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Nuclear track studies of alpha-recoil damage in nature: Relation to isotopic disequilibrium and leaching of radionuclides
Nucl. Tracks hr. .I. Radial.
Rodiar. Appl.
Meas., Insrrum.,
Vol. 14, No. Parr D
4, pp.
431-446,
0191-278X/88
1988
163.00 + 0.00
PergamonPressplc
Printedin Great Britain
NUCLEAR TRACK STUDIES OF ALPHA-RECOIL DAMAGE IN NATURE: RELATION TO ISOTOPIC DISEQUILIBRIUM AND LEACHING OF RADIONUCLIDES ROBERT
L.
FLEISCHER
General Electric Research and Development Center, Schenectady, NY 12301, U.S.A. (Received 17 March 1988) Abstract--One of the special characteristics of nuclear track studies is the wide variety of phenomena for which track etching provides useful information. This paper describes such a case, with applications ranging from isotope geochemistry, to mineral exploration, seismology, and health physics. Independent work relating to the danger from inhaled radioactive aerosols (Raabe ef al., 1973) nucleated these studies, which later led to other areas. What Raabe and co-workers did was to observe large differences between the solubilities of isotopically different plutonium dioxides. That result 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 two specific microscopic mechanisms that have been shown, using nuclear track techniques, to be causes of isotopic disequilibrium in the U and Th decay series. A similar alpha-decay process (of 226Ra)causes leaching to play a significant role in releasing ***Rn from natural substances, 222Rnbeing the immediate decay product of 226Ra.The average annealing time in nature of the alpha-recoil damage sites can be inferred from the extent of isotopic disequilibrium for different isotopic pairs in the Th and U decay chains.
1. INTRODUCTION TRACK etching is useful in an astonishing variety of fields. The work described in this article is an exam-
ple. It gives evidence as to how three very different processes occur: how natural isotopic disequilibrium begins within heavy-element decay chains, how the rare gas ***Rn is released from the earth, and how differential solubility occurs between the nuclear materials 23%02 and 23BPu02. 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. A well known behavior of radioactive species in nature is the release of products of alpha-decay from rocks and soils. This common phenomenon is evidenced by release of *=Rn from the ground and by the remarkable variability of 234U/23*U ratios on earth. A brief review by Szabo (1969) shows 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 turns out to be critical in these processes. Although other evidence will be noted, the single most convincing demonstration is the finding that 2wU/238U activity ratios are unity on the moon (Rosholt and Tatsumoto, 1970, 1971), a place where there is no water to effect disequilibrium. On Earth N.T.
M/4-8
437
the usual situation is that water which has permeated soil or rock becomes enriched in *W activity relative to 238U, and the solid is partially depleted in *%l. Among the mechanisms that have been proposed, two are unproved and two are established. Each depends on the fact that the residual recoil atom that results from alpha-decay is given kinetic energy that is sufficient to move it many atomic distances from its original location. There is no clear evidence for these two mechanisms: first, Rosholt et al. (1963) suggested that recoiling *%U 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 U+6 would reach surfaces to be dissolved. Second, Kigoshi (1971) proposed that atoms which recoil into adjacent grains might have an easy diffusional path to the nearby surfaces. Although no evidence for the action of these mechanisms exists, there is no proof that they do not act. One mechanism of *“U 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 *qh, each atom of which receives about 72 keV from the alpha-decay of *“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,
ROBERT
438
L. FLEISCHER
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. It is noted that there are also major chemical sources of large-scale disequilibrium such as is produced, for example, between 234U and 23@Thin the oceans, arising because the negligible solubility of thorium causes it to precipitate, whereas the uranium remains in solution. 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 describe the release of isotopes from mineral grains, not subsequent larger-scale chemical or physical separations.
2. DISSOLUTIONS
OF PuO, AEROSOLS
In studying how long inhaled radioactive aerosols might reside in the lungs, a startling isotopic effect was observed in the dissolution of Pu02 (Raabe et al., 1973), an experiment in which dissolution was compared for 239Pu02 and 23*Pu02. They found that in water and other aqueous solutions the oxide containing 23*Pu dissolved 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 (dm - 1); and the discrepancy of N 100,000 x 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 23*Pu 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 dissolution 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. 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 (within a recoil range 6 of the surface) can
WATER
Pu0, PIPTIC
FIG. 1. Dissolution of PuO, particles was measured by Raabe er 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” RrO,.
contact the liquid. This geometric consideration leads to the observed variation of dissolution rate, which is roughly as (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 work of Raabe et al. (1973) 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 finding that the solvent contained not just isolated atoms of Pu but an assortment of clusters that contained up to lo4 Pu atoms (Fleischer and Raabe, 1977). Figure 3 shows evidence of a larger 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 x 10i9/cm2. The sensitivity which results from that fluence is such that a pair of tracks from a common point locates what is typically a cluster of _ 60 atoms of 239Pu(in short, particles with less than 3 x 10-x’gm of Pu02 were observed). The background of fission from natural 235Uin the quartz is negligible because the uranium content is so low (N 3 wt parts/10i2). 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 Figure 3 could be artifacts of coagulation in water either prior to
Table 1. Relative dissolution lifetimes of 239Fu02 and 238Pu02 Case Observed (at 37°C) (from Raabe er al.,
T239/T238
Medians of 1973)
untreated particles* Medians of
140(f90)
EDTA-Treated
255 (* 190)
particles*
ExDected
From chemical kinetics From radiation damage
1.0021 235 (f IO)?
*Lifetime proportional to particle radius assumed. TRatio of decay rates, corrected 7% for greater energy of 2’8pudecay, and 20% for known isotopic purity of u8Fu0, particles (Fleischer, 1975).
