Health Physics Pergamon Press 1975. Vol. 29 (August), pp. 291-300. Printed in Northern Ireland

FALLOUT FROM THE 15TH CHINESE NUCLEAR TEST HIROMICHI NAKAHARA, TAKESHI SOTOBAYASHI, OSAMU NITHO, TOSHIO SUZUKI and SEITARO KOYAMA

Department of Chemistry, Niigata University, Igarashi, Niigata City, Japan 950-2 1 and SHIGEMASA TONOUCHI

Environmental Research Institute, Niigata Prefecture, Niigata City, Niigata, Japan 950-2 1

(Received 14 March 1974; accejted 3 December 1974) Abstract-Fallout particles and radioactive rains from the 15th Chinese nuclear test were studied by an optical microscope, gamma-ray spectrometry, and an electron microprobe X-ray analyzer. The particles were found to be very brittle and of irregular shape. Gamma-ray spectrometry showed that the fractionation behavior of nuclides was extraordinary when compared with those observed for fallout particles in the past. The lo6Rh gamma rays could be observed unexpectedly. Fractionation between the A = 103 and A = 105 mass chains is discussed in terms of the particle formation time and the temperature. The 239Np/237U ratios varied from 0.8 to 4.0 and an attempt was made to correlate the ratios with the type of bomb. Study of particle host materials showed that the hosts were composed of earth materials from the bomb testing site and small amounts of iron. It is noteworthy that sulphur and bromine were also found among the host materials. INTRODUCTION only radioactive rain could be detected from THE15th Chinese nuclear test was carried out the 12th explosion test whereas only two fallout a t 3:55 GMT on 27 June 1973. According to particles in 10 m2 could be collected from the

news sources, the test was performed in air in western China, probably for the purpose of testing the nuclear warhead of a n intercontinental ballistic missile. The magnitude of the bomb was in the range of 1-3 megatons. The first radioactive rain was detected on I July and many radioactive fallout particles could be collected for several days beginning 2 July. Area density of these particles was roughly 1-3/m2 and their radioactivity ranged from 300 to 11,000 counts/min with a GM survey meter of an average counting efficiency of about 10%. The next rainfall came on 11 July, and a large amount of activity could still be detected even 2 weeks after the explosion. In the past 3 yr, three other tests were conducted by China, namely, the 12th test of about 20 kilotons on 18 November 1971, from a tower, the 13th test of less than 20 kilotons on 7 January 1972, in the atmosphere and the 14th of 20-200 kilotons on 18 March 1972, in the atmosphere (ZANDERand ARASKOG, 1973). Here, at Niigata, situated a t a latitude of 39" and a longitude of 138"E, facing the Japan Sea,

13th test. Neither radioactive particles nor rain could be observed from the 14th test although MOOREet al. (1973) reported fallout particles from the 14th. The purpose of this paper is to report on the extraordinary experimental findings on nuclear debris collected from the 15th test. More than 50 fallout particles were spotted, but only 10 of them were collected successfully, because of fragile characteristics of the particles under mechanical stress. These particles and rain samples were submitted to the following three kinds of experiments: (1) observation with an optical microscope, (2) gamma ray spectrometry for determination of radioactive species, and (3) elemental analysis of the particle host by use of an electron microprobe X-ray analyzer (EMX). EXPERIMENTS AND RESULTS

(1) Observation under an optical microscope Only 10 fallout particles could be transferred onto slide glasses for observation under a n optical microscope with its magnification factor

