**3. Experimental**

#### **3.1 Sampling and samples conditioning**

Therefore, according with the idea to consider radioactivity as a quite natural phenomenon, supported by the existence of Primordial, Cosmogenic and Radiogenic radioisotopes, as well as the Oklo phenomenon, it is proposed to identify the natural radioactivity by Primordial radioisotope 40K, based on the fact that it is present in one of more abundant elements on earth, as it can be seen in Fig. 2, and as a consequence is found in the

Radioactive contamination started on the planet in 1945, when the first nuclear test was performed in Alamo Gordo, New Mexico, followed by the war actions in Hiroshima and Nagasaki. Since then, radioactive contamination at global level has been variable, depending on repeated nuclear tests, few accidents such as Three Mile Island and Chernobyl, and minor failures in nuclear power plants. These contaminants are produced mainly by fission products from 235U, which according their fission yielding and half lives, they remain radioactive during a time span from seconds to a great number of eons (1 eon = 1 x 109 years). But certainly, burned nuclear fuels which are under control and stored accordingly the safest techniques to guarantee they will always be confined and never disseminated in the environment, same case that residues of artificially produced radioisotopes used in medicine, industry or any other purpose, they should not be considered as radioactive contaminants, as much as they are under safe enough surveillance. So, approximately 30-40% all of known radioisotopes are fission products, which when they come into environment by deliberate nuclear explosion, severe accident or failure in nuclear plant, they represent the so called radioactive contamination. From this perspective, it seems that radioactive contamination has been growing up from its beginning, with rather short equilibrium periods. Also, if it is considered that sea water represents approximately 80% of planet surface, plus the action of wind, rain and rivers current, the main repository of radioactive contamination should be the sea. However, radioactive contamination is only added to natural radioactivity. From the first elements in the Periodic Table: 3H, 10Be and 14C, natural radioisotopes are either continuously produced by nuclear reactions in the earthly atmosphere, or they were created at same time that non radioactive ones, in the mixture of isotopes forming elements such as 40K, 50V and 87Rb. And then from Bi to beyond uranium elements, every isotope is radioactive with no exception. Therefore, it seems that to properly quantify the importance at planet level of any radioactive contamination, it should be done on the basis of radioactivity already present since the planet birth, whose decaying becomes the most evident sign of earth evolution and it is still taking place. In this way, 0.0118% isotopic abundance, 1.28 x 109 years half life, 40K is the natural radioisotope most abundant in the earth crust and also in the numerous salts dissolved in sea water. So, the radioactivity due to 40K might be the most suitable measurement, in order to have one basis of natural radioactivity to be compared with that of any artificial radioisotope. Among these, the fission product 137Cs presents the highest yielding in the fission of 235U , and it is the most common radioactive pollutant found in nuclear accidents due to its half life equal to 30.07 years, and γ rays easy to detect with higher efficiency due to a low energy equal to 662 Kev. Figure 1 represents the fission products yielding from 235U vs. mass number (A) and Fig. 2

represents percentage of elements on earth vs atomic number (Z).

Therefore, according with the idea to consider radioactivity as a quite natural phenomenon, supported by the existence of Primordial, Cosmogenic and Radiogenic radioisotopes, as well as the Oklo phenomenon, it is proposed to identify the natural radioactivity by Primordial radioisotope 40K, based on the fact that it is present in one of more abundant elements on earth, as it can be seen in Fig. 2, and as a consequence is found in the

**3. Experimental** 

**3.1 Sampling and samples conditioning** 

**2. Radioactive contamination** 

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

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

Radioactivity in Marine Salts and Sediments 235

24 hours detection time. Also, sediment marine samples have been picked up from 40-60 meters depth in three zones: Gulf of Mexico, to south east of Veracruz port and Laguna Verde Nuclear Power Plant, around Grijalva and Usumacinta delta rivers, as well as north, near the border with territorial USA sea water, and in Pacific Ocean between Cortés sea and Mazatlán port. Samples were taken by two ships: Puma in the Gulf and Justo Sierra in the Pacific Ocean, both at service of Sea Science and Limnology Institute, from National University of Mexico. Figure 3 presents the Puma ship. Figure 4 the Justo Sierra ship. These ships work in Oceanography research, for Institute of Sea Science and Limnology, in the National University of Mexico. Figure 5 presents one sediment sample conditioned in the Marinelli container. Figure 6 presents the low background scintillation set and Figure 7

presents the low background semi-conductor set.

