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Measurement of Al26 in stone meteorites and its use in the derivation of orbital elements
(teochimica etCosmoehimica Acts,1974, Vol.38,pp.SSQtoSOQ.Pergamon Press.Printed inNorthern Ireland
descent
and ifs
use
in the
of AP in stonemeteorites derivation
of orbital elements
IAN R. CAMERON and ZAFER TOP Department of Physics, University of New Brunswick, Saint J’ohn, New Brunswick, Canada (Received 20 J&y 1973; accepted in revised form 2 Janzlary x974) Ab&&--The Alzs activity has been measured by gamma-ray coincidence spectrometry in a total of 30 stony meteorites (13 bronzite, 13 hype&hen%, 1 carbonaceous and 1 enstatite chondrite, and 2 achon~ites). The measured Alz8 content has been compared with calculated values based on the method developed by LAYWJKEINAand USMNOVA (1972a, 1972b), which takes account of the modulation of the grdaotic cosmic r&y intensity along the meteorite orbit due to sol&r magnetic activity. The predictions have been modified empirically to allow for the difference in chemical composition between H- and L-ehondrites. Comparison of the measured value with that predicted in the absence of solar modulation permits the estimation of the aphelion of the orbit of the meteorite. The great majority of the derived aphelia lie within the range 2.05-2.45 AU. While this result must be treated with considerable reserve owing to the uncertainties associated with the model, it is consistent with recent data from photometric observations of the asteroid belt.
of the levels of radioactive A12*produced in meteorites by exposure to the oosmic radiation have been used to obtain a variety of information both on the history of the me~o~tes themselves and on the vacations of the cosmic ray flux in space and time. A number of workers (e.g. ANDERS, 1960, 1962; BOGARD et al., 1971; CRESSY, 1971; FUSE and ANDERS, 1969; HERPERS et ak, 1969; HERZOG and ANDERS, 1971a; MEASUREMENTS
HEYMANN and ANDERS, 1967; HONDA etal., 1961;
Lmwmr!rzetaZ.,
1965; SPANNAGEL
and HEUSSER, 1969 ; TOBAEEN et al., 1971; VISTE and ANDERS, 1962) have measured Alg8in meteorites and tektites, and have used the data to estimate exposure ages and AP production rates for different classes of meteorite. The study of meteorites whose exposure ages, based on A12*,are of the same order as the half-life of that isotope (0.74 m.y.) has led to a suggested revision of the accepted rates of production of cosmogenic He3 and Ne21in stone meteorites, and henee to 8 significant change in their calculated rare gas exposure ages (HERZOG and ANDERS, 1971b). The interpretation of AP activity in meteorite samples is complicated by the variation of its production rate with depth below the meteorite surface, Since Al26 is close in mass to the principal target nuclei leading to its production, its distribution reflects the local flux of cosmic ray secondaries generated in the meteorite. The A126 concentration will be lowest either near the centre of a large meteorite or in the region just below the surface; the importance of the latter effect is reduced since the surface layer of the meteorite is largely lost by ablation during its passage through the atmosphere. Another effect which influences the levels of cosmogenio n&ides in meteorites is the variation of the incident cosmic ray flux, either with time or with position on the meteorite orbit. Shop-~rrn flux variations have a direct effect on the specific 899
900
IAN R. CAMERONand ZAFER TOP
activities of cosmogenic nuclides of relatively short half-life, such as Ar3’ (half-life 34 days). Information about the space and time variations of the cosmic-ray flux has been obtained by comparing the concentrations, in the same meteorite, of Ar3’ and A? (BEDECK, 1972 ; BEUEMANN and W&IKE, 1969; FIRE&IAN and DEFELIGE, I 960 ; FIREMXNand GOEBEL,1970 ; FIREMANand SPANNA~EL, I971 ; FORMANet al., 1971; STOENNER et al., 1960) and of Naz2 and Al26 (FIREMIAN, 1967). Solar magnetic activity produces a modulation of the cosmic-ray intensity which decreases with distance from the Sun. A meteorite moving in an orbit which is relatively close to the Sun, say with an aphelion of the order of 2 AU, will therefore be subject to an average cosmic-ray flux which is lower than that experienced by a meteorite moving in a higher aphelion orbit which results in its spending a large propo~ion of its time at much greater distances from the Sun. The activity level of a long-lived oosmogenic nuclide, such as AZ28,the concentration of which reflects the average cosmic-ray intensity over a very large number of orbital periods, will consequently be lower in the meteorite of smaller aphelion. A compensating effect is produced by the increase in the flux of low energy cosmic rays due to solar flares, but this enhancement of the cosmogenic production rate is significant only in the outer few centimeters of the meteorite, which are usually lost by ablation. LAVRUKHINAand USTI-NOVA (1972a, b) suggested a method for deriving the aphelion value of a meteorite’s orbit from its AlZ6 content, on the assumption that the outer radius of the region of solar modulation is at 1*9 AU from the Sun. Beyond this radius, the radiation intensity is assumed to have its unmodulated value of O-39 pa~icles~crna set sterad irrespective of the solar activity phase, while at distances less than l-9 AU the intensity, averaged over a solar cycle, is reduced by the solar activity to 70 per cent of its unmodulated value. These authors have calculated the average activity of Al26in stone meteorites as a function of their size, first of all on the assumption that the intensity of the vacating flux is the maximum (~modula~d) value. The actual Alz6 activity expected for a meteorite in a given orbit will then depend on the relative proportion of time it has spent in the modulated and unmodulated regions. Conversely, by comparing the measured Al28 activity with the value predicted for a meteorite of that size subject to an unmodu~a~d flux, it is possible to estimate the dimensions of its orbit. LAVRU~A and USTINOVA(1972b) applied this technique to virtually all the stone meteorites for which Al26 activities were currently available, and derived the result that the aphelia ~st~bution for chondrites has a maximum in the range 1%2~5 AU. The present paper presents the results of Al26 measurements in some 30 meteorites, most of whioh have not been measured before, while some of the others are characterized by divergent values in the literature. The analysis developed by LAVRUKHINA and USTIKOVA(1972b) has been modified and applied to extend the available data on the distribution of aphelia. PROCEDURE All measumm%nts repofted here were made with a gamma-ray spectrometer described by
TOP (1972). This consisted of two 12.7 x IO.2 cm NaI(T1) scintillators operating in coincidence to detect the annihilation gamma-rays arising from the positron decay of AP. The sointillators werespecial low-background assemblies supplied by Quartz et Silice. The detector system wm shielded by 10 cm of low a&iv&y lead. Meteorite and calibration samples were mounted on the front face of one of the scintillator housings.
Measurement of Alz6 in stone meteorites
901
The output from one of the scintillation channels was fed to an Ortec 420 single channel analyser, the window of which was set at 0.61 ri: 0.06 MeV. The pulses from the other scintillator in coincidence with the SCA output were recorded on either a Nuclear Data 1100 or 2400 multichannel analyser. The meteorite sample was counted alternately with background for 1330 min intervals. The total counting time for each sample was at least 16,000 min. Calibration runs were carried out using mockups containing a mixture of dunite and iron powder with known amounts of Alz6 added. The mockup shell was made in the usual manner by covering the specimen with thin aluminum foil and coating with epoxy resin. The proportions of dunite and iron used for the filling were adjusted to simulate the electron density of the meteorite itself. Most of the c~bratio~ were done using sources of A.l26supplied by the U.S. National Bureau of Standards. Some of the earlier measurements employed sources of Naz2 and Alz6 from commercial suppliers, but the calibrations quoted for these were not always reliable when compared with the NBS standards. All measurements made with the commercial sources have been corrected to the NBS calibrations. The gaiu of the multichamel analyser wss monitored before each run and small adjustments were made as necessary to maintain the 0.51 &leV ovation peak in the same channel through. out the me~~ment sequence. Similar checks were made before each run to minimize drift in the trigger channel. An extensive set of tests was carried out to establish the reproducibility of the calibration technique. Possible h-homogeneity of the mockup filling was checked by repeating some of the calibration runs after the mockup shell has been emptied and refilled with the same mixture. In some asses, the calibration runs were repeated with mockups which were completely independent in all respects from the original. Various materials were tried for the blank samples used for background counting. In all cases, the effects observed were within the statist,ical errors associated with the meteorite counting runs themselves. The ratio between the activities of meteorite and calibration mockup w&s obtained by a computer fit of their respective annihilation peaks.
