Biswas, S

Durgaprasad, N

Dutta, A

Goswami, J N

Mitra, B

Singh, R K

Vahia, M N

IACS Institutional Repository

Indian J. Phys. 64A  (3) 175-181 (1990)Cosmic ray propagation studies from sub-iron and iron abundances in Spacelab-3 Anuradha experimentN Durgaprasad, S  Biswas, B Mitra, R K Singh and M  N Vahia Tata Institute of Fundam ental R esearch . C o lab a, B o m bay-400 00 5 , India  andA  Dutta and J N GoswamiP h y sica l Research  Laboratory, A h m edabad-380 009 , IndiaA b s t r a c t : So lid  state nuclear track detectors su ch  a s C R - 3 9  and Lexan P o lycar­bonate, have been used here to study the properties of low  energy co sm ic rays. E sp ec ia lly  study of the low  energy region (nam ely 3 0 - 1 0 0  M eV/am u), not hitherto easily  a cce ss ib le  for other typ es of detectors and for iron group n ucle i, w a s made po ssib le  in th is  w ork, by flying th ese p lastics  in sp ace  v eh ic le s  outside  the earth's atm osphere. The instrum ent Anuradha flo w n  in S p ace lab -3  space  shuttle w a s  em ployed for th is  purpose. An enhanced ratio of 1.1 ± 0 .3  w as  observed for the ratio of (scand ium  to chrom ium ) to iron nuclei in 5 0 -1 0 0  MeV/ amu region. Nonstandard m o dels of propagation of iron group nuclei in space  includ ing  reacceleration  p ro cess w ould  be needed for exp lain ing  the observed  data.K e y w o rd s  : C o sm ic  rays, Anuradha experim ent, iron group n ucle i.P A C S N o s :  9 4 .4 0 . - i, 9 4 .8 0 . f gI . IntroductionOver the past few years, solid state nuclear track detectors (SSNTD) were found to be extremely useful in the study of low energy cosmic rays (E j^ IOO  MeV/amu), Of especial importance is the most sensitive plastic CR-39 capable of detecting and measuring charge (Z) and energy (E) of particles Z  /3>6 where is the velocity of the particle with respect to light. Such a plastic was flown in Anuradha Spacelab-3 in 1985. The aim of the work reported in this paper is to measure the charge (Z ^ 2 1 )  and energy ( E = 5 0 -1 0 0  MeV/amu) of iron group nuclei. Plastics exposed in satellites to outer space have a unique advantage in this regard as compared to electronic detectors flown in balloons, rockets and satellites. We report in the succeeding pages the experimental details of the methods used and the results obtained. Our previous Skylab experiment using Lexan deiectors yielded a significantly large value for the ratio of iron secondaries ( S c -C r )  to iron (Fe -C o ) in 5 0 -1 5 0  MeV/amu (Durgaprasad and Biswas 1988). We measured again this ratio in this experiment and discussed its significance.176 N Durgaprasad et al2. The experimental procedureThe Anuradha instrument was flown on board the space shuttle Spacelab-3 on a six day mission in 1985 at an altitude of 350 km and with an orbital inclination of 57 ’ to the equator. The solar activity was approaching solar minimum during this period. The instrument consisted of two stacks of circular plastic sheets of dimensions 40 cm diameter, the top stack being stationary and the bottom one of 149 CR-39 sheets rotating slowly. The rotation was used to get the time resolu­tion, the advantage of which had been used in the succeeding investigation reported by Mitra et af (1990). The present investigation was aimed to study primary and secondary iron group nuclei. Even though relativistic iron nuclei can be recorded in CR-39 sheets, only particles below the energy 400 MeV/amu can be brought to rest in the 4.5 cm thick stack of 149 sheets. We have studied at present, particles that stop within the stack and have energy less than 100 MeV,/ amu, as a first attempt. In