Determination of cosmic radiation characteristics aboard “salute - 7” orbital station

Nuclear Tracks, Vol. 12, Nos I-6, pp. 489-492, 1986. Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain.

0191-278X/86 $3.00+.00 Pergamon Journals Ltd.

DETERMINATION OF COSMIC RADIATION CHARACTERISTICS ABOARD "SALUTE - 7" ORBITAL STATION

A.B.Akopova,= V.E.Dudkin, O.N.Karpov, L.V.Melkumyan, Yu.V.Potapov, Sh.B.Rshtuni Yerevan Physics Institute, 375036 Yerevan 36, ~arkaryan St. 2, Armenia, U.S.S.R.

ABSTRACT

The particles fluence was measured, the integral LET spectrum of cosmic radiation in the range of ( 7.55"102 t 4.9-104 ) ~eV/cm in the emulsion as well as the integral

spectrum of charge distribution

of stopping n u c l e i with

Z ~ 5 were obtained with the help of registration controlled nuclear emulsion exposed on board the "Salute - 7" orbital station during more than 8 months.

KEYWORDS

Nuclear emulsions; development threshold, integral spectrum; linear energy transfer; particle fluence; radiation dose.

The problem of radiation safety of a space vehicle crew is highly important nowadays, when the range and sspeolally the duration of space missions increase and is as great as several months and more. Hence, any accurate and reliable appraisal of the radiation hazard for astronauts exposed to the action of oharged particles is of great practloal interest.

A l l t h e a v a i l a b l e d a t a on t h e most i m p o r t a n t c h a r a c t e r i s t i c s used in t h e dosimetry, the LET spectrum and the rate of absorbed radiation dose, were o b t a i n e d d u r i n g c o m p a r a t i v e l y s h o r t m i s s i o n s of d i f f e r e n t space v e h i c l e s for less than 20 days ( e.g., in Cosmos - 782, 936, 1129 artificial Earth satellites e t c . ) r e p o r t e d by K o v a l e v (1979, 1982), t h a t gave p o o r s t a t i s t i c s 489

490

A. B. AKOPOVA et a~.

I. CHOICE 0F

d,,,'= tC ~-R

~

FUNCTION

The charge identification

of heavy nuclei was carried out using the method b a s e d on t h e d e p e n d e n c e o f g r a i n d e n s i t y ~/)on the residual r a n g e JR}. -12 .. 20 To find this dependence the emulsions were calibrated with He , u~ , ~ s l ~ ions with E = 9 . 1 M e V / n u c l , accelerated in Dubna. The dependence ~'/t/=/["/~) for each ion was plotted for the layers with different registration thrsIh~J was found for the olds, and the experimental dependence ~--~_ energy loss range corresponding to that of calibration ions. In each case 20 and more identical tracks were measured to mln~mize the statistical errors within + 0.5. It was necessary to compare the grain density d e t e r m i n e d on t h e p a r t o f t h e t r a c k o f some f i n i t e length with the specific ionization l o s s on t h e same l e n g t h . T h i s c o n d i t i o n d e t e r m i n e d t h e choice of the "step" of measurement, I 0 / ~ .

//4/) As is seen from experimental results in Pig. I, the function ~-~--z F ( ~ / for particles with Z >/1 i s c o r r e c t l y described by the expression found by Blau (1949)

provided that the total specific ionization losses are replaced by "local" energy losses causing the production of ~ -electrons with E _z 5 KeV , w h i c h make t h e d e v e l o p a b l e l a t e n t i m a g e c e n t e r s t o show up a l o n g t h e t r a ~E Jectory. The p a r a m e t e r -Az~o h a s t h e m e a n i n g o f t h e t h r e s h o l d of emulsion sensitivity and is determined from extrapolation of the empiric curve to -mAll values of grain density. It is necessary to note that the value of experimental threshold is always higher than ~ o . This discrepancy is due to the fact that the experimentally

determined threshold is the energy

loss on the length corresponding to minimum countable grain density - 2 grains per I 0 ~ . So, the experimental threshold has the meaning of the

registration

threshold

of the track,

while

the

extrapolation

threshold

is

sensitivity threshold.

