Papers by Ifechukwude Okwechime
2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop (NSS/MIC/RTSD), 2016
Cadmium zinc telluride (CdZnTe) X-rays and gamma-rays detectors have found applications in radiol... more Cadmium zinc telluride (CdZnTe) X-rays and gamma-rays detectors have found applications in radiological and nuclear security, environmental protection, astrophysics, and nuclear medical imaging. Cadmium manganese telluride (CdMnTe) has the potential for similar success as CdZnTe in the detection of ionizing radiations. The detection of radiological and nuclear threats would greatly benefit from detector wafers with large-volume, high-performance, and low-cost of production. This paper presents recent advances in the development of CdZnTe and CdMnTe nuclear detectors with the focus on the detection of radiological and nuclear threats. Efforts on improving the performance of CdZnTe and CdMnTe detectors were made in three major areas: crystal growth, post-growth annealing, and device fabrication. In crystal growth, we optimized ampoule carbon coating for CdZnTe and eliminated detector performance-limiting secondary phase particles in CdMnTe. In post-growth annealing of CdMnTe and CdZnT...
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2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2017
Cadmium zinc telluride (CdZnTe) radiation detectors have found applications in medical imaging an... more Cadmium zinc telluride (CdZnTe) radiation detectors have found applications in medical imaging and in the detection of radiological and nuclear threats. While there has been success in developing CdZnTe detectors with less than 1% FWHM energy resolution at the 662 keV gamma energy, charge collection can be improved through optimization of surface treatment techniques. The dominant surface species on CdZnTe are cadmium, tellurium, and tellurium oxide. The tellurium oxide is expected to produce a more chemically stable surface. This paper presents the quantification of the tellurium oxide species on CdZnTe wafers that are freshly etched in a mixture of hydrogen bromide in hydrogen peroxide and ethylene glycol solution. It also presents the effects of the tellurium oxide on surface current and detector performance. X-ray photoelectron spectroscopy (XPS) was used to quantify the tellurium oxide species on the wafer surface. The formation of tellurium oxide on the CdZnTe wafers immediate...
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Surface passivation and contact deposition are important processes in the fabrication of cadmium ... more Surface passivation and contact deposition are important processes in the fabrication of cadmium zinc telluride (CdZnTe) X-rays and gamma-rays detectors. Surface passivation is used to reduce the effects of fabrication-induced surface defects. It is also used to reduce defects that could result from aging of the detector. Another important factor in charge collection is the interface between the electrical contacts and the detector wafer. In this paper, we present the results of the effects of chemo-mechanically polishing CdZnTe wafers with a hydrogen bromide in hydrogen peroxide solution followed by passivation in a mixture of potassium hydroxide in hydrogen peroxide solution by monitoring their energy resolutions over a 4-day period. The results showed that the 59.5-keV gamma peak of Am-241 remains under the same channel over the 4 days.
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Background Individuals with pre-diabetes and diabetes have increased risks of developing macro-va... more Background Individuals with pre-diabetes and diabetes have increased risks of developing macro-vas-cular complications including heart disease and stroke; which are the leading causes of death globally. The objective of this study was to estimate the prevalence of pre-diabetes and diabetes, and to investigate their predictors among adults18 years in Florida. Methods Data covering the time period January-December 2013, were obtained from Florida’s Behavioral Risk Factor Surveillance System (BRFSS). Survey design of the study was declared using SVYSET statement of STATA 13.1. Descriptive analyses were performed to estimate the prevalence of pre-diabetes and diabetes. Predictors of pre-diabetes and diabe-tes were investigated using multinomial logistic regression model. Model goodness-of-fit was evaluated using both the multinomial goodness-of-fit test proposed by Fagerland, Hos-mer, and Bofin, as well as, the Hosmer-Lemeshow’s goodness of fit test. Results
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Surface defects caused during cutting and polishing in the fabrication of cad-mium zinc telluride... more Surface defects caused during cutting and polishing in the fabrication of cad-mium zinc telluride (CdZnTe) nuclear detectors limit their spectral performance. Chemical treatments are often used to remove surface damages and defects. In this paper, we present the analysis of Te and TeO 2 species on the surfaces of CdZnTe nuclear detectors treated with hydrogen bromide and ammonium-based solutions. The CdZnTe wafers were chemo-mechanically polished in a mixture of hydrogen bromide in hydrogen peroxide and ethy-lene glycol, followed by a chemical passivation in a mixture of ammonium fluoride and hydrogen peroxide solution. X-ray photoelectron spectroscopy showed significant conversion of Te to TeO 2 , thus producing a more chemically stable surface. The resistivity of the CdZnTe samples is in the order of 10 10 ohms-cm. The current for a given applied voltage increased following the passivation and decreased after a 3-hour period. Results from spectral response measurements showed that the 59.5-keV gamma-peak of Am-241 was stable under the same channel for the surface treatment processes.
