This research study introduces a multi-layered square-shaped metamaterial (MSM) structure for the electromagnetic (EM) absorption reduction in wireless mobile devices. Usually, wireless devices, for example, a cellular phone emits radiofrequency (RF) energy to the surroundings when used it. Moreover, fast-growing wireless communication technologies that support cellular data networks have also motivated this study. Hence, the focus of the research was to reduce the Specific Absorption Rate (SAR) for the Sub-6 frequency range by designing a multi-layered and compact, 10 × 10mm2 sized metamaterial structure that can be attached inside a mobile phone by avowing any overlapping with existing parts. Overall, six distinct square-shaped metamaterials were constructed on 0.25 mm thick Rogers RO3006 substrate material to reach the target of this investigation. Furthermore, numerical simulations of the proposed metamaterial electromagnetic properties and SAR reduction values were performed by adopting Computer Simulation Technology (CST) Microwave Studio 2019 software. From these simulations, the proposed MSM structure exhibited multi-band resonance frequencies accurately at 1.200, 1.458, 1.560, 1.896 GHz (at L-band), 2.268, 2.683 2.940, 3.580 GHz (at S-band) and 5.872 GHz (at C-band). Simultaneously, the proposed MSM structure was simulated in High-Frequency Structure Simulator (HFSS) to authenticate the numerical simulation data. The comparison of simulation data shows that only the primary and last resonance frequencies were reduced by 0.02 and 0.012 GHz, whereas the rest of the frequencies were increased by 0.042, 0.030, 0.040, 0.032, 0.107, 0.080, and 0.020 GHz in sequential order. In addition, the introduced MSM structure manifests left-handed behaviour at all the resonance frequencies. Nevertheless, the highest recorded SAR values were 98.136% and 98.283% at 1.560 GHz for 1 g and 10 g of tissue volumes. In conclusion, the proposed MSM met the objectives of this research study and can be employed in EM absorption reduction applications.
This paper presents the design and analysis of a novel split-H-shaped metamaterial unit cell structure that is applicable in a multi-band frequency range and that exhibits negative permeability and permittivity in those frequency bands. In the basic design, the separate split-square resonators are joined by a metal link to form an H-shaped unit structure. Moreover, an analysis and a comparison of the 1 × 1 array and 2 × 2 array structures and the 1 × 1 and 2 × 2 unit cell configurations were performed. All of these configurations demonstrate multi-band operating frequencies (S-band, C-band, X-band and Ku-band) with double-negative characteristics. The equivalent circuit model and measured result for each unit cell are presented to validate the resonant behavior. The commercially available finite-difference time-domain (FDTD)-based simulation software, Computer Simulation Technology (CST) Microwave Studio, was used to obtain the reflection and transmission parameters of each unit cell. This is a novel and promising design in the electromagnetic paradigm for its simplicity, scalability, double-negative characteristics and multi-band operation.
The paper reveals the design of a unit cell of a metamaterial that shows more than 2 GHz wideband near zero refractive index (NZRI) property in the C-band region of microwave spectra. The two arms of the unit cell were splitted in such a way that forms a near-pi-shape structure on epoxy resin fiber (FR-4) substrate material. The reflection and transmission characteristics of the unit cell were achieved by utilizing finite integration technique based simulation software. Measured results were presented, which complied well with simulated results. The unit cell was then applied to build a single layer rectangular-shaped cloak that operates in the C-band region where a metal cylinder was perfectly hidden electromagnetically by reducing the scattering width below zero. Moreover, the unit cell shows NZRI property there. The experimental result for the cloak operation was presented in terms of S-parameters as well. In addition, the same metamaterial shell was also adopted for designing an eye-shaped and triangular-shaped cloak structure to cloak the same object, and cloaking operation is achieved in the C-band, as well with slightly better cloaking performance. The novel design, NZRI property, and single layer C-band cloaking operation has made the design a promising one in the electromagnetic paradigm.
