Radiological hazard assessments of radionuclides in building materials, soils and sands from the Gaza Strip and the north of Sinai Peninsula

Radiological hazards to the residents of the Gaza Strip, Palestine and the north of the Sinai Peninsula, Egypt, were determined using the naturally occurring radionuclides (226Ra, 232Th and 40K) in 69 samples of building materials (demolition debris, plasters, concretes, from recycling plants and raw cements from suppliers), soils and sands collected in the field. The radiological hazard indices and dose rates calculated with the activity concentrations of radionuclides in those materials determined by gamma-ray spectrometry indicate that the values are all within the global limits recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation 2000 and European Commission 1999. The results of Spearman's correlation and hierarchical cluster analysis for 210Pb in the building materials, soils and sands suggest that the samples include 210Pb from the atmospheric fallout. The medium correlation between 232Th and 40K in demolition debris implies that their activity concentrations are characteristic of the building materials and constituents of the demolition debris. Non-natural ratio of 238U/235U was found in the soil and sand samples collected in the Gaza Strip. Furthermore, 137Cs and 241Am were detected in some soil, sand and demolition debris samples analyzed in this study. The origins of those anthropogenic radionuclides were considered.

All living organisms are continually exposed to ionizing radiation, which has always existed naturally 1 . The sources of that exposure are cosmic rays that come from outer space and from the surface of the Sun, terrestrial radionuclides that occur in the Earth's crust, in building materials and in air, water and foods and in the human body itself 1 . The radiation exposure in dwellings is mainly due to building material gamma-ray emission from primordial radionuclides such as 40 K and 238 U, 232 Th, 235 U and their decay products. The typical range of annual radiation dose from natural sources in soil and building materials is reported as from 0.3 to 1 mSv, with the worldwide mean value of 0.48 mSv. The value depends on the type of building materials and the geographical location 2 . Accordingly, it is important to assess and understand the radiation dose due to dwellings for human health and the studies on the radiation exposure from naturally occurring radionuclides have been performed for various regions.
For the Egyptian region, radiological hazard assessments from various building materials were implemented in a previous study. The results showed that radiological hazards of the materials were below the safety limit, expect the granite which showed extremely high activity concentrations 3 . In another study, the radiation levels around Sharm El-Sheikh in South Sinai were estimated. The results indicate that the absorbed dose rates and gamma radiation hazard indices in all locations were higher than the world mean value and unity, respectively 4 .
In the Arabic regions, the activity concentration of 238 U, 232 Th and 40 K in soil samples collected in populated areas of Jordan were reported to be of normal level except for the Zarqa area where significant high 238 U concentration was found in soil samples originating from phosphate rocks and the values exceed recommended safety levels 5 . In another study, a model of a typical room in Jordanian houses was simulated to calculate the annual effective dose rate for various parts of the room using the Monte Carlo Simulation Program. The results show that the total annual effective dose for Jordanian house was 0.50 mSv year −1 with 238 U contributing about 44% of the total annual effective dose equivalent (AEDE) 6 .
The study on radiological hazard assessment of naturally occurring radionuclides was carried out analyzing 226 Ra, 232 Th and 40 K in the soil and building material samples collected in two cities in Saudi Arabia. The radiological hazard indices were less than the recommended safety limits except for five ceramic samples 7,8 . In the soil samples collected in northern Al Jubail 137 Cs, ranging from 0.107 to 0.916 Bq kg −1 , was also detected 7 . Likewise, a radiological map of the whole Kuwait country was established showing the activity concentrations of naturally occurring radionuclides lower than the mean value of worldwide 9 .
The activity concentrations of naturally occurring radionuclides were determined for building materials collected in Taiz and Hodeida, Yemen. They found high activity concentrations of 226 Ra and 232 Th in granite and cement bricks. The highest activity concentrations of 226 Ra and 232 Th in granites were 154.22 ± 6.97 Bq kg −1 and 229.14 ± 3.40 Bq kg −1 and in cement bricks were 180.95 ± 6.92 Bq kg −1 and 252.85 ± 3.94 Bq kg −1 , respectively 10 .
