Assessment of natural radionuclides in soil sample

As useful soil is to human, it also has its own short comings becuase it can be contaminated. Due to its nature, it is capable to house some naturally producing toxic and radioactive substances known as natural radionuclides. Definitely, human activities also contribute to the production of these substances, such as extensive use of fertilizers rich in phosphates for agricultural purposes serves as another sources of radioactivity in soils other than those of natural origin (UNSCEAR, 2000).

In this post, I will try as much as possible to focus on natural radionuclide.

Mainly the natural radioactivity in soil is derived from the 238U and 232Th parent series and natural 40K. Natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions, thereby, causing dosage variation. Study of soil radioactivity can provide reference data for future observation of anthropomorphic impact and associated radiological risk to human health.

Before we proceed to the assessment of these naturally occurring radionuclides, and their hazardous effects on lives, I think we needed to have the knowledge of some facts about them, Therefore, each radionuclides are described below one after the other. Hope you'll find this post educative!

Here we go!

What is RADIUM-226
Radium: This guy is a silvery-white metal and it is formed by the radioactive decay of uranium and thorium. The most common isotope of radium and the longest lived radium is 226Ra with a half-life of 1,600 years.

It is found in nature and it can strongly attach to materials such as rocks, soil, water and It can also be found in air. It releases energy in the form of rays, waves or particles. Radium in the soil may be taken in by plants, thereby contaminating plant ecosystem. it can also build up in fish and in other living organisms including humans.

These are but a little hand works of 226Ra!

During the radioactive decay process of Radium, alpha, beta, and gamma radiation are
released. Alpha particles are capable of traveling only a short distance, therefore, it can not travel through the skin. Beta particles are capable to go through the skin, but they cannot pass through the whole body, while Gamma radiation is capable to pass through the body.

Hope I am getting things right, if I'm not, please correct me.

Humans could be exposed to radium through breathing of radium contaminated air, drinking water containing radium (higher levels of Radium are found in well-water), eating of radium contaminated plant products, also, Miners who work with uranium and hard rock are exposed to higher levels of radium. Radium has been considered as cancer-causing substance. High level exposure to radium can lead to higher chances of bone, liver and breast cancer.

If actually, radionuclides are doing more harm than good, the question is that, can't we do without them? I will like my fellow physicist to do justice to this question so that people can learn more. Thanks

What is Thorium-232?

All known thorium isotopes are unstable. The most stable isotope and the only naturally occurring isotope of thorium are 232Th. It is radioactive and has a half-life of 14.05 billion years. It decays very slowly through alpha decay, starting a decay chain named the thorium series that ends at stable 208Pb. The radioactive decay series of 232Th is a complex reaction and produces alpha, beta, and gamma radiation. As a corollary, the alpha or beta particles emitted as a result of the decay of a parent isotope, gamma rays are emitted by most of the daughter isotopes. Intensities most gamma rays are very low.

They have energies of the range near zero to greater than 2500 keV but it is not usable for gamma-ray spectrometry with NaI(Tl) detectors.

What is potassium-40?
After 232Th and 238U, the radioactive decay of potassium (40K) is ranked third in the Earth's mantle, as the source of radiogenic heat. An unspecified amount of radiogenic sources are also likely contained in the core. It has been proposed that significant core radioactivity may be caused by high levels of U, Th, and K.

40K is considered the largest source of natural radioactivity in animals including humans with a half-life of 1.277 × E9 years. One out of every 8500 potassium atoms is radioactive.

This is a large amount!

For example, averagely, a 70 kg human body contains about 140 grams of potassium in his body, and hence about 0.0164 grams of 40K; whose decay produces about 4,300 disintegrations per second (becquerels) continuously throughout the life of the body. This amount of activity is equal to 0.11 µCi.

Perhaps we should wear a radioactive sign around our necks!

As much as 0.4% potassium is present in fruits and vegetables, while an average soil sample contains 2% potassium. Therefore, 1 Kg of soil has an activity of 0.016 µCi due to the content of potassium alone. When taking a measurement of gamma spectrum of soil and food samples for long times, one or two hours, an accuracy of as good as 10% is attributed to the potassium content. Therefore, 40K is considered as the largest contributor to our natural background radiation.

The detector: gamma ray spectrometer

A high-purity germanium (HPGe) semiconductor detector can be used to detect and measure radionuclides by analysis of the gamma-ray energy spectrum produced in a gamma-ray spectrometer. HPGe detector has the ability to measure gamma radiation directly from the original sample without the need for chemical separation and high resolving power, this made it to be outstanding. A germanium detector gives excellent energy resolution when applied to gamma-ray spectra to separate two adjacent peaks in a γ-ray spectrum.

Germanium system is more superior in energy resolution, permitting many closely spaced gamma-ray energies that are unresolved in by NaI(Ti) spectra to be separated.
Sample measurements were obtained using a γ-ray spectrometer with a p-type coaxial intrinsic HPGe detector. It has a relative efficiency of 30% and 1.83 keV resolution (FWHM) for the 60Co 1332 keV γ-ray energy in nuclear energy Measurement.

A PC-based multi-channel analyser (PCMCA Card: TRUMP-PCI-8K) was coupled to the detector. A window-based software (MAESTRO-32, ver. 5.30 [A65-B32], ORTEC, U.S.) that matched the gamma energies at various energy levels to a library of possible isotopes was used for gamma-ray spectral analysis. To reduce system background, the detector was shielded by lead on all sides. Comparism check of the count rates in each peak of spectra from a source of known radioactivity to the count rates expected from the known intensities of each gamma ray is a measure of its efficiency.

My Investigation: A case study of LAUTECH farm land

A total of 10 samples were collected and assayed from Ladoke Akintola University of Technology (LAUTECH) Ogbomoso, Oyo State Nigeria farmland. At each location, about 500g soil samples were collected randomly at a depth of 30cm which was placed in polythene bags and transferred to laboratory.

Each soil samples were thoroughly air dried and crushed, ground into powder, homogenized and sieved through 0.5mm mesh sieve to remove extraneous items and to obtain a fine-grained sample that would present a uniform matrix to the detector.

The powdered soil samples were stored in tight plastic containers for about four (4) weeks before measurement to attain a state of secular radioactive equilibrium between the 238U and 232Th series and their respective progenies to prepare for measurements.

Detection and measurement
The gamma-ray energies of daughters products were used to determine the radioactivity concentrations of the radionuclides. 226Ra concentrations were determined from the gamma-ray energies of its daughters 214Pb (352.92 and 295.21 keV) and 214Bi (609.31, 1120.30 and 1764.50 keV), while 232Th concentrations were determined from the gamma-ray energies of its daughters 212Pb (238.63 keV), 208Tl (583.14 and 510.84 keV). Gamma-ray energies of 1460.80 keV and 662 keV were used to determine the radioactivity concentrations of 40K and 137Cs respectively.

The activity concentrations were calculated using the formula below:

The table below presents the concentrations (Bq kg–1) of natural radionuclides as investigated in soil samples at a 30cm depth of LAUTECH farm land.

Table showing concentrations of natural radionuclides in Bq kg–1

The results obtained in this study were compared with the world average concentration level of 35Bqkg-1 for 226Ra, 30Bqkg-1 for 232Th, and 400Bqkg-1 for 40K set by UNSCEAR, 2000, it was found out that only one sample (sample H) for 40k slightly exceeded the average level. The slight excess in concentration of 40K might due to the extensive use of fertilizers rich in phosphates for agricultural purposes(UNSCEAR, 2000).

Thanks for your time

References
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