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PLOS ONE, 2015
Individuals with pre-diabetes and diabetes have increased risks of developing macro-vascular comp... more Individuals with pre-diabetes and diabetes have increased risks of developing macro-vascular complications including heart disease and stroke; which are the leading causes of death globally. The objective of this study was to estimate the prevalence of pre-diabetes and diabetes, and to investigate their predictors among adults ≥18 years in Florida. Data covering the time period January-December 2013, were obtained from Florida's Behavioral Risk Factor Surveillance System (BRFSS). Survey design of the study was declared using SVYSET statement of STATA 13.1. Descriptive analyses were performed to estimate the prevalence of pre-diabetes and diabetes. Predictors of pre-diabetes and diabetes were investigated using multinomial logistic regression model. Model goodness-of-fit was evaluated using both the multinomial goodness-of-fit test proposed by Fagerland, Hosmer, and Bofin, as well as, the Hosmer-Lemeshow's goodness of fit test. There were approximately 2,983 (7.3%) and 5,189 (12.1%) adults in Florida diagnosed with pre-diabetes and diabetes, respectively. Over half of the study respondents were white, married and over the age of 45 years while 36.4% reported being physically inactive, overweight (36.4%) or obese (26.4%), hypertensive (34.6%), hypercholesteremic (40.3%), and 26% were arthritic. Based on the final multivariable multinomial model, only being overweight (Relative Risk Ratio [RRR] = 1.85, 95% Confidence Interval [95% CI] = 1.41, 2.42), obese (RRR = 3.41, 95% CI = 2.61, 4.45), hypertensive (RRR = 1.69, 95% CI = 1.33, 2.15), hypercholesterolemic (RRR = 1.94, 95% CI = 1.55, 2.43), and arthritic (RRR = 1.24, 95% CI = 1.00, 1.55) had significant associations with pre-diabetes. However, more predictors had significant associations with diabetes and the strengths of associations tended to be higher than for the association with pre-diabetes. For instance, the relative risk ratios for the association between diabetes and being overweight (RRR = 2.00, 95% CI = 1.55, 2.57), or obese (RRR = 4.04, 95% CI = 3.22, 5.07), hypertensive (RRR = 2.66, 95% CI = 2.08, 3.41), hypercholesterolemic (RRR = 1.98, 95% CI = 1.61, 2.45) and arthritic (RRR = 1.28, 95% CI = 1.04, 1.58) were all further away from the null than their associations with pre-diabetes. Moreover, a number of variables such as age, income level, sex, and level of physical activity had significant association with diabetes but not pre-diabetes. The risk of diabetes increased with increasing age, lower income, in males, and with physical inactivity. Insufficient physical activity had no significant association with the risk of diabetes or pre-diabetes. There is evidence of differences in the strength of association of the predictors across levels of diabetes status (pre-diabetes and diabetes) among adults ≥18 years in Florida. It is important to monitor populations at high risk for pre-diabetes and diabetes, so as to help guide health programming decisions and resource allocations to control the condition.
