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Doses from galactic cosmic ray particles under spacecraft shielding
0735-245X/92 S5.00+ .OO Pcrgmon Pressplc
Nucl. TracksRadiat.Meas.,Vol. 20. No. 1, pp. 33-39, 1992 ht. 3. Radial.Appl. Inmum., Port D Printed in Great Britain
DOSES FROM GALACTIC COSMIC RAY PARTICLES UNDER SPACECRAFT SHIELDING V. E. DUDK~Nand Yu. V. PCITAPOV Research Center for Spacecraft Radiation Safety, U.S.S.R. Ministry of Public Health, Moscow 123182, U.S.S.R. (Receiued 2 March
1991)
Abstract-This work presents the results of calculating the absorbed and equivalent doses from galactic cosmic ray (GCR) particles under a spacecraft shielding of up to 5Ogcn1-~ in the space environment beyond the Earth’s magnetic field during solar minimum and maximum. The calculations are made for standard geometry, namely, normal incidence of a broad particle beam onto a semi-infinite plane shielding layer. The doses are calculated at a point under shielding in biological tissue. The GCR doses are calculated for primary protons, for alpha particles, and for Be, N, Si, and Fe nuclei which are representative of all the charged groups of GCR particles. The particle passages through matter are calculated by solving the radiation transfer equation, making allowance for nuclear collisions and for the contribution of the main secondary radiation components to the total dose. The input data (the inelastic interaction cross-sections, the fragmentation parameters, the mean multiplicities of secondaries, etc.) are used with either prescribed values or values published elsewhere. The equivalent OCR particle doses am calculated making allowance for two types of the dependence of quality factor (QF) on linear energy transfer (LET) of particles in biological tissue. The component composition of the particle doses and the contributions of secondary components to the total dose are analysed as functions of the shielding thickness. The attenuation curves of the GCR particle doses defined by different forms of the differential GCR energy spectra are compared with each other. The resultant values of the GCR particle doses beyond the Earth’s magnetic field are compared with the values found elsewhere and with the standard doses adopted in the U.S.S.R. for space flights of up to 3 yr.
the absorbed and equivalent GCR doses under shieldings of up to 10 g cmm2 Al were estimated. It should be noted that the calculation results were obtained elsewhere using different GCR particle spectra and different techniques and, therefore, proved to be often in disagreement with each other. In the present work, an attempt will be made to unify the input data and the calculation techniques (Dudkin et al., 1975, 1989; Vikhrov et al., 1978). At present, some All-Union State Standards and Procedure Guidelines have been prepared and adopted in the U.S.S.R. to standardize different aspects of the above-mentioned types of calculations involving, first of all, the norms for radiation safety of space flights of a duration of up to 3 yr (SCRSDSF, 1986c). In SCRSDSF (1985a, 1986a), the techniques are standardized for calculating the absorbed and equivalent doses from protons and from multiply charged ions, respectively. In SCRSDSF (1985b, 1986b) the characteristics of nuclear interactions (cross-sections, collision-free paths, fragmentation parameters, etc.) of protons (SCRSDSF, 1986b) and multiply charged ions (SCRSDSF, 1985b) are prescribed. In Energy Spectra (1985a+ 1989) copious experimental data are used to standardize the values of the differential energy spectra of GCR particles, from hydrogen to nickel, during solar minima and maxima, and to prescribe
1. TNTRODUCTION THE PENDING interplanetary
space missions require that the radiation environment along the future spacecraft trajectories should be thoroughly appraised. During long-term space flights, galactic cosmic rays (GCR) constitute one of the main permanent sources of radiation hazard. The doses from the GCR particles were calculated on several occasions (Wilkinson and Curtis, 1972; Alsmiller et al., 1972; Dudkin et al., 1975, 1989; Letaw et al., 1986, 1987; Townsend and Wilson, 1990). In the initial works (Wilkinson and Curtis, 1972; Alsmiller et al., 1972), the absorbed GCR particle doses were calculated disregarding the contribution from secondary radiation to the total dose. Our work (Dudkin et al., 1975) was the first to show that the particles with charges 2 2 10 make the major contribution to the GCR particle doses. The contribution has proved to be particularly substantial in the calculated equivalent doses because of the high LET values and, hence, high quality factors (up to 20) of the Z > 10 particles. In subsequent works (Letaw et al., 1986, 1987; Dudkin et al., 1989; Townsend and Wilson, 1990), the GCR particle doses were calculated for different solar activity periods and for different geometrics. In Townsend and Wilson (1990) the contributions from some types of secondary radiation to 33
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‘A
m
sr!In01NH3ZLI.
NOILVTWIV3
(INV V.LVa UldNI
‘Z
‘(9861) sau.mf pue sruepy u! pasodoJd ewads aql 01 JE[!I.U!S allnbala EJlaads.&SJaua paz!pJEpuElS aql‘ULXOjJ!aql UI 'EUI!U!KU pUEEUl!XHUJIe[OS UaaMlaiJ e~laadsap!lJed~I3E)aqlSu!lelnalea~oj aepwuoj aql PUE NIXana
PE
2 ‘A
DOSES FROM
GCR PARTICLES
UNDER
where j = 1, up to i = 1; S,,,(E) is the ionization loss of the representative nuclei of i, j group with energy E (see SCRSDSF, 1985c); k is the number of a group of nuclei from which the nuclei of j group are produced; Pu and A, are the parameters of fragmentation of the k-group nuclei into the j-group nuclei and the interaction free path, respectively. The values of P and 1 presented in SCRSDSF (1985b), have been inferred from copious experimental data and are independent of the energy of projectile nuclei within experimental errors, and where
Fj(0, E) = q’(E) is the initial total energy spectrum of i group nuclei; S(x) is the delta-function. The set (3) consists of one-coordinate equations for radiation transfer. The first equation describes the radiation transfer by the nuclei from the primary i group only. The second equation makes allowance for the radiation transfer by the j-group nuclei produced in fragmentation of the i-group nuclei (j < i). The second term in the expression (2) was calculated by the formula k-i
Dj=C
c
Pkj
k-j+1
x
Fl(x*,
E*)exp
where E* is the energy of fragments produced at an interaction point located at depth x*; energy E* is found from the range @)-energy relation: E = R;‘[R,(E*)
+ (x -x*)1.
(5)
The upper limit of energy integration E& for the kth group is inferred from the relation E& = RL’[Rk(E;t,,)
-x+],
(6)
where Ek_ is the highest energy of primaries incident into the layer. The doses from secondary radiations were calculated making allowance not only for the projectile fragments (including neutrons), but also for the fast (“cascade”) nucleons emitted from either of the interacting nuclei, for the slow (“evaporation”) nucleons
SPACECRAFT
35
SHIELDING
emitted from a target-nucleus, for the recoil nuclei, and for x-mesons. In the calculations, the values of specific doses (the dose per unit flux of each particle species) presented in Vikhrov et al. (1978) were used for the fast particles, for the slow neutrons, and for n-mesons, while the slow charged particles were assumed to be locally absorbed at the point where they were produced. The necessary data on the characteristics of secondary radiation were inferred from both the results of our experiments and the results obtained elsewhere.