ALPHA-RECOIL
DAMAGE
IN NATURE
439 EMITTED NUCLEUS
EMITTED l
‘He NUCLEUS
FIG. 2. Recoil ejection of atoms. Alpha decay of 23% might eject the recoil 235Unucleus 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 (Fleischer and Raabe, 1978a) shows that the excavation of the damage region occurs from the later action of water rather than by direct recoil ejection.
or during the droplet-evaporation process. Experimental evidence contradicts this hypothesis. The size distributions seen by Fleischer 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, as shown in a study of stream water (Fleischer and Delany, 1976). Therefore, no significant coagulation occurred from the time of sample collection till 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 2sgPu 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 PuO, was expected to eject PuO, 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 ejected particles that would consist of one 235U recoil and N accompanying 23gPu 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 235Urecoils (n) from alpha-decay of 239Pu,and the known fission cross sections c235and 4239for 23sU and 239Pu, the value of N is determined from N = (aza + Nu,,,)@n 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 side of Fig. 5 and in Table 2, the observed counts were those of solely the 235Urecoil nuclei that are ejected by alpha-decay of
239Pu. Instead of one 23sU atom and 400 23gPuatoms per recoil that crossed the surface of the PuO,, 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, the diagram on the right in Fig. 5 shows clear signal above the background. Calculation showed an experimentally insignificant amount, only 0.40 f 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 239Pu that entered the solution in the earlier experiments. The conclusion is this: the accelerated solution of 238Pu02 is not from direct ejection but from subsequent etching of the stored radiation damage from the exiting 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 (Fleischer and Raabe, 1978a). 3. DEMONSTRATION OF CHEMICAL RELEASE OF RECOILS IMPLANTED IN MINERALS The results that were just discussed imply 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 im-
ROBERT
L. FLEISCHER
FIG. 3. Plutonium cluster found in the residue from evaporating a drop of water. “%‘u fission fragment tracks were etched in quartz that was irradiated with 5 x 10I9 thermal neutrons/cm2. The star with -400 prongs represents a particle with about 104 239Puatoms. The tracks are about 10 pm in length (Fleischer and Raabe, 1977).
planted 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 PuO, provided a specially convenient system. They used the decay 239Pu-4He + 235U. Thus a layer of 239Pu02 will emit 235Urecoil atoms at velocities which will implant them in an adjacent
target mineral. The locations of 23sU atoms be sensitively located and counted by induced fission and subsequent scanning tracks (Fleischer, 1980). Figure 7 shows the in the experiments, which allow measurement
can then neutronof fission four steps of both
the “‘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 released remain in the minerals. Both the solutions and the minerals are then analyzed by fission track induction. Using 23sU as an implanted nuclide has another advantage beyond being easily traced by inducing fission tracks: it is similar to 234Uin mass, chemical behavior, and the energy given it by alpha-decay. It is therefore closely relevant to the natural 234U/238U disequilibrium that is one of the major interests here.
ALPHA-RECOIL
DAMAGE
NUMBER OF PRONGS PER PARTICLE AT 5 X IO” nvt
IN NATURE
441
Table 2. 23sU recoils and ejected 239Pu atoms* Inferred 239Ruatoms per
Calculated 23sU recoils 3.76 3.58 3.48 3.30 3.27
x x x x x
23SUrecoil
10’ IO’ IO’ 10’ 10’
0.41 f -0.10 f 0.94 + -0.04 f 0.72 f
0.27 0.16 0.39 0.17 0.34
‘Fleischer and Raabe (1978a).
3.1. Sample results on minerals
oood I
IO
IO2
IO3
104’
NUMBER OF 23s Pu ATOMS PER PARTICLE FIG. 4. Size distribution of 239F’u-containing sub-particles in water in which PUO, 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.
Figures 8 and 9 give results for the minerals quartz and muscovite respectively as a function of the concentrations of solutions of HCl and NaOH, and for water of neutral pH. Consider Fig. 8 (Fleischer, 1980). After a 24 h soak, the amount of 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 as implanted, the second (the higher of the two controls) was rinsed for 5 s in water, and shows that the 235U was not easily removed and therefore was not superficially located. The solutions typically removed 40% of the implanted uranium, and most of the
SINGLE 235lJ ATOMS RECOIL FROM23gPu
-
CATCHER , DETECTOR PUO,
THE CATCHER-DETECTOR IS NEUTRON-IRRADIATED AND THEN SCANNED TO COUNT THE RECOILING 235U ATOMS
OOW
Y-POSITION (mm) 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 f 0.005 cm from the substrate. Right-fission track counts across the ama that was adjacent to 0.85 &i of 23sPu02 particles for 16 hr and was then exposed to 6 x lOI9thermal neutrons per cm2. Only single *‘rlJ recoils are observed (Fleischer and Raabe, 1978a).
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442
L. FLEISCHER
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. In contrast, recoiling nuclei from alpha-decay are low-velocity, lightly ionizing ions that are not expected to produce tracks through ionization. Only the alternative of atomic collisions appears tenable as a mechanism for damage in all the materials for which
l?Ec~;L$TO
STOPPEDIN PORES
ETCHlNG OF TRACKS
I
FIG. 6. Mechanisms of recoil release from solid grains. 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 *3*Th, half life 24 days (above); by the time an implanted 234Th recoil in dry soil is later released by water it usually has decayed to W (below) (Fleischer and Raabe, 1978a).
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 alpharecoil 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. (Fleischer, 1982a). For Fig. 9 only the implanted samples were measured. In spite of appreciable scatter, which defines the reproducibility of these experiments, it can be seen that the amount of 235U retained decreases for the stronger solutions, either acidic or basic. The effect of time in increasing the z3sU 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 11 different time intervals given in Fig. 12 of that paper shows that loss of *W 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 continuous damage that connects with a surface; hence less than 100% of the recoils will be preferentially leachable. 4. MECHANISM
AND PERMANENCE RADIATION DAMAGE
OF
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 thought to be produced by Coulomb repulsions of nearby ions or
IMPLANTATION
OF TRACEABLE
ATOMS
IE CHEMICAL TREATMENT TO PRODUCE POSSIBLE TRACK LEACHING
1
I
01 SAMPLES III
NEUTRON IRRADIATION TO INDUCE FISSION
DRIED SOLUTION
Ip
ETCHING AND READOUT OF INDUCED FISSION TRACKS
ETCHED DETECTOR
5?l ETCHED MINERAL RG.