A double-negative metamaterial-inspired antenna is presented for mobile wireless applications. The antenna consists of a semi-circular radiating patch and a 3 × 4 hexagonal shaped metamaterial unit cell array in the ground plane. The antenna is fed with a 50 Ω microstrip feed line. The electric dimensions of the proposed antenna are 0.20λ × 0.26λ × 0.004λ, at the low-end frequency. The proposed antenna achieves a -10 dB impedance with a bandwidth of 2.29 GHz at the lower band and 1.28 GHz at the upper band and can operate for most of the mobile applications such as upper GSM bands, WiMAX, Bluetooth, and wireless local area network (WLAN) frequency bands. The focused novelties of the proposed antenna are its small size, multi-standard operating bands, and electromagnetic absorption reduction at all the operating frequencies using the double-negative metamaterial ground plane.
This study explores the effect of symmetrical square shaped metamaterial design for microwave frequency applications. The latest technology demands of advanced performance and research studies of metamaterial integration in the related bands are increasing tremendously. Therefore, this motivates us to explore the metamaterial design structure that has a high possibility to be applied in more than two resonance bands using a compact design structure. This study emphasis on a compact 14 × 14 mm2 and 1.524 mm thick substrate material known as Rogers RT6002. Seven distinct square shaped metamaterial (SQM) rings were constructed on the substrate material to achieve the goal of this research study. Besides that, the investigations of the metamaterial electromagnetic properties and effective medium parameters were carried out by utilising the Computer Simulation Technology Microwave Studio (CST) software. According to the numerical simulation results, the proposed SQM unit cell manifested quintuple resonance frequencies precisely at 3.384 (S band), 5.436, 7.002 (C band), 11.664 (X band), and 17.838 GHz (Ku band). Meanwhile, for the validation process, the comparison between the simulation and measurement results was analysed and data showed that the first and third resonance frequencies were increased by 0.336 and 0.139 GHz, respectively while other frequencies were reduced by 0.186, 0.081, and 0.709 GHz in sequential order. The numerical simulation of the metamaterial design was conducted in a High Frequency Structure Simulator (HFSS) to further validate the results. Furthermore, the proposed SQM manifested left handed characteristics at the second to fifth resonance bands. In a nutshell, the SQM successfully achieves the objectives of this research work and can be applied to multi band applications.
This study aims to demonstrate the feasibility of metamaterial application in absorption reduction of 5G electromagnetic (EM) energy in the human head tissue. In a general sense, the radio frequency (RF) energy that received by wireless mobile phone from the base station, will emit to surrounding when the devices are in active mode. Since the latest fifth generation technology standard for cellular networks is upon us, the emission of radiation from any wireless devices needs to be taken into consideration. This motivation helps to prepare this paper that focuses on construction of novel and compact square-shaped metamaterial (SM) design to reduce electromagnetic exposure to humans. The commercially available substrate material known as FR-4 with thickness of 1.6 mm was selected to place the metamaterial design on it. The electromagnetic properties and Specific Absorption Rate (SAR) analyses were carried out numerically by utilising high-performance 3D EM analysis, Computer Simulation Technology Studio (CST) software. Meanwhile, for the validation purpose, the metamaterial designs for both unit and array cells were fabricated to measure the electromagnetic properties of the material. From the numerical simulation, the introduced SM design manifested quadruple resonance frequencies in multi bands precisely at 1.246 (at L-band), 3.052, 3.794 (at S-band), and 4.858 (C-band) GHz. However, the comparison of numerically simulated and measured data reveals a slight difference between them where only the second resonance frequency was decreased by 0.009 GHz while other frequencies were increased by 0.002, 0.045, and 0.117 GHz in sequential order. Moreover, the SAR analysis recorded high values at 3.794 GHz with 61.16% and 70.33% for 1 g and 10 g of tissue volumes, respectively. Overall, our results demonstrate strong SAR reduction effects, and the proposed SM design may be considered a promising aspect in the telecommunication field.
