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1.
Abstract— Core samples were obtained from various locations of the ～ 105-kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic-ray-produced nuclides. Relations between measured abundances of cosmogenic nuclides (10Be, 26Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic 22Ne/21Ne ratio were determined and compared with recent model predictions of production rates. The measured 22Ne/21Ne ratios (1.06-1.08) and significant variations observed in concentrations of cosmogenic 21Ne and 3He suggest an ～40-cm shielding gradient across Chico and irradiation within a large object (> 100-cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of 10Be (average = 20.7 dpm/kg) and 26Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either 22Ne/21Ne or 21Ne. Activities of 10Be and 26Al suggest irradiation in a smaller object (～40–80 cm radius). The 26Al activity and the 26Al/10Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two-stage irradiation with t1 ～ 104 Ma and t2 ～ 4 Ma and a second-stage body the size of Knyahinya. The single stage, 10Be/21Ne exposure age for Chico is 65 Ma. The 22Ne/21Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with 22Ne/21Ne ratios significantly less than ～ 1.055.  相似文献   

2.
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.  相似文献   

3.
Abstract— The Peekskill H6 meteorite fell on 1992 October 9. We report extensive measurements of cosmic-ray produced stable nuclides of He, Ne, and Ar, of the radionuclides 22Na, 60Co, 14C, 36Cl, 26Al, and 10Be, and of cosmic-ray track densities. After correction for shielding via the 22Ne/21Ne ratio, the concentrations of cosmic-ray produced 3He, 21Ne and 38Ar give an average exposure age of 25 Ma, which is considered to be a lower limit on the true value. The 10Be/21Ne age is 32 Ma and falls onto a peak in the H-chondrite exposure age distribution. The activities of 26Al, 14C, 36Cl, and 10Be are all close to the maximum values expected for H-chondrites. Together with cosmic-ray track densities and the 22Ne/21Ne ratio, these radionuclide data place the samples at a depth >20 cm in a meteoroid with a radius >40 cm. In contrast, the 60Co activity requires a near-surface location and/or a much smaller body. Calculations show that a flattened geometry for the Peekskill meteoroid does not explain the observations in the context of a one-stage irradiation. A two-stage model can account for the data. We estimate an upper bound of 70 cm on the radius of the earlier stage of irradiation and conclude that Peekskill's radius was <70 cm when it entered the Earth's atmosphere. This size limit is somewhat smaller than the dynamic determinations (Brown et al., 1994).  相似文献   

4.
Abstract— We have measured the concentrations of the cosmogenic radionuclides 10Be, 26Al and 36Cl (half-lives 1.51 Ma, 716 ka, and 300 ka, respectively) in two different laboratories by accelerator mass spectrometry (AMS) techniques, as well as concentrations and isotopic compositions of stable He, Ne and Ar in the Antarctic H-chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmospheric radius of (20 ± 5) cm and a shielding depth for the analyzed samples of between 4 and 8 cm. Using calculated and experimentally determined production rates of cosmogenic nuclides, an exposure age of ～40 Ma is obtained from cosmogenic 21Ne and 38Ar. The extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 ± 0.4 Ma. A similarly long terrestrial age was found so far only for the Antarctic L-chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.  相似文献   

5.
Abstract— Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids. We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides 10Be, 26Al, and 36Cl in an attempt to constrain the cosmic‐ray exposure history of two members of the acapulcoite‐lodranite clan recovered in Antarctica: Frontier Mountain (FRO) 95029 and Graves Nunataks (GRA) 95209. From cosmic‐ray‐produced 3He, 21Ne, and 38Ar and appropriate production rates, we derive parent‐body breakup times of 4.59 ± 0.60 and 6.82 ± 0.60 Ma for FOR 95029 and GRA 95209, respectively. These times are consistent with those obtained from the pairs 10Be‐21Ne and 26Al‐21Ne; whereas the times inferred from the pair 36Cl‐36Ar are slightly longer, perhaps because the 36Cl activities decreased as a result of decay on Earth. Terrestrial ages up to ～50 ka for the two meteorites are consistent with the measured 36Cl activities of the metal phases. All acapulcoites and lodranites dated until now show cosmic‐ray exposure ages in the range of 4–10 Ma. This is the same range as that found for the major exposure age cluster of the H chondrites. As a common parent body is improbable on the basis of the O‐isotopic systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.  相似文献   