3. CALCULATION RESULTS DISCUSSION
AND
Presented below are the results of calculating the GCR particle doses obtained by the techniques described in Section 2 above, under “standard conditions with a tissue-equivalent geometry” absorber. Tables 1 and 2 present the absorbed and equivalent doses from GCR particles under shielding beyond the Earth’s magnetic field during solar minimum and maximum for the energy spectra of primary particles proposed elsewhere (Curtis and Wilkinson, 1968; Kovalev et al., 1978; Adams et al., 1981; Adams and James, 1986) and for the values of the spectra standardized in the U.S.S.R. (Energy Spectra, 1985a-c, 1989). For comparison, the second columns of Tables 1 and 2 present the GCR particle flux in free space during two solar activity periods. The tabulated values of the doses indicate that the most substantial x-dependent changes and model-defined differences are observed at small values of shielding thickness x. These variations occur when the GCR energy spectra include a great number of low-energy particles (up to approximately tens of MeV per nucleon (Energy Spectra, 1985a+, 1989; Adams er al., 1981; Adams and James, 1986)). As the shielding thickness increases, the absorbed dose remains nearly constant, while the equivalent dose decreases monotonically. At the depths x Z 10 g cm-* the doses differ from the mean dose by + 3-5% during solar minimum and by f 8-12% during solar maximum. It should be noted also that the doses prove to be similar in the case of the spectra proposed by Adams (Adams and James,
Table 1. Absorbed dose rates as functions of shielding thickness x for different forms of GCR spectra (cGy yr-I)
Ref. Curtis and Wilkinson (1968) Kovalev et ul. (1978) Adams er al. (1981) Adams and James (1986) Energy Spectra (1985a-c. 1989) Kovalev et al. (1978) Adams er al. (1981) Adams and James (1986) Energy Spectra (1985a-c, 1989)
Particle flux, Particle Cm2s-1 3.36 3.90 5.16 4.40 4.53 2.28 2.36 1.72 1.68
Solar activity Period Min.
Max.
x,
gem-2
0
0.1
1.0
5
10
20
30
18.4 16.5 29.1 23.4 21.3 8.6 13.0 8.7 6.5
18.3 16.4 22.0 21.0 18.2 8.6 10.0 7.0 6.4
17.1 16.1 19.6 16.8 17.3 8.7 8.8 6.5 6.1
14.2 15.2 17.6 15.7 16.0 9.1 7.7 6.0 5.9
13.7 15.0 17.1 15.0 15.6 9.6 7.8 6.3 6.4
14.6 16.4 19.3 16.9 17.3 10.6 8.7 7.1 7.1
14.2 15.8 18.0 16.1 15.3 11.0 9.2 7.4 7.3
36
V. E. DUDKIN
and YU. V. POTAPOV
Table 2. Equivalent dose rates as functions of shielding thickness x for different forms of GCR spectra (CSVyr-‘) Particle flux,
particle Ref.
C&s-’
Curtis and Wilkinson (1968) Kovalev et al. (1978) Adams ef al. (1981) Adams and James (1986) Energy Spectra (1985a-c 1989) Kovalev el al. (1978) Adams ef al. (1981) Adams and James (1986) Energy Spectra (1985ac. 1989)
3.36 3.90 5.16 4.40 4.53 2.28 2.36 1.72 1.68
Solar activity period Min.
Max.
1986) and in the case of the spectra standardized in the U.S.S.R. (Energy Spectra, l985a-c 1989). Figures 1 and 2 are the plots of the decreasing absorbed and equivalent GCR dose rates beyond the Earth’s magnetic field during solar minimum for the U.S.S.R.-standardized spectra (Energy Spectra, 1985a-c, 1989). The plots disregard the doses due to the L-group particles (Z = 3-5) because the relevant analysis has shown that the contribution of the L-group nuclei to the total dose is below 1% under shieldings of all thicknesses. From Figs 1 and 2 it is seen that the component composition of the doses varies substantially with increasing thickness (x). For example, the contribution to the dose under small shielding thicknesses, x, is mainly from the GCR particles with Z > 3 ( - 50% of the absorbed dose and -80% of the equivalent dose), whereas the contribution of these particles under thicknesses x = 30 g cm-’ decreases to 10 and 25%, respectively. It should be noted that more than a third of the
1
0
I 20
I IO
x(g
x,
gem-2
0
0.1
1.0
5
10
20
30
122 96.5 134 115 120 49.5 54.9 40.7 35.6
I21 96.0 109 100 101 49.4 47.8 37.4 33.0
104 88.1 100 90.2 89.0 47.3 45.2 33.0 30.0
72.2 64.0 75.0 64.0 63.5 40.1 36. I 26.8 25.4
53.6 51.1 57.1 49.8 49.6 34.4 28.0 21.6 22.1
37.2 39.1 44.2 38.5 36.8 27.7 22.4 17.8 17.9
28.9 32.8 35.7 31.3 32.5 24.0 19.4 15.6 15.6
equivalent dose under small shielding thicknesses is from the Z 2 20 particles (vH-group) due to the high values of LET and QF for the given particles. During solar maximum, the dependence of dose rate on the shielding thickness, x, is less pronounced than during solar minimum (see Tables 1 and 2 and curves 1 and 7 in Figs 1 and 2). As the thickness x increases from 0 to 30 g cme2, the H dose decreases by a factor of 3.7 during solar minimum and by a factor of 2.3 during solar maximum. The difference in the factors can be attributed mainly to a lower relative fraction of low-energy particles in the fluence of GCR nuclei during solar maximum. At present, the National Radiation Protection Commission of the U.S.S.R. is revising the standardized dependence of the quality factor QF =f(LET,) in terms of the microdosimetric approach and making allowance for the fresh radiobiological data (Atvetisov et al., 1988). The eventual effect of the new representation of quality factors on the GCR dose
I
30
Cm-z)
FIG. 1. The component composition and the total equivalent dose rates under different shielding thicknesses x during solar minimum (solid lines) and maximum (dashed line). Curves I and 7: total doses; curve 2: proton dose; curve 3: vH-group dose; curve 4: LH-group dose; curve 5: helium dose; curve 6: M-group dose.
0
I 20
I 10
x (g
I 30
cm-2)
FIG. 2. The same as in Fig. I for the absorbed dose rate. Curves I and 7: total doses; curve 2: proton dose; curve 3: helium dose; curve 4: M-group dose; curve 5: LH-group dose; curve 6: vH-group dose.
