Development of an autoradiographic method of investigation of hot particles from the chernobyl nuclear power plant

Nucl. Tracks Radial. Meas., Vol. 21, No. 3, pp. 323-328, 1993 Printed in Great Britain

0969-8078/93 $6.00 + .00 © 1993 Pergamon Press Ltd

DEVELOPMENT OF AN AUTORADIOGRAPHIC METHOD OF INVESTIGATION OF HOT PARTICLES FROM THE CHERNOBYL NUCLEAR POWER PLANT A. B. AKOPOVA, N. V. VIKTOROVA, V. M. KmSHCmAN, N, V. MAGRADZE, K. M. OVNANIAN, K. I.TUMANIAN and T. S. CHALABIAN

Yerevan Physics Institute, Alikhanyan Brothers Str. 2, 375036, Yerevan, Armenia Abstract--A method of autoradiographic investigation of hot particles that have risen into the atmosphere and precipitated on the soil and leaves after the Chernobyl accident of April 1986 has been developed. Particular objects of investigation are the a-active radionuclides with their dimensions and activity varying from 0.2 to 200~m and from 10-6 to 10-I Bq, respectively. BYa-2 nuclear emulsions were used as detectors. The activity distribution function of particles of different dimension, A = f(d), and the leaf contamination density distribution in the vertical profile are obtained.

1. INTRODUCTION AT PRESENT the parameters of ~- and //-active hot particles (HP) resulting from the Chernobyl accident are being thoroughly investigated. But the a-active HP, especially those having the dimension d < 5 #m, which are biologically dangerous because they may be inhaled (Bykhovsky and Zaraev, 1974), are poorly investigated as the radiochemical method used for this purpose provides data only on the total content of ~-radionuclides in the samples. The purpose of the present work is to develop a method of investigation of the parameters of ,,-active HP with various dimensions. The results of investigation will allow us to create a biological model for estimation of the contribution of a-active HP to the radiation dose.

2. M E T H O D A thin (0.08 g c m -2) calcinated soil sample taken in 1989 at a distance of 5 km north-west from the Chernobyl nuclear power plant (ChP) and leaves from the herbarium gathered in 1986 near the river of Pripyat at a distance of 1 km from the plant were used as sources of a-active radionuclides. The investigations were carried out on BYa-type nuclear emulsions sensitive to protons of energies less than 50 MeV, by the sample-emulsion contact method of exposure. The exposure was determined experimentally. It lasted from several hours to several days for particles with high (from 10 -1 to 10-4Bq) and low (from 10 -2 to 10 -7 Bq) activity, respectively. The alpha-particles released from HP form a characteristic fan pattern in the nuclear emulsion, with the tracks radially dispersing from the central

part of the autoradiogram (Fig. l(a)). The central part of the pattern is outlined by the tracks of a-particles released in the plane perpendicular to the emulsion surface by the radionuclides located at the HP edges. The distance between these imaginary planes formed by the ~-particle tracks determines the particle size. Similarly, the distance between the maximally distant parallel tracks of a-particles emitted by the radionuclides at any angle of the emulsion's surface, determines the diameter of the HP, as is shown in Fig. 2 which presents a schematic view of a spherical HP. Since spherical HP are very rare, up to 16 parameters of each particle have been measured by step by step rotation of the microscope's table for 30 ° around the center of the particle radiograph. The mean value of all measurements is taken as the effective diameter of the particle. The accuracy of the HP diameter measurement by this method is estimated by the mean square deviation and is no less than 85%. But the present method developed by us (Akopova et al., 1990) is, in fact, unsuitable for measuring the small-size particles with activity from 10 -6 to 10 -7 Bq, because the formation of a characteristic fan pattern with radially dispersing tracks in their autoradiogram required unwarranted long-lasting exposures. Therefore, we used a special technique (Styro et al., 1963) of particle size estimation (d < 5 #m) by several (3 _
324

A.B.

AKOPOVA

et al.

(a)

(b)

FIG. 1. Microphotos of HP with an activity of: (a) ~ 10 -t Bq; and (b) 10 -4 Bq.

HOT PARTICLES

FROM

THE CHERNOBYL

flaming laym"

POWER

325

PLANT

Table 1. Activity distribution of particles of different size in soil samples

FIG. 2. Schematic diagram of size measurement of HP with < 10 -4 Bq activity.

