- Email: [email protected]
Applications of nuclear track detectors in space radiation dosimetry
Nuclear Tracks, Vol. 12, Nos I-6, pp. 505-508, 1986. Int. J. Radiat. AppI. Instrum., Part D Printed in Great Britain.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
APPLICATIONS OF NUCLEAR TRACK DETECTORS IN SPACE RADIATION DOSIMETRY
E. V. Benton* and Robert G. Richmond** *Department of Physics, University of San Francisco San Francisco, California 94117, U.S.A. **NASA-Lyndon B. Johnson Space Center Houston, Texas 77058, U.S.A.
ABSTRACT Over the past twenty years, the radiation environment of manned spacecraft has been measured by a variety of detector types. The high-LET (linear energy transfer) spectrum produced by HZE particles has been measured by plastic nuclear track detectors which included different combinations of CR-39, polycarbonate and cellulose nitrate sheets. These detectors are lightweight, compact, and have provided an effective means of measuring the high LET on all the space shuttle flights to date, including Spacelab-l, as well as the past missions of Apollo and the Skylab series. For low LET, absorbed dose and dose rates as a function of such parameters as inclination, altitude, spacecraft type and shielding have been measured with thermoluminescent detectors (TLD's). For low Earth-orbit missions the dose encountered was found to be strongly altitude-dependent, with a smaller dependence on orbit inclination. This paper presents radiation data gathered on the last several space shuttle flights and discusses further experiments which are being planned. INTRODUCTION As long-duration spaceflights become increasingly frequent and the possibility of the first U.S. space station becomes real, the constraints imposed by the space radiation environment must be addressed. Research has already shown that the highly penetrating nature of certain components of the space radiation field, such as galactic cosmic rays, makes effective shielding of spacecraft crews impractical. At the same time, the possible effects of these radiations on biomedical experiments, electronics, computers and materials in space must be studied. These effects will have to be taken into account when evaluating the results of such experiments. Detailed experimental data on radiation levels and their variation inside and outside orbiting spacecraft is still limited to comparatively few studies /Benton and Henke, 1983a; Janni,1969; Petrov et al., 1975; Bailey, 1977; Jordan, 1983/. On Apollo, Skylab and more recently on the Space Shuttle, the HZE particle exposure for individual astronauts has been measured by means of plastic nuclear track detectors (PNTD's), while the thermoluminescent detector (TLD) method was used for the low LET component. On Apollo, the HZE detectors were small sheets of Lexan. On Skylab, cellulose nitrate was added while on the Space Shuttle the more sensitive CR-39 is being used. The data gathered on Apollo and Skylab has been described previously /Benton et al., 1975; Benton et al., 1977/. This paper will focus on more recent results obtained from detectors flown on the Space Shuttle. RADIATION AND DOSE RATES In order to illustrate the dose and dose rates encountered during manned space flight, the data from earlier and more current U.S. flights is shown in Table 1 and 2. Here it can be seen that for orbital flights about the Earth, the dose rates vary from about 5-6 mrads/day for some typical Space Transportation System (STS) flights up to nearly 90 mrad/day for higher altitude and greater orbital inclination in Skylab 4. For the Apollo, the dose rates went up to ~127 mrad/day for Apollo 14 which traversed a "hot" portion of the radiation belt. The average dose rate inside the heavily shielded film vaults of Skylab, drawers B (16-30 g/cm 2) and F (30--50 g/cm 2) of Skylabs 2 and 3 were 39.5 and 33.5 mrad/day respectively, suggesting that even very heavy shielding is ineffective in reducing the dose rate from galactic cosmic 505
506
E.V.