RESULTS The meteorite samples me~ured are listed, along with their Ala6 activities, in Table 1. The classification of meteorite types is that of VAN SCHMOSand WOOD (1967). To aid comparison with other authors, the identification number of each sample is given, since some variation of Alzs activity will be found from one sample to another of the same meteorite owing to shielding elects. The importance of these effects in the interpretation of the results is discussed in the following section. The unce~~nties quoted are statistical counting errors (la) only. The NBS sources used for calibration had a quoted uncertainty of 1-l per cent, which in all cases is considerably less than the error from the counting statistics, For some of the earlier measurements, an additional error has had to be included because of the need to interca&brate these sources against NBS standards. For two of the meteorites, B~derheim and Pueblito de Allende, a,correction to the measured an~ation activity was required owing to the presence of residual Naaa (half-life 2.60 yr). The corrections were made using the data of HoNnA et aE.(1961) and RANCITELLIet al. (1969), respectively. Several of the more recent falls also required a correction for positron decay of Sc44(the daughter of 47 yr Ti44); this was s, minor correction in the range 04-l dpm/kg in all cases. Cont~butions in the an~hil~tion peak region from the uranium and thorium series were negligible for the chondrite measurements; for the two Ca-rich achondrites, Sioux County and Pasamonte, corrections in the range 045-04 per cent were applied, based on measurements with standard sources and the conwntration data given for the two meteorites by K&r@ and WXNKE (1959) and Rows et al. (1963).
IAN
902
Table
Meteorite
Claeeifioation (Van sahmue and Wood)
Date of fall
R.
CAMERON and ZAFER TOP
1. Measurements
of Alz6 activity Sample
Semple Sample wed eource
in this work
Al*’ activity (dpm/kg)
xna~
Others
This work
(g)
Hype&hem chondritm (L) Baxter Benton Bjurbale Bmderheim
L LL6 L4 L6
1916 1949 1899 1960
ASU GSC ASU GSC
419.1x 0218101 148 0220106
239 274 252 277
50 64 69 68
&2 f2 &2 f2
Bill-ea Chantonnay Colby (Wie.) Holbrook
L L6 L6 L6
1946 1812 1917 1912
ASU ASU GSC GSC
734 341.2 0333101 0805109
190 309 148 320
58 64 69.5 57.5
f3 &2 f 3 f 2
Homestead Modoc Otis Potter
L5 L6 L L6
1875 1905 Find Find
GSC ASU ASU GSC
0807101 81a 467.1~ 1607101
123 491 314 176
67 59 43 64.5
-&3 *2 f 1.6 f 4
Saratov Bronzite chondrites (H)
L4
1918
ASU
740
373
71
*2
Bledsoe Clovis No 2 Dimmitt
H H”;3, 4)
Find Find Find
AML GSC ASU
H 121.35 0335101 584.49
130 96.5 474
52 72 44
f3 &4 * 1.5
Dokachi Forest City (1)
H5 H5
1903 1890
ASU GSC
624.1 0602101
164 312
61 44
&2 &-2
Forest City (2) Gilgoin McrIla Mills Puituek Queen’s Mercy Richardton
H5 H5 H H H5 H6 H5
1890 Find 1920 Find 1868 1926 1918
ASU ASU ASU AML GSC ASU ASU
49f 51a 611.1 H120.4 1608101 765 1OOK
291 196.5 180 204 324 208 248
43.5 53.5 70 58 61 58 69
f 2 f 3 *a &4 f2 f2 f3
Rose City VUlCaIl Carbonaceous chondrite
H H
1921 Find
ASU GSC
301-1x 2206102
243 386
66 53
f 3-b f2
Pueblito de Allende Enstatite ahondrite
c3
1969
GSC
1612103
593
61
f2
Abw Achondrites Passemoote Sioux County
E4
1952
GSC
011710
177
68-6 f
Ach Ach
1933 1933
ASU ASU
19788 198a
119 207
101 100
66-7 f 2.7 (1) 72 f 4 (2) 68 f 3(3) 66 (4) 60 f 6 (5) 57 f 2 (6)
4-5
f 6 f4
68 f 79 f
9, 73 f 7 (2) 9 (6)s 44 f 6 (7)
71 & 8 (6) 60.5 f 4.3 (1) 63 & 5 (3) 73 f 8, 68 f 7 (6) 52 f 3 (7) 71 (8)
28.8 & I.7 (1) 58.4 f 36 (10)
2 (9)
46 f 5, 48 f 5 (6) 60.2 f 3.6 (11)
50.2 +
I.9 (11)
71 f 46 f
8. 39 f 4 (6) 3 (7) 63 & 4 (12)
69 f
2 (13)
69 f
8 (6) 63 -& 7 (14)
99 f 11 (6) 101.6 f 3.8 (9) 122 f 13 (6) 99.6 f 6 (9)
* The value quoted in Ref. (6) is in error. Corrected value is ae shown (Rowe, private communication) Sources of meteorite specimens: AML, American Meteroite Laboratory; ASU, Arizona State University; GSC, Geological Survey of Canada. References: (1) (2) (3) (4) (5) (6, (7)
HEXPEES et al. (1969); BISWAE ef al. (1963); Frmxm (1967); CRESSY (1971); HONDA et d. (1961); ROWE and CLARK (1971); CFZSSY (1964);
(8) (9) (IO) (11) (12) (13) (14)
SPANNA~ELand HEWSSEX (1969); FUSE and ANDERS (1969); SPAI?NAOEL(1973); HEBMAX and ANDEBB 11973); EEXANN and KOE~UET(1968); R&-XmELLI et al. (1969); HEand ANDERS (1967).
Measurementof Al26in stone meteorites
903
A few of the samples measured, such as Bruderheim, were chosen because their activity values had been reliably established on the basis of several independent measurements; they therefore acted as a check on the absence of significant systematic error in the present work. The agreement of our measurements with the previously published results is satisfactory in general. Apart from the peculiar case of Dimmitt, and a disagreement which is just outside the experimental errors with HERPERSet al. (1969) for Bjurbijle and BISWAS et al. (1963) for Holbrook, the remaining discrepancies are cases where our results are lower than those of ROWE and CLARK(1971) or higher than those of CRESSY(1964). The final results quoted by the former have been obtained by a somewhat indirect calibration relative to the K40 content of the samples, and it is possible that the correction factor may be too high in some cases, leading to an excessive estimate of the Al2s activity. (This does not apply to the Bruderheim result, where a direct Al26 calibration was performed.) The results of CRESSY(1964), on the other hand, as pointed out by HEY-N and ANDERS (1967), may be systematically low, since they were made on chemically isolated aluminium fractions, and some difficulty waa experienced in the yield determinations. It should perhaps be pointed out that one factor which may produce minor differences in the comparison with the results of other authors is that the great majority of such results have been obtained using Nas2 as the calibration isotope. The results in such cases will depend on the relative intensity of the annihilation gamma-rays emitted in the decays of Alasand Na 2*. Most of the comparison activities quoted in Table 1 are based on an assumed 84-85 per cent intensity for the positron decay of Al26; that there may be an appreciable uncertainty in thia intensity, however, is suggested by the recent work of SAMWORTH et al. (1972), which yielded a value of 82-l per cent for this branch. Our value for Dimmitt lies half-way between the two widely differing results of HERPERSet al. (1969) and FUSEand ANDERS(1969), and therefore does nothing to throw light on this discrepancy. Our value would be consistent with the Hes-age of about l-5 m.y. quoted by EBERHARDTet ab. (1966) ; this low age, however, is very probably due to helium diffusion loss since, as pointed out by FUSE and ANDERS (1969), the Ne21 content of Dimmitt suggests that its age is great enough to allow saturation of the Al26 activity. Apart from Dimmitt, the only H-chondrite with an activity of less than 50 dpm/kg is Forest City ; the result obtained for this meteorite seems particularly reliable in view of the good agreement found with two separate specimens. The low value obtained both by other workers and the authors is not due to non-saturation, since both the He3 and Ne21 contents, as given by KJRSTENet al. (1963), lead to an exposure age of the order of 4-5 m.y. It is possible that the low value is a shielding effect, but the recovered mass of Forest City is not remarkably high (122 kg) ; the low A12sactivity could only be explained as a shielding effect if the ablation had been unusually great. For the L-chondrites, the only remarkably low results are for Baxter and Otis. The low Alzs activity of the former is presumably due to its small size. The Otis result may be partly due to a size effect (the recovered mass was only 2.6 kg) ; again, it would not appear to be unsaturated since its He3 and Nell contents, as given by ZXHRIN~ER(1966), suggest an exposure age of at least 12 m.y.