DOSES FROM
GCR PARTICLES
UNDER
SPACECRAFT
SHIELDING
31
*----------__
0
I IO
2y 20 I
I 40
30 (p
L 50
x
cK2)
FIG. 3. The attenuation curves for the GCR equivalent dose rate during solar minimum (curves 1) and maximum (curves 2). The solid and dashed lines have been obtained with QF, and QF2, respectively. attenuation curves was estimated by calculating the equivalent doses on the basis of the new dependence
of QF on LET,. Figure 3 compares the attenuation curves obtained for QF, (the former dependence) and for QFz (the new dependence on LET,). It is seen that the attenuation curves for QF, and QF2 during solar minimum meet each other at x m 3 g crne2. This effect is due to a high contribution under shielding thicknesses of < 3 g cm-* from the low-energy particles with Z > 10 and LET, > 1600 MeVcm-‘, for which QF2 < QF, ( w 40% of the H-dose is from the vH-group nuclei). That is why the H-doses obtained using the QF2 values prove to be smaller. As the shielding thickness increases, the contribution from the vH-group component decreases and the particles with LET e = 20-1600 MeV cm-‘, for which QF2 > QF, , become more important. The soft sides of the energy spectra of the Z > 10 nuclei are much smaller during solar maximum compared with solar minimum, so the effect of the difference in the QF,,, values on the dose attenuation curves becomes noticeable at x 2 10 g cm-* only. The excess of the H-dose obtained with QF2 over the dose calculated with QF, is lo-15% at x > 10gcme2 during solar maximum. Absorbed doses are usually measured when making dosimetric studies in space. It is of interest, therefore, to obtain the dependencies of the mean @ values on the shielding thickness x. Table 3 presents the values of @ during solar minimum and maximum, which have been calculated for the dependence QF, = f (LET,). The @ values for QF, (LET,) Table 3. Values of w
Solar min. Solar max.
(g cm21
FIG. 4. A comparison between the attenuation curves for the GCR equivalent dose rate during solar minimum obtained from the present work (curve I), from Letaw et ul. (1987) (curve 2), and Townsend and Wilson (1990) (curve 3).
differ from the tabulated values by not more than 10%. Figure 4 compares our calculated GCR dose attenuation curves with the results obtained elsewhere (Letaw et al., 1987; Townsend and Wilson, 1990). All the calculations were made for solar minimum under the identical geometric conditions (the “standard geometry”). It is seen that all the results are similar because, probably, of the similar calculational models used and the similar values of the input data. For example, the energy spectra used in Letaw et al. (1987) and Townsend and Wilson (1990) (see Adams and James, 1986) are close in their form and absolute values to our spectra (see Energy Spectra, 1985a-c, 1989). The minor differences between the attenuation curves obtained by us and those in Letaw et al. (1987) are probably due to the inaccuracy in approximating the QF value as a function of LET,. Compared with the standardized values of QF = f (LET,), the QF values used in Letaw et al. (1986, 1987) are lower by 20-30%, so the difference between the latter’s results and our data is most substantial at x Q 15 g cmm2, i.e. when the highest contribution to the total H-dose is from the particles with the highest QF value. It should be noted that the values of H-doses at x < 5 g cm-* are omitted in Letaw et al. (1987) because the calculations there were made for bloodgenerating organs whose mean location depth was taken to be z5 g cm-*. An even smaller difference in the GCR attenuation doses is noted when comparing our data with the results of Townsend and Wilson (1990). In this case,
as a function of x (gcme2)
0
I
5
10
20
30
50
5.65 5.57
4.93 4.82
3.94 4.08
3.16 3.44
2.23 2.52
2.02 2.18
1.90 2.05
V. E. DUDKIN
38
and YU. V. POTAPOV
as seen from Fig. 4, the differences (by up to 20% at a maximum of x = 30 g cm-*) are probably due to different contributions of secondary radiation and to somewhat different input data. To clarify the matter, we have estimated the contribution from the secondary radiation to the total H-dose, as was done in Townsend and Wilson (1990). Analysing the estimates has shown that, under small shielding thicknesses (up to x = S-10 gem-*), up to 90% of the H-dose is due to primary radiation and to the fragments of primary nuclei, while the remaining N 10% is due mainly to low-energy charged particles (residual target nuclei) and to neutrons. As the shielding thickness increases, the contribution from secondary radiation as a whole, and particularly from neutrons, is also increased. At x = 30 g cm-*, for example, the secondary radiation is responsible for - 50% of the total H-dose, of which some 30% is due to neutrons. The shielding thicknesses required for effective radiation safety for spacecraft crews in flights of different durations in interplanetary space were estimated using the attenuation curve obtained for solar minimum and on the basis of the data of SCRSDSF (1986c). The result obtained is presented in Fig. 5 and demonstrates that the expected equivalent dose will not exceed 66.5, I183 and 162.5 CSV during flights of
I-, 2-, and 3-yr durations if the crew is protected with shieldines of 4.0. 6.3. and 8.0 acm-* thicknesses. respectively. It should be borne in mind that the above estimates have been obtained for the GCR particles alone without making allowance for the contribution to the dose from on-board radiation
sources, from the Earth’s radiation belts, and from other cosmic ray sources (solar flares). Therefore, the actual
thickness
of shielding
0
Ill, 405063
I
I
x (g
required
I
60
I IO
for the living-
I 15
err?)
FIG. 5. The annual (curve 1), biennial (curve 2). and 3-yr (curve 3) GCR equivalent dose rates vs shielding thickness x of tissue-equivalent material.
cabin of spacecraft during an interplanetary mission for duration of 3 yr may be much greater than -8 g cm-*. The American researchers Letaw et al. (1987) estimate such a required thickness to be _ 20 g crt -*, which involves _ 30 tons of weight for any reasonable size of the living-cabin, thereby making the technological realization of any manned interplanetary mission very difficult. An alternative approach proposed recently is to carry out long-term manned interplanetary missions during solar maxima when the GCR dose is decreased by a factor of -2. A specially designed radiation-protected shelter is suggested for spacecraft crew during solar flares.
REFERENCES Adams J. H. Jr and James H. (1986) Cosmic ray effects on microelectronics. Part IV: NRL Memo. Renort 5901. 31 December. Adams J. H. Jr. Silhcrherg R. and Tsao C. H. (1981) Cosmic ray effects on microelectronics, part I: the near-Earth particle environment. NRL Memorandum Report 4506, August 25. Alsmiller R. G. Jr, Santoro R. T., Barish J. and Claibome S. (1972) Shielding of manned space vehicles against protons and alpha particles. ORNL-RSIC-35, pp. 493-507. Atvetisov G. M., Gubin A. T. er al. (1988) Recommended dependence of quality factor on LET for the new
radiation safety norms. Hygiene and Sanirorion 10, 32-36. Curtis S. B. and Wilkinson M. C. (1968) Study of radiation hazards to man on extended missions. NASA, CR-1037, Washington, pp. 89-96. Dudkin V. E., Kovalev E. E., Nefedov N. A., Potapov, Yu. V. and Boedanov S. D. (19891 Calculations of galactic cosmic riy doses under‘shielhing during longterm space flights. Abstracts of Reports at V All-Union Conf. on Protection of Nuclear Technological Installations Against Ionizing Radiations. D. 180. Protvino. Dudkin V. E.: Kuznetsov V. G.. Po&ov Yu. V. and Sysoeva 0. V. (1975) Formation of dose fields in biological tissue from multiply-charged ions. In Problems qf Dosimetry and Protection Against Radiations, No. 14 (Edited by Kimel R. L. and Sakharov V. K.), pp. 72-77. Atomrzdat. Moscow. Energy Spectra (1985a) Galactic cosmic ray protons. AllUnion State Standard 25645, pp. 122-185. Standartizdat. Moscou. Energy Spectra (I 985b) Galactic cosmic ray helium nuclei. All-Union State Standard 25645, pp. 123-185. Standartlzdat. Moscow. Energy Spectra ( 1985~) Galactic cosmic ray middle nuclei. All-Union State Standard 25645, pp. 124-185. Standartlzdat. Moscow. Energy Spectra (1989) Galactic cosmic ray light, heavy, and very heavy nuclei. All-Union State Standard 25645, pp. 144188. Standartizdat, Moscow. Interaction of Ionizing Radiation with Matter (1979) Chemical composition of tissue-equivalent matter. AllUmon State Standard 18622-29. Standartizdat, Moscow. Kovalev E. E.. Kolomensky A. V., Muratova I. A. and Petrov V. M. (1978) Model description of differential spectra of galactic 42, 923-926.
cosmic rays. Izv. Akad. Nouk SSSR
Lctaw J. R.. Silberhcrg R. and Tsao C. H. (1986) Natural radiation hazards on the manned Mars mission. NASA MOOZ,pp. 642-655.
