**1. Introduction**

224 Radioisotopes – Applications in Physical Sciences

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Radioactivity is a natural phenomenon always taking place in our planet and in the whole universe. In the very beginning of matter, which it is evolving till now, some radioactive isotopes were created, among others, to form either in a mixture or as a single one, the ninety material units known as elements, which combined in a huge number of ways represent what is called matter, nature and universe. This sort of radioisotopes are, for example, 40K, 50V and 87Rb, as well as every radioisotope found from bismuth to uranium, all of them radioactive, classified by Mendeleieff according their atomic number and weight in the Periodic Chart. These natural radioisotopes are called Primordial and are shown in Table 1.


Table 1. Radioisotopes in the isotopic mixture of elements from K to Bi (Primordial) (Choppin a, 1980)

Radioactivity in Marine Salts and Sediments 227

decreased as much as 0.4%. An explanation of that anomaly was that some mass of 235U had suffered fission for some time in the past. Residues of fission products with longer half-life were thus looked for in the site, and were surprisingly found. The minimum concentration of highly fissionable 235U to have the critical mass for fission chain reaction is 1%. As this radioactive isotope half-life is 700 million years, while that of 238U is 4.5 billion years, the necessary time span to get that minimum concentration finished 400 million years ago. But age of fission products' residues found in the field were also coincident within a much larger time span, around 2 billion years ago, which is also a common order of magnitude for some other minerals with some radioactive isotope in its composition, such as 40K and 87Rb. At that time, 235U concentration in minerals should have been much greater, and thus very likely to make fission possible. Geographical conditions are favourable as well as rain may have washed out uranium minerals found in the surrounding hills, which could have then concentrated at the bottom of a lake. This lake could have then dried out as a result of a change in the rain cycle, or possibly as a consequence of fission heat, from which sediments can be found at Oklo mine. Therefore, the Oklo phenomenon is a fact that supports the idea of radioactivity as a natural component of material reality, and should by no means cause major concerns if the phenomenon is adequately managed, as happens with fire, explosives, acids, fuels, speed, pressure, electricity and so on. As Chang, the great chemist says: "humans are not necessarily the innovators, but merely the imitators of nature" (Chang,

Finally, some natural radioisotopes with comparable half life to planet age, such as heavy 232Th, 235U and 238U are decaying into radioisotopes which linking one to another make a radioactive chain, each link created by decaying of the previous one and evolving to next one by its own decaying, to finish with a stable Pb isotope. These sort of natural radioisotopes are called Radiogenic and are shown in Tables 3, 4 and 5. So, they have as a link, for example, 215At and 218At, radioisotopes with extremely short half lives, but in spite of it always present in nature because they are continuously created in the 235U and 238U radioactive chains. Pu radioisotopes are formed by 238U irradiated with thermal neutrons and successive beta decay. Among them, 239Pu (t1/2 = 24,400 years) and 241Pu (t1/2 = 13.2 years) are the most important, because they have a great cross section for fission with thermal neutrons, and so they are the origin of the so called breeding reactors, where calorific energy is obtained at same time that a new fissionable, nuclear fuel is

232Th radioactive chain is called (4n) because the mass number of every link is a multiple of 4. In the same way, as radioactive chains of 238U and 235U show links whose mass numbers are reproduced by algebraic expressions (4n+2) and (4n+3), where n is an entire number, they are called in this manner. While 241Pu radioactive chain, which is not natural, but produced in modern enriched uranium nuclear reactors, is called (4n+1) by same reason. It is noticeable from Tables 3, 4 and 5 the presence of links with Ra, Rn and Po isotopes. Ra and Po were the first radioisotopes isolated from pechblenda minerals by Pierre and Marie Curie, while Rn radioisotopes are also found there, all of them with different half lives and radiation energies. These radioisotopes of heaviest noble gas have been always a radioactive component of earth atmosphere everywhere, specially concentrated in those indoor places where their α and γ radiations are now detected. Therefore, emissions produced by natural radioisotopes have always been in air, earth and sea, but quite a different matter is the environmental contamination produced today by 235U and maybe tomorrow by 239Pu and

2005).

produced.

241Pu fission products.

Two vacancies are shown in the Periodic Chart: Tc and Pm, elements not present in nature, because when they are produced by nuclear reactions, only short half life radioisotopes are produced, and so, if they have existed some time, they were quickly transformed into their neighbour elements. But nuclear reactions are taking place continuously in the earthly atmosphere by the interaction between light elements in gaseous state and nuclear particles such as α particles, fast neutrons, protons and deuterons coming from stratosphere. The products of these nuclear reactions are also radioisotopes, which are pulled down to the planet mainly by rain water and wind with no interruption. Radioisotopes of this sort are, for example: 3H, 10Be and 14C, which in spite of their short half lives, compared with the age of solar system, reach an equilibrium state between their rates of production and decaying. These natural radioisotopes are called Cosmogenic and are shown in Table 2.


