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Estimation of error in the determination of Al26 in stone meteorites by indirect γ-ray spectrometry
Cfeochimica et Cosmochimica
Acts, 1971,
Vol. 35, pp. 727 to 730. Pergamon Press.
Printedin NorthernIreland
NOTES
Estimation of error in the determination of A126in stone meteorites by indirect pray spectrometry M. W. ROWE and R. S. CLARK Department of Chemistry, Texas A & M University, College Station, Texas 77843 Radiation Counting Laboratory, National Aeronautics and Space Administration Lunar Receiving Laboratory, Houston, Texas 77058 (Received
20 October 1970; accepted
in
revisedform 9 February
1971)
Abstract-The error involved in the determination of the A126 concentration by the method of Rowe and his co-workers was measured and the values they reported should be increased by a factor of l-36 f O-05. The potassium contents of the meteorites measured by ROWE et al., are not affected by this error and can be used with confidence. HEYMANN AND ANDERS (1967) first called attention to a systematic error in the A126 measurements in stone meteorites by Rowe and his colleagues (ROWE and VAN DILLA, 1961; ROWE et al., 1963; ANDERSON et al., 1964; ROWE and MANUEL, 1964). In these reports, the A126 was calculated indirectly by comparing the 1*83-MeV Al26 photopeak with the l-46-MeV photopeak of K40 in a KC1 spiked standard. Small corrections were applied to account for differences in efficiency, photofraction, peak width, and self-absorption between the two y-ray energies. Rowe and his co-workers neglected, however, to correct for the loss of the 1*83MeV y-rays which summed with O-51-MeV y-rays. The 0*51-MeV y-rays result from annihilation of positrons emitted by AP in coincidence with the l-83-MeV y-rays. When a 0*51-MeV y-ray and a 1.83-MeV y-ray interact with the detector simultaneously, only a single event is recorded-a sum pulse of 2*34-MeV or less depending on whether the y-quantum lose their energy by photo-effect or Compton interaction. The number of events appearing in the 1*83-MeV photopeak is reduced by such cases; they occur instead in another energy region. HEYMANN and ANDERS (1967) pointed out that the average A126 content in ordinary chondrites measured by them was about 20-30 per cent higher than that of ROWE et al. (1963). ROWE (1967) replied with the limited (and apparently confused) information that he had available that the error due to lack of summing correction seemed to account for a discrepancy of only about 7 per cent. It is the purpose of this paper to present results of a direct measurement of the magnitude of the summing error involved in the method of Rowe and his co-workers. We measured several A12’j mockups of different sizes and shapes which had been previously prepared at the National Aeronautics and Space Administration Radiation Counting Laboratory at Houston as well as potassium standards with known activities in identical geometries. The crystal originally used by ROWE et al., was only 7-l/2 x 4 in. whereas the crystal used in this work was 8 x 4 in. The original crystal was not available for this work but the difference will not significantly 727
728
Notes Table
1. Description
of A126 and K mockups
measured
on an 8 x 4 in. NaI(T1)
Mockup weight Geometric
(8) 300 309 304 613 904 897
Right circular Right circular Right circular Right circular Right circular Irregular *
* This was the mockup
shape
Activity of A126 (dpm)
Dimensions
cylinder cylinder cylinder cylinder cylinder
100mm 25 mm 25 mm 50 mm 75 mm
x x x x x
50mmdia. 90 mm dia. 90 mm dia. 90 mm dia. 90 mm dia.
for lunar rock 10057 (P.E.T.,
crystal
Science
6.38 2.83 2.88 5.71 2.88 7.59
x x x x x x
gK
lo3 103 lo3 10s lo3 103
15.75 15.73 15.73 31.46 31.46 7.865
165, 1211-1227, 1969).
affect the results presented here. The mockups used for these measurements are described in Table 1. We calculated the Alz6 in the exact manner employed by ROWE et al., at Los Alamos. This value was then compared with the known AP6 concentration. These data are set forth in Table 2 and indicate that the Alz6 values of ROWE et al., Table 2. Comparisons of known Alz6 in mockup with that measured by the method of ROWE et al., i.e. by comparison of the 1*83-MeV y-ray of A126 with 1*46-MeV y-ray of known amount of K
Mockup 304 309 300 613 613 897 897
g g g g g g g
(on side) (on side) (front side) (back side)
904 g weighted* average
Known A12s (dpm) 2.88 2.83 6.38 5.71 5.71 7.59 7.59 2.88
x x x x x x x x
lo3 lo3 lo3 10s 103 103 103 lo3
Measured Alz6 (dpm) by method of ROWE et al. 2.08 1.97 4.79 4.09 4.18 5.87 5.86 2.06
x x x x x x x x
lo3 103 10s lo3 lo3 lo3 lo3 lo3
Known A12s Measured A12s 1.38 1.44 I.33 1.39 1.37 1.29 1.29 1.40 1.36 k 0.05
* Since the 304 g and 309 g samples were virtually identical in geometry, they were considered as one sample in the average. Since no difference was observed in 613 g and 897 g samples with geometry changed, they were each considered as one sample. The unweighted average is also 1.36.