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This study investigated the consequences of exposure to crude oil on components of the immune sys... more This study investigated the consequences of exposure to crude oil on components of the immune system of male rats. A total of 40 male albino rats were used for the experiment. Exposure to crude oil was achieved by oral administration of increasing doses (low, medium and high) of Nigerian Qua Iboe Brent crude oil to the rats every other day for 4 weeks. Haematological parameters, relative weights and histomorphological features of the spleen of rats that received the crude oil were compared to control rats. The results showed that packed cell volume values (P<0.05), erythrocyte counts (P<0.05), absolute neutrophil counts (P<0.01) and absolute monocyte counts (P<0.01) were significantly reduced in crude oil-exposed rats. The total leukocyte counts (P<0.05) and absolute lymphocyte counts (P<0.01) were increased at the low dose of crude oil, but were subsequently reduced with increase in dose of crude oil. Splenic morphology was severely altered in rats that consumed c...
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Journal of Pediatric and Adolescent Gynecology, 2011
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Mechanically polishing cadmium zinc telluride (CdZnTe) wafers for x-ray and gamma-ray detectors o... more Mechanically polishing cadmium zinc telluride (CdZnTe) wafers for x-ray and gamma-ray detectors often is inadequate in removing surface defects caused by cutting them from the ingots. Fabrication-induced defects, such as surface roughness, dangling bonds, and nonstoichiometric surfaces, often are reduced through polishing and etching the surface. In our earlier studies of mechanical polishing with alumina powder, etching with hydrogen bromide in hydrogen peroxide solution, and chemomechanical polishing with bromine–methanol–ethylene glycol solution, we found that the chemomechanical polishing process produced the least surface leakage current. In this research, we focused on using two chemicals to chemomechanically polish CdZnTe wafers after mechanical polishing, viz. bromine–methanol–ethylene glycol (BME) solution, and hydrogen bromide (HBr) in a hydrogen peroxide and ethylene–glycol solution. We used x-ray photoelectron spectroscopy (XPS), current–voltage (I–V) measurements, and Am-241 spectral response measurements to characterize and compare the effects of each solution. The results show that the HBr-based solution produced lower leakage current than the BME solution. Results from using the same chemomechanical polishing solution on two samples confirmed that the surface treatment affects the measured bulk current (a combination of bulk and surface currents). XPS results indicate that the tellurium oxide to tellurium peak ratios for the mechanical polishing process were reduced significantly by chemomechanical polishing using the BME solution (78.9% for Te 3d 5/2O2 and 76.7% for Te 3d 3/2O2) compared with the HBr-based solution (27.6% for Te 3d 5/2O2 and 35.8% for Te 3d 3/2O2). Spectral response measurements showed that the 59.5-keV peak of Am-241 remained under the same channel number for all three CdZnTe samples. While the BME-based solution gave a better performance of 7.15% full-width at half-maximum (FWHM) compared with 7.59% FWHM for the HBr-based solution, the latter showed a smaller variation in performance of 0.39% FWHM over 7 days compared with 0.69% for the BME-based solution.
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OBJECTIVES:
To examine the impact of the Central Hillsborough Healthy Start Project (CHHS) on hu... more OBJECTIVES:
To examine the impact of the Central Hillsborough Healthy Start Project (CHHS) on human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) diagnosis rates in women in Hillsborough County, Florida.
METHODS:
Project records were linked to hospital discharge data and vital statistics (Florida, 1998-2007; N = 1,696,221). The χ(2) test was used to compare rates for HIV/AIDS and pregnancy-related complications for mothers within the CHHS service area with mothers in Hillsborough County and the rest of Florida.
RESULTS:
During a 10-year period, HIV/AIDS diagnosis rates among women in the CHHS service area declined by 56.3% (P = 0.01). The observed decline was most evident among black women. HIV/AIDS diagnosis rates in the rest of Hillsborough County and Florida remained unchanged (P = 0.48).
CONCLUSIONS:
Lessons learned from the CHHS Project can be used to develop effective and comprehensive models for addressing the HIV epidemic.