In another study, the radiological hazard indices were assessed for 20 soil samples collected at different depths in Hadhramout, Yemen. The mean value of the absorbed dose rate was estimated as 463.12 nGy h −1 , ranging from 17.22 to 600.16 nGy h −1 . This value is about eight times higher than the mean global value 11 .
Although there are intensive studies on the assessment of radiological hazards due to residents in various regions, the study relating to the Gaza Strip as well as the north of the Sinai Peninsula is still rare. One of the scarce studies on the Gaza Strip was carried out using solid state nuclear track detector CR-39 to monitor the radon activity concentration and the spatial distribution of radon in beach sand samples of the Gaza Strip. They found a positive correlation between the decrease of grain size of the sediments and radon concentration. The analytical results of radium concentration of the beach sand varied from 0.56 to 8.46 Bq kg −1 with the mean value of 1.66 Bq kg −1 , elevating the activity concentration toward the northern part of the Gaza Strip 12 . For the north of the Sinai Peninsula, the studies on characteristics, spatial distribution and vertical profile of gamma-ray emitting radionuclides in the local harbor, Al-Arish valley and Zaranik for 42 locations were reported. The results indicate that the activities of 232 Th and 226 Ra series in samples from the harbor and along the beach were higher than those from non-coastal sites, although no risk exists for public health based on the calculated effective dose equivalent and the recommended limit of 5 mSv year −1 , except for two locations in the Zaranik 13 .
The past and ongoing siege of the Gaza Strip has led to a severe shortage of building materials since 2007. Many houses, buildings and infrastructure were destroyed and produced huge amounts of demolition debris after the war in the Gaza Strip in 2008. As a result, reuse of the degraded building materials has been required in the past and present. According to the United Nations Development Program (UNDP), the amount of demolition debris in the Gaza Strip, produced from 2008 through 2009, was estimated to be 600,000 tons 14 . The demolition debris was crushed, sieved and reused as recycled building materials. The radioactive substances in those recycled building materials originate mainly from naturally occurring radionuclides in their components such as sandstone, plaster, granite, and others, and anthropogenic radionuclides. Although there are many crushers to recycle demolition debris in the Gaza Strip 15,16 , no information about the radiological hazard from the recycled building materials were reported until now. It is important to understand the radiogenic characters of building materials from the point of view of radiation protection for the residents.
In this study we assessed the radiological hazards from naturally occurring radionuclides ( 226 Ra, 232 Th, and 40 K) in the building materials, soils and sands collected from the Gaza Strip, Palestine and the north of the Sinai Peninsula, Egypt to discuss the health safety issues due to dwelling in those areas. The study on the recycled building materials is especially important for the area where such materials are required to build houses. The activity concentrations were analyzed by gamma spectrometry and the analytical results were evaluated using various radiological assessment methods. Furthermore, Spearman's correlation and hierarchical cluster analysis were implemented to clarify the correlation among the concentration of radionuclides. Additionally, 210 Pb, 235 U, and anthropogenic radionuclides 137 Cs and 241 Am were also determined and the results are discussed. The results obtained in this study could form part of database for radiological hazard assessments.