Fig. 3. Ship Puma, samples collector in Pacific Ocean

Fig. 4. Ship Justo Sierra, samples collector in Gulf of Mexico

radioactive background all over the world, while the present radioactive contamination can be easily represented by 137Cs, fission product of 235U. Besides, both radioisotopes are electromagnetic radiation emitters with suitable energies to be easily detected, and so one way to measure the intensity of present radioactive contamination should be to obtain a radioactive contamination factor (RCF), by dividing specific radioactivity of 40K by that of 137Cs in solid samples, that is to say disintegrations per time and weight units measured in both radioisotopes. This present radioactive contamination background, even when proceeds from limited portions on earth surface, where it has remained for long time as a well located radioactive source which must be left away by population and conveniently shielded, it has been unavoidable that a fraction of it spreads out to atmosphere in the gas and dust form, which can travel long distances to be finally carried down mainly by rain water on earth surface as either solutions or suspensions. But as sea represents the much larger proportion of planet surface, about 80%, and it is also the main factor of rain cycle, out of control radioactive pollutants produced anywhere in considerable amounts reach always the sea water in concentrations which can be easily measured by γ rays detection. Therefore, it seems that it is in sea water and marine sediments where global radioactive contamination should be searched and evaluated, because it is there where planet radioactive contamination has mainly created a growing deposit since the last world war. However, if it is assumed the sea water volume approximately as 1.4 x 1018 m3, then it might be considered as an enormous natural radioactive source, not at all by contamination, but because it contains in solution an important concentration of K salts and its natural radioisotope 40K (β and γ rays emitter after electronic capture, half life 1.28 x 109 years, 0.0118% isotopic abundance), which represents the main source of natural radioactivity as much in solid minerals (excepting those of heavy metals from Pb on), as in sea water and marine sediments. In this way, in order to asses the importance of any present or future radioactive contamination at planet scale, it might be compared by some radioactive contamination factor or some other way with natural radioactivity, which has been increased at certain extent by radioactive contamination. We are talking here about radioactivity spread out to environment from a local point, which must be immediately attended in situ, whereas that diluted in environment and reaching far away places usually produces great panic, even when it has never before been compared with natural, already existent radioactivity since the beginning of solar system. On the other hand, 40K radioactivity as well as K concentration salts in sea water increases with ocean depth till a maximum value, and then decreases before reaching the bottom till a value usually lower than that at surface, as it happens with every mineral salt dissolved in sea water (Vázquez, 2001). So, it is quite possible to characterize superficial sea water in different coasts in terms of 40K specific radioactivity, by sampling at about one kilometre from the coast, where it keeps constant for parallel much longer distances on the littoral, and obviously is easier to do it that in high sea, useful figure to calculate the concentration of elementary K in that particular sea zone. The way to do it is quite simple: 6-8 litres of sea water must be boiled, in order to get a suitable volume of sea salt to fill up a Marinelli container, usually about half a litre, necessary to perform low background radioactive detection. Once the dry salt sample is weighed and conditioned in the Marinelli container, it is ready to measure its natural as well as polluting radioactivity, by making use first of one heavily shielded scintillation set (NaI, Tl activated), and then one equally shielded hyper-pure Ge detector(HPGe), during 12-