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The purpose of this paper is to provide a logical rationale on the need to increase country-speci... more The purpose of this paper is to provide a logical rationale on the need to increase country-specific research on cardiovascular diseases (CVD) in West Africa. Using health data from the World Health Organization's Health Statistics and Health Information Systems, the burden of cardiovascular diseases among West African countries was described. PubMed's database and search tools were used with specific key words, to retrieve articles referencing cardiovascular disease for the study period 1992 to 2011. The total number of publications from each West African country was obtained and the percentage of each country's contribution was estimated. An annual trend of research productivity was then plotted for each country and also for the region. The major contributors to CVD research in West Africa were Nigeria, Ghana, and Senegal. An overall trend of increasing CVD research output was evident during the study period, driven mainly by research from Nigeria. This cross-sectional study revealed the need to increase country-specific research on CVD in West Africa. Conducting CVD research provides evidence of the epidemiology, mechanisms, and determinants of CVDs. This study is important as it may serve as a basis for developing regionally tailored policies and interventions for West Africa.
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Bridgman-grown cadmium zinc telluride (CdZnTe or CZT) and cadmium manganese telluride (CdMnTe or... more Bridgman-grown cadmium zinc telluride (CdZnTe or CZT) and cadmium manganese telluride (CdMnTe or CMT) crystals often have Te inclusions that limit their performances as X-ray- and gamma-ray-detectors. We present here the results of post-growth thermal annealing aimed at reducing and eliminating Te inclusions in them. In a 2D analysis, we observed that the sizes of the Te inclusions declined to 92% during a 60-h annealing of CZT at 510 °C under Cd vapor. Further, tellurium inclusions were eliminated completely in CMT samples annealed at 570 °C in Cd vapor for 26 h, whilst their electrical resistivity fell by an order of 102. During the temperature-gradient annealing of CMT at 730 °C and an 18 °C/cm temperature gradient for 18 h in a vacuum of 10−5 mbar, we observed the diffusion of Te from the sample, so causing a reduction in size of the Te inclusions. For CZT samples annealed at 700 °C in a 10 °C/cm temperature gradient, we observed the migration of Te inclusions from a low-temperature region to a high one at 0.022 μm/s. During the temperature-gradient annealing of CZT in a vacuum of 10−5 mbar at 570 °C and 30 °C/cm for 18 h, some Te inclusions moved toward the high-temperature side of the wafer, while other inclusions of the same size, i.e., 10 µm in diameter, remained in the same position. These results show that the migration, diffusion, and reaction of Te with Cd in the matrix of CZT- and CMT-wafers are complex phenomena that depend on the conditions in local regions, such as composition and structure, as well as on the annealing conditions.
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Surface damages occur in Cadmium zinc telluride (CdZnTe) wafers for radiation detection devices ... more Surface damages occur in Cadmium zinc telluride (CdZnTe) wafers for radiation detection devices during dicing and polishing. This often results in increased leakage current that limits the performance of the detector. An effective method of removing the surface damage and thus reducing the leakage current is through the use of chemical treatments. The effects discussed in this study include: chemical polishing with a mixture of hydrogen bromide solution followed by passivation with ammonium fluoride in a hydrogen peroxide solution. The effects on the current-voltage measurements and the spectral response were monitored over a 2-week period. X-ray photoelectron spectroscopy (XPS) was also obtained to observe the formation of chemical species on treated surfaces. The resistivity of the treated CdZnTe samples is on the order of 1010 ohm-cm. The current in the I-V measurements increased rapidly immediately following the chemical polishing and surface passivation, and decreased steadily afterwards. The spectral response showed that the 59.5-keV peak of Am-241 was stable in the same position over the test period.
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Surface damages occur in Cadmium zinc telluride (CdZnTe) wafers for radiation detection devices d... more Surface damages occur in Cadmium zinc telluride (CdZnTe) wafers for radiation detection devices during dicing and polishing. This often results in increased leakage current that limits the performance of the detector. An effective method of removing the surface damage and thus reducing the leakage current is through the use of chemical treatments. The effects discussed in this study include: chemical polishing with a mixture of hydrogen bromide solution followed by passivation with ammonium fluoride in a hydrogen peroxide solution. The effects on the current-voltage measurements and the spectral response were monitored over a 2-week period. X-ray photoelectron spectroscopy (XPS) was also obtained to observe the formation of chemical species on treated surfaces. The resistivity of the treated CdZnTe samples is on the order of 1010 ohm-cm. The current in the I-V measurements increased rapidly immediately following the chemical polishing and surface passivation, and decreased steadily afterwards. The spectral response showed that the 59.5-keV peak of Am-241 was stable in the same position over the test period.