should be multiplied by a factor of 1.36 f 0.05 to correct for their error. Our determination of the size of this error agrees with the estimate of HEYMANN and ANDERS (1967) and also FUSE and ANDERS (1969) who continued the Al26 measurements at The University of Chicago. RO;WE’S (1967) estimate of 7 & 4 per cent for the summing correction definitely understates the error. Table 3 lists the corrected data of ROWE et al., with data of others for comparison.
729
Notes Table 3. Alas concentrations of meteorites measured by ROWE et al., corrected for systematic error Alzs (dpmjkg) Meteorite
Chondrites Abee Achilles Archie Beard&y
(ROWE et al. times 1.36 f O-05)
I II III Bruderheim
69 68 57 68 75 53 57
f i f * & + &
8 7 6 7 8 6 2
Calhham Cavour Cherokee Springs I II Ehole Forest City I II Hamlet
72 62 54 56 45 46 48 52
* + f f f f + *
8 7 6 6 5 5 5 7
Hark&on
42 f 3
Holbrook
58 f 6
Others (reference)
56 f 5(l) 44 f 4(2)
60 f 6(l) 58 + 3(3)
70 f 7(4)
64 60 50 45 50 44
f i f ?c f +
6(Q) 5(3) 4(5) 5(4) 3(3) S(2)
69 f Q(6) Kernouve Indarth Ladder Creek I IL La Lande Mocs Modoc Morland Ness Pantar Plainview Potter I II Richardton I II St. Chinian Searsmont Sylaeauga Tysnes Island Carbonaceous ohondrites Felix Mighei Murray
61 54 39 49 67 71 71 64 76 72 76
f? f 6 f 3 & 5 f 7 i8 *s f 7 f 8 f 8 If 8
73 68 71 39 71 61 81 80
f 8 & 7 f8 f 4 f8 &7 +Q 3 9
52 & 6 35 ^^ f 4 _ 60 & 7
74 + 6(6) 60.6 f 2*8(‘7)
69 f 5(2)
38 54 52 63 45
i W) zh 3(Q) i 3(2) z!z5(3) z!z3(2)
54 zh 4(9) 35.6 f 2(7) 42 f 4(Q)
730
Notes Table
3 (continued) Alzs (dpm/kg) Meteorite
(ROWE
et&. tim0S
1.36 & 0.05)
Achondrites Bishopville Juvinas Moore Co. Norton Co. Nuevo Laredo Pasamonte Pena Blanoa Spring Sioux County Stannern References: (1) HONDA e2 nl. (1961). (2) CRESSY (1964). (3) FIREMAN (1967). (4) HONDA and ARNOLD (1964). (5) SHEDLOVSKY et al. (1967).
86 133 75 72 82 99 65 122 130 (6) (7) (8) (9)
f + * f * & & + *
9 15 8 8 9 11 7 13 14
Others (reference) 82.3 & 2*7(7) 105.8 * 4*0(7) 79-4 f
4*0(8)
101.5 * 3+3(7) 65.2 i. 3+3(7) 99 * 5(7)
BISWAS et aZ. (1963). FUSE and ANDERS (1969). HERZO~; and ANDERS (1971). HEYMANN and ANDERS (1967).