radioactive background all over the world, while the present radioactive contamination can be easily represented by 137Cs, fission product of 235U. Besides, both radioisotopes are electromagnetic radiation emitters with suitable energies to be easily detected, and so one way to measure the intensity of present radioactive contamination should be to obtain a radioactive contamination factor (RCF), by dividing specific radioactivity of 40K by that of 137Cs in solid samples, that is to say disintegrations per time and weight units measured in both radioisotopes. This present radioactive contamination background, even when proceeds from limited portions on earth surface, where it has remained for long time as a well located radioactive source which must be left away by population and conveniently shielded, it has been unavoidable that a fraction of it spreads out to atmosphere in the gas and dust form, which can travel long distances to be finally carried down mainly by rain water on earth surface as either solutions or suspensions. But as sea represents the much larger proportion of planet surface, about 80%, and it is also the main factor of rain cycle, out of control radioactive pollutants produced anywhere in considerable amounts reach always the sea water in concentrations which can be easily measured by γ rays detection. Therefore, it seems that it is in sea water and marine sediments where global radioactive contamination should be searched and evaluated, because it is there where planet radioactive contamination has mainly created a growing deposit since the last world war. However, if it is assumed the sea water volume approximately as 1.4 x 1018 m3, then it might be considered as an enormous natural radioactive source, not at all by contamination, but because it contains in solution an important concentration of K salts and its natural

and γ rays emitter after electronic capture, half life 1.28 x 109 years,

0.0118% isotopic abundance), which represents the main source of natural radioactivity as much in solid minerals (excepting those of heavy metals from Pb on), as in sea water and marine sediments. In this way, in order to asses the importance of any present or future radioactive contamination at planet scale, it might be compared by some radioactive contamination factor or some other way with natural radioactivity, which has been increased at certain extent by radioactive contamination. We are talking here about radioactivity spread out to environment from a local point, which must be immediately attended in situ, whereas that diluted in environment and reaching far away places usually produces great panic, even when it has never before been compared with natural, already existent radioactivity since the beginning of solar system. On the other hand, 40K radioactivity as well as K concentration salts in sea water increases with ocean depth till a maximum value, and then decreases before reaching the bottom till a value usually lower than that at surface, as it happens with every mineral salt dissolved in sea water (Vázquez, 2001). So, it is quite possible to characterize superficial sea water in different coasts in terms of 40K specific radioactivity, by sampling at about one kilometre from the coast, where it keeps constant for parallel much longer distances on the littoral, and obviously is easier to do it that in high sea, useful figure to calculate the concentration of elementary K in that particular sea zone. The way to do it is quite simple: 6-8 litres of sea water must be boiled, in order to get a suitable volume of sea salt to fill up a Marinelli container, usually about half a litre, necessary to perform low background radioactive detection. Once the dry salt sample is weighed and conditioned in the Marinelli container, it is ready to measure its natural as well as polluting radioactivity, by making use first of one heavily shielded scintillation set (NaI, Tl activated), and then one equally shielded hyper-pure Ge detector(HPGe), during 12-

radioisotope 40K (β-

24 hours detection time. Also, sediment marine samples have been picked up from 40-60 meters depth in three zones: Gulf of Mexico, to south east of Veracruz port and Laguna Verde Nuclear Power Plant, around Grijalva and Usumacinta delta rivers, as well as north, near the border with territorial USA sea water, and in Pacific Ocean between Cortés sea and Mazatlán port. Samples were taken by two ships: Puma in the Gulf and Justo Sierra in the Pacific Ocean, both at service of Sea Science and Limnology Institute, from National University of Mexico. Figure 3 presents the Puma ship. Figure 4 the Justo Sierra ship. These ships work in Oceanography research, for Institute of Sea Science and Limnology, in the National University of Mexico. Figure 5 presents one sediment sample conditioned in the Marinelli container. Figure 6 presents the low background scintillation set and Figure 7 presents the low background semi-conductor set.

