210Pb Analysis Research Articles

Analysis of 210Po, 210Bi, and 210Pb in atmospheric and oceanic samples by simultaneously auto-plating 210Po and 210Bi onto a nickel disc

210Po and 210Pb are commonly measured to study particle cycling and particulate organic carbon export (POC) flux from the upper ocean. 210Bi is a potential oceanographic tracer. However, no convenient and rapid analytical method for 210Bi has been developed in the marine environment due to its short half-life (5 days). The aims are to study factors influencing the simultaneous auto-plating of 210Po and 210Bi onto nickel disc and to develop an effective ship-board procedure for the rapid measurement of 210Po, 210Bi, and 210 Pb at sea. The results suggest that the optimal conditions for auto-deposition of 210Po and 210Bi were achieved by using a 25 mm diameter nickel disc in 60 mL acidic solution (pH = 0.5) for 16 h at 25 ± 1 °C. By adding 209Po and 207Bi as yield tracers to the sediments, recoveries of Po and Bi were >90%. If 210Po or 210Bi were in equilibrium with 210Pb, 210Po and 210Bi could also be potential proxies for 210Pb in sedimentary chronology. The overall recoveries of 209Po and 207Bi were >70% for rainwaters and >60% for seawater samples, respectively, which indicates this analytical procedure is also applicable to marine and atmospheric environments. This method allows the rapid measurement of 210Bi on a ship and help to obtain its vertical profile or spatial distribution in the marine environment within 1–2 days.

Long-lived radionuclides in marine sediments from the Beibu Gulf, South China Sea: Spatial distribution, controlling factors, and proxy for transport pathway

The source and transport process of sediments are of great concern in marine sedimentology. In this study, naturally occurring radionuclides (238U, 226Ra, 228Ra, and 40K) in surface sediments and sediment cores were comprehensively analysed using high-purity germanium (HPGe) γ spectrometry in order to explore radionuclide fingerprints as novel proxies for tracking the source and transport pathway of sediments in the Beibu Gulf. We found that the radioactivity level of sediments in the Beibu Gulf was lower than that in other Chinese seas, following a comprehensive compilation and comparison of radioactivity in the region, indicating the Beibu Gulf is a high-quality marine environment. An interesting “butterfly pattern” was exhibited in the horizontal distribution of radionuclides in surface sediments from the Beibu Gulf, mainly attributable to the controlling factors of the non-linear negative regulation of sediment grain size and positive role of total organic carbon in sediment. Furthermore, the vertical distribution of radionuclides in sediment cores was affected by the redox state and a physical/biological disturbance, which were demonstrated by the simultaneous analysis of redox-sensitive element Mn and 210Pb, respectively. Additionally, the 226Ra238U endmember model and 232Th238U endmember model were proposed for quantifying local input (85%) and trans-region input (15%) of marine sediments in the Beibu Gulf in the context of the Guangdong-Qiongzhou Strait-Guangxi framework. We inferred that the Beibu Gulf was at risk of contamination by artificial radionuclides derived from nearby Guangdong Province, which has the highest number of nuclear power plants in China, through a trans-region input by ocean currents. Our study not only provided spatiotemporal distributions of radionuclides in the Beibu Gulf and their controlling factors (sediment grain size, total organic carbon, redox state, physical/biological disturbance, and ocean currents) but also indicated the radionuclide fingerprints (226Ra/238U and 232Th/238U) as potential geochemical proxies for quantifying the distinct sources of radionuclides and other associated chemicals (e.g., heavy metals, organic pollutants) in sediments.