TABLE i
Duration (hrs/days)
Flight Gemini 4 Gemini 6 Apollo 7* Apollo 8 Apollo 9 Apollo i0 Apollo I I Apollo 12 Apollo 13 Apollo 14 Apollo 15 Apollo 16 Apollo 17 Skylab 2** Skylab 3 Skylab 4 Apollo-Soyuz Test Project
BENTON and ROBERT G. RICHMOND
Dosimetry Data from U.S. Manned Spaceflights
Inclination (deg)
97.3 hrs 25.3 hrs 260.1 hrs 147.0 hrs 241.0 hrs 192.0 hrs 194.0 hrs 244.5 hrs 142.9 hrs 216.0 hrs 295.0 hrs 265.8 hrs 301.8 hrs 28 days 59 days 90 days 9 days
Apogee-Perigee (km)
32.5 28.9
Average Dose (mrad)
296 - 166 311 - 283 lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t a l t i t u d e = 435 " ~ 435 . ~ 435
50 50 50 50
"
Average dose rate (mrad/day)
46 25 160 160 200 480 180 580 240 1140 300 510 550 1596 3835 7740
11 23 15 26 20 60 22 57 40 127 24 46 44 57 ± 3 65 ± 5 86 ± 9
106
12
= 220
*Doses quoted for Apollo flights are skin TLD doses. The doses to the blood-forming organs are approximately 40% lower than the values measured at the body surface. **Mean thermoluminescent dosimeter (TLD) Skylab dose rates from crew dosimeters.
TABLE 2 STS Mission Number 1 2 3 4 5 6 7 8 41A¢ 41B 41C 41D 41G 51A 51C 51D 51B 51G 51F 51!
Crew Doses and Mission Parameters for the Space Shuttle Flights.
Spacecraft
Launch Date
Mission Duration (hr)
Alt.(nm)
Columbia Columbia Columbia Columbia Columbia Challenger Challenger Challenger Columbia Challenger Challenger Discovery Challenger Discovery Discovery Discovery Challenger Discovery Challenger Discovery
04-12-81 ll-12-81 03-22-82 06-27-82 11-11-82 04-04-83 06-18-83 08-30-83 11-28-83 02-03-84 04-06-84 08-30-84 10-05-84 11-08-84 01-24-85 04-12-85 04-29-85 06-17-85 07-29-85 08-27-85
54 58 195 169 122 120 143 145 248 191 168 145 197 192 74 168 166 170 191 192
145 137 151 160 153 158 160 160(max) !35 160 285(max) 160 190(max) 160x190 160x180 160x245 190 205(max) 174x164 240(max)
Number of Inc.( ° ) Crew 40 38 38 28.5 28.5 28.5 28.5 28.5 57 28.5 28.5 28.5 57 28.5 28.5 28.5 57 28.5 49.5 28.5
2 2 2 2 4 4 5 5 6 5 5 6 7 5 5 7 7 7 7 5
Range of Crew Doses (mrad)
Average Crew Dose (mrad)
not avail. 6-18(USF) 42-47(USF) 38-41(USF) 22-25(USF) 24-27 43-46 38-41 119-141 45-49 441-622 51-53 84-92 88-159 35-41 303-472 127-160 105-152 112-167 99-120
15 II(USF) 45(USF) 40(USF) 24(USF) 25±2*** 44±1 39i-i 125±2 48!i 519±5 52~-i 88±1 115±2 39±1 381!5 148±2 130±1 138±2 106±1
Stated uncertainty represents measurement precision, i o of the mean, rather than absolute accuracy. Previous measurements of absolute accuracy suggest that the given values are accurate to within 6-8%. %STS-9.