Fig. 3. Ship Puma, samples collector in Pacific Ocean

Fig. 4. Ship Justo Sierra, samples collector in Gulf of Mexico

Radioactivity in Marine Salts and Sediments 237

In order to obtain our results either of natural or contaminant radioactivity in Bq per gram of sea salts and marine sediments, we must calculate the detection efficiency of both, scintillation and HPGe detector systems. It is easier and more precise to use one 40K calibrated source formed by a known weight of KCl, and by separate one 137Cs calibrated source. Detection efficiency for the 1461 Kev γ rays peak emitted by 40K was determined by a standard made out by filling a Marinelli container with a weighed mass of KCl salt, AR grade. Detection time of 10-20 minutes was enough to get ±1% as statistical error. Then, the counts accumulated in the peak expressed as counts per second (cps), when divided by the specific activity expressed as disintegrations per second per gram (dps/g = Bq/g) of either KCl or elementary K, and multiplied by 100, is obtained detection efficiency for scintillation and semiconductor systems in the same way. Equations 1 and 2 show the calculation to get the specific activity of KCl and elementary K respectively, due to 11% of 40K decaying nucleus by electron capture to 40Ar and emitting γ rays with an energy of 1461Kev. Equation 3 show the calculation to get the total specific activity of elementary K, due to 0.0118% isotopic abundance of 40K (β- emitter 89%, EC and γ rays emitter 11%), constant value that

Bq K Ar/gKCl = 0.693x6.02x10 x0.0118 x11 / 1.28x10 x365x24x60x60x100x100x74.5

Bq K Ar/gK = 0.693x6.02x10 x0.0118x11 / 1.28x10 x365x24x60x60x100x100x39.1

(1)

(2)

Fig. 7. HPGe Semiconductor Detetection set

**3.2 Radioactive detection** 

will be used to characterize sea salts.

→

→

40 40 23 9 40 40

40 40 23 9 40 40

= 3.4 Bq K Ar/ gK

→

= 1.8 Bq K Ar / g KCl

→

Fig. 5. Marinelli container with sediments

Fig. 6. Scintillation Detection set

Fig. 5. Marinelli container with sediments

Fig. 6. Scintillation Detection set

Fig. 7. HPGe Semiconductor Detetection set

#### **3.2 Radioactive detection**

In order to obtain our results either of natural or contaminant radioactivity in Bq per gram of sea salts and marine sediments, we must calculate the detection efficiency of both, scintillation and HPGe detector systems. It is easier and more precise to use one 40K calibrated source formed by a known weight of KCl, and by separate one 137Cs calibrated source. Detection efficiency for the 1461 Kev γ rays peak emitted by 40K was determined by a standard made out by filling a Marinelli container with a weighed mass of KCl salt, AR grade. Detection time of 10-20 minutes was enough to get ±1% as statistical error. Then, the counts accumulated in the peak expressed as counts per second (cps), when divided by the specific activity expressed as disintegrations per second per gram (dps/g = Bq/g) of either KCl or elementary K, and multiplied by 100, is obtained detection efficiency for scintillation and semiconductor systems in the same way. Equations 1 and 2 show the calculation to get the specific activity of KCl and elementary K respectively, due to 11% of 40K decaying nucleus by electron capture to 40Ar and emitting γ rays with an energy of 1461Kev. Equation 3 show the calculation to get the total specific activity of elementary K, due to 0.0118% isotopic abundance of 40K (β- emitter 89%, EC and γ rays emitter 11%), constant value that will be used to characterize sea salts.

40 40 23 9 40 40 Bq K Ar/gKCl = 0.693x6.02x10 x0.0118 x11 / 1.28x10 x365x24x60x60x100x100x74.5 = 1.8 Bq K Ar / g KCl → → (1)

$$\begin{aligned} \text{Bq}^{40}\text{K} & \rightarrow ^{40}\text{Ar/gK} = 0.693 \times 6.02 \times 10^{23} \times 0.0118 \times 11 \text{ / } 1.28 \times 10^9 \times 365 \times 24 \text{x60} \times 60 \times 100 \times 100 \times 39.1 \\ &= 3.4 \text{ Bq}^{40}\text{K} \rightarrow ^{40}\text{Ar/gK} \end{aligned} \tag{2}$$

Radioactivity in Marine Salts and Sediments 239

sediments are detected during much longer time periods, from 20 to 24 hours, but with