Measuring bismuth‐210, its parent, and daughter in aquatic systems

Abstract210Bi (t1/2: 5.01 d)—the daughter of 210Pb and parent of 210Po—has rarely been measured in aquatic systems, and its behavior in the water column is poorly understood. In this article, I present a method for quickly measuring 210Pb, 210Bi, and 210Po in aquatic samples, where (1) 210Bi and 210Po are scavenged onto an anion solid‐phase extraction disk within 15 min of pretreating the sample; (2) beta decay of 210Bi is counted on the disk immediately thereafter; (3) 210Po is subsequently removed from the disk and redeposited on a copper plate for α‐spectroscopy; and (4) 210Pb is determined via the ingrowth of 210Bi. I present decay‐corrected calculations for total, dissolved, and particle‐bound fractions of each nuclide and conclude with an analysis of 210Pb, 210Bi, and 210Po activities in rain, dreissenid (quagga) mussels, and water samples from the Milwaukee Inner Harbor in Lake Michigan. Results show that the loss of lead on the anion solid‐phase extraction disks was negligible (0.2% ± 2.1%; ± 1 SD, n = 4), and the sorption of bismuth was complete (99% ± 2%; ± 1 SD, n = 16). Relative mean absolute deviations of duplicate sample analyses of lake water were 2.4% ± 1.9% for 210Pb (geometric mean of total sample activity: 3.0 disintegrations per minute [dpm], n = 6), 7.7% ± 5.8% for 210Bi (geometric mean of total sample activity: 2.6 dpm, n = 8), and 2.7% ± 1.7% for 210Po (geometric mean of total sample activity: 1.4 dpm, n = 8).

Atmospheric deposition of 7Be, 210Pb and 210Po during typhoons and thunderstorm in Shanghai, China and global data synthesis

Atmospherically-delivered 7Be, 210Po and 210Pb in bulk precipitation and air samples collected around the globe have provided valuable quantification on the rates of removal, as well as proportional mixing of attendant air masses; however, such studies during thunderstorm and typhoon events are limited. We report the first continuous time-series rainwater sampling and analysis of 7Be, 210Pb and 210Po from two typhoons and one thunderstorm during 2015 summer in Shanghai. The depositional fluxes within individual rain events of typhoons and thunderstorms varied by a factor of 10 for 7Be, 5.7 for 210Pb, 7.4 for 210Po, and 7.0 for 7Be/210Pb activity ratios (AR). Such large observed variations in the depositional fluxes of 7Be, 210Pb, 210Po and 7Be/210Pb activity ratios were attributed to air masses injected from surrounding high pressure system adjoining the typhoon to low pressure system within the typhoon. Based on 7Be/210Pb activity ratios, we estimated the variations in the fraction of maritime and continental air masses into the typhoon. Observed constancy in the 210Po/210Pb AR indicates that the residence times of air masses contributing to the typhoon during heavy rain are similar. From a synthesis of global fallout of 7Be and 210Pb during pulse events (precipitation≥50 mm from single rainout event), we quantify the importance of pulse events in the atmospheric fallout of these radionuclides.

Direct analysis of 210Pb in drinking water by liquid scintillation counting after sulfate precipitation

Lead-210 in drinking water can be rapidly determined by liquid scintillation counting (LSC) using a new sulfate precipitation method. In this method, 210Pb was first preconcentrated from water using iron hydroxide co-precipitation followed by sulfate precipitation to decontaminate most of non-alkaline earth elements. The Pb in the sulfate precipitate was then dissolved in strong alkaline solution, while alkaline earth elements (Sr, Ba, Ra) were separated as the sulfate/carbonate precipitate. To optimize the method, the influences of different acids and sulfate concentration on sulfate precipitation as well as the effects of pH, the added SO42−/CO32− concentrations and the Sr/Ba contents in the sulfate precipitate on the dissolution of PbSO4 were studied. In addition, X-ray fluorescence (XRF) analysis was applied for rapid determination of the chemical recovery and triple-to-double coincidence ratio (TDCR) quench correction curve for counting of 210Pb by LSC was established. The method was validated using 5 L of tap water samples and minimum detectable activity concentration (MDC) of ∼0.016 Bq·L−1 was achieved, which is sufficiently sensitive to meet the guidance level of 0.1 Bq·L−1 for 210Pb in drinking water as recommended by the World Health Organization (WHO).

Cryoconites from Alpine glaciers: Radionuclide accumulation and age estimation with Pu and Cs isotopes and 210Pb

