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Cosmogenic radioisotopes in the Dhajala chondrite: implications to variations of cosmic ray fluxes in the interplanetary space
194
Earth and Planetary Science Letters, 40 (1978) 194-202 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[6]
COSMOGENIC RADIOISOTOPES IN THE DHAJALA CHONDRITE: IMPLICATIONS TO VARIATIONS OF COSMIC RAY FLUXES IN THE INTERPLANETARY SPACE N. BHANDARI, S.K. BHATTACHARYA and B.L.K. SOMAYAJULU Physical Research Laboratory, Ahmedabad (India)
Received January 17, 1978 Revised version received April 10, 1978
Activities of a suite of radioisotopes ranging in half-life from 5.6 days (S2Mn) to 3.7 m.y. (S3Mn) have been measured in the Dhajala chondrite. The results show that all the radioactivities are close to the expected levels except 54Mn and 22Na. Their activities are higher than those based on the interplanetary fluxes at 1 A.U. near the ecliptic, expected immediately before the fall of Dhajala, corresponding to the time of solar minimum. Furthermore, activity ratios of S4Mn/saMn and 22Na/26A1 are higher by 30-50% than expected. The departure from the expected values is discussed in terms of spatial variations of cosmic rays based on the computed orbital parameters of the meteoroid. If the galactic cosmic ray fluxes in the equatorial region (+15 °) are assumed to be the same as in the ecliptic plane then these results suggest higher fluxes by 33 -+7% at heliographic latitudes 15-40°S, during solar minimum.
1. Introduction The production rate, and hence the activity, of a radioisotope in a meteorite resulting from the nuclear interactions of cosmic rays is proportional to its flux along the orbital segment of the meteoroid covered mainly during a mean life o f the radioisotope. Radioisotopes with different half-lives, therefore, offer a time parameter which can be used for evaluation of the temporal and spatial variations of cosmic rays along the meteoroid orbit. Based on comparison of such measurements in meteorites and lunar samples, some aspects of solar and galactic cosmic ray characteristics in the past, their variation with solar cycle and with heliocentric distance in the solar system have been evaluated [ 1 - 3 ]. In this context the Dhajala meteorite [4] is of special interest in view of the fact that (1) its orbital parameters have been computed reasonably well [5], (2) it fell at the time of solar minimum, and (3) it was recovered immediately after the fall, making measurements on short-lived isotopes possible. The Dhajala meteorite fell on 28 January 1976 at about 8.40 p.m. Indian Standard Time in Gujarat,
India. The phenomena associated with the fall of the meteorite, field observations and meteorite characteristics have been described earlier [4,6]. The first few fragments o f the meteorite arrived in our laboratory within a day o f the fall. Immediately afterwards a variety of investigations on rare gas isotopes [7], cosmic ray tracks [8] and chemical and mineralogical compositions were carried out. Based on these studies, the meteorite is classified as a H3 bronzite chondrite having a cosmic ray exposure age o f about 6.2 m.y. Systematic measurements of several radioisotopes ranging in half-life from 5.6 days (S2Mn) to 3.7 m.y. (S3Mn) have been carried out. Some preliminary data were reported earlier [4,9]. The final results are presented here and discussed in terms of the characterlstics of galactic cosmic rays in the interplanetary space.
2. Experimental details Radioisotopes 26A1, S4Mn and 22Na were measured in a few fragments using a non-destructive gamma-ray spectrometer (Table 1). In three selected fragments,
195 T-1 l, T-67 and T-68, several radioisotopes have been measured after radiochemical separation allowing us to determine the activity of 7Be, l°Be, 22Na, 26A1, 32p, 33p, 51Cr' ~;2Mn' SaMn' S4Mn' SVCo and 6°Co (Table 2). The gammay-ray analysis was performed using a 7.5 cm X 7.5 cm NaI(T1) crystal. The system has been described earli.er [3]. Clear signals were obtained at 511,835, 1460 and 1810 keV and minor peaks due to 6°Co were also seen. The 1460-keV signal due to 4°K was used as a relative internal standard for estimating the effective mass contributing to the signals. It has been shown earlier that this procedure is valid in the mass range of 1 15 kg [10]. After Compton correction due to the 1810-keV signal, the ratio of the 1810-keV signal to tile 1460-keV signal was calculated for each fragment and for the meteorite Bansur where 26A1 is known to be 50 -+ 5 dpm/kg [3]. Z6AI dpm in each fragment was then obtained directly from comparison of this ratio. Due corrections for the potassium content of the meteorites (Dhajala = 840 ppm, Bansur = 870 ppm) were taken into account. In the same way 22Na was calculated from tile 511-keV signal, after subtracting the 26A1 contribution. The S4Mn activity was determined from the 835-keV signal using the same procedure. The average of the ratios of the 835-keV signal to the 1460-keV signal were taken to correspond to 144 dpm/kg, the value measured in T-11. For radiochemical investigation, a sample (T-11) of 350 g was crushed to <230/lm (60 mesh) and 320 g of this sample was dissolved using HF, HNO3 and HC1 having preserved the remaining 30 g for mass spectrometric, fossil track and chemical studies. About 45 g remained undissolved, which was fused with Na2CO3 and the melt was dissolved and mixed to the main solution. An aliquot was taken for the chemical estimation of elements and carriers of Be (100 mg -= BeO), P (40 mg ~ Mg2P2OT) and Co (100 rag) were added and equilibrated by vigorous boiling. Standard chemical procedures were followed for separation and radiochemical purification of the elements. Briefly, Fe was removed by ether extraction in 9M ttC1. Aluminium, Na and Mn were precipitated as chlorides by Gooch-Haven's method. Manganese was separated as MnO2 in a HNO3-NaBrO3 medium. Hydroxide precipitates in a Na202-NaOH medium
enabled A1, Be, Cr and P to be removed from Fe, Ni and Co. Final separations were done on ion exchange columns [11]. The radiochemical purification was performed using characteristic precipitations or ion exchange methods. Table 2 lists the final forms in which samples were counted. 13 g of another sample (T-67) was also processed for 26A1 and 14 g of T-68 for 1°Be and 26A1 after extracting argon for agAr analysis, which is in progress. In addition to the counting systems described earlier, proportional and GM counters were used for estimating the radioactivity of X-ray emitters (S3Mn in sample T-11) and beta emitters (32p, 33p and 1°Be). S2Mn, 26Al and 6°Co were counted on beta-gamma coincidence detectors and were identified by their characteristic gamma radiations. S2Mn and a2p were additionally confirmed by their characteristic decay rates. Absorption measurements over several weeks made on the phosphorus sample enabled us to resolve the a2p and aap signals.
3. Results and discussions Eight fragments have been studied in the present work. The results of non-destructive gamma counting of various fragments are given in Table 1. Extreme variations of 1.31 for 22Na, 1.38 for 26A1 and 1.24 for S4Mn have been observed. There is no systematic change in radioactivity or tracks in a fragment with its location in the strewn field. One of these fragments, T-68, was also analysed for radionuclides 1°Be and 26A1 after radiochemical purification. In addition, two other fragments, T-11 and T-67, were also radiochemically studied as described earlier. These results are shown in Table 2. There is a general agreement between repeated measurements on the same fragment by different methods.