291

292

FALLOUT FROM THE 15TH CHINESE NUCLEAR TEST

of 400. All of them wcre of light color or (2) Gamma ray sflectrometry colorless and no dark colored particles wcre Identification and determination of radiofound in contrast to a previous report on the active species in each fallout particle was done by 3rd Chinese nuclear explosion (MAMURO et al., direct gamma ray spectrometry. The detector 1968). As mentioned above, most of the used was a 15 cm3 Ge(Li) semiconductor system particles were very brittle and either a light connected onto a n 800 channel pulse height poking by a sewing needle or the application of analyzer. Resolution of the counting system a light weight, such as that of a small cover was about 3 keV for 6OCo y-ray. The detector glass, was strong enough to crush a seemingly was calibrated by y-standards of BOCo, 54Mn, single entity into pieces. This brittleness is well 137Cs, 133Ba and Is2Ta. As a n example, a demonstrated in Fig. 1 (a and b) where pictures single y-spectrum of GP-4 is shown in Fig. 2, of GP-8 (see Table 1) are shown before (a) and and for comparison, a single y-spectrum of a after (b) crushing. fallout particle from the 13th Chinese nuclear Sizes and shapes varied markedly among the test observed in our laboratory is given in 10 particles, and only two (GP-2 and GP-10) Fig. 3. I n Fig. 2, a photopeak of 319 keV were nearly spherical, others being of irregular which decays with a half-life of about 35 hr is shape. These observations are quite contrary to clearly seen and can be assigned to the lo5Rh repeated experiences (see, for example, MAMUROnuclide together with the photopeak at 306 keV e t al. (1965, 1968) who observed mostly only which is somewhat contaminated with the spherical particles). Description of particle 305-keV rays of 140Ba (la7Sd also emits y-rays sizes was attempted, thercfore, in terms of their of 3 19 keV with the abundance of 1.5 % whose geometrical cross-section. Results of the optical contribution to the lo5Rh peak is, however, studies on the samples discussed in this paper negligible since there are neither the 91- nor are summarized in Table 1. 53 1-keV peaks, characteristic of 14Wd,detected Determination of the specific gravity of the as shown in the figure. The 316-keV peak of two spherical particles was tried by use of 239Np is distinguishable.) The spectrum of Stoke's method (FUJITA et al., 1967; KOBAYASHI Fig. 2 is typical of all other spectra taken of the and SASAKI,1964), but reproducible results fallout particles from the 15th test and they are could not be obtained with ethanol or water as rich in 103Ru 105Rh 132Te -1321 and 14oBa-140La a medium. in contrast to the ones from the 13th test which are enriched in S7Zr, D5Zr-96Nb,l47Nd. Figure 4 TobL 1. Discriplion of ramplrs shows the spectrum taken from the rain sample . __~ collected at the time of the 12th test. It is quite 'Time after Particle similar to the one shown in Fig. 2 although, it size Comments Sample detonation (days) (m? should be pointed out, no lo5Rh is present in the former spectrum. All the results from the 2 1. of rain n-2 4.5 I 1 July 1973 y-ray studies are summarized in Table 2 in terms 50 1. of rain R-50 5.61 1 July 1973 of number of atoms present in each sample a t Filter residue RK-50 16.59 1 I July 1973 the time of detonation. Also included in the 844 Pentaqonal GP-I 6.60 98.5 Spherical GP-2 5.62 table are the results from the 12th and 13th CP-3 6.11 710 Square 2090 .-f CP-4 7.57 Chinese nuclear tests for comparison. The Very brittle GP-5 8.01 I : errors quoted are based on the number of 540 Vrry brittle GP-6 8.59 0 551 GP-7 9.00 c counts observed under the photopeaks, taking GP-8 10.16 940 Very brittle ln 617 GP-9 11.55 into consideration the magnitude and fluctua3 40 Spherical GP-10 10.71 __ Fine particles GP-I' 9.69 tions in Compton and background levels. - .. .-

-

5zq

K-7

6.79

R-13

6.19

-

7 I. or rain 24 Sovembcr 1971 13 1. of rain 23-24 h-ovcmber 1971

(3) Elemental analysis by an electron-microprobe X-ray analyzer Several papers have been reported on elemental analysis of fallout particles from the past nuclear bomb tests (MAMURO et al., 1965,

FIG. 1. The optical microscope picture of the GP-8 sample: (a) before the specimen was crushed, and (b) after the specimen was crushed.

