1 Direct Pb isotopic analysis of a nuclear fallout debris particle from the 2 nuclear test 3 4 5 Jeremy J. Bellucci1*, Joshua F. Snape1, Martin W. Whitehouse1, Alexander A. Nemchin1,2 6 7 *Corresponding author, email address: [email protected] 8 9 1Department of Geosciences, Swedish Museum of Natural History, SE-104 05 10 Stockholm, Sweden

11 2 Department of Applied Geology, Curtin University, Perth, WA 6845, Australia

12 13 Abstract 14 15 The Pb isotope composition of a nuclear fallout debris particle has been directly 16 measured in post-detonation materials produced during the Trinity nuclear test by a 17 secondary ion mass spectrometry (SIMS) scanning ion image technique (SII). This 18 technique permits the visual assessment of the spatial distribution of Pb and can be used 19 to obtain full Pb isotope compositions in user-defined regions in a 70 µm x 70 µm 20 analytical window. In conjunction with BSE and EDS mapping of the same particle, the 21 Pb measured in this fallout particle cannot be from a major phase in the precursor arkosic 22 sand. Similarly, the Pb isotope composition of the particle is resolvable from the 23 surrounding at the 2σ uncertainty level. The Pb isotope composition measured in 24 the particle here is in excellent agreement with that inferred from measurements of green 25 and red trinitite, suggesting that these types of particles are responsible for the Pb isotope 26 compositions measured in both trinitite . 27 28 29 Keywords: 30 31 Trinitite, Trinity Test, Post Detonation Nuclear Forensics, Pb isotopes, SIMS, Scanning 32 Ion Imaging, Fallout Debris, Forensics 33 34 35 Introduction 36

37 In the event of a non-state sanctioned nuclear attack, forensic investigations will

38 be necessary to determine the provenance of the weapon and the materials used in

39 construction of the device. To determine accurately the isotopic compositions of the fuel

40 and the device, the materials present after detonation must be analyzed carefully to

1 41 delineate any anthropogenic signatures from those indigenous to the blast site1-9. The

42 only post-detonation materials available for public study are those that were produced

43 from the first atomic weapons test, codenamed “Trinity”. The Trinity test took place on

44 July 16, 1945 at 5:29:45 a.m. (GMT-7) at the White Sands Proving Grounds, just south of

45 Alamogordo, NM. The first nuclear device, referred to as the “Gadget”, was driven by the

46 fusion reaction of super-grade Pu (240Pu/239Pu = 0.01-0.033,5,9-11). The core of the Gadget

47 contained a 2.5 cm diameter Be neutron initiator, surrounded by a 9.2 cm diameter Pu-Ga

48 alloy fuel shell, a 22 cm diameter tamper shell constructed from natural U, and a 22.9 cm

49 diameter boron-plastic shell12. The core of the device was surrounded by the implosion

50 assembly, which consisted of traditional explosives and aluminum shells12. The

51 traditional explosives used to initiate the compression of the core, and thus initiate the

52 critical nuclear reaction in the device were of RDX, TNT, and Baratol, which is a mixture

12 53 of TNT and Ba(NO3)2 . Other materials associated with the device include an unknown

54 amount of electronics, wiring, and other components. Before detonation, the Gadget was

55 elevated onto a steel tower with a height of 30.5 m. After initial ignition, a 21 kiloton

56 explosion was produced yielding a mushroom cloud with a height of 15.2 to 21.3 km and

57 a temperature of ~8430°K13,14. The materials produced in this test, a mix of

58 anthropogenic and natural, were given the name “Trinitite.”

59 Trinitite, generally, is composed of two phases: melt glass and relict

60 grains1,5-7,14-15. The upper most surface of trinitite is mostly glass, grading down into

61 arkosic sand, the surface lithology at the test site. The melt glass is extremely

62 heterogeneous on a micrometer scale and contains the vast majority of radioactive

63 elements and elemental enrichments attributed to the device or test materials1-9,14-15.