DOSES FROM
GCR PARTICLES
Letaw J. R., Silberberg R. and Tsao C. H. (1987) Radiation hazards on space missions. Nurure 330, 709-710. Radiation Safety Norms RSN-76/87 (1988) Energoizdat, Moscow. Recommendations ICRP (1971) Publication 21. Pergamon Press, Oxford. . ’ Space Crew Radiation Safety During Space Flight (SCRSDSFl. (1985a) Methods for calculating absorbed and’ equivalent doses from multiply-charged cosmic ray ions. Procedure Guideline RD 50-25645, pp. 207-285. Standartizdat, Moscow. SCRSDSF. (1985b) Nuclear interaction characteristics of multiply-charged ions. All-Union State Standard 25645, pp. 212-285. Standartizdat, Moscow. SCRSDSF. (1985~) Methods for calculating ionization loss and paths of heavy charged particles. Procedure Guideline RD 50-25645, pp. 206-284. Standartizdat, Moscow. SCRSDSF. (1986a) Methods for calculating absorbed and equivalent dose from cosmic ray protons under shielding. Procedure Guideline RD 50-25645, pp. 208-286. Standartizdat. Moscow.
NT 20/l-D
UNDER
SPACECRAFT
SHIELDING
39
SCRSDSF. (1986b) Nuclear interaction characteristics of protons. All-Union State Standard 25645, pp. 211-288. Standartizdat, Moscow. SCRSDSF. (1986c) Safety norms with flight duration of up to three years. All-Union State Standard 25645, pp. 215-285. Standartizdat, Moscow. Townsend L. W. and Wilson J. W. (1990) Interaction of spaa radiation with matter. Report on 41st Congress of the International Astronautical Federation, Dresden, Germany, pp. l-11. Vikhrov A. I., Dudkin V. E., Kovalev E. E., Komochkov M. M., Lebedev V. N., Litvinova E. G., Mitrinaya V. G., Potapov Yu. V., Potemkin E. L., Sychev B.S. and Frolov V. V. (1978) Atlas of Dose Ckaracteristics
of
External
lonihg
Rahion,
Handbook
(Edited by Kovalev E. E.), pp. 47-51. Atomizdat, Moscow. Wilkinson M. C. and Curtis S. B. (1972) Galactic cosmic ray heavy primary and secondary doses. In Proc. Nat. Symp. Natural Man Made Radial. Space, pp. 104-l 10. Las Vegas, Nevada.
Nucl. TracksRadiat.Meas.,Vol. 20. No. 1, pp. 33-39, 1992 ht. 3. Radial.Appl. Inmum., Port D Printed in Great Britain
DOSES FROM GALACTIC COSMIC RAY PARTICLES UNDER SPACECRAFT SHIELDING V. E. DUDK~Nand Yu. V. PCITAPOV Research Center for Spacecraft Radiation Safety, U.S.S.R. Ministry of Public Health, Moscow 123182, U.S.S.R. (Receiued 2 March
1991)
Abstract-This work presents the results of calculating the absorbed and equivalent doses from galactic cosmic ray (GCR) particles under a spacecraft shielding of up to 5Ogcn1-~ in the space environment beyond the Earth’s magnetic field during solar minimum and maximum. The calculations are made for standard geometry, namely, normal incidence of a broad particle beam onto a semi-infinite plane shielding layer. The doses are calculated at a point under shielding in biological tissue. The GCR doses are calculated for primary protons, for alpha particles, and for Be, N, Si, and Fe nuclei which are representative of all the charged groups of GCR particles. The particle passages through matter are calculated by solving the radiation transfer equation, making allowance for nuclear collisions and for the contribution of the main secondary radiation components to the total dose. The input data (the inelastic interaction cross-sections, the fragmentation parameters, the mean multiplicities of secondaries, etc.) are used with either prescribed values or values published elsewhere. The equivalent OCR particle doses am calculated making allowance for two types of the dependence of quality factor (QF) on linear energy transfer (LET) of particles in biological tissue. The component composition of the particle doses and the contributions of secondary components to the total dose are analysed as functions of the shielding thickness. The attenuation curves of the GCR particle doses defined by different forms of the differential GCR energy spectra are compared with each other. The resultant values of the GCR particle doses beyond the Earth’s magnetic field are compared with the values found elsewhere and with the standard doses adopted in the U.S.S.R. for space flights of up to 3 yr.
