Neutron Activation Analysis

Introduction

Within the Berkeley Radwatch program, we have generally focused on measurements to study - mostly naturally occurring - radioactivity in samples that are part of environment and food (see Gamma Sampling Data). This was done by so-called passive means employing very sensitive high-purity germanium (HPGe) gamma-ray spectrometers. We can extend these measurements using nuclear technology to look beyond the naturally occurring radioactive materials in samples and study the presence of non-radioactive elements that can be made radioactive through irradiation with neutrons in a process called neutron activation.

We hope that these new measurements will help to illustrate some of the powerful methods for understanding our environment utilizing nuclear technologies and improve awareness of some of the non-radioactive materials we are exposed to in our environment and food which could potentially provide a health hazard.

Global Impact

While we focus on samples collected through local Bay Area fish markets, it is important to recognize the availability of marine life sourced from around the world. To that end, a portion of our investigation has focused on uncovering details of the elemental composition of marine life at a global scale. In this process, global and local trends are explored; these trends include the impact of geological phenomena on the regional aquatic life as well as the correlation between sample size and elemental composition.

Background

This Neutron Activation Analysis (\(N_{AA}\)) provides very sensitive means to determine the elemental composition, specifically the concentrations of trace metals in a wide range of objects and samples. We have initiated measurements to study the composition of some of the samples we have previously analyzed with passive counting, including kelp, fish, and other sea life along the west coast of the Americas. The analysis of the \(N_{AA}\) data enables the quantification of trace metals such as mercury or arsenic in these samples down to below ppm concentrations, dependent on the specific element. The \(N_{AA}\) results also provide the opportunity to compare trace metal concentrations in samples across different locations and over time. In addition, we can compare the results to other measurements and to regulatory limits to see whether the concentrations we observe can potentially pose a health hazard.

\(N_{AA}\) is a non-damaging and relatively simple analysis method that allows for very accurate and precise measurements of chemical composition. Samples are irradiated with a high neutron flux, which activates all neutron capturing isotopes within the sample. These activated isotopes then decay, emitting gamma rays at characteristic energies specific to that isotopes, similar to the passive method using HPGe spectrometers. Based on the known natural abundance of the observed isotopes we can estimate the total elemental composition of the corresponding element in the sample. The details of this analysis technique are described in the Technical Details section.

Does this method pose a risk to the analyzers?

Most activated isotopes usually have half-lives roughly a day long, which means that these isotopes will remain radio-active long enough to acquire a measurement with good statistical accuracy. While these half-lives allow for accurate measurements, they can also present a safety hazard due to radiation exposure. However, we do not expect to deal with hazardous isotopes nor activities large enough to pose significant radiation hazards.

Summary of Results

These results demonstrate the effectiveness of this analysis method, allowing us to determine the elemental composition of many sea samples. Using a relatively simple activation method, we were able to determine the concentrations of thirteen different elements, including arsenic, mercury, and bromine. Normally, it's difficult to chemically measure bromine, for example, due to interference from other halogens like chlorine. Neutron activation occurs regardless of chemical properties since the only relevant property is the neutron capture cross section.

In order to map out a global distribution of elemental composition, 16 different aquatic samples from places all around the globe were collected. The map below (Figure 1) show the results at the approximate locations where our samples were originally sourced. Click on a given location to see the fish type and a summary of the results for that sample. These results will be discussed in more detail below.

Reference and Limit Comparisons

The charts below show the elemental concentrations in parts-per-million (ppm) for each of the 16 fish samples with comparisons to related reference samples as well as environmental or federally regulated limits when applicable. Referenes for these comparisons are obtained from a range of sources, with sample specific details described in the figure text for that sample. The same FDA reference, which provides mercury (Hg) levels across dozens of species, was used for all samples [1]. For samples containing cesium (Cs), the comparison was referenced from a fish otolith study [2]. Comparisons of our results with this reference are not expected to show good agreement because cesium behaves like potassium, getting absorbed into the nervous system and muscle tissue, leading to wide variations across samples. In most cases, the arsenic (As) references are from a publication on metal concentrations in aquatic life in China [3]. In instances where no suitable reference for the sample fish species was provided, the environmental limits were used. The cobalt (Co) and selenium (Se) references came from an environmental study [4]. The potassium (K), rubidium (Ru), and iron (Fe) references were extracted from a study of trace metals in fish [5]. The sodium (Na) was referenced from a journal article on mineral content in common fish species [6].