6.
Abstract— The Lohawat meteorite is a texturally heterogeneous breccia having a variety of mineral and lithic fragments. Among mineral fragments, pyroxenes show a wide range of composition (Wo0.011–0.17En0.37–0.78Fs0.21–0.60) whereas plagioclase is anorthitic (An0.92Ab0.07Or0.007). Abundant rounded “chondrule‐like” objects ranging in size up to ～7 mm, some with concentric layering, have been observed. Petrographic features, trace element composition and rare earth element patterns show the presence of eucritic and diogenitic components confirming that it is a typical howardite. Cosmogenic tracks, rare gases (He, Ne, and Ar) and radionuclides (22Na and 26Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 106/cm2. 38Ar exposure age is estimated to be ～110 Ma, being the highest among howardites. The track production rates correspond to ablation of 9 to 15 cm, implying a radius for its preatmospheric size of ～27 cm. 22Na/26Al ～ 1, as expected from the production models and solar modulation of galactic cosmic‐ray fluxes before its fall, suggesting that the meteoroid did not undergo any fragmentation during the past ～2 Ma in interplanetary space. The radiogenic age based on K‐Ar method is 4.3 Ga while the U‐Th‐4He age is 3.3 Ga indicating partial loss of He.  相似文献   

7.
Abstract— Glass-rich separates were prepared from a sample of the basaltic lunar meteorite EET87521 rich in dark glass. Noble gas isotopic abundances and 26Al and 10Be activities were measured to find out whether shock effects associated with lunar launch helped to assemble these phases. Similar 10Be and 26Al activities indicate that all materials in EET87521 had a common exposure history in the last few million years before launch. However, the glass contains much higher concentrations of trapped gases and records a much longer cosmic-ray exposure, 100 Ma–150 Ma, in the lunar regolith than does the bulk sample. The different histories show that the glass existed long before the ejection of EET87521. The trapped 40Ar/36Ar ratio of 1.6 ± 0.1 implies that the lunar exposure that produced most of the stable cosmogenic noble gases began 500 Ma ago. Cosmogenic and trapped noble gas components correlate strongly in various temperature-release fractions and phases of EET87521, which is probably because the glass contains most of the gas. The trapped solar ratios, 20Ne/22Ne = 12.68 ± 0.20 and 36Ar/38Ar = 5.24 ± 0.05 can be understood as resulting from a mixture consisting of ～60% solar wind and 40% solar energetic particles (SEP). All EET87521 phases show a 40K-40Ar gas retention age of ～3300 Ma, which is in the range of typical lunar mare basalts.  相似文献   

8.
Abstract— We measured cosmogenic radionuclides (10Be, 26Al, and 36Cl) and noble gases (He, Ne, and Ar) in 10 specimens of the Mocs L6 chondrite to determine the exposure history and preatmospheric relationship among fragments from known locations in the strewn field. Cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15.2 Ma. However, Mocs has very low 22Ne/21Ne ratios indicative of deep burial in a large meteoroid, but radionuclide levels at saturation values typical for much smaller meteoroids: this paradox suggests a possible complex exposure. For the latter case, we propose a two‐stage exposure history in which Mocs initially was deeply buried in a large object for 110 Ma, followed by exposure in a 65 cm object for 10.5 Ma. Relative shielding was inferred from the measured 22Ne/21Ne ratios assuming constant 22Ne/21Ne production for all samples during the first stage. These shielding levels, which are supported by estimates based on 36Cl production by neutron capture, indicate a possible relationship between depth of samples in the Mocs meteoroid and fall location in the strewn field.  相似文献   

