios in leachants used on the minerals monazite (EYAL, 1982; EYAL and KAUFMAN, 1982), uraninite and thorianite (EYAL and FLEISCHER,1985a,b), betafite (EYALet al., 1985), and thoritemelanovite (EWING et al., 1986). The usual observation in these minerals is that the enhancement in the 234U/238Uactivity ratio, (R& in the leachate is - 10% and that for 228Tl-l/ 232Th, (R8 *) is much larger, i.e., factors of 2X to 7X. Time is a factor because the half life of 234Uis 245,000 y, while that of ‘**Th is 1.91 y. For 228Th recoil atoms, only a few years pass before they are transmuted, and therefore all 1400

, , , , , ,

,

/

, , ,, , ,

800 ~

2001

_

153‘AV’-

1

MECHANISM AND PERMANENCE OF RADIATION DAMAGE Recoil tracks are different from most particle tracks that are studied by preferential etching. The more usual tracks are caused by ionization followed by atomic displacements that are produced by coulomb repulsions of nearby ions or other charged species (FLEISCHER et al., 1965, 1975). Since ionization bursts dissipate rapidly in good conductors (metals and semiconductors), atomic displacements from induced ionization do not occur there, and etchable tracks are absent.

FIG. 8. Track densities per area of 235U-implanted surface for quartz 0 and for solutions in which quartz was immersed a for 24 hours at 23°C. Separate average readings are given for control samples and those that were exposed to the solutions. For the solutions the “average” excludes the sample with obvious contamination, indicated by lower limits. R signifies a 5-sec. water rinse (ELEISCHER, 1980).

R. L. Fieischer

1464 1400 rl-T-rn

r-l

of the samples (HUANG and WALKER, 1967). One would expect the two activity ratios in Eqn. (I) to he identical in solutions after leaching of mica, but the low U and Th concentrations make it unlikely that the sensitivity exists which would allow a clean, unambiguous measurement to be done.

r--T-TT-l-TJ

MUSCOVITE

I

EFFECT

0

LLLLLL! u t._L_l...t_1 .i_l 0 -2 -4 -6 H;O 1 -6 -4 -2 0

FIG. 9. Similar to Fig. 8, except for mica. Squares give results for a one-week exposure to the solutions, In this case only the recoils in the mineral were measured (FLEISCHER,1982a). 228Th that is removed by leaching was at fresh, unannealed radiation-damage sites. However, if on the average, damage he&d itself in lo4 years, only a small fraction of the 234U atoms in a mineral will normally be located in fresh damage. Hence only that small fraction would be preferentially leachable. From this physical picture it is clear that the two activity ratios that are measured in the etching solution will differ when lifetimes for annealing of damage lie between the two decay lifetimes. EVALand FLEISCHER( 1985a,b) inferred that the annealing time, r1\ at ambient temperatures of alpha recoil damage is given by TA

=

74M4.s

-

~)/&2

-

11,

(11

where r4 is the mean life of 234U, 353,000 y. Using Eqn. (1), the mean lifetimes given in Table 3 result. For the four uranium and thorium-rich minerals that are amenable to the measurements of the isotopic ratios, ages range from 400 years for a metamict mineral to 2,000 to 25,000 years for several crystalline minerals. In contrast, for mica it is known that alpha-recoil tracks exist over the ages

-_-."~.. Lifetime (years) 400

nonszite

18,000

>100,000,000

.-

Reference for 1saropic Data Eyal et et (1985)

Eyal(1982) Eysl and Kaufmn(l982) Huang and Walker(1967)

14,000, 7,800

Eyal and Fleischer(l985a.b)

3,000 to b,ooO

Ewing e+ aL.(1986) Ewing e,t el.(f986)

25,000

Eyal and Fleischerfl985e.b)

OF MOISTURE

As noted in the introduction, water is critical to the production of isotopic disequilibrium and the release of alpharecoil nuclei from rocks and soils. Figure 6 makes evident two roles of water. As the upper diagram shows. if there is sufficient thickness of water in the pore spaces. ejected nuclei are brought to rest in the liquid and are thus transferred from the solid. The second role, as shown in the lower diagram, is as a leachant, which only can be effective after implantation of the recoil nucleus through a surface. In wet soil or rock only narrow pores (5.20 nm) allow the recoiling nucleus to cross the water and be imbedded in an adjacent grain. In dry materials such implantation is common, but release requires subsequent wetting of the surface. The relative fractions of the observed effects from the two factors is not intuitively obvious, but they have been calculated for certain model cases thought to be descriptive of soils and rock (FIXISCHER, 1983). For wet steady-state conditions most of the transfer to water from coarse soil is by recoil stopping; for fine soil or for rock both mechanisms contribute: and for oscillating wet-dry conditions the contribution from leaching can become generally dominant.