Material and methods
Studying area. The present study was carried out with the materials collected in the Gaza Strip, Palestine and the north of the Sinai Peninsula, Egypt, as shown in Fig. 1. The area of the Gaza Strip is 365 km 2 with around 2.05 million residents 17 . It is located in the southwest of Palestine and on the southeast coast of the Mediterranean Sea. The weather of the Gaza Strip is characterized by a semi-arid climate 18 . The sampling in the Gaza Strip was performed at Beit Lahia, Jabalia, El Sheikh Radwan, El-Tuffah, Al-Shejaiya, Nuseirat, Khan Yunis and Rafah. In the north of the Sinai Peninsula, the samples were collected at Negela, Qatia and Rommana. Rommana located on the coast of the Mediterranean Sea, and Qatia and Negela villages are in a desert plateaus area. The sampling locations are shown in Fig. 1 with blue symbols. 69 samples of building materials, demolition debris, sands and soils were collected: 14 demolition debris, 6 concretes and 5 recycled plasters (KhanYunis, Al Shejaiya and Jabalia), 1 fresh concrete and 2 raw cements (suppliers in the Gaza Strip), 3 raw cements (suppliers in Egypt), 24 soils and sands (18 samples from previous bombed regions and 6 samples from random places in the Gaza Strip) and 14 sand samples (the north of the Sinai Peninsula).
An example of pile of demolition debris and sieving in the Gaza Strip is shown in Fig. 2.
Sample preparation. The demolition debris and concrete samples were initially broken coarsely using a hammer. Then, the coarse parts were pulverized using a mechanical grinder. The soil samples were packed into polyethylene bags after removing any stones and transferred to the laboratory for analysis. All samples were dried at 85-95 °C until reaching constant weight for each. The dried samples were sieved with a mesh size of Self-attenuation and summing coincident correction. The self-attenuation correction factor was experimentally determined through the transmission of gamma-rays emitted from the external point sources   www.nature.com/scientificreports/ ( 241 Am, 152 Eu) through each of the 4 mol L −1 HCl mixed standard solution and the sample. The point sources, which cover most of the range of gamma-ray energies, were positioned about 11 cm distant either from the standard or the sample. The measured output gamma-ray intensity (counts s −1 ) was corrected to the measurement date. The self-attenuation correction factor was determined for various energies using the following equation 22 : where T sample is the attenuated output gamma-ray intensity (counts s −1 ) from the sample corrected to the measurement date. T 0 is the unattenuated output gamma-ray intensity from the reference 4 mol L −1 HCl standard solution (counts s −1 ). The final results were calculated using the efficiency for each energy line, the self-attenuation resulting from the differences of the chemical composition and the density between the 4 mol L −1 HCL standard and the sample. The radioactivity concentration for a radionuclide in the samples A si (Bq kg −1 ) was calculated using Eq. (2): where C net is the net counts corrected with self-attenuation, interference and background, γ is the emission probability for each certain energy photo peak, ε(Eγ) is the absolute photo-peak efficiency of the germanium detector at this energy, m is the mass of the measured sample in kg, t is the measurement time (s).
For the most of radionuclides analyzed in this study, the coincidence summing correction was not required. Only for 609.31 keV ( 214 Bi) and 583.19 keV ( 208 Tl), the sum coincidence correction was required and performed using Genie 2000 software.
Determination of activity concentration of 226 Ra, 232 Th, 40  where C net ( 234 Th) is the net counts of 234 Th at 63.28 keV peak, C total is the total counts at 63 keV Peak corrected with the background, A( 232 Th) is the weighted mean activity concentration of 232 Th calculated from its progeny, γ is the emission probability for 63.18 kev energy line of 232 Th, ε(Eγ) is absolute photo-peak efficiency of the used germanium detector at this energy and f k is the self-attenuation correction factor. If the two gamma lines were measured, weighted mean of their activity concentrations was used to determine the activity concentration of 238 U. Otherwise, the activity concentration of 238 U was determined using the 63.28 keV line after correction of self-attenuation and interferences.
