Radiological assessment of different monazite grades after mechanical separation from black sand

In Egyptian black sands, monazite is a precious mineral characterized by its composition, which includes crucial constituents such as thorium, trace amounts of uranium, and rare earth elements. It is essential to evaluate and quantify the extent of gamma-ray exposure resulting from the presence of primordial radionuclides. This necessity arises from human activities that extract and retrieve raw materials in uranium and thorium mining operations. The current study focuses on the radiological assessment of Monazite raw material in various grades and calculates the associated hazard indices. A hyper pure Germanium detector (HPGe) determined the particular activity. For grade, 90% Monazite samples, the average activities for 232Th, 238U, and 40K were 348,008 ± 1406, 69,299 ± 2086, and 27,510 ± 245 Bq/kg, respectively. For grade 75% Monazite samples, the average activities were 219,000 ± 901, 55,000 ± 500, and 18,300 ± 86 Bq/kg, while for grade 50% Monazite samples, it was 43,294 ± 1549, 9593 ± 629, and 4000 ± 211 Bq/kg for the same element, respectively. Also, 138La’s inherent radioactivity was taken into account. The computed effective and absorbed dosages exceed the worker’s exempt limit of 20 mSv/y. The calculated hazard parameters are higher than the maximum recommended limits. Therefore, it is imperative to employ radiation safety measures to mitigate the potential hazards of ionizing radiation.


Materials and methods
Sample collection.The raw sand samples were collected from a black sand beach on the Mediterranean coastline near Abu Khashabah, approximately 7 km east of the Rosetta estuary.The provided sample consists of a 2-km segment of unprocessed sand, exhibiting a range of widths extending from a few meters to 20 m.A quantity of sand ranging from 10 to 30 cm was manually removed from the surface.The geographical representation of the area under investigation is depicted in Fig. 1.The image of the map was acquired via the online tool Scribble Maps (https://m.scrib blema ps.com/) 26 .The sample was processed by examining physical disparities among the commercially available minerals included in black sand 27,28 .Figure 2 illustrates the Physical Techniques employed for the Separation of Monazite and other Valuable Minerals from Black Sands.Following the concentration and separation of monazite samples with varying grades, the samples were suitably prepared for gamma-ray measurements using an HPGe detector.
Measurement procedures.The Monazite samples examined in this study were meticulously prepared for laboratory measurements.It underwent a process of air-drying, grinding, and subsequent sieving to achieve a particle size of 200 mesh.Three samples were tested for each grade of Monazite, and the following measurements were conducted.The resulting average value was calculated.The samples were subjected to a sealing process with a duration of 4 weeks to achieve a state of secular equilibrium.This equilibrium is characterized by the daughter radionuclides undergoing disintegration at a rate equivalent to that of the parent radionuclides.Implementing this method is crucial to ensure the adequate confinement of decay products and radon gas within the container www.nature.com/scientificreports/and the sample.The containers utilized for the analyzed samples were identical in design to those employed for the reference material during efficiency calibration, ensuring consistent geometric properties.The gamma-ray activity was determined using a gamma-ray spectrometry system comprising a high-resolution n-type HPGe detector, specifically the GR3021 Canberra model with Serial number 2966076.This detector is designed with a reverse electrode closed-end coaxial geometry, featuring a diameter of 55.5 mm, a length of 52.5 mm, and a distance of 5 mm from the window.The detector has a relative efficiency of 30% and a resolution of 2.1 keV (FWHM) at the energy line of 1.33 MeV.The cryostat model used is 7500SL, the preamplifier model is 2002CSL, and the multi-channel analyzer (MCA) model is MP2-1U with a serial number of 05040700.The data analysis is performed using Genie 2000 software.The placement of the detector occurred within a cylindrical chamber that was insulated with lead, including a permanent bottom and a portable cover at the top.This arrangement was implemented to mitigate the impact of background radiation effectively.The samples were subjected to a counting process lasting 22 h, during which the background counts were also recorded for the same duration to determine the net count.The spectra that were acquired were analyzed utilizing the Genie 2000 software.
The energy calibration of the HPGe detector was conducted by employing certified sealed point sources obtained from Amersham, England.These sources included various emitters, such as 60 Co (with energies of 1173.2 and 1332.5 keV), 137 Cs (with an energy of 661.7 keV), and 226 Ra (with energies ranging from 186 to 2450 keV).These sources were selected based on their wide energy range, making them suitable for calibration.The certified reference materials required for calibrating the efficiency of measuring 232 Th, 238 U, and 40 K activity were obtained from the International Atomic Energy Agency.The materials under consideration include RGTh-1, RGU-1, and RGK-1.
The average activities of 212 Pb (238.6 keV), 208 Ti (583.1 and 2614.7 keV), and 228 Ac (911.1 and 968.9 keV) in the samples were utilized to determine the activity of 232 Th.The concentration of 238 U was estimated using the average activity of the decay products 214 Pb (295.2 and 351.9 keV) and 214 Bi (609.3, 1120, and 1764.5 keV).The concentration of the radionuclide 40 K was determined using the gamma line of 1460 keV.The equation utilized to calculate the radioactivity present in the Monazite samples was as follows 5,19 : In the given context, the variables are defined as follows: A denotes the activity of a particular nuclide measured in Bq/kg, C n represents the net area counts observed in the corresponding photo peak after background subtraction, ɛ γ signifies the absolute efficiency at the energy of the photo peak, P γ denotes the probability of gamma-ray emission that matches the intensity of the photo peak energy, t represents the duration of counting in seconds (79,200 s), and m represents the mass of the samples in kilograms.