Metamaterial with negative permittivity demonstrate excellent performance in cutting-edge technology. Thus, this study modified the double dumbbell-shaped split-ring resonator (MDD-SRR) based negative permittivity for satellite communications. The proposed MDD-SRR unit cell comprises a square-shaped split-ring resonator and two dumbbell-shaped rings. Some parts of the outer square ring were extended to enlarge the electrical length which altered the inductance of the metamaterial unit cell. The dimension of the proposed unit cell is 9 × 9 × 1.524 mm3, fabricated on a Rogers RT6002 (lossy) substrate material. Based on the results, five resonances for the transmission coefficient were achieved at frequencies of 2.896 GHz, 8.11 GHz, 9.76 GHz, 12.48 GHz and 13.49 GHz, including the S, X and Ku band satellite communication frequency bands through numerical simulation in a high-frequency electromagnetic simulator Computer Simulation Technology (CST) microwave studio. Negative permittivity at frequencies ranging from 2.896-3.76 GHz, 8.11-8.592 GHz, 9.76-10.784 GHz, 12.496-12.768 GHz, 13.504-14.4 GHz, were observed and extracted using the Robust and Nicolson-Ross-Weir (NRW) methods. Meanwhile, an effective medium ratio (EMR) measured at 11.51 to 2.896 GHz specified the goodness of the metamaterial unit cell for satellite communication with higher bandwidth and gain. The simulated, circuit model and measured results that were compared for validation purposes indicated that the simulation results, the equivalent circuit model results and measured results occupied each other. Moreover, the numerical simulation of the double dumbbell-shaped metamaterial unit cell was performed using a High-Frequency Structure Simulator (HFSS) to confirm the results. To evaluate the parametric study, the proposed unit cell was subjected to change different substrate types, change of split gap of rings, change of direction of electromagnetic field propagation, and structural optimization. In conclusion, the S, X and Ku-bands in the proposed metamaterial are competent for satellite communications as they are also investigated using an array of a unit cell.
Various techniques are commonly used to produce nano-crystalline NiAl2O4 materials; however, their practical applications in the microwave region remain very limited. In this work, flexible substrates for metamaterials containing two different concentrations of NiAl2O4 (labelled Ni36 and Ni42) have been synthesised using a sol-gel method. The formation of spinel structures in the synthesised materials is confirmed, and their crystalline sizes are determined using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray techniques. The dielectric properties, conductivities, loss tangents, and other parameters of the NiAl2O4-based substrates are analysed to evaluate their applicability as dielectric materials for the microwave frequency range. The obtained results show that the fabricated Ni36 and Ni42 nickel aluminates possess dielectric constants of 4.94 and 4.97 and loss tangents of 0.01 and 0.007, respectively; in addition, they exhibit high flexibility and light weight, which make them suitable for applications as metamaterial substrates. The synthesised structures are also validated experimentally using a commercially available electromagnetic simulator; as a result, double negative behaviour of the flexible metamaterials has been observed. Furthermore, it is found that the prepared NiAl2O4 substrates can be used in the S-, C-, and X-bands of the microwave frequency region.
A new perfect metamaterial absorber (PMA) with high fractional bandwidth (FBW) is examined and verified for solar energy harvesting. Solar cells based on perfect metamaterial give a chance to increase the efficiency of the system by intensifying the solar electromagnetic wave that incident on the device. The designed structure is mostly offered in the visible frequency range so as to exploit the solar's energy efficiently. Parametric investigations with regard to the measurements of the design structure are fulfilled to characterize the absorber. The finite-difference time-domain (FDTD) method-based CST simulator was used to keep the pattern parameters and absorbance analysis. The metamaterial shows almost 99.96% and 99.60% perfect absorption at 523.84 THz and 674.12 THz resonance frequencies. Moreover, absorption's FBW is studied, and 39.22% FBW is found. The results confirm that the designed PMA can attain very high absorption peak at two modes such as transverse electric (TE) and transverse magnetic (TM) mode. Other than the numerical outcomes demonstrated that the suggested configuration was also polarization angle insensitive. In addition, the change of absorbance of the structure has provided a new kind of sensor applications in these frequency ranges. Therefore, the suggested metamaterial absorber offers perfect absorption for visible frequency ranges and can be used for renewable solar energy harvesting applications.
Solar energy is one of the ambient sources where energy can be scavenged easily without pollution. Intent scavenging by the solar cell to recollect energy requires a state-of-the-art technique to expedite energy absorption to electron flow for producing more electricity. Structures of the solar cell have been researched to improve absorption efficiency, though most of them can only efficiently absorb with narrow-angle tolerance and polarization sensitivity. So, there is a strong demand for broadband absorption with minimal polarization sensitivity absorber, which is required for effective solar energy harvesting. In this paper, we proposed a new Split Hexagonal Patch Array (SHPA) shape metamaterial absorber with Double-negative (DNG) characteristics, which will provide a wide absorption band with low polarization sensitivity for solar spectrum energy harvesting. The proposed new SHPA shape consists of six nano-arms with a single vertical split which with arrowhead symmetry. This arm will steer electromagnetic (EM) resonance to acquire absolute negative permittivity and permeability, ensuring DNG property. This DNG metamaterial features analyzed based on the photoconversion quantum method for maximum photon absorption. The symmetric characteristics of the proposed structure enable the absorber to show polarization insensitivity and wide incident angle absorption capabilities. Simulated SHPA shows a visible and ultraviolet (UV) spectrum electromagnetic wave absorption capacity of more than 95%. The quantum method gives an advantage in the conversion efficiency of the absorber, and the numerical analysis of the proposed SHPA structure provides absorbance quality for THz regime energy harvesting through solar cell or photonic application.