later attempts this energy will be extended to 400 MeV, amu by studying lower layers of the bottom stack. The region of energy 5 0 -1 0 0  MeV amu is scantily studied so far.The experimental procedure adopted is as follows. First the CR-39 and Lexan sheets were etched in a solution of 6.25 N NaOH for six hours at 70'C. After drying they were cut into four quarters. These sheets were later mounted on perspex pieces and scanned for cones in the Leitz Ortholux microscopes. An area of 337 cm® in sheet 1 - 2  was scanned so far. These cones were later followed from sheet to sheet till they left the twentieth sheet and only those particles that stopped in these sheets were measured. From these measurements the values of Vt ;Vg (=C.L., B, where Vj is the track etch rate, V„ the bulk etch rate, C.L. is the cone length and B is the bulk material removed since to the etch starting time)were calculated using the relations first obtained by Henke and Benton (1971) and as was obtained by us previously (see for example, Vijayadamodar and Durgaprasad 1988). In this paper, was also described the procedure adopted for determining the charge (2) and energy (E) of these particles. The charge calibration was done using the beams of 140 MeV/amu iron nuclei to which the stacks were exposed, and stopped in the 16 -1 8  sheets depending on dip angle and the thickness of the sheet, at that point. From Z  and £ values, the fluences (f) and the abundances related to iron were determined using the procedure adopted by us previously (see for example Durgaprasad and Biswas 1988).3. Results and discussionWe restrict in this work a study of particles of charge greater than calcium and lessthan cobalt and estimate the S c -C r/ F e -C o  ratios in cosmic rays. This ratio is asensitive indicator of the propagation history of cosmic ray heavy nuclei in theCosmic ray propagation studies etc 177galaxy. This is because the S c - C r  particles are supposed to be rare in the sources and results in cosmic rays due to their traversal of matter in the source regions and interstellar space. The results on the carbon to calcium nuclei will be discussed elsewhere (Mitra et al 1990).The histogram of about ninety nuclei obtained from the present data is given in Figure 1. It should be noted that the data do not represent equi-energy intervals scanned for all the ions, as the response function for nuclei is not the same. This response increases rapidly with atomic number. That is why we found more iron nuclei and less secondaries, whereas in reality it is much more. At this energyATOMIC NO ZFigure I. T h e  m easured charge spectrum  of 93 observed n ucle i. The relative  num bers do not reflect their relative ab und ances s in ce  the energy in terva ls of observation  are not th e sam e.range of 5 0 -1 0 0  MeV amu, direct access for fully stripped iron group nuclei of galactic cosmic rays is possible in the high latitude segments of the Spacelab-3 orbit. Anomalous cosmic rays come either from the heliosphere or local interstellar space. They are likely to be not included in the sample since they traverse little matter and are not likely to produce secondaries.From this data, the sub-iron (Z  =  2 1 -2 4 )  to iron ( Z -  2 6 -2 7 )  ratio was obtained as fo llow s:We used two methods of obtaining this ratio. Both gave almost the same result, we now outline below these two methods.3.1. Method I : In the first method, we calculate the maximum and minimum energies (from the computed ranges) of the titanium, chromium and iron nuclei, that are recorded in this stack. From this, the flux ratio, r, is defined 2r=N (21-24)/N (26  -  27)=  (1/N[26 -  