SPACE RADIATION DOSIMETRY
507
rays. A comparison of the radiation doses and dose rates measured on Spacelab-I (STS-9) is also shown /Benton et al., 1985/. For low Earth orbit, the effect of the greater orbital inclination of STS-9 a-ndSTS-41G (570 ) compared with other flights of the Space Shuttle (28.5 °) is clearly seen. Even though STS-9 was at a somewhat lower altitude (241 km) than several previous flights (284--297 km), the low-LET dose rate is nearly double that previously recorded. The effect is even more dramatic when the dose-equivalents are compared: ~150 mrem for Spacelab-i and ~50 m~em for the previous STS flights. This difference is the result of a substantial increase in the fluences of high-LET HZE particles having high values of radiobiological effectiveness (RBE). HZE PARTICLE MEASUREMENTS On the Apollo and Skylab missions, the HZE particle exposure for individual astronauts was measured by means of plastic nuclear track detectors. On Apollo, the detectors were located in the passive dosimetry packs carried on the chest, thigh and ankle of each astronaut /Benton et al., 1975/. On Skylab, each astronaut wore a single passive dosimeter on either the wrist or the ankle /Benton et al., 1977/. On Apollo missions 8--12, each plastic packet consisted of two 190 ~m-thick layers of type 8070-112 Lexan. On Apollo 13 and subsequent missions, three layers were used. The addition of the third layer was found to significantly improve the charge resolution of the detector stack. Each detector had an area of ~8 cm 2. In Skylab, in addition to Lexan, several layers of cellulose nitrate were included. This allowed the measurement of lower-LET particles. The data obtained from the Lexan detectors contained in the Apollo passive personnel dosimeters are shown in an earlier paper /Benton, 1983b/. The most heavily instrumented Apollo mission was that of Apollo 17. In addition to the personnel passive dosimeters, five other biologically related experiments were instrumented to contain plastic nuclear track detectors. The five experiments included the HZE Dosimeter, the Biostack, the Alfmed and the Biocore. Detailed results from the cellulose nitrate detectors are also described in Benton 1983b. The data clearly show the influence of shielding on the HZE exposure with the lightly shielded HZE Dosimeter recording nearly four times the flux recorded with the Biocore detectors. At the same time it is clear that even for a very heavily shielded situation, such as the Biocore (30-40 g/cm 2 is a conservative estimate), there are still a significant number of HZE particle hits. Several observations can be realized from the early HZE dosimetry. Because Skylab was in a relatively low Earth orbit and due to the presence of the Earth's geomagnetic field, the flux of high-LET, HZE particles within Apollo is considerably higher than that of Skylab. At the same time, a comparison of track fluences measured on later Apollo missions with those of earlier missions indicates an increase in the HZE cosmic-ray flux partially due to a decrease in the degree of solar modulation. Also, considerable variation has been found to exist in recorded HZE fluences as a function of detector location on the astronaut's body and in the spacecraft, caused by variations in the amount of shielding. The shielding also shifts the measured Z spectrum toward the lighter particles, even though the PNTD's are heavily biased toward the higher-Z particles. Fewer Z=26 (Fe) particles are observed than expected as compared with lighter particles, suggesting fragmentation of Fe particles in passage through the spacecraft shielding. LET SPECTRA In Figure 1 is shown a summary of integral LET spectra for HZE particles inside Spacelab-l, Skylab, ASTP (Apollo-Soyuz Test Project), and Apollo 17 missions, as measured by plastic nuclear track detectors containing CR-39, cellulose nitrate, and Lexan polycarbonate. The effective LET threshold for track registration for the three different detector types is ~20, I00 and 225 keV/~m in water respectively. The highest curve in Figure 1 represents the calculation for cosmic-ray iron (Fe) nuclei at solar minimum and without shielding, as in free space. As expected, the Apollo (lunar) missions produced a higher HZE particle flux than those of near-Earth orbit owing to the effects of shielding by the Earth and the geomagnetic field on the lower-orbiting missions. Increased shielding, causing a substantial decrease in HZE particle flux compared with lunar missions, was produced by the stowing of dosimeters in well-shielded film vault drawers. This effect is indicated by the slope of the LET curves for the Skylab astronauts, command module, and film vault drawers B and F, which steepens somewhat with increased shielding thickness, being steepest for drawer F.
508
E.V.
10-4
,
~
;pacelab I 10-5
BENTON and ROBERT G. RICHMOND
Fig. i. Integral LET spectrum of HZE particles inside spacecraft measured on lunar (Apollo) and near-Earth (Skylab, ASTP, SL-I) missions, calculated for cosmic-ray iron (Fe) nuclei in free space at solar minimum. Detectors used were cellulose nitrate, Lexan polycarbonate and (on SL-I) CR-39.
I
Col ic-rayFe ( s o l a r m i n , , no sh|eldlng)
-
Skyl~
astronau~--.~
• COmmand ) t o d u l ~ .~ A I' STPastronaut--a' Skylab Film Vault O raweB---o. r ~
L~ Ll L~ L~ L1
SU~MARY AND CONCLUSIONS Nuclear track detectors continue to provide useful information in the field of space dosimetry. The relatively recent addition of the CR-39 detector helps to lower the effective threshold for track registration from ~80 keV/wm for cellulose nitrate to ~i0--20 keV/~m for currently used materials. Continued refinement of the technique suggests that this threshold can be further reduced down to perhaps 3-5 keV/~m.