The major gamma line used for determination of 235 U in all samples was 185.72 keV, (ep: 57.2%). The line is, however, interfered by 226 Ra line of 186.21 keV (ep: 3.59%). The gamma line 143.76 keV of 235 U with low emission probability 10.96%, which is about 20% of the emission probability of 185.72 keV gamma line 20 , was not detected in all measured samples. The correction with a spectral interference by 223 Ra (t 1/2 = 11.435d, 144.23 keV, ep: 3.22%) for 235 U at 143.76 keV peak, if detected, is also required. 223 Ra was eliminated by subtracting its contributing counts calculated using its 323.87 keV line or the counts of 219 Rn at 401.81 keV. 219 Rn is a decay product of 223 Ra. Since the half-life time of 219 Rn is very low (t 1/2 = 3.96 s), it grows rapidly into secular equilibrium with its parent nuclide 23 and can be used for 223 Ra activity amount. The interference in the net counts of 235 U were corrected using Eqs. (4) and/or (5).
(1) www.nature.com/scientificreports/ where C net ( 235 U) is the net counts of 235 U radionuclide in a peak, C total is the total counts at this peak, A( 226 Ra) weighted mean is the weighted mean activity concentration of 226 Ra calculated from its daughters gamma lines, A( 223 Ra) is the activity concentration of 223 Ra calculated using its gamma line 323.87 keV and/or 401.81 keV of 219 Rn decay product, γ is the emission probability of the corresponding gamma line, ε(Eγ ) is the absolute photo-peak efficiency of the used germanium detector at this energy and m is the mass of measured sample in kg, t is the measurement time in seconds and f k is the self-attenuation correction factor for this energy. If these two gamma lines of 235 U were detected, weighted mean of their activities was calculated as for the activity concentration of 235 U. Otherwise, the 235 U activity concentration was calculated using the 185.72 keV line after correction of background and self-attenuation and interferences.
Background measurements for all energy area were performed regularly, and the results were used in the calculation of final analytical result for each nuclide.
Calculation of uncertainty. Uncertainty in the activity concentration of radionuclide i was calculated using error propagation and Eq. (6): is the uncertainty of the net counts corrected to background counts and the interfering radionuclides, A i is the activity of radionuclide in Bq, U t , U b and U in are the uncertainties of the total counts at a certain peak, background counts and interfering radionuclide counts, respectively. U(γ ) is the uncertainty of the emission probability, U( f k ) is the uncertainty of the self-attenuation correction factor, m is the mass of the measured sample.
External (U ext ) and internal (U int ) uncertainties were calculated using Eqs. (7) and (8), respectively, and the larger uncertainty of them is reported in the results.
where A w is the weighted mean activity concentration (Bq kg −1 ),U cal is the relative uncertainty of efficiency calibration, N is the non-zero weights, A si is the activity concentration of radionuclide i (Bq kg -1 ) contributing to the weighted mean, and wi is the contribution weight of radionuclide i to the weighted mean, equals to 1 Calculation of radiological hazard indices. The radiological hazard indices were calculated using the activity concentrations of 226 Ra, 232 Th and 40 K obtained in this study expressed in the symbols A Ra , A Th and A K , respectively, in the following equations.
The gamma index (I γ ) is introduced by the European Commission 1999 24 and is calculated from Eq. (9) 24,25 : The internal hazard index (H in ) is calculated using Eq. (10) 26 : The external hazard index H ex based on a criterion of Krieger model (1981) was applied and calculated using Eq. (11) 26 : The radium equivalent activity (Bq kg −1 ) is calculated using Eq. (12) 27 : The permissible maximum value of the radium equivalent activity is 370 Bq kg −1 , which corresponds to an effective dose of 1 mSv to the public.
The indoor absorbed dose rate D indoor (nGy h −1 ) can be calculated using Eq. (13) 24,25 applying conversion factor of 0.92, 1.1 and 0.08 nGy h −1 per Bq kg −1 for 226 Ra ( 238 U), 232 Th and 40 K, respectively.  www.nature.com/scientificreports/ The outdoor absorbed dose rate D outdoor (nGy h −1 ) at 1 meterabove the ground surface is determined using the following equation 28 : where the conversion factors used for the calculation of the absorbed dose rate in air (nGy h −1 ) are 0.463, 0.604, 0.0417 for 238 U, 232 Th and 40 K, respectively.