Cryoconites (“cold dust”, derived from the Greek) are aeolian sediments accumulated on glacier surfaces. In cryoconites from the surface of the Stubacher Sonnblickkees, a temperate Austrian glacier, extremely high activity concentrations of artificial and natural radionuclides were found. Artificial radionuclides stem from two clearly distinguishable sources, global fallout from the nuclear weapons testing era deposited over a period of years until roughly 1966 and the fallout from Chernobyl in 1986, which was essentially deposited as a single input during one week. Anthropogenic radionuclides identified were 137Cs, 134Cs, 238Pu, 239+240Pu, 90Sr, 241Am, 60Co, 125Sb, 154Eu, and 207Bi. The naturally occurring radionuclides detected were the long-lived radon decay product 210Pb, the primordial radionuclide 4 K and the cosmogenic 7Be.Isotopic ratios of 134Cs/137Cs and 239+240Pu/238Pu were used to separate the nuclide inventory into the contributions of the two aforementioned sources, which show varying degrees of mixing and provide information on the mixing age of the cryoconites. Since isotopic ratios of Pu often have high uncertainties due to low absolute concentrations, age estimation based on this method can be quite inaccurate. Additional information about the age of cryoconites was obtained through analysis of 210Pb, which is constantly deposited over time.

Historical Characteristics of Contamination and Risk of Heavy Metals in Weishan Lake, China.

Combined with the dating analysis of 210Pb and 137Cs, the historical concentration and risk of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn in the core sediment of Weishan Lake were determined. Pollution concentrations, enrichment factor, geo-accumulation index, and ecological risk index of these metals and C/N ratio indicate that the natural inputs prevailed till early 1900s. From late 1970s onwards, the risk posed by these metals was significantly increased. Cu and Hg might cause adverse biological effects. The increase of soil erosion and wastewater discharge in the watershed promoted input and accumulation of these metals in the lake. The coal and petroleum industry in the catchment led the more accumulation of Hg, Cd and Cu in the lake.

Quantitation of lead-210 (210Pb) using lead-203 (203Pb) as a “Massless” yield tracer

Determination of Pb-210 (210Pb) in aqueous solution is a common radioanalytical challenge in environmental science. Widely used methods for undertaking these analyses (e.g., ASTM D7535) rely on the use of stable lead (Pb) as a yield tracer that takes into account losses of 210Pb that inevitably occur during elemental/radiochemical separations of the procedures. Although effective, these methods introduce technical challenges that can be difficult to track and potentially introduce uncertainty that can be difficult to quantify. Examples of these challenges include interference from endogenous stable Pb in complex sample matrices; contamination of stable Pb carrier with 210Pb; and high detection limits due to counting efficiency limitations. We hypothesized that many of these challenges could be avoided by the use of the electron-capture, gamma-emitting isotope, 203Pb as a chemical yield tracer in the analysis of 210Pb.A series of experiments were performed to evaluate the efficacy of 203Pb as a tracer. Four different matrices were analyzed, including a complex matrix (hydraulic-fracturing produced fluids); and samples comprising less complicated matrices (i.e., river water, deionized water, and tap water). Separation techniques and counting methodologies were also compared and optimized. Due to a relatively short-half life (52 h), 203Pb tracer is effectively massless for the purposes of chemical separations, allowing for reduced chromatography column resin bed volumes. Because 203Pb is a gamma emitter (279 keV; 81% intensity), recovery can be determined non-destructively in a variety of matrices, including liquid scintillation cocktail. The use of liquid scintillation as a counting methodology allowed for determination of 210Pb activities via 210Pb or 210Po; and recoveries of greater than 90% are routinely achievable using this approach. The improved method for the analysis of 210Pb in aqueous matrices allows for the analysis of complex matrices, at reduced cost, while providing greater counting flexibility in achieving acceptable detections limits.

Analysis of 210Pb in water samples with plastic scintillation resins

210Pb is a radioactive lead isotope present in the environment as member of the 238U decay chain. Since it is a relatively long-lived radionuclide (T1/2 = 22.2 years), its analysis is of interest in radiation protection and the geochronology of sediments and artwork. Here, we present a method for analysing 210Pb using plastic scintillation resins (PSresins) packaged in solid-phase extraction columns (SPE cartridge). The advantages of this method are its selectivity, the low limit of detection, as well as reductions in the amount of time and reagents required for analysis and the quantity of waste generated. The PSresins used in this study were composed of a selective extractant (4′,4″(5″)-Di-tert-butyldicyclohexano-18-crown-6 in 1-octanol) covering the surface of plastic scintillation microspheres. Once the amount of extractant (1:1/4) and medium of separation (2 M HNO3) were optimised, PSresins in SPE cartridges were calibrated with a standard solution of 210Pb. 210Pb could be fully separated from its daughters, 210Bi and 210Po, with a recovery value of 91(3)% and detection efficiency of 44(3)%. Three spiked water samples (one underground and two river water samples) were analysed in triplicates with deviations lower than 10%, demonstrating the validity of the PS resin method for 210Pb analysis.

