GAMMA RAYS emitted in radioactive decay of the nucleus can originate from a number of sources ranging from the cosmos such as neutron stars, supernova explosions, and provinces of black holes to terrestrial nuclear explosions, lightning, and naturally occurring radioisotopes (Eisenbud and Gesell 1997 ; Oni et al. 2011 ; Petalas et al. 2005). Also, the Earth’s crust contains radioactive elements, which are a major influence in natural radioactivity aside from cosmogenic sources. To this end, there are numerous sources of terrestrial radionuclides including uranium and radium in rocks, soil, food, air, and water. Also, phosphate fertilizers, fossil fuels (coals), and some common construction materials contain activity levels from 238U, 232Th, and 40K (Knoll 2010 ; Eisenbud and Gesell 1997). 40K is widely distributed throughout the terrestrial environment; e.g., in crustal rocks, plants, animals, and humans (Cember and Johnson 2009). Cosmic rays consisting mostly of protons can produce cosmogenic radionuclides by nuclear reactions in the upper atmosphere that give rise to cosmic ray showers including muons, electrons, gamma radiation, and also neutrons within the environment (Knoll 2010). For instance, examples of gamma-emitting radionuclide products impacting human exposures from cosmological sources include 22Na and 7Be (Eisenbud and Gesell 1997 ; UNSCEAR 2000, 2010). Studies have also shown that the average dose rate of natural radiation is higher among mountain terrains in the United States (U.S.) than that of dose rates in Atlantic and Gulf coastal plains due to elevation levels in the U.S. (Wahl 2010). The purpose of this study was to have students involved with the TSU health physics program measure and report the dose rate from background gamma radiation at designated locations in Houston, TX, using the Canberra InSpectorTM 1000 Digital Hand-Held Multichannel Analyzer (Mirion Technologies Canberra, Inc, 800 Research Parkway, Meriden, CT 06450) and compare the results with the average gamma dose rate on Earth. The Department of Physics at TSU currently boasts the only undergraduate health physics program in the Greater Houston area. A specialized program in health physics at the undergraduate level is unique, particularly at an HBCU. The curriculum covers the basics of radiation physics theory and experimental measurement techniques, which are typically designed for first or second year graduate students.
MATERIALS AND METHODS
Education and research mentoring
Students take two semesters of laboratory classes highlighting specific hands-on experiments, which involve different radiation detection apparatuses (e.g., Geiger-Mueller and NaI Scintillation detector systems), analyses, and techniques. In addition to health physics courses and parallel classes at TSU, student education and research mentoring included weekly update meetings, where students gave brief PowerPoint presentations to their research mentor and peers on the current status of their project. For this project, students were provided with several specific guidelines emphasizing the following: (1) literature search, (2) data measurement and analysis, and (3) manuscript preparation. Students had access to high impact factor journal publications through the Texas Medical Center library system. They were instructed to read published literature on environmental gamma radiation studies and outline a measurement plan that included experimental designs, project timeline, and survey site selection for data collection mainly close to parks within the I‐610 loop in Houston.
The cumulative dose and dose rate were measured during the winter and spring months of 2016 at nine distinct locations in Houston, TX, using the Canberra InSpectorTM 1000 Digital Hand-Held Multichannel Analyzer. The Canberra InSpectorTM 1000 used in these measurements has numerous features and components, which include a LaBr3 scintillation detector, neutron detection, and radionuclide identification capabilities using the Genie software (Canberra Industries). The resolution of the LaBr3 scintillation detector is about 3.5% FWHM at 662 keV and also has a higher light output, a faster light decay time output pulse, and improved energy resolution and efficiency compared with a standard NaI(TI) scintillation detector. In addition, the LaBr3 scintillation detector has temperature stabilization capabilities and also a higher maximum throughput for high input count rates (Canberra Industries 2007).
