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Determination of short-lived radionuclides in fresh fall-out debris for identification of nuclear weapon tests
DETERMINATION OF SHORT-LIVED RADIONUCLIDES IN FRESH FALL-OUT DEBRIS FOR IDENTIFICATION OF NUCLEAR WEAPON TESTS S. B. HINGORANI,R. N.
KHANDEKAR
and S. J. S. ANAND
Air Monitoring Section, Bhabha Atomic Research Centre, Trombay, Bombay-400085, India (Received I April 1975. Accepted 9 September 1975)
Summary-Radiochemical procedures for the assay, of short-lived fission and activation products are described. They are rapid and quantitative aid the radionuclides separated are radiochemically pure. Ratios of some of the short-lived radionuclides obtained by these measurements for selected Chinese and French nuclear tests are given and provide information about the fissile material used in the
The programmes of nuclear weapon testing by China and France contribute considerable quantities of short-lived radionuclides to the atmosphere. The
measurement of this short-lived activity by y-ray spectrometry alone is not sufficient for the identification of a weapon test, so a chemical separation of the fission and induced products is necessary. The chemical methods selected must take into account the complicated composition of the sample, be rapid and quantitative and have high chemical yield. The methods presented in this paper meet these criteria and also permit joint assay of various radionuclides from a single sample. The sample material is a swab collected by rubbing the surface of commercial aircraft with cotton soaked in organic solvent. These samples are routinely collected on arrival of the aircraft at Bombay to estimate the background levels of the atmosphere before and after nuclear tests. The results are used to get information about detonation, e.g., fissile material used, mode of detonation etc., by comparison of the activity ratios of selected fission products such as “Sr, lllAg, 13’Te and “MO. These fission products, being shortlived, do not suffer interference by products from earlier tests. There are two important activation products which help in deciding whether the test was with a conventional H-bomb using a three-stage device with 238U in the third stage. These are neptunium-239 and uranium-237 which are produced from 23sU by (n, y) and (n, 2n) reactions respectively. 237U, in a nuclear weapon, is also produced from 235U by double neutron capture but its production by this reaction is very small compared with that from the (n, 2n) reaG tion in a conventional H-bomb test. Other activation products also help in determining parameters of the weapon test. However, the samples collected for our studies gave hardly any measurable quantities of these activation products and therefore only 237U and ‘jgNp are discussed in this paper. 313
EXPERIMENTAL Sample preparation
The sampling method collects considerable quantities of insoluble silica, iron, sulphate and phosphate compounds. Because of the difficulty with which these compounds dissolve, a leaching procedure for the dissolution of the sample is used.’ The cotton swab is digested with a mixture of 3M hydrochloric acid and O.lM hydrofluoric acid in the presence of known amounts of carriers. The heating time is reduced to a minimum to reduce the attack on the glassware. The solution is filtered through a Whatman No. 54 paper on a Biichner funnel. This process is repeated 3 or 4 times and finally the residue is leached with 6M nitric acid and filtered ofl. It is found that more than 95% of the total activity is leached out by this process. All parts of the filtrate are then pooled and a suitable aliquot is taken for the determination of each individual nuclide. Chemical procedures