27])-[N(21 -2 2 )  ■ (^£(26)/^£(22)) +  N(23 -  24) (d£(26)/J£(24)]178 N Durgaprasad et al(1)where N(Z) denotes the number of observed nuclei of charge Z. In the above equation J£(Z ) is taken as [£ max ( Z ) - £  min (Z)]. The minimum energy £ (min) is calculated from the minimum range (R min) asR (min) =  [t(AI +  mylar)+ 1(1 -  l) +  t( 1 - 2 ) ] /sin 8=  (182 -f 337 +  235)/sin 40 ’ =  1173 /xm (2)where t(N) is the thickness of the sheet N, 8 is the average dip angle of the tracks taken in this investigation as 40 degrees. The value of R (min) turns out to be 1173/xm. From this £ (min) is calculated using the range energy relation of Henke and Benton (1971).The minimum recordable etch rate ratio Vg is taken as 2.5 in the top sheet 1 - 2  at top surface. The residual range corresponding to this etch rate ratio is obtained from Vt/Vg versus RR. Let us call this value as Rt(Z). Then R (max) (Z) is computed from the relationR (max) (Z )= R  (min) (Z )+R «  (Z) (3)£ (max) (Z) is obtained from R (max) (Z) and thus also J£(Z).N(Z) is the actual number of observed nuclei and are taken from the experi­mental data. Then using eq. (1), the ratio r is calculated. These values of R (min) (Z), R (max) (Z) and d£(Z) are given in Table 1.T a b le  I . V a lu es of ^ E , M eV/am u obtained from method 1.z VtiVg R. (Z )(^m)Rma. (Z )  (//m)Rinln (Z )  (/m )Enmr (Z )  (M eV /am u)E™,„ (Z )  J £  (Z )  (M eV /am u) (M eV /am u)22 2.5 2400 3573 1173 73.3 39.3 34.024 2.5 3200 4373 1173 86.2 41.2 45.026 2.5 4000 5173 1173 99.0 43.1 55.93.2, Method 2 : In the second method, d£ (Z) is calculated from the actually observed values of £ (min) (Z) and £ (max) (Z) in the experimental sample and d£ is also calculated. The rest of procedure followed is same. In Table 2 are given these values. In Table 3 are given the values of the relative abundances of the nuclei [Z = (2 1  -2 7 )]  obtained by both the methods.The observed ratio was 1.1 ±  0.3 for S c —Cr/Fe—Co particles. It will be noted that a similar high value for sub-iron to iron abundance ratio at low energy w asCosmic roy propagation studies etc 179also observed in the Skylab experiment (Biswas et al 1975, Durgaprasad andBiswas 1988).Table 2. V alu es of JE M eV/am u obtained from method 2.Z O bserved va lues  of £fuiu (M eV /am u)O bserved  va lues j £  of Emitx (M eV /am u)(M eV/am u)22 51 78 2724 55 89 3426 55 97 42In Figure 2, we give the variation of this ratio as a functional energy. In thisfigure, the results from other experiments at higher energies are included. Theseexperiments were conducted with various kinds of detectors flown in balloons and satellites (HEAO-C2 for high energy and IM P  7 and 8 for low energy). It sh .w s a strong energy dependence of path length traversed by cosmic rays. However, it should be mentioned here, that data of Garcia-Munoz et al (1987) were measured in a large energy interval of 87 — 400 MeV/amu. In Figure 2, curve 1 is based onr i ’Tm♦oA7* 0  -ft0I 0o e0 60 4  -02>I r r i ■! 11 tHlSWORK (SPACCLA6-9)$K YLAB.OURQAFRASAO CT AL (1966) HCA0-C2.90UT0UL ETAL (1969) MACHLET AL(I977)YOUNG ET A L (I9 6 I)WEBBER (1962)LEZNIAK 6  WEBBER (I976»I979) GARCIA-MUNOZ ET AL (1964) BHATIA a  SINGH (I960)0 iLuuajuuJ90,i.. i .l.xaa, JL lL100 5000 OOCX>900  OOO K ENERGY, MtV/Nfigure 2. O bserved  ratios of Z --21 2 4 / 2 — 2 6 --2 7  nucle i a s  a function  of theirk in e t ic  energy (M eV /am u ). The com puted curves from theoretical m odels are a lso  sh o w n  (see  text for d e ta ils ) .the fit obtained by Soutoul et al (1985) for B /C +0  ratios, length is varied with energy as follows :Aesc=Xb-j8*R-» (R > 5 .5 G V )A esc»Ab*/3 '5 . S - ' (R < 