Results and discussion
Activity calculation.The radioactivity concentrations of various grades of monazite ore were determined by measuring the gamma rays released by the primordial radionuclides in the ore.Three samples were chosen from each monazite grade and subjected to gamma spectroscopic analysis to evaluate their radioactivity.The resulting measurements were then averaged for each set of three samples Table 1 displays the recorded activity of the primordial radionuclides 232 Th, 238 U, and 40 K in the examined materials.The activity for each grade is determined by calculating the average of three samples.The levels of radioactivity exhibited by the isotopes 232 Th, 238 U, and 40 K are found to be significantly elevated in all samples of Monazite.These samples demonstrate varying degrees of concentration, ranging from approximately 50% for lower grades to 90% for higher grades, with an intermediate grade of 75% observed in certain Monazite specimens.The average activity of 232 Th, 238 U, and 40 K in grade 90% Monazite samples was 348,008 ± 1406, 69,299 ± 2086, and 27,510 ± 245 Bq/ kg, respectively.The average activity of Monazite samples with a grade of 75% was found to be 219,000 ± 901, 55,000 ± 500, and 18,300 ± 86 Bq/kg.In contrast, Monazite samples with a grade of 50% exhibited an average activity of 43,294 ± 1549, 9593 ± 629, and 4000 ± 211 Bq/kg.The recorded activity values in all samples exceed the established exemption levels of 35, 30, and 420 Bq/kg for 232 Th, 238 U, and 40 K, respectively 21 .The pattern that has been observed indicates a positive correlation between the grade of the Monazite samples and the concentration of radioactivity.Furthermore, it is seen that the levels of 232 Th activity are consistently higher than those of both 238 U and 40 K in all analyzed samples.The high concentration of thorium (Th) relative to uranium (U) in Monazite is attributed to its composition.Specifically, Monazite typically has 0.2-0.4% uranium oxide (U 3 O 8 ) (1) www.nature.com/scientificreports/and 5.0-10.0%thorium oxide (ThO 2 ) in its component ratios.The Monazite resource has a range of thorium ratios from 6 to 10%.While variations in the uranium ratio range from 0.20 to 1.0% exist [22][23][24] .
The data for 232 Th, 238 U, and 40 K concentrations in parts per million (ppm) and percentages (%) were derived using the conversion factor documented in the current literature.In the case of 232 Th, the conversion factor is 1 part per million (ppm) equals 4.06 Becquerels per kilogram (Bq/kg).Similarly, for 226 Ra, the conversion factor is one ppm equals 12.35 Bq/kg.Lastly, for 40 K, the conversion factor is 1 percent equals 313 Bq/kg 18,19,29 .For a grade of 90%, the concentration values (in parts per million) are 85,716 ppm for 232 Th, 5611 ppm for 238 U, and 88% for 40 K.The obtained grade is 75%.The measured values for the isotopes 232 Th, 238 U, and 40 K are 54,000, 4454 ppm, and 59%, respectively.The obtained grade is 50%, with the measured values of 10,664 and 777 parts per million (ppm) for the isotopes 232 Th and 238 U, respectively.Additionally, the isotope 40 K was found to have a concentration of 13%.
Based on the assessment of 138 La's inherent radioactivity, its decay behavior can be described as follows: The visibility of the two gamma lines of 138 La at 788.7 and 1435.8 keV in the spectrum of Monazite, namely the lines at 786.3 and 1434.1 keV of 234 Pa, may have been hindered by the interference caused by the gamma lines of both daughters of 232 Th (782.1, 1434.2, and 1438 keV of 228 Ac) and 238 U.However, the La sample obtained from the Monazite exhibited distinct visibility of the two gamma lines [27][28][29] .The activity of 138 La is around 400 Bq/kg.