The chemical procedures developed in our laboratory are described below. Barium-140 is usually determined directly from the gamma-ray spectrum through its daughter ““‘La. For this a known fraction of the leach solution (usually a tenth of the total volume) is evaporated in a beaker to minimum volume and transferred to a 5.0-cm diameter “Perspex” planchette which, after drying, is sealed and counted by a 256channel pulse-height analyser, using a Ge(Li) detector. In general, the final isolation of the precipitate is carried out by filtration with a special filter funnel using a l-in. Whatman No. 542 paper and the chemical yield is obtained by weighing the precipitate. The sample is then mounted on a planchette and counted conventionally. The filter funnel is made of “Perspex”, and has a threaded base, a 2.5-cm diameter circular perforated disc and a vertical reservoir 2.4 cm i.d. and 12 cm in height. Separation of‘ strontium and barium.’ A known volume of the leach solution with known quantities of carriers (150 mg of Sr + 100 mg of Ba) is placed in a beaker. The solution is warmed and anhydrous sodium carbonate added. The insoluble carbonates are filtered off (Whatman No. 540 paper) and redissolved in 6M nitric acid. The volume of the solution is reduced to 40 ml by evaporation and 65 ml of fuming nitric acid (sg. 1.52) are added slowly to the ice-cooled solution. The crystals of barium and strontium nitrates are allowed to settle. The supernatant liquid is decanted and the crystals are dissolved in distilled water. The volume of the solution is again brought to 40 ml and the nitrate precipitation
314
S. B. HINGORANI.R. N. KHANDEKAR and S. J. S. ANAND
repeated twice with 60 ml of fuming nitric acid, The nitrate crystals are then dissolved in water and strontium and barium precipitated by addition of ammonium carbonate. The solution is warmed for about 10 min to coagulate the carbonates and finally filtered. The precipitate is dissolved in 6M hydrochloric acid and barium is precipitated as chromate by addition of 10 ml of l.SM sodium chromate and adjustment of the pH to 3.54. The chemical yield and activity are then measured. The filtrate from the chromate separation is warmed and ammonium carbonate added to precipitate strontium. The precipitate is filtered off (Whatman No. 540 paper) and dissolved in 6M hydrochloric acid. The solution is warmed with absolute alcohol (2 or 3 drops) to reduce any chromate. Twenty mg of yttrium carrier are added and the yttrium is precipitated by slow addition of freshly prepared ammonia solution to remove yttrium-90 and other rareearth activity. The solution is warmed, cooled and filtered and the time noted. Ammonium carbonate (2-3 g) is added to the filtrate, the solution is warmed and the strontium carbonate allowed to settle. The activity is then determined after filtration. After the counting of s%r and ‘%r activity, the precipitate is ignited to constant weight in a platinum crucibIe to obtain the chemical yield. The residue is dissolved in 3M hydrochloric acid and stored with 20 mg of yttrium carrier for equilibration and subsequent milkings. Separution of silver and molybdenum.” A known volume of the leach solution, with known amounts of carriers, is placed in a beaker and evaporated almost to dryness. To the residue are added 2 ml of cont. nitric acid and 2 ml of bromine water, and the resultant solution is slowly heated to oxidize tin and molybdenum. The solution is cooled and diluted and a few drops of 6M hydrochloric acid are added, with constant stirring, to precipitate insoluble chlorides. The precipitate is centrifuged, washed with distilled water and dissolved in ammonia solution. Ten mg of iron carrier are added to the solution and iron hydroxide scavenging is carried out to remove impurities such as Sr and Ce. To the filtrate are added 10 ml of saturated thioacetamide solution and silver sulphide is precipitated on gentle heating. The precipitate is centrifuged, dissolved in 5 ml of cont. nitric acid and boiled to remove sulphur completely. The solution is then cooled. The scavenging cycle with iron hydroxide and precipitation as silver sulphide is repeated. The filtrate is finally adjusted to pH 3.5 with 6M hydrochloric acid. The precipitate is digested by gentle heating and allowed to settle. It is then filtered off and the chemical yield and activity are measured. The filtrate from the precipitation of silver chloride is adjusted to pH 8.5 with ammonia solution. To the solution are added 10 mg of iron carrier and hydroxide scavenging is carried out to remove Cr and Nb impurities. The filtrate is adjusted to pH 35 with 6M nitric acid and 10 ml of 4”/; ~-benzoinoxime solution in ethanol are added. The precipitate is centrifuged and dissolved in a mixture of 10 ml of fuming