The annual effective dose equivalent (AEDE) is calculated using the conversion coefficient from absorbed dose in air to effective dose, 0.7 Sv Gy −1 , which is found in United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 1993 29,30 . For indoor and outdoor measurements, the occupancy factor is approximately 0.8 and 0.2, respectively 29 , as shown in Eqs. (15) and (16).
Specific absorbed dose rate. The specific absorbed dose rate (D specific ) from the materials of pile can be calculated as a source of gamma radiation 25 . The gamma exposure from the materials of pile depends on the facing area (m 2 ) and the distance from the pile. In the crushers' case, facing area is assumed to be infinity and the separation distance is 1 m, so the specific absorbed dose rate in air due to the pile of materials is calculated using Eq. (17).
Assuming that the workers in crushers work for seven hours per day and six days a week, the annual effective dose (μSv y -1 ) is calculated using Eq. (18).

Multivariate statistical analysis. The multivariate statistical analysis is used in environmental studies
to interpret relations between different variables 31 . Spearman's correlation and cluster analysis were carried out using the IBM SPSS software (version 18.0) to deduce the correlation among the various radionuclides.

Results and discussion
Analytical results of certified reference material. To check a reliability of the analytical method used in this study, the standard reference soil, NIST 4353A (Rocky flat soil), provided by the National Institute of standards and technology (NIST) of the US Environmental Measurements Laboratory of the US Departments was analyzed. The reference sample was prepared using the same cylindrical containers for the samples. The analytical results of NIST 4353A were compared to the certified values. The range of the results for all radionuclides analyzed with the 5 detectors used for the sample analysis are corresponding to the 95/95 tolerance limit within the uncertainties as shown in Table 1, indicating a reliability of the analytical method used in this study.
Activity concentrations of 226 Ra, 232 Th, 40 Table 2. The activity    www.nature.com/scientificreports/ concentration of 226 Ra in the building materials is generally higher than that of soils and sands. It was observed that the concentration of 238 U, 235 U and 232 Th in the recycled concretes collected from crushers were two times higher than that of the fresh concrete collected from the local supplier in the Gaza Strip, indicating some concentration of those nuclides during recycling processes. Besides, there is no significant variation of the activity concentration of radionuclides among the demolition debris, recycled plaster and cement within the uncertainties. The radioactivity concentrations of soils collected in the Gaza Strip are nearly two times higher than those in the sands from the same area. The activity concentrations of 226 Ra, 232 Th and 40 K in the cement, soil and sand samples from the Gaza Strip and various areas reported previously are shown in Table 3 for comparison. The concentration of 226 Ra in the cement from the Gaza Strip is within the worldwide range, although that in the soils and sands from the Gaza Strip and the north of the Sinai Peninsula is lower than sample from the other areas by the factor of 3-7, except for Australia, Cyprus and Qatar 27,32,33 . The mean concentration of 232 Th for the various samples in this study is significantly lower than that of the other areas expect for Cyprus and Qatar. A similar tendency is observed www.nature.com/scientificreports/ also for 40 K and its concentration in the cements, soil and sand is clearly lower than that of the world mean and other regions 29 . Activity ratio of 238 U/ 235 U in soil samples from the Gaza Strip. Natural uranium consists of the long-lived radioactive isotopes, mainly, 234 U, 235 U and 238 U, with molar abundances of 0.0057, 0.72, and 99.27%, respectively. In natural uranium, the activity ratio of 238 U to 235 U is reported as 21.5 46,47 . In the present study, the activity ratio of 238 U/ 235 U was determined by gamma-ray spectrometry for 24 soil and sand samples collected in the Gaza Strip to understand one of their characters. The activity ratio of 238 U/ 235 U was found as the mean value of 13.2 ± 2.8 and median of 12.7, implying that the soils contain some enriched uranium. In previous study, enriched uranium was observed in crater soil sample collected during the war in the Gaza Strip in 2008. They reported the mean mass ratio of 238 U/ 235 U for three samples based on 12 measurements as 116 ± 16 (natural U: 137.88) in the soils 48 . It can be assumed that the enriched uranium found in this study also contain the spread materials into the atmosphere in the Gaza Strip in relevant events.  