If samples from Oklo uranium mine were considered as marine sediments, in order to evaluate the radiation danger they represent, it is very likely that radioactivity from natural radioisotopes of heavy metals such as 232Th, 235U and 238U, origin of radioactive chains with several short half life radioisotopes in their links, were substantially higher than that from 40K, natural radioisotope present almost everywhere, and by sure in Oklo minerals too. Since also in marine sediments have been found radioactive heavy metals, similarity between these two mineral samples becomes more understandable, besides the hypotheses that Oklo mine was a huge lake, probably of salted water in its origin. So, even when radioactive contamination by 137Cs is not possible to confirm in Oklo due to its relatively short half life, it should be very easily detected in marine salts in the case of recent contamination, such as that in Fukushima, Japan, which at present should be in the mixture of natural marine salts, and in the near future

similar dead time in detectors to that produced by KCl source.

will be in marine sediments, accompanying heavy metals and of course 40K.

as Bq/g of salt is obtained, according the equation 6:

( ) [ ] [ ]

40

corrected by background Ws = Salt sample weigh

0.11 = Fraction of K

**radioactivity** 

Where:

**3.3 Characterization of marine salts and sediments through natural and pollutant** 

Samples were taken in two points of Gulf of Mexico. One is to the south east of Laguna Verde Nuclear Plant, between delta of Usumacinta and Grijalva rivers, and the other to the north east of the Gulf, near the line with territorial USA waters. In the Pacific Ocean, samples were taken from Cortés Sea to Mazatlán port. In order to characterize sea waters by its K concentration, 5-6 litres of water samples were boiled to obtain about half a Kilogram of salt to fill up one Marinelli container. The weight of salt obtained and divided by the number of litres evaporated gives us one first figure equal to g/L, which means salinity. When counts accumulated during 20-24 hours in a low background detection system, either scintillation or HPGe, are expressed as counts per second, corrected for background in same units (cps) and divided by salt sample weight, detection efficiency for 1461 Kev γ rays (2.8% in our scintillation system and 0.22% in our HPGe detector) and the fraction of 40K nucleus decaying to 40Ar by EC and γ rays emission (11/100), total specific activity of 40K expressed

Bq/g salt = (cps Sample – cps Background ) /Ws x Det. Eff. x 0.11 (6) [ ] [ ]


γ

 ( ) <sup>40</sup> nucleus decaying to Ar by EC and rays emission 11%

<sup>40</sup> - Bq/g salt = Specific activity of sea salt due to K total decaying β 89% , rays 11%

cps sample – cps Background = counts accumulated per second by sample and

t expressed in grams

expressed as fractions Scintillation 2.8x10 , HPGe 0.22x10

( )

γ

Det. Eff = Detection efficiency for 1461 Kev rays emitted by K in our detection systems,

In this way, when salinity is multiplied by specific activity of sea salt, activity per litre of sea water is obtained. Also, when specific activity of sea salt is divided by specific activity of

( ) [] []

γ

40

mentary

$$\begin{aligned} \text{Bq}^{\text{w}} \text{K}/\text{g}\text{K} &= 0.693 \times 6.02 \times 10^{13} \times 0.0118 / 1.28 \times 10^9 \times 365 \times 24 \times 365 \times 60 \times 60 \times 100 \times 39.1 \\ &= 31.19 \text{ Bq/gK} \end{aligned} \tag{3}$$

Where:

23 40 40 40 40 9 9 Ln 2 = 0.693 Avogadro's number = 6.02x10 Isotopic abundance of K = 0.0118/100 Decay yielding of K Ar = 11/100 Half life of K = 1.28x10 years = 1.28x10 x365x24x60x60 seconds KCl molecular weight = 74 → .5 K atomic weight = 39.1

Therefore, detection efficiency for counts accumulated in either scintillation or semiconductor detector, produced by γ rays with energy 1461 Kev, emitted by 40K, is given alternatively by equations 4 and 5.

$$\text{Det. Eff.} \left( \text{electromagnetic radiation} \right) \left( \% \right) = \text{cpsx}100/1.8 \text{xW}\_{\text{s1}} \tag{4}$$

$$\text{Det. Eff.} \left( \text{electromagnetic radiation} \right) \left( \% \right) = \text{cpsx}100/3.4 \text{xW}\_{\text{s2}} \tag{5}$$