Dramatic loss of glacier accumulation area on the Tibetan Plateau revealed by ice core tritium and mercury records

Abstract. Two ice cores were retrieved from high elevations (~5800 m a.s.l.) at Mt. Nyainqêntanglha and Mt. Geladaindong in the southern and central Tibetan Plateau region. The combined tracer analysis of tritium (3H), 210Pb and mercury, along with other chemical records, provided multiple lines of evidence supporting that the two coring sites had not received net ice accumulation since at least the 1950s and 1980s, respectively. These results implied an annual ice loss rate of more than several hundred millimeter water equivalent over the past 30–60 years. Both mass balance modeling at the sites and in situ data from the nearby glaciers confirmed a continuously negative mass balance (or mass loss) in the region due to dramatic warming in recent decades. Along with a recent report on Naimona'nyi Glacier in the Himalayas, the findings suggest that the loss of accumulation area of glacier is a possibility from the southern to central Tibetan Plateau at high elevations, probably up to about 5800 m a.s.l. This mass loss raises concerns over the rapid rate of glacier ice loss and associated changes in surface glacier runoff, water availability, and sea levels.

Fast procedure for self-absorption correction for low γ energy radionuclide 210Pb determination in solid environmental samples.

Low-energy X and γ radiations (for example of 210Pb: E γ = 46.5 keV) are effectively self-absorbed even in thin environmental samples, including air filters with captured dust or contaminated soil, as well as in bottom sediment matrixes with limited quantities of the samples. In this paper, a simple method for the direct analysis of 210Pb (T 1/2 = 22.3 years) by gamma-ray spectrometry in environmental samples with self-absorption correction is described. The method is based on the comparison of two γ peak activities coming from other natural radionuclides, usually present in environmental samples. We have analyzed the dependence of the self-absorption correction factor for the 210Pb activity on the activity ratios of 911 and 209 keV peaks and 609 and 295 keV peaks coming from nuclides of 238U or 232Th rows, present in typical environmental samples.

A Determination of the Concentration Level of Lead 210PB Isotope in Solid Samples for the Assessment of Radiation Risk Occuring in Coal Mines

Abstract Lead 210Pb, an element of the natural uranium radioactive decay series, is not currently considered a source of radiation risk, especially in a radiation protection system in underground mines in Poland. However, it could be a completely independent element of the radioactive series due to its physical and chemical properties. Routine measurements showed significantly higher than expected concentrations of 210Pb in underground radium rich sediments, based only on the radioactive decay law. This phenomenon implies a need of 210Pb concentration monitoring in such sediments. Nevertheless, the laboratory analysis of 210Pb by gamma radiation spectroscopy is connected with a particular hindrance, the self-attenuation of 210Pb radiation in samples. Current work describes a practical method for obtaining the self-attenuation correction factor in the case of 210Pb concentration analysis. Experimentally obtained correction factors range between 0.51–6.96 cm2/g. Neglecting this factor can cause a significant error or underestimations in radiological risk assessment.

A decade of 7Be and 210Pb activity in surface aerosols measured over the Western Iberian Peninsula

This work presents the analysis of 7Be and 210Pb continuously measured near Lisbon, Portugal, and their correlation with meteorological parameters, between 2001 and 2010. The values are compared against other similar works over the Iberian Peninsula and Eastern Mediterranean countries. Relative Humidity shows the strongest correlation sign, negative. Higher 7Be activity was found in the years 2001, 2003, 2007 and 2009 being the year 2001 a singular one regarding extremes. The use of 7Be as a stratospheric tracer can be interesting to enhance the radionuclide sample frequency due to the meso to local scale of this phenomenon.

Atmospheric mercury deposition recorded in an ombrotrophic peat core from Xiaoxing'an Mountain, Northeast China

The historical mercury accumulation rates (Hg AR) resulting from atmospheric deposition to Xiaoxing'an Mountain were determined via analysis of 210Pb- and 14C-dated cores up to 5000 years old. Natural Hg AR background, pre-industrial Hg AR and maximum industrial Hg AR in Northeast China were 2.2 ±1.0μg/m2/yr for 5100–4500BP, 5.7μg/m2/yr and 112.4μg/m2/yr, respectively. We assumed that the increase in Hg deposition in the Xiaoxing'an mountain area during industrial time was mainly attributed to local anthropogenic emissions around this peat bog.