FIG.5(a). A secondary electron image of the GP-3 sample. (b) A mapping of the elemental distribution in the GP-3 sampIe.

293

HIROMICHI KAKAHARA et al. 2x10

eV

Counted for

112

5L.?OO

SOC.

Te 226

I

1.:

I/ 1.(

O.!

0

ENERGY l i ( e V )

FIG. 2. The single gamma spectrum of the GP-4 sample taken at 7.57 days after the detonation. 15 cm3 Ge(Li).

1966, 1968). According to these reports, most of them were composed mainly of Fe and/or A1 or Si. Thus, it was felt worthwhile to investigate the elemental composition of the host material of such irregular fallout particles.

Due to the brittle nature of the particles collected, only three particles (GP-3, GP-4 and a fraction of GP-8) could be transferred to a well polished copper plate for qualitative analysis of the host material. Each sample was

5 x 10'

Counted for

10,000 sac

4

3

2

1

i 0

tm

200

300

400

500

600

700

800

900

.ENERGY [KeVl

FIG.3. The single gamma spectrum of the G-1 sample taken at 4.33 days after the detonation; 13th test.

294

FALLOUT FROM THE 15TH CHINESE NUCLEAR TEST

fixed by a vacuum evaporated carbon film of a few tens hgstroms thick. The terminal voltage of the electron gun was 25 keV and the probe current was 0.015-0.02 A. Analyzing crystals used were LiF for the wave length range of 1.07-3.75 pA, ADP for 2.7-10.0 pA and RAP for detection of the Na-K X-rays. The detector was a proportional counter with a 25-pmthick Be window. A secondary electron image of GP-3 is given in Fig. 5(a) and mapping of elements of the same sample in Fig. 5(b). As a typical example, the X-ray spectrum of GP-4 is shown in Figs 6(a) and (b) where the presence of sulphur is noteworthy. The elements thus observed are summarized in Table 3 with their approximate relative abundances and comments. Rough relative abundances were estimated by applying theoretical corrections to the observed K, X-ray peak areas. The presence of bromine in GP-8 was confirmed from the La and L, X-ray peaks (Fig. 7). I t should be noted that there was almost no chlorine detected in any of the three samples.

explain those extraordinary features : (1) Chemical fractzonation with respect to 99M0 Chemical fractionation is defined here as the number of atoms ratio with respect to 99M0 at the time of detonation, corrected for the expected differences in fission yields, namely,

where NAodenotes the number of atoms, at the time of detonation, of species A , and Ya denotes the cumulative fission yield of the species A. Results are shown in Table 4. According to the literature (EDVARSON et al., 1959; FREILING and KAY, 1966; MAMVRO et al., 1965, 1968), fallout particles are generally enriched in s7Zr, 95Zr, 143Ce and 14*Ce, namely, refractory elements, whereas very fine particles and rain samples are enriched in 103Ru, lS1I and 132Te, i.e. volatile elements. This trend can be observed for the particles from the 13th test and for the rain from 12th test, although it is not followed by the samples collected from the 15th test. They DISCUSSION are enriched in volatile elements with the only In the foIIowing, some new features found in one exception of GP- 10 sample. (The GP-I 0 is this work are presented. We shall attempt to spherical in shape and its chemical fractionation Counted for

o

100

200

300

400

500

600

700

eoo

goo

50,000 sec

19~)

ENERGY I KeV 1 FIG.4. The single gamma spectrum of the R-13 sample taken at 7.47 days after the detonation; 12th test.

%

G- 1 G-2

9.10 f 1.007 9.30 f 1.127

2.08 f 0.12" 1.28 f 0.13*

N.D. N.D.

N.D. N.D.