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KEYWORDS Nuclear track detectors; space radiation; dosimetry; applications of track detectors.
io-B ACKNOWLEDGMENT
8 LL~
Partial support for the preparation of this paper was provided by NASA under contract No. NAS9-17389.
A Z
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LET®(keV/~m.H20) REFERENCES
Bailey J. V. (1977) Dosimetry during space missions. IEEE Trans. Nucl. Sci. NS-23, 1379-1387. Benton E. V., Ilenke R. P. and Bailey J. V. (1975) Heavy cosmic-ray exposure of Apollo astronauts. Science 187, 263. Benton E. V., Peterson D. D., Bailey J. V. and Parnell T. A. (1977) High-LET particle exposure of Skylab astronauts. Health Phys. 32, 15. Benton E. V. and Henke R. P. (1983a) Radiation exposures during space flight and their measurements. Adv. Space Res. 3, No. 8, 171-185. Benton E. V. (1983b) Dosimetric radiation measurements in space. Nucl. Tracks and Rad. Meas. 7, Nos. 1/2, I-ii. Benton E. V., Frank A. L., Parnell T. A., Watts J. W. Jr. and Gregory J. C. (1985) Radiation environment of Spacelab-l. Paper to be presented at the AIAA conference "Shuttle Environment and Operations II," Houston TX, Nov. 13-15, 1985. Janni J. (1969) A review of Soviet manned space flight dosimetry results. Aerospace Med. 40, No. 12, Section II, Chap. XIII, 1547-1556. Jordan T. M. (1983) Radiation protection for manned space activities. JPL Publication 83-26, Jet Propulsion Laboratory, Pasadena, CA. Petrov V., Akatov Y., Kozlova S., Markelov V., Nesterov V., Redko V., Smirenny L., Khortsev A. and Chernikh I. (1975) The study of the radiation environment in near-Earth space. Space Res. 13, 128.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
APPLICATIONS OF NUCLEAR TRACK DETECTORS IN SPACE RADIATION DOSIMETRY
E. V. Benton* and Robert G. Richmond** *Department of Physics, University of San Francisco San Francisco, California 94117, U.S.A. **NASA-Lyndon B. Johnson Space Center Houston, Texas 77058, U.S.A.
ABSTRACT Over the past twenty years, the radiation environment of manned spacecraft has been measured by a variety of detector types. The high-LET (linear energy transfer) spectrum produced by HZE particles has been measured by plastic nuclear track detectors which included different combinations of CR-39, polycarbonate and cellulose nitrate sheets. These detectors are lightweight, compact, and have provided an effective means of measuring the high LET on all the space shuttle flights to date, including Spacelab-l, as well as the past missions of Apollo and the Skylab series. For low LET, absorbed dose and dose rates as a function of such parameters as inclination, altitude, spacecraft type and shielding have been measured with thermoluminescent detectors (TLD's). For low Earth-orbit missions the dose encountered was found to be strongly altitude-dependent, with a smaller dependence on orbit inclination. This paper presents radiation data gathered on the last several space shuttle flights and discusses further experiments which are being planned. INTRODUCTION As long-duration spaceflights become increasingly frequent and the possibility of the first U.S. space station becomes real, the constraints imposed by the space radiation environment must be addressed. Research has already shown that the highly penetrating nature of certain components of the space radiation field, such as galactic cosmic rays, makes effective shielding of spacecraft crews impractical. At the same time, the possible effects of these radiations on biomedical experiments, electronics, computers and materials in space must be studied. These effects will have to be taken into account when evaluating the results of such experiments. Detailed experimental data on radiation levels and their variation inside and outside orbiting spacecraft is still limited to comparatively few studies /Benton and Henke, 1983a; Janni,1969; Petrov et al., 1975; Bailey, 1977; Jordan, 1983/. On Apollo, Skylab and more recently on the Space Shuttle, the HZE particle