Methods: All "Std" GM tube sensor values for radiation monitoring sites in the US were tabulated in an excel spreadsheet for the two radiation monitoring web sites during the Oct 19-20, 2015, period. "Pancake" data from RadiationNetwork and identified "pancake" (N=1) and "beta" counting sites from NETC were not included. The RadiationNetwork values appeared to be updated about every minute whereas the ten minute average is reported for each monitoring site by NETC and this more stable value was used. The .xls file of raw data is available upon request from the author (jamesjkeene_at_gmail.com).
Results:
RadiationNetwork About one half of the sites displayed were identified as "Std" Geiger counters (N = 24) with the remainder having more sensitive "pancake" sensors. This data categorization by RadiationNetwork fit well with the observed distribution of CPM (counts per minute). All the Std counts were less than 20 CPM and all of the pancake counts were greater than 25 CPM. That is, the two distributions did not overlap; none of the reported values were between 21 and 24 CPM inclusive. It appears that most of the Std units reporting data may have been products of seintl.com (SI). The mean (average) CPM for the pancake counters was 2.85 times that of the Std counters. The two peaks in the histogram (blue points) in the figure show the distribution of Std (left) and pancake (right) sites.
The author manages a radiation monitoring station in the Commonwealth of Dominica, Windward Islands -- apparently the only such site in the Caribbean. The public may view its CPM and ten minute average on the NETC site which is presently about 10 CPM. The Geiger counter used is a seintl.com product and therefore, a comparison of this 10 CPM value with the RadiationNetwork values may be most appropriate. As the figure above shows, the mean CPM of 13.58 is about 35% greater in the US sites compared to the Dominica average.
NETC The Nuclear Emergency Tracking Center (NETC) data appears to be almost all Std Geiger counters (N = 56) made by gqelectronicsllc.com (GQ), except at least one identified as "pancake". The NETC mean of the 56 monitoring sites was 19.49, about 6 CPM above the RadiationNetwork value of 13.58. This difference is evident in the histogram (pink points) of the NETC sites.
Discussion: The higher CPM values for NETC Std Geiger counters may be due to several factors. First, it may be that the "effective" area of the GM tubes in most of the NETC values may be larger than that in the typical units used by RadiationNetwork monitors. In addition, the units may have consistent differences in calibration. Most of the Std Geiger counters reporting to RadiationNetwork were SI units, while almost all of the Std units on NETC were made by CQ.
There was not a "match" of the monitoring sites between the two web sites, just a tabulation of all of the radiation monitoring sites on the days sampled for which data was available. However, both maps from the two web sites are generally similar in having monitoring stations spanning the east to the west coasts of the US.
Given the numerous methodological issues which one can cite which might add error to the reported measurements, it may be noteworthy that a rather consistent average difference was found between the maps of the two web sites. Hence, regardless of the exact reason for the approximately 6 CPM difference in the higher NETC values, compared to RadiationNetwork values, for Std sensors, the present report may be helpful in further analysis, perhaps where data from both maps are aggregated.
Finally, the lower radiation level in Dominica is highly significant statistically (t-test) and may be good news for those seeking some refuge from Fukushima fallout. This difference between Dominica and the US might be even greater for the northern US states which tend to have higher CPM than many southeastern states, perhaps due to fallout thanks to jet stream location, a point that can be statistically verified with the spreadsheet data. Meanwhile, for those in the US, still a "land of plenty" at least for man-made radioisotopes, enjoy that extra dose of radiation while you can.
How to use both RadiationNetwork (RN) and NETC data in studies of radiation levels? This methodological question is important because investigators like to have the largest samples possible which enables statistical analysis to reveal smaller effects, which might nonetheless be important scientifically or health-wise. Two quite different approaches might be suggested.
ReplyDeleteBased on the present pilot study above, four categories of radiation sensors may be identified;
1. RN "Std" Geiger counters mostly made by seintl.com (SI).
2. RN "Pancake" Geiger counters.
3. NETC "Std" Geiger counters mostly made by gqelectronicsllc.com (GQ).
4. NETC "EPA beta" counters.
Option 1: z scores. A z score is a data standardization method where each member of a group of numbers (CPM) is processed by subtracting the group average (mean) and then dividing by the group standard deviation. Thus, the group average z score is zero. A z score of 1.5 indicates a value 1.5 standard deviations above the average within that group, and so on.
The z score option would allow combining the four groups of CPM counters listed above. In this case, differences between the groups would be lost, but relatively high and low CPM value variation would be included in any subsequent analysis.
Option 2: CPM calibration. This option would not suffer the disadvantage of Option 1, where between-group variability is lost.
1. Pick an arbitrary group from the 4 listed above to be a "baseline group".
2. "Calibrate" or "adjust" the CPM of the other three groups to our arbitrary baseline group as in the following example.
Example:
1. Pick the CQ Std NETC units as our baseline group.
2a. Since the baseline average CPM is about 1.44 times greater (19.59 / 13.58) than the SI Std RN unit CMPs, according to the data reported above, each of these values would be multiplied by 1.44 to be comparable to our baseline group and aggregated with it.
2b. The SI Pancake RN units reportedly averaged CPMs 2.85 times greater than the SI Std RN units. Thus, these values would be divided by 2.85 and then multiplied by 1.44 to be added to our baseline group (or simply multiplied by 0.505, which equals 1.44 / 2.85).
2c. The average of all of the EPA beta NETC monitors can be computed and used to adjust those CPM values to our baseline group in a similar manner.
Option 2 has the advantage of inclusion of both within-group variation and between-group variation in CPM values allowing more comparisons as a function of other variables such as location, time, etc. The weakness of Option 2 is the validity of conclusions depends on accuracy of the calibration factors used. This accuracy would increase as the present pilot study is replicated a number of times to home in on a stable set of reliable calibration factors.