5.5 GV)The mean escape path(4)(5)where R and jS are interstellar values of the rigidity and velocity relative to that of the light. They find that Ab =  24 g/cm® and S=0.65  for a fit at high energies (1 - 1 5  GeV/amu) for the ratio B/C+0. Curve 2 is based on the values of escape length (Soutoul et al 1985) as given belowAesc =  Af iS (1+0.4, Ekin)' R - ‘ (R > 5.5 GV) (6)A e s c = : (1 +0.4, Ekin) •S.S-* (R < 5.5 GV) (7)T a b le  3. The relative abundances of sub-iron  ( Z -  21 - 2 4 )  to iron ( Z = 2 6 - 2 7 )  nuclei.180 N Durgaprasad et alC harge O bserved  of nuclei num ber of Z  nucle i NM ethod 1R atio  relative toM ethod 2  Ratio  relative toZ  = 26 Z = 2 6 - 2 7  Z - 2 5 - 2 7  Z - 2 6  2 - 2 6 - 2 721 22 2 3 - 2 4  21 24  262 6 - 2 72 5 - 2 71611273335390 .800.411.211.001.061.190 .7 50 .3 91.140 .941.001.110 .6 70 .3 51.020 .850 .9 01.000 .750.411 .161.001 .061.190.710 .391.100 .9 41.001.110 ,6 40 .3 50 .990 .8 50 .9 01.00where Ekin is the interstellar kinetic energy in GeV/amu. The data agrees well at high energies (£ > 1 GeV/amu) for values of Af=26.8 g/cm® and 8=0.65. Curve 3 is based on the expected ratios for the constant value of mean free path as 7 g, cm® in 2 00 -1000  MeV/amu energy interval as deduced from B/C +0  ratios. Curve 4 shows the ratios for a mean escape path length of 11 g/cm® for the same energy as deduced from high energies. Curve 5 is given for an infinite path length (complete confinement). The enhanced ratio observed in our experiment can not be understood in terms of the standard model of cosmic ray propagation. This suggests that iron group nuclei have different time history as compared to other nuclei. Non-standard models including reacceleration process or a two stage acceleration process would be needed for explaining the measured data. These models include large traversal of matter in the source region and adiabatic deceleration in the source such as during early phase of the expansion of the supernova remnant. Further work on these problems is continuing here.ReferencesB isw a s  S , N evatia J ,  Durgaprasad N and Venkatavaradan V S  1975 N ature (London ; 259 4 0 9  B hatia  V  S  and S ing h  G  1980 Astrophys. Space S c i. 69  461 D urgaprasad N and B isw a s  S  1988 Astrophys. Space Sci 149 16 3D urgaprasad N, Yad av J  S  and B isw a s  S  19 85  Proc. 16th In t. Cosmic Roy Conf. Kyoto  I 29 6  G arc ia-M u n o z M , S im p so n  J  A , G u z ik  T  G . W e fe lJ  P and M argo lis S  R 19 87  Astrophyi.J. Supp l. 64 26 9G arc ia -M u n o z M, G u zik  T  G , S im pson  J  A  and W efel J  P 1984 A stro p h y s ,} . 2 8 0 L 1 3Cosmic roy propogatlon studies etc 181Henke R P and Benton E V 1971 N u c l, In str . Methods 97  483 Lezniak J  A  and Webber W R 1978 A strophys.} .  223 676 — 1979 Astrophys. Spoce Sci. 63 35Maehl R C, Ormes J  F , Fisher A  J  and Hagen F A 1977 Astrophys. Space Sc i. 47 163 Mitra Banashree, Biswas S, Durgaprasad N , Singh R K, Vahia M N, Dutta A  and Goswami J  N m o  Indian.]. Phys. 64 201Soutoul A , Englemann J  J ,  Ferrando P H , Kochmirand L , Masse P and Webber W R 1985 Proc. 19th In t, Cosmic Ray Conf. La Jo lla  2 8 Viiayadatnodar G V and Durgaprasad N 1988 In d ia n ]. Phys. 62B1 Webber W R 1982 Astrophys. j. 252 386Young J  S, Freier S P, Waddington C J ,  Brewster N C and Fickle R K 1981 A strophys.]. 246 1014

Cosmic Ray Propagation Studies from Sub-Iron and Iron Abundances in Spacelab-3 Anuradha Experiment

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Cosmic Ray Propagation Studies from Sub-Iron and Iron Abundances in Spacelab-3 Anuradha Experiment

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