3.1. Shielding depths Spot samples from all the fragments studied here were also analyzed for cosmic ray tracks [8]. From these data, using the exposure age of 6.2 m.y. [7] and the production rates given by Bhattacharya et al. [12], we obtain the shielding depth of each fragment within the meteoroid body during its exposure to
196 TABLE 1 26A1, S4Mn, 22Na in fragments of the Dhajala meteorite Sample
Weight (kg)
T-67 G-1 T-78 T-16 T-17 T-92
1.5 2.39 1.63 2.02 2.05 6.5
Distance along the fallout track * (km)
Track density ** (cm-2)
Shielding depth
Activity (dpm/kg) ***
(cm)
26A1
S4Mn
22N a
8 17 15 14 9 17
2.3 6.7 1.4 3 3 3
23 18 13 12 12 12
52-+5 62-+6 48-+5 55 -+ 5 45-+5 53 -+ 5
123-+ 8 152 -+ 10 150 -+ 10
96-+10 120 -+ 10 126 -+ 12
53
144
111
× × × × × ×
103 10 a 104 104 104 104
Average * Bhandari et al. [4], Lal and Trivedi [6]. ** From Bagolia et al. [8]. *** Errors are l o counting statistics. c o s m i c rays. T h e t r a c k d e n s i t i e s f o r s o m e f r a g m e n t s
r e s p e c t i v e l y . All t h e o t h e r s a m p l e s , listed in T a b l e 1,
are given in T a b l e 1. In T-11 a n d T-68 t h e t r a c k d e n sities in olivines w e r e 4 × 104 c m -2 a n d 1.5 X 104
h a d s h i e l d i n g d e p t h s o f 1 2 - 2 3 c m . Since m o s t o f t h e fragments analysed here were tens of centimeters, s p o t values o f d e p t h s m a y n o t b e r e p r e s e n t a t i v e f o r
c m -2 l e a d i n g t o s h i e l d i n g d e p t h s o f 11 a n d 13 c m , TABLE 2 Chemical and counting details of samples Radionuclide
Elemental abundance (%)
Chemical form
Amount expected (g)
Amount counted (g)
Detection mode
Counting efficiency (ideal)
Background (cpm)
Signal (cpm)
Activity at the time of fall (dpm/kg)
BeO BeO BeO A1203 Mg2P207 Mg2P207 Cr203 MnO2 MnO2 MnO2 Co203 Co203
0.111 0.111 0.111 6.954 1.449 1.449 1.006 1.256 1.256 1.256 0.321 0.321
0.026 0.026 0.050 0.148 0.414 0.414 0.392 0.456 0.295 0.321 0.187 0.194
3" /313/3+"r /3t33' /3÷-~ X-ray "r X-ray #--'r
0.094 0.27 0.24 0.075 0.36 0.15 0.122 0.085 0.027 0.148 0.033 0.040
0.015 0.138 0.033 0.0086 0.1 0.026 0.010 0.24 2.27 0.2 0.0027
0.046 0.30 0.07 0.017 0.51 0.04 0.062 0.012 0.24 2.61 0.04 0.016
132 16.0 17.3 45 22 7.2 51 28 76 144 27 7.3
BeO
A12O3
0.111 0.646
0.025 0.266
/3/3÷-3'
0.24 0.075
0.036 0.0086
0.040 0.038
18 61
-+ 2 -+ 7
A1203
0.246
0.088
/3÷-0'
0.075
0.0086
0.016
56
-+ 12
Sample T-11 7Be * 10Be 1°Be ** 26A1 a2p *** 33p *** SlCr * S2Mn SaMn S4Mn STCo 6°Co
1.15 0.126 0.126 0.215 0.248 0.248 0.248 0.04 0.04
-+ 12 -+ 1.5 -+ 4.3 -+ 9 -+ 1.7 -+ 2.4 -+ 14 ± 4 -+ 9 -+ 8 -+ 8 -+ 1
Sample T-68 1°Be 26A1
1.15
Sample T-6 7 26A1
1.15
* These samples were measured by Dr. C.P. Kohl. ** Duplicate sample. *** 32p and a3p were calculated by BiUer's plot.
197 the whole sample. It is, therefore, not possible to construct a depth profile of the radioactive isotopes from the measurements given in Tables 1 and 2. For the discussion of the average observed activity of the various radionuclides we shall, therefore, compare them with the mean values expected at depths between 10 and 30 cm.
3.2. Expected activity of radionuclides To understand the observed activities and their variations in terms of cosmic ray intensity and shielding by overlying material, we compare them with the calculated profiles. These are not known well for all the isotopes studied here. Lavrukhina and Utsinova [1] and Reedy and Arnold [13] have made calculations of the isotope production in chondrites and Moon, respectively. In addition, Kohman and Bender [14] have calculated the rate of production of radionuclides in iron meteorites. Trivedi and Goel [ 15], following the procedure of Kohman and Bender, have given production rates of 2 2Na and 3H in spherical chondrites of various radii. The expected activities of various other nuclides like S4Mn, S3Mn, 26A1 were
calculated from an extension of the Reedy-Arnold method to meteorites. The Reedy-Arnold parameters were obtained from the 22Na profile given by Trivedi and Goel and will be discussed elsewhere. Here we only mention that the observed ratios of 22Na/26A1 and S4Mn/SaMn in various meteorites range between 1.2 to 1.6 and 0.80 to 1.14, respectively, and agree well with our calculations of production ratios for meteorites of different radii. The calculations, shown in Fig. 1, are based on the composition of the Dhajala meteorite as given by Noonan et al. [16]. It is seen that, for S3Mn, S4Mn, 22Na and 26A1, the production rate increases with depth towards the center of the body, provided the radius of the meteoroid is less than 30 cm. In a 40 or 50-cm chondrite, after a depth of about 10 cm the variation of isotope production with depth is small, within about 10% over the mean value. For larger bodies the isotope production shows a well-defined maximum at about 15 cm which shifts to shallower depths as the radius of the meteoroid increases. After this broad maximum the isotope production decreases slowly towards the center of the body. In the following we use these profiles to determine the preatmospheric size of Dhajala.
3.3. Comparison with calculation 180
I
I GO
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E
i
T
[
,
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'
12(3-
'
I
~
I
'
O 22No O 26 AI • 53Mn • 54Mn
/
140
E c.
I
~
'00--80
5
54Mn
22N0
4~
m 26AI
20 0
0
~ I L I
I0
20
t I
30
~oocm26Al I
I
4,0
t I I [
DEPTH
50
60
t [
70
I
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80
90
(cm)
Fig. 1. Depth variation o f the calculated activities o f S4Mn, SaMn, 26A1 and 22Na in a spherical bronzite o f 50 cm radius. s 3Mn profile takes into account undersaturation during 6.2 m.y. exposure. The 26A1 profile is also given for a sphere of 40 and 100 cm radius for comparison. The observed activities in various fragments are shown.
The observed average values for the four isotopes are S3Mn = 76 dpm/kg, 26A1 = 53 dpm/kg, S4Mn = 144 dpm/kg and 22Na = 111 dpm/kg and they differ from the mean calculated values by +13%, -2%, +69% and +35% respectively (Table 3). The levels of the short-lived activities like 22Na and SaMn are affected by the solar cycle variations in the galactic cosmic ray (GCR) fluxes, which have not been taken into account in the calculations given above; these will be discussed later. The 26A1 activity, representing the average fluxes, indicates that its observed activity levels are similar to those expected for a body of about 50 cm radius. For smaller bodies the activity levels of 26A1 and S3Mn will be much higher than those observed. The variation of observed 26A1 activity in different fragments also suggests that the preatmospheric radius of the meteoroid was 50 cm. This estimate is slightly higher than the results of Bagolia et al. [8] who, based on an extensive study of cosmic ray tracks, give a value of--~ 40 cm. The longest-lived cosmogenic isotope measured
198 TABLE 3 Cosmogenic radioactivities in fragments of the Dhajala meteorite Isotope SaMn l°Be 26A1 60Co 22Na S4Mn STCo 7Be 51Cr 33p 32p S2Mn
Half-life 3.7X106 years 1.6X106 years 7.4 X l0 s years 5.2 years 2.6 years 312 days 271 days 53.3 days 27.7 days 25.3 days 14.2 days 5.6 days
T-11 (dpm/kg) 76 16 45
-+ 9 -+ 1.5 +- 9
7.3 + 1 144 ± 8 27 ± 8 132 + 12 5l ± 14 7.2 ± 2.4 22 ± 1.7 28 ± 4
T-67 (dpm/kg) 52 ± 5, 56 -+ 12 96 ± 10 123 ± 8 -
T-68 (dpm/kg) -
18 -+2 61 ± 7
Expected * activity (dpm/kg) 67 16 52
82 85
* Mean activity at depths between 10 and 30 cm for a 50-cm bronzite, based on the average GCR flux J (E > 1 GeV) = 1.7 protons/cm 2 • s - 4n.
here is S3Mn (half-life = 3.7 m.y.). In view of the 6.2m.y. exposure age of this meteorite, it should attain 69% of its secular equilibrium value. The observed S3Mn activity of 76 + 9 dpm/kg leads to a production rate of 405 + 50 atoms/min • kg Fe. The calculated mean production rate is 360 atoms/min • kg Fe. The 1°Be (half-life = 1.6 m.y.) should acquire 93% of its saturation level. Its activity of 17 + 1 dpm/kg is similar to the activities found in other stony meteorites and also similar to the calculated value. The same is tile case with 26A1 whose activity was found to range from 45 to 62 dpm/kg as discussed earlier. For short-lived isotopes, the modulation of the GCR fluxes due to solar cycle has to be considered.