nitric and 3 ml of perchloric acid and the solution evaporated slowly to dryness. Iron hydroxide scavenging is once again carried out to remove Zr, Nb, 12 etc. The filtrate is finally adjusted to pH 3 with 6M nitric acid. Three ml of saturated lead nitrate solution are added to the solution, which is then gently heated to precipitate lead molybdate, which is collected, weighed and counted. Separation of tellurium. A known fraction of the leach solution with 40 mg of tellurium carrier is evaporated to dryness. The residue is taken up in a small quantity of hvdrochloric acid and evanorated again to dryness. This is-repeated 2 or 3 times to bring the t&due into the chloride form. The residue is then taken un in 3M hydrochloric acid and freshly prepared sul~hu~ dioxide- is passed through it. Elemental tellurium is precipitated. It is then dissolved in clqaa reyiu. reprecipitated and redissolved in
uqua regia. Ten mg of iron carrier are added to the solution and pure tellurium is precipitated, dried, weighed and counted. Separation of’neptunium and uranium.* A measured fraction of the leach solution with known amounts of *j3U, 237Np and 236~ tracers is taken in a beaker along with 1 mg of iron carrier, 1 g of hydroxylamine hydrochloride and 10 ml, for every 100 ml of leach solution. of a 4:1 v/v mixture of saturated boric acid solution and cont. nitric acid. The solution is warmed and ammonia solution added slowly to precipitate hydroxides. The precipitate is filtered off, washed and dissolved in 6M nitric acid and the solution evaporated almost to dryness, The residue is taken up in 15 ml of 1M nitric acid and neotunium is converted into oxidation state IV by addition’of I ml of 15% hydroxylamine hydrochloride solution and 0.3 ml of 3M ferrous sulphamate. Neptunium is twice extracted with 05M thenoyotrifluoroacetone in xylene by shaking for about 10 min in a lOO-ml separating funnel. The organic phase is washed with an equal volume of 1M nitric acid and the washings are added to the aqueous phase. Neptunium is stripped from the organic phase with an equal volume of 8M nitric acid, which is then evsaporated to dryness with 1M hydrochI~ric acid and the residue taken up with two 3-m! portions of electrolyte solution (TM ammonium chloride, O.OlM oxalic acid) and &aced in an electrolysis cell. Neptunium is deposited on a 2-5-cm diameter stainless-steel disc for 20 min at a current density of l-1.2 A. The aqueous phase left after the neptunium extraction is three times evaporated to dryness -with a little nitric acid and the residue is finallv taken ua in 10 ml of 1M nitric acid. Ten g of aluminmm nitrate, 3 or 4 drops of cerium carrier (10 mg of Ce4’/ml) and i&l5 drops of hydrogen peroxide are added to the solution and uranium is extracted by shaking with an equal volume of ethyl acetate for 1 min. The organic phase is slowly evaporated on a hot-plate. The residue is taken up in 10 ml of 1M nitric acid and the uranium extraction with ethyl acetate is repeated twice more. Finally the uranium is electroplated on a stainless-steel disc similarly to neptunium.
RESULTS AND DISCUSSION The chemical procedures described above are rapid and quantitative and the radionuclides separated are free from contamination by other fission products. The B-emitting isotopes are counted in a low-background assembly using an end-window GM counter with a lower thickness of the sample. The necessary corrections for the ioss due to self-absorption etc. are made and the final count-rate is used to estimate the con~ntrat~on of each nuchde. The purity of each separated nuchde is checked by following the radioactive decay and in the case of B-emitting nuclides by using appropriate aluminium absorbers. The chemical yields of uranium and neptunium are determined radiometrically by adding 233U and 237Np tracers and in the case of fission products the yields are determined gravimetrically by adding an appropriate inactive carrier. The chemical yields of separated radionuclides are 65.-SOoi, for strontium and barium, 8590% for silver, 50-557: for molybdenum 7@800/, for tellurium, 82-85S$ for neptunium and .5%60~( for uranium. The overall errors in the estimation of these nuclides are within lo:/,. The time taken for the entire analysis is 510 hr.