904
IAN R.
CAMERON
and
&FER
TOP
CALCULATION OBMETEORITE ORBITS The interpretation of Alas activity in terms of meteorite orbits is subject to considerable error from a variety of factors. The most important of these are: (i) The considerable confusion which exists regarding the detailed variation of cosmic ray intensity over the region of the meteorite orbits ; (ii) Shielding effects, which are particularly significant for meteorites of small pre-atmospheric size or for surviving fragments from the interior of a highly-ablated large meteorite; (iii) Variations in induced activity due to differences in chemical composition between meteorites of the same or different classes. The current information on variations of cosmic ray intensity as a function of energy and spatialposition has been summarized in a review by O’GALLAOHER (1972). A recent review of measurements with the Pioneer 8 and 9 space craft over the distance range of 0.75-1.10 AU from the Sun has been given by WEBBER and LEZNIAK (1973). The results of the various determinations at lower energies show very large discrepancies, and even in the higher energy range, which is of principal interest in the production of cosmogenic nuclides in meteorites, the only conclusion that can be drawn is that the heliocentric gradient is positive with a magnitude somewhere between a few per cent and a few tens of per cent per astronomicalunit. Various authors have attempted to derive the magnitude of the cosmic ray gradient from the ratio of A?’ to Ar3s in the Lost City meteorite, the orbit of which was such that it had a time-average distance from the Sun of 1.8 AU. FIREMANand SPANNAGEL (1971) and FORMANet al. (1971) found a positive gradient of about 60 per cent/AU at energies around 1 GeV, whereas BEOEMANN (1972) derived a much smaller value, in the order of 15 per cent/AU. CRESSY (1971) suggested that the approximate equality of the Al26 activities of the two meteorites Pribram and Lost City, whose orbital elements are accurately known, could be explained on the assumption that a step change of the cosmic ray flux occurred at around 1.02 AU from the Sun. The tentative nature of this model, and the variety of possible sources of error, such as the rather large uncertainty in the measured ratio of the meteorite activities, are emphasized by the author himself. One of the most comprehensive analyses of the relation of Al26 activity to the spatial variation of cosmic ray flux is that of LAVRUKECINA and USTINOVA (19728, 1972b). By comparing the measured and calculated activities of radionuclides with different half-lives in several chondrites, they concluded that the upper limit of the solar modulation region is at l-9 AU from the Sun. They also derived a radial gradient of about 70 per cent/AU, in reasonable agreement with the Lost City work referred to above. One of the reasons for applying the model of Lavrukhina and Ustinova to the present set of Alz6 activities is the relative ease with which the parameters of the model can be changed to accommodate more precise information about, for example, the extent of the modulation region or the predicted levels of Alzs activity. In this method, the Al26 activity is determined by the parameter 2 = tmod/torb, where tori,is the orbital period of the meteorite and tmoais the time which it spends during each revolution in the modulated region of the cosmic ray flux (taken as having its boundary at 1.9 AU from the Sun). The observed Alaa activity, Hobe, can then
Measurement of Al= in stone meteorites
906
be expressed as
(1) where H,,, is the activity which the meteorite would have attained if it had been exposed throughout to an unmodulated flux and Hmin, that which it would have had if its orbit had been entirely within the modulation region. Hmin was taken to be 70 per cent of H,,, as determined by the Russian authors. The value of H marfor a given meteorite can be calculated using the data presented in their paper for the average Al26activity in stone meteorites aa a function of their radius. It should be noted that in their work a single curve is presented for chondrite activity, no distinction being made between H- and L-chondrites. That the chemical variation between the two types accounts for a significant difference in Ala6activity is suggested by the work of CRE~SY(197 1) on chemically separated samples of the Bruderheim chondrite. Combining the elemental cross-sections derived by Cressy with the average composition data for H- and L-chondrites quoted by ~SON (1971), one finds that the average ratio of Alasin L-chondrites to that in H-chondrites is l-09. The ratio of average Al2e activities measured in the present work, omitting the few anomalously low values, is in the neighbourhood of 1.08. A similar ratio between the H- and L-chondrite activities has been found by HEYMANNand ANDERS(1967). It would therefore seem probable that the most important factor in the variation of Al26activity between the two types of chondrite is the chemical composition rather than the difference in orbit parameters. This effect has been added empirically to the analysis by increasing the H,, values of LALVRUIIRINA and USTINOVAby 4.5 per cent for the L-chondrites and reducing their H,,, values for H-chondrites by a similar amount. While this procedure is crude, it does permit some reasonable allowance to be made for the effect of chemical variation between the classes. Since the calculated Al26activity is a function of the preatmospheric radius of the meteorite, some assumption has to be made about the extent of ablation. The preatmospheric radii were calculated for three assumed values of the extent of ablation: 92, 85 and 60 per cent. In each case, 2 was calculated from equation (l), using our measured Alz6 activity. The aphelion of the meteorite orbit was then calculated using the relation which Lavrukhina and Ustinova obtained by fitting to the known orbits of Pribram and Lost City, and two plausible orbits for Bruderheim based on the method of visible radiants. The relation is Q’ = 1.26 + 0.132 + 0X532-‘.
(2) The aphelia derived using this formula are given in Table 2. Allowing for the error in the measurement of activity, all but 6 of the meteorites measured have aphelia in the range 2.05-2.45 AU. The relatively small spread is not unexpected, since the range of Alz6activity variation is itself not very large, with the exception of a few anomalous values. The values of Q’ lying outside the quoted range arise partly from the low activities for Otis, Dimmitt and Forest City, which were discussed in the previous section, and partly from the rather high activities of Saratov, Merua and Clovis. Confidence in the first of these high activity values is enhanced by the identical value found by SPANNAQEL and HEUSSER(1969).
906
IAN
R.
CAXERON
and
ZAJ?ER
TOP
Table 2. Aphelia of meteorite orbits calculated under conditions of 92, 85 and 60% ablation Aphelion(q’) in AU
Alss activity Total mass Meteorite
(kg)
dpmlkg (present work)
92 % ablation Our velue Ref. 1
85 ‘A eblstion Our velue Ref. 1
60 % ablation Our value Ref. 1
Hypersthene chondrites (L) Baxter Benton Bjurbiile Bruderheim BUl%a, Chsntonney Colby (Wis.) Holbrook Homestead Modoa Otis potter saratov Bronzite chondrites (H) Bledsoe Clovia No. 2 Dimmitt Dokachi Forest City (Sample 1) Forest City (Semple 2) Gilgoin MIXUS Mills P&tusk Queen’s Meroy Richerdton Rose City VulCcLn Enstatite chondrites Abee
0.61 2.84 330 303 25 31.5 110 220 226 30 26 261 328
30.5 13.3 13.5 3.84 122 122 147.5 71.4 2 >200 w250t 90.9 10.6 19 107
50 64 59 58 58 64 69.5 57.5 67 59 43 64.6 71
1.97 2.28 3.28 2.85 1,96 2.20 7.2 2.41 54 1.99 <1.9 10.7 co
52 72 45.5, 52+ 45* 44*5* 55* 70 58 51 59* 59 56 53
1.96 17.8
2.44 2.27 11.8 2.80 2.06 1.95 2.40
-27.8 4.06 -14.5 -4.39 ~11.86 co
2.03 1.92 1.92 -
2.18 -4,8
2.07 2.54 2.48 2.30 1.94 2.16 3.25 2.16 3.86 1.97 (1.9 3.19 co
1.93 co
8.83 3.93 2.66 3.69 3.32 co
2.29 4.19 2.10 2.05 1.97 2.16 2,54 1.99 2.51 1.99 (1.9 2.40 4.19
2.27 -4.58 2.27 -11.9
-
2.20
2.06
1.96 co <1*9 2.26 <1.9
2.18
2.90
2.04
3.06 2.32 -
Ca-Riah aahondritss Posamonts Sioux County
3.6 4.1
* Al*O activity has been corrected for non-saturation due to short exposure age. t Estimeted from approximate size of piece observed from fall (HEY, 1966). Ref. USTINOVA (1972b).