3.4. Solar cycle effect The magnitude of the solar cycle effect can be easily estimated from the shorter-lived isotopes with a mean life g 1 year, by comparing their observed activities in different meteorites, after considering the target element abundances, preatmospheric size of the meteorites and shielding effect. 7Be, mainly produced from oxygen, does not depend on the type of meteorite and can be ideally used for estimating cosmic ray fluxes during different phases of the solar cycle, lts value of 132 -+. 12 dpm/kg in T-11 is 14%
higher than 116 + 12 dpm/kg for the Canon City meteorite [ 17] which fell in 1973 during solar maximum. These measurements suggest that the solar cycle effect is atmost +20% over the mean. Fireman [2] considered 22Na and came to a similar conclusion that its production does not vary by more than +15% from the mean value. For other isotopes the production rates have not been calculated. Considering the uncertainty of the shielding effects the activities of 3ap (7.2 + 2.6) and 32p (22.1 + 1.7) dpm/kg compare favourably with 18 + 9 and 14 + 5 dpm/kg found in the St. Severin meteorite, which also fell at the time of solar minimum in 1966 [18]. The 27-day S~Cr has an activity of 51 -+ 14 dpm/kg which is also similar to the expected values. The 5.6-day S2Mn shows an activity of 103 + 15 dpm/kg Fe which is much lower than the value 172 -+ 36 dpm/kg Fe in St. Severin [19]. This may partially be due to size effects, but the production rates are not known. The measured proton fluxes [20,21 ] indicate that J (GCR fluxes >1 GeV) for 1975 was 38% higher than the mean fluxes (1.7 proton/cm 2 • s • 4rr) used to calculate the production rates given in Fig. 1. Considering the measured GCR fluxes during previous years we estimate that the solar cycle effect will result in a 28% and 12% higher production for S4Mn and 22Na, respectively. In summary, the above dis-
199 cussion shows that the m a g n i t u d e o f the solar cycle effect is t o o small to a c c o u n t for the observed excess ~ctivities o f 22Na and S4Mn.
3.5. Excess activity of 22Na and S4Mn Many uncertainties in calculations can be r e m o v e d if we consider the ratio o f the activities o f isotopes, p r o d u c e d in similar reactions, rather than the individual radionuclides. T.he 22Na/26A1 (observed production ratio) in Dhajala is 2.1 and S4Mn/S3Mn = 1.3. These are higher than those r e p o r t e d in any o t h e r m e t e o r i t e where 22Na/26A1 is usually b e t w e e n 1.2 and 1.6 and 54Mn/S3Mn is around 1.0. The excess o f 22Na and S4Mn by about 20% and 30% over and above the solar m i n i m u m effect, therefore, appears to be genuine. A comparison of the observed activities o f S4Mn and 22Na with o t h e r m e t e o r i t e s is made in Table 4. Since S4Mn is almost totally p r o d u c e d f r o m Fe, a direct c o m p a r i s o n o f activity per gram of target elem e n t is possible if we ignore the shielding effects for the different meteorites. Table 4 shows that Dhajala has the highest S4Mn d p m / k g Fe value observed. F u r t h e r m o r e , the activity does n o t correlate inversely
with the solar cycle b u t seems to depend on the orbital inclination o f the meteorites. This p o i n t is discussed later. F r o m the above discussion, it is c o n c l u d e d that the long-term (m.y.) average G C R flux in the interplanetary space encompassed by the Dhajala orbit has been the same as around the Moon. The activity o f shorterlived isotopes ( 5 - 5 3 days) also seems to be consistent with the same flux, although for some o f t h e m the calculations are n o t reliable, nor do adequate observations exist in other meteorites or the M o o n to allow an accurate comparison. The solar cycle effect from minima to m a x i m a is generally within -+20% of the mean flux a d o p t e d here. For the isotopes 54Mn and 22Na with half-lives o f around a year, however, there seems to be a clear excess above the values e x p e c t e d from the G C R fluxes at the time o f solar m i n i m u m near the ecliptic.
3.5. Spatial variation of galactic cosmic ray fluxes To explain the excess activity o f S4Mn and 22Na we n o w consider the possibility of an abnormally high G C R flux over the space segment o f the Dhajala orbit during the last few years. Ballabh et al. [5]
TABLE 4 Comparison of 54Mn and 22Na radioactivity with solar cycle and orbital inclination in different meteorites and a Moon rock
Type Date of fall V~ (km/s) Inclination (°) Shielding depth (cm) 54Mn (dpm/kg)
St. Severin
Allende
Murchison
Lost City
Ucera
Dhajala
Moon rock 14321
LL6 27/6/1966
C3 21/2/1969
C2 28/9/1969
H5 3/1/1970
H5 16/1/1970
H3 28/1/1976
5/2/1971
14 a ~1 a 2-5 e
20 b >~20 e
_ ~>10 e
81 ± 2 g
99-128 h (-+4) 23.5 503 69-81 (52) h
83 ± 8 i 101 ± 2 j 25.43 361 80±8 i
65 ± 6 i 85 ± 4 J 27.63 270 80±8 i
max (1.64)
max (1.52)
max (1.47)
Fe content (%) 19.2 S4Mn (dpm/kg Fe) 427 22Na(dpm/kg) 72-108g,m,n,o Solar activity (J) * rain (2.01)
14.2 c 12 c 5-12 e
_ _ ~20 e 61 ± 4 J
21 d 28 d 11-23 f
_ 0 13.3
144 ± 8 k
27 -+5 1
26.43 230 60±4J
27.2 525 111 ±6 k
10.5 260 3254 1
max (1.47)
min (2.34)
max (1.14)
* J is the average GCR flux (E > 1 GeV) over one year previous to the fall (protons/cm 2 • s - 4n). a Nordemann et al. [35]. b McCrosky et al. [36]. c McCrosky et al. [33]. d Ballabh et al. [5]. e Bhandari et al. (in preparation) f Bagolia e t al. [8]. g Cressy [19]. h Rancitelli et al. [30]. i Bogard et al. [32]. J Cressy [31]. k This work. 1Wahlen e t al. [34]. m Fireman [2]. n Tobailem et al. [29]. o Spannagel and Sonntag [28].
200
N2 o ~ 40
I
I
I
f
I
I
I
I
r/~l
t
J
Comparison of the calculated and expected activities of 22 Na and S4Mn in the Dhajala meteorite
o
~~J
TABLE 5
HELIOCENTRIC HELPOGRAPHIC
20
40
t) D3
0
22Na
54Mn
111 82 35% +12%
144 85 69% +28%
22N°"~rOd}
,/0
ZOO I 3i00 4i0 1500 6/0 ?'0[0 8010 9/0 IO00 ` DAYS BEFORE IMPACT
I100 I ....
Fig. 2. The variation of heliocentric distance and heliographic and heliocentric latitude as a function of time calculated backwards from the time of impact. The orbital parameters given by Ballabh et al. [5] are used for these computations. The mean life of some of the isotopes measured here is marked.
have computed the orbital elements of the meteoroid based on eyewitness accounts of the meteor trail. It has been shown that the inclination of the orbit was 28 -+ 4°to the ecliptic. Their best choice of orbital parameters are: a = 1.8 A.U., e = 0.59, q = 0.74, i = 27.6 °, co'= 109.1 ° and g2 = 307.8 ° and the orbital period was about 2.5 years. The heliographic latitude and the heliocentric distance as a function of time before collision with the earth, computed on the basis of these orbital parameters, are shown in Fig. 2. From this figure it appears that the meteoroid spent a considerable fraction of its period off the ecliptic covering 20°N to 38°S heliolatitudes. The aphelion of the meteoroid is in the southern hemisphere and the highest heliographic latitude traversed is about 38°S. It is estimated that the meteoroid spent 80% and 75% of the mean life of S4Mn and 22Na, respectively, beyond heliolatitude 15 ° (Fig. 2). For shorter-lived isotopes like 32p, a3p, 7Be ' most of the observed activity was produced when the meteoroid was close to the ecliptic in the northern hemisphere. The meteoroid also covered 0 . 7 - 2 . 8 A.U. of heliocentric distance during its orbit. We first consider the increase of GCR fluxes with radial distance from the Sun. Two lines of evidence suggest that this increase is rather small. A comparison of several radioisotopes produced in lunar rocks and meteorites covering 1 - 4 A.U. showed close agreement with the GCR fluxes used by Reedy and Arnold [13]. This led Bhattacharya and Bhan-
Mean observed (dpm/kg) Expected at ecliptic a Excess Effect of solar minimum b ( 1 9 7 5 - 1 9 7 6 ) at ecliptic Radial gradient c (2.8%/A.U.) Residual excess Required increase in GCR flux at latitude > 15 ° d
+5% 15% 26% e
+4% 28% 40%e
a Based on GCR flux J (E > 1 GeV) = 1.7 protons/cm 2 • s 4rr. b Based on observed GCR fluxes since 1965 (Garcia-Munoz et al. [ 20,21 ] and references therein). c Mckibben et at. [22]. d Assuming that the production within +-15° heliolatitude is the same as at the ecliptic and higher at latitudes >15 ° . The irradiation periods in these two zones are based on Fig. 2. e This increase is in addition to the 38% enhancement in the ecliptic during solar minimum ( 1 9 7 5 - 1 9 7 6 ) .