Radiological effects.
Various radiation parameters were estimated based on the concentrations of radioactivity, To gain insight into the radiological risks associated with the Monazite samples under investigation.These included the radium equivalent activity (Ra eq ), The gamma-absorbed dose rate (DR), The annual effective dose rate (AEDR), the external hazard index (H ex ), the internal hazard index (H in ), representative gamma level index (I γr ), The alpha index (I α ), and the annual gonadal dose equivalent (AGDE).
The radium equivalent activity (Ra eq ) is a widely used parameter for assessing the radiological effects of naturally occurring radionuclides such as 226 Ra, 232 Th, and 40 K.It comprehensively measures the radiation hazards associated with these elements by combining their activities into a single quantity.The Ra eq parameter is related to assessing both external gamma exposure and internal dosage resulting from radon and its progeny.The Ra eq model is based on the premise that the levels of gamma radiation dosage resulting from 1 Bq/kg of 226 Ra, 0.7 Bq/ kg of 232 Th, and 13 Bq/kg of 40 K are considered equal.The subsequent content presents the index: The abbreviations C Ra , C Th , and C K indicate the average activity in Bq/kg of 226 Ra, 232 Th, and 40 K, respectively.The obtained results for the radium equivalent (Ra eq ) of monazite grades at 50%, 75%, and 90% are 71,798, 369,556, and 568,719 Bq/kg, respectively.The estimated values exceeded the allowable limit of 370 Bq/kg as suggested by the International Atomic Energy Agency (IAEA) 16,21,30 .Two supplementary parameters were constructed to represent the gamma-absorbed and annual effective dose rates, and their corresponding values were presented in Table 2.The gamma-absorbed dose in the open air or at a height of one meter above the ground was determined for the examined materials by utilizing the specific activity values of 40 K, 226 Ra, and 232 Th.The computation was executed using the following formula 21 ; The investigated samples' gamma absorbed dose rates for Monazite with grades 50%, 75%, and 90% are 31,564.6,162,537.2, and 249,730.8nGy/h.These values exceed the acceptable limit of 57 nGy/h, as established by the United Nations Scientific Committee on the Effects of Atomic Radiation 21 .Depending on the values of gamma absorbed dose rates, The annual effective dose rate was calculated in units of µSv/yr using the formula below 21   www.nature.com/scientificreports/ The determined annual effective radiation rate exceeds the established worker exemption level defined by the International Commission on Radiological Protection (ICRP) 31 .The significantly increased value can be attributed to the high activity of Monazite.
Two additional parameters signify the risks associated with external and internal radiation.The external hazard index (H ex ) value must remain below unity to provide a minimal level of radiation hazard.Its formula is 31 ; On the other hand, the internal hazard index (H in ) quantitatively measures internal exposure to carcinogenic radon and its short-lived progeny.The calculation is performed using the subsequent formula: The variables A Ra , A Th , and A K denote the activity of 226 Ra, 232 Th, and 40 K, respectively, measured in (Bq/ kg).The potential radiation threat should remain minimal if the two index values are less than unity.Based on the data shown in Table 3, it can be observed that the calculated H ex and H in values for the monazite samples exceed a value of one.A supplementary index known as the gamma index (I γ ) has been formulated to account for the collective effects of 226 Ra, 232 Th, and 40 K in terms of their radiological hazards concerning the exposure of employees and the general public to naturally occurring radioactive materials (NORM).The values for the I γ index were estimated by employing the formula described in references 32,33 .
If the gamma index values exceed unity, the effective dosage received by workers or the general population will be higher than one mSv/y 33 .Based on the data presented in Table 3, it can be observed that the expected values of the gamma index for all the samples analyzed remain higher than one, indicating statistical significance.Another index, the alpha index, focuses on the potential risks associated with inhaling short-lived decay products of 222 Rn.This issue is of significant concern with indoor radiation exposure 1 .The alpha index (I α ) is utilized to assess the internal risk arising from the alpha activity of a substance and is expressed as the following numerical value: where A Ra is the concentration of 226 Ra's activity Bq/kg.I α must be less than unity for safety.The average values of I α for the measured samples at grades 90%, 75%, and 50% are 835, 550, and 102 Bq/kg, respectively.To evaluate the influence of the activity on organs such as the gonads, active bone marrow, and bone surface cells, it is necessary to quantify the annual gonadal dose equivalent (AGDE) in millisieverts per year (mSv/y).It was determined using the methodology outlined by 34 .
The AGDE factor was calculated to be 1671.5 mSv/y, 1087.8 mSv/y, and 211.2 mSv/y for grade levels of 90%, 75%, and 50%, respectively.These values are significantly more than the worldwide average value for AGDE 0.3 mSv/y 21 .
Based on the data derived from this study concerning the activity levels of 232 Th, 238 U, and 40 K and their comparison with previous research on Egyptian Monazite and Monazite samples from other nations, as presented in Table 4, it is evident that the recorded activity values exceed the acceptable level for radiation safety exemption.Therefore, it is imperative to examine radiation safety measures to mitigate the potential adverse radiological consequences associated with rare earth elements derived from Egyptian Monazite.The radiation protection must be applied so that the magnitude of individual doses must be as low as reasonably achievable, with economic and social factors being considered.Individual dose assessment for workers required the estimation of levels of exposure by applying personal monitoring and workplace monitoring 35 . (5)