1 L~vnn~ax~a
and
The results of Lavrukhina and Ustinova showed a general tendency for the q’ values of L-chondrites to be greater than those of H-chondrites. Our results, again ignoring the few anomalous values, give average aphelia which are similar for the two types of chondrite. This is, of course, to be expected, since the empirical correction to the production rates to allow for the chemical composition difference implies that the difference in mean activity between the two groups is principally due to this effect rather than to orbital differences. It should be noted that, although the method of Lavrukhina allows for shielding effects in the calculation of the average activity in a meteorite of given size, there will still be an appreciable error, owing to the fact that the A12s activities of the samples measured will, in general, deviate from the mean activity of the meteorite as a whole. The production rates of low-energy products, such as Alzs and MI?, on the other hand, are rather insensitive to depth and size. For example, the results of KOHMAN
Mw.wwemen% of Al28 in stone meteorites
907
and BENDER (1967)show a range of variation for Mns3of only f 15 per cent from the mean over a range of radius variation equivalent to 24-70 cm for stone meteorites. Fusx and ANBERS f1969) showed that serious variation of APB activity due to shielding was rare among randomly selected samples of meteorites covering a wide range of recovered masses. CBESSY (1972) noted that the AlS8 concentration in Bueblito de Allende showed a variation of only f 10 per cent over a depth range of up to 40 cm. These results suggest that, in general, unless one is dealing with samples from the interior of meteotites of very large or very small preatmosph~~~ size, the assumption that the AP activity of the sample measured is representative of the mean value of the meteorite is not likely to lead to serious error. The effect of any errors which do arise will be to increase the spread of the aphelion values rather than to produce a systematic trend, since any given sample is equally likely to be high or low compared to the average calculated by Lavrukhina and U.&nova. The necessity for calculating aphelion values for a range of possible ablations can be avoided if measurements are available from which an ablation estimate can be made. Only a very few data of this kind exist (e.g. BHANDARI,1969; M~RTI et al., 1969) and only one such value (about 90 per oent, for Baxter) applies to the meteorites measured in the present work. It should be pointed out that the variation of the average aphelion value produced by varying the a.blation over the range 60-92 per cent is only f5 per cent for both types of ohondrite, The absolute values of the aphelia predicted must be treated with considerable reserve, since the model used is subject to two major sources of uncertainty which can be resolved only when more reliable information becomes available. The first of these is the lack of detailed information about the heliocentric cosmic ray gradient. In the present calculations, for example, changing the radius of the outer boundary assumed for the mod~atiu~ region by f5 per cent produced a percentage change of approximately the same amount in q’. The second major possible source of error is in the calculation of the absolute production rates of APE, on which the model is directly dependent. The significance of this effect is illustrated by the fact that the modification of j&5 per cent made to the Z=, valaes to allow for the different compositions of the L- and R-chondrites had the effect of changing the mean aphelion value for each group by about 9 per cent. While the model leaves much to be desired, however, it is interesting that the rather low aphelion values predicted are consistent with the supposition that the majority of chondritic meteorites are derived from asteroids of the Apollo type rather than from the main asteroid belt. This supposition has found recent support If)?3 ; from photSometricstudies (e.g. Crrep~~rret aI., 1973 ; CIIIApmx and SAL~~BURP, EGANet a$., 1973). AoknowledgemesS--We are in&&tadto the Geologica;l Survey of Canada for providing specimens on loan from the National Meteorite Collection of Canada, and to Dr. C. B. MOOREfor the loan of samples from the collections of Arizona State University. The work was carried out, under a grant fA5710) from the National Eesesrch Council of Canada.
ANDERS E. (1980) The record in the meteorites -II. meteorites and tektites. Qeo&%. Oomochk. Act&
On the presence of aluminum-26
19,53-62,
in
908
Lu
R. C-RON
and Z~L~FER TOP
ANDERSE. (1962) Two meteorites of unusually short cosmic-ray exposure age. Science 188, 431-433.
BEQE~N F. (1972) Argon 37/argon 39 activity ratios in meteorites and the spatial constancy of the cosmic radiation. J. aeophys. Res. 77, 3650-3659. BESET F. and W&~KE H. (1969) 3sK(7a,P)-produced 39Ar in chondrites; new data and their interpretationin terms of size, exposure age and orbital elements. In MeteoriteResearch, (editor P. M. Millman), pp. 363-371. Reidel. BHANDARIN. (1969) On orbital velocities of some stone meteorites. Proc. of the 11thSy~osizmz on Cosmic Rays, Astrophysics, C?eophysics and Elementary Particle Physics, Delhi, October ‘7-11, 1969, (Vol. II) pp. 327-331. Department of Atomic Energy, Government of India. Brswas M. M., MAYER-B~RICKE C. and GENTNER W. (1963) Cosmic ray produced Naz2 and Al26 activities in chondrites. In Eurth Science and Meteor&s, (compilers J. Geiss and E. D. Goldberg), Chap. 11, pp. 207-218. North Holland. BOUARDD. D., CLARKR. S., KEITH J. E. and REYNOLDSM. A. (1971) Noble gases and radionuclides in Lost City and other recently fallen meteorites. J. Geophys. Res. 76, 4076-4083. CHAPMAN C.R., MOCORDT. B. and PIETER~C. (1973) Minor planets and related objects. X. Spectrophotometricstudy of the composition of (1686) Toro. As&on. J. 78, 602-506. C~~APMAN C. R. and SALISBTJRY J. W. (1973) Comparisons of meteorite and asteroid spectral reflectivities. Icwzrr,19, 607-622. CRESSYP. J., JR. (1964) Cosmogenic radionuclidesin stone meteorites. Ph.D. dissertation, USAEC Report NYO-8924. CRESSYP. J. (1972) Cosmogenic radionuclides in the Allende and Murchison carbonaceous chondrites. J. ffeophys. Res. 77, 490.54911. CRESSYP. J. (1971) The production rate of Al 26 from target elements in the Bruderheim chondrite. Geoohim. Cosmochim. Acta 55, 1283-1296. EBERH~RDTP., EUUSTERO., GEISSJ. and MA~TI K. (1966) Rare gas measurementsin 30 stone meteorites. 2. Naturforsch. 21a,414-426. Eom W. G., VEVER~AJ., NOLANIIM. and HIL~EMA~T. (1973) Photometric and polarimetric properties of the Bruderheim chondritic meteorite. Icorm 19, 366-371. Eu W. D. and KOW~ANT. P. (1958) Cosmic-ray-inducedradioactivitiesin meteorites---II. A120,BelO and Coso, aerolites, siderites and tektites. cfeochim. Cosmochim. Aota 14,364-369. FIREMANE. L. (1967) Radioactivites in meteorites and cosmic-ray variations. Beochim. Cosmochim. Acta 81,1691-1700. FIRBUN E. L. and DE~ELICEJ. (1960) Argon 37, argon 39, and tritium in meteorites and the spatial constancy of cosmic rays. J. Geophys. Res. a, 3036-3041. FIREE. L. and GOEBELR. (1970) Argon 37 and argon 39 in recently fallen meteorites and cosmic-ray variations. J. Gwphys. Res. 76, 2116-2124. FIREMANE. L. and SPANNAQEL G. (1971) Radial gradient of cosmic rays from the Lost City meteorite. J. Beophys. Res. 76, 4127-4134. FORMANM. A., STOENNERR. W and DAVIS R. (1971) Cosmic-ray gradient measured by the argon 37/argon 39 ratio in the Lost City meteorite. J. Geophys. Rea. 78, 4109-4120. FUSE K. and ANDERS E. (1969) Aluminum-26 in meteorites-VI. Achondrites. Cfeochim. Cosmoohim. Acta 33, 653-670. HERMANG. F. and ANDER~E. (1973) Unpublished work. HERPERSU., HERRW. and W~LBZE R. (1969) Evaluation of 5sMnby (?t,y) activation, 2sAl and special trace elements in meteorites by y-coincidence techniques. In Meteorite Research, (editor P. M. Millman), pp. 387-396. Reidel. HERZOUG. F. and ANDERSE. (1971a) Radiation age of the Norton County meteorite. Geochim. Cosmoohim. Acta 35, 239-244. HERZOQG. F. and ANDERSE. (1971b) Absolute scale for radiation ages of stony meteorites. Beochim. Coswwchim. Acta 85, 605-611. HJZY M. H. (1966) Catalogof Meteorites (Third Edition), p. 397. Alden Press. HEYMANND. and ANDERSE. (1967) Meteorites with short cosmic-ray exposure ages, as determined from their Al26 content. Cfeochim. Cosnwchim. Acta 21,1793-1810. HONDAM., U~MOTO S. and ARNOLDJ. R. (1961) Radioactive species produced by cosmic rays in Bruderheim and other stone meteorites. J. Qeophys. Res. 66, 3541-3546.