dari [3] to conclude that the long-term (m.y.) radial gradient of cosmic rays ( > 1 0 0 MeV) are ~<+7%/A.U. Over the last few years Pioneer 10 and 11 have made direct observations of cosmic ray intensity gradient. The results [22] indicate that the gradient (E > 70 MeV) is 1.4 -+ 0.2%/A.U. during 1975-76 and about 3.5% during other periods. The aphelion of Dhajala was about 2.8 A.U. and thus this gradient can atmost contribute 5% to the production of S4Mn and 22Na. The data on all the radionuclides taken together can be understood if there is a higher flux of galactic cosmic rays at high heliolatitudes during a solar minimum. The data on other meteorites except Allende (i ~ 20 °) are consistent with the ecliptic fluxes determined from lunar samples (Table 4). The increase in the GCR fluxes should therefore occur beyond 15 or 20 ° heliolatitude. In view of the exponential build up of the isotope activity, it is hard to estimate the gradient in cosmic ray fluxes with latitude but a value of +30% above the ecliptic value may be consistent with the observations of 22Na and S4Mn (Table 5). Earlier estimates of the cosmic ray gradient perpendicular to the ecliptic are based on neutron monitor data but are confined to +7 ° [25], above and below the solar equatorial plane, as covered by the
201 inclination of Earth's orbit from September to March [23,24]. According to the analysis by Antonucci and Marocchi [25] of data from 1962 to 1972, the cosmic ray intensity (E > 10 GeV) is higher at higher latitudes at the time o f solar maximum, and the intensity minimum is obtained at the solar equatorial plane. The situation is reversed around solar minimum when the cosmic ray maximum is detected near the solar equatorial plane while minima exist at +7 ° latitude. There is also an asymmetrical component in the gradient across the ecliptic. When the two components are in phase, a gradient of the order of 20%/ A.U. has been deduced by them. The asymmetrical component alone gives a value of 2 - 8 % / A . U . as deduced by Dorman and Fischer [26] and Barker and Hatton [23]. With the Dhajala meteorite we are able to cover a larger heliolatitude band because of its higher inclination. Our observations indicate that the situation is significantly different at high heliolatitudes. The flux o f GCR particles is much higher at solar minimum at latitudes exceeding 1 5 - 2 0 ° S , compared to that at ecliptic. Such an increase in GCR fluxes is qualitatively expected from sunspot activity and coronal green line intensity which are less at high heliolatitudes, particularly in the south during solar minimum [27]. The observed activities of the twelve radionuclides covering 5.6 days to 3.7 m.y. half-life thus indicate that (1) the average GCR fluxes over a period of million years is same at heliolatitudes up to 40°S as near the ecliptic, and (2) during solar minima the GCR fluxes enhance by about 30% at 1 5 - 4 0 ° S over and above the 39% observed at the ecliptic. This would imply that during the solar maxima, the fluxes are correspondingly less and during the solar minima, the modulation of GCR fluxes is very small beyond 15°S. The second conclusion is valid even if the meteoroid orbit has altered in the past. In conclusion, the data indicate that the magnitude of GCR modulation with solar cycle b e y o n d 15°S results in a higher flux by +70% as compared to -+40% observed near the ecliptic.
Acknowledgements We are grateful to Professor D. Lal for taking an active interest and to Drs. G. Castagnoli and G. Sub-
ramaniam for discussions. We thank Dr. C.P. Kohl for 7Be and s 1Cr measurements which were carried out in Professor J.R. Arnold's laboratory at the University o f California, San Diego. Our appreciation is due to Dr. R. Finkel, Dr. G.M. Ballabh, Shri J.T. Padia, Professor S. Krishnaswami, Shri M.M. Sarin and Shri J.R. Trivedi for their help at various stages of this work.
References 1 A.K. Lavrukhina and G.K. Utsinova, Cosmogenic radionuclides in stones and meteorite orbits, Earth Planet. Sci. Lett. 15 (1972) 347. 2 E.L. Fireman, Radioactivies in meteorites and cosmic ray variations, Geochim. Cosmochim. Acta 31 (1967) 1691. 3 S.K. Bhattacharya and N. Bhandari, Modulation of galactic cosmic rays from the study of cosmogenic radioisotopes in meteorites, Proc. Symp. Sol. Planet. Phys., January, Ahmedabad, 2 (1976) 161. 4 N. Bhandari, D. Lal, J.R. Trivedi and A. Bhatnagar, The Dhajala meteorite shower, Meteoritics 11 (1976) 137. 5 G.M. Ballabh, A. Bhatnagar and N. Bhandari, The Orbit of the Dhajala meteorite, Icarus 33 (1978) 361. 6 D."Lal and J.R. Trivedi, Observations on the spatial distribution of Dhajala meteorite fragments in the strewn field, Proc. Ind. Acad. Sci. 86A (1977) 393. 7 K. Gopalan, M.N. Rao, K.M. Suthar and T.R. Venkatesan, Cosmogenic and radiogenic noble gases in the Dhajala chondrite, Earth Planet. Sci. Lett. 36 (1977) 341. 8 C. Bagolia, N. Doshi, S.K. Gupta, S. Kumar, D. Lal and J.R. Trivedi, The Dhajala meteorite shower: atmospheric fragmentation and ablation based on cosmic ray track studies, Nucl. Track Detect. 1 (1977), 83. 9 N. Bhandari, S.K. Bhattacharya, S. Krishnaswami and B.L.K. Somayajulu, Variation of cosmogenic radioactivity in Dhajala fragments, Meteoritics 11 (1976) 250. 10 N. Bhandari and H.R. Prabhakara, Cosmic ray records in some meteorites, Space Sci. Symp. Waltair, India, (1978) abstract. 11 R. Finkel, Depth profile of galactic cosmic ray produced radionuclide in lunar samples, Ph.D. Thesis, University of California, San Diego, Calif. (1972). 12 S.K. Bhattacharya, J.N. Goswami and D. Lal, Semi-empirical rates of formation of cosmic ray tracks in spherical objects exposed in space: preatmospheric and post-atmospheric depth profiles, J. Geophys. Res. 78 (1973) 8356. 13 R.C. Reedy and J.R. Arnold, Interaction of solar and galactic cosmic ray particles with the moon, J. Geophys. Res. 77 (1972) 537. 14 T.P. Kohman and M. Bender, Nuclear production by cosmic rays in meteorites and on the moon, in: High Energy Nuclear Reactions in Astrophysics, B.S.P. Shen, ed. (1967) 169.