Table 2. Some radioisotopes found in rain water (Cosmogenic) (Choppin b, 1980)

This is a very general and rather schematic description of natural radioactivity, always existent and main indicator of earth and universe evolution, since intensity of every radioactive source is always decreasing as time goes by, that is to say, the number of nucleus decaying by unit time when emitting nuclear radiations is inversely proportional to half life, and directly proportional to mass of every radioisotope, either natural or by human creation. But over the unavoidable and omnipresent natural radioactivity, it has been added that created by man. First radioisotopes of short half life were created, such as 13N and 30P with half lives of 9.9 and 2.5 minutes respectively, by the irradiation of B and Al with α particles emitted by Po. This discovery was made by Frederic Joliot and his wife Irene Curie in 1934. Since then, more than 2,000 artificial radioisotopes have been created, either as a research field itself or by a huge number of technological applications.

#### **1.1 The Oklo phenomenon, a nuclear reactor in nature**

In 1972, one mine of uranium minerals called Oklo, situated in the young country of Gabon, in Western Africa, was being fully exploited. Its minerals were sent to Pierrelate Centre for industrial uranium enrichment in France. Surprisingly, some samples showed a lower 235U concentration than elsewhere in the world, that is to say 0.7%, which in some cases

Two vacancies are shown in the Periodic Chart: Tc and Pm, elements not present in nature, because when they are produced by nuclear reactions, only short half life radioisotopes are produced, and so, if they have existed some time, they were quickly transformed into their neighbour elements. But nuclear reactions are taking place continuously in the earthly atmosphere by the interaction between light elements in gaseous state and nuclear particles such as α particles, fast neutrons, protons and deuterons coming from stratosphere. The products of these nuclear reactions are also radioisotopes, which are pulled down to the planet mainly by rain water and wind with no interruption. Radioisotopes of this sort are, for example: 3H, 10Be and 14C, which in spite of their short half lives, compared with the age of solar system, reach an equilibrium state between their rates of production and decaying.

> H 12.35 years 2500 Be 53.4 days 81 Be 1.6x10**<sup>6</sup>** years 360 C 5715 years 22000 Na 2.6 years 0.6 Al 7.16x10**6** years 1.7

**<sup>35</sup>**S 87.5 days 14 **<sup>36</sup>**Cl 3x10**5** years 11

This is a very general and rather schematic description of natural radioactivity, always existent and main indicator of earth and universe evolution, since intensity of every radioactive source is always decreasing as time goes by, that is to say, the number of nucleus decaying by unit time when emitting nuclear radiations is inversely proportional to half life, and directly proportional to mass of every radioisotope, either natural or by human creation. But over the unavoidable and omnipresent natural radioactivity, it has been added that created by man. First radioisotopes of short half life were created, such as 13N and 30P with half lives of 9.9 and 2.5 minutes respectively, by the irradiation of B and Al with α particles emitted by Po. This discovery was made by Frederic Joliot and his wife Irene Curie in 1934. Since then, more than 2,000 artificial radioisotopes have been created, either as a

In 1972, one mine of uranium minerals called Oklo, situated in the young country of Gabon, in Western Africa, was being fully exploited. Its minerals were sent to Pierrelate Centre for industrial uranium enrichment in France. Surprisingly, some samples showed a lower 235U concentration than elsewhere in the world, that is to say 0.7%, which in some cases

Table 2. Some radioisotopes found in rain water (Cosmogenic) (Choppin b, 1980)

Production rate in the atmosphere (nucleus/ m2-s)

These natural radioisotopes are called Cosmogenic and are shown in Table 2.

Radiosotope Half Life

**<sup>32</sup>**Si 280 years **<sup>32</sup>**P 14.3 days **<sup>33</sup>**P 25.3 days

**<sup>39</sup>**Ar 269 years

research field itself or by a huge number of technological applications.