2 64 These anthropogenic contributions to the melt glass have also been observed on the

65 surface of melt glass in the form of debris particles and therefore, were likely added to

66 melt glass while it was still molten, or cooling2. Due to the degree of heterogeneity

67 present in the trinitite melt glass, in situ analytical methods provide the most effective

68 means to target specific domains in the melt glass at the 10 to 100 micrometer scale. The

69 spatial context provided by performing in situ measurements of the glass make it easier to

70 constrain and discuss mixing end-members present2-9,15.

71 The best forensic evidence to determine the provenance of a nuclear device lies in

72 the isotopic composition of heavy metals (Pb, Pu, and U1,3-9,16). Specifically, the isotopic

73 composition of these elements reflects the ores, enrichment procedures, and fuel cycle

74 used in creating the fuel and/or components for the device. One of these metals, Pb, has

75 been observed as a particle and the composition of Pb has been inferred by the

76 measurement of green and red trinitite glasses by laser ablation multi-collector

77 inductively coupled mass spectrometry4,6,8. Based on the Pb isotope compositions

78 measured in the trinitite glasses, a mixing end member was inferred and attributed to the

79 Buchans mine, which was owned and operated by an American mining company at the

80 time the test was conducted4,8 .

81 The heavy metal Pb is composed of four isotopes 208Pb, 207Pb, 206Pb, and 204Pb.

82 208Pb, 207Pb, and 206Pb are the decay products of the long-lived isotopes 232Th, 235U, and

83 238U, respectively. 204Pb is neither a parent nor daughter isotope in a radioactive decay

84 chain. As a consequence, the Pb isotopic composition of a given , rock, ore, or

85 other material is dependent on its initial Pb isotopic composition, its age, and its time-

86 integrated Th-U-Pb history. Due to the large, natural variations in the geochemical

3 87 behavior of U, Th, and Pb, the Pb isotopic composition of rocks, , and ores have

88 readily quantifiable natural deviations. This advantage of the Pb isotope system can be

89 exploited by assigning isotopic signatures or fingerprints to materials. As such, these

90 signatures can be used to link Pb ores and the industrial (anthropogenic) materials

91 produced from them4,8,17-19. The techniques to determine the Pb isotope composition of

92 post detonation materials are critical for forensic investigations because Pb could easily

93 be substituted as a tamper/neutron reflector in a hypothetical improvised nuclear device1.

94 Lead has been observed as an oxide (PbO) inclusion on the surface of trinitite2,

95 seen in metal alloys14,15, and Pb enrichments have been observed to correlate positively

96 with those for Cu in trinitite glass6,8,14,15. Additionally, Pb is found to be concentrated

97 along with Cu in red trinitite (responsible for the red color), which is likely a remnant of

98 the wiring used in the device8,14,15. Despite these multiple lines of evidence, the exact

99 origin or usage of industrial Pb in the Trinity test is unknown because there was no

100 documented use of Pb in the manufacturing of the device or at the test site12. However,

101 the presence of Pb in trinitite has been attributed to Pb bricks for radiation shielding,

102 solder, pigs, or some other anthropogenic component related to the experiment4,8,14,15.

103 The composition of the Pb used in the test has been estimated by several indirect methods,

104 but to date has not been directly measured in a particle entrained in the trinitite glass.

105 Therefore, the main focus of this study is to image and measure directly the Pb isotopic

106 composition of a Pb-rich fallout inclusion within Trinitite glass.

107 To accomplish this goal, scanning ion imaging (SII) secondary ion mass

108 spectrometry (SIMS) was used. SII provides isotopic imaging in e.g., 70 x 70 µm

109 domains for Pb at a spatial resolution of 20 µm2, after which regions of interest (ROIs)

4 110 can be assigned and full Pb isotope compositions can be obtained18-21. This technique is

111 preferred because the limits of detection are much lower (ng/g-µg/g) than other imaging

112 techniques (e.g., EDS at wt %). Since fallout is entrained in the trinitite glass as particles

113 in the upper most surface of the trinitite glass, this is the most important area to search for

114 these debris particles. Therefore, in contrast to previous trinitite studies3-9, the sample

115 here was mounted in epoxy, glass side up (instead of a cross section) to maximize the

116 trinitite glass surface area and provide a higher likelihood of accessing a fallout particle

117 entrained within the glass.