Temporal changes of 7Be, 137Cs and 210Pb activity concentrations in surface air at Monaco and their correlation with meteorological parameters

Results of analysis of 7Be, 137Cs and 210Pb on aerosol filters carried out from 1998 to 2010 in Monaco show that a weak correlation between activity concentrations of these radionuclides in the atmosphere and meteorological parameters has been found for 7Be and temperature ( r = 0.50), 210Pb and temperature and humidity ( r = 0.43 and 0.41, respectively), and 137Cs and precipitation ( r = 0.51). The minimum and maximum 7Be activity concentrations were observed during 2000 and 2009, corresponding with the maximum and minimum solar activity, respectively. The maximum 137Cs activity concentration found in May–June 1998 was due to the accident at Algeciras in Spain. The deposition velocities of 7Be, 137Cs and 210Pb depended on the precipitation rate, and attained maximum values during dry seasons. The investigated radionuclides may be used as atmospheric tracers, especially in long-term periods.

Distribution and biokinetic analysis of 210Pb and 210Po in poultry due to ingestion of dicalcium phosphate

Dicalcium phosphate (DCP) is used as a calcium supplement for food producing animals (i.e., cattle, poultry and pig). When DCP is produced via wet acid digestion of the phosphate rock and depending on the acid used in the industrial process, the final product can result in enhanced 210Pb and 210Po specific activities (∼ 2000 Bq·kg − 1). Both 210Pb and 210Po are of great interest because their contribution to the dose received by ingestion is potentially large. The aims of this work are to examine the accumulation of 210Pb and 210Po in chicken tissues during the first 42 days of life and to build a suitable single-compartment biokinetic model to understand the behavior of both radionuclides within the entire animal using the experimental results. Three commercial corn-soybean-based diets containing different amounts and sources of DCP were fed to broilers during a period of 42 days. The results show that diets containing enhanced concentrations of 210Pb and 210Po lead to larger specific accumulation in broiler tissues compared to the blank diet. Radionuclides do not accumulate homogeneously within the animal body: 210Pb follows the calcium pathways to some extent and accumulates largely in bones, while 210Po accumulates to a large extent in liver and kidneys. However, the total amount of radionuclide accumulation in tissues is small compared to the amounts excreted in feces. The single-compartment non-linear biokinetic model proposed here for 210Pb and 210Po in the whole animal takes into account the size evolution and is self-consistent in that no fitting parameterization of intake and excretions rates is required.

Historical accumulation rates of mercury in four Scottish ombrotrophic peat bogs over the past 2000 years

The historical accumulation rates of mercury resulting from atmospheric deposition to four Scottish ombrotrophic peat bogs, Turclossie Moss (northeast Scotland), Flanders Moss (west-central), Red Moss of Balerno (east-central) and Carsegowan Moss (southwest), were determined via analysis of 210Pb- and 14C-dated cores up to 2000 years old. Average pre-industrial rates of mercury accumulation of 4.5 and 3.7 μg m − 2 y − 1 were obtained for Flanders Moss (A.D. 1–1800) and Red Moss of Balerno (A.D. 800–1800), respectively. Thereafter, mercury accumulation rates increased to typical maximum values of 51, 61, 77 and 85 μg m − 2 y − 1 , recorded at different times possibly reflecting local/regional influences during the first 70 years of the 20th century, at the four sites (TM, FM, RM, CM), before declining to a mean value of 27 ± 15 μg m − 2 y − 1 during the late 1990s/early 2000s. Comparison of such trends for mercury with those for lead and arsenic in the cores and also with direct data for the declining UK emissions of these three elements since 1970 suggested that a substantial proportion of the mercury deposited at these sites over the past few decades originated from outwith the UK, with contributions to wet and dry deposition arising from long-range transport of mercury released by sources such as combustion of coal. Confidence in the chronological reliability of these core-derived trends in absolute and relative accumulation of mercury, at least since the 19th century, was provided by the excellent agreement between the corresponding detailed and characteristic temporal trends in the 206Pb/ 207Pb isotopic ratio of lead in the 210Pb-dated Turclossie Moss core and those in archival Scottish Sphagnum moss samples of known date of collection. The possibility of some longer-term loss of volatile mercury released from diagenetically altered older peat cannot, however, be excluded by the findings of this study.