R-7 R-13

f 0.05t f 0.02' f 0.06' f 0.06t i 0.067 -I 0.515 f 0.09t f 0.02; f 0.07t f 0.16t f 0.23t f 1.24p f 0.01 f 0.02"

1.C2 f 0.07" 8.55 f 0.62t

1.01 f 0.117 1.80 f 0.17t

8.23 1.83 1.20 1.24 4.46 8.30 7.41 1.65 1.19 2.17 6.79 8.67 1.48 2.19

140 keV

N.D. N.D.

N.D. N.D.

8.03 f 1.206 1.00 f 0.3t

8.46 f 0.596 1.02 f 0.067

1.73 f 0.26t 1.84 rt 0.26t

6.35 k 1.919 1.01 f 0.20t

4.73 f 0.52t 6.57 f 0.52t

1.87 i 0.07t 2.68 f 0.38t 2.35 f 0.07t 1.57 f 0.20t

3.17 5 0.05t 3.45 f 0.087

N.D. N.D.

2.58 f 0.03t 3.13 f 0.297

+

5.89 f 0.14t 1.62 f O.C5* 2.33 f 0.08" 2.43 f 0.177 4.13 f 0.03. N.D. 5.81 f 0.20+ 1.68 f 0.24t 9.43 f 1.576 2.66 f 0.28t 4.89 f 0.427 3.13 f 0.217 2.94 f 0.18t 3.19 f 0.04'

N.D. N.D. 1.25 f 0.05" 1.55 f 0.08t 9.80 f 1.988 1.93 f 0.23t 5.41 f 0.27t 7.23 f 0.44t 2.89 f 0.25t 8.54 f 0.52t 1.25 f 0.06* 1.67 f 0.19t 6.42 f 0.22t 2.90 i 0.03'

*

2.32 f 0.10t 3.69 0.33t 2.29 f 0.12' 2.21 f 0.14t 5.17 i 0.16t 1.64 f 0.16t 1.55 f 0.03* 3.04 f 0.30t 2.9c f 0.22t 1.35 f 0.06" 6.04 f 0.61 t 2.05 f 0.28t 3.16 f 0.267 3.61 f 0.06"

5.51 1.34 f 0.027 0.04" 3.67 f 0.287 N.D. 4.04 f O.llt N.D. 4.31 f 0.13t N.D. N.D. 2.85 f 0.207 N.D. 2.31 f 0.728 N.D. 5.71 f 0.17t

1.91 f 0.227 N.D. N.D.: 1.79 f 0.06" N.D. 1.16 f 0.07* 4.93 f 0.15' 1.74 f 0.17" 1.17 f 0.14* 2.91 f 0.46* 4.14 f 0.698 N.D.t N.D. 1.61 f 0.0611

1.11 f 0.20t N.D. 4.98 f 0.08" 1.02 f 0.04" 3.81 f 0.297 6.67 f 0.467 4.48 f 0.07" 7.56 0.54t 6.91 f 0.35t 8.54 f 0.61t 9.58 f 0.72t 7.09 f 0.31t N.D. 8.69 f 0.09'

137u

208 keV

14Og,-l40~~

481 keV

13a~~-i321

667 keV

''1 365 keV

l'JQu-"'Rh 319 keV

WSRU 497 keV

Table 2 . Number of atoms present at the time of detonation

99Mo-98mTc

.D. Not detected Probably decayed out by the time of measurement. actors attached to the numbers are abbreviated by the symbols as follows: i o ~ ,* 108, t 107, B 106.

2

z

-u

N.D. N.D. N.D.

W.D.

2.68 f 0.357 5.7 f 1.8t N.D.: N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

N.D. N.D. 2.04 f 0.17" N.D. N.D. N.D. N.D. N.D. N.D. N.D.