ETCHED DETECTOR es UNETCHED SUBSTRATE
7. Technique for implanting 23JU in minerals and for locating the nuclei after treatment of the mineral with a solution. The 235Unuclei that recoil from the alpha-decay of ?u are implanted (step I), and uranium nuclei are found at the end of recoil tracks (iust as 2yU is located in nature). The implanted samples are separately exposed to different solutions (step II), which may release 235U.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 235U nuclei; encircled n, neutrons; ff, fission fragments. In step IV, the detectors are etched and the induced fission tracks counted to measure how much 2JsU was retained in the mineral and how much went into the solution. Test-tube-shaped sites that are open are where recoils are released; shaded sites represent unleached sites where recoils are retained (Fleischer, 1980).
ALPHA-RECOIL
‘400r
DAMAGE
r-l T
R o CONTROL 0 608AV
FIG. 8. Track densities per area of 235U-implanted surface for quartz and for solutions in which quartz was immersed for 24 hr 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 5 set water rinse (Fleischer, 1980).
experiments such as are reported in Figs 8 and 9 have been done. The fact that the preferential leaching
occurs in silicon (Fleischer, 1982b) is telling, since ionization-produced tracks are not expected nor observed there (Fleischer et al., 1965). This statement is supported by Fig. 10 (Fleischer et al., 1975), which reminds the reader that atomic displacements produced by collisions become dominant at the low velocities of recoil nuclei, where ionization is negli-
443
IN NATURE
gible, 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. The following section describes experiments that do not involve direct observation of etched recoil tracks, but do test their annealing properties. 4.1. Radiometric
measurement
of recoil-track
per -
manence
The observed extent of isotopic disequilibrium in the 23*Uand 232Th decay series can be used to infer how long alpha-recoil damage is retained in certain minerals. Eyal and co-workers have observed 22sTh/232Thand 234U/238Uratios in solutions after the following minerals were soaked: monazite (Eyal, 1982; Eyal and Kaufman, 1982), uraninite and thorianite (Eyal and Fleischer, 1985a, b), betafite (Eyal et al., 1985), and thorite-melanovite (Ewing et al., 1986). The usual observation in these minerals is that the enhancement in the 2uU/23BUactivity ratio, (4, r) in the leachate is _ 10% and that for. u8Th/232Th, (Rs *) is much larger-i.e. factors of 2 x to 7 x . Time is a factor because the half life of 234U is 245,000 yr, while that of 22*Th is 1.91 yr. For =*Th recoil atoms, only a few years pass before they are transmuted, and therefore all 228Th that is removed by leaching was at fresh, unannealed radiationdamage sites. However, if on the average, damage healed itself in 104yr, only a small fraction of the 2yU 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
HYPERSTHENE DIRECT DISPLACEMENTS
~24HALEMlE62AVl 0 168 HR LEMi (525AVI-
d
10 RESIDUAL
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).
RANGE
(pm)
FIG. 10. Etchable damage from a stopping heavy nucleus partitioned into the effects of ionization at high energies and of atomic displacements at low energies. Only for the detectors with low thresholds, such as Lexan polycarbonate, is a range deficit not observed (schematic from Fleischer et al., 1975).
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444
L. FLEISCHER
Table 3. Mean lifetimes of alpha-recoil damage Lifetime (yr)
Mineral Betafite (pyrochlore, metamict) Monazite
400 18,000 > 1oo,ooo,ooo 14,000 Sc 7800 3OOu-6000 25,000
Muscovite Thorianite Thorite-melanocerite Uraninite
damage lie between the two decay lifetimes. Eyal and Fleischer (1985a, b) inferred that the annealing time, T*. at ambient temperatures of alpha-recoil damage is given by 7A =
7,(R,,,
-
l)/(R,,
-
I),
(1)
where 7q is the mean life of 2uU, 353,000 yr. Using equation (l), the mean lifetimes given in Table 3 result. For the four uranium and thoriumrich minerals that are amenable to the measurements of the isotopic ratios, ages range from 400 yr for a metamict mineral to 2000-25,000 yr for several crystalline minerals. In contrast, for mica it is known that alpha-recoil tracks exist over the ages of the samples (Huang and Walker, 1967). One would expect the two activity ratios in equation (1) to be 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 using presently available radiochemical techniques.
5. EFFECT
Reference for isotopic data
OF MOISTURE
As noted in the introduction, water is critical to the production of isotopic disequilibrium and the release of alpha-recoil 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
Eyal er al. (1985) Eyal (1982) Eyal and Kaufman (I 982) Huang and Walker (1967) Eyal and Fleischer (1985a, b) Ewing et al. (1986) Eyal and Fleischer (1985a, b)
5.1. Radon emanation The discussion so far has emphasized isotopic disequilibrium of *%I_lrelative to *%J, and to a lesser degree **‘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 would be affected by unusual values. Similarly important, however, is the separation of **‘Rn from its parent 226Ra. Much more dramatic variations occur, and they clearly will be governed by the same mechanisms. The emanation of “*Rn 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 (Fleischer et al., 1972) and for gas flows that may be attendant to earth strains prior to earthquakes (Fleischer and Mogro-Campero, 1985) and to the existence of subsurface hydrocarbon deposits (Fleischer and Turner, 1984). Since =*Rn 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. 11. It also indicates that more radon is released by wet rock I,
1
t
I
I
I
,
,
ab
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 (< 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 (Fleischer, 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.