Table 4. 137 Cs are generated by nuclear activities and migrated in the environment. 137 Cs has the half-life time of 30.17 year and strongly adsorbed to clay minerals, organic materials in the soil, and animal and plant tissues 45,49 . The range of activity concentration of 137 Cs is 0.06-6.2 Bq kg −1 with the highest activity concentration of 6.22 ± 0.22 Bq kg −1 observed in a sand sample from Jabal Al Sourani located in the east of El Tuffah, the Gaza Strip. The highest activity concentration in the sand samples from the north of Sinai Peninsula was obtained as 0.27 ± 0.03 Bq kg −1 and the value is one order of magnitude lower than that from Jabl Al Sourani. 137 Cs was detected in 11 demolition debris samples with the highest activity concentration as 2.72 ± 0.10 Bq kg −1 . The range of concentration for all materials obtained in this study is 0.06-6.2 Bq kg −1 . In Previous studies, the mean activity concentration of 137 Cs in top soil samples (0-5 cm depth) collected in Palestine and Lebanon was reported as 7.8 and 23 Bq kg −1 , respectively 50,51 . 241 Am has the half-life time of 432.2 y and emits both alpha particles and gamma rays. 241 Am is a decay product of 241 Pu (half-life: 14.325 ± 0.006 year) 52 and its concentration in the environment continues to increase. It www.nature.com/scientificreports/ was estimated that 241 Am would reach its maximum activity in the middle of the twenty-first century, supposing no further significant releases of radionuclides would be happened 53 . A quantitative determination of 241 Am can be carried out by non-destructive analysis using gamma spectrometry, although chemical separation is required for low activity concentrations of 241 Am 54 . 241 Am was detected in 3 demolition debris, 6 soil and sand samples from the Gaza Strip. The highest concentration of 0.715 ± 0.054 Bq kg −1 for demolition debris sample collected from crushers at KhanYunis. In previous study, the range of 241 Am concentrations in collected soil samples from Saudi Arabia was reported as 0.021-0.49 Bq kg −155 and the values are corresponding to the results obtained in this study. It was reported that 241 Am was detected in various locations in Algeria as a resultant of radioactive residue from the former French nuclear testing sites in Algeria located about 3200 km from the Gaza Strip 56 . In our previous study, a high Pu concentration in freshly fallen snow was found on Mt. Zugspitze in Germany on the 24th of February 2016, together with a high concentration of PM10 (38 μg m −3 ) compared to the other samples (< 1 μg kg −1 ) from other sampling dates. The modeled backward trajectories on the 23rd of February 2016 indicated that the airborne contribution was transported from the north of the African Continent. Together with the data on activity ratio 239+240 Pu/ 137 Cs, we suggested that the elevated Pu concentration was influenced by the transported Saharan dusts presumably from the source of nuclear tests in Algeria to the European alpine area over a distance of about 2000 km 57 . 241 Am found in the samples were possibly transported from the former French nuclear testing sites in Algeria or due to other reasons. However, a definitive explanation of the 241 Am origins was not facilitated in this study.

Activity concentration of
Gamma index. The gamma index I γ is taken into account the dose criteria, distinction of bulk or superficial materials and amounts of the used building materials. For material used in bulk amounts, I γ ≤ 1 corresponds to an absorbed gamma dose rate of 1 mSv/y, while I γ ≤ 0.5 corresponds to an absorbed gamma dose rate of 0.3 mSv year −124 . The calculated mean of gamma index for various kinds of sample are shown in Table 5. The results indicate that all values are smaller than the maximum limit value of 0.5 corresponding to dose criteria of 0.3 mSv year −1 .

Internal hazard index.