the absorbed and equivalent GCR doses under shieldings of up to 10 g cmm2 Al were estimated. It should be noted that the calculation results were obtained elsewhere using different GCR particle spectra and different techniques and, therefore, proved to be often in disagreement with each other. In the present work, an attempt will be made to unify the input data and the calculation techniques (Dudkin et al., 1975, 1989; Vikhrov et al., 1978). At present, some All-Union State Standards and Procedure Guidelines have been prepared and adopted in the U.S.S.R. to standardize different aspects of the above-mentioned types of calculations involving, first of all, the norms for radiation safety of space flights of a duration of up to 3 yr (SCRSDSF, 1986c). In SCRSDSF (1985a, 1986a), the techniques are standardized for calculating the absorbed and equivalent doses from protons and from multiply charged ions, respectively. In SCRSDSF (1985b, 1986b) the characteristics of nuclear interactions (cross-sections, collision-free paths, fragmentation parameters, etc.) of protons (SCRSDSF, 1986b) and multiply charged ions (SCRSDSF, 1985b) are prescribed. In Energy Spectra (1985a+ 1989) copious experimental data are used to standardize the values of the differential energy spectra of GCR particles, from hydrogen to nickel, during solar minima and maxima, and to prescribe
1. TNTRODUCTION THE PENDING interplanetary
space missions require that the radiation environment along the future spacecraft trajectories should be thoroughly appraised. During long-term space flights, galactic cosmic rays (GCR) constitute one of the main permanent sources of radiation hazard. The doses from the GCR particles were calculated on several occasions (Wilkinson and Curtis, 1972; Alsmiller et al., 1972; Dudkin et al., 1975, 1989; Letaw et al., 1986, 1987; Townsend and Wilson, 1990). In the initial works (Wilkinson and Curtis, 1972; Alsmiller et al., 1972), the absorbed GCR particle doses were calculated disregarding the contribution from secondary radiation to the total dose. Our work (Dudkin et al., 1975) was the first to show that the particles with charges 2 2 10 make the major contribution to the GCR particle doses. The contribution has proved to be particularly substantial in the calculated equivalent doses because of the high LET values and, hence, high quality factors (up to 20) of the Z > 10 particles. In subsequent works (Letaw et al., 1986, 1987; Dudkin et al., 1989; Townsend and Wilson, 1990), the GCR particle doses were calculated for different solar activity periods and for different geometrics. In Townsend and Wilson (1990) the contributions from some types of secondary radiation to 33
(I)
‘5'P (3 ‘xhH.(A’)t~
“‘b
,
‘“~~ ‘3 = cx)H s “‘“3 ,
‘3P(3‘X)J7.(3)!~ ,~9~ I=(w)a s
aepu.u.toj aql 6q pauy.u.Ialap ale x ssauq3!ql 30 Su!pla!qs hre.wqJt! Japun sapyvd 8139 ruo~j sasop ((x)H) lualvnba pue ((x)a) paqJosqe woi aql u! pampoJd sna[mu heuyd e 30 swaruSeq aql apnls suoymba aql ‘03 uoyqos -u! ‘.nqny~ed u! ‘qxqh suoJ]nau pue sa!mpuoJas aql s! (3 dx)fd uopmnj aq~ ‘(3 ‘x){~ ‘a.! ‘amang paS.wqD 30 1Cla!JEA ieaJ% e 01 as!1 a@ !almu ql!M bhua-[eyeds 30 uoymnj .mauq e SE pap&a1 Su!aq sapg’ed paSleq3 h.eaq JO suoy~o~ yselau! aql x qldap it? asop aql ql!~ ‘(~861) gsax3s u! pasod .asop p2loi aql 01 uo!le!pe~ -oJd poqlatu aql dq pala[n31m alaM (Z) uo!ssaJdxa hpuo3as 30 uo!lnq!4uos aql ~03 pue suogmalu! aql jo suuai omi isly aqi ‘Z e z qi!m sap!ved lealmu Jo3 acmsmolp2 Syyem ‘palyye3 aJaM ~a1m.u LAeaq Lmuud tuoq sasop aql Supt?~n~~e~ uaqM qSnoJq1 !alXIu JO sa%essed aqL ‘(6~61 ‘.Iallew qly uo!la!pe~ Su!z!uo~ 30 uoy3waluI) paz!plepuels osle 30 luapuadapu! aq 01 paumsse SBM snapnu-label e s! uog!sodtuos asoqm amelsqns ]uapXyba-ansyl u! s ruoq pallya sapyed aql 103 uuoj [emads aql (A) Su!pla!qs lapun lu!od e le palslmlm aJaM sasop lua tpampold aJIe daql aJaqm lu!od aql -[eA!nba put! paqJosqe aqL ‘(x) sassauyXq1 iualajyp 30 Jalleur JO lalCq aue[d al!uyu!-!ruas e OIUO weaq ap!lJedprzoJq e 30 amappu! [euuou ‘a.! ‘suo!l!puo~ LlaruoaS pXpwls Jo3 pale1n31e9 alaw sasop aql .!a13nu aJaM dnok? le ,Qe~of paq.xosqEale snalanu-la%21e uxoq sap!1 -led,,uogEJodEAa,,patheqa Lhaua-mol aql leqluo!l -dmnssa aql uo palepym SBM asop anssg aql (A!) !SUO!l??~tIa~T3 aql U! PaAlOAU!aJaM suoqnau pue suolold Jealanuequ!JO slaqurnulepux!sql!M
aa!leluasaIdaJ
nqrcy~ed
aql 01 paqym
!apnu 30 suoyntlalu! aql asnmaq paylsnf al!nb s! UaA!S E jo uxwaads k%laUa 1~101 aql JO ULIOJaql uogdumsse s!qL .suoloJd 30 asm aql u! se awes aql pue asuang aql ‘(8Z 01 ()Z ~013 SaS.Ieqa ‘dnoA!i HA) aq 01 pauxnsse aJaM sue! Lrepuoms 30 suuoj uoynq 8za.Jsr pue ‘(6101 ()Iu10q saSJeq3‘dnol%~1) pi!ssz -ys!p k%aua pua 3EInSuE aqi pue pla!Laql (un) ‘(601 9 UrOJjSaSlEqa‘dno&I fi) iNpI ‘(5 01 E LUO3j !paprESaJs!p aJaM spua qlad uo! le %ul.rallEas aid!1 sa%Eqa ‘dnoJ% 7) *aa, ‘(uxnyaq) laHt ‘(suolold) ‘H, -Inuraqi put2suogenpny SSOIuopezgxo!aql (u) ‘A[auxeu ‘sdnod aql jo qaaajo !apnu aA!leluasaJdaJ fsnalanu Lxewud aql SE ~oj palvva aJaM =OP aq.L ?ss86r) gsams uoalanulad Blaua acuesagi qlp pue uoymp ames 01 Su~p~o~trsdno~Solu!ua~o~qse~umJlsadsaSDi?q3 aql u! mag snalmu al!l@old e 30 sluauxSe~3 aql wql apgn?d ~3f) aqJ_ .aSue.I ,_uoa[mu Aaf) o'oz-10'0 almpu! 01 paumssv alahi elep pwauxuadxa (!) aql 01 Suo~aq 01 uayvl alaM e~lsads LSlaua :suoyduxnsse a!sEqSu!mol~ojaql uo (z) 01 JEI!LU!Sapg.IEda3f) aql ‘a[" 12SV '(SSfj[) 'I"IJ AOS!laAlV lauuew E u! palElnap33 SBM asop lualEA!nba aq_L. u! pasodoJd aauapuadap Mau aql pue (1~61 dx3I .!a[anu Jaqrosqeql!M dnolS f JO !alanu suo~lepuaunuo3a~)d~~~aqlICqpapuaunuo3a~(886~ aAgeluasaldaJ JO suo!s!~oau! paanpold (SluauISeq ‘surroNdlajesuo!le!pe~)aauapuadap paz!pJEpuEls ldaaxa)sagepuocw 01 anp asop aql s! (a‘x)Ta -.x*s.s.naql ‘Qaureu ‘anssgu! ~37 uo ~6 JO sap !!alsnuJaqrosqo qly !alanudnofi-! hreuxud jo -uapuadapo~l Su!sn pale~na[t?aaJaMsasoplUa[EA~nba suo!pE~alu!u! paanpoJd siuaur%eyaJEqa!qm dnoJS/ aql wmur!xEux pue uxnux!u!ux 3elos10j palo1nafe3 30 !a[mu aAymmsaJdar pue dnoti ! jo !apnu aAp alaM salt?1 asop (H) lualEA!nbapue (a) paqJosqE -eluasaJdar 30 ssol uoy~o! 01 anp x qldap la sasop aqL 3afpnlsuogE~nap3 snoueizu! pasn alafi puv aql ‘LlaAyadsaJ ‘aJa (3 &x):a pua (3 ‘x)@ aJaqM poqlaru p~epuelspay!un E dq ‘plays!lauSEur s,qllEg aql puolcaq %npla!qs lapun sasop 8139 aql alE[nalEs 01 alq!ssod l!ayew aAoqa pauoguatu (6861 (2) ‘(3 ‘WYa+ (23‘x)fa (g ‘x>;a = (3 ‘xha ‘a-asS6~‘EwadS r(haug !a-~9861 ‘>esSfj~‘dS(3 -saps) saugap!nSaJnpmoJd pua sp~epuvlsaqL
‘If’+
e[nurroj aql 1cq paltym[e3 am 1 C z qly sap!lred paSJeq3 hvaq luappu! UIOJJsasop aqL '3 sal%JauaououI JOJ paltqna1wdnolg ! JO snapnu aay?luasaJdaJ B uxo33 x Su!p[a!qs lapun sasop luale -Agba pue paqlosqe aql are (a ‘x)!H put2 (a ‘x)!a WA ‘~7 ‘w ‘T ‘aH ‘d = !) ! dnod 01 Su@uolaq sap!yed 30 e~lmk aql30 uxns aql 'a.!‘dnorS.tjo !a[anujo umwtads LSJauaIeguaJagp p301 aql s!(g),& aJaqM hodvmd
‘A
m
sr!In01NH3ZLI.