A new design and analysis of a wide-band double-negative metamaterial, considering a frequency range of 0.5 to 7 GHz, is presented in this paper. Four different unit cells with varying design parameters are analyzed to evaluate the effects of the unit-cell size on the resonance frequencies of the metamaterial. Moreover, open and interconnected 2 × 2 array structures of unit cells are analyzed. The finite-difference time-domain (FDTD) method, based on the Computer Simulation Technology (CST) Microwave Studio, is utilized in the majority of this investigation. The experimental portion of the study was performed in a semi-anechoic chamber. Good agreement is observed between the simulated and measured S parameters of the developed unit cell and array. The designed unit cell exhibits negative permittivity and permeability simultaneously at S-band (2.95 GHz to 4.00 GHz) microwave frequencies. In addition, the designed unit cell can also operate as a double-negative medium throughout the C band (4.00 GHz to 4.95 GHz and 5.00 GHz to 5.57 GHz). At a number of other frequencies, it exhibits a single negative value. The two array configurations cause a slight shift in the resonance frequencies of the metamaterial and hence lead to a slight shift of the single- and double-negative frequency ranges of the metamaterial.
A new metasurface superstrate structure (MSS)-loaded dual band microstrip line-fed small patch antenna is presented in this paper. The proposed antenna was designed on a ceramic-filled bioplastic sandwich substrate with a high dielectric constant. The proposed 7 × 6 element, square-shaped, single-sided MSS significantly improved the bandwidth and gain of the proposed antenna. The proposed MSS incorporated a slotted patch antenna that effectively increased the measured operating bandwidth from 13.3% to 18.8% and from 14.8% to 23.2% in the lower and upper bands, respectively. Moreover, the average gain of the proposed MSS-based antenna was enhanced from 2.12 dBi to 3.02 dBi in the lower band and from 4.10 dBi to 5.28 dBi in the upper band compared to the patch antenna alone. In addition to the bandwidth and gain improvements, more directive radiation characteristics were also observed from the MSS antenna compared to the patch itself. The effects of the MSS elements and the ground plane length on the reflection coefficient of the antenna were analyzed and optimized. The overall performance makes the proposed antenna appropriate for RFID and WLAN applications.
The aim of this paper is to introduce a compact double-negative (DNG) metamaterial that exhibits a negative refractive index (NRI) bandwidth of more than 3.6 GHz considering the frequency from 2 to 14 GHz. In this framework, two arms of the designed unit cell are split in a way that forms a Modified-Z-shape structure of the FR-4 substrate material. The finite integration technique (FIT)-based Computer Simulation Technology (CST) Microwave Studio is applied for computation, and the experimental setup for measuring the performance is performed inside two waveguide ports. Therefore, the measured data complies well with the simulated data of the unit cell at 0-degree and 90-degree rotation angles. The designed unit cell shows a negative refractive index from 3.482 to 7.096 GHz (bandwidth of 3.61 GHz), 7.876 to 10.047 GHz (bandwidth of 2.171 GHz), and 11.594 to 14 GHz (bandwidth of 2.406 GHz) in the microwave spectra. The design also exhibits almost the same wide negative refractive index bandwidth in the major region of the C-band and X-band if it is rotated 90 degrees. However, the novelty of the proposed structure lies in its effective medium ratio of more than 4, wide bandwidth, and compact size.