Measurement of Alz6 in stone meteorites
909
Kalium B~t~ungen Kmsm~ T., KRANKOWSKYD. and Z&~IN GIER J. (1963) Edelg~-Ed an einer grosseren Zahl von Steinmeteoriten. ~~ch~~. Cabal. Acta fl, 13-42. KOEWAX T. P. and BENDER M. L. (1967) Nuclide production by cosmic rays in meteorites and on the moon. In High-Energy Nwbersr Reactions in Astrophysics, (editor B. S.P. Shen), Chap. 7, pp. 169-245. W. A. Benjamin. KiSmo H. and W&KE H. (1959) Uranbestimmungen an Steinmeteoriten mittels neutronenaktivierung uber die Xenon-Isotopen 133 und 135. 2. Natzlrfor&. 14a, 866-869. LA~~~KF~INAA. K. and USTINOVA G. K. (1972a) Isotope test for estimating elongation of meteorite orbits. Dokl. Akad. Nauk SSSR 204, 316-319. Trans. in Sow. Phye.-Dokl. 17, 409-412. LA~~~JEE~NAA. K. and USTINOVA G. K. (197215) Cosmogenic radionuclides in stones and meteorite orbits. Earth Planet. Sci. Lett. 15, 347-360. LIPSCEUTZM. E., SIGNERP. and .fkXDERS E. (1965) Cosmic-ray exposure ages of iron meteorites by the Ne~~~z6 method. J. #eophg8. Res. 70,1473-1489. MARTI K., SEEDL~VSKYJ. P., LINDSTRO~ R. M., ARNOLD J. R. and B~ARI N. G. (1969) Cosmic-ray produced radionuchdes and rare gases near the surface of Saint-Severin meteorite. In Meteorite Research, (editor P. M. Millman), pp. 246-266. Reidel. MANN B. (1971) Handbook of Elemmtd Abundance8 in &fetemites, pp. 524,541. Gordon &Breach. O’G~A~EER J. J. (1972) Observations of the radial gradient of gala&c cosmic radiation over a solar cycle. Rev. aeophye. Space Whys. 10, 821-834. RANCITELLI L. A., PERKINS R. W., COOPERJ. A., KAYE J. H. and Woom N. A. (1969) Radionuclide composition of the Allende meteorite from non-destructive gamma-ray spectrometric analysis. Science 166, 1296-1271. RO’WE M. W. and CLARIER. S. (1971) Estimation of error in the determination of Al26 in stone meteorites by indirect y-ray spectrometry. &ochim ~osmochim. Aota 85, 727-730. ROLE M. W., VAN DILLA M. A. and ANDEBON E. C. (1963) On the radioactivity of stone meteorites. #eoeh~m. sorghum, Acta 27, 983-1001. S-WORTH E. A., WALTON E. K. and OLNESSJ. W. (1972) Beta decay of the Alz6 ground state. Phy8. Rev. 05, 138-142. SPA~A~EL G. (1973) Unpublished work. SPANNAQELG. and HEUSSER G. (1969) Radiation ages of chondrites. In Meteorite Research, (editor P. M. Millman) pp. 372-386. Reidel. STOENNERR. W., SCHBEFFER0. A. and DAVIS R. (1960) Meteorites as space probes for testing the spatial constancy of cosmio rays. J. Geophy8. Re8.85,3026-3034. TOBAILEMJ., NORDED~ANN D. and G~YEBINET. (1971) Gamma emitters and exposure age of the Bovedy meteorite of the fall of April 25,1969. Nature Phya. Soi. 229,118-119. TOP Z. (1972) Construction of a gamma-gamma coincidence counting system and the measurements of cosmogenic Al26 activities in stone meteorites. M.Sc. Thesis, University of New Brunswick. Vu SCHMUSW. R. and WOOD J. A. (1967) ~he~cal-petrolo~e clas&cation for the chondritic meteorites. ~eo~h~m. ~o~~c~~rn. Acta 31, 747-765. VISTE E. and ANoERs E. (1962) Cosmic-ray exposure history of tektites. J. #eophy8. Rea. 67, 2913-2920. WEBBER W. R. and LEZNIAK J. A. (1973) Interplanetary radial gradients of galactic cosmic ray protons and helium nuclei: Pioneer 8 and 9 measurements from 0.75 to 1.10 AU. J. a”eophy8. Res. 78, 1979-2000. ZHRRINCERJ. (1966) Chronology of chondrites with rare gas isotopes. Meteoritika 27, 25-40.
descent
and ifs
use
in the
of AP in stonemeteorites derivation
of orbital elements
IAN R. CAMERON and ZAFER TOP Department of Physics, University of New Brunswick, Saint J’ohn, New Brunswick, Canada (Received 20 J&y 1973; accepted in revised form 2 Janzlary x974) Ab&&--The Alzs activity has been measured by gamma-ray coincidence spectrometry in a total of 30 stony meteorites (13 bronzite, 13 hype&hen%, 1 carbonaceous and 1 enstatite chondrite, and 2 achon~ites). The measured Alz8 content has been compared with calculated values based on the method developed by LAYWJKEINAand USMNOVA (1972a, 1972b), which takes account of the modulation of the grdaotic cosmic r&y intensity along the meteorite orbit due to sol&r magnetic activity. The predictions have been modified empirically to allow for the difference in chemical composition between H- and L-ehondrites. Comparison of the measured value with that predicted in the absence of solar modulation permits the estimation of the aphelion of the orbit of the meteorite. The great majority of the derived aphelia lie within the range 2.05-2.45 AU. While this result must be treated with considerable reserve owing to the uncertainties associated with the model, it is consistent with recent data from photometric observations of the asteroid belt.