Where:

( ) 40 40 40 40 40 40 cps = counts accumulated per second 1.8 = specific activity of K Ar by EC, rays emission per gram of KCl Bq K Ar / g KCl 3.4 = specific activity of K Ar by EC, rays emission per gram of ele γ γ → → → ( ) ( ) 40 40 s1 s2 K Bq K Ar / g K W = Weight of KCl in the Marinelli container W = Weight of K in the Marinelli container 52.48% of KCl →

In order to obtain the detection efficiency for gamma rays (662 Kev) emitted by radioactive contaminant 137Cs, it has been used a calibrated multinuclide standard source in an identical Marinelli container to that used with KCl. In this case, calculation is only to divide counts per second accumulated in the corresponding peak (662 Kev), multiply by 100 and divide by the 137Cs certificate activity in Bq at a given date and corrected to present time by decaying factor. To calculate detection efficiency by separate of γ rays from 40K (1461 Kev) and γ rays from 137Cs (662Kev), it is easier and more precise in our project, that to find that corresponding to 40K from a graph efficiency versus energy, plotted with data obtained from the calibrated multinuclide source, because in this later case Compton distribution is much higher than in natural samples, such as KCl, marine salts and sediments. So, background correction in both detections has revealed as almost irrelevant when detections efficiencies are obtained, while on the other hand it is extremely important when marine salts and

<sup>40</sup> <sup>23</sup> <sup>9</sup> Bq K/gK = 0.693x6.02x10 x0.0118/1.28x10 x365x24x365x60x60x100x39.1

23

Half life of K = 1.28x10 years = 1.28x10 x365x24x60x60 seconds

Therefore, detection efficiency for counts accumulated in either scintillation or semiconductor detector, produced by γ rays with energy 1461 Kev, emitted by 40K, is given

Det. Eff. electromagnetic radiation ( ) ( ) % = cpsx100/1.8xW s1 (4)

Det. Eff. electromagnetic radiation ( ) ( ) % = cpsx100/3.4xW s2 (5)

( )

mentary

40 40 40

→

Isotopic abundance of K = 0.0118/100 Decay yielding of K Ar = 11/100

Avogadro's number = 6.02x10

KCl molecular weight = 74

40 40

→

40 40

W = Weight of KCl in the Marinelli container

→

W = Weight of K in the Marinelli container 52.48% of KCl

K atomic weight = 39.1

40 9 9

.5

1.8 = specific activity of K Ar by EC, rays emission per gram of KCl

γ

γ

In order to obtain the detection efficiency for gamma rays (662 Kev) emitted by radioactive contaminant 137Cs, it has been used a calibrated multinuclide standard source in an identical Marinelli container to that used with KCl. In this case, calculation is only to divide counts per second accumulated in the corresponding peak (662 Kev), multiply by 100 and divide by the 137Cs certificate activity in Bq at a given date and corrected to present time by decaying factor. To calculate detection efficiency by separate of γ rays from 40K (1461 Kev) and γ rays from 137Cs (662Kev), it is easier and more precise in our project, that to find that corresponding to 40K from a graph efficiency versus energy, plotted with data obtained from the calibrated multinuclide source, because in this later case Compton distribution is much higher than in natural samples, such as KCl, marine salts and sediments. So, background correction in both detections has revealed as almost irrelevant when detections efficiencies are obtained, while on the other hand it is extremely important when marine salts and

3.4 = specific activity of K Ar by EC, rays emission per gram of ele

Where:

Where:

Ln 2 = 0.693

alternatively by equations 4 and 5.

( )

( )

Bq K Ar / g KCl

cps = counts accumulated per second

40 40

s1 s2 →

40 40

K Bq K Ar / g K

→

= 31.19 Bq/gK (3)

sediments are detected during much longer time periods, from 20 to 24 hours, but with similar dead time in detectors to that produced by KCl source.