exposure for individual astronauts has been measured by means of plastic nuclear track detectors (PNTD's), while the thermoluminescent detector (TLD) method was used for the low LET component. On Apollo, the HZE detectors were small sheets of Lexan. On Skylab, cellulose nitrate was added while on the Space Shuttle the more sensitive CR-39 is being used. The data gathered on Apollo and Skylab has been described previously /Benton et al., 1975; Benton et al., 1977/. This paper will focus on more recent results obtained from detectors flown on the Space Shuttle. RADIATION AND DOSE RATES In order to illustrate the dose and dose rates encountered during manned space flight, the data from earlier and more current U.S. flights is shown in Table 1 and 2. Here it can be seen that for orbital flights about the Earth, the dose rates vary from about 5-6 mrads/day for some typical Space Transportation System (STS) flights up to nearly 90 mrad/day for higher altitude and greater orbital inclination in Skylab 4. For the Apollo, the dose rates went up to ~127 mrad/day for Apollo 14 which traversed a "hot" portion of the radiation belt. The average dose rate inside the heavily shielded film vaults of Skylab, drawers B (16-30 g/cm 2) and F (30--50 g/cm 2) of Skylabs 2 and 3 were 39.5 and 33.5 mrad/day respectively, suggesting that even very heavy shielding is ineffective in reducing the dose rate from galactic cosmic 505
506
E.V.
TABLE i
Duration (hrs/days)
Flight Gemini 4 Gemini 6 Apollo 7* Apollo 8 Apollo 9 Apollo i0 Apollo I I Apollo 12 Apollo 13 Apollo 14 Apollo 15 Apollo 16 Apollo 17 Skylab 2** Skylab 3 Skylab 4 Apollo-Soyuz Test Project
BENTON and ROBERT G. RICHMOND
Dosimetry Data from U.S. Manned Spaceflights
Inclination (deg)
97.3 hrs 25.3 hrs 260.1 hrs 147.0 hrs 241.0 hrs 192.0 hrs 194.0 hrs 244.5 hrs 142.9 hrs 216.0 hrs 295.0 hrs 265.8 hrs 301.8 hrs 28 days 59 days 90 days 9 days
Apogee-Perigee (km)
32.5 28.9
Average Dose (mrad)
296 - 166 311 - 283 lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t lunar o r b i t a l f l i g h t a l t i t u d e = 435 " ~ 435 . ~ 435
50 50 50 50
"
Average dose rate (mrad/day)
46 25 160 160 200 480 180 580 240 1140 300 510 550 1596 3835 7740
11 23 15 26 20 60 22 57 40 127 24 46 44 57 ± 3 65 ± 5 86 ± 9
106
12
= 220
*Doses quoted for Apollo flights are skin TLD doses. The doses to the blood-forming organs are approximately 40% lower than the values measured at the body surface. **Mean thermoluminescent dosimeter (TLD) Skylab dose rates from crew dosimeters.
TABLE 2 STS Mission Number 1 2 3 4 5 6 7 8 41A¢ 41B 41C 41D 41G 51A 51C 51D 51B 51G 51F 51!
Crew Doses and Mission Parameters for the Space Shuttle Flights.
Spacecraft
Launch Date
Mission Duration (hr)
Alt.(nm)
Columbia Columbia Columbia Columbia Columbia Challenger Challenger Challenger Columbia Challenger Challenger Discovery Challenger Discovery Discovery Discovery Challenger Discovery Challenger Discovery
04-12-81 ll-12-81 03-22-82 06-27-82 11-11-82 04-04-83 06-18-83 08-30-83 11-28-83 02-03-84 04-06-84 08-30-84 10-05-84 11-08-84 01-24-85 04-12-85 04-29-85 06-17-85 07-29-85 08-27-85
54 58 195 169 122 120 143 145 248 191 168 145 197 192 74 168 166 170 191 192
145 137 151 160 153 158 160 160(max) !35 160 285(max) 160 190(max) 160x190 160x180 160x245 190 205(max) 174x164 240(max)
Number of Inc.( ° ) Crew 40 38 38 28.5 28.5 28.5 28.5 28.5 57 28.5 28.5 28.5 57 28.5 28.5 28.5 57 28.5 49.5 28.5
2 2 2 2 4 4 5 5 6 5 5 6 7 5 5 7 7 7 7 5
Range of Crew Doses (mrad)
Average Crew Dose (mrad)
not avail. 6-18(USF) 42-47(USF) 38-41(USF) 22-25(USF) 24-27 43-46 38-41 119-141 45-49 441-622 51-53 84-92 88-159 35-41 303-472 127-160 105-152 112-167 99-120
15 II(USF) 45(USF) 40(USF) 24(USF) 25±2*** 44±1 39i-i 125±2 48!i 519±5 52~-i 88±1 115±2 39±1 381!5 148±2 130±1 138±2 106±1
Stated uncertainty represents measurement precision, i o of the mean, rather than absolute accuracy. Previous measurements of absolute accuracy suggest that the given values are accurate to within 6-8%. %STS-9.