IO
20 so 40 so 60 TIME IDAYSI
70
FIG. 11. 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 14.8cm water barrier” means that there is a 14.8cm thick region of water between the rock and the air (Fleischer, 1983).
ALPHA-RECOIL I
z
DAMAGE
IN NATURE
445
I
DETECTOR
01
0
4 cm
FIG. 12. Apparatus used to measure emanation of radon from soil as a function of the moisture content of the soil. A visual color-changing humidity card is used to record the
humidity level.
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 of water or soil gas in the ground might increase radon transport to the surface, but effects are expected to be small. Mogro-Camper0 and 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 N 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 rarely satisfied in nature. In summary, moisture can either increase or decrease the observed radon signal. Emanation is maximixed 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. The moisture effect was examined using the simple apparatus sketched in Fig. 12 (Fleischer and Mogro-Campero, 1978). In these experiments ordinary soil was used as a radon emitter. In this closed system about 1.8 cm thickness of soil (100 g) 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 + 10%. Radon concentrations were monitored with ally1 diglycol carbonate track detectors, which have been calibrated against well established standards (Fleischer et al., 1980). Variability is likely to be largely the result of
I 0.50 AVERAGEHUMIDITYIN SOIL
I 1.00
FIG. 13.Variation of track density (as a measure of radon concentration) with humidity above the soil in laboratory experiments like those described by Fleischer and MogoCamper0 (1978), using sealed cylindrical containers such as are shown in Fig. 12 (Fleischer, 1987).
the soil samples being somewhat inhomogeneous, since a few highly emanating grains can have a striking effect on a soil sample. Figure 13 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 that in this case is empirically found to be -2.5 (Fleischer, 1987). The same data in terms of water content show the increase caused by the water up to 20-40 wt % water in soil, followed by an expected, but not very clearly documented, drop-off caused by water becoming a diffusive barrier. Diffusion distances and the derivation of the attenuation factor are given by Fleischer and Mogro-Camper0 (1978). Additional recent examples of moisture affecting radon emanation are given by Strong and Levins (1982), Strandon et al. (1984), Greiner (1985), and Gan et al. (1986).
6. 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 origin of the observed isotopic disequilibrium 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. Acknowledgemenu-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). This article is a variation on
446
ROBERT L. FLEISCHER
the theme of one that was written for geochemists. That version is published in Geochimica et Cosmochimicu Acra.
REFERENCES Ewing R. C., Lumpkin G. R. and Eyal Y. (1986) Spontaneous annealing of alpha-recoil damage in metamict thorites. Materials Research Society Fall Meeting, Abstract LlO.1, p. 478. Eyal Y. (1982) Isotopic fractionixation of thorium and uranium upon leaching of monazite: alpha-recoil damage effects. In Scientific Basis fir Nuclear Waste Managemenr-V (Edited by Lutze W.), pp. 399-408, Elsevier, Amsterdam. Eyal Y., Lumpkin G. R. and Ewing R. C. (1985) Alpha recoil effect on the dissolution of betafite: rapid natural annealing of radiation damage within a metamict phase. Materials Research Societv IXth Meetina on Scientific Basis for Nuclear Waste Management, St&kholm, 911 September. Eyal Y. and Fleischer R. L. (1985a) Radiation damage by alpha recoils: natural annealing retards leaching of ancient nuclear materials. Nurure 314, 518-520. Eyal Y. and Fleischer R. L. (198Sb) Preferential leaching and the age of radiation damage from alpha-decay in minerals. Geochim. cosmochim. Acta 49, 1155-l 164. Eyal Y. and Kaufman A. (1982) Alpha-recoil damage in monazite: preferential dissolution of the radiogenic actinide isotopes. Nucl. Technol. 58, 77-83. Fleischer R. L. (1975) On the dissolution of respirable PuO, particles. Hirh Phys. 29, 69-73. Fleischer R. L. (1980) Isotooic disequilibrium of uranium: alpha-recoil damage and preferential solution effects. Science 207, 979-98 1. Fleischer R. L. (1982a) Alpha recoil damage and solution effects in minerals: isotopic disequilibrium and radon release. Geochim. cosmochim. Acfa 46, 2191-2201. Fleischer R. L. (1982b) Nature of alpha-recoil damage: evidence from preferential solution effects. Nucl. Trucks 6, 35-42.
Fleischer R. L. (1983) Theory of alpha-recoil effects on radon release and isotopic disequilibrium. Geochim. cosmochim. Acta 41, 779-784.
Fleischer R. L. I19871 Moisture and **‘Rn emanation. Hlth Phys. 52, 793-799.
Fleischer R. L., Price P. B. and Walker R. M. (1965) The ion explosion spike mechanism for formation of charged particle tracks in solids. J. uppl. Phys. 36, 3645-3652. Fleischer R. L., Alter H. W., Furman S. C., Price P. B. and Walker R. M. (1972) Technological applications of science: the case of particle track etching. Science 178, 255-263. Fleischer R. L., Giard W. R., Mogro-Camper0 A., Turner L. G.. Alter H. W. and Ginarich J. E. (1980) Dosimetry of environmental radon: methods and theory for low-dose, integrated measurements. Hlth Phys. 39, 957-962.
Fleischer R. L., Price P. B. and Walker R. M. (1975) Nuclear Tracks in Solids. University of California Press, Berkeley. Fleischer R. L. and Delany A. C. (1976) Determination of suspended and dissolved uranium in water. Analyr. Chem. 48,642-645.