A new meta-surface structure (MSS) with a near-zero refractive index (NZRI) is proposed to enhance the performance of a square loop antenna array. The main challenge to improve the antenna performance is increment of the overall antenna volume that is mitigated by assimilating the planar NZRI MSS at the back of the antenna structure. The proposed NZRI MSS-loaded CPW-fed (Co-Planar Waveguide) four-element array antenna is designed on ceramic-bioplastic-ceramic sandwich substrate using high-frequency structure simulator (HFSS), a finite-element-method-based simulation tool. The gain and directivity of the antenna are significantly enhanced by incorporating the NZRI MSS with a 7 × 6 set of elements at the back of the antenna structure. Measurement results show that the maximum gains of the antenna increased from 6.21 dBi to 8.25 dBi, from 6.52 dBi to 9.05 dBi and from 10.54 dBi to 12.15 dBi in the first, second and third bands, respectively. The effect of the slot configuration in the ground plane on the reflection coefficient of the antenna was analyzed and optimized. The overall performance makes the proposed antenna appropriate for UHFFM (Ultra High Frequency Frequency Modulation) telemetry-based space applications as well as mobile satellite, microwave radiometry and radio astronomy applications.
A compact metamaterial inspired antenna operate at LTE, Bluetooth and WiMAX frequency band is introduced in this paper. For the lower band, the design utilizes an outer square metallic strip forcing the patch to radiate as an equivalent magnetic-current loop. For the upper band, another magnetic current loop is created by adding metamaterial structure near the feed line on the patch. The metamaterial inspired antenna dimension of 42 × 32 mm2 compatible to wireless devices. Finite integration technique based CST Microwave Studio simulator has been used to design and numerical investigation as well as lumped circuit model of the metamaterial antenna is explained with proper mathematical derivation. The achieved measured dual band operation of the conventional antenna are sequentially, 0.561~0.578 GHz, 2.346~2.906 GHz, and 2.91~3.49 GHz, whereas the metamaterial inspired antenna shows dual-band operation from 0.60~0.64 GHz, 2.67~3.40 GHz and 3.61~3.67 GHz, respectively. Therefore, the metamaterial antenna is applicable for LTE and WiMAX applications. Besides, the measured metamaterial antenna gains of 0.15~3.81 dBi and 3.47~3.75 dBi, respectively for the frequency band of 2.67~3.40 GHz and 3.61~3.67 GHz.
This work focused on the novel symmetrical left-handed split ring resonator metamaterial for terahertz frequency applications. A compact substrate material known as Silicon with a dimension of 5 µm was adopted in this research investigation. Moreover, several parameter studies were investigated, such as clockwise rotation, array and layer structure designs, larger-scale metamaterials, novel design structure comparisons and electric field distribution analysis. Meanwhile, two types of square-shaped metamaterial designs were proposed in this work. The proposed designs exhibit double and single resonance frequencies respectively, likely at 3.32 and 9.24 THz with magnitude values of - 16.43 and - 17.33 for the first design, while the second design exhibits a response at 3.03 THz with a magnitude value of - 19.90. Moreover, the verification of these results by adopting High-frequency Structure Simulator software indicates only slight discrepancies which are less than 5%. Furthermore, the initial response of the proposed designs was successfully altered by simply rotating the design clockwise or even increasing the dimension of the design. For instance, the first resonance frequency is shifted to the lower band when the first proposed design was rotated 90°. On the other hand, by increasing the size of the metamaterial, more than nine resonance frequencies were gained in each symmetric design. Furthermore, the symmetric metamaterial with a similar width and length of 10 µm dimension was adopted for both design structures to construct an equivalent circuit model by utilising Advanced Design System software. Finally, both unit cell designs were utilised to explore the absorption performances which exhibit four and five peak points. Overall, the altering behaviour by changing physical properties and compact design with acceptable responses become one of the novelties of this research investigation. In a nutshell, the proposed designs can be utilised in terahertz frequency which gives optimistic or advantageous feedback and is relatively suitable for the adopted frequency range.