658 keV

R-2 R-50 RR-50 GP-1 GP-2 GP-3 GP-4 GP-5 GP-6 GP-7 GP-8 GP-9 GP-I0 GP-F

9%

757 keV

Sample

97zr-97~

4.78 f 0.62t 3.27 i 0.49t

8.61 f 0.86t 1.61 f 0.24'

5.59 f 0.17"

4.69 i 0.23t 0.10* 1.49 N.D.t 2.14 f 0.38t 6.89 =t0.06' N.D. 8.94 f 0.75t 5.42 f 0.98t 2.31 f 0.65t 1.05 f 0.16; 1.90 f 0.27' 1.07t 8.63 5.72 f 1.307

239Np 278 keV

FALLOUT FROM THE 15TH CHINESE NUCLEAR TEST

296

500 cps

. n

0

Q1 c

m

2

4

5

i! 3

0

50

u

I

3

I

4

5

6

Wave length ?A)

I

I

L

8

9

10

FIG.6(a and b). The X-ray spectra of the GP-4 sample. The analyzing crystals used are LIF and ADP, respectively.

297

HIROMICHI NAKAHARA et al. .-~

. .....-

Observrd elements

-

Comments

Na, Ca > S,Al & Fe, K Ca 9 Fe,Si S > Mu Fe > K, AI,Br, Si > Na,Ca,Mg 9 C1

GP-3

c, P 4 GP-8

__

Table 3. RrrullJ of E M X ~ t u d u s

__

.-

Sample

Rich in Na, Ca; poor in Fe; no Br, CI Rich in Ca; presence of S; no A1 Rich in Fc; presence o f B r ; little CI ~

.~.

behavior agrees well with that expected from the past experience.) As for the rain samples, it has to be mentioned that they were prepared for y-ray measurements by simply evaporating to dryness. However, the RR-50 sample (see Tables 2 and 4) is the residue left on the filter paper when 50 1. of the rain collected on 11 July was filtered through a millipore filter of 0.45 pm porosity. The filtrate of this rain did not show any appreciable amount of radioactivity and it may be deduced that, together with the similarity in chemical fractionation behavior, the rain samples from the 15th test contained fallout particles. (2) ratio It is reasonable to assume that there is no chemical fractionation between 23QNpand 237U since the former nuclide can be considered to be taken into the particles at the stage of its precursor, 23eU. Therefore, the number of

L 3

I

I

I

4

5

6

atoms ratio between the two nuclides at the time of the particle formation, 23QNp/237U (atomic ratio), will be useful for judging if the tested weapon is of a thermonuclear or fission type (KOYAMA, 1971). When the bomb tamper is made of depleted uranium, as is usually the case, the ratio would be smaller for thermonuclear type bombs. The ratio varies approximately from 0.8 to 4.0 among the samples from the 15th test with a mean ratio evaluated by weighting with the respective standard deviation of 1.49 -& 0.19. According to SARUHASHI et al. (1973), the rain collected on 2 July 1973 in Tokyo showed the 23QNp/237U ratio of 1.23 which agrees well with the present result. In Table 5 are summarized the 23eNp/237Uratios observed from the past Chinese nuclear tests together with the remarks taken from ZANDER and ARASKOG (1973). I t is clearly seen that the ratios are less than 10 for all the thermonuclear type bombs and the ratio from the 15th

Wave length

I

iA>

I

I

I

0

9

x)

FIG.7. The X-ray spectrum of the GP-8sample showing the presence of the bromine L X-rays.

298

FALLOUT FROM T H E 15TH CHINESE NUCLEAR TEST Table 4. Chemical fractionation with respect to s9Mo and r3sNp/237U ratio Nuclide sample

5%;

97Zr 5.32%

9sMo 5.48%

103Ru 4.90%

R-2 R-50 RR-50 GP-1 GP-2 GP-3 GP-4 GP-5 GP-6 GP-7 GP-8 GP-9 GP- 10 GP-F

N.D. N.D. 1.83 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

0.34 0.32 N.D.’ N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

1.oo 1.00 1.oo 1.00 1.oo I .oo 1.oo 1.oo 1.oo 1.oo 1.oo 1.oo 1.oo 1.oo

0.15 N.D. 4.64 9.20 0.96 8.99 6.76 0.51 5.81 4.40 1.58 9.15 N.D. 4.44

N.D. N.D.