9.
Abstract— The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described. Its mean mineral composition is defined by olivine (Fa31.4), orthopyroxene (Fs25.1,Wo2.0), clinopyroxene (Wo45En45.6Fs9.4) and plagioclase (An10.6Ab83.6Or5.8). The meteorite shows moderate shock features, which indicate that it belongs to the S4 category. Based on mineralogical and chemical criteria the meteorite is classified as an LL6 brecciated veined chondrite. Several cosmogenic radioisotopes (46Sc, 7Be, 54Mn, 22Na and 26Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes, and particle tracks density have been measured. Concentrations of cosmogenic 21Ne and 38Ar indicate that its cosmic‐ray exposure age is 24.8 Ma. Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present. The track density in olivines is found to be (1.3 ± 0.3) × 106/cm2. Activities of most of the short‐lived isotopes are lower than those expected from solar cycle variation. 22Na/26Al (1.12 ± 0.02) is found to be significantly anomalous, being ?25% lower than expected from the Climax neutron monitor data. These results indicate that the cosmic‐ray flux during the terminal segment of the meteoroid orbit was low. The activities of 26Al and 60Co and the track density indicate small meteoroid size with a radius ?15 cm.  相似文献   

10.
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11.
Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (10Be, 26Al, 36Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm?2, consistent with the preatmospheric size of asteroid 2008 TC3 that produced the Almahata Sitta strewn field; (2) both have, within error, a 21Ne/26Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic 38Ar. We also discuss the approximately 3.8 Ga (4He) and approximately 4.6 Ga (40Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC3 is derived has not experienced a large break‐up event in the last 3.8 Ga.  相似文献   

12.
Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (<6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, 36Arn, is attributed to production by thermal neutrons. From the values of 36Arn, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 1016 neutrons cm?2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2–1.4 × 1016 neutrons cm?2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.  相似文献   

13.
Abstract— We present a purely physical model for the calculation of depth‐ and size‐dependent production rates of cosmogenic nuclides by galactic cosmic‐ray (GCR) particles. besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on only one free parameter—the integral number of gcr particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth‐ and size‐dependent production rates for 10Be, 14C, 26Al, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semi‐empirical estimates by up to a factor of 4 but agree within ～20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26Al vs. 10Be, 26Al vs. 53Mn, 10Be/21Ne vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.  相似文献   

14.
Abstract— We measured the concentrations and isotopic compositions of He, Ne, and Ar in 29 bulk samples from 11 different strewn field fragments of the large Jiddat al Harasis (JaH) 073 L6 chondrite shower, including 7 samples from known locations within the main mass. In addition, we measured the concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in 10 samples. All fragments of this shower are characterized by low 10Be concentrations (7.6–12.8 dpm/kg), high 26Al/10Be ratios (3.5‐5), large contributions of neutron capture 41Ca (200–1800 dpm/kgCa), low 3He/21Ne ratios (1.5‐3.0), large variations in cosmogenic 21Ne (1.2–12) × 10?8cm3STP/g, and significant contributions of neutron‐capture 36Ar. Stepwise heating experiments show that neutron‐capture produced 36Ar is predominantly released between 1000–1200 °C. All these results are consistent with a first‐stage exposure of ?65 Ma within ?20 cm of the surface of the L‐chondrite parent body, followed by ejection of a 1.5‐2 m large object, which was then delivered to Earth within about 0.5 and 0.7 Ma. The cosmogenic nuclide data in JaH 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history. The observed 3He/21Ne ratios of 2.5‐3.0 in the most shielded samples (including those of the main mass) are lower than predicted by model calculations, but similar to the lowest values found in the large Gold Basin L‐chondrite shower. The Bern plot, which gives a linear correlation for 3He/21Ne versus 22Ne/21Ne, is evidently not valid for very high shielding. Some of our measured 22Ne/21Ne ratios in JaH 073 are lower than 1.06, which is not well understood, but might be explained by loss of cosmogenic neon from shocked sodium‐rich plagioclase during terrestrial weathering. The amount of trapped atmospheric argon in the JaH 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments, which range from <100 g to >50 kg. Finally, low concentrations of radiogenic 4He and 40Ar indicate incomplete degassing < 1 Ga ago, probably at the main collision event on the L‐chondrite parent body ?480 Ma ago.  相似文献   