Radon emanation

The discussion so far has emphasized isotopic disequilibrium of 23aU relative to 238U. and to a lesser degree l%Th relative to 232Th. These ratios and other related ratios not only are useful in interpreting geological processes, but are part of certain dating methods that wouid be affected by unusual values. Similarly important, however. is the separation of *12Rn from its parent 22bRa.Much more dramatic variations occur. and they clearly will be governed by the same mechanisms. The emanation of 222Rn and its entry into homes are matters of widespread public-health concern (NERO and LOWDER, 1983), and radon in the ground is a tracer for subsurface uranium (FLEISCHERef al., 1972) and for gas flows that may be attendant to earth strains prior to earthquakes (FLEISCHER and MOCRO-CAMPERO,1985) and to the existence of subsurface hydrocarbon deposits (FLEISCHER and TURNER, 1984). Since 222Rn has a mean life of 5.5 days, in soil that is dry for a week ***Rn has built up to nearly its dry steady-state value of implanted recoils. When water then enters, a surge of radon release is expected as illustrated by the calculations shown in Fig. 10. It also indicates that more radon is released by wet rock than dry and that if there is additional water which the radon must diffuse across (in this case a 14.8 cm thickness of water), the radon concentration will be attenuated by radioactive decay during the diffusion time necessary to cross the water barrier. Convection in the ground might increase radon transport to the surface, but effects are expected to be small. MOGRO-

Radionuclide alpha-recoil damage

w

200-

1465 1 ,

I

I

I

I I

I

I

I

I

I

Y - 20,0

E

I

‘Oo’

I 0.5

WATElf CONTEiT

I I I IO 20 50 [gm/ IOOgm SOIL]

I

100

FlG. 12. Effect of water content on radon release from soil (measured from counts of nuclear tracks). The initial increase is attributed to preferential dissolution of radiation-damaged sites. The calculated ultimate decrease at high water contents (produced by water being a strong diffisional barrier to radon escape) is consistent with the data, I

G

D

IO

but because of scatter, is not well supported experimentally. The curve is given by Eqn. (2) (FLEISCHER, 1987).

I

20

30 40 TIME IDAYSI

50

._

FIG. 10. Calculated radon release for a rock model, wet and dry. Water increases the release into pore spaces but attenuates its escape from the rock. “Wet” means that pore spaces are filled with water; “wet with 14.8 cm water barrier” means that there is a 14.8 cm thick region of water between the rock and the air (FLEISCHER, 1983).

CAMPEROand FLEISCHER (1977), have examined models of convection. They conclude that although convection could occur under normal geothermal gradients, high gas permeabilities, and circulation depths that extend to -300 m, the velocity of gas motion would be high enough only if the thickness of the high permeability region were at least 5 km. Otherwise, radon decay during transport reduces the effect strongly. Such multiple conditions are thought to be satisfied rarely in nature. In summary, moisture can either increase or decrease the observed radon signal. Emanation is maximized by solid grains being covered by a thin film of water that is sufficient to stop recoils and leach recoil damage. On the other hand, excess water attenuates the apparent radon emanation. Figures 11 and 12 are examples (FLEISCHER, 1987). In these experiments ordinary soil was used as a radon emitter. In a closed cylindrical system about 1.8 cm thickness of soil ( 100

1501

OOO

AVERAGE HUYlOlTY

E

(2)

where puay is the track density found for dry soil, the enhancement factor E describes the greater emanation in wet soil relative to dry soil, d the depth of water in the soil, 6 the soil thickness (1.8 cm), and z, the mean diffusion distance for radon in soil that is saturated with water. When the water depth exceeds 6, the result for Eqn. (2) with d = 6 is multiplied by exp(-h/zn,o), where h is the depth of the water above the soil and znp is the mean diffusion distance (2.18 cm) for ***Rn in water. The diffusion distances and the derivation of the tanh factor are given by FLEISCHERand MOGRO-CAMPER0 (1978). Additional recent examples of moisture affecting radon emanation are given by STRONGand LEVINS( 1982), STFWNDEN et al. (1984), GREINER (1985), and GAN et al. (1986).

1.00

IN SOIL

FIG. 11. Variation of track density (as a measure of radon concentration) with humidity above the soil in laboratory experiments like those described by FLEISCHER and MOGR~XAMPERO(1978), using sealed cylindrical containers such as are shown in their Fig. 4 (FLEISCHER, 1987).

gm) with an additional air space of 300 ml was present along with specified amounts of water. Humidity was monitored by color-changing humidity cards, which can be read to about +lO%. Radon concentrations were monitored with ally1 diglycol carbonate track detectors, which have been calibrated against well established standards (FLZISCHERet al., 1980). Variability is likely to be largely the result of the soil samples being somewhat inhomogeneous, since a few highly emanating grains can have a striking effect on a soil sample. Figure 11 shows that when the humidity in the soil gas is less than lOO%, radon is at a constant low level. Once 100% is reached, water films are possible and the emanation is enhanced by a factor E that in this case is empirically found to be -2.5. The same data in terms of water content (Fig. 12) shows the increase caused by the water up to 20 to 40 wt% water in soil, followed by the expected, but not very clearly documented, drop-off caused by water becoming a diffusive barrier. The behavior is fit by Eqn. (2):

CONCLUSIONS

Nuclei that result from alpha decay in solids may be released either by recoil injection into surrounding fluids or by injection into neighboring solids, followed by leaching. These effects are sufficient to account for much of the microscopic

1466

R. L. Fleischer

origin of the observed isotopic di~uiiib~um

in the U and Th decay series and many of the observed properties of 222Rn emanation. Chemical and physical processes can produce additional, macroscopic separations that follow the release of isotopes from mineral grains. -Icknawledgements-The work described here was done in part in collaboration with 0. G. Raabe and Y. Eyal. Extensive experimental help was supplied by W. R. Giard and L. G. Turner. The author is also grateful to E. A. Martell for calling his attention to the surprising observations of RAABE et al. (1973). Although the study of isotopic disequilibrium using nuclear tracks as a tool was not a research area of G. J. Wasserburg, to whom this paper and issue are dedicated, the author is in his debt. Wasserburg’s enthusiastic demonstration of the power, versatility, and importance of isotopic information led to my interest in this area. Editorial handling: D. 1. Papanastassiou

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