Fleischer R. L. and Moro-Camper0 A. (1978) Mapping of integrated radon emanation for detection of longdistance migration of gases within the earth: techniques and principles. J. geophys. Res. 83, 3539-3549. Fleischer R. L. and Mogro-Camper0 A. (1985) Association of subsurface radon changes in Alaska and the
northeastern United States with earthquakes. Geochim. cosmochim. Acla 49, 1061-1071. Fleischer R. L. and Raabe 0. G. (1977) Fragmentation of respirable PUO, particles in water ‘by alpha-decay-a mode of “dissolution”. Hlrh Phvs. 32. 253-257. Fleischer R. L. and Raabe 0. *G. (1978a) Recoiling alpha-emitting nuclei- mechanisms for uranium series disequilibrium. Geochim. cosmochim. Acta 42, 973-978. Fleischer R. L. and Raabe 0. G. (1978b) On the mechanism of “dissolution” in liquids of PuO, by alpha-decay. Hlth Phys. 35, 545-548. Fleischer R. L. and Turner L. G. (1984) Correlations of radon and carbon isotopic measurements with petroleum and natural gas at Cement, Oklahoma. Geophysics 49, 810-871. Gan T. H., Mason G. C., Wise K. N., Whittlestone S. and Wyllie H. A. (1986) Desotption of Rn by moisture and heat. Hlth Phys. SO, 407-410. Greiner N. R. (1985) Radon emanation from coals: effects of moisture and particle size. Hlth Phys. 48, 283-288. Huang W. H. and Walker R. M. (1967) Fossil alpha particle tracks: a new method of age determination. Science 155, 1103-l 106. Kigoshi K. (1971) Alpha-recoil *‘YTh:dissolution into water and the zwU/23*Udisequilibrium in nature. Science 173, 47-48.
Kronfeld J. (1974) Uranium deposition and Th-234 alpharecoil: an explanation for extreme U-234/U-238 fractionation within the Trinity aquifer. Earth planer. Sci. Lerr. 21, 327-330.
Mogro-Camper0 A. and Fleischer R. L. (1977) Subterrestrial fluid convection: a hypothesis for longdistance migration within the earth. Eurrh planer. Sci. L&t. 34, 321-325.
Nero A. V. and Lowder W. M. (Eds) (1983) Indoor radon. Hlth Phys. 45, 273-56 1. Raabe A. G., Kanapilly G. M. and Boyd H. A. (1973) Studies of in vitro solubility of respirable particles of *‘sPu and *19Pu oxides and on accidentally released aerosol containing 239Pu. Inhalation Toxicology Research Institute Annual Report, Lovelace Foundation, Report LF-46, UC-48, pp. 24-30. Rosholt J. N., Shields W. R. and Gamer E. L. (1963) Isotope fractionation of uranium in sandstone. Science 139, 224-226. Rosholt J. N. and Tatsumoto M. (1970) Isotopic composition of uranium and thorium in Apollo 11 samples. Proc. Apollo II Lunar Sci. Conf., Geochim. cosmochim. Acla 2, Suppl. 1, 1499-1502.
Rosholt J. N. and Tatsumoto M. (1971) Isotopic composition of thorium and uranium in Apollo 12 samples. Proc. Apollo I2 Lunar Sci. ConfI, Geochim. cosmochim. Acfa 2;Suppl. 2, 1577-1584. -
Seitz F. (1949) On the disordering of solids by action of fast massive particles. Disc. Par&zy Sot. 5, 271-282. Sheludko A. (1966) Colloid Chemisrry, p. 223. Elsevier Press, Amsterdam. Sheng Z. Z. and Kuroda P. K. (1984) Alpha-recoil effects of uranium isotopes in radioactive minerals. Radiochimica Acra 37, 93-98.
Stranden E., Kolston A. K. and Lind B. (1984) Radon exhalation: moisture and temperature dependence. Hlth Phys. 47, 480484.
Strong K. P. and Levins D. M. (1982) Effect of moisture content on radon emanation from uranium ore and tailings. Hlth Phys. 42, 27-32. Sxabo B. J. (1969) Uranium-series dating of quaternary successions. In Proc. VIII INQUA Con/., Paris, pp. 941-949. Union International for Quatemary Studies.
Rodiar. Appl.
Meas., Insrrum.,
Vol. 14, No. Parr D
4, pp.
431-446,
0191-278X/88
1988
163.00 + 0.00
PergamonPressplc
Printedin Great Britain
NUCLEAR TRACK STUDIES OF ALPHA-RECOIL DAMAGE IN NATURE: RELATION TO ISOTOPIC DISEQUILIBRIUM AND LEACHING OF RADIONUCLIDES ROBERT
L.
FLEISCHER
General Electric Research and Development Center, Schenectady, NY 12301, U.S.A. (Received 17 March 1988) Abstract--One of the special characteristics of nuclear track studies is the wide variety of phenomena for which track etching provides useful information. This paper describes such a case, with applications ranging from isotope geochemistry, to mineral exploration, seismology, and health physics. Independent work relating to the danger from inhaled radioactive aerosols (Raabe ef al., 1973) nucleated these studies, which later led to other areas. What Raabe and co-workers did was to observe large differences between the solubilities of isotopically different plutonium dioxides. That result 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 two specific microscopic mechanisms that have been shown, using nuclear track techniques, to be causes of isotopic disequilibrium in the U and Th decay series. A similar alpha-decay process (of 226Ra)causes leaching to play a significant role in releasing ***Rn from natural substances, 222Rnbeing the immediate decay product of 226Ra.The average annealing time in nature of the alpha-recoil damage sites can be inferred from the extent of isotopic disequilibrium for different isotopic pairs in the Th and U decay chains.
1. INTRODUCTION TRACK etching is useful in an astonishing variety of fields. The work described in this article is an exam-
ple. It gives evidence as to how three very different processes occur: how natural isotopic disequilibrium begins within heavy-element decay chains, how the rare gas ***Rn is released from the earth, and how differential solubility occurs between the nuclear materials 23%02 and 23BPu02. 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. A well known behavior of radioactive species in nature is the release of products of alpha-decay from rocks and soils. This common phenomenon is evidenced by release of *=Rn from the ground and by the remarkable variability of 234U/23*U ratios on earth. A brief review by Szabo (1969) shows 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 turns out to be critical in these processes. Although other evidence will be noted, the single most convincing demonstration is the finding that 2wU/238U activity ratios are unity on the moon (Rosholt and Tatsumoto, 1970, 1971), a place where there is no water to effect disequilibrium. On Earth N.T.