118

119 Analytical Methods

120 The trinitite sample studied here was purchased from the Mineralogical Research

121 Corporation (www.minresco.com). The sample was mounted glass side up in epoxy and

122 polished, to maximize the available upper glass surface area. After the sample was

123 mounted, polished, and cleaned, it was carbon coated for SEM analysis, similarly cleaned

124 and then coated with 30 nm of Au for SIMS analyses. Backscatter electron images (BSE)

125 and energy dispersive x-ray maps (EDS) were made using a FEI Quanta FEG 650

126 scanning electron microscope at Stockholm University, Sweden, operating with an

127 accelerating voltage of 20 kV at a working distance of 10 mm. All SIMS SII analyses

128 were performed using a CAMECA IMS1280 large-geometry ion microprobe located at

129 the NordSIMS facility, Swedish Museum of Natural History, Stockholm, Sweden,

130 modified from those previously described18-21. Before the start of each SII analysis, an

- 131 area of 80 x 80 µm was sputtered for 10 minutes using a ~20 µm, ~10 nA O2 primary

5 132 beam to remove the gold coating and minor residual surface contamination. To obtain

133 high spatial resolution for the ion images, a 50 µm primary beam aperture was used,

134 resulting in a projected beam diameter of ~5 µm on the sample surface and a beam

135 current of ~250 pA. This spot size, which corresponds to a surface area of 20 µm2,

136 defines the minimum spatial resolution at which the technique can confidently resolve

137 individual, non-mixed chemical domains. The secondary ion beam from the imaged 70 x

138 70 µm area (smaller than the pre-sputter area) was processed using the dynamic transfer

139 optical system (DTOS), which is a synchronized raster in the transfer section of the

140 instrument that deflects the ions back onto the ion optic axis of the instrument regardless

141 of their original point of sputtering from the sample, allowing both image acquisition and

142 collection of secondary ions at high mass resolution, in this case 4860 M/ΔM (sufficient

143 to resolve Pb from known molecular interferences). A NMR field sensor was used in

144 regulation mode to control the stability of the magnetic field over the duration of all

145 image analyses. Secondary ions of 204Pb, 206Pb, 207Pb, and 208Pb were collected

146 simultaneously using a multi-collector array consisting of low-noise discreet dynode

147 Hamamatsu R4146-04 electron multipliers. The typical backgrounds for these detectors

148 are <0.005 cps, removing the need for any background correction. Prior to image

149 acquisition, the secondary ion beam was centered in the field aperture, optimized for

150 maximum energy distribution in a 45 eV energy window, and mass calibrated using the

151 208Pb signal. Pb isotopes were collected in 80 blocks of 20s integrations. Post acquisition,

152 the images were processed using the CAMECA WinImage2 software package. Regions

153 of interest (ROIs) were selected in each of three images. ROIs were selected based on the

154 ‘hot spots’ in 208Pb in Images 1 and 2, and over the whole glassy area in Image 3. Once

6 155 the ROIs were determined for each image, the isotope ratios of 207Pb/206Pb, 208Pb/206Pb,

156 and 204Pb/206Pb were calculated in WinImage2 and exported to Microsoft Excel where all

157 isotope ratios were corrected for gain differences between the electron multipliers.

158 A single, linear correction factor that accounts for Pb isotope mass fractionation

159 and gain differences between electron multipliers was determined by performing

160 conventional spot analyses of USGS basaltic glass reference material BCR-2G (~11 µg/g

- 161 Pb) before the imaging session, using a ~20 µm, ~10 nA O2 primary beam. Gain

162 correction values were then determined by comparison to the accepted Pb isotope ratios22.

163 External precision for the measurements of ratios 207Pb/206Pb, 208Pb/206Pb, and 204Pb/206Pb

164 was 0.16%, 0.30%, and 0.32%, respectively (2σ).