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Passivation and improved contact deposition techniques are known to reduce the surface leakage cu... more Passivation and improved contact deposition techniques are known to reduce the surface leakage current and decrease noise levels of CdZnTe X-Ray and gamrna-ray detectors by improving their spectral energy resolutions. This paper presents a comparative study of surface passivation process in CdZnTe X-Ray and gamma-ray detectors. The experimental study compares three surface passivation processes: mechanically polished with a O.9μm Alumina Powder (AI2O3) finishing; mechanically polished with a O.9μm Alumina Powder (AI2O3) and etched with HBr + H2O2 solution for 2 minutes; mechanically polished with a O.9,.m Alumina Powder (AI2O3) and chemomechanically polished with bromine-methanol-ethylene glycol solution. The results show that chemo-mechanical polishing with bromine-methanol-ethylene glycol solution proved to be the best method out of the three for reducing surface leakage current. The preliminary results of aging studies on H2O2 (Hydrogen Peroxide), NH4F (Ammonium Fluoride) + H2O2 + H2O and (NH4hS (Ammonium Sulfide) passivation agents are also presented.
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Conference Presentations by Ifechukwude Okwechime
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Surface damages caused during cutting and polishing in the fabrication of cadmium zinc telluride ... more Surface damages caused during cutting and polishing in the fabrication of cadmium zinc telluride (CZT) nuclear detectors often result in high leakage current that limits the spectral performance of the material. Chemical treatment is an effective technique for removing the surface damages and thus reducing the leakage current. In this study, we report the effects of chemo-mechanical polishing with a mixture of hydrogen bromide in hydrogen peroxide and ethylene glycol solution, followed by passivation with potassium hydroxide in hydrogen peroxide on CZT detector performance. The treatment effects were studied using current-voltage (I-V) measurements, spectral response and x-ray photoelectron spectroscopy (XPS). The resistivity of the CZT samples is in the order of 1010 ohms-cm. The currents in the I-V measurements increased rapidly following the chemo-mechanical polishing and passivation and decreased steadily over a 14-day period. The spectral response results show that the 59.5-keV peak of Am-241 was stable under the same channel over the 14-day period.
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Papers by Ifechukwude Okwechime
To examine the impact of the Central Hillsborough Healthy Start Project (CHHS) on human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) diagnosis rates in women in Hillsborough County, Florida.
METHODS:
Project records were linked to hospital discharge data and vital statistics (Florida, 1998-2007; N = 1,696,221). The χ(2) test was used to compare rates for HIV/AIDS and pregnancy-related complications for mothers within the CHHS service area with mothers in Hillsborough County and the rest of Florida.
RESULTS:
During a 10-year period, HIV/AIDS diagnosis rates among women in the CHHS service area declined by 56.3% (P = 0.01). The observed decline was most evident among black women. HIV/AIDS diagnosis rates in the rest of Hillsborough County and Florida remained unchanged (P = 0.48).
CONCLUSIONS:
Lessons learned from the CHHS Project can be used to develop effective and comprehensive models for addressing the HIV epidemic.
Conference Presentations by Ifechukwude Okwechime
To examine the impact of the Central Hillsborough Healthy Start Project (CHHS) on human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) diagnosis rates in women in Hillsborough County, Florida.
METHODS:
Project records were linked to hospital discharge data and vital statistics (Florida, 1998-2007; N = 1,696,221). The χ(2) test was used to compare rates for HIV/AIDS and pregnancy-related complications for mothers within the CHHS service area with mothers in Hillsborough County and the rest of Florida.
RESULTS:
During a 10-year period, HIV/AIDS diagnosis rates among women in the CHHS service area declined by 56.3% (P = 0.01). The observed decline was most evident among black women. HIV/AIDS diagnosis rates in the rest of Hillsborough County and Florida remained unchanged (P = 0.48).
CONCLUSIONS:
Lessons learned from the CHHS Project can be used to develop effective and comprehensive models for addressing the HIV epidemic.