If samples from Oklo uranium mine were considered as marine sediments, in order to evaluate the radiation danger they represent, it is very likely that radioactivity from natural radioisotopes of heavy metals such as 232Th, 235U and 238U, origin of radioactive chains with several short half life radioisotopes in their links, were substantially higher than that from 40K, natural radioisotope present almost everywhere, and by sure in Oklo minerals too. Since also in marine sediments have been found radioactive heavy metals, similarity between these two mineral samples becomes more understandable, besides the hypotheses that Oklo mine was a huge lake, probably of salted water in its origin. So, even when radioactive contamination by 137Cs is not possible to confirm in Oklo due to its relatively short half life, it should be very easily detected in marine salts in the case of recent contamination, such as that in Fukushima, Japan, which at present should be in the mixture of natural marine salts, and in the near future will be in marine sediments, accompanying heavy metals and of course 40K.

#### **3.3 Characterization of marine salts and sediments through natural and pollutant radioactivity**

Samples were taken in two points of Gulf of Mexico. One is to the south east of Laguna Verde Nuclear Plant, between delta of Usumacinta and Grijalva rivers, and the other to the north east of the Gulf, near the line with territorial USA waters. In the Pacific Ocean, samples were taken from Cortés Sea to Mazatlán port. In order to characterize sea waters by its K concentration, 5-6 litres of water samples were boiled to obtain about half a Kilogram of salt to fill up one Marinelli container. The weight of salt obtained and divided by the number of litres evaporated gives us one first figure equal to g/L, which means salinity. When counts accumulated during 20-24 hours in a low background detection system, either scintillation or HPGe, are expressed as counts per second, corrected for background in same units (cps) and divided by salt sample weight, detection efficiency for 1461 Kev γ rays (2.8% in our scintillation system and 0.22% in our HPGe detector) and the fraction of 40K nucleus decaying to 40Ar by EC and γ rays emission (11/100), total specific activity of 40K expressed as Bq/g of salt is obtained, according the equation 6:

$$\text{cBq/g salt} = \text{(cps[Sample] - cps[Background]) / Ws} \times \text{Det. Eff.} \times 0.11 \tag{6}$$

Where:

( ) [] [] <sup>40</sup> - Bq/g salt = Specific activity of sea salt due to K total decaying β 89% , rays 11% γ

( ) [ ] [ ] cps sample – cps Background = counts accumulated per second by sample and

corrected by background

Ws = Salt sample weigh t expressed in grams

( ) 40 -2 -2 Det. Eff = Detection efficiency for 1461 Kev rays emitted by K in our detection systems, expressed as fractions Scintillation 2.8x10 , HPGe 0.22x10 γ 

40 0.11 = Fraction of K ( ) <sup>40</sup> nucleus decaying to Ar by EC and rays emission 11% γ

In this way, when salinity is multiplied by specific activity of sea salt, activity per litre of sea water is obtained. Also, when specific activity of sea salt is divided by specific activity of

Radioactivity in Marine Salts and Sediments 241

in sea salts as in marine sediments, to have a reliable and easy to understand figure to evaluate the magnitude of recent pollution as well as to size up the possible growing or decreasing

Figures 8 and 9 show the background and electromagnetic radiation (γ rays) of marine sediments picked up at Gulf of Mexico North, obtained with a low background scintillation

Figures 10 and 11 show the background and electromagnetic radiation (γ rays) of marine sediments picked up at Gulf of Mexico North, obtained with a low background

Table 7 shows the results obtained from sea salts samples taken up in Pacific Ocean North, between Cortes sea and Mazatlan port, and Gulf of Mexico North and South East, as well as sediments pollution measured by RCF (Radioactive Contamination Factor), where

These results have been obtained within statistical variations given by Maestro Program I and II, maximum ± 15 % to minimum ± 1% of counts accumulated in both detection systems during detection times from 3.96 X 104 to 8 x 104 seconds or 11 and 22.2 hours. So, when subtracting background and dividing activity due to 137Cs by that due to 40K , statistical

> Peak: 996.57 = 1475.99 keV FWHM: 73.93 FW(1/5)M: 119.41 Library: Ag-110M at 1475.76 ; 6.01 cA

Live: 37444.12 Dead: 5.44%

Gross Area: 21654 Net Area: 8980 ±565 Gross Count Rate: 0.58 cps

Real: 39600.00

semiconductor detector, HPGe, coupled to a PC charged with Maestro Program II.

rate in already existing radioactive pollution in marine sediments.