**1.1 The Oklo phenomenon, a nuclear reactor in nature** 

decreased as much as 0.4%. An explanation of that anomaly was that some mass of 235U had suffered fission for some time in the past. Residues of fission products with longer half-life were thus looked for in the site, and were surprisingly found. The minimum concentration of highly fissionable 235U to have the critical mass for fission chain reaction is 1%. As this radioactive isotope half-life is 700 million years, while that of 238U is 4.5 billion years, the necessary time span to get that minimum concentration finished 400 million years ago. But age of fission products' residues found in the field were also coincident within a much larger time span, around 2 billion years ago, which is also a common order of magnitude for some other minerals with some radioactive isotope in its composition, such as 40K and 87Rb. At that time, 235U concentration in minerals should have been much greater, and thus very likely to make fission possible. Geographical conditions are favourable as well as rain may have washed out uranium minerals found in the surrounding hills, which could have then concentrated at the bottom of a lake. This lake could have then dried out as a result of a change in the rain cycle, or possibly as a consequence of fission heat, from which sediments can be found at Oklo mine. Therefore, the Oklo phenomenon is a fact that supports the idea of radioactivity as a natural component of material reality, and should by no means cause major concerns if the phenomenon is adequately managed, as happens with fire, explosives, acids, fuels, speed, pressure, electricity and so on. As Chang, the great chemist says: "humans are not necessarily the innovators, but merely the imitators of nature" (Chang, 2005).

Finally, some natural radioisotopes with comparable half life to planet age, such as heavy 232Th, 235U and 238U are decaying into radioisotopes which linking one to another make a radioactive chain, each link created by decaying of the previous one and evolving to next one by its own decaying, to finish with a stable Pb isotope. These sort of natural radioisotopes are called Radiogenic and are shown in Tables 3, 4 and 5. So, they have as a link, for example, 215At and 218At, radioisotopes with extremely short half lives, but in spite of it always present in nature because they are continuously created in the 235U and 238U radioactive chains. Pu radioisotopes are formed by 238U irradiated with thermal neutrons and successive beta decay. Among them, 239Pu (t1/2 = 24,400 years) and 241Pu (t1/2 = 13.2 years) are the most important, because they have a great cross section for fission with thermal neutrons, and so they are the origin of the so called breeding reactors, where calorific energy is obtained at same time that a new fissionable, nuclear fuel is produced.

232Th radioactive chain is called (4n) because the mass number of every link is a multiple of 4. In the same way, as radioactive chains of 238U and 235U show links whose mass numbers are reproduced by algebraic expressions (4n+2) and (4n+3), where n is an entire number, they are called in this manner. While 241Pu radioactive chain, which is not natural, but produced in modern enriched uranium nuclear reactors, is called (4n+1) by same reason. It is noticeable from Tables 3, 4 and 5 the presence of links with Ra, Rn and Po isotopes. Ra and Po were the first radioisotopes isolated from pechblenda minerals by Pierre and Marie Curie, while Rn radioisotopes are also found there, all of them with different half lives and radiation energies. These radioisotopes of heaviest noble gas have been always a radioactive component of earth atmosphere everywhere, specially concentrated in those indoor places where their α and γ radiations are now detected. Therefore, emissions produced by natural radioisotopes have always been in air, earth and sea, but quite a different matter is the environmental contamination produced today by 235U and maybe tomorrow by 239Pu and 241Pu fission products.

Radioactivity in Marine Salts and Sediments 229

Radioisotope Half Life Historical Name Type of

6.75 hours Uranium Z β-

2.5x10**5** years Uranium II α , γ

8x10**4** years Ionium α , γ

1602 years Radium α , γ

3.8 days Radon (emanation) α , γ

3.05 minutes Radium A α , β-

26.8 microseconds Radium B β- , γ

2 seconds Astatine α

19.7 minutes Radium C α , β-

164 microseconds Radium C´ α , β-

1.3 minutes Radium C´´ β- , γ

21 years Radium D β- , γ

5.01 years Radium E α , β-

138.4 days Radium F α

4.19 minutes Radium E´ β-

**<sup>206</sup>**Pb stable Radium G \_\_\_

**<sup>234</sup>**Pa

**<sup>234</sup>**U

**<sup>230</sup>**Th

**<sup>226</sup>**Ra

**<sup>222</sup>**Rn

**<sup>218</sup>**Po

**<sup>214</sup>**Pb (99.98%)

**<sup>218</sup>**At (0.02%)

**<sup>214</sup>**Bi

**<sup>214</sup>**Po (99.98%)

**<sup>210</sup>**T1(0.02%)

**<sup>210</sup>**Pb

**<sup>210</sup>**Bi

**<sup>210</sup>**Po (100%)

**<sup>206</sup>**T1 (0.00013%)

Table 4. **238**U radioactive chain (4n + 2)

radioactive decay

, γ

, γ


**<sup>234</sup>**mPa 1.17 minutes Uranium X**<sup>2</sup>** β- , γ

**<sup>232</sup>**Th

**<sup>228</sup>**Ra

**<sup>228</sup>**Ac

**<sup>228</sup>**Th

**<sup>224</sup>**Ra

**<sup>220</sup>**Rn

**<sup>216</sup>**Po

**<sup>212</sup>**Pb

**<sup>212</sup>**Bi

**<sup>212</sup>**Po (64%)

**<sup>238</sup>**U

**<sup>234</sup>**Th

**<sup>234</sup>**mPa

Table 3. **232**Th radioactive chain (4n)