165 Results

166 Three scanning ion images of trinitite glass were obtained and are shown in

167 Figure 1 along with corresponding ROIs. In Images 1 and 2, ROIs were selected on the

168 basis of 208Pb intensity, whereas Image 3 was uniform in 208Pb intensity and the ROI was

169 placed over the whole image. Following the scanning ion imaging, BSE and EDS maps

170 were made of the inclusion in question and are shown in Figure 2. The Pb isotopic

171 compositions of ROIs are presented in Table 1 and shown in Figure 3. The Pb isotopic

172 composition of the regions of interest in the inclusion are resolvable from the surrounding

173 glass at the 2σ uncertainty level and match those from previous estimates of the

174 anthropogenic Pb in trinitite4 and from red trinitite glass8 (Figure 3). Additionally, the

175 two orders of magnitude enrichment in Pb in the inclusion vs. the glass is similar to that

176 seen in the laser ablation data in similar samples (Figure 1, Image 2)4,6.

7 177 Discussion

178 The inclusion here has an amorphous, bleb morphology similar to those

179 previously seen in trinitite2 and is bright in BSE, slightly enriched in Ca and Fe, but

180 depleted in K, Al, and Si (Figure 2). Similar increases in anthropogenic end-members,

181 specifically enriched U, have been observed with corresponding enrichments in Ca and

182 Fe in other post detonation materials23. The inclusion is surrounded by a rim that appears

183 dark in the BSE image and is also evident the element maps (Figure 2). This feature is

184 interpreted as a diffusion rim, and is also seen in the visual distribution of Pb in the SII

185 (Figure 1). Similarly, there is a clear gradual increase over 20 µm in Pb concentrations in

186 the line traverse cross section. This gradual increase in Pb cps is indicative of absorption

187 into the glass because there is not a sharp contrast between the glass and the inclusion

188 (Figure 1). Since the particle’s Pb isotope composition is resolvable from the surrounding

189 glass outside of 2σ uncertainty, it is assumed to be a unique addition to the post

190 detonation glass.

191 The measured Pb isotopic composition of trinitite glass is a reflection of at least

192 three mixing end-members: 1) the main host for Pb in arkosic sand, the precursor

193 material for the trinitite, are feldspars24, 2) phases that contain amounts of radiogenic Pb

194 such as zircon, monazite, and apatite, and 3) any anthropogenic contribution from the test.

195 have a solid solution with the three major end-members being K-

196 (KAlSi3O4), Albite (NaAlSi3O4), and Anorthite (CaAl2Si2O4). The particle area is

197 depleted in K and Al, therefore, the inclusion could not have been a feldspar or feldspar

198 remnant. There are no other precursor phases in arkosic sand that could contain enough

8 199 Pb to account for the amount of enrichment seen in Pb in the SII (Figure 1). Additionally,

200 the unradiogenic nature of the composition measured here, strongly suggests that this is

201 not from any U-bearing radiogenic phases originally present in the sandstone. The

202 morphology of the inclusion here and in previous studies2, the association of Pb and Cu,

203 which has been linked explicitly with test materials2,6,8,14,15, the late additions of Pb seen

204 as inclusions on the surface of the trinitite2, and the diffusion rim, strongly suggests this

205 particle represents a late-stage addition to the trinitite glass that was being absorbed into

206 the glass and quenched, preserving the texture and composition of Pb particles from the

207 test. Since the particle seems to be frozen in the process of absorption to the glass, it is

208 assumed that mixing between the particle and surrounding glass was limited. These

209 particles are likely pieces of the melted Pb bricks, solder, pigs, or some other unknown

210 test component previously inferred to have been at the test site. Lastly, it is unlikely that

211 the Pb isotope composition of this particle was altered by the neutron flux from the

212 trinitite test due to the small neutron absorption cross section of Pb. Thus, the

213 composition of this particle likely represents the closest composition yet measured to the

214 actual Pb materials used in the test and is in very good agreement with previous estimates

215 of the anthropogenic contribution to the trinitite glass4,8.

216 Conclusions

217 Utilizing a SIMS SII method, the morphology and isotopic composition of a Pb-

218 rich nuclear fallout particle has been determined. This particle was likely being absorbed

219 into the melt glass when it was then quenched, preserving its bleb morphology and strong

220 enrichment in Pb concentration. The same magnitude of Pb enrichment was seen in laser

221 ablation data of similar samples4,6. Furthermore, the Pb isotopic composition of the

9 222 nuclear fallout particle measured here is in excellent agreement with previous estimates

223 of the Pb isotopic composition of anthropogenic materials used in the Trinity test4,8. The

224 correlation between the Pb isotopic compositions measured using these two different

225 analytical techniques (laser ablation vs. SIMS) on three different trinitite sample

226 mediums (trinitite glass, red glass, and the fallout particle in this study) greatly improve

227 the confidence in the determination of the isotopic composition associated with the

228 anthropogenic Pb component of post detonation materials.