Most of the elements highlighted were found to have regulatory limits, which are also shown in comparison with the elemental concentrations measured in our samples. In most cases, the elemental composition of our samples fell bellow the regulatory limits, with a few exceptions. In particular, in some cases our samples exceded the limits for antimony. The regulations on antimony (Sb) are a bit vague as noted by the W.H.O. since its contributions to health are still unclear [7]. It is believed that antimony may be a carcinogen and it is best to set a lower limit for this element for precautionary reasons, so current limits are very conservative. Not all elements (such as scandium) have references or regulatory limits but those elements do not pose a threat to the consumer since they are not present in hazardous quantities.

Although the regulatory limit for potassium and sodium is not available, the restrictions applicable to these elements are dietary and therefore it is not necessary to impose a regulatory limit on ppm in individual fish species. Bromine, similar to rubidium, doesn’t have a meaningful regulatory limit since the amount present in seafood is not present in quantities considered a risk potential. Rubidium’s regulatory limit of 23 ppm is only an estimate of a dietary average but can be exceeded. Also, non-radioactive cesium, which in only very midly toxic, is not expected to occur in quantities high enough to be harmful. Because potassium can be confused with cesium in the body, the only real risk of excess cesium would be potassium deficiency, but at levels not generally encountered in natural sources [cite].

The sources for reference samples and environmental limits are summarized in our References section.

Mercury vs Fish Size

Aside from the geographical significance of the samples, there appeared to be a correlation between sample size and mercury concentration. This result is expected since mercury bioaccumulates [11]. This means that mercury is absorbed faster than it is excreted. Therefore, the farther up the food chain a species lies, the more mercury it will accumulate from species below it. The graph in figure 3 demonstrates the bioaccumulant trend found in our samples.

Figure 3: Comparison of fish size versus the mercury concentration in the 14 samples with detected mercury. In blue are the results for all fish samples sourced from the wild, and in red are those sourced from farmed fish supplies. The size ranges for each fish type, indicated by the horizontal error bars, were obtained from [30-41].

The horizontal uncertainty on the graph signifies the range of size the fish species has with the center point being the average. The vertical uncertainty is the uncertainty of the calculated measurements. Although not very explicit, the general trend seems to show that as the average weight increases, so does the mercury concentration. It also appears that farmed samples contain less mercury than the wild samples. The reason for this is likely due to the controlled diets of the farmed fish. Large farmed fish are fed herring and anchovies while smaller fish are fed corn [12].

Global Trends

The maps below (Figure 4) show the relative concentration for the indicated heavy metal at the approximate locations where the samples containing that element were originally sourced. The relative concentrations are indicated by the size of the circles at each location, and clicking on the circle will show the measured concentration at that location. Select locations will provide a more detailed description of why the elemental concentrations may vary significantly from other locations.

We found a good degree of geographical significance for several elements. In those cases, the concentration were found to be much higher in certain regions. This is likely due to local mining or agricultural activity or an increase in the ores for that element in that region. We have highlighted these in the selection of which elements are focused on in Figure 4, specifically Arsenic, Bromine, Mercury, Cesium, and Rubidium.