of the levels of radioactive A12*produced in meteorites by exposure to the oosmic radiation have been used to obtain a variety of information both on the history of the me~o~tes themselves and on the vacations of the cosmic ray flux in space and time. A number of workers (e.g. ANDERS, 1960, 1962; BOGARD et al., 1971; CRESSY, 1971; FUSE and ANDERS, 1969; HERPERS et ak, 1969; HERZOG and ANDERS, 1971a; MEASUREMENTS
HEYMANN and ANDERS, 1967; HONDA etal., 1961;
Lmwmr!rzetaZ.,
1965; SPANNAGEL
and HEUSSER, 1969 ; TOBAEEN et al., 1971; VISTE and ANDERS, 1962) have measured Alg8in meteorites and tektites, and have used the data to estimate exposure ages and AP production rates for different classes of meteorite. The study of meteorites whose exposure ages, based on A12*,are of the same order as the half-life of that isotope (0.74 m.y.) has led to a suggested revision of the accepted rates of production of cosmogenic He3 and Ne21in stone meteorites, and henee to 8 significant change in their calculated rare gas exposure ages (HERZOG and ANDERS, 1971b). The interpretation of AP activity in meteorite samples is complicated by the variation of its production rate with depth below the meteorite surface, Since Al26 is close in mass to the principal target nuclei leading to its production, its distribution reflects the local flux of cosmic ray secondaries generated in the meteorite. The A126 concentration will be lowest either near the centre of a large meteorite or in the region just below the surface; the importance of the latter effect is reduced since the surface layer of the meteorite is largely lost by ablation during its passage through the atmosphere. Another effect which influences the levels of cosmogenio n&ides in meteorites is the variation of the incident cosmic ray flux, either with time or with position on the meteorite orbit. Shop-~rrn flux variations have a direct effect on the specific 899
900
IAN R. CAMERONand ZAFER TOP
activities of cosmogenic nuclides of relatively short half-life, such as Ar3’ (half-life 34 days). Information about the space and time variations of the cosmic-ray flux has been obtained by comparing the concentrations, in the same meteorite, of Ar3’ and A? (BEDECK, 1972 ; BEUEMANN and W&IKE, 1969; FIRE&IAN and DEFELIGE, I 960 ; FIREMXNand GOEBEL,1970 ; FIREMANand SPANNA~EL, I971 ; FORMANet al., 1971; STOENNER et al., 1960) and of Naz2 and Al26 (FIREMIAN, 1967). Solar magnetic activity produces a modulation of the cosmic-ray intensity which decreases with distance from the Sun. A meteorite moving in an orbit which is relatively close to the Sun, say with an aphelion of the order of 2 AU, will therefore be subject to an average cosmic-ray flux which is lower than that experienced by a meteorite moving in a higher aphelion orbit which results in its spending a large propo~ion of its time at much greater distances from the Sun. The activity level of a long-lived oosmogenic nuclide, such as AZ28,the concentration of which reflects the average cosmic-ray intensity over a very large number of orbital periods, will consequently be lower in the meteorite of smaller aphelion. A compensating effect is produced by the increase in the flux of low energy cosmic rays due to solar flares, but this enhancement of the cosmogenic production rate is significant only in the outer few centimeters of the meteorite, which are usually lost by ablation. LAVRUKHINAand USTI-NOVA (1972a, b) suggested a method for deriving the aphelion value of a meteorite’s orbit from its AlZ6 content, on the assumption that the outer radius of the region of solar modulation is at 1*9 AU from the Sun. Beyond this radius, the radiation intensity is assumed to have its unmodulated value of O-39 pa~icles~crna set sterad irrespective of the solar activity phase, while at distances less than l-9 AU the intensity, averaged over a solar cycle, is reduced by the solar activity to 70 per cent of its unmodulated value. These authors have calculated the average activity of Al26in stone meteorites as a function of their size, first of all on the assumption that the intensity of the vacating flux is the maximum (~modula~d) value. The actual Alz6 activity expected for a meteorite in a given orbit will then depend on the relative proportion of time it has spent in the modulated and unmodulated regions. Conversely, by comparing the measured Al28 activity with the value predicted for a meteorite of that size subject to an unmodu~a~d flux, it is possible to estimate the dimensions of its orbit. LAVRU~A and USTINOVA(1972b) applied this technique to virtually all the stone meteorites for which Al26 activities were currently available, and derived the result that the aphelia ~st~bution for chondrites has a maximum in the range 1%2~5 AU. The present paper presents the results of Al26 measurements in some 30 meteorites, most of whioh have not been measured before, while some of the others are characterized by divergent values in the literature. The analysis developed by LAVRUKHINA and USTIKOVA(1972b) has been modified and applied to extend the available data on the distribution of aphelia. PROCEDURE All measumm%nts repofted here were made with a gamma-ray spectrometer described by
TOP (1972). This consisted of two 12.7 x IO.2 cm NaI(T1) scintillators operating in coincidence to detect the annihilation gamma-rays arising from the positron decay of AP. The sointillators werespecial low-background assemblies supplied by Quartz et Silice. The detector system wm shielded by 10 cm of low a&iv&y lead. Meteorite and calibration samples were mounted on the front face of one of the scintillator housings.
Measurement of Alz6 in stone meteorites
901
The output from one of the scintillation channels was fed to an Ortec 420 single channel analyser, the window of which was set at 0.61 ri: 0.06 MeV. The pulses from the other scintillator in coincidence with the SCA output were recorded on either a Nuclear Data 1100 or 2400 multichannel analyser. The meteorite sample was counted alternately with background for 1330 min intervals. The total counting time for each sample was at least 16,000 min. Calibration runs were carried out using mockups containing a mixture of dunite and iron powder with known amounts of Alz6 added. The mockup shell was made in the usual manner by covering the specimen with thin aluminum foil and coating with epoxy resin. The proportions of dunite and iron used for the filling were adjusted to simulate the electron density of the meteorite itself. Most of the c~bratio~ were done using sources of A.l26supplied by the U.S. National Bureau of Standards. Some of the earlier measurements employed sources of Naz2 and Alz6 from commercial suppliers, but the calibrations quoted for these were not always reliable when compared with the NBS standards. All measurements made with the commercial sources have been corrected to the NBS calibrations. The gaiu of the multichamel analyser wss monitored before each run and small adjustments were made as necessary to maintain the 0.51 &leV ovation peak in the same channel through. out the me~~ment sequence. Similar checks were made before each run to minimize drift in the trigger channel. An extensive set of tests was carried out to establish the reproducibility of the calibration technique. Possible h-homogeneity of the mockup filling was checked by repeating some of the calibration runs after the mockup shell has been emptied and refilled with the same mixture. In some asses, the calibration runs were repeated with mockups which were completely independent in all respects from the original. Various materials were tried for the blank samples used for background counting. In all cases, the effects observed were within the statist,ical errors associated with the meteorite counting runs themselves. The ratio between the activities of meteorite and calibration mockup w&s obtained by a computer fit of their respective annihilation peaks.
RESULTS The meteorite samples me~ured are listed, along with their Ala6 activities, in Table 1. The classification of meteorite types is that of VAN SCHMOSand WOOD (1967). To aid comparison with other authors, the identification number of each sample is given, since some variation of Alzs activity will be found from one sample to another of the same meteorite owing to shielding elects. The importance of these effects in the interpretation of the results is discussed in the following section. The unce~~nties quoted are statistical counting errors (la) only. The NBS sources used for calibration had a quoted uncertainty of 1-l per cent, which in all cases is considerably less than the error from the counting statistics, For some of the earlier measurements, an additional error has had to be included because of the need to interca&brate these sources against NBS standards. For two of the meteorites, B~derheim and Pueblito de Allende, a,correction to the measured an~ation activity was required owing to the presence of residual Naaa (half-life 2.60 yr). The corrections were made using the data of HoNnA et aE.(1961) and RANCITELLIet al. (1969), respectively. Several of the more recent falls also required a correction for positron decay of Sc44(the daughter of 47 yr Ti44); this was s, minor correction in the range 04-l dpm/kg in all cases. Cont~butions in the an~hil~tion peak region from the uranium and thorium series were negligible for the chondrite measurements; for the two Ca-rich achondrites, Sioux County and Pasamonte, corrections in the range 045-04 per cent were applied, based on measurements with standard sources and the conwntration data given for the two meteorites by K&r@ and WXNKE (1959) and Rows et al. (1963).
IAN
902
Table
Meteorite
Claeeifioation (Van sahmue and Wood)
Date of fall
R.
CAMERON and ZAFER TOP
1. Measurements
of Alz6 activity Sample
Semple Sample wed eource
in this work
Al*’ activity (dpm/kg)
xna~
Others
This work
(g)
Hype&hem chondritm (L) Baxter Benton Bjurbale Bmderheim
L LL6 L4 L6
1916 1949 1899 1960
ASU GSC ASU GSC
419.1x 0218101 148 0220106
239 274 252 277
50 64 69 68
&2 f2 &2 f2
Bill-ea Chantonnay Colby (Wie.) Holbrook
L L6 L6 L6
1946 1812 1917 1912
ASU ASU GSC GSC
734 341.2 0333101 0805109
190 309 148 320
58 64 69.5 57.5
f3 &2 f 3 f 2
Homestead Modoc Otis Potter
L5 L6 L L6
1875 1905 Find Find
GSC ASU ASU GSC
0807101 81a 467.1~ 1607101
123 491 314 176
67 59 43 64.5
-&3 *2 f 1.6 f 4
Saratov Bronzite chondrites (H)
L4
1918
ASU
740
373
71
*2
Bledsoe Clovis No 2 Dimmitt
H H”;3, 4)
Find Find Find
AML GSC ASU
H 121.35 0335101 584.49
130 96.5 474
52 72 44
f3 &4 * 1.5
Dokachi Forest City (1)
H5 H5
1903 1890
ASU GSC
624.1 0602101
164 312
61 44
&2 &-2
Forest City (2) Gilgoin McrIla Mills Puituek Queen’s Mercy Richardton
H5 H5 H H H5 H6 H5
1890 Find 1920 Find 1868 1926 1918
ASU ASU ASU AML GSC ASU ASU
49f 51a 611.1 H120.4 1608101 765 1OOK
291 196.5 180 204 324 208 248
43.5 53.5 70 58 61 58 69
f 2 f 3 *a &4 f2 f2 f3
Rose City VUlCaIl Carbonaceous chondrite
H H
1921 Find
ASU GSC
301-1x 2206102
243 386
66 53
f 3-b f2
Pueblito de Allende Enstatite ahondrite
c3
1969
GSC
1612103
593
61
f2
Abw Achondrites Passemoote Sioux County
E4
1952
GSC
011710
177
68-6 f
Ach Ach
1933 1933
ASU ASU
19788 198a
119 207
101 100
66-7 f 2.7 (1) 72 f 4 (2) 68 f 3(3) 66 (4) 60 f 6 (5) 57 f 2 (6)
4-5
f 6 f4
68 f 79 f
9, 73 f 7 (2) 9 (6)s 44 f 6 (7)
71 & 8 (6) 60.5 f 4.3 (1) 63 & 5 (3) 73 f 8, 68 f 7 (6) 52 f 3 (7) 71 (8)
28.8 & I.7 (1) 58.4 f 36 (10)
2 (9)
46 f 5, 48 f 5 (6) 60.2 f 3.6 (11)
50.2 +
I.9 (11)
71 f 46 f
8. 39 f 4 (6) 3 (7) 63 & 4 (12)
69 f
2 (13)
69 f
8 (6) 63 -& 7 (14)
99 f 11 (6) 101.6 f 3.8 (9) 122 f 13 (6) 99.6 f 6 (9)
* The value quoted in Ref. (6) is in error. Corrected value is ae shown (Rowe, private communication) Sources of meteorite specimens: AML, American Meteroite Laboratory; ASU, Arizona State University; GSC, Geological Survey of Canada. References: (1) (2) (3) (4) (5) (6, (7)
HEXPEES et al. (1969); BISWAE ef al. (1963); Frmxm (1967); CRESSY (1971); HONDA et d. (1961); ROWE and CLARK (1971); CFZSSY (1964);
(8) (9) (IO) (11) (12) (13) (14)
SPANNA~ELand HEWSSEX (1969); FUSE and ANDERS (1969); SPAI?NAOEL(1973); HEBMAX and ANDEBB 11973); EEXANN and KOE~UET(1968); R&-XmELLI et al. (1969); HEand ANDERS (1967).