Determination
315
of short-lived radionuclides
Table 1. Expected activity ratios of selected fission products at the time of fission’5’ Isotope ratio 111Ag18QSr
’ 1iAg/t4’Ba 99Mo/‘11A I g
235”
Fission-spectrum
0.05 001 465
neutron fission of 238U 239Pu
I4-MeV neutron fission of 239pu 235U 238U
0.23 O-03 168
1.97 042 12.2
1.46 0.12 41.0
2.5 0.40 14.8
52 0.90 80
Table 2. Observed activity ratios in fall-out samples from selected Chinese tests Isotope ratio
Observed ratio
“rAg/sgSr
0.116 0.068 0.213 0.84 0.98 0023 0.010 0.045 0,023 0.028 0.23 0.16 111.0 153.0 31.8 38.8 0.16 0.15 2.82 i.57 O-30 0.10 42.5 19.2
1ilA
‘@Ba gl’
‘“Mo/r”Ag
237U/140Ba
239Np/‘40Ba
Test number and date of test First Second Third Sixth Eighth First Second Third Fourth Fifth Sixth Eighth Fifth Seventh Eighth Tenth Second Fourth Sixth Tenth Second Fourth Sixth Eighth
(16.1064) (14.5.65) (9.566) (17.6.67) (27.1268) (16.1064) (14.5.65) (9.566) (27.10.66) (28.12.66) (17.6.67) (27.12.68) (28.12.66) (24.12.67) (27.12.68) (29.9.69) (14.565) (27.10.66) (17.6.67) (29.9.69) (14.565) (27.10.66) (17.6.67) (27.12.68)
Type of bomb deduced A-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb A-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb
using z35U using 235U using *% using using using using using
z3SU 23*U z3sU 235U 235U
using “%J using z35U using 235U using zJsU using 235U using 23’U
Table 3. Observed activity ratios in fall-out samples from selected French tests
Isotope ratio iilAgfSgSr
I1*Ag/t4’Ba
99Mo/“‘Ag
237U/‘40Ba 23gNp/‘40Ba
Observed ratio
Test date
1.40 1.06 0.95 0.78 0.14 0.10 0.10 0.013 24.8 30.8 257.0 48.2 0.02 O-02 4.3 4.1
187.66 11.9.66 5.6.67 2.7.67 18.7.66 11.9.66 5.6.67 24.8.68 5.6.67 2.7.67 7.7.68 24.8.68 18.7.66 11.9.66 187.66 11.9.66
Type of bomb concluded A-bomb A-bomb A-bomb A-bomb A-bomb A-bomb A-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb A-bomb A-bomb
using using using using using using using
z39Pu 239Pu 239F’u 239Pu 23gPu *39Eu 239Pu
using 239Pu using 239Pu using using using using
z39F’n 239Pu z39Pu 239Pu
316
S. B.
HINGORANI,R. N. KHANDEKARand
The expected activity ratios of selected nuclides at the time of fission are given in Table 1. Tables 2 and 3 give the observed ratios from some Chinese and French tests. The fission product ratios measured correspond with those expected. Furthermore, the mode of detonation and the fissile material used in the weapon are also determined by comparing these measured ratios with the expected activity ratios given in Table 1. The most sensitive ratio for the determination of fissile material is i1 'Ag/*'Sr, as 235U and 23gPu fission in A-bomb tests give ratios which differ by a factor of 29. A thermonuclear reaction is identified by comparing the observed ratios 238U 14MeV fission ratios, with corresponding though more significant are the ratios of 237U and 23gNp to 14’Ba. This is because the 237U/140Ba value varies between 0.02 and 0.2 for purely fission devices, depending on whether 23gPu or 235U is used as fissile material, and for thermonuclear devices it is usually higher than 2.0. Similarly the 23gNp/140Ba value is higher for H-bombs as the amount of 238U is much larger and the number of neutrons liberated in the reaction is also about 10 times that for an A-bomb of the same energy.4 Some variations in the ratios are observed. For example, the expected ratio for ’ 1‘Ag/“‘Ba for the fission-spectrum neutron fission of 235U is O.O18,5 while for the second and third Chinese A-bomb tests these values are 0.01 and OGl5 respectively. Similarly
S. J. S.
ANAND
the 111Ag/140Ba values for the sixth and eighth Chinese tests are 0.23 and 0.16 respectively, compared with the value of 040 for 14-MeV fission of 238U.5 These variations could be due to a difference in neutron energies at the time of detonation. If the tests are held close together, the activity collected will contain short-lived isotopes from earlier tests, thus making it difficult to make detailed measurements of the sample. In such cases the fission products “MO and ‘32Te (tlj2 = 67 and 77 hr respectively) are used to express activity ratios relative to other fission products. Acknowledgements-The authors wish to express their thanks to Dr K. G. Vohra, Head, Division of Radiological Protection for guiding the project and to Dr U. C. Mishra, Head, Air Monitoring Section for his valued suggestions and discussions. Thanks are also due to B. Y. Lalit, S. Sadasivan, S. Rajan and S. B. Santani for their help in activity measurements and chemical analysis.
REFERENCES 1. A. L. Boni, Anal. Chem., 1960, 32, 599 2. S. B. Hingorani, At. Energy Establ. Trombay Report, AEETIAM-17, 1960. 3. S. J. S. Anand and M. S. Rae, Proc. Chemistry Symposium, Madras, 1970, Vol. II, 339. 4. R. N. Khandekar. S. B. Santani and M. S. Rao. ibid. 1970, Vol. II, 347: 5. U. C. Mishra, B. Y. Lalit, R. K. Varma and S. Sadasivan, J. Sci. Ind. Rex (India), 1974, 33, 216.