229

230 Acknowledgements

231 The authors would like to acknowledge Marianne Ahlbom for access to the SEM

232 at Stockholm University. This manuscript was greatly improved from discussions with Dr.

233 Greg Brennecka and the authors would like to thank an anonymous reviewer for further

234 constructive comments. This work was funded by grants from the Knut and Alice

235 Wallenberg Foundation (2012.0097) and the Swedish Research Council grants VR 621-

236 2012-4370 to MJW, AAN, and VR 2016-03371 to JJB. The NordSIMS ion microprobe

237 facility operates as a Nordic infrastructure funded by Iceland and Sweden. This is

238 NordSIMS publication # 487.

239

240 References:

241 1. Fahey, A.J.; Zeissler, C.J.; Newbury, D.E.; Davis, J.; and Lindstrom, R.M. PNAS. 2010, 242 107, 20207-20212. 243 244 2. Bellucci, J.J.; Simonetti, A. J. Radioanal. Nucl. Chem. 2012, 293, 313-319. 245 246 3. Bellucci, J.J.; Simonetti, A.; Wallace, C.; Koeman, E.C.; and Burns, P.C. Anal. Chem. 247 2013, 85, 4195- 4198.

10 248 249 4. Bellucci, J.J.; Simonetti, A.; Wallace, C.; Koeman, E.C.; Burns, P.C. Anal. Chem. 250 2013, 85, 7588-7593. 251 252 5. Wallace, C.; Bellucci, J.J.; Simonetti, A.; Koeman, C.; Hainley, T.; Burns, P.C. J. 253 Radioanal. Nucl. Chem. 2013, 295, 2049-2057. 254 255 6. Bellucci, J.J.; Simonetti, A.; Koeman, E.C.; Wallace, C.; Burns, P.C. Chem. Geo. 2014, 256 365, 69-86. 257 258 7. Sharp, N.; McDonough, W.F.; Ticknor, B.W.; Ash, R.D.; Piccoli, P.M.; Borg, D.T. J. 259 Radioanal. Nucl. Chem. 2014, 302, 57-67. 260 261 8. Koeman, E.C.; Simonetti, A.; Burns, P.C. Anal. Chem. 2015, 87, 5380-5386. 262 263 9. Donahue, P.H.; Simonetti, A.; Koeman, E.C.; Mana, S.; Burns, P.C. J. Radioanal. Nucl. 264 Chem. 2015, 306, 457-467. 265 266 10. Parekh, P.P.; Semkow, T.M.; Torres, M.A.; Haines, D.K.; Cooper, J.M.; Rosenburg, 267 P.M.; Kitto, M.E. J. Environ. Radioact. 2006, 85, 103-120.

268 11. Nygren, U.; Rameback, H.; Nilsson, C. J. Radioanal. Nucl. Chem. 2007, 272, 45-51.

269 12. Rhodes, R. The Making of the Atomic Bomb. 1986. 575–577, 657. 270 271 13. Semkow, T. M.; Parekh, P. P.; Haines, D. K. Modeling the Effects of the Trinity Test. 272 2006, 142. 273 274 14. Eby, N.; Hermes, R.; Charnley, N.; Smoliga, J.A. Geology Today. 2010, 5, 180-185. 275 276 15. Eby, G.N.; Charnley, N.; Pirre, D.; Hermes, R.; Smoliga, J.; Rollinson, G. Am. 277 Mineral. 2015, 100, 427-441. 278 279 16. 12. Joint Working Group of the American Physical Society and the American 280 Association for the Advancement of Science. Nuclear Forensics: Role, State of the Art, 281 and Program Needs. 2010, ISBN: 978-0-87168-720-3 282 http://cstsp.aaas.org/files/Complete.pdf). 283 284 17. Brill, R.H.; Wampler, J.M., Am. J. Archaeol. 1967, 81, 63-77.