Conclusion
The present study aims to examine the radiological assessments of primordial radionuclides in various grades of Monazite.The activity levels of 232 Th, 238 U, and 40 K in all the analyzed monazite samples were higher than those considered exempt.All computed radiological hazard parameters exceed the allowable limits.Hence, it is imperative to mandate radiation safety protocols throughout the processing of Monazite for the extraction of industrially and nuclearly relevant metals, such as rare earth elements (REE s ), thorium (Th), and uranium (U).To ensure the safety of workers involved in processing raw materials, it is imperative to implement a radiation safety program that adheres to the following guidelines: the justification of practices, the limitation of radiation doses to individuals, and the optimization of radiation protection.The acquired data can be employed as a repository to oversee the natural radioactivity of monazite processing and its corresponding index of radiological dangers.

Figure 1 .
Figure 1.The geographical representation of the area under investigation (Abu Khashabah) region along the Mediterranean coastline.

Figure 2 .
Figure 2. The above flow sheet outlines the procedure for concentrating and separating monazite samples of varying grades.

Table 2 .
The estimated absorbed dose rates nGy/h and the annual effective doses μSv/yr for Monazite samples.

Table 3 .
Different hazard indexes Ra eq , H ex , H in , I γ , I α , and AGED in Monazite samples.

Table 4 .
Shows the activity of 232 Th, 238 U, and 40 K of Egyptian Monazite compared with some countries.