NOILVTWIV3
(INV V.LVa UldNI
‘Z
‘(9861) sau.mf pue sruepy u! pasodoJd ewads aql 01 JE[!I.U!S allnbala EJlaads.&SJaua paz!pJEpuElS aql‘ULXOjJ!aql UI 'EUI!U!KU pUEEUl!XHUJIe[OS UaaMlaiJ e~laadsap!lJed~I3E)aqlSu!lelnalea~oj aepwuoj aql PUE NIXana
PE
2 ‘A
DOSES FROM
GCR PARTICLES
UNDER
where j = 1, up to i = 1; S,,,(E) is the ionization loss of the representative nuclei of i, j group with energy E (see SCRSDSF, 1985c); k is the number of a group of nuclei from which the nuclei of j group are produced; Pu and A, are the parameters of fragmentation of the k-group nuclei into the j-group nuclei and the interaction free path, respectively. The values of P and 1 presented in SCRSDSF (1985b), have been inferred from copious experimental data and are independent of the energy of projectile nuclei within experimental errors, and where
Fj(0, E) = q’(E) is the initial total energy spectrum of i group nuclei; S(x) is the delta-function. The set (3) consists of one-coordinate equations for radiation transfer. The first equation describes the radiation transfer by the nuclei from the primary i group only. The second equation makes allowance for the radiation transfer by the j-group nuclei produced in fragmentation of the i-group nuclei (j < i). The second term in the expression (2) was calculated by the formula k-i
Dj=C
c
Pkj
k-j+1
x
Fl(x*,
E*)exp
where E* is the energy of fragments produced at an interaction point located at depth x*; energy E* is found from the range @)-energy relation: E = R;‘[R,(E*)
+ (x -x*)1.
(5)
The upper limit of energy integration E& for the kth group is inferred from the relation E& = RL’[Rk(E;t,,)
-x+],
(6)
where Ek_ is the highest energy of primaries incident into the layer. The doses from secondary radiations were calculated making allowance not only for the projectile fragments (including neutrons), but also for the fast (“cascade”) nucleons emitted from either of the interacting nuclei, for the slow (“evaporation”) nucleons
SPACECRAFT
35
SHIELDING
emitted from a target-nucleus, for the recoil nuclei, and for x-mesons. In the calculations, the values of specific doses (the dose per unit flux of each particle species) presented in Vikhrov et al. (1978) were used for the fast particles, for the slow neutrons, and for n-mesons, while the slow charged particles were assumed to be locally absorbed at the point where they were produced. The necessary data on the characteristics of secondary radiation were inferred from both the results of our experiments and the results obtained elsewhere.
3. CALCULATION RESULTS DISCUSSION
AND
Presented below are the results of calculating the GCR particle doses obtained by the techniques described in Section 2 above, under “standard conditions with a tissue-equivalent geometry” absorber. Tables 1 and 2 present the absorbed and equivalent doses from GCR particles under shielding beyond the Earth’s magnetic field during solar minimum and maximum for the energy spectra of primary particles proposed elsewhere (Curtis and Wilkinson, 1968; Kovalev et al., 1978; Adams et al., 1981; Adams and James, 1986) and for the values of the spectra standardized in the U.S.S.R. (Energy Spectra, 1985a-c, 1989). For comparison, the second columns of Tables 1 and 2 present the GCR particle flux in free space during two solar activity periods. The tabulated values of the doses indicate that the most substantial x-dependent changes and model-defined differences are observed at small values of shielding thickness x. These variations occur when the GCR energy spectra include a great number of low-energy particles (up to approximately tens of MeV per nucleon (Energy Spectra, 1985a+, 1989; Adams er al., 1981; Adams and James, 1986)). As the shielding thickness increases, the absorbed dose remains nearly constant, while the equivalent dose decreases monotonically. At the depths x Z 10 g cm-* the doses differ from the mean dose by + 3-5% during solar minimum and by f 8-12% during solar maximum. It should be noted also that the doses prove to be similar in the case of the spectra proposed by Adams (Adams and James,
Table 1. Absorbed dose rates as functions of shielding thickness x for different forms of GCR spectra (cGy yr-I)
Ref. Curtis and Wilkinson (1968) Kovalev et ul. (1978) Adams er al. (1981) Adams and James (1986) Energy Spectra (1985a-c. 1989) Kovalev et al. (1978) Adams er al. (1981) Adams and James (1986) Energy Spectra (1985a-c, 1989)
Particle flux, Particle Cm2s-1 3.36 3.90 5.16 4.40 4.53 2.28 2.36 1.72 1.68
Solar activity Period Min.
Max.
x,
gem-2
0
0.1
1.0
5
10
20
30
18.4 16.5 29.1 23.4 21.3 8.6 13.0 8.7 6.5
18.3 16.4 22.0 21.0 18.2 8.6 10.0 7.0 6.4
17.1 16.1 19.6 16.8 17.3 8.7 8.8 6.5 6.1
14.2 15.2 17.6 15.7 16.0 9.1 7.7 6.0 5.9
13.7 15.0 17.1 15.0 15.6 9.6 7.8 6.3 6.4
14.6 16.4 19.3 16.9 17.3 10.6 8.7 7.1 7.1
14.2 15.8 18.0 16.1 15.3 11.0 9.2 7.4 7.3
36
V. E. DUDKIN
and YU. V. POTAPOV
Table 2. Equivalent dose rates as functions of shielding thickness x for different forms of GCR spectra (CSVyr-‘) Particle flux,
particle Ref.
C&s-’
Curtis and Wilkinson (1968) Kovalev et al. (1978) Adams ef al. (1981) Adams and James (1986) Energy Spectra (1985a-c 1989) Kovalev el al. (1978) Adams ef al. (1981) Adams and James (1986) Energy Spectra (1985ac. 1989)
3.36 3.90 5.16 4.40 4.53 2.28 2.36 1.72 1.68
Solar activity period Min.
Max.
