**4. Results**

240 Radioisotopes – Applications in Physical Sciences

elementary K and multiplied by 100, concentration of K in sea salt is obtained as

40

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

= ( ) [] []

Bq /L Activity per litre of sea water due to K total decaying 89% , rays 11%

<sup>−</sup> =

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

40 40

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

Bq/L = g/L x Bq/g salt (7)

( ) [] [] <sup>40</sup>

K total decaying 89% , rays 11%

−

β

 γ

> γ

β

%K = Bq/g salt x 100 / 31.19 Bq/g K (8)

percentage, according the equations 7 and 8:

Bq /g salt Specific activity of sea salt due to

=

%K K concentration of sea salt expressed as percentage

Where:

β

−

=

=

( ) [] []

 γ

89% , rays 11%

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

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

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

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 variations were always below ± 15%.

Fig. 8. Background spectrum in Scintillation Detection System

Radioactivity in Marine Salts and Sediments 243

Fig. 11. Gulf of Mexico North East sediment spectrum sample, HPGe Detection System

g salt/L sea water

0.276 10.1 36.7 0.88 0.89

0.073 2.5 34.8 0.23 0.58

0.173 7.3 42.5 0.55 0.93

Conclusion of research results is based in several points, however reduced in samples

Table 7. Results of natural radioactivity (40K) in sea salt samples and %RCF in marine

number and extent too, when referring to very large littorals at Mexico.

Bq 40K/g salt

Gulf of Mexico South East

Pacific Ocean North

Gulf of Mexico North East

sediment samples

**5. Conclusion** 

Bq 40K/L sea water

Peak: 5235.85 = 662.05 keV FWHM: 0.13 FW(1/5)M: 0.19 Library:Cs-137 (Cesium) at

661.66; 0.00Bq Gross Area: 5753 Net Area: 2013±180 Gross/Net Count Rate: 0.07/0.03 cps

Sea Salt Samples Marine Sediment Samples

Peak: 11559.77 = 1460.31 keV FWHM: 6.37 FW(1/5)M: 10.64 Library:K-40 (Potassium) at

1460.75; 0.00Bq Gross Area: 11167 Net Area: 9609±176 Gross/Net Count Rate: 0.14/0.12 cps

%K in sea salt

137 40 Bq Cs/g %RCF x100 Bq K/g <sup>=</sup>

Fig. 9. Gulf of Mexico North East, sea salt spectrum sample, Scintillation Detection System

Fig. 10. Background spectrum in HPGe Detection System

Peak: 974.97 = 1456.26 keV FWHM: 79.19 FW(1/5)M: 119.20 Library: J-134 at 1455.50: 31.23 cA

Live: 76 209.64 Dead 4.74%

Peak: 11365.69 = 1462.42 keV FWHM: 4.17 FW(1/5)M: 6.29 Library:K-40 (Potassium) at 1460.75;

Gross/Net Count Rate: 0.10/0.08 cps

0.00Bq Gross Area: 7841 Net Area: 6521±158

Gross Area: 109771 Net Area: 70449 ± 943 Gross Count Rate: 1.44 cps

Real: 80 000.00

Fig. 9. Gulf of Mexico North East, sea salt spectrum sample, Scintillation Detection System

Fig. 10. Background spectrum in HPGe Detection System

Peak: 5145.46 = 662.65 keV FWHM: 2.55 FW(1/5)M: 3.80 Library:Cs-137 (Cesium) at 661.66;

Gross/Net Count Rate: 0.07/0.02 cps

0.00Bq Gross Area: 5553 Net Area: 1288±191

Fig. 11. Gulf of Mexico North East sediment spectrum sample, HPGe Detection System


Table 7. Results of natural radioactivity (40K) in sea salt samples and %RCF in marine sediment samples