KHANDEKAR
and S. J. S. ANAND
Air Monitoring Section, Bhabha Atomic Research Centre, Trombay, Bombay-400085, India (Received I April 1975. Accepted 9 September 1975)
Summary-Radiochemical procedures for the assay, of short-lived fission and activation products are described. They are rapid and quantitative aid the radionuclides separated are radiochemically pure. Ratios of some of the short-lived radionuclides obtained by these measurements for selected Chinese and French nuclear tests are given and provide information about the fissile material used in the
The programmes of nuclear weapon testing by China and France contribute considerable quantities of short-lived radionuclides to the atmosphere. The
measurement of this short-lived activity by y-ray spectrometry alone is not sufficient for the identification of a weapon test, so a chemical separation of the fission and induced products is necessary. The chemical methods selected must take into account the complicated composition of the sample, be rapid and quantitative and have high chemical yield. The methods presented in this paper meet these criteria and also permit joint assay of various radionuclides from a single sample. The sample material is a swab collected by rubbing the surface of commercial aircraft with cotton soaked in organic solvent. These samples are routinely collected on arrival of the aircraft at Bombay to estimate the background levels of the atmosphere before and after nuclear tests. The results are used to get information about detonation, e.g., fissile material used, mode of detonation etc., by comparison of the activity ratios of selected fission products such as “Sr, lllAg, 13’Te and “MO. These fission products, being shortlived, do not suffer interference by products from earlier tests. There are two important activation products which help in deciding whether the test was with a conventional H-bomb using a three-stage device with 238U in the third stage. These are neptunium-239 and uranium-237 which are produced from 23sU by (n, y) and (n, 2n) reactions respectively. 237U, in a nuclear weapon, is also produced from 235U by double neutron capture but its production by this reaction is very small compared with that from the (n, 2n) reaG tion in a conventional H-bomb test. Other activation products also help in determining parameters of the weapon test. However, the samples collected for our studies gave hardly any measurable quantities of these activation products and therefore only 237U and ‘jgNp are discussed in this paper. 313
EXPERIMENTAL Sample preparation
The sampling method collects considerable quantities of insoluble silica, iron, sulphate and phosphate compounds. Because of the difficulty with which these compounds dissolve, a leaching procedure for the dissolution of the sample is used.’ The cotton swab is digested with a mixture of 3M hydrochloric acid and O.lM hydrofluoric acid in the presence of known amounts of carriers. The heating time is reduced to a minimum to reduce the attack on the glassware. The solution is filtered through a Whatman No. 54 paper on a Biichner funnel. This process is repeated 3 or 4 times and finally the residue is leached with 6M nitric acid and filtered ofl. It is found that more than 95% of the total activity is leached out by this process. All parts of the filtrate are then pooled and a suitable aliquot is taken for the determination of each individual nuclide. Chemical procedures
The chemical procedures developed in our laboratory are described below. Barium-140 is usually determined directly from the gamma-ray spectrum through its daughter ““‘La. For this a known fraction of the leach solution (usually a tenth of the total volume) is evaporated in a beaker to minimum volume and transferred to a 5.0-cm diameter “Perspex” planchette which, after drying, is sealed and counted by a 256channel pulse-height analyser, using a Ge(Li) detector. In general, the final isolation of the precipitate is carried out by filtration with a special filter funnel using a l-in. Whatman No. 542 paper and the chemical yield is obtained by weighing the precipitate. The sample is then mounted on a planchette and counted conventionally. The filter funnel is made of “Perspex”, and has a threaded base, a 2.5-cm diameter circular perforated disc and a vertical reservoir 2.4 cm i.d. and 12 cm in height. Separation of‘ strontium and barium.’ A known volume of the leach solution with known quantities of carriers (150 mg of Sr + 100 mg of Ba) is placed in a beaker. The solution is warmed and anhydrous sodium carbonate added. The insoluble carbonates are filtered off (Whatman No. 540 paper) and redissolved in 6M nitric acid. The volume of the solution is reduced to 40 ml by evaporation and 65 ml of fuming nitric acid (sg. 1.52) are added slowly to the ice-cooled solution. The crystals of barium and strontium nitrates are allowed to settle. The supernatant liquid is decanted and the crystals are dissolved in distilled water. The volume of the solution is again brought to 40 ml and the nitrate precipitation
314
S. B. HINGORANI.R. N. KHANDEKAR and S. J. S. ANAND
repeated twice with 60 ml of fuming nitric acid, The nitrate crystals are then dissolved in water and strontium and barium precipitated by addition of ammonium carbonate. The solution is warmed for about 10 min to coagulate the carbonates and finally filtered. The precipitate is dissolved in 6M hydrochloric acid and barium is precipitated as chromate by addition of 10 ml of l.SM sodium chromate and adjustment of the pH to 3.54. The chemical yield and activity are then measured. The filtrate from the chromate separation is warmed and ammonium carbonate added to precipitate strontium. The precipitate is filtered off (Whatman No. 540 paper) and dissolved in 6M hydrochloric acid. The solution is warmed with absolute alcohol (2 or 3 drops) to reduce any chromate. Twenty mg of yttrium carrier are added and the yttrium is precipitated by slow addition of freshly prepared ammonia solution to remove yttrium-90 and other rareearth activity. The solution is warmed, cooled and filtered and the time noted. Ammonium carbonate (2-3 g) is added to the filtrate, the solution is warmed and the strontium carbonate allowed to settle. The activity is then determined after filtration. After the counting of s%r and ‘%r activity, the precipitate is ignited to constant weight in a platinum crucibIe to obtain the chemical yield. The residue is dissolved in 3M hydrochloric acid and stored with 20 mg of yttrium carrier for equilibration and subsequent milkings. Separution of silver and molybdenum.” A known volume of the leach solution, with known amounts of carriers, is placed in a beaker and evaporated almost to dryness. To the residue are added 2 ml of cont. nitric acid and 2 ml of bromine water, and the resultant solution is slowly heated to oxidize tin and molybdenum. The solution is cooled and diluted and a few drops of 6M hydrochloric acid are added, with constant stirring, to precipitate insoluble chlorides. The precipitate is centrifuged, washed with distilled water and dissolved in ammonia solution. Ten mg of iron carrier are added to the solution and iron hydroxide scavenging is carried out to remove impurities such as Sr and Ce. To the filtrate are added 10 ml of saturated thioacetamide solution and silver sulphide is precipitated on gentle heating. The precipitate is centrifuged, dissolved in 5 ml of cont. nitric acid and boiled to remove sulphur completely. The solution is then cooled. The scavenging cycle with iron hydroxide and precipitation as silver sulphide is repeated. The filtrate is finally adjusted to pH 3.5 with 6M hydrochloric acid. The precipitate is digested by gentle heating and allowed to settle. It is then filtered off and the chemical yield and activity are measured. The filtrate from the precipitation of silver chloride is adjusted to pH 8.5 with ammonia solution. To the solution are added 10 mg of iron carrier and hydroxide scavenging is carried out to remove Cr and Nb impurities. The filtrate is adjusted to pH 35 with 6M nitric acid and 10 ml of 4”/; ~-benzoinoxime solution in ethanol are added. The precipitate is centrifuged and dissolved in a mixture of 10 ml of fuming nitric and 3 ml of perchloric acid and the solution evaporated slowly to dryness. Iron hydroxide scavenging is once again carried out to remove Zr, Nb, 12 etc. The filtrate is finally adjusted to pH 3 with 6M nitric acid. Three ml of saturated lead nitrate solution are added to the solution, which is then gently heated to precipitate lead molybdate, which is collected, weighed and counted. Separation of tellurium. A known fraction of the leach solution with 40 mg of tellurium carrier is evaporated to dryness. The residue is taken up in a small quantity of hvdrochloric acid and evanorated again to dryness. This is-repeated 2 or 3 times to bring the t&due into the chloride form. The residue is then taken un in 3M hydrochloric acid and freshly prepared sul~hu~ dioxide- is passed through it. Elemental tellurium is precipitated. It is then dissolved in clqaa reyiu. reprecipitated and redissolved in