There was a strong trend found for the presence and concentration of cesium, which was most abundant in the coastal regions of the northern Pacific. It is important to clarify that the cesium found in these fish samples is not a result of the Fukushima accident. This was carefully studied by looking at the concentration of cesium-137 (Cs-137) in these samples. Cesium-137 is a key indicator of waste contamination from nuclear reactors or nuclear bomb testing. The lack of Cs-137 found in these samples confirms that the cesium found in this study is a result of environmental factors unrelated to radiological sources. One likely explanation is the precence of higher than average levels of naturally occurring cesium in Cananda, where there are large deposits in the form of pullcite ore [13]. This cesium is then absorbed by fish, where it can mimic potassium in the body and concentrate in the muscle tissue and nervous system. In small amounts, both cesium and rubidium can help aid the nervous system, only becoming toxic at much higher levels. Both elements, but especially rubidium, can help to prevent cell toxicity and cell damage [14].

We also found interesting geographical variations in the level of arsenic, one of the more toxic heavy metals we investigated. Several samples were found to have significant levels of arsenic, though only one exceeded the regulatory limits, a Korean sample collected from the East China Sea. In the regions where higher levels of arsenic were found, it is typical to see high levels of arsenic as a result of human activity. For example, there are several arsenic mines in East Asian countries, such as the Koreas, which have lead to elevated levels of arsenic in local species. There have been several warnings released in regions affected by arsenic mines, and one study noted "The oxidation process of arsenic-bearing sulfide ores has been noted as a rsk factor for the release of inorganic arsenic into soil and water in the vicinity of mines" [15]. In Norway, elevated levels of arsenic have caused local residents to closely monitor their diet and reduce their fish intake, especially in children.

We found a higher concentration of rubdium around the South China Sea. Rubidium is a non-toxic trace metal and is usually absorbed into the body as a potassium substitute. Levels in this region are likely high as a result of tin-bearing granite deposits, which were discovered in the Malay peninsula. These deposits are rich in rubidium and other elements such as lithium and boron [16], which may be released into the nearby waters.

Beyond mines and ore deposits, element concentrations can be influenced by the species' particular habitat. Bromine, an element found in abundance in the Earth's crust, is prominent in several fish species, but in particular in those species whose habitat is generally close to the sediment. For example, the Arctic Chilean Sea Bass live in deep water during the winter and migrate towards shallow water in the warmer seasons [17]. This species spends most of its life near the coast, which could explain the high levels of bromine seen in this sample. Similarly, shrimp spend most of their time near the seafloor and estuaries, where large amounts of sediment are present, and the Venezualian HD shrimp sample collected was found to have the highest levels of bromine of all our samples.

There is no regulatory limit for bromine relevant to these samples. Naturally occurring bromine in aquatic life is not present in concentrations considered potentially harmful. However, there is a potential risk of toxicity due to organic bromine consumption. Naturally occurring, inorganic bromine is located in sediments and is of the type that species that tend to stay near the ocean floor, such as shrimp, would consume. Organic bromine, on the other hand, is man-made and its occurrence in the environment is due to pollution. Organic bromine is used for insecticides and fireproofing materials. The widespread use of organic bromine has caused an accumulation of this toxic material in the environment. The liquid form of organic bromine can be corrosive to skin and the vapor form is toxic when inhaled [18]. Neutron activation cannot differentiate between organic and inorganic bromine, as there is a lack of information regarding the particular molecule inwhich the bromine is present. Organic bromine is created in compounds containing carbon, which is difficult to detect using \(N_{AA}\). Such limitations prevent the classification of the bromine measured, however due to the relationship between habitat and bromine concentrations seen in these samples, it is safe to assume that most of the bromine detected is due to naturally occurring inorgainc bromine, and no cause for concern.

Limitations of the \(N_{AA}\) Method

While neutron activation can be a powerful tool for determining the presence and concentration of heavy metals in samples, avoiding complicated chemical extraction procedures and giving access to elements undetectable through other means. However, it is important to note that there are limitations to the power of this procedure. The primary limitation of \(N_{AA}\) is that it and cannot provide information about certain light elements that would allow for a more detailed exploration of the particular sources of the elements found, including Boron, Oxygen, Carbon, and Aluminum, due to their extremely short half-lives. The need for a high flux neutron source also limits the accessibility of this method.