1986) and in the case of the spectra standardized in the U.S.S.R. (Energy Spectra, l985a-c 1989). Figures 1 and 2 are the plots of the decreasing absorbed and equivalent GCR dose rates beyond the Earth’s magnetic field during solar minimum for the U.S.S.R.-standardized spectra (Energy Spectra, 1985a-c, 1989). The plots disregard the doses due to the L-group particles (Z = 3-5) because the relevant analysis has shown that the contribution of the L-group nuclei to the total dose is below 1% under shieldings of all thicknesses. From Figs 1 and 2 it is seen that the component composition of the doses varies substantially with increasing thickness (x). For example, the contribution to the dose under small shielding thicknesses, x, is mainly from the GCR particles with Z > 3 ( - 50% of the absorbed dose and -80% of the equivalent dose), whereas the contribution of these particles under thicknesses x = 30 g cm-’ decreases to 10 and 25%, respectively. It should be noted that more than a third of the
1
0
I 20
I IO
x(g
x,
gem-2
0
0.1
1.0
5
10
20
30
122 96.5 134 115 120 49.5 54.9 40.7 35.6
I21 96.0 109 100 101 49.4 47.8 37.4 33.0
104 88.1 100 90.2 89.0 47.3 45.2 33.0 30.0
72.2 64.0 75.0 64.0 63.5 40.1 36. I 26.8 25.4
53.6 51.1 57.1 49.8 49.6 34.4 28.0 21.6 22.1
37.2 39.1 44.2 38.5 36.8 27.7 22.4 17.8 17.9
28.9 32.8 35.7 31.3 32.5 24.0 19.4 15.6 15.6
equivalent dose under small shielding thicknesses is from the Z 2 20 particles (vH-group) due to the high values of LET and QF for the given particles. During solar maximum, the dependence of dose rate on the shielding thickness, x, is less pronounced than during solar minimum (see Tables 1 and 2 and curves 1 and 7 in Figs 1 and 2). As the thickness x increases from 0 to 30 g cme2, the H dose decreases by a factor of 3.7 during solar minimum and by a factor of 2.3 during solar maximum. The difference in the factors can be attributed mainly to a lower relative fraction of low-energy particles in the fluence of GCR nuclei during solar maximum. At present, the National Radiation Protection Commission of the U.S.S.R. is revising the standardized dependence of the quality factor QF =f(LET,) in terms of the microdosimetric approach and making allowance for the fresh radiobiological data (Atvetisov et al., 1988). The eventual effect of the new representation of quality factors on the GCR dose
I
30
Cm-z)
FIG. 1. The component composition and the total equivalent dose rates under different shielding thicknesses x during solar minimum (solid lines) and maximum (dashed line). Curves I and 7: total doses; curve 2: proton dose; curve 3: vH-group dose; curve 4: LH-group dose; curve 5: helium dose; curve 6: M-group dose.
0
I 20
I 10
x (g
I 30
cm-2)
FIG. 2. The same as in Fig. I for the absorbed dose rate. Curves I and 7: total doses; curve 2: proton dose; curve 3: helium dose; curve 4: M-group dose; curve 5: LH-group dose; curve 6: vH-group dose.
DOSES FROM
GCR PARTICLES
UNDER
SPACECRAFT
SHIELDING
31
*----------__
0
I IO
2y 20 I
I 40
30 (p
L 50
x
cK2)
FIG. 3. The attenuation curves for the GCR equivalent dose rate during solar minimum (curves 1) and maximum (curves 2). The solid and dashed lines have been obtained with QF, and QF2, respectively. attenuation curves was estimated by calculating the equivalent doses on the basis of the new dependence
of QF on LET,. Figure 3 compares the attenuation curves obtained for QF, (the former dependence) and for QFz (the new dependence on LET,). It is seen that the attenuation curves for QF, and QF2 during solar minimum meet each other at x m 3 g crne2. This effect is due to a high contribution under shielding thicknesses of < 3 g cm-* from the low-energy particles with Z > 10 and LET, > 1600 MeVcm-‘, for which QF2 < QF, ( w 40% of the H-dose is from the vH-group nuclei). That is why the H-doses obtained using the QF2 values prove to be smaller. As the shielding thickness increases, the contribution from the vH-group component decreases and the particles with LET e = 20-1600 MeV cm-‘, for which QF2 > QF, , become more important. The soft sides of the energy spectra of the Z > 10 nuclei are much smaller during solar maximum compared with solar minimum, so the effect of the difference in the QF,,, values on the dose attenuation curves becomes noticeable at x 2 10 g cm-* only. The excess of the H-dose obtained with QF2 over the dose calculated with QF, is lo-15% at x > 10gcme2 during solar maximum. Absorbed doses are usually measured when making dosimetric studies in space. It is of interest, therefore, to obtain the dependencies of the mean @ values on the shielding thickness x. Table 3 presents the values of @ during solar minimum and maximum, which have been calculated for the dependence QF, = f (LET,). The @ values for QF, (LET,) Table 3. Values of w
Solar min. Solar max.
(g cm21
FIG. 4. A comparison between the attenuation curves for the GCR equivalent dose rate during solar minimum obtained from the present work (curve I), from Letaw et ul. (1987) (curve 2), and Townsend and Wilson (1990) (curve 3).
differ from the tabulated values by not more than 10%. Figure 4 compares our calculated GCR dose attenuation curves with the results obtained elsewhere (Letaw et al., 1987; Townsend and Wilson, 1990). All the calculations were made for solar minimum under the identical geometric conditions (the “standard geometry”). It is seen that all the results are similar because, probably, of the similar calculational models used and the similar values of the input data. For example, the energy spectra used in Letaw et al. (1987) and Townsend and Wilson (1990) (see Adams and James, 1986) are close in their form and absolute values to our spectra (see Energy Spectra, 1985a-c, 1989). The minor differences between the attenuation curves obtained by us and those in Letaw et al. (1987) are probably due to the inaccuracy in approximating the QF value as a function of LET,. Compared with the standardized values of QF = f (LET,), the QF values used in Letaw et al. (1986, 1987) are lower by 20-30%, so the difference between the latter’s results and our data is most substantial at x Q 15 g cmm2, i.e. when the highest contribution to the total H-dose is from the particles with the highest QF value. It should be noted that the values of H-doses at x < 5 g cm-* are omitted in Letaw et al. (1987) because the calculations there were made for bloodgenerating organs whose mean location depth was taken to be z5 g cm-*. An even smaller difference in the GCR attenuation doses is noted when comparing our data with the results of Townsend and Wilson (1990). In this case,
as a function of x (gcme2)
0
I
5
10
20
30
50
5.65 5.57
4.93 4.82
3.94 4.08
3.16 3.44
2.23 2.52
2.02 2.18
1.90 2.05
V. E. DUDKIN
38
and YU. V. POTAPOV
as seen from Fig. 4, the differences (by up to 20% at a maximum of x = 30 g cm-*) are probably due to different contributions of secondary radiation and to somewhat different input data. To clarify the matter, we have estimated the contribution from the secondary radiation to the total H-dose, as was done in Townsend and Wilson (1990). Analysing the estimates has shown that, under small shielding thicknesses (up to x = S-10 gem-*), up to 90% of the H-dose is due to primary radiation and to the fragments of primary nuclei, while the remaining N 10% is due mainly to low-energy charged particles (residual target nuclei) and to neutrons. As the shielding thickness increases, the contribution from secondary radiation as a whole, and particularly from neutrons, is also increased. At x = 30 g cm-*, for example, the secondary radiation is responsible for - 50% of the total H-dose, of which some 30% is due to neutrons. The shielding thicknesses required for effective radiation safety for spacecraft crews in flights of different durations in interplanetary space were estimated using the attenuation curve obtained for solar minimum and on the basis of the data of SCRSDSF (1986c). The result obtained is presented in Fig. 5 and demonstrates that the expected equivalent dose will not exceed 66.5, I183 and 162.5 CSV during flights of
I-, 2-, and 3-yr durations if the crew is protected with shieldines of 4.0. 6.3. and 8.0 acm-* thicknesses. respectively. It should be borne in mind that the above estimates have been obtained for the GCR particles alone without making allowance for the contribution to the dose from on-board radiation
sources, from the Earth’s radiation belts, and from other cosmic ray sources (solar flares). Therefore, the actual
thickness
of shielding
0
Ill, 405063
I
I
x (g
required
I
60
I IO
for the living-
I 15
err?)