M/4-8
437
the usual situation is that water which has permeated soil or rock becomes enriched in *W activity relative to 238U, and the solid is partially depleted in *%l. Among the mechanisms that have been proposed, two are unproved and two are established. Each depends on the fact that the residual recoil atom that results from alpha-decay is given kinetic energy that is sufficient to move it many atomic distances from its original location. There is no clear evidence for these two mechanisms: first, Rosholt et al. (1963) suggested that recoiling *%U 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 U+6 would reach surfaces to be dissolved. Second, Kigoshi (1971) proposed that atoms which recoil into adjacent grains might have an easy diffusional path to the nearby surfaces. Although no evidence for the action of these mechanisms exists, there is no proof that they do not act. One mechanism of *“U 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 *qh, each atom of which receives about 72 keV from the alpha-decay of *“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,
ROBERT
438
L. FLEISCHER
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. It is noted that there are also major chemical sources of large-scale disequilibrium such as is produced, for example, between 234U and 23@Thin the oceans, arising because the negligible solubility of thorium causes it to precipitate, whereas the uranium remains in solution. 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 describe the release of isotopes from mineral grains, not subsequent larger-scale chemical or physical separations.
2. DISSOLUTIONS
OF PuO, AEROSOLS
In studying how long inhaled radioactive aerosols might reside in the lungs, a startling isotopic effect was observed in the dissolution of Pu02 (Raabe et al., 1973), an experiment in which dissolution was compared for 239Pu02 and 23*Pu02. They found that in water and other aqueous solutions the oxide containing 23*Pu dissolved 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 (dm - 1); and the discrepancy of N 100,000 x 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 23*Pu 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 dissolution 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. 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 (within a recoil range 6 of the surface) can
WATER
Pu0, PIPTIC
FIG. 1. Dissolution of PuO, particles was measured by Raabe er 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” RrO,.
contact the liquid. This geometric consideration leads to the observed variation of dissolution rate, which is roughly as (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 work of Raabe et al. (1973) 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 finding that the solvent contained not just isolated atoms of Pu but an assortment of clusters that contained up to lo4 Pu atoms (Fleischer and Raabe, 1977). Figure 3 shows evidence of a larger 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 x 10i9/cm2. The sensitivity which results from that fluence is such that a pair of tracks from a common point locates what is typically a cluster of _ 60 atoms of 239Pu(in short, particles with less than 3 x 10-x’gm of Pu02 were observed). The background of fission from natural 235Uin the quartz is negligible because the uranium content is so low (N 3 wt parts/10i2). 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 Figure 3 could be artifacts of coagulation in water either prior to
Table 1. Relative dissolution lifetimes of 239Fu02 and 238Pu02 Case Observed (at 37°C) (from Raabe er al.,
T239/T238
Medians of 1973)
untreated particles* Medians of
140(f90)
EDTA-Treated
255 (* 190)
particles*
ExDected
From chemical kinetics From radiation damage
1.0021 235 (f IO)?
*Lifetime proportional to particle radius assumed. TRatio of decay rates, corrected 7% for greater energy of 2’8pudecay, and 20% for known isotopic purity of u8Fu0, particles (Fleischer, 1975).
ALPHA-RECOIL
DAMAGE
IN NATURE
439 EMITTED NUCLEUS
EMITTED l
‘He NUCLEUS
FIG. 2. Recoil ejection of atoms. Alpha decay of 23% might eject the recoil 235Unucleus 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 (Fleischer and Raabe, 1978a) shows that the excavation of the damage region occurs from the later action of water rather than by direct recoil ejection.
or during the droplet-evaporation process. Experimental evidence contradicts this hypothesis. The size distributions seen by Fleischer 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, as shown in a study of stream water (Fleischer and Delany, 1976). Therefore, no significant coagulation occurred from the time of sample collection till 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 2sgPu 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 PuO, was expected to eject PuO, 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 ejected particles that would consist of one 235U recoil and N accompanying 23gPu 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 235Urecoils (n) from alpha-decay of 239Pu,and the known fission cross sections c235and 4239for 23sU and 239Pu, the value of N is determined from N = (aza + Nu,,,)@n 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 side of Fig. 5 and in Table 2, the observed counts were those of solely the 235Urecoil nuclei that are ejected by alpha-decay of
239Pu. Instead of one 23sU atom and 400 23gPuatoms per recoil that crossed the surface of the PuO,, 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, the diagram on the right in Fig. 5 shows clear signal above the background. Calculation showed an experimentally insignificant amount, only 0.40 f 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 239Pu that entered the solution in the earlier experiments. The conclusion is this: the accelerated solution of 238Pu02 is not from direct ejection but from subsequent etching of the stored radiation damage from the exiting 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 (Fleischer and Raabe, 1978a). 3. DEMONSTRATION OF CHEMICAL RELEASE OF RECOILS IMPLANTED IN MINERALS The results that were just discussed imply 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 im-
ROBERT
L. FLEISCHER
FIG. 3. Plutonium cluster found in the residue from evaporating a drop of water. “%‘u fission fragment tracks were etched in quartz that was irradiated with 5 x 10I9 thermal neutrons/cm2. The star with -400 prongs represents a particle with about 104 239Puatoms. The tracks are about 10 pm in length (Fleischer and Raabe, 1977).
planted 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 PuO, provided a specially convenient system. They used the decay 239Pu-4He + 235U. Thus a layer of 239Pu02 will emit 235Urecoil atoms at velocities which will implant them in an adjacent
target mineral. The locations of 23sU atoms be sensitively located and counted by induced fission and subsequent scanning tracks (Fleischer, 1980). Figure 7 shows the in the experiments, which allow measurement
can then neutronof fission four steps of both
the “‘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 released remain in the minerals. Both the solutions and the minerals are then analyzed by fission track induction. Using 23sU as an implanted nuclide has another advantage beyond being easily traced by inducing fission tracks: it is similar to 234Uin mass, chemical behavior, and the energy given it by alpha-decay. It is therefore closely relevant to the natural 234U/238U disequilibrium that is one of the major interests here.