Measurement and meteorological analysis of 7Be and 210Pb in aerosol at Waliguan Observatory

Beryllium-7 (7Be) and lead-210 (210Pb) activities were measured from October 2002 to January 2004 at Waliguan Observatory (WO: 36.287°N, 100.898°E, 3816 m a.s.l (above sea level) in northwest China. 7Be and 210Pb activities are high with overall averages of 14.7±3.5 mBq m−3 and 1.8±0.8 mBq m−3 respectively. For both 7Be and 210Pb, there are significant short-term and seasonal variations with a commonly low value in summer (May-September) and a monthly maximum in April (for 7Be) and in December (for 210Pb). The ratio of 7Be/210Pb showed a broad maximum extending from April to July, coinciding with a seasonal peak in surface ozone (O3). The seasonal cycles of 7Be and 210Pb activities were greatly influenced by precipitation and thermal dynamical conditions over the boundary layer, especially for 210Pb. The vertical mixing process between the boundary layer and the aloft air modulates the variations of 7Be and 210Pb at WO in summer. It is indicated that air mass had longer residence time and originated from higher altitudes at WO in the spring-summer time and the winter in 2003. During an event with extremely high weeklyaveraged 7Be concentration (24.8 mBq m−3) together with high O3 levels and low water mixing ratio, we found that air masses had been convectively transported a long distance to WO from high latitude source regions in central Asia, where significant subsiding motions were observed. In another case with the extreme 210Pb activity of 5.7 mBq m−3 high CO2 level and specific humidity (in winter), air masses had come from south China and north Indian regions where 222Rn activities were high. This study, using 7Be and 210Pb as atmospheric tracers, has revealed that complex interactions of convective mixing from the upper troposphere and long-range transports exist at WO.

HISTORICAL CHANGES AND SEDIMENTARY ENVIRONMENT OF IMAZU TIDAL FLAT

The purpose of this study is to demonstrate the process of deposition and influence of human and natural impacts to sediment in Imazu tidal flat. In this study, various impacts on Imazu tidal flat were examined with the use of topographic, aerial photographs, rainfall record. Then, analysis of core sample's gradation, ignition loss, 137Cs and 210Pb was studied to clarify the age of sedimentation and historical change of environment of sediment. The results were as follows. The urban development around Imazu tidal flat has had worse influences on the sedimentary environment. The sedimentation rate of Imazu tidal flat was calculated 0.47-1.12cm/y since 1963, and in 1954-1963 was 1.56-3.56cm/y by the analysis of core samples. It was calculated that 5640-13440m3 sediment was discharged from catchment area annually, and 47520-333720m3 sediment flowed in Imazu tidal flat in 1953 by these results. It was assumed that this was caused by the flood in 1953.

An efficiency calibration for 210Pb and 7Be measurements by gamma-ray spectrometry in atmospheric filters

Natural radionuclides, especially 210Pb and 7Be, condensed on suspended particulate matter in the atmosphere (commonly called aerosols) have been widely used to study a large variety of relevant atmospheric processes. A rapid and non-destructive analysis of 210Pb and 7Be is possible by direct measurement of their 46.5 and 477.6 keV γ-rays, respectively. In this work we have developed an original method to find an empirical function which relates the full-energy peak efficiency with energy in a rectangular geometry for energies above 150 keV. For the energy range below 150 keV only the 46.5 keV of the gamma photon of 210Pb was of interest in this work. A dusty Reference Material (phosphate rock) containing high levels of 238U-series radionuclides, especially 226Ra, was used as calibration sample. This solid matrix (grain size smaller than 63 μm) was taken to impregnate homogenously the filters so that the real geometry of measurement was reproduced. 210Pb and 7Be activity concentrations in surface air were determined in samples collected from July 2004 to June 2005 at three different locations of the Huelva province (South West of Spain): urban, rural, and industrialized areas. 7Be and 210Pb activities have been determined in the PM 10 and TSP fractions of surface aerosols. The radiometric measurements were performed by low-level gamma spectrometry with a thin window coaxial germanium detector.