Fig. 8. Background spectrum in Scintillation Detection System

detector, NaI(Tl), 3X3", coupled to a PC charged with Maestro Program.

**4. Results** 

RFC = Bq 137Cs x 100/g / Bq 40K/g .

variations were always below ± 15%.

elementary K and multiplied by 100, concentration of K in sea salt is obtained as percentage, according the equations 7 and 8:

$$\mathbf{Bq/L = g/L \ge Bq/g \ge \text{salt}}\tag{7}$$

$$2\% \text{K} = \text{Bq/g salt} \times 100 \text{ / } 31.19 \text{ Bq/g K} \tag{8}$$

Where:

( ) [] [] <sup>40</sup> Bq /L Activity per litre of sea water due to K total decaying 89% , rays 11% β γ<sup>−</sup> =

g /L Salinity of seawater expressed in grams per litre of sea water =

40 Bq /g salt Specific activity of sea salt due to = ( ) [] [] K total decaying 89% , rays 11% β γ −

%K K concentration of sea salt expressed as percentage =

40 40 31.19 Bq K / g K Specific activity of elementary K due to K total decaying =

( ) [] [] 89% , rays 11% β γ −

So, when these figures are experimentally obtained, a great portion of sea water may be characterized from the 40K natural decaying of its salt, data which should be very useful to detect and evaluate any recent contamination, such as that occurred in Fukushima, Japan, at present, and in the past those of Three Miles Island in USA, and Chernobyl in Russia, even when the nuclear accident or failure might have occurred at a large distance from the sea site. In any case, radioactive contamination should be represented by some fission product, most probably 137Cs, due to its high fission yielding and easy detection of 662 Kev γ rays emission.

Nevertheless, and even when 137Cs has not been detected in Mexican marine salts till now, it has been detected in every marine sediment tested in samples picked up at 60-80 meters deep. This fact maybe becomes enough evidence that it does already exists a radioactive contamination at sea bottom, creating one background from now on, which should be very important to evaluate in order to compare how it is growing up or maybe decaying when time goes by, and with no doubt nuclear power will have a great development all over the world. The main origin of this radioactive background at sea bottom, should be the test nuclear explosions at Alamo Gordo and Bikini, as well as the war actions in Hiroshima and Nagasaki, followed by nuclear test explosions performed by several countries since then, and only in a minor proportion by accidents and failure events of nuclear plants, considering that from 1945 to present day only 2.2 time spans of 30.07 years (half life of 137Cs) have passed away. 137Cs has not been detected so far in sea salt samples taken up from Mexican littorals, neither Pacific Ocean nor Gulf of Mexico. On the contrary, every sediment picked up from 60-80 meters depth, seems to have accumulated a small amount of 137Cs, creating a certain pollutant radioactivity over the natural radioactive background present at sea bottom, which is represented mainly by 40K and 232Th, 235U and 238U radioactive chains. So, fission product 137Cs should have been first dissolved in sea water, among a great diversity of ions in there, and then settled down on sediments as time goes by, because it is a rather heavy ion. In this way, 137Cs present in sea salts should be indicating some recent pollution, while in marine sediments should be one of the main contributors to increase its natural background. Therefore, the proportion expressed as percentage of specific pollutant radioactivity Bq137Cs/g multiplied by 100 and divided by specific natural radioactivity (Bq40K/g), should be as useful in sea salts as in marine sediments, to have a reliable and easy to understand figure to evaluate the magnitude of recent pollution as well as to size up the possible growing or decreasing rate in already existing radioactive pollution in marine sediments.