ALPHA-RECOIL
DAMAGE
NUMBER OF PRONGS PER PARTICLE AT 5 X IO” nvt
IN NATURE
441
Table 2. 23sU recoils and ejected 239Pu atoms* Inferred 239Ruatoms per
Calculated 23sU recoils 3.76 3.58 3.48 3.30 3.27
x x x x x
23SUrecoil
10’ IO’ IO’ 10’ 10’
0.41 f -0.10 f 0.94 + -0.04 f 0.72 f
0.27 0.16 0.39 0.17 0.34
‘Fleischer and Raabe (1978a).
3.1. Sample results on minerals
oood I
IO
IO2
IO3
104’
NUMBER OF 23s Pu ATOMS PER PARTICLE FIG. 4. Size distribution of 239F’u-containing sub-particles in water in which PUO, 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.
Figures 8 and 9 give results for the minerals quartz and muscovite respectively as a function of the concentrations of solutions of HCl and NaOH, and for water of neutral pH. Consider Fig. 8 (Fleischer, 1980). After a 24 h soak, the amount of 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 as implanted, the second (the higher of the two controls) was rinsed for 5 s in water, and shows that the 235U was not easily removed and therefore was not superficially located. The solutions typically removed 40% of the implanted uranium, and most of the
SINGLE 235lJ ATOMS RECOIL FROM23gPu
-
CATCHER , DETECTOR PUO,
THE CATCHER-DETECTOR IS NEUTRON-IRRADIATED AND THEN SCANNED TO COUNT THE RECOILING 235U ATOMS
OOW
Y-POSITION (mm) 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 f 0.005 cm from the substrate. Right-fission track counts across the ama that was adjacent to 0.85 &i of 23sPu02 particles for 16 hr and was then exposed to 6 x lOI9thermal neutrons per cm2. Only single *‘rlJ recoils are observed (Fleischer and Raabe, 1978a).
ROBERT
442
L. FLEISCHER
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. In contrast, recoiling nuclei from alpha-decay are low-velocity, lightly ionizing ions that are not expected to produce tracks through ionization. Only the alternative of atomic collisions appears tenable as a mechanism for damage in all the materials for which
l?Ec~;L$TO
STOPPEDIN PORES
ETCHlNG OF TRACKS
I
FIG. 6. Mechanisms of recoil release from solid grains. 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 *3*Th, half life 24 days (above); by the time an implanted 234Th recoil in dry soil is later released by water it usually has decayed to W (below) (Fleischer and Raabe, 1978a).
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 alpharecoil 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. (Fleischer, 1982a). For Fig. 9 only the implanted samples were measured. In spite of appreciable scatter, which defines the reproducibility of these experiments, it can be seen that the amount of 235U retained decreases for the stronger solutions, either acidic or basic. The effect of time in increasing the z3sU 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 11 different time intervals given in Fig. 12 of that paper shows that loss of *W 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 continuous damage that connects with a surface; hence less than 100% of the recoils will be preferentially leachable. 4. MECHANISM
AND PERMANENCE RADIATION DAMAGE
OF
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 thought to be produced by Coulomb repulsions of nearby ions or
IMPLANTATION
OF TRACEABLE
ATOMS
IE CHEMICAL TREATMENT TO PRODUCE POSSIBLE TRACK LEACHING
1
I
01 SAMPLES III
NEUTRON IRRADIATION TO INDUCE FISSION
DRIED SOLUTION
Ip
ETCHING AND READOUT OF INDUCED FISSION TRACKS
ETCHED DETECTOR
5?l ETCHED MINERAL RG.
ETCHED DETECTOR es UNETCHED SUBSTRATE
7. Technique for implanting 23JU in minerals and for locating the nuclei after treatment of the mineral with a solution. The 235Unuclei that recoil from the alpha-decay of ?u are implanted (step I), and uranium nuclei are found at the end of recoil tracks (iust as 2yU is located in nature). The implanted samples are separately exposed to different solutions (step II), which may release 235U.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 235U nuclei; encircled n, neutrons; ff, fission fragments. In step IV, the detectors are etched and the induced fission tracks counted to measure how much 2JsU was retained in the mineral and how much went into the solution. Test-tube-shaped sites that are open are where recoils are released; shaded sites represent unleached sites where recoils are retained (Fleischer, 1980).
ALPHA-RECOIL
‘400r
DAMAGE
r-l T
R o CONTROL 0 608AV
FIG. 8. Track densities per area of 235U-implanted surface for quartz and for solutions in which quartz was immersed for 24 hr 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 5 set water rinse (Fleischer, 1980).