The e x t r a p o l a t i o n t o maximum p o s s i b l e g r a i n d e n s i t y " c " ( w h i c h i S a p p r o x i mately equal to the number of undeveloped grains per unit length) determin e s t h e maximum p o s s i b l e v a l u e o f s p e c i f i c ionization in the grain counting technique. The d e n s i t y r a t i o 1/10 determines the disoriminational nature of latent image centers development along the track and is an indication of effective transfer of the ionization gradient. The p a r a m e t e r " b " i n (1) describes t h e mean e f f i c i e n c y of the development of latent image cent e r s i n t h e g i v e n d e v e l o p m e n t r e g i m e . The f i t t i n g of (1) to experimental res u l t s w a s made b y m e a n s o f t h e m e t h o d o f l e a s t s q u a r e s u s i n g t h e PUMILI s u b -

DETERMINATION

OF C O S M I C

RADIATION

CHARACTERISTICS

491

!

to t

~*

Fig. 2. The spectrum of 11-

Fig. I. Empirical ~-~= / [ ~

for different

)

empirical c es for particles with differ-

curves thresh-

ent charges.

olds. program,

The curves in Fig. I represent several functions for different thresholds. Experimental points on the curves show that the expression (I) gives a good fit to experimental data for arbitrary values of thresholds and charges. So, the obtained function allows one to find the dependences of grain density on the residual range for particles with different charges. In Fig. 2 the spectrum of these curves for the sensitivity threshold of 1129 MeV/cm is given.

2. THE METHOD OF IDENTIFICATION

To determine the particle charge, the grain density in two parts of the track ( A /~/I and n/t~ ) and the distance between them ~ R ) were measured. Then the charge was determined by checking these values against the curves analogous to those shown in Fig. 2. This method allowed us to identify the charge of low energy particles with sufficient accuracy, provided that one can determine the grain density at different values of ~ and make several independent measurements on the same track. If the density does not change much along the track, we may obtain the upper and lower limits for Z.

3- THE E X P ~ T M E N T A L

Two

200~

thick layers of

giving the registration

RESULTS

6 ~ - type emulsions were developed in the regime

thresholds

of

100 and 2200 MeV/cm.

The thresholds

492

A.B.

AKOPOVA

et al.

correspond to the first and third curves in Fig. I. To md~e the calibration curves more exact, we selected the tracks of stopped particles, the grain density of which in the initial part was minimal (2 grains per 1 0 / ~ ) and this part was deeper by 15 - 2 0 / ~ f r o m the layer surface. This allowed us to attribute the threshold loss to the first grains of the track. So, the energy loss and the residual range allowed to identify the particle without measuring the grain density distribution along the track. As is seen in Fig. 3, the tracks of the proton (in the layer with 100 MeV/cm registration threshold) and carbon (in the layer with 2200 MeV/cm threshold) identified in such a way, are also correctly described by corresponding empirical curves. a# f6" f6

tq ¢2 8"

¢o

6o g-

....

Pig. 3. The empirical c u r v e ~ w and experimental data with root mean square errors for 5 tracks of H~ (curve I) and 4 tracks of C~ 2

(curve 2).

,~. . . .

ko ....

~

.....

P.,s~

Fig. 4. Experimental and empirical curves of grain density vs. residual range. The confidence limit was: 99% for Z=6 (curve I); 82% for Z=7 (2) and 25% for Z=5 (curve 3).

The correspondence between the hypothetical distribution and measurements was verified for each track using Pearson's criterion. For this purpose the track was compared with the empirical curve ~ N z / / R ) for the supposed values of Z

and Z + 1. In Pig.4 we present an experimentalW~ -dependence of Z~/V vs. for the track which was identified as the carbon track. REPERENCES

Akopova A.B., Magradze N.¥., Moiseenko A.A., Muradyan S.Kh., Hovnanyan K. (1983), Method of Selective Development of Thick-Layer Nuclear Emulsions, ~ -671(61)-83. Blau M. (1949) Grain Density in Photographic Tracks of Heavy Particles, Phys.Rev. V.75, p.279.