N.D. N.D.

1.00 1.00

0.96 1.16

2.10 1.52

1.00 1.00

106Ru 3.89%

1 3 2 ~ ~ 1 4 0 ~ ~ 237U

1311

239Np

3.90%

4.35%

5.22%

t

t

0.33 N.D. N.D.. 20.3 N.D. 19.7 9.37 1.48 13.9 18.9 8.59 N.D. N.D. 10.4

0.94 1.03 0.43 N.D. 1.28 N.D. 0.82 N.D. N.D. 1.85 N.D. 0.37 N.D. 0.37

0.36 0.25 2.40 2.25 1.46 2.49 2.64 0.23 3.07 7.90 1.12 2.98 0.27 2.08

N.D. N.D. 1.09 1.31 0.23 2.44 0.77 0.46 2.55 4.13 1.93 2.02 0.46 1.38

0.72 0.89 1.94 1.96 9.26 N.D. 0.78 0.10 c.79 1.23 0.72 3.61 0.20 1.46

0.57 0.81 N.D.’ 1.72 15.4 N.D. 1.21 0.33 4.84 2.8C 6.60 0.58 2.55

1.53; 3.23 2.45 3.95 3.89 1.83 2.94 1.75

2.86 1.94

N.D. N.D.

4.41 2.69

2.93 1.10

1.94 1.56

4.68 3.65

8.52 8.94

1.82 f 0.27 2.45 i. 0.43

N.D. N.D.

N.D. N.D.

0.11 0.16

0.10 0.15

0.087 0.22

0.062 G.12

0.47 0.38

7.53 f 2.5 3.24i-0.8

N.D. Not detected. * Probably decayed out by the time of measurement.

t Number of atoms ratio with

=sNp/wJ 0.8 i 0.04 0.92 i 0.07 0.88 i 0.17 0.02 1.67

*

1 94.

0.14 f 0.74 i 0.80 i 0.74 i 0.65 i 0.44 f 0.41 ZJZ 0.05

respect to gsMo.

test is the smallest. I t is perhaps worthy of note here that the observed scattering of the 239Np/ 237U data among the samples may have been caused either by leaching or weathering during the time between formation and measurement, or by the difference in neutron energy distribution at the outer sphere of the bomb, namely, at the tamper.

(1) for the definition of chemical fractionation) and a correlation seems to exist between Io3Ru and 105R~-105Rh.This enrichment is astonishingly large and can be related to the particle formation processes and various environmental factors under which the detonation was conducted. In Fig. 8 are plotted 1 0 5 R ~ / 1 0 3 R ~ ratios as a function of 23sNp/237U.A correlation seems to exist between the two ratios, and the (3) Detection of Io5Rh 1 0 5 R ~ / 1 0 3 ratio R ~ increases somewhat as the lo5Rhy-rays were observed from the particles 239Np/237Uratio increases. This correlation, of the 15th test. From Table 4, the chemical however, depends largely on the GP-7 and fractionation of lo5RuJo5Rh with respect to GP-8 data, and it is not certain if it is a forggMoamounts to as much as 20 (see equation tuitous correlation or a true one. The Io5Ru/ Table 5. Chinese nuclear fesfs and a30Np1z37Urafw Date

Time (GMT)

Type of burst

1

641016

0700

Tower

23 4 5

650514 660509 661027 661228

0200 0800

Air Air drop drop Missile, not HHS Tower

6 7

670617 671224

0019

8 9 10 11 12 13 14 15

681227 690922 690929 701014 711118 720107 720318 730627

0730 1615 0840 0730 0600 0700 0600 0355

* SARUHASHI et 51. (1973).

Air drop Air drop Air drop Underground Air drop Air drop Tower Atmospheric Atmospheric Missile ?

Yield “20 ktons

>20 ktons 200-500 ktons