uqua regia. Ten mg of iron carrier are added to the solution and pure tellurium is precipitated, dried, weighed and counted. Separation of’neptunium and uranium.* A measured fraction of the leach solution with known amounts of *j3U, 237Np and 236~ tracers is taken in a beaker along with 1 mg of iron carrier, 1 g of hydroxylamine hydrochloride and 10 ml, for every 100 ml of leach solution. of a 4:1 v/v mixture of saturated boric acid solution and cont. nitric acid. The solution is warmed and ammonia solution added slowly to precipitate hydroxides. The precipitate is filtered off, washed and dissolved in 6M nitric acid and the solution evaporated almost to dryness, The residue is taken up in 15 ml of 1M nitric acid and neotunium is converted into oxidation state IV by addition’of I ml of 15% hydroxylamine hydrochloride solution and 0.3 ml of 3M ferrous sulphamate. Neptunium is twice extracted with 05M thenoyotrifluoroacetone in xylene by shaking for about 10 min in a lOO-ml separating funnel. The organic phase is washed with an equal volume of 1M nitric acid and the washings are added to the aqueous phase. Neptunium is stripped from the organic phase with an equal volume of 8M nitric acid, which is then evsaporated to dryness with 1M hydrochI~ric acid and the residue taken up with two 3-m! portions of electrolyte solution (TM ammonium chloride, O.OlM oxalic acid) and &aced in an electrolysis cell. Neptunium is deposited on a 2-5-cm diameter stainless-steel disc for 20 min at a current density of l-1.2 A. The aqueous phase left after the neptunium extraction is three times evaporated to dryness -with a little nitric acid and the residue is finallv taken ua in 10 ml of 1M nitric acid. Ten g of aluminmm nitrate, 3 or 4 drops of cerium carrier (10 mg of Ce4’/ml) and i&l5 drops of hydrogen peroxide are added to the solution and uranium is extracted by shaking with an equal volume of ethyl acetate for 1 min. The organic phase is slowly evaporated on a hot-plate. The residue is taken up in 10 ml of 1M nitric acid and the uranium extraction with ethyl acetate is repeated twice more. Finally the uranium is electroplated on a stainless-steel disc similarly to neptunium.
RESULTS AND DISCUSSION The chemical procedures described above are rapid and quantitative and the radionuclides separated are free from contamination by other fission products. The B-emitting isotopes are counted in a low-background assembly using an end-window GM counter with a lower thickness of the sample. The necessary corrections for the ioss due to self-absorption etc. are made and the final count-rate is used to estimate the con~ntrat~on of each nuchde. The purity of each separated nuchde is checked by following the radioactive decay and in the case of B-emitting nuclides by using appropriate aluminium absorbers. The chemical yields of uranium and neptunium are determined radiometrically by adding 233U and 237Np tracers and in the case of fission products the yields are determined gravimetrically by adding an appropriate inactive carrier. The chemical yields of separated radionuclides are 65.-SOoi, for strontium and barium, 8590% for silver, 50-557: for molybdenum 7@800/, for tellurium, 82-85S$ for neptunium and .5%60~( for uranium. The overall errors in the estimation of these nuclides are within lo:/,. The time taken for the entire analysis is 510 hr.
Determination
315
of short-lived radionuclides
Table 1. Expected activity ratios of selected fission products at the time of fission’5’ Isotope ratio 111Ag18QSr
’ 1iAg/t4’Ba 99Mo/‘11A I g
235”
Fission-spectrum
0.05 001 465
neutron fission of 238U 239Pu
I4-MeV neutron fission of 239pu 235U 238U
0.23 O-03 168
1.97 042 12.2
1.46 0.12 41.0
2.5 0.40 14.8
52 0.90 80
Table 2. Observed activity ratios in fall-out samples from selected Chinese tests Isotope ratio
Observed ratio
“rAg/sgSr
0.116 0.068 0.213 0.84 0.98 0023 0.010 0.045 0,023 0.028 0.23 0.16 111.0 153.0 31.8 38.8 0.16 0.15 2.82 i.57 O-30 0.10 42.5 19.2
