Gamma-ray analysis of 2011, 2012, and 2013 Red Salmon Samples
January 30, 2014
This summarizes the gamma-ray analyses of three Red Salmon samples caught in the Kenai River in Alaska. Three samples, one from 2011, 2012, and 2013 were analyzed at the LBNL Low Background Facility. A small amount of Fukushima-sourced radio-cesium was found in the July 2011 sample, likely from the initial airborne releases of radioactivity in Spring of 2011. No Fukushima-related radioisotopes were found in the 2012 and 2013 samples. In all samples, naturally radioactive potassium absolutely dominated the gamma spectrum. Additional commentary is included as explanation for those not as familiar with gamma counting.
Table of Results:
|Sample Type||Location||Date Collected||Date Measured||Sample Size||Cs-134 [Bq/kg]||Cs-137 [Bq/kg]||K-40 (natural) [Bq/kg]||Pb-210 (natural) [Bq/kg]||Bi-214 (natural) [Bq/kg]|
|Red Salmon||Kenai River, AK||July 2011||01/07/2014||370.9 g||0.04(1)||0.23(1)||111.5(7)||0.36(4)||0.092(14)|
|Red Salmon||Kenai River, AK||July 2012||01/09/2014||629.8 g||n.d. < 0.006||0.14(1)||121.4(6)||0.48(5)||0.017(4)|
|Red Salmon||Kenai River, AK||July 2013||01/17/2014||771.6||n.d. < 0.008||0.14(1)||121.1(8)||0.45(7)||0.034(12)|
** Note: n.d. indicates that the listed isotope was 'not detected' and a one sigma upper limit is listed.
The 2011, 2012, and 2013 Red Sockeye Salmon samples were analyzed at the LBNL Low Background Facility. The samples were sent via overnight shipping from Alaska to Berkeley, CA. Each fish sample was caught in July of its respective year. The samples consisted of: one fillet from 2011, several filets from 2012, and three fillets from 2013; all caught in the Kenai River. The analysis for these samples was inspired, in part, by a YouTube video showing salmon samples being tested with a Geiger counter and showing excess counts over background. What that gentleman was seeing, most likely, was variation in backgrounds in the room or perhaps 40K in the fish.
All samples were counted in the Low Background Facility, on a High Purity Germanium (HPGe) detector. This detector is set up to be a very low background system, which allows it to detect very small amounts of radioactivity as compared to other systems. It is in a laboratory space constructed of very thick, low-radioactivity concrete and is equipped with an active veto system to reduce cosmic-ray muon induced backgrounds. What a system such as this offers over typical Geiger counters, in addition to the low background environment that increases its sensitivity, is that it carries spectroscopic capabilities. A Geiger counter will only detect radioactivity—but not tell you anything about it—was it natural, unnatural, or what energy did the gamma ray carry? HPGe detectors allow you to separate gamma rays according to energy, so one can make precise identification and quantification of its source. At low levels, natural variations of primordial radionuclides found in essentially anything can produce excess counts on a Geiger counter that can easily be interpreted as ‘contamination.’ Or for instance, a Geiger counter would not see a small amount of man-made radioactivity if there is also a large amount of natural radioactivity present. HPGe detectors, however, have the ability to separate these sources and produce more detailed information.
Note that what is discussed here will simply be the radioactivity of the sources and not the induced dose. Often these are quite different. In all of the samples pictured here, the dominant activity dominating the gamma-ray spectra was natural potassium 40K. Also detected are varying activities of uranium and thorium series isotopes, which contain many α emitters. Alpha particles quite often present the highest dose to tissues, as they do the largest amount of damages to cells. Outside of the body, these are often not a major issue as they cannot make it through the skin, for instance. But inside the body is another story. One of the significant sources of a natural yearly dose comes from the inhalation of radon and radon daughters, found in both the uranium and thorium series as 222Rn and 220Rn, respectively. In the case of 222Rn, it decays through a series of α and β emitters that present their primary exposure when they embed themselves in the lungs and emit these particles directly upon sensitive tissues. It is the same with isotopes ingested as food into the body. With these salmon samples, although 134,137Cs was detectable in some samples, it was far below the levels of natural 40K. After converting to dose, however, you would find that even the 40K dose would be far below the dose from natural α emitters in the samples. One isotope alone, 210Po, was found by Fischer, et. al.  to dominate the dose from tuna found with Fukushima-derived radioisotopes by many orders of magnitude. (We highly recommend reading that paper.) Their dose calculations were based off of the migratory studies by the same group in the papers from Madigan et. al.  & .