Measurementof Al26in stone meteorites
903
A few of the samples measured, such as Bruderheim, were chosen because their activity values had been reliably established on the basis of several independent measurements; they therefore acted as a check on the absence of significant systematic error in the present work. The agreement of our measurements with the previously published results is satisfactory in general. Apart from the peculiar case of Dimmitt, and a disagreement which is just outside the experimental errors with HERPERSet al. (1969) for Bjurbijle and BISWAS et al. (1963) for Holbrook, the remaining discrepancies are cases where our results are lower than those of ROWE and CLARK(1971) or higher than those of CRESSY(1964). The final results quoted by the former have been obtained by a somewhat indirect calibration relative to the K40 content of the samples, and it is possible that the correction factor may be too high in some cases, leading to an excessive estimate of the Al2s activity. (This does not apply to the Bruderheim result, where a direct Al26 calibration was performed.) The results of CRESSY(1964), on the other hand, as pointed out by HEY-N and ANDERS (1967), may be systematically low, since they were made on chemically isolated aluminium fractions, and some difficulty waa experienced in the yield determinations. It should perhaps be pointed out that one factor which may produce minor differences in the comparison with the results of other authors is that the great majority of such results have been obtained using Nas2 as the calibration isotope. The results in such cases will depend on the relative intensity of the annihilation gamma-rays emitted in the decays of Alasand Na 2*. Most of the comparison activities quoted in Table 1 are based on an assumed 84-85 per cent intensity for the positron decay of Al26; that there may be an appreciable uncertainty in thia intensity, however, is suggested by the recent work of SAMWORTH et al. (1972), which yielded a value of 82-l per cent for this branch. Our value for Dimmitt lies half-way between the two widely differing results of HERPERSet al. (1969) and FUSEand ANDERS(1969), and therefore does nothing to throw light on this discrepancy. Our value would be consistent with the Hes-age of about l-5 m.y. quoted by EBERHARDTet ab. (1966) ; this low age, however, is very probably due to helium diffusion loss since, as pointed out by FUSE and ANDERS (1969), the Ne21 content of Dimmitt suggests that its age is great enough to allow saturation of the Al26 activity. Apart from Dimmitt, the only H-chondrite with an activity of less than 50 dpm/kg is Forest City ; the result obtained for this meteorite seems particularly reliable in view of the good agreement found with two separate specimens. The low value obtained both by other workers and the authors is not due to non-saturation, since both the He3 and Ne21 contents, as given by KJRSTENet al. (1963), lead to an exposure age of the order of 4-5 m.y. It is possible that the low value is a shielding effect, but the recovered mass of Forest City is not remarkably high (122 kg) ; the low A12sactivity could only be explained as a shielding effect if the ablation had been unusually great. For the L-chondrites, the only remarkably low results are for Baxter and Otis. The low Alzs activity of the former is presumably due to its small size. The Otis result may be partly due to a size effect (the recovered mass was only 2.6 kg) ; again, it would not appear to be unsaturated since its He3 and Nell contents, as given by ZXHRIN~ER(1966), suggest an exposure age of at least 12 m.y.
904
IAN R.
CAMERON
and
&FER
TOP
CALCULATION OBMETEORITE ORBITS The interpretation of Alas activity in terms of meteorite orbits is subject to considerable error from a variety of factors. The most important of these are: (i) The considerable confusion which exists regarding the detailed variation of cosmic ray intensity over the region of the meteorite orbits ; (ii) Shielding effects, which are particularly significant for meteorites of small pre-atmospheric size or for surviving fragments from the interior of a highly-ablated large meteorite; (iii) Variations in induced activity due to differences in chemical composition between meteorites of the same or different classes. The current information on variations of cosmic ray intensity as a function of energy and spatialposition has been summarized in a review by O’GALLAOHER (1972). A recent review of measurements with the Pioneer 8 and 9 space craft over the distance range of 0.75-1.10 AU from the Sun has been given by WEBBER and LEZNIAK (1973). The results of the various determinations at lower energies show very large discrepancies, and even in the higher energy range, which is of principal interest in the production of cosmogenic nuclides in meteorites, the only conclusion that can be drawn is that the heliocentric gradient is positive with a magnitude somewhere between a few per cent and a few tens of per cent per astronomicalunit. Various authors have attempted to derive the magnitude of the cosmic ray gradient from the ratio of A?’ to Ar3s in the Lost City meteorite, the orbit of which was such that it had a time-average distance from the Sun of 1.8 AU. FIREMANand SPANNAGEL (1971) and FORMANet al. (1971) found a positive gradient of about 60 per cent/AU at energies around 1 GeV, whereas BEOEMANN (1972) derived a much smaller value, in the order of 15 per cent/AU. CRESSY (1971) suggested that the approximate equality of the Al26 activities of the two meteorites Pribram and Lost City, whose orbital elements are accurately known, could be explained on the assumption that a step change of the cosmic ray flux occurred at around 1.02 AU from the Sun. The tentative nature of this model, and the variety of possible sources of error, such as the rather large uncertainty in the measured ratio of the meteorite activities, are emphasized by the author himself. One of the most comprehensive analyses of the relation of Al26 activity to the spatial variation of cosmic ray flux is that of LAVRUKECINA and USTINOVA (19728, 1972b). By comparing the measured and calculated activities of radionuclides with different half-lives in several chondrites, they concluded that the upper limit of the solar modulation region is at l-9 AU from the Sun. They also derived a radial gradient of about 70 per cent/AU, in reasonable agreement with the Lost City work referred to above. One of the reasons for applying the model of Lavrukhina and Ustinova to the present set of Alz6 activities is the relative ease with which the parameters of the model can be changed to accommodate more precise information about, for example, the extent of the modulation region or the predicted levels of Alzs activity. In this method, the Al26 activity is determined by the parameter 2 = tmod/torb, where tori,is the orbital period of the meteorite and tmoais the time which it spends during each revolution in the modulated region of the cosmic ray flux (taken as having its boundary at 1.9 AU from the Sun). The observed Alaa activity, Hobe, can then
Measurement of Al= in stone meteorites
906
be expressed as
(1) where H,,, is the activity which the meteorite would have attained if it had been exposed throughout to an unmodulated flux and Hmin, that which it would have had if its orbit had been entirely within the modulation region. Hmin was taken to be 70 per cent of H,,, as determined by the Russian authors. The value of H marfor a given meteorite can be calculated using the data presented in their paper for the average Al26activity in stone meteorites aa a function of their radius. It should be noted that in their work a single curve is presented for chondrite activity, no distinction being made between H- and L-chondrites. That the chemical variation between the two types accounts for a significant difference in Ala6activity is suggested by the work of CRE~SY(197 1) on chemically separated samples of the Bruderheim chondrite. Combining the elemental cross-sections derived by Cressy with the average composition data for H- and L-chondrites quoted by ~SON (1971), one finds that the average ratio of Alasin L-chondrites to that in H-chondrites is l-09. The ratio of average Al2e activities measured in the present work, omitting the few anomalously low values, is in the neighbourhood of 1.08. A similar ratio between the H- and L-chondrite activities has been found by HEYMANNand ANDERS(1967). It would therefore seem probable that the most important factor in the variation of Al26activity between the two types of chondrite is the chemical composition rather than the difference in orbit parameters. This effect has been added empirically to the analysis by increasing the H,, values of LALVRUIIRINA and USTINOVAby 4.5 per cent for the L-chondrites and reducing their H,,, values for H-chondrites by a similar amount. While this procedure is crude, it does permit some reasonable allowance to be made for the effect of chemical variation between the classes. Since the calculated Al26activity is a function of the preatmospheric radius of the meteorite, some assumption has to be made about the extent of ablation. The preatmospheric radii were calculated for three assumed values of the extent of ablation: 92, 85 and 60 per cent. In each case, 2 was calculated from equation (l), using our measured Alz6 activity. The aphelion of the meteorite orbit was then calculated using the relation which Lavrukhina and Ustinova obtained by fitting to the known orbits of Pribram and Lost City, and two plausible orbits for Bruderheim based on the method of visible radiants. The relation is Q’ = 1.26 + 0.132 + 0X532-‘.