FIG. 5. The annual (curve 1), biennial (curve 2). and 3-yr (curve 3) GCR equivalent dose rates vs shielding thickness x of tissue-equivalent material.
cabin of spacecraft during an interplanetary mission for duration of 3 yr may be much greater than -8 g cm-*. The American researchers Letaw et al. (1987) estimate such a required thickness to be _ 20 g crt -*, which involves _ 30 tons of weight for any reasonable size of the living-cabin, thereby making the technological realization of any manned interplanetary mission very difficult. An alternative approach proposed recently is to carry out long-term manned interplanetary missions during solar maxima when the GCR dose is decreased by a factor of -2. A specially designed radiation-protected shelter is suggested for spacecraft crew during solar flares.
REFERENCES Adams J. H. Jr and James H. (1986) Cosmic ray effects on microelectronics. Part IV: NRL Memo. Renort 5901. 31 December. Adams J. H. Jr. Silhcrherg R. and Tsao C. H. (1981) Cosmic ray effects on microelectronics, part I: the near-Earth particle environment. NRL Memorandum Report 4506, August 25. Alsmiller R. G. Jr, Santoro R. T., Barish J. and Claibome S. (1972) Shielding of manned space vehicles against protons and alpha particles. ORNL-RSIC-35, pp. 493-507. Atvetisov G. M., Gubin A. T. er al. (1988) Recommended dependence of quality factor on LET for the new
radiation safety norms. Hygiene and Sanirorion 10, 32-36. Curtis S. B. and Wilkinson M. C. (1968) Study of radiation hazards to man on extended missions. NASA, CR-1037, Washington, pp. 89-96. Dudkin V. E., Kovalev E. E., Nefedov N. A., Potapov, Yu. V. and Boedanov S. D. (19891 Calculations of galactic cosmic riy doses under‘shielhing during longterm space flights. Abstracts of Reports at V All-Union Conf. on Protection of Nuclear Technological Installations Against Ionizing Radiations. D. 180. Protvino. Dudkin V. E.: Kuznetsov V. G.. Po&ov Yu. V. and Sysoeva 0. V. (1975) Formation of dose fields in biological tissue from multiply-charged ions. In Problems qf Dosimetry and Protection Against Radiations, No. 14 (Edited by Kimel R. L. and Sakharov V. K.), pp. 72-77. Atomrzdat. Moscow. Energy Spectra (1985a) Galactic cosmic ray protons. AllUnion State Standard 25645, pp. 122-185. Standartizdat. Moscou. Energy Spectra (I 985b) Galactic cosmic ray helium nuclei. All-Union State Standard 25645, pp. 123-185. Standartlzdat. Moscow. Energy Spectra ( 1985~) Galactic cosmic ray middle nuclei. All-Union State Standard 25645, pp. 124-185. Standartlzdat. Moscow. Energy Spectra (1989) Galactic cosmic ray light, heavy, and very heavy nuclei. All-Union State Standard 25645, pp. 144188. Standartizdat, Moscow. Interaction of Ionizing Radiation with Matter (1979) Chemical composition of tissue-equivalent matter. AllUmon State Standard 18622-29. Standartizdat, Moscow. Kovalev E. E.. Kolomensky A. V., Muratova I. A. and Petrov V. M. (1978) Model description of differential spectra of galactic 42, 923-926.
cosmic rays. Izv. Akad. Nouk SSSR
Lctaw J. R.. Silberhcrg R. and Tsao C. H. (1986) Natural radiation hazards on the manned Mars mission. NASA MOOZ,pp. 642-655.
DOSES FROM
GCR PARTICLES
Letaw J. R., Silberberg R. and Tsao C. H. (1987) Radiation hazards on space missions. Nurure 330, 709-710. Radiation Safety Norms RSN-76/87 (1988) Energoizdat, Moscow. Recommendations ICRP (1971) Publication 21. Pergamon Press, Oxford. . ’ Space Crew Radiation Safety During Space Flight (SCRSDSFl. (1985a) Methods for calculating absorbed and’ equivalent doses from multiply-charged cosmic ray ions. Procedure Guideline RD 50-25645, pp. 207-285. Standartizdat, Moscow. SCRSDSF. (1985b) Nuclear interaction characteristics of multiply-charged ions. All-Union State Standard 25645, pp. 212-285. Standartizdat, Moscow. SCRSDSF. (1985~) Methods for calculating ionization loss and paths of heavy charged particles. Procedure Guideline RD 50-25645, pp. 206-284. Standartizdat, Moscow. SCRSDSF. (1986a) Methods for calculating absorbed and equivalent dose from cosmic ray protons under shielding. Procedure Guideline RD 50-25645, pp. 208-286. Standartizdat. Moscow.
NT 20/l-D
UNDER
SPACECRAFT
SHIELDING
39
SCRSDSF. (1986b) Nuclear interaction characteristics of protons. All-Union State Standard 25645, pp. 211-288. Standartizdat, Moscow. SCRSDSF. (1986c) Safety norms with flight duration of up to three years. All-Union State Standard 25645, pp. 215-285. Standartizdat, Moscow. Townsend L. W. and Wilson J. W. (1990) Interaction of spaa radiation with matter. Report on 41st Congress of the International Astronautical Federation, Dresden, Germany, pp. l-11. Vikhrov A. I., Dudkin V. E., Kovalev E. E., Komochkov M. M., Lebedev V. N., Litvinova E. G., Mitrinaya V. G., Potapov Yu. V., Potemkin E. L., Sychev B.S. and Frolov V. V. (1978) Atlas of Dose Ckaracteristics
of
External
lonihg
Rahion,
Handbook
(Edited by Kovalev E. E.), pp. 47-51. Atomizdat, Moscow. Wilkinson M. C. and Curtis S. B. (1972) Galactic cosmic ray heavy primary and secondary doses. In Proc. Nat. Symp. Natural Man Made Radial. Space, pp. 104-l 10. Las Vegas, Nevada.