The samples arrived via FedEx overnight to Berkeley, CA. Upon arrival, they were immediately stored in a freezer in the Nuclear Engineering department until they could be processed. They were all still frozen upon arrival. Although they could have been analyzed raw, we first baked them to drive off as much moisture as possible, for the sake of keeping the samples fresher. Each was counted for more than 24 hours, so leaving them raw in our containers would have allowed them to start decaying. By drying them out, they could hold up to being out in the open for several days without refrigeration. The mass used in the analysis was the wet or frozen weight of the fish — not the dry ’baked’ weight, which was considerably less. The processed samples, consisting of one or two fillets each, were baked for at least an hour at 400◦F to dry them out as much as possible. After baking, they were pulled apart into small pieces and placed into a beaker for analysis, as seen in Figure A. Both the scales and muscle tissue were used in the gamma counting.
Left: Preparation of the 2013 salmon fillets, being weighed prior to baking.
Right: The 2013 salmon fillets after baking and loaded into an analysis container for gamma counting.
Discussion on each Sample:
2011 Salmon: The sample analyzed consisted of one fillet. The natural radioactivity of the potassium in the fish is over 500 times the 137Cs, and nearly 2500 times the activity of the 134Cs.
The FDA Derived Intervention Level for imported foods is 1200 Bq kg−1 (137Cs + 134Cs combined)  — this sample is below that by a factor of over 4000. The critical limit set by the FDA for either cesium isotope is 370 Bq kg−1. Even invoking very strict limits, such as one in Japan right now for seafood at 100 Bq kg−1 combined, also agrees that the values for Cs are quite small. (This fish is still below that by a factor of nearly 400.)
Based upon the latter samples, Fukushima is responsible for around 60% of the 137Cs in this fish, and all of the 134Cs. The rest of the 137Cs is from pre-Fukushima, legacy sources (surface weapons testing in the cold war).
Given that this fish was caught in July 2011, and the primary Fukushima release was airborne in March of 2011, and that the fish was caught in a river — initial thought is that the 134Cs and 137Cs in the fish that is Fukushima-derived may be a result of the fish being exposed to it in freshwater. The airborne fallout likely came down across wide regions and the annual runoff of melting snow later concentrated it into the rivers and streams during the spring melt where the fish absorbed it. Therefore, salmon's exposure may not have been from its life in the Pacific Ocean, but rather from the airborne fallout collecting/concentrating (still at small levels) in river water. However, depending on the migration habits of the fish, it is also possible that it could have been a function of migration such as the tuna study from Madigan, et. Al [2, 3].
2012 Salmon: The sample analyzed consisted of two fillets. 134Cs was not detected in this sample. A one sigma upper limit is provided. The 137Cs present is attributed to only pre-Fukushima, legacy sources. See Figures 6 and 7.
2013 Salmon: The sample analyzed consisted of two fillets. 134Cs was not detected in this sample. A one sigma upper limit is provided. The 137Cs present is attributed to only pre-Fukushima, legacy sources. See Figures 8 and 9.
With these samples, although 134Cs and 137Cs are >detectable this does not mean they are very strong. Rather, it is only because our low background HPGe detector is so sensitive at detecting these minuscule activities that we can even see them at all. We also tested a series of fish purchased in Bay Area retail locations. It is currently in the process of being typeset in a journal, but a proof can be found on arXiv.org . Other Fukushima-related measurements by our local groups can be found in  and .