Measurements were performed at nine equidistant positions spaced about 5.1 m apart covering a square area of 103 m2 at each location in Houston. The detector was placed about a foot above the surface of the ground, and each measurement point was identified with position markers and verified with GPS latitude and longitude coordinates within an accuracy of 5 m (Night Sky Tools, 2016). Two measurements were performed for 10 min at each position to accumulate sufficient statistics to record a uniform cumulative dose and dose rate. The dead time of the detector was less than 0.2% for all measurements considered in this study. Radionuclide identification was also performed using the Genie2000 Spectroscopy Software with a fixed gain shift tolerance (i.e., maximum allowed shift of gain in nuclide analysis) of 5% and correlation threshold of 0.8. Note that the correlation threshold is the minimum correlation coefficient necessary to ensure nuclide identification. A value of 1 indicates a perfect correlation (Canberra Industries 2012). Another criterion used in the spectral analysis was the requirement of a single gamma energy emission line from a radionuclide within a 5‐keV energy tolerance [i.e., a full width at half maximum (FWHM) multiplier]. The FWHM multiplier was numerically set to 2 to the left and right of the region of interest (ROI) and, thus, used to resolve peak identification in the spectra. In this study, any radionuclide that passed all analysis identification criteria at the 95% confidence level or greater was identified in our spectra. The 40K peak was the only radionuclide that was clearly identified in our measured spectra.
RESULTS AND DISCUSSION
Fig. 1 shows the measured gamma dose rate versus the latitude and longitude coordinates combined with a geographical map of the selected measurement sites in Houston, TX. The zero was suppressed on the vertical axis to enhance differences in the gamma dose rates.
The gamma dose rate was averaged over all measurements using the following fundamental expression:
is the average dose rate,
is the dose rate of the i-th measurement, and N is the total number of measurements. The standard deviation, σ, of each measured value was calculated using the following expression:
The dose rate per unit area was also determined by normalizing the average dose rate to the total measurement area (i.e., 103 m2) at each location. Of note is that the measured gamma dose rate varied less than 10% across all sites. Table 1 lists the measured results including the latitude and longitude coordinates, dose rate, and dose rate per unit area.
An example of a measured gamma spectrum at the Gulfgate location in Houston is shown in Fig. 2 indicating the pronounced 40K spectral peak. The 40K peak was the only radionuclide identified by the analysis software from Canberra in the spectra at all measurement sites.
Students at Texas Southern University were mentored on a research project contiguous with its health physics program, where the background gamma dose rate was measured at designated locations within the I‐610 loop in Houston, TX. The gamma dose rate per unit area was observed to be about 1 nanosievert per square meter per hour on average. The data were nearly uniform at all measurement sites; though, a maximum gamma dose rate of 0.120 ± 0.003 μSv h−1 was measured at the Washington Avenue location. 40K is the most widespread radionuclide of naturally-occurring potassium and was consistently observed in the measured spectra. Additional radionuclide identification was mainly limited by the energy resolution of the Canberra InSpectorTM 1000’s LaBr3 scintillation crystal. On average, the gamma dose rate was collectively 0.114 ± 0.001 μSv h−1 for all measurement sites considered in this study. The mean gamma dose rates measured in this work were nearly a factor of two larger than the world average of 0.060 μSv h−1 (UNSCEAR 2000, 2010), yet consistent with dedicated regional measurements elsewhere (Yoshimura et al. 2004 ; Jones and Paulus 2008 ; Anjos et al. 2011 ; Gaso et al. 2013 ; Manohar et al. 2013 ; Zhang et al. 2013 ; Garba et al. 2015 ; Rangaswamy et al. 2015 ; Abdalhamid et al. 2017). Further studies are planned for the future with a high purity germanium detector to enhance the spectral peak resolution for better radionuclide identification and also to pinpoint the source of the larger gamma dose rates within the greater Houston area compared with the world average.
The authors would like to thank Daniel Vrinceanu for assistance with Mathematica plotting usage. Also, the authors thank Robert Emery for helpful comments on this manuscript. Specifically, this study was financially supported by the U.S. Nuclear Regulatory Commission (Grant No. NRC-HQ‐84‐14‐G‐0018). The authors report no conflicts of interest and are responsible for the writing and content of this paper.
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