202 15 B.M.P. Trivedi and P.S. Goel, Nuclide production rates in stone meteorites and lunar samples by galactic cosmic radiation, J. Geophys. Res. 78 (1973) 4885. 16 A.F. Noonan, K. Frederiksson, E. Jerosewich and P. Brenner, Mineraology and bulk, chondrule, size-fraction chemistry, Meteoritics 11 (1976) 340. 17 C.P. Kohl, Galactic cosmic ray produced radioactivity in lunar and chondritic materials, Ph.D. Thesis, University of California, San Diego, Calif. (1975). 18 K. Marti, J.P. Shedlovsky, R.M. Lindstrom, J.R. Arnold and N.G. Bhandari, Cosmic ray produced radionuclides and rare gases near the surface of St. S~v~rin meteorite, in: Meteorite Research, P. Millman, ed. (1969) 275. 19 P.J. Cressy, Multiparameter analysis of gamma radiation from the Barwell, St. S~v~rin and Tatlith meteorites, Geochim. Cosmochim. Acta 34 (1970) 771. 20 M. Garcia-Munoz, G.M. Mason and J.A. Simpson, New aspects of the cosmic ray modulation in 1974-1975 near solar minimum, Ap. J. 213 (1977) 263. 21 M. Garcia-Munoz, G.M. Mason and J.A. Simpson, The appearances of superfluxes of quiet time cosmic rays, Proc. 15th Int. Conf. Cosmic Rays 3 (1977) 209. 22 R.B. Mckibben, J.J. O'Gallagher, K.R. Pyle and J.A. Simpson, Cosmic ray intensity gradients in the outer solar system measured by Pioneer 10 and 11, Proc. 15th Int. Conf. Cosmic Rays 3 (1977) 240. 23 M.C. Barker and C.J. Hatton, Evidence for a cosmic ray gradient perpendicular to the solar equatorial plane, Planet. Space Sci. 19 (1971) 549. 24 A. Hashim and M. Barcovitch, A cosmic ray density gradient perpendicular to the ecliptic, Planet. Space Sci. 20 (1972) 791. 25 E. Antonucci and D. Marocchi, Cosmic ray perpendicular gradient during 1962-1972, J. Geophys. Res. 81 (1976) 4627. 26 L.I. Dorman and S. Fischer, Influence of the changes in the earth's heliolatitude on the cosmic ray intensity in 1963-64 and evaluation of the cosmic ray intensity gradient perpendicular to the ecliptic plane, Proc. 9th Int. Conf. Cosmic Rays (1966) 239.
27 L.I. Dorman and R.T. Gushchina, Relation of the 11 year and annual cosmic ray variations to the heliolatitude index of solar activity according to the spots and the green coronal line, Proc. 15th. Int. Conf. Cosmic Rays 3 (1977) 263. 28 G. Spannagel and C. Sonntag, Cosmic ray produced activities in chondrites, in: Radioactive Dating and Methods of Low Level Counting (IAEA, Vienna, 1967) 231. 29 J. Tobailem, B. David and D. Nordemann, Radioactivit6 induite par le rayonnement cosmique dans la m6t~orite Saint S~v~rin, in: Radioactive Dating and Methods of Low Level Counting (IAEA, Vienna, 1967) 207. 30 L.A. Rancitelli, R.W. Perkins, J.A. Cooper, J.H. Kaye and N.A. Wogman, Radionuclide composition of the Allende meteorite from nondestructive gamma-ray spectrometric analysis, Science 166 (1969) 1269. 31 P.J. Cressy, Jr., Cosmogenic radionuelides in the Lost City and Ucera meteorites, J. Geophys. Res. 76 (1971) 4072. 32 D.D. Bogard, R.S. Clarke, J.E. Keith and M.A. Reynolds, Noble gases and radionuclides in Lost City and other recently fallen meteorites, J. Geophys. Res. 76 (1971) 4076. 33 R.E. McCrosky, A. Posen, G. Schwartz and C.Y. Shao, Lost City meteorite, its recovery and a comparison with other fireballs, Smithsonian Astrophys. Lab. Spec. Rep. 336 (1971). 34 M. Wahlen, M. Honda, M. Imamura, J.S. Fruchter, R.C. Finkel, C.P. Kohl, J.R. Arnold and R.C. Reedy, Cosmogenic nuclides in football size rocks, Proc. 3rd Lunar Sci. Conf. (1972) 1719. 35 D. Nordemann, J. Tobailem, C.H.L. St.-Genies, La m6t6orite Saint S6v6rin recherche de la trajectoire atmospherique et de l'orbite, CEA-R-4045 Rapp. Paris (1970). 36 R.E. McCrosky, A. Posen, G. Schwartz and G.A. Tougas, Preliminary comments on the trajectory, orbit, and initial mass of the Allende Meteorite. EOS, Trans. Am. Geophys. Union 50 (1969) 458.
Earth and Planetary Science Letters, 40 (1978) 194-202 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[6]
COSMOGENIC RADIOISOTOPES IN THE DHAJALA CHONDRITE: IMPLICATIONS TO VARIATIONS OF COSMIC RAY FLUXES IN THE INTERPLANETARY SPACE N. BHANDARI, S.K. BHATTACHARYA and B.L.K. SOMAYAJULU Physical Research Laboratory, Ahmedabad (India)
Received January 17, 1978 Revised version received April 10, 1978
Activities of a suite of radioisotopes ranging in half-life from 5.6 days (S2Mn) to 3.7 m.y. (S3Mn) have been measured in the Dhajala chondrite. The results show that all the radioactivities are close to the expected levels except 54Mn and 22Na. Their activities are higher than those based on the interplanetary fluxes at 1 A.U. near the ecliptic, expected immediately before the fall of Dhajala, corresponding to the time of solar minimum. Furthermore, activity ratios of S4Mn/saMn and 22Na/26A1 are higher by 30-50% than expected. The departure from the expected values is discussed in terms of spatial variations of cosmic rays based on the computed orbital parameters of the meteoroid. If the galactic cosmic ray fluxes in the equatorial region (+15 °) are assumed to be the same as in the ecliptic plane then these results suggest higher fluxes by 33 -+7% at heliographic latitudes 15-40°S, during solar minimum.
1. Introduction The production rate, and hence the activity, of a radioisotope in a meteorite resulting from the nuclear interactions of cosmic rays is proportional to its flux along the orbital segment of the meteoroid covered mainly during a mean life o f the radioisotope. Radioisotopes with different half-lives, therefore, offer a time parameter which can be used for evaluation of the temporal and spatial variations of cosmic rays along the meteoroid orbit. Based on comparison of such measurements in meteorites and lunar samples, some aspects of solar and galactic cosmic ray characteristics in the past, their variation with solar cycle and with heliocentric distance in the solar system have been evaluated [ 1 - 3 ]. In this context the Dhajala meteorite [4] is of special interest in view of the fact that (1) its orbital parameters have been computed reasonably well [5], (2) it fell at the time of solar minimum, and (3) it was recovered immediately after the fall, making measurements on short-lived isotopes possible. The Dhajala meteorite fell on 28 January 1976 at about 8.40 p.m. Indian Standard Time in Gujarat,
India. The phenomena associated with the fall of the meteorite, field observations and meteorite characteristics have been described earlier [4,6]. The first few fragments o f the meteorite arrived in our laboratory within a day o f the fall. Immediately afterwards a variety of investigations on rare gas isotopes [7], cosmic ray tracks [8] and chemical and mineralogical compositions were carried out. Based on these studies, the meteorite is classified as a H3 bronzite chondrite having a cosmic ray exposure age o f about 6.2 m.y. Systematic measurements of several radioisotopes ranging in half-life from 5.6 days (S2Mn) to 3.7 m.y. (S3Mn) have been carried out. Some preliminary data were reported earlier [4,9]. The final results are presented here and discussed in terms of the characterlstics of galactic cosmic rays in the interplanetary space.