1ilA
‘@Ba gl’
‘“Mo/r”Ag
237U/140Ba
239Np/‘40Ba
Test number and date of test First Second Third Sixth Eighth First Second Third Fourth Fifth Sixth Eighth Fifth Seventh Eighth Tenth Second Fourth Sixth Tenth Second Fourth Sixth Eighth
(16.1064) (14.5.65) (9.566) (17.6.67) (27.1268) (16.1064) (14.5.65) (9.566) (27.10.66) (28.12.66) (17.6.67) (27.12.68) (28.12.66) (24.12.67) (27.12.68) (29.9.69) (14.565) (27.10.66) (17.6.67) (29.9.69) (14.565) (27.10.66) (17.6.67) (27.12.68)
Type of bomb deduced A-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb A-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb H-bomb H-bomb
using z35U using 235U using *% using using using using using
z3SU 23*U z3sU 235U 235U
using “%J using z35U using 235U using zJsU using 235U using 23’U
Table 3. Observed activity ratios in fall-out samples from selected French tests
Isotope ratio iilAgfSgSr
I1*Ag/t4’Ba
99Mo/“‘Ag
237U/‘40Ba 23gNp/‘40Ba
Observed ratio
Test date
1.40 1.06 0.95 0.78 0.14 0.10 0.10 0.013 24.8 30.8 257.0 48.2 0.02 O-02 4.3 4.1
187.66 11.9.66 5.6.67 2.7.67 18.7.66 11.9.66 5.6.67 24.8.68 5.6.67 2.7.67 7.7.68 24.8.68 18.7.66 11.9.66 187.66 11.9.66
Type of bomb concluded A-bomb A-bomb A-bomb A-bomb A-bomb A-bomb A-bomb H-bomb A-bomb A-bomb H-bomb H-bomb A-bomb A-bomb A-bomb A-bomb
using using using using using using using
z39Pu 239Pu 239F’u 239Pu 23gPu *39Eu 239Pu
using 239Pu using 239Pu using using using using
z39F’n 239Pu z39Pu 239Pu
316
S. B.
HINGORANI,R. N. KHANDEKARand
The expected activity ratios of selected nuclides at the time of fission are given in Table 1. Tables 2 and 3 give the observed ratios from some Chinese and French tests. The fission product ratios measured correspond with those expected. Furthermore, the mode of detonation and the fissile material used in the weapon are also determined by comparing these measured ratios with the expected activity ratios given in Table 1. The most sensitive ratio for the determination of fissile material is i1 'Ag/*'Sr, as 235U and 23gPu fission in A-bomb tests give ratios which differ by a factor of 29. A thermonuclear reaction is identified by comparing the observed ratios 238U 14MeV fission ratios, with corresponding though more significant are the ratios of 237U and 23gNp to 14’Ba. This is because the 237U/140Ba value varies between 0.02 and 0.2 for purely fission devices, depending on whether 23gPu or 235U is used as fissile material, and for thermonuclear devices it is usually higher than 2.0. Similarly the 23gNp/140Ba value is higher for H-bombs as the amount of 238U is much larger and the number of neutrons liberated in the reaction is also about 10 times that for an A-bomb of the same energy.4 Some variations in the ratios are observed. For example, the expected ratio for ’ 1‘Ag/“‘Ba for the fission-spectrum neutron fission of 235U is O.O18,5 while for the second and third Chinese A-bomb tests these values are 0.01 and OGl5 respectively. Similarly
S. J. S.
ANAND
the 111Ag/140Ba values for the sixth and eighth Chinese tests are 0.23 and 0.16 respectively, compared with the value of 040 for 14-MeV fission of 238U.5 These variations could be due to a difference in neutron energies at the time of detonation. If the tests are held close together, the activity collected will contain short-lived isotopes from earlier tests, thus making it difficult to make detailed measurements of the sample. In such cases the fission products “MO and ‘32Te (tlj2 = 67 and 77 hr respectively) are used to express activity ratios relative to other fission products. Acknowledgements-The authors wish to express their thanks to Dr K. G. Vohra, Head, Division of Radiological Protection for guiding the project and to Dr U. C. Mishra, Head, Air Monitoring Section for his valued suggestions and discussions. Thanks are also due to B. Y. Lalit, S. Sadasivan, S. Rajan and S. B. Santani for their help in activity measurements and chemical analysis.
REFERENCES 1. A. L. Boni, Anal. Chem., 1960, 32, 599 2. S. B. Hingorani, At. Energy Establ. Trombay Report, AEETIAM-17, 1960. 3. S. J. S. Anand and M. S. Rae, Proc. Chemistry Symposium, Madras, 1970, Vol. II, 339. 4. R. N. Khandekar. S. B. Santani and M. S. Rao. ibid. 1970, Vol. II, 347: 5. U. C. Mishra, B. Y. Lalit, R. K. Varma and S. Sadasivan, J. Sci. Ind. Rex (India), 1974, 33, 216.