SPACE RADIATION DOSIMETRY
507
rays. A comparison of the radiation doses and dose rates measured on Spacelab-I (STS-9) is also shown /Benton et al., 1985/. For low Earth orbit, the effect of the greater orbital inclination of STS-9 a-ndSTS-41G (570 ) compared with other flights of the Space Shuttle (28.5 °) is clearly seen. Even though STS-9 was at a somewhat lower altitude (241 km) than several previous flights (284--297 km), the low-LET dose rate is nearly double that previously recorded. The effect is even more dramatic when the dose-equivalents are compared: ~150 mrem for Spacelab-i and ~50 m~em for the previous STS flights. This difference is the result of a substantial increase in the fluences of high-LET HZE particles having high values of radiobiological effectiveness (RBE). HZE PARTICLE MEASUREMENTS On the Apollo and Skylab missions, the HZE particle exposure for individual astronauts was measured by means of plastic nuclear track detectors. On Apollo, the detectors were located in the passive dosimetry packs carried on the chest, thigh and ankle of each astronaut /Benton et al., 1975/. On Skylab, each astronaut wore a single passive dosimeter on either the wrist or the ankle /Benton et al., 1977/. On Apollo missions 8--12, each plastic packet consisted of two 190 ~m-thick layers of type 8070-112 Lexan. On Apollo 13 and subsequent missions, three layers were used. The addition of the third layer was found to significantly improve the charge resolution of the detector stack. Each detector had an area of ~8 cm 2. In Skylab, in addition to Lexan, several layers of cellulose nitrate were included. This allowed the measurement of lower-LET particles. The data obtained from the Lexan detectors contained in the Apollo passive personnel dosimeters are shown in an earlier paper /Benton, 1983b/. The most heavily instrumented Apollo mission was that of Apollo 17. In addition to the personnel passive dosimeters, five other biologically related experiments were instrumented to contain plastic nuclear track detectors. The five experiments included the HZE Dosimeter, the Biostack, the Alfmed and the Biocore. Detailed results from the cellulose nitrate detectors are also described in Benton 1983b. The data clearly show the influence of shielding on the HZE exposure with the lightly shielded HZE Dosimeter recording nearly four times the flux recorded with the Biocore detectors. At the same time it is clear that even for a very heavily shielded situation, such as the Biocore (30-40 g/cm 2 is a conservative estimate), there are still a significant number of HZE particle hits. Several observations can be realized from the early HZE dosimetry. Because Skylab was in a relatively low Earth orbit and due to the presence of the Earth's geomagnetic field, the flux of high-LET, HZE particles within Apollo is considerably higher than that of Skylab. At the same time, a comparison of track fluences measured on later Apollo missions with those of earlier missions indicates an increase in the HZE cosmic-ray flux partially due to a decrease in the degree of solar modulation. Also, considerable variation has been found to exist in recorded HZE fluences as a function of detector location on the astronaut's body and in the spacecraft, caused by variations in the amount of shielding. The shielding also shifts the measured Z spectrum toward the lighter particles, even though the PNTD's are heavily biased toward the higher-Z particles. Fewer Z=26 (Fe) particles are observed than expected as compared with lighter particles, suggesting fragmentation of Fe particles in passage through the spacecraft shielding. LET SPECTRA In Figure 1 is shown a summary of integral LET spectra for HZE particles inside Spacelab-l, Skylab, ASTP (Apollo-Soyuz Test Project), and Apollo 17 missions, as measured by plastic nuclear track detectors containing CR-39, cellulose nitrate, and Lexan polycarbonate. The effective LET threshold for track registration for the three different detector types is ~20, I00 and 225 keV/~m in water respectively. The highest curve in Figure 1 represents the calculation for cosmic-ray iron (Fe) nuclei at solar minimum and without shielding, as in free space. As expected, the Apollo (lunar) missions produced a higher HZE particle flux than those of near-Earth orbit owing to the effects of shielding by the Earth and the geomagnetic field on the lower-orbiting missions. Increased shielding, causing a substantial decrease in HZE particle flux compared with lunar missions, was produced by the stowing of dosimeters in well-shielded film vault drawers. This effect is indicated by the slope of the LET curves for the Skylab astronauts, command module, and film vault drawers B and F, which steepens somewhat with increased shielding thickness, being steepest for drawer F.