2. Experimental details Radioisotopes 26A1, S4Mn and 22Na were measured in a few fragments using a non-destructive gamma-ray spectrometer (Table 1). In three selected fragments,
195 T-1 l, T-67 and T-68, several radioisotopes have been measured after radiochemical separation allowing us to determine the activity of 7Be, l°Be, 22Na, 26A1, 32p, 33p, 51Cr' ~;2Mn' SaMn' S4Mn' SVCo and 6°Co (Table 2). The gammay-ray analysis was performed using a 7.5 cm X 7.5 cm NaI(T1) crystal. The system has been described earli.er [3]. Clear signals were obtained at 511,835, 1460 and 1810 keV and minor peaks due to 6°Co were also seen. The 1460-keV signal due to 4°K was used as a relative internal standard for estimating the effective mass contributing to the signals. It has been shown earlier that this procedure is valid in the mass range of 1 15 kg [10]. After Compton correction due to the 1810-keV signal, the ratio of the 1810-keV signal to tile 1460-keV signal was calculated for each fragment and for the meteorite Bansur where 26A1 is known to be 50 -+ 5 dpm/kg [3]. Z6AI dpm in each fragment was then obtained directly from comparison of this ratio. Due corrections for the potassium content of the meteorites (Dhajala = 840 ppm, Bansur = 870 ppm) were taken into account. In the same way 22Na was calculated from tile 511-keV signal, after subtracting the 26A1 contribution. The S4Mn activity was determined from the 835-keV signal using the same procedure. The average of the ratios of the 835-keV signal to the 1460-keV signal were taken to correspond to 144 dpm/kg, the value measured in T-11. For radiochemical investigation, a sample (T-11) of 350 g was crushed to <230/lm (60 mesh) and 320 g of this sample was dissolved using HF, HNO3 and HC1 having preserved the remaining 30 g for mass spectrometric, fossil track and chemical studies. About 45 g remained undissolved, which was fused with Na2CO3 and the melt was dissolved and mixed to the main solution. An aliquot was taken for the chemical estimation of elements and carriers of Be (100 mg -= BeO), P (40 mg ~ Mg2P2OT) and Co (100 rag) were added and equilibrated by vigorous boiling. Standard chemical procedures were followed for separation and radiochemical purification of the elements. Briefly, Fe was removed by ether extraction in 9M ttC1. Aluminium, Na and Mn were precipitated as chlorides by Gooch-Haven's method. Manganese was separated as MnO2 in a HNO3-NaBrO3 medium. Hydroxide precipitates in a Na202-NaOH medium
enabled A1, Be, Cr and P to be removed from Fe, Ni and Co. Final separations were done on ion exchange columns [11]. The radiochemical purification was performed using characteristic precipitations or ion exchange methods. Table 2 lists the final forms in which samples were counted. 13 g of another sample (T-67) was also processed for 26A1 and 14 g of T-68 for 1°Be and 26A1 after extracting argon for agAr analysis, which is in progress. In addition to the counting systems described earlier, proportional and GM counters were used for estimating the radioactivity of X-ray emitters (S3Mn in sample T-11) and beta emitters (32p, 33p and 1°Be). S2Mn, 26Al and 6°Co were counted on beta-gamma coincidence detectors and were identified by their characteristic gamma radiations. S2Mn and a2p were additionally confirmed by their characteristic decay rates. Absorption measurements over several weeks made on the phosphorus sample enabled us to resolve the a2p and aap signals.
3. Results and discussions Eight fragments have been studied in the present work. The results of non-destructive gamma counting of various fragments are given in Table 1. Extreme variations of 1.31 for 22Na, 1.38 for 26A1 and 1.24 for S4Mn have been observed. There is no systematic change in radioactivity or tracks in a fragment with its location in the strewn field. One of these fragments, T-68, was also analysed for radionuclides 1°Be and 26A1 after radiochemical purification. In addition, two other fragments, T-11 and T-67, were also radiochemically studied as described earlier. These results are shown in Table 2. There is a general agreement between repeated measurements on the same fragment by different methods.
3.1. Shielding depths Spot samples from all the fragments studied here were also analyzed for cosmic ray tracks [8]. From these data, using the exposure age of 6.2 m.y. [7] and the production rates given by Bhattacharya et al. [12], we obtain the shielding depth of each fragment within the meteoroid body during its exposure to
196 TABLE 1 26A1, S4Mn, 22Na in fragments of the Dhajala meteorite Sample
Weight (kg)
T-67 G-1 T-78 T-16 T-17 T-92
1.5 2.39 1.63 2.02 2.05 6.5
Distance along the fallout track * (km)
Track density ** (cm-2)
Shielding depth
Activity (dpm/kg) ***
(cm)
26A1
S4Mn
22N a
8 17 15 14 9 17
2.3 6.7 1.4 3 3 3
23 18 13 12 12 12
52-+5 62-+6 48-+5 55 -+ 5 45-+5 53 -+ 5
123-+ 8 152 -+ 10 150 -+ 10
96-+10 120 -+ 10 126 -+ 12
53
144
111
× × × × × ×
103 10 a 104 104 104 104
Average * Bhandari et al. [4], Lal and Trivedi [6]. ** From Bagolia et al. [8]. *** Errors are l o counting statistics. c o s m i c rays. T h e t r a c k d e n s i t i e s f o r s o m e f r a g m e n t s
r e s p e c t i v e l y . All t h e o t h e r s a m p l e s , listed in T a b l e 1,
are given in T a b l e 1. In T-11 a n d T-68 t h e t r a c k d e n sities in olivines w e r e 4 × 104 c m -2 a n d 1.5 X 104
h a d s h i e l d i n g d e p t h s o f 1 2 - 2 3 c m . Since m o s t o f t h e fragments analysed here were tens of centimeters, s p o t values o f d e p t h s m a y n o t b e r e p r e s e n t a t i v e f o r
c m -2 l e a d i n g t o s h i e l d i n g d e p t h s o f 11 a n d 13 c m , TABLE 2 Chemical and counting details of samples Radionuclide
Elemental abundance (%)
Chemical form
Amount expected (g)
Amount counted (g)
Detection mode
Counting efficiency (ideal)
Background (cpm)
Signal (cpm)
Activity at the time of fall (dpm/kg)
BeO BeO BeO A1203 Mg2P207 Mg2P207 Cr203 MnO2 MnO2 MnO2 Co203 Co203
0.111 0.111 0.111 6.954 1.449 1.449 1.006 1.256 1.256 1.256 0.321 0.321
0.026 0.026 0.050 0.148 0.414 0.414 0.392 0.456 0.295 0.321 0.187 0.194
3" /313/3+"r /3t33' /3÷-~ X-ray "r X-ray #--'r
0.094 0.27 0.24 0.075 0.36 0.15 0.122 0.085 0.027 0.148 0.033 0.040
0.015 0.138 0.033 0.0086 0.1 0.026 0.010 0.24 2.27 0.2 0.0027
0.046 0.30 0.07 0.017 0.51 0.04 0.062 0.012 0.24 2.61 0.04 0.016
132 16.0 17.3 45 22 7.2 51 28 76 144 27 7.3
BeO
A12O3
0.111 0.646
0.025 0.266
/3/3÷-3'
0.24 0.075
0.036 0.0086
0.040 0.038
18 61
-+ 2 -+ 7
A1203
0.246
0.088
/3÷-0'
0.075
0.0086
0.016
56
-+ 12
Sample T-11 7Be * 10Be 1°Be ** 26A1 a2p *** 33p *** SlCr * S2Mn SaMn S4Mn STCo 6°Co
1.15 0.126 0.126 0.215 0.248 0.248 0.248 0.04 0.04
-+ 12 -+ 1.5 -+ 4.3 -+ 9 -+ 1.7 -+ 2.4 -+ 14 ± 4 -+ 9 -+ 8 -+ 8 -+ 1
Sample T-68 1°Be 26A1
1.15
Sample T-6 7 26A1
1.15
* These samples were measured by Dr. C.P. Kohl. ** Duplicate sample. *** 32p and a3p were calculated by BiUer's plot.
197 the whole sample. It is, therefore, not possible to construct a depth profile of the radioactive isotopes from the measurements given in Tables 1 and 2. For the discussion of the average observed activity of the various radionuclides we shall, therefore, compare them with the mean values expected at depths between 10 and 30 cm.
3.2. Expected activity of radionuclides To understand the observed activities and their variations in terms of cosmic ray intensity and shielding by overlying material, we compare them with the calculated profiles. These are not known well for all the isotopes studied here. Lavrukhina and Utsinova [1] and Reedy and Arnold [13] have made calculations of the isotope production in chondrites and Moon, respectively. In addition, Kohman and Bender [14] have calculated the rate of production of radionuclides in iron meteorites. Trivedi and Goel [ 15], following the procedure of Kohman and Bender, have given production rates of 2 2Na and 3H in spherical chondrites of various radii. The expected activities of various other nuclides like S4Mn, S3Mn, 26A1 were
calculated from an extension of the Reedy-Arnold method to meteorites. The Reedy-Arnold parameters were obtained from the 22Na profile given by Trivedi and Goel and will be discussed elsewhere. Here we only mention that the observed ratios of 22Na/26A1 and S4Mn/SaMn in various meteorites range between 1.2 to 1.6 and 0.80 to 1.14, respectively, and agree well with our calculations of production ratios for meteorites of different radii. The calculations, shown in Fig. 1, are based on the composition of the Dhajala meteorite as given by Noonan et al. [16]. It is seen that, for S3Mn, S4Mn, 22Na and 26A1, the production rate increases with depth towards the center of the body, provided the radius of the meteoroid is less than 30 cm. In a 40 or 50-cm chondrite, after a depth of about 10 cm the variation of isotope production with depth is small, within about 10% over the mean value. For larger bodies the isotope production shows a well-defined maximum at about 15 cm which shifts to shallower depths as the radius of the meteoroid increases. After this broad maximum the isotope production decreases slowly towards the center of the body. In the following we use these profiles to determine the preatmospheric size of Dhajala.