Radioisotope Half Life Historical Name Type of radioactive

1.4x10**10** years Thorium α

6.7 years Mesothorium I β-

6.13 hours Mesothorium II β-

1.9 years Radiothorium α , γ

3.64 days Thorium X α , γ

55 seconds Toron (emanation) α , γ

0.15 seconds Thorium A α

10.6 hours Thorium B β-

60.6 minutes Thorium C α , β-

304 nanoseconds Thorium C**´** α

4.5 x 109 years Uranium I α

24.1 days Uranium X**<sup>1</sup>** β- , γ

1.17 minutes Uranium X**<sup>2</sup>** β- , γ

**<sup>208</sup>**T1 (36%) 3.1 minutes Thorium C´´ <sup>β</sup>- , <sup>γ</sup>

**<sup>208</sup>**Pb stable Thorium D \_\_\_

Radioisotope Half Life Historical Name Type of

decay

, γ

radioactive decay

, γ


Table 4. **238**U radioactive chain (4n + 2)

Radioactivity in Marine Salts and Sediments 231

**<sup>241</sup>**Pu

**<sup>241</sup>**Am(100%)

**<sup>237</sup>**U(0.0023%)

**<sup>237</sup>**Np

**<sup>233</sup>**Pa

**<sup>233</sup>**U

**<sup>229</sup>**Th

**<sup>225</sup>**Ra

**<sup>225</sup>**Ac

**<sup>221</sup>**Fr

**<sup>217</sup>**At

**<sup>213</sup>**Bi

**<sup>209</sup>**T1(2.2%)

**<sup>209</sup>**Pb

Table 6. **241**Pu radioactive chain (4n + 1)

**<sup>213</sup>**Po (97.8%) 4.2

Radioisotope Half Life Name Type of radioactive

13.2 years Plutonium α , β- , γ

458 years Americium α , γ

6.75 days Uranium β- , γ

2.14x10**6** years Neptunium α , γ

1.6x10**5** years Uranium α , γ

7340 years Thorium α , γ

14.8 days Radium β- , γ

10 days Actinium α , γ

4.8 minutes Francium α , γ

0.032 seconds Astatine α

microseconds Polonium <sup>α</sup>

2.2 minutes Thallium β-

3.3 hours Lead β-

**<sup>209</sup>**Bi > 2x10**18** years Bismuth α ?

47 minutes Bismuth α , β- , γ

27 days Protactinium β- , γ

decay

, γ

, γ


Table 5. **235**U radioactive chain (4n + 3)

**<sup>235</sup>**U

**<sup>231</sup>**Th

**<sup>231</sup>**Pa

**<sup>227</sup>**Ac

**<sup>227</sup>**Th(98.6%)

**<sup>223</sup>**Fr(1.4%)

**<sup>223</sup>**Ra

**<sup>219</sup>**Rn

**<sup>215</sup>**Po

**<sup>211</sup>**Pb (100%)

**<sup>215</sup>**At(0.00023%)

**<sup>211</sup>**Bi

**<sup>211</sup>**Po(0.28%)

**<sup>207</sup>**T1 (99.7%)

Table 5. **235**U radioactive chain (4n + 3)

Radioisotope Half Life Historical Name Type of radioactive

7.1x10**8** years Actinouranium α , γ

25.2 hours Uranium Y β- , γ

3.25x10**4** years Protoactinium α , γ

21.6 years Actinium α , β- , γ

18.2 days Radioactinium α , γ

22 minutes Actinium K β- , γ

11.43 days Actinium X α , γ

1.8 milliseconds Actinium A α , β-

36.1 minutes Actinium B β- , γ

2.15 minutes Actinium C α , β-

4.79 minutes Actinium C´´ β- , γ

(emanation) α , <sup>γ</sup>

4 seconds Actinium

0.1 millisecond Astatine

0.52 seconds Actinium C´

**<sup>207</sup>**Pb stable Actinium D \_\_\_

decay

α

α , γ

, γ


Table 6. **241**Pu radioactive chain (4n + 1)

Radioactivity in Marine Salts and Sediments 233

Fig. 1. 235U Fission Products Yielding vs. Mass Number (A)

Fig. 2. Abundance of elements in earth (%) vs. Atomic Number (Choppin c, 1980)