285 18. Kusiak, M. A.; Whitehouse, M. J.; Wilde, S. A.; Dunkley, D. J.; Menneken, M.; 286 Nemchin, A. A; Clark, C. Am. J. Sci. 2013, 313, 933-967

287 19. Kusiak M.A.; Whitehouse M.J.; Wilde S.A.; Nemchin, A.A.; Clark, C. Geology. 2013, 288 41, 291-294. 289 290 20. Whitehouse, M.J.; Kumar, G.R.R.; Rimša, A. Contrib Mineral Petrol. 2014, 168,

11 291 1042

292 21. Bellucci J. J.; Whitehouse M. J.; Nemchin A. A.; Snape J. F.; Pidgeon R.T.; Grange 293 M.L.; Reddy S.M.; Timms N. Chem. Geo. 2016, 438, 112–122

294 22. Woodhead, J.D.; Hergt, J.M. Geostand. Geoanal. Res. 2000, 24, 33-38. 295 296 23. Weisz, D.G.; Jackobsen, B.; Marks, N.E.; Knight, K.B.; Isselhardt, B.H.; Matzel, J.E.; 297 Weber, P.K.; Prussin, S.G.; Hutcheon, I.D. Geochim. Cosmochim. Acta. (in press) DOI: 298 http://dx.doi.org/10.1016/j.gca.2016.10.036 299 300 24. Pettijohn, F.J. Chapter S: U.S. Geological Survey Professional Paper, 1963, 440-s, 301 19 p. 302 303 304 Figures 305 306 Figure 1. Scanning ion images of a Pb inclusion in trinitite glass. ROIs were selected to

307 encompass the glass and the inclusion in images 1 and 2, while the whole area was

308 selected in image 3. A line traverse shown in image 2 with a spot size of approximately 4

309 µm was used in calculating 208Pb cps vs. µm.

310

311 Figure 2. Backscatter electron and EDS maps of the inclusion. Red box represents area

312 imaged by Figure 1, Image 2. There are no enrichments in major elements other than Ca

313 and Fe, indicating that the increased Pb concentration cannot be explained by any of the

314 mineral phases present in the arkosic sand from which the glass was formed.

315

316 Figure 3. Pb isotopic diagram of trinitite and estimates of the device compositions from

317 the literature.

12 Table 1. Isotopic Compositions of Trinitite from literature and Scanning Ion Imaging Sample Name 204Pb/206Pb 2 207Pb/206Pb 2 208Pb/206Pb 2 Image 1 Glass ROI 1 0.061 0.002 0.916 0.012 2.17 0.02 Image 1 Inclusion ROI 2 0.057 0.004 0.879 0.018 2.11 0.04 Image 3 Glass ROI 1 0.063 0.004 0.930 0.029 2.16 0.04 Image 2 Inclusion ROI 1 0.056 0.001 0.869 0.004 2.11 0.01 Image 2 Inclusion ROI 2 0.055 0.002 0.873 0.008 2.13 0.02

K-Feldspar4 0.062 0.946 2.21 Estimated Fallout4 0.056 0.874 2.12 8 318 Estimated Fallout 0.056 0.870 2.11

13 Image 1 2 1

Image 2 1

2

Image 3

1.E+05 Cross section from Image 2

1.E+04 Pb cps 1.E+03

1 208

1.E+02 0 20 40 60 80 100 μm BSE Al K

100 μm

Ca Fe Si 2.22 Trinitite Glass Image 1 Trinitite Glass Image 3 2.20 Pb Inclusion Image 1 Pb Inclusion Image 2 2.18 Estimated Fallout4 Estimated Fallout8 K-Feldspar 4 2.16

2.14

Pb/ Pb 2.12 208 206 2.10

2.08

0.98

0.96

0.94

0.92

0.90 Pb/ Pb 0.88 207 206

0.86

Error bars represent 2σ uncertainty 0.84 0.050 0.052 0.054 0.056 0.058 0.060 0.062 0.064 0.066 0.068 0.070 204 Pb/ 206 Pb