licknowlcdgements-ale are grateful to Professors E. ANDERS and D. HEYMANN, who first brought the summing error to our attention. We appreciate the use of the Los Alamos Scientific Laboratory’s 8 x 4 in. NaI (Tl) crystal and Dr. I’. N. DEAN’S assistance in its operation. We also wish to acknowledge the technical assistance of Mr. M. K. ROBBINS of Brown and RootNorthrup. This work was supported in part by the National Science Foundation Grant GP18716 and in part by the National Aeronautics and Space Administration. REFERENCES ANDERSON E. C., ROWE M. W. and UR~Y H. C. (1964) Potassium and aluminum 26 contents of three bronzite chondrites. J. Geophys. Res. 69, 564-565. BISWAS M. M., MAYER-B• RICKE C. and GENTNER W. (1963) Cosmic-ray produced Naaz and AlSo activities in chondrites. In Earth Science and Meteotitics (editors J. Geiss and E. D. Goldberg), pp. 207-215. North-Holland. See also: MAYER-B• RICEE C., BISWAS M. M. and GENTNER W. 2. Nuturforsch. ITa, 921-924. (1962) y-spektroskopische Untersuchungen an Steinmeteoriten. CRESSY P. J., JR. (1964) Cosmogenic radionuclidcs in stone meteorites. Ph.D. Dissertation, U.S. AEC Report NYO-8924. FIREMAN E. L. (1967) Radioactivities in meteorites and cosmic-ray variations. Geochim. Cosmochim. Acta 31,1691-1700. FUSE K. and ANDERS E. (1969) Aluminum-26 in meteorites-VI. Achondrites. Geochim. Cosmochim. Acta 33, 653-670. HERZOC G. F. and ANDERS E. (1971) Radiation age of the Norton County meteorite. Geochim. Cosmochim. Acta, 35, 239-244. HEYMANN D. and ANDERS E. (1967) Meteorites with short cosmic-ray exposure ages, as determined from their Alz6 content. Geochim. Cosmochim. Acta 33, 1793-1808. HONDA M. and ARNOLD J. R. (1964) Effects of cosmic rays on meteorites. Scielace 143, 203-212. HONDA M., UMEMOTO S. and ARNOLD J. R. (1961) Radioactive species produced by cosmic rays in Bruderheim and other stone meteorites. J. Geophye. Rec. 66, 3541-3546. ROWE M. W. (1967) A126 determination in stone meteorites by y-ray spectrometry. Geochim. Cosmochim. Acta 31, 1808-1809. ROWE M. W. and MANUEL 0. K. (1964) y-Radioactivity in the Fayetteville meteorite. J. Geophys. Res. 69, 1944-1946. ROWE M. W. and VAN DILLA M. A. (1961) On the radioactivity of the Bruderheim chondrite. .J. Geophys. Res. 66, 3553-3556. Rowe M. W., VAN DILLA M. A. and ANDERSON E. C. (1963) On the radioactivity of stone meteorites. Geochim. Cosmochim. Acta 27, 983-1001. SHEDLOVSEY J. R., CRESSY I?. ._I., JR. and KOHMAN T. P. (1967) Radioactivities in the Peace River and Harleton chondrites. J. Geophys. Res. 72, 5051-5058.
Acts, 1971,
Vol. 35, pp. 727 to 730. Pergamon Press.
Printedin NorthernIreland
NOTES
Estimation of error in the determination of A126in stone meteorites by indirect pray spectrometry M. W. ROWE and R. S. CLARK Department of Chemistry, Texas A & M University, College Station, Texas 77843 Radiation Counting Laboratory, National Aeronautics and Space Administration Lunar Receiving Laboratory, Houston, Texas 77058 (Received
20 October 1970; accepted
in
revisedform 9 February
1971)
Abstract-The error involved in the determination of the A126 concentration by the method of Rowe and his co-workers was measured and the values they reported should be increased by a factor of l-36 f O-05. The potassium contents of the meteorites measured by ROWE et al., are not affected by this error and can be used with confidence. HEYMANN AND ANDERS (1967) first called attention to a systematic error in the A126 measurements in stone meteorites by Rowe and his colleagues (ROWE and VAN DILLA, 1961; ROWE et al., 1963; ANDERSON et al., 1964; ROWE and MANUEL, 1964). In these reports, the A126 was calculated indirectly by comparing the 1*83-MeV Al26 photopeak with the l-46-MeV photopeak of K40 in a KC1 spiked standard. Small corrections were applied to account for differences in efficiency, photofraction, peak width, and self-absorption between the two y-ray energies. Rowe and his co-workers neglected, however, to correct for the loss of the 1*83MeV y-rays which summed with O-51-MeV y-rays. The 0*51-MeV y-rays result from annihilation of positrons emitted by AP in coincidence with the l-83-MeV y-rays. When a 0*51-MeV y-ray and a 1.83-MeV y-ray interact with the detector simultaneously, only a single event is recorded-a sum pulse of 2*34-MeV or less depending on whether the y-quantum lose their energy by photo-effect or Compton interaction. The number of events appearing in the 1*83-MeV photopeak is reduced by such cases; they occur instead in another energy region. HEYMANN and ANDERS (1967) pointed out that the average A126 content in ordinary chondrites measured by them was about 20-30 per cent higher than that of ROWE et al. (1963). ROWE (1967) replied with the limited (and apparently confused) information that he had available that the error due to lack of summing correction seemed to account for a discrepancy of only about 7 per cent. It is the purpose of this paper to present results of a direct measurement of the magnitude of the summing error involved in the method of Rowe and his co-workers. We measured several A12’j mockups of different sizes and shapes which had been previously prepared at the National Aeronautics and Space Administration Radiation Counting Laboratory at Houston as well as potassium standards with known activities in identical geometries. The crystal originally used by ROWE et al., was only 7-l/2 x 4 in. whereas the crystal used in this work was 8 x 4 in. The original crystal was not available for this work but the difference will not significantly 727
728
Notes Table
1. Description
of A126 and K mockups
measured
on an 8 x 4 in. NaI(T1)
Mockup weight Geometric
(8) 300 309 304 613 904 897
Right circular Right circular Right circular Right circular Right circular Irregular *
* This was the mockup
shape
Activity of A126 (dpm)
Dimensions
cylinder cylinder cylinder cylinder cylinder
100mm 25 mm 25 mm 50 mm 75 mm
x x x x x
50mmdia. 90 mm dia. 90 mm dia. 90 mm dia. 90 mm dia.