experiments such as are reported in Figs 8 and 9 have been done. The fact that the preferential leaching
occurs in silicon (Fleischer, 1982b) is telling, since ionization-produced tracks are not expected nor observed there (Fleischer et al., 1965). This statement is supported by Fig. 10 (Fleischer et al., 1975), which reminds the reader that atomic displacements produced by collisions become dominant at the low velocities of recoil nuclei, where ionization is negli-
443
IN NATURE
gible, 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. The following section describes experiments that do not involve direct observation of etched recoil tracks, but do test their annealing properties. 4.1. Radiometric
measurement
of recoil-track
per -
manence
The observed extent of isotopic disequilibrium in the 23*Uand 232Th decay series can be used to infer how long alpha-recoil damage is retained in certain minerals. Eyal and co-workers have observed 22sTh/232Thand 234U/238Uratios in solutions after the following minerals were soaked: monazite (Eyal, 1982; Eyal and Kaufman, 1982), uraninite and thorianite (Eyal and Fleischer, 1985a, b), betafite (Eyal et al., 1985), and thorite-melanovite (Ewing et al., 1986). The usual observation in these minerals is that the enhancement in the 2uU/23BUactivity ratio, (4, r) in the leachate is _ 10% and that for. u8Th/232Th, (Rs *) is much larger-i.e. factors of 2 x to 7 x . Time is a factor because the half life of 234U is 245,000 yr, while that of 22*Th is 1.91 yr. For =*Th recoil atoms, only a few years pass before they are transmuted, and therefore all 228Th that is removed by leaching was at fresh, unannealed radiationdamage sites. However, if on the average, damage healed itself in 104yr, only a small fraction of the 2yU 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
HYPERSTHENE DIRECT DISPLACEMENTS
~24HALEMlE62AVl 0 168 HR LEMi (525AVI-
d
10 RESIDUAL
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).
RANGE
(pm)
FIG. 10. Etchable damage from a stopping heavy nucleus partitioned into the effects of ionization at high energies and of atomic displacements at low energies. Only for the detectors with low thresholds, such as Lexan polycarbonate, is a range deficit not observed (schematic from Fleischer et al., 1975).
ROBERT
444
L. FLEISCHER
Table 3. Mean lifetimes of alpha-recoil damage Lifetime (yr)
Mineral Betafite (pyrochlore, metamict) Monazite
400 18,000 > 1oo,ooo,ooo 14,000 Sc 7800 3OOu-6000 25,000
Muscovite Thorianite Thorite-melanocerite Uraninite
damage lie between the two decay lifetimes. Eyal and Fleischer (1985a, b) inferred that the annealing time, T*. at ambient temperatures of alpha-recoil damage is given by 7A =
7,(R,,,
-
l)/(R,,
-
I),
(1)
where 7q is the mean life of 2uU, 353,000 yr. Using equation (l), the mean lifetimes given in Table 3 result. For the four uranium and thoriumrich minerals that are amenable to the measurements of the isotopic ratios, ages range from 400 yr for a metamict mineral to 2000-25,000 yr for several crystalline minerals. In contrast, for mica it is known that alpha-recoil tracks exist over the ages of the samples (Huang and Walker, 1967). One would expect the two activity ratios in equation (1) to be 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 using presently available radiochemical techniques.
5. EFFECT
Reference for isotopic data
OF MOISTURE
As noted in the introduction, water is critical to the production of isotopic disequilibrium and the release of alpha-recoil 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
Eyal er al. (1985) Eyal (1982) Eyal and Kaufman (I 982) Huang and Walker (1967) Eyal and Fleischer (1985a, b) Ewing et al. (1986) Eyal and Fleischer (1985a, b)
5.1. Radon emanation The discussion so far has emphasized isotopic disequilibrium of *%I_lrelative to *%J, and to a lesser degree **‘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 would be affected by unusual values. Similarly important, however, is the separation of **‘Rn from its parent 226Ra. Much more dramatic variations occur, and they clearly will be governed by the same mechanisms. The emanation of “*Rn 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 (Fleischer et al., 1972) and for gas flows that may be attendant to earth strains prior to earthquakes (Fleischer and Mogro-Campero, 1985) and to the existence of subsurface hydrocarbon deposits (Fleischer and Turner, 1984). Since =*Rn 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. 11. It also indicates that more radon is released by wet rock I,
1
t
I
I
I
,
,
ab
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 (< 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 (Fleischer, 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.
IO
20 so 40 so 60 TIME IDAYSI
70
FIG. 11. 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 14.8cm water barrier” means that there is a 14.8cm thick region of water between the rock and the air (Fleischer, 1983).
ALPHA-RECOIL I
z
DAMAGE
IN NATURE
445
I
DETECTOR
01
0
4 cm
FIG. 12. Apparatus used to measure emanation of radon from soil as a function of the moisture content of the soil. A visual color-changing humidity card is used to record the
humidity level.
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 of water or soil gas in the ground might increase radon transport to the surface, but effects are expected to be small. Mogro-Camper0 and 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 N 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 rarely satisfied in nature. In summary, moisture can either increase or decrease the observed radon signal. Emanation is maximixed 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. The moisture effect was examined using the simple apparatus sketched in Fig. 12 (Fleischer and Mogro-Campero, 1978). In these experiments ordinary soil was used as a radon emitter. In this closed system about 1.8 cm thickness of soil (100 g) 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 + 10%. Radon concentrations were monitored with ally1 diglycol carbonate track detectors, which have been calibrated against well established standards (Fleischer et al., 1980). Variability is likely to be largely the result of
I 0.50 AVERAGEHUMIDITYIN SOIL
I 1.00
FIG. 13.Variation of track density (as a measure of radon concentration) with humidity above the soil in laboratory experiments like those described by Fleischer and MogoCamper0 (1978), using sealed cylindrical containers such as are shown in Fig. 12 (Fleischer, 1987).
the soil samples being somewhat inhomogeneous, since a few highly emanating grains can have a striking effect on a soil sample. Figure 13 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 that in this case is empirically found to be -2.5 (Fleischer, 1987). The same data in terms of water content show the increase caused by the water up to 20-40 wt % water in soil, followed by an expected, but not very clearly documented, drop-off caused by water becoming a diffusive barrier. Diffusion distances and the derivation of the attenuation factor are given by Fleischer and Mogro-Camper0 (1978). Additional recent examples of moisture affecting radon emanation are given by Strong and Levins (1982), Strandon et al. (1984), Greiner (1985), and Gan et al. (1986).
6. 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 origin of the observed isotopic disequilibrium 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. Acknowledgemenu-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). This article is a variation on
446
ROBERT L. FLEISCHER
the theme of one that was written for geochemists. That version is published in Geochimica et Cosmochimicu Acra.
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