-6.~-

NUCLEAR

I

-3~9

I

I

I

- 11..~

9.72

30.'r/

x (t~m)

A (Bq)

d ~um)

9.6 x 10 -1 5.9 x 10 -I 5.9 x 10 -m 5.6 x 10 -I 4.8 x 10 -j 4.8 x 10 -I 4.1 x 10 -I 3.7 x 10 -t 3.2 x 10 -t 3.1 x 10 -I 1.8 x 10 -I 1.4 x 10 -1 1.3 x 10 -I 1.1 x 10 -m 1.0 x 10 -l 9.6 X 10 -2 6.7 X 10 -2 2.0 X 10 -2 1.7 X 10 -2 1.1 X 10 -2 7.8 x 10 -3 6.3 x 10 -3 5.9 x 10 -3

201 107 148 i 18 105 105 111 84 103 92 80 73 87 60 54 48 60 32 83 41 35 35 60

A (Bq) 5.9 5.6 5.2 5.2 5.2 4.4 4.4 4.1 3.7 3.1 2.7 2.2 1.6 1.7 1.5 1.3 1.2 1.1 1.1 9.6 4.4 3.4

x 10 -3 × 10 -3 x 10 -3 x 10 -3 × 10 -3 x 10 -3 x 10 -3 x 10 -3 x 10 -3 x 10 -3 x 10 -3 x 10 -3 x 10 -3 X 10 .3 X 10 -3 X 10 -3 × 10 -3 X 10 -3 X 10 -3 X 10 -4 x 10 -4 x 10 -4

FIG. 3. Schematic diagram of size measurement of HP with > 10 -6 Bq activity. Table

2.

Size and depth of penetration o f HP with an a-activity (0.9-6.0) x 10-16 Bq into blackberry, willow and walnut leaves

Blackberry

R (~m) 0.10 0.40 0.43 0.44 0.58 0.74 0.76 0.90 1.34 1.39 1.67 1.92 ZOO 2.49 2.66 2.88 2.89 3.25 4.28 4.16 4.48 5.38 5.67 5.73 6.03 6.74 8.28 12.53 12.73 13.48 16.14 20.59 22.48 23.94 25.31

Depth of penetration (~m) 5.7-7.0 7.0-8.0 7.5-8.6 6.6-8.0 3.0-4.0 4.8-6.3 5.7-6.5 1.4-10.0 7.8-10.0 0.0-4.5 6.0-9.1 3.5-12.6 3.2-7.0 1.0-11.0 1.0-6.0 0.0-2.5 7.0-45.0 15.6-23.0 22.0-58.0 8.0-2.5 14.3--40.0 5.0-7.0 2.5-5.0 191.3-15,0 9.0-17,7 9.0-17.2 11.3--60.0 20.0-27,0 3.0-40.3 14.9-16,2 24.8-45.0 17.5-19.8

Willow

R ~m) 0.80 0.82 0.84 0.85 1.16 1.19 1.30 1.69 1.81 1.94 2.85 2.86 3.26 3.36 3.56 3.67 4.08 4.08 4.05 5.72 5.83 6.58 11.63 12.52

Depth of penetration ~m) 0.2-7.7 0.8-3.4 6.5-9.0 13.0-15.5 6.5-12.5 9.5-10.5 7.0-14.0 0.25-5.14 13.0-17.0 9.5-19.0 10.0-18.0 0.2-18.0 12.3-24.5 13.86-24.4 3.5-6.5 5.3-19.5 7.5-14.4 15.4-28.7 1.6-8.0 10.0-22.0 21.0-59.0 37.6-59.4

Walnut

R ~m) 0.19 0.19 0.27 0.44 0.46 0.90 1.06 1.08 1.47 1.68 1.77 2.28 2.48 2.66 3.28 3.48 5.47 7.48 15.87 22.35

Depth of penetration ~m) 3.0-5.0 9.5-10.0 2.0-6.0 0.6-0.8 0.1-1.5 1.0-5.7 0.0-4.0 2.5--4.0 5.0-6.8 61.06.0-18.0 2.0-8.0 1.2-2.8 5.5-2.9 0.0-8.0 0.0-18.0 3.0-18.0 11.5-39.7 12.0-18.0

of

d ~m) 22 19 20 48 35 31 34 24 13 14 62 12 -25 22 22 11 23 23 11 55 --

326

A . B . A K O P O V A et al. selected all the cases with three or more radially dispersing tracks of ~-particles on that face of the nuclear emulsion that was in close contact with the samples. In each selected case, the particle paths were reconstructed by the measured X, Y and Z-coordinates of all the g-tracks. The HP site in the sample was taken to be the region of the closest approach of all reconstructed tracks. The effective particle radius was taken to be the radius of the circle, which had an equal area with the triangle, the vertices of which were the midpoints on the closest approach paths.

10-1

1o-21 -

7"

10-3

•

"

3. RESULTS AND D I S C U S S I O N 10-4

The results of measurements on the walnut, willow and blackberry leaf samples were processed by an EC-1046 computer. Figure 3 shows the results from a 3-~ autoradiograph. The particle activity was determined by the total number of ~-tracks on the autoradiographs. From the results of HP size and activity measurements in all samples, a very important characteristic was discovered--especially from the point of view of radiation hazard connected with the inhalation of H P - - t h e activity distribution function of particles of different size (see Tables 1 and 2, and Fig. 4). In spite of the spread in experimental data, a clear dependence of activity on the particle size was observed. This dependence was strongest in the region of small-sized particles and weakened with increasing particle size. The shape of the curve could have been due to an essentially strong self-absorption of g-radiation in large particles and its absence in particles

'

10-61

I

I

100

d

.........