This study presented a unique, miniaturised asymmetric interconnected vertical stripe (IVS) design for terahertz (THz) frequency applications. Therefore, this research aimed to achieve a frequency response of 0 to 10 THz using a 5 × 5 µm2 Silicon (Si) substrate material. Meanwhile, various parametric examinations were conducted to investigate variations in the performance. For example, the unit cell selection process was carefully examined by using various design structures and modifying the structure by adding split gaps and connecting bars between vertical stripes. Furthermore, the proposed sandwich structure design was used to compute the absorbance and reflectance properties. All the analytical examinations were executed utilising the Computer Simulation Technology (CST) 2019 software. The introduced IVS metamaterial exhibits negative index behaviour and has a single resonance frequency of 5.23 THz with an acceptable magnitude of - 24.38 dB. Additionally, the quadruple-layer IVS structure exhibits optimised transmission coefficient behaviour between 3 and 6 THz and 7 to 9 THz, respectively. However, the magnitude of the transmission coefficient increased with the number of material layers. Besides that, the absorbance study shows that using a quadruple-layer structure obtains unique and promising results. Overall, the proposed asymmetric IVS metamaterial design achieves the required performance by using a compact structure rather than extending the dimensions of the design.
We created an ultra-thin, triple-band incident angle-insensitive perfect metamaterial absorber (MMA) with a metallic patch and a continuous metal ground isolated by a central dielectric substrate. The top metallic patch, placed across the edges of the 0.58 mm thickness Rogers RO4003C (lossy) substrate, forms the bulk of the projected absorber's ultra-thin layer. Nonetheless, absorption is exceedingly strong, covering C-band, X-band and K-band and reaching levels of 97.8%, 99.9%, and 99.9%, respectively, under normal and even oblique (0° to 45°) incident conditions. In chosen ranges of frequency of 6.24, 10.608, and 18.624 GHz for both TM and TE mode, the displayed Q-factors were 62.4, 17.68, and 26.61, respectively. We correspondingly calculated the RAB (relative absorption bandwidth) to evaluate absorption performance. An equivalent circuit proved its performance capabilities, indicating that it would produce a high-quality MMA from ADS software. Furthermore, the absorber's performance has been verified in free space on a sample being tested using a different array of unit cells. Moreover, the proposed structures with HFSS simulators to display the MMA's absolute absorption at each absorption peak are somewhat inconsistent with the results of the CST simulator. Because of its superior performance, the ultra-thin absorber is suited for a wide range of applications, including satellite applications such as radar systems, stealth technology, imaging, and electromagnetic interference reduction.
Compelling facial expression recognition (FER) processes have been utilized in very successful fields like computer vision, robotics, artificial intelligence, and dynamic texture recognition. However, the FER's critical problem with traditional local binary pattern (LBP) is the loss of neighboring pixels related to different scales that can affect the texture of facial images. To overcome such limitations, this study describes a new extended LBP method to extract feature vectors from images, detecting each image from facial expressions. The proposed method is based on the bitwise AND operation of two rotational kernels applied on LBP(8,1) and LBP(8,2) and utilizes two accessible datasets. Firstly, the facial parts are detected and the essential components of a face are observed, such as eyes, nose, and lips. The portion of the face is then cropped to reduce the dimensions and an unsharp masking kernel is applied to sharpen the image. The filtered images then go through the feature extraction method and wait for the classification process. Four machine learning classifiers were used to verify the proposed method. This study shows that the proposed multi-scale featured local binary pattern (MSFLBP), together with Support Vector Machine (SVM), outperformed the recent LBP-based state-of-the-art approaches resulting in an accuracy of 99.12% for the Extended Cohn-Kanade (CK+) dataset and 89.08% for the Karolinska Directed Emotional Faces (KDEF) dataset.
Batu Dam is of considerable importance to the metropolis of Kuala Lumpur, its existence and the quality assessment of its waters being essential in helping to maintain the lives of a large sector of the Malaysian population. Concerning the level of naturally occurring radioactivity contained within its waters, a well characterised HPGe γ-ray technique has been used in making measurements of the concentrations of primordial radionuclides in samples of surface water from the Dam. Based on the mean individual daily consumption of dam water, estimation has been made of the concomitant radiation dose. Activity concentrations, in units of Bq l-1, have been found to be in the range 2.4-3.2 for 226Ra, 1.1-1.3 for 232Th and 22.7-40.7 for 40K, in line with literature data for surface waters. The total annual ingestion dose for infants (<1 y) and adolescents (12-17 y) are found to be significant and greater than the World Health Organization recommended maximum dose of 0.1 mSv y-1 from the imbibing of drinking water. However, the Dam water does not pose a threat to public health, the Dam water not being used as the sole source of drinking water. Noting that this is the only known study of water from Batu Dam, the reported levels allow for evaluation of future changes in the natural radioactivity profile.