15.
Abstract— We report here a chance find of a meteorite in the sand dunes of Ararki village of Hanumangarh district in the Rajasthan desert of northwest India. Chemical and petrological evidence in conjunction with isotopic composition of oxygen indicate that it is an L5 chondrite. The fayalite content of olivines is 26.3 mol%. The meteorite has some serpentinized olivines and 0.3% carbon having a terrestrial isotopic composition, indicating that it is moderately weathered. The absence of 22Na indicate that the meteorite fell to Earth more than a decade ago. The cosmic‐ray exposure age based on cosmogenic 21Ne is 7.2 Ma. Low density of cosmic‐ray heavy nuclei tracks, low 26A1 activity, the shielding parameter [(22Ne/21Ne)C = 1.094] and absence of neutron capture effects indicate cosmic‐ray shielding in a meteoroid having radius of about 16 cm, implying a meteoroid mass of about 60 kg and ablation of about 93%. The gas retention ages, based on U/Th‐4He and K‐40Ar are 1.1 and 0.58 Ga, respectively, suggesting a heating and degassing event late in the history of this meteorite.  相似文献   

16.
Abstract— We measured the concentrations of the cosmogenic radionuclides 10Be (half-life = 1.51 × 106 a), 26Al (7.05 × 105 a) and 36Cl (3.01 × 105 a) in Lewis Cliff (LEW) 86360, an L-chondrite from the Lewis Cliff stranding area, East Antarctica. In addition, the concentrations and isotopic compositions of He, Ne and Ar were measured. The combined results yield a terrestrial age of 2.35 ± 0.15 Ma. Only one other stony meteorite with a similar terrestrial age (～2 Ma) is known from the Allan Hills stranding area (ALH 88019), whereas all previously dated stony meteorites from Antarctica are younger than 1 Ma. We argue that LEW 86360 spent most of its terrestrial residence time deep inside the ice, near the base of the glacier, where ice flow rates are much lower than at the surface. The terrestrial ages of LEW 86360 and ALH 88019 are consistent with existing hypotheses concerning the stability and persistence of the East Antarctic ice sheet.  相似文献   

17.
Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and 10Be and 26Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar 20Ne/22Ne ratio of ?12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic 21Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic 21Ne concentrations in FRO 90174 show no simple correlation with 10Be and 26Al activities. We found 21Ne excesses between 0.3‐1.1 × 10?8cm3STP/g in 6 of the 17 samples. Since excess 21Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess 21Ne and solar 20Ne, we conclude that excess 21Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess 21Ne. This excess 21Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.  相似文献   

18.
Abstract— We measured the concentrations of the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of 10Be, 26Al, and 41Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the 22Ne/21Ne ratio at very high shielding. The 21Ne/10Be and 21Ne/26Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of 21Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture 36Ar in most samples are consistent with measured concentrations of neutron‐capture 36Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture 36Ar were found in those samples with an excess of 21Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced 21Ne and neutron‐capture‐produced 36Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic 4He and 40Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.  相似文献   

19.
Abstract— We measured abundances and isotopic compositions of noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite. The concentrations of noble gases in Acuña metal are very low compared to those reported so far for other iron meteorites. The isotopic ratios of He, Ne and Ar indicate that they are mostly of cosmogenic origin. Cosmogenic components are even present in Kr and Xe, which could not have been produced from Fe, Ni and P and are probably due to the spallation of trace elements of higher masses. The high 4He/21Ne ratio of 420 in Acuña metal indicates that the samples were at a deep position within a very large meteoroid. The exposure ages of Acuña were estimated to be 50–200 Ma from 3He, 21Ne and 38Ar abundances and by utilizing the diagrams of production rates vs. the 4He/21Ne ratio based on the Signer-Nier model. The low exposure age of Acuña may indicate a history different from that of other IIIAB irons whose exposure ages cluster at ～670 Ma. Otherwise, Acuña may be one of the samples with the low production rate, which can not be estimated from the diagrams of the Signer-Nier model.  相似文献   

20.
We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides 10Be, 26Al, and 36Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic 36Cl and 36Ar measured in the metal separates, we determined 36Cl‐36Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation 3He/21Ne versus 22Ne/21Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation 3He/21Ne versus 22Ne/21Ne breaks down at high shielding. Using 36Cl‐36Ar exposure ages and measured noble gas concentrations, we determine 21Ne and 38Ar production rates as a function of 22Ne/21Ne. The new data agree with recent model calculations for the relationship between 21Ne and 38Ar production rates and the 22Ne/21Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for 21Ne and 38Ar production rates as a function of the shielding indicator 22Ne/21Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the 10Be/21Ne and 26Al/21Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.  相似文献   

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