200

0un)

FIG. 4. HP activity as a function of particle diameter. After a close-contact exposure of the nuclear emulsion layers to the source of radionuclides and their chemical treatment, each nuclear emulsion layer was scanned for an g-active HP autoradiograph. We

(a)

I

[ | I s ~-] I I 16

I - Blackberry leaf 2 - Willow leaf 3 - Walnut leaf

14 12 n

10

|--! 2 I-

!

i

;

-I

*" J ,-* " _

. I

0

2

4

6

$

. I

10

12

14

R 0~m)

Fig. 5(a).

,-,

I"1 ~. . . . -'

I

16

, I

18

20

22

24

26

28

HOT PARTICLES FROM THE CHERNOBYL NUCLEAR POWER PLANT

I

(b)

'A-k

n0 2

l

4

I

I

I0

2o

I'~ 30

40

I

!

50

R ~)

327

\.

10 4 -

FIG. 5. H P c~-activitydistributionin size for: (a) inhalable; and (h) non-inhalable particles.

\ I 1

1o4

with d < 40 #m. This dependence was satisfactorily approximated by the exponential

I 2

h (m)

A = Area x - - I " 1 -a2dc

where Amax is the maximum activity obtained experimentally; A is the activity of particles; I, B, C are parameters. The expressions were fitted to the experimental data by means of sequential approximations using Fumili subroutines. A good fit of X 2= 18.8 on 112 points was obtained at three values of the variables I = 1.7, B = 5.0, C = 1.1. The data on size distribution of radionuclides are very important for estimation of the biological hazard and determination of the mechanism of HP formation. The distribution curves are asymmetric, with a steeper slope towards small particles (Fig. 5(a), (b)). In the probabilistic-logarithmicscale the particle size distribution lies satisfactorily on a straight line, which allows us to consider it as a normal logarithmic distribution law and to estimate its parameters. The median radius and the r.m.s, deviations were 2.17 + 0.7 # m and 12 _+ 2.8 # m for the leaf and soil samples, respectively. The four available soil and leaf samples enabled measurement of the contamination density distribution in the vertical profile. Analysis of the radioactive fallout distribution carried out by the data presented in Table 3 and Fig. 6 shows that the contamination density increases with decreasing distance to the ground and reaches a maximum in the soil. It is supposed that large HP are easily washed from the leaves by rainfall and deposit in the soil, while

FIG. 6. Distribution density of a-contamination of plants from different tiers in the vertical profile.

a-

to~

10-s

t

10-d

10"7

1~0-~

I

toe A(Bq)

FIG. 7. The contamination density as a function of ~-activity of HP from the leaf samples of: (i) blackberry (upper curve); (ii) willow (middle curve); (iii) walnut (lower curve). small particles, as follows from Table 2, are arrested at various depths in the leaves. They are found at depths from 0 to 60 #m. It should be noted that no HP larger than 25 # m were found in the leaf samples.

Table 3. Some physical characteristics of the sources of 0t-active HP Distance from the ground

I

10.5

S

Total activity

Deposition density 3.2 × 102

Source

(m)

(cm2)

Soil Blackberry Willow Walnut

0 0.1 1 2

64

2.1 x 104

(Sq)

18

3.2 x 10 -3

1.8

30

1.7 × 10-3

5.5 x 10-s

58

2.4

3.7 x 10 -6

X 10 -4

(Bq cm -2) × 10 -4

328

A . B . AKOPOVA et al.

The fallout density estimation (Fig. 7) shows that the main contamination in the samples is due to the large number of small-sized a-active particles, which are most dangerous from the biological point of view. Thus, the most important task is to study the parameters of an individual ~t-particle from the inhalable fraction. Analyzing the results obtained, one may consider that the technique developed allows us to obtain important information about each individual ~t-active particle, about its size, activity, depth of penetration into the sample in a wide range of diameters from 0.2 to 200#m and activities from 10 -5 to 10=7Bq, as distinct from the radiochemical method, which gives

the total content of ~-active radionuclides in the sample investigated. REFERENCES Akopova A. B., Magradze N. V., Moiseenko A. A., Chalabian T. S., Viktorova N. V. and Garger E. K. (199 I) Autoradiographic investigation of radionuclide alpha-activity in soil and plant samples from the Chernobyl zone. Nucl. Tracks Radiat. Meas. 19, 733-738. Bykhovsky A. V. and Zaraev O. Z. (1974) Hot Aerosol Particles in Technical Application, p. 28. Atomizdat, Moscow. Styro B. I., Garbaluuskas Ch. A. et al. (1963) On the presence of a-emitting hot aerosol particles in the atmosphere. A E 1963 15, 262.