for lunar rock 10057 (P.E.T.,
crystal
Science
6.38 2.83 2.88 5.71 2.88 7.59
x x x x x x
gK
lo3 103 lo3 10s lo3 103
15.75 15.73 15.73 31.46 31.46 7.865
165, 1211-1227, 1969).
affect the results presented here. The mockups used for these measurements are described in Table 1. We calculated the Alz6 in the exact manner employed by ROWE et al., at Los Alamos. This value was then compared with the known AP6 concentration. These data are set forth in Table 2 and indicate that the Alz6 values of ROWE et al., Table 2. Comparisons of known Alz6 in mockup with that measured by the method of ROWE et al., i.e. by comparison of the 1*83-MeV y-ray of A126 with 1*46-MeV y-ray of known amount of K
Mockup 304 309 300 613 613 897 897
g g g g g g g
(on side) (on side) (front side) (back side)
904 g weighted* average
Known A12s (dpm) 2.88 2.83 6.38 5.71 5.71 7.59 7.59 2.88
x x x x x x x x
lo3 lo3 lo3 10s 103 103 103 lo3
Measured Alz6 (dpm) by method of ROWE et al. 2.08 1.97 4.79 4.09 4.18 5.87 5.86 2.06
x x x x x x x x
lo3 103 10s lo3 lo3 lo3 lo3 lo3
Known A12s Measured A12s 1.38 1.44 I.33 1.39 1.37 1.29 1.29 1.40 1.36 k 0.05
* Since the 304 g and 309 g samples were virtually identical in geometry, they were considered as one sample in the average. Since no difference was observed in 613 g and 897 g samples with geometry changed, they were each considered as one sample. The unweighted average is also 1.36.
should be multiplied by a factor of 1.36 f 0.05 to correct for their error. Our determination of the size of this error agrees with the estimate of HEYMANN and ANDERS (1967) and also FUSE and ANDERS (1969) who continued the Al26 measurements at The University of Chicago. RO;WE’S (1967) estimate of 7 & 4 per cent for the summing correction definitely understates the error. Table 3 lists the corrected data of ROWE et al., with data of others for comparison.
729
Notes Table 3. Alas concentrations of meteorites measured by ROWE et al., corrected for systematic error Alzs (dpmjkg) Meteorite
Chondrites Abee Achilles Archie Beard&y
(ROWE et al. times 1.36 f O-05)
I II III Bruderheim
69 68 57 68 75 53 57
f i f * & + &
8 7 6 7 8 6 2
Calhham Cavour Cherokee Springs I II Ehole Forest City I II Hamlet
72 62 54 56 45 46 48 52
* + f f f f + *
8 7 6 6 5 5 5 7
Hark&on
42 f 3
Holbrook
58 f 6
Others (reference)
56 f 5(l) 44 f 4(2)
60 f 6(l) 58 + 3(3)
70 f 7(4)
64 60 50 45 50 44
f i f ?c f +
6(Q) 5(3) 4(5) 5(4) 3(3) S(2)
69 f Q(6) Kernouve Indarth Ladder Creek I IL La Lande Mocs Modoc Morland Ness Pantar Plainview Potter I II Richardton I II St. Chinian Searsmont Sylaeauga Tysnes Island Carbonaceous ohondrites Felix Mighei Murray
61 54 39 49 67 71 71 64 76 72 76
f? f 6 f 3 & 5 f 7 i8 *s f 7 f 8 f 8 If 8
73 68 71 39 71 61 81 80
f 8 & 7 f8 f 4 f8 &7 +Q 3 9
52 & 6 35 ^^ f 4 _ 60 & 7
74 + 6(6) 60.6 f 2*8(‘7)
69 f 5(2)
38 54 52 63 45
i W) zh 3(Q) i 3(2) z!z5(3) z!z3(2)
54 zh 4(9) 35.6 f 2(7) 42 f 4(Q)
730
Notes Table
3 (continued) Alzs (dpm/kg) Meteorite
(ROWE
et&. tim0S
1.36 & 0.05)
Achondrites Bishopville Juvinas Moore Co. Norton Co. Nuevo Laredo Pasamonte Pena Blanoa Spring Sioux County Stannern References: (1) HONDA e2 nl. (1961). (2) CRESSY (1964). (3) FIREMAN (1967). (4) HONDA and ARNOLD (1964). (5) SHEDLOVSKY et al. (1967).