(2) The aphelia derived using this formula are given in Table 2. Allowing for the error in the measurement of activity, all but 6 of the meteorites measured have aphelia in the range 2.05-2.45 AU. The relatively small spread is not unexpected, since the range of Alz6activity variation is itself not very large, with the exception of a few anomalous values. The values of Q’ lying outside the quoted range arise partly from the low activities for Otis, Dimmitt and Forest City, which were discussed in the previous section, and partly from the rather high activities of Saratov, Merua and Clovis. Confidence in the first of these high activity values is enhanced by the identical value found by SPANNAQEL and HEUSSER(1969).
906
IAN
R.
CAXERON
and
ZAJ?ER
TOP
Table 2. Aphelia of meteorite orbits calculated under conditions of 92, 85 and 60% ablation Aphelion(q’) in AU
Alss activity Total mass Meteorite
(kg)
dpmlkg (present work)
92 % ablation Our velue Ref. 1
85 ‘A eblstion Our velue Ref. 1
60 % ablation Our value Ref. 1
Hypersthene chondrites (L) Baxter Benton Bjurbiile Bruderheim BUl%a, Chsntonney Colby (Wis.) Holbrook Homestead Modoa Otis potter saratov Bronzite chondrites (H) Bledsoe Clovia No. 2 Dimmitt Dokachi Forest City (Sample 1) Forest City (Semple 2) Gilgoin MIXUS Mills P&tusk Queen’s Meroy Richerdton Rose City VulCcLn Enstatite chondrites Abee
0.61 2.84 330 303 25 31.5 110 220 226 30 26 261 328
30.5 13.3 13.5 3.84 122 122 147.5 71.4 2 >200 w250t 90.9 10.6 19 107
50 64 59 58 58 64 69.5 57.5 67 59 43 64.6 71
1.97 2.28 3.28 2.85 1,96 2.20 7.2 2.41 54 1.99 <1.9 10.7 co
52 72 45.5, 52+ 45* 44*5* 55* 70 58 51 59* 59 56 53
1.96 17.8
2.44 2.27 11.8 2.80 2.06 1.95 2.40
-27.8 4.06 -14.5 -4.39 ~11.86 co
2.03 1.92 1.92 -
2.18 -4,8
2.07 2.54 2.48 2.30 1.94 2.16 3.25 2.16 3.86 1.97 (1.9 3.19 co
1.93 co
8.83 3.93 2.66 3.69 3.32 co
2.29 4.19 2.10 2.05 1.97 2.16 2,54 1.99 2.51 1.99 (1.9 2.40 4.19
2.27 -4.58 2.27 -11.9
-
2.20
2.06
1.96 co <1*9 2.26 <1.9
2.18
2.90
2.04
3.06 2.32 -
Ca-Riah aahondritss Posamonts Sioux County
3.6 4.1
* Al*O activity has been corrected for non-saturation due to short exposure age. t Estimeted from approximate size of piece observed from fall (HEY, 1966). Ref. USTINOVA (1972b).
1 L~vnn~ax~a
and
The results of Lavrukhina and Ustinova showed a general tendency for the q’ values of L-chondrites to be greater than those of H-chondrites. Our results, again ignoring the few anomalous values, give average aphelia which are similar for the two types of chondrite. This is, of course, to be expected, since the empirical correction to the production rates to allow for the chemical composition difference implies that the difference in mean activity between the two groups is principally due to this effect rather than to orbital differences. It should be noted that, although the method of Lavrukhina allows for shielding effects in the calculation of the average activity in a meteorite of given size, there will still be an appreciable error, owing to the fact that the A12s activities of the samples measured will, in general, deviate from the mean activity of the meteorite as a whole. The production rates of low-energy products, such as Alzs and MI?, on the other hand, are rather insensitive to depth and size. For example, the results of KOHMAN
Mw.wwemen% of Al28 in stone meteorites
907
and BENDER (1967)show a range of variation for Mns3of only f 15 per cent from the mean over a range of radius variation equivalent to 24-70 cm for stone meteorites. Fusx and ANBERS f1969) showed that serious variation of APB activity due to shielding was rare among randomly selected samples of meteorites covering a wide range of recovered masses. CBESSY (1972) noted that the AlS8 concentration in Bueblito de Allende showed a variation of only f 10 per cent over a depth range of up to 40 cm. These results suggest that, in general, unless one is dealing with samples from the interior of meteotites of very large or very small preatmosph~~~ size, the assumption that the AP activity of the sample measured is representative of the mean value of the meteorite is not likely to lead to serious error. The effect of any errors which do arise will be to increase the spread of the aphelion values rather than to produce a systematic trend, since any given sample is equally likely to be high or low compared to the average calculated by Lavrukhina and U.&nova. The necessity for calculating aphelion values for a range of possible ablations can be avoided if measurements are available from which an ablation estimate can be made. Only a very few data of this kind exist (e.g. BHANDARI,1969; M~RTI et al., 1969) and only one such value (about 90 per oent, for Baxter) applies to the meteorites measured in the present work. It should be pointed out that the variation of the average aphelion value produced by varying the a.blation over the range 60-92 per cent is only f5 per cent for both types of ohondrite, The absolute values of the aphelia predicted must be treated with considerable reserve, since the model used is subject to two major sources of uncertainty which can be resolved only when more reliable information becomes available. The first of these is the lack of detailed information about the heliocentric cosmic ray gradient. In the present calculations, for example, changing the radius of the outer boundary assumed for the mod~atiu~ region by f5 per cent produced a percentage change of approximately the same amount in q’. The second major possible source of error is in the calculation of the absolute production rates of APE, on which the model is directly dependent. The significance of this effect is illustrated by the fact that the modification of j&5 per cent made to the Z=, valaes to allow for the different compositions of the L- and R-chondrites had the effect of changing the mean aphelion value for each group by about 9 per cent. While the model leaves much to be desired, however, it is interesting that the rather low aphelion values predicted are consistent with the supposition that the majority of chondritic meteorites are derived from asteroids of the Apollo type rather than from the main asteroid belt. This supposition has found recent support If)?3 ; from photSometricstudies (e.g. Crrep~~rret aI., 1973 ; CIIIApmx and SAL~~BURP, EGANet a$., 1973). AoknowledgemesS--We are in&&tadto the Geologica;l Survey of Canada for providing specimens on loan from the National Meteorite Collection of Canada, and to Dr. C. B. MOOREfor the loan of samples from the collections of Arizona State University. The work was carried out, under a grant fA5710) from the National Eesesrch Council of Canada.
ANDERS E. (1980) The record in the meteorites -II. meteorites and tektites. Qeo&%. Oomochk. Act&
On the presence of aluminum-26
19,53-62,
in
908
Lu
R. C-RON
and Z~L~FER TOP
ANDERSE. (1962) Two meteorites of unusually short cosmic-ray exposure age. Science 188, 431-433.
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