3.3. Comparison with calculation 180
I
I GO
I
E
i
T
[
,
I
'
12(3-
'
I
~
I
'
O 22No O 26 AI • 53Mn • 54Mn
/
140
E c.
I
~
'00--80
5
54Mn
22N0
4~
m 26AI
20 0
0
~ I L I
I0
20
t I
30
~oocm26Al I
I
4,0
t I I [
DEPTH
50
60
t [
70
I
I L
80
90
(cm)
Fig. 1. Depth variation o f the calculated activities o f S4Mn, SaMn, 26A1 and 22Na in a spherical bronzite o f 50 cm radius. s 3Mn profile takes into account undersaturation during 6.2 m.y. exposure. The 26A1 profile is also given for a sphere of 40 and 100 cm radius for comparison. The observed activities in various fragments are shown.
The observed average values for the four isotopes are S3Mn = 76 dpm/kg, 26A1 = 53 dpm/kg, S4Mn = 144 dpm/kg and 22Na = 111 dpm/kg and they differ from the mean calculated values by +13%, -2%, +69% and +35% respectively (Table 3). The levels of the short-lived activities like 22Na and SaMn are affected by the solar cycle variations in the galactic cosmic ray (GCR) fluxes, which have not been taken into account in the calculations given above; these will be discussed later. The 26A1 activity, representing the average fluxes, indicates that its observed activity levels are similar to those expected for a body of about 50 cm radius. For smaller bodies the activity levels of 26A1 and S3Mn will be much higher than those observed. The variation of observed 26A1 activity in different fragments also suggests that the preatmospheric radius of the meteoroid was 50 cm. This estimate is slightly higher than the results of Bagolia et al. [8] who, based on an extensive study of cosmic ray tracks, give a value of--~ 40 cm. The longest-lived cosmogenic isotope measured
198 TABLE 3 Cosmogenic radioactivities in fragments of the Dhajala meteorite Isotope SaMn l°Be 26A1 60Co 22Na S4Mn STCo 7Be 51Cr 33p 32p S2Mn
Half-life 3.7X106 years 1.6X106 years 7.4 X l0 s years 5.2 years 2.6 years 312 days 271 days 53.3 days 27.7 days 25.3 days 14.2 days 5.6 days
T-11 (dpm/kg) 76 16 45
-+ 9 -+ 1.5 +- 9
7.3 + 1 144 ± 8 27 ± 8 132 + 12 5l ± 14 7.2 ± 2.4 22 ± 1.7 28 ± 4
T-67 (dpm/kg) 52 ± 5, 56 -+ 12 96 ± 10 123 ± 8 -
T-68 (dpm/kg) -
18 -+2 61 ± 7
Expected * activity (dpm/kg) 67 16 52
82 85
* Mean activity at depths between 10 and 30 cm for a 50-cm bronzite, based on the average GCR flux J (E > 1 GeV) = 1.7 protons/cm 2 • s - 4n.
here is S3Mn (half-life = 3.7 m.y.). In view of the 6.2m.y. exposure age of this meteorite, it should attain 69% of its secular equilibrium value. The observed S3Mn activity of 76 + 9 dpm/kg leads to a production rate of 405 + 50 atoms/min • kg Fe. The calculated mean production rate is 360 atoms/min • kg Fe. The 1°Be (half-life = 1.6 m.y.) should acquire 93% of its saturation level. Its activity of 17 + 1 dpm/kg is similar to the activities found in other stony meteorites and also similar to the calculated value. The same is tile case with 26A1 whose activity was found to range from 45 to 62 dpm/kg as discussed earlier. For short-lived isotopes, the modulation of the GCR fluxes due to solar cycle has to be considered.
3.4. Solar cycle effect The magnitude of the solar cycle effect can be easily estimated from the shorter-lived isotopes with a mean life g 1 year, by comparing their observed activities in different meteorites, after considering the target element abundances, preatmospheric size of the meteorites and shielding effect. 7Be, mainly produced from oxygen, does not depend on the type of meteorite and can be ideally used for estimating cosmic ray fluxes during different phases of the solar cycle, lts value of 132 -+. 12 dpm/kg in T-11 is 14%
higher than 116 + 12 dpm/kg for the Canon City meteorite [ 17] which fell in 1973 during solar maximum. These measurements suggest that the solar cycle effect is atmost +20% over the mean. Fireman [2] considered 22Na and came to a similar conclusion that its production does not vary by more than +15% from the mean value. For other isotopes the production rates have not been calculated. Considering the uncertainty of the shielding effects the activities of 3ap (7.2 + 2.6) and 32p (22.1 + 1.7) dpm/kg compare favourably with 18 + 9 and 14 + 5 dpm/kg found in the St. Severin meteorite, which also fell at the time of solar minimum in 1966 [18]. The 27-day S~Cr has an activity of 51 -+ 14 dpm/kg which is also similar to the expected values. The 5.6-day S2Mn shows an activity of 103 + 15 dpm/kg Fe which is much lower than the value 172 -+ 36 dpm/kg Fe in St. Severin [19]. This may partially be due to size effects, but the production rates are not known. The measured proton fluxes [20,21 ] indicate that J (GCR fluxes >1 GeV) for 1975 was 38% higher than the mean fluxes (1.7 proton/cm 2 • s • 4rr) used to calculate the production rates given in Fig. 1. Considering the measured GCR fluxes during previous years we estimate that the solar cycle effect will result in a 28% and 12% higher production for S4Mn and 22Na, respectively. In summary, the above dis-
199 cussion shows that the m a g n i t u d e o f the solar cycle effect is t o o small to a c c o u n t for the observed excess ~ctivities o f 22Na and S4Mn.
3.5. Excess activity of 22Na and S4Mn Many uncertainties in calculations can be r e m o v e d if we consider the ratio o f the activities o f isotopes, p r o d u c e d in similar reactions, rather than the individual radionuclides. T.he 22Na/26A1 (observed production ratio) in Dhajala is 2.1 and S4Mn/S3Mn = 1.3. These are higher than those r e p o r t e d in any o t h e r m e t e o r i t e where 22Na/26A1 is usually b e t w e e n 1.2 and 1.6 and 54Mn/S3Mn is around 1.0. The excess o f 22Na and S4Mn by about 20% and 30% over and above the solar m i n i m u m effect, therefore, appears to be genuine. A comparison of the observed activities o f S4Mn and 22Na with o t h e r m e t e o r i t e s is made in Table 4. Since S4Mn is almost totally p r o d u c e d f r o m Fe, a direct c o m p a r i s o n o f activity per gram of target elem e n t is possible if we ignore the shielding effects for the different meteorites. Table 4 shows that Dhajala has the highest S4Mn d p m / k g Fe value observed. F u r t h e r m o r e , the activity does n o t correlate inversely
with the solar cycle b u t seems to depend on the orbital inclination o f the meteorites. This p o i n t is discussed later. F r o m the above discussion, it is c o n c l u d e d that the long-term (m.y.) average G C R flux in the interplanetary space encompassed by the Dhajala orbit has been the same as around the Moon. The activity o f shorterlived isotopes ( 5 - 5 3 days) also seems to be consistent with the same flux, although for some o f t h e m the calculations are n o t reliable, nor do adequate observations exist in other meteorites or the M o o n to allow an accurate comparison. The solar cycle effect from minima to m a x i m a is generally within -+20% of the mean flux a d o p t e d here. For the isotopes 54Mn and 22Na with half-lives o f around a year, however, there seems to be a clear excess above the values e x p e c t e d from the G C R fluxes at the time o f solar m i n i m u m near the ecliptic.