86 133 75 72 82 99 65 122 130 (6) (7) (8) (9)
f + * f * & & + *
9 15 8 8 9 11 7 13 14
Others (reference) 82.3 & 2*7(7) 105.8 * 4*0(7) 79-4 f
4*0(8)
101.5 * 3+3(7) 65.2 i. 3+3(7) 99 * 5(7)
BISWAS et aZ. (1963). FUSE and ANDERS (1969). HERZO~; and ANDERS (1971). HEYMANN and ANDERS (1967).
licknowlcdgements-ale are grateful to Professors E. ANDERS and D. HEYMANN, who first brought the summing error to our attention. We appreciate the use of the Los Alamos Scientific Laboratory’s 8 x 4 in. NaI (Tl) crystal and Dr. I’. N. DEAN’S assistance in its operation. We also wish to acknowledge the technical assistance of Mr. M. K. ROBBINS of Brown and RootNorthrup. This work was supported in part by the National Science Foundation Grant GP18716 and in part by the National Aeronautics and Space Administration. REFERENCES ANDERSON E. C., ROWE M. W. and UR~Y H. C. (1964) Potassium and aluminum 26 contents of three bronzite chondrites. J. Geophys. Res. 69, 564-565. BISWAS M. M., MAYER-B• RICKE C. and GENTNER W. (1963) Cosmic-ray produced Naaz and AlSo activities in chondrites. In Earth Science and Meteotitics (editors J. Geiss and E. D. Goldberg), pp. 207-215. North-Holland. See also: MAYER-B• RICEE C., BISWAS M. M. and GENTNER W. 2. Nuturforsch. ITa, 921-924. (1962) y-spektroskopische Untersuchungen an Steinmeteoriten. CRESSY P. J., JR. (1964) Cosmogenic radionuclidcs in stone meteorites. Ph.D. Dissertation, U.S. AEC Report NYO-8924. FIREMAN E. L. (1967) Radioactivities in meteorites and cosmic-ray variations. Geochim. Cosmochim. Acta 31,1691-1700. FUSE K. and ANDERS E. (1969) Aluminum-26 in meteorites-VI. Achondrites. Geochim. Cosmochim. Acta 33, 653-670. HERZOC G. F. and ANDERS E. (1971) Radiation age of the Norton County meteorite. Geochim. Cosmochim. Acta, 35, 239-244. HEYMANN D. and ANDERS E. (1967) Meteorites with short cosmic-ray exposure ages, as determined from their Alz6 content. Geochim. Cosmochim. Acta 33, 1793-1808. HONDA M. and ARNOLD J. R. (1964) Effects of cosmic rays on meteorites. Scielace 143, 203-212. HONDA M., UMEMOTO S. and ARNOLD J. R. (1961) Radioactive species produced by cosmic rays in Bruderheim and other stone meteorites. J. Geophye. Rec. 66, 3541-3546. ROWE M. W. (1967) A126 determination in stone meteorites by y-ray spectrometry. Geochim. Cosmochim. Acta 31, 1808-1809. ROWE M. W. and MANUEL 0. K. (1964) y-Radioactivity in the Fayetteville meteorite. J. Geophys. Res. 69, 1944-1946. ROWE M. W. and VAN DILLA M. A. (1961) On the radioactivity of the Bruderheim chondrite. .J. Geophys. Res. 66, 3553-3556. Rowe M. W., VAN DILLA M. A. and ANDERSON E. C. (1963) On the radioactivity of stone meteorites. Geochim. Cosmochim. Acta 27, 983-1001. SHEDLOVSEY J. R., CRESSY I?. ._I., JR. and KOHMAN T. P. (1967) Radioactivities in the Peace River and Harleton chondrites. J. Geophys. Res. 72, 5051-5058.