508
E.V.
10-4
,
~
;pacelab I 10-5
BENTON and ROBERT G. RICHMOND
Fig. i. Integral LET spectrum of HZE particles inside spacecraft measured on lunar (Apollo) and near-Earth (Skylab, ASTP, SL-I) missions, calculated for cosmic-ray iron (Fe) nuclei in free space at solar minimum. Detectors used were cellulose nitrate, Lexan polycarbonate and (on SL-I) CR-39.
I
Col ic-rayFe ( s o l a r m i n , , no sh|eldlng)
-
Skyl~
astronau~--.~
• COmmand ) t o d u l ~ .~ A I' STPastronaut--a' Skylab Film Vault O raweB---o. r ~
L~ Ll L~ L~ L1
SU~MARY AND CONCLUSIONS Nuclear track detectors continue to provide useful information in the field of space dosimetry. The relatively recent addition of the CR-39 detector helps to lower the effective threshold for track registration from ~80 keV/wm for cellulose nitrate to ~i0--20 keV/~m for currently used materials. Continued refinement of the technique suggests that this threshold can be further reduced down to perhaps 3-5 keV/~m.
L~
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I
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11 ~°o. '
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j~ •
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I ' Ap01lo I I astronauC5
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e ($Cosmic*ray kylabo8 glcFm
KEYWORDS Nuclear track detectors; space radiation; dosimetry; applications of track detectors.
io-B ACKNOWLEDGMENT
8 LL~
Partial support for the preparation of this paper was provided by NASA under contract No. NAS9-17389.
A Z
lo -9 I
i01
I
i
L
102
I
103
'
]
104
LET®(keV/~m.H20) REFERENCES
Bailey J. V. (1977) Dosimetry during space missions. IEEE Trans. Nucl. Sci. NS-23, 1379-1387. Benton E. V., Ilenke R. P. and Bailey J. V. (1975) Heavy cosmic-ray exposure of Apollo astronauts. Science 187, 263. Benton E. V., Peterson D. D., Bailey J. V. and Parnell T. A. (1977) High-LET particle exposure of Skylab astronauts. Health Phys. 32, 15. Benton E. V. and Henke R. P. (1983a) Radiation exposures during space flight and their measurements. Adv. Space Res. 3, No. 8, 171-185. Benton E. V. (1983b) Dosimetric radiation measurements in space. Nucl. Tracks and Rad. Meas. 7, Nos. 1/2, I-ii. Benton E. V., Frank A. L., Parnell T. A., Watts J. W. Jr. and Gregory J. C. (1985) Radiation environment of Spacelab-l. Paper to be presented at the AIAA conference "Shuttle Environment and Operations II," Houston TX, Nov. 13-15, 1985. Janni J. (1969) A review of Soviet manned space flight dosimetry results. Aerospace Med. 40, No. 12, Section II, Chap. XIII, 1547-1556. Jordan T. M. (1983) Radiation protection for manned space activities. JPL Publication 83-26, Jet Propulsion Laboratory, Pasadena, CA. Petrov V., Akatov Y., Kozlova S., Markelov V., Nesterov V., Redko V., Smirenny L., Khortsev A. and Chernikh I. (1975) The study of the radiation environment in near-Earth space. Space Res. 13, 128.