3.5. Spatial variation of galactic cosmic ray fluxes To explain the excess activity o f S4Mn and 22Na we n o w consider the possibility of an abnormally high G C R flux over the space segment o f the Dhajala orbit during the last few years. Ballabh et al. [5]
TABLE 4 Comparison of 54Mn and 22Na radioactivity with solar cycle and orbital inclination in different meteorites and a Moon rock
Type Date of fall V~ (km/s) Inclination (°) Shielding depth (cm) 54Mn (dpm/kg)
St. Severin
Allende
Murchison
Lost City
Ucera
Dhajala
Moon rock 14321
LL6 27/6/1966
C3 21/2/1969
C2 28/9/1969
H5 3/1/1970
H5 16/1/1970
H3 28/1/1976
5/2/1971
14 a ~1 a 2-5 e
20 b >~20 e
_ ~>10 e
81 ± 2 g
99-128 h (-+4) 23.5 503 69-81 (52) h
83 ± 8 i 101 ± 2 j 25.43 361 80±8 i
65 ± 6 i 85 ± 4 J 27.63 270 80±8 i
max (1.64)
max (1.52)
max (1.47)
Fe content (%) 19.2 S4Mn (dpm/kg Fe) 427 22Na(dpm/kg) 72-108g,m,n,o Solar activity (J) * rain (2.01)
14.2 c 12 c 5-12 e
_ _ ~20 e 61 ± 4 J
21 d 28 d 11-23 f
_ 0 13.3
144 ± 8 k
27 -+5 1
26.43 230 60±4J
27.2 525 111 ±6 k
10.5 260 3254 1
max (1.47)
min (2.34)
max (1.14)
* J is the average GCR flux (E > 1 GeV) over one year previous to the fall (protons/cm 2 • s - 4n). a Nordemann et al. [35]. b McCrosky et al. [36]. c McCrosky et al. [33]. d Ballabh et al. [5]. e Bhandari et al. (in preparation) f Bagolia e t al. [8]. g Cressy [19]. h Rancitelli et al. [30]. i Bogard et al. [32]. J Cressy [31]. k This work. 1Wahlen e t al. [34]. m Fireman [2]. n Tobailem et al. [29]. o Spannagel and Sonntag [28].
200
N2 o ~ 40
I
I
I
f
I
I
I
I
r/~l
t
J
Comparison of the calculated and expected activities of 22 Na and S4Mn in the Dhajala meteorite
o
~~J
TABLE 5
HELIOCENTRIC HELPOGRAPHIC
20
40
t) D3
0
22Na
54Mn
111 82 35% +12%
144 85 69% +28%
22N°"~rOd}
,/0
ZOO I 3i00 4i0 1500 6/0 ?'0[0 8010 9/0 IO00 ` DAYS BEFORE IMPACT
I100 I ....
Fig. 2. The variation of heliocentric distance and heliographic and heliocentric latitude as a function of time calculated backwards from the time of impact. The orbital parameters given by Ballabh et al. [5] are used for these computations. The mean life of some of the isotopes measured here is marked.
have computed the orbital elements of the meteoroid based on eyewitness accounts of the meteor trail. It has been shown that the inclination of the orbit was 28 -+ 4°to the ecliptic. Their best choice of orbital parameters are: a = 1.8 A.U., e = 0.59, q = 0.74, i = 27.6 °, co'= 109.1 ° and g2 = 307.8 ° and the orbital period was about 2.5 years. The heliographic latitude and the heliocentric distance as a function of time before collision with the earth, computed on the basis of these orbital parameters, are shown in Fig. 2. From this figure it appears that the meteoroid spent a considerable fraction of its period off the ecliptic covering 20°N to 38°S heliolatitudes. The aphelion of the meteoroid is in the southern hemisphere and the highest heliographic latitude traversed is about 38°S. It is estimated that the meteoroid spent 80% and 75% of the mean life of S4Mn and 22Na, respectively, beyond heliolatitude 15 ° (Fig. 2). For shorter-lived isotopes like 32p, a3p, 7Be ' most of the observed activity was produced when the meteoroid was close to the ecliptic in the northern hemisphere. The meteoroid also covered 0 . 7 - 2 . 8 A.U. of heliocentric distance during its orbit. We first consider the increase of GCR fluxes with radial distance from the Sun. Two lines of evidence suggest that this increase is rather small. A comparison of several radioisotopes produced in lunar rocks and meteorites covering 1 - 4 A.U. showed close agreement with the GCR fluxes used by Reedy and Arnold [13]. This led Bhattacharya and Bhan-
Mean observed (dpm/kg) Expected at ecliptic a Excess Effect of solar minimum b ( 1 9 7 5 - 1 9 7 6 ) at ecliptic Radial gradient c (2.8%/A.U.) Residual excess Required increase in GCR flux at latitude > 15 ° d
+5% 15% 26% e
+4% 28% 40%e
a Based on GCR flux J (E > 1 GeV) = 1.7 protons/cm 2 • s 4rr. b Based on observed GCR fluxes since 1965 (Garcia-Munoz et al. [ 20,21 ] and references therein). c Mckibben et at. [22]. d Assuming that the production within +-15° heliolatitude is the same as at the ecliptic and higher at latitudes >15 ° . The irradiation periods in these two zones are based on Fig. 2. e This increase is in addition to the 38% enhancement in the ecliptic during solar minimum ( 1 9 7 5 - 1 9 7 6 ) .
dari [3] to conclude that the long-term (m.y.) radial gradient of cosmic rays ( > 1 0 0 MeV) are ~<+7%/A.U. Over the last few years Pioneer 10 and 11 have made direct observations of cosmic ray intensity gradient. The results [22] indicate that the gradient (E > 70 MeV) is 1.4 -+ 0.2%/A.U. during 1975-76 and about 3.5% during other periods. The aphelion of Dhajala was about 2.8 A.U. and thus this gradient can atmost contribute 5% to the production of S4Mn and 22Na. The data on all the radionuclides taken together can be understood if there is a higher flux of galactic cosmic rays at high heliolatitudes during a solar minimum. The data on other meteorites except Allende (i ~ 20 °) are consistent with the ecliptic fluxes determined from lunar samples (Table 4). The increase in the GCR fluxes should therefore occur beyond 15 or 20 ° heliolatitude. In view of the exponential build up of the isotope activity, it is hard to estimate the gradient in cosmic ray fluxes with latitude but a value of +30% above the ecliptic value may be consistent with the observations of 22Na and S4Mn (Table 5). Earlier estimates of the cosmic ray gradient perpendicular to the ecliptic are based on neutron monitor data but are confined to +7 ° [25], above and below the solar equatorial plane, as covered by the
201 inclination of Earth's orbit from September to March [23,24]. According to the analysis by Antonucci and Marocchi [25] of data from 1962 to 1972, the cosmic ray intensity (E > 10 GeV) is higher at higher latitudes at the time o f solar maximum, and the intensity minimum is obtained at the solar equatorial plane. The situation is reversed around solar minimum when the cosmic ray maximum is detected near the solar equatorial plane while minima exist at +7 ° latitude. There is also an asymmetrical component in the gradient across the ecliptic. When the two components are in phase, a gradient of the order of 20%/ A.U. has been deduced by them. The asymmetrical component alone gives a value of 2 - 8 % / A . U . as deduced by Dorman and Fischer [26] and Barker and Hatton [23]. With the Dhajala meteorite we are able to cover a larger heliolatitude band because of its higher inclination. Our observations indicate that the situation is significantly different at high heliolatitudes. The flux o f GCR particles is much higher at solar minimum at latitudes exceeding 1 5 - 2 0 ° S , compared to that at ecliptic. Such an increase in GCR fluxes is qualitatively expected from sunspot activity and coronal green line intensity which are less at high heliolatitudes, particularly in the south during solar minimum [27]. The observed activities of the twelve radionuclides covering 5.6 days to 3.7 m.y. half-life thus indicate that (1) the average GCR fluxes over a period of million years is same at heliolatitudes up to 40°S as near the ecliptic, and (2) during solar minima the GCR fluxes enhance by about 30% at 1 5 - 4 0 ° S over and above the 39% observed at the ecliptic. This would imply that during the solar maxima, the fluxes are correspondingly less and during the solar minima, the modulation of GCR fluxes is very small beyond 15°S. The second conclusion is valid even if the meteoroid orbit has altered in the past. In conclusion, the data indicate that the magnitude of GCR modulation with solar cycle b e y o n d 15°S results in a higher flux by +70% as compared to -+40% observed near the ecliptic.
Acknowledgements We are grateful to Professor D. Lal for taking an active interest and to Drs. G. Castagnoli and G. Sub-
ramaniam for discussions. We thank Dr. C.P. Kohl for 7Be and s 1Cr measurements which were carried out in Professor J.R. Arnold's laboratory at the University o f California, San Diego. Our appreciation is due to Dr. R. Finkel, Dr. G.M. Ballabh, Shri J.T. Padia, Professor S. Krishnaswami, Shri M.M. Sarin and Shri J.R. Trivedi for their help at various stages of this work.
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