Internal hazard index H in quantifies the internal exposure of 222 Rn and its decay products. 222 Rn is a decay product of 226 Ra, which has a short half-life (3.82 days) and emits alpha particles. The index value H in should be less than 1 to ensure safe from the exposure of 222 Rn and their short-lived decay products for the respiratory system of people living in the dwellings. The results of H in obtained in this study shown in Table 5 are all less than 1. Outdoor absorbed dose rate. To assess the exposure to radiation attributed by naturally occurring radionuclides in soil, the outdoor absorbed dose rate (D outdoor ) at 1 m above the ground surface was determined. The results are shown in Table 5. The mean of absorbed dose rate of the world is 58 nGy h −129 . The values obtained in this study are all below the mean of the world.
Annual effective dose equivalent. The annual effective dose equivalent (AEDE) indicates the effective dose received by an adult annually due to the activity concentration of radionuclides present in the building materials. The annual effective dose equivalent for various kinds of samples obtained in this study is shown in 3 μSv year −1 (fresh concrete) to 155 ± 18 μSv year −1 (recycled concrete), indicating that the mean AEDE indoor for recycled concrete is about two times higher than that of the fresh concrete. This result is corresponding to the values obtained in our previous study using the simulation model for a resident room using Monte Carlo N-Particle Transport(MCNP) 34 .
The UNSCEAR estimated that the mean worldwide exposure to natural radiation sources is as 2400 μSv year −1 . Total external terrestrial radiation accounts for 480 μSv year −129 . The mean annual doses in the Gaza Strip and the north of Sinai Peninsula obtained in this study are below this reference level.
Specific absorbed dose rate of crushers containing demolition debris. Crushers in the Gaza Strip contain huge amounts of demolition debris to be recycled as concrete blocks. The specific absorbed dose rate in air from material pile and the corresponding AEDE due to the materials in the crushers are shown in Table 6. The obtained results for the various materials used in the crushers are considered within safe levels. Inhalation doses from the dust generated by crushing the demolition debris and analysis of various demolition debris after the recent war in the Gaza Strip in 2021 are left to the future study. Th and 40 K) in the various kinds of samples studied in this study, the analysis of Spearman's correlation was carried out. The Spearman's correlation coefficients among the radionuclides are listed in Table 7. A significantly positive correlation is observed among 226 Ra, 238 U, 210 Pb and 235 U in the building materials and soil samples of the Gaza Strip, implying a common origin of those radionuclides. In the building material samples, 232 Th shows an insignificant weak correlation with 226 Ra, 235 U and 238 U and this suggest that they are from various sources of building materials. The medium positive correlation was observed between 40 K and other radionuclides in soil samples collected from the Gaza Strip.
Hierarchical cluster analysis. The interrelations among the radionuclides can be well classified and qualitatively visualized using hierarchical cluster analysis (HCA). In HCA, similar variables/cases are classified in a same group. Clustering is repeated with the next most similar variables till all of them are clustered 31 .
Visual representation of the distance at which clusters are combined can be carried out using dendrogram, reading from left to right. Vertical lines show joined clusters. The position on the scale indicates the distance at which the clusters are joined. The observed distances are rescaled (not actual distance) and it ranges from 1 to 25. The ratio of the rescaled distances within the dendrogram is the same as the ratio of original distances. The dendrogram was agglomerated with the nearest neighbor method using squared Euclidean distance. The derived dendrogram for building materials collected from the Gaza Strip is shown in Fig. 4. The radionuclides in the dendrogram are classified into two clusters. The blue line includes single radionuclide 40 K, not cluster. Cluster 1 (red line), consisting of 210 Pb and 226 Ra, shows little similarity with the other radionuclides, as 210 Pb derives not only from the decay of 238 U series but also from atmospheric fallout. Cluster 2 (green line) includes 238 U, 226 Ra, 232 Th and 235 U, indicative of their main origin in the natural radioactive series of the earth crust.  www.nature.com/scientificreports/