Within this document, we choose to emphasize the natural 40K activity as a direct comparison to 134,137Cs because: (1) referencing governmental limits are often quite confusing to the public; (2) comparing and discussing dose rates are confusing to the public, and sometimes the calculations for them change over time as new standards are put into place; (3) potassium and cesium behave similarly so they serve as good proxies for each other since organisms effective at absorbing Cs would also be expected to absorb K well, and vice versa. The combined 134+137Cs found in the salmon was far, far below the 1200 Bq/kg Derived Intervention Level by the FDA . Also, please note that we are not affiliated with the FDA, EPA, or any other accredited analysis laboratory related to food, etc. However, we do have great expertise in the measurement of small amounts of radioactivity.
Frequently Asked Questions
Q: Are the radiation levels found in the Salmon safe to eat?
A: Yes. The levels of Cs-134,137 are far below even the strictest of limits, and are minuscule compared to the natural radioactivity present.
Figure 1: A bar chart showing the activities within the Salmon samples. Note that the y-axis is in a log scale, which allows you to more easily see small and large values at the same time. Figure 2shows the same plot in linear scale, which demonstrates how dramatic the 40K activity is over the others.
Figure 2: A bar chart showing the activities within the Salmon samples. Note that the y-axis is in a linear scale, which shows how small the other activities are compared to the 40K activity.
Figure 3: The 2011 Salmon sample, at the full gamma energy range. Note the y-axis is in linear scale, which clearly demonstrates the amount of K40 relative to other radioisotopes present.
Figure 5: The 2011 Salmon sample, zoomed upon the energy region where Cs-134, 137 isotopes are present.
Figure 6: The 2012 Salmon sample, showing the full energy spectrum. Only the Cs-137 peak from legacy sources was detected, the rest are from natural radioactivity.
Figure 7: The 2012 Salmon sample, showing energy spectrum zoomed upon the Cs region. Only the Cs-137 peak from legacy sources was detected, the rest are from natural radioactivity. No Cs-134 was present in this sample.
Figure 8: The 2013 Salmon sample, showing energy spectrum zoomed upon the Cs region. Only the Cs-137 peak from legacy sources was detected, the rest are from natural radioactivity. No Cs-134 was present in this sample.
Figure 9: The 2012 Salmon sample, showing the full energy spectrum. Only the Cs-137 peak from legacy sources was detected, the rest are from natural radioactivity.
 N.S.Fisher, K.Beaugelin-Seiller, T.G.Hinton, Z.Baumann, D.J.Madigan, J.Garnier-Laplace, Evaluation of radiation doses and associated risk from the Fukushima nuclear accident to marine biota and human consumers of seafood, Proceedings of the National Academy of Sciences (2013). http://dx.doi.org/10.1073/pnas.1221834110.
 D.J.Madigan, Z.Baumann, N.S.Fisher, Pacific bluefintuna transport Fukushima-derived radionuclides from Japan to California, Proceedings of the National Academy of Sciences (2012). http://dx.doi.org/10.1073/pnas.1204859109.
 D.J.Madigan, Z.Baumann, O.E.Snodgrass, H.A.Ergl, H.Dewar, N.S.Fisher, Radiocesium in Pacific bluefin tuna thunnus orientalis in 2012 validates new tracer technique, Environmental Science & Technology 47 (2013) 2287–2294. http://dx.doi.org/10.1021/es4002423.
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 E.B.Norman, C.T.Angell, P.A.Chodash, Observations of fallout from the Fukushima reactor accident in San Francisco Bay Area rainwater, PLoS ONE 6 (2011) e24330. http://dx.doi.org/10.1371/journal.pone.0024330.
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 U.S. Food and Drug Administration, CPG Sec. 560.750 Radionuclides in Imported Foods - Levels of Concern, 2005. Accessed 2013-11-07. http://www.fda.gov/ICECI/ComplianceManuals/CompliancePolicyGuidanceManua....