As you all know, I have always maintained that there is Fukushima fallout in the rain… but that the levels (even if they are unsafe) are too low for a Geiger counter to detect.
My sensitive Gamma Spectrometer has now (I believe) detected Cs137 in a rain water collection bucket which concentrates, or so it seems, the Cs137.
Most of the radiation detected by Geiger counters from rain is from Radon Washout, a processes whereby radon in the air (decays from natural uranium around the world) is “washed” out and falls to the ground in the rain. The decay chain is sudden and very quick, providing a few hours of potent readings before falling to background.
Inspector (regular or EXP) Sensitivity to Iodine 125:
0.02 µCi = 740Bq = 44,400Bq/60seconds
(At contact for I-125)
Iodine -125 Electron Capture
Gamma – 35.49 keV 6.60 %
X-Ray – 27.47 keV 75.7 %
Best energy range for detection by LND7317 probe:
10 keV = 100 keV (max)
The range where detector efficency falls rapidly (Cs137 is also in this range):
100 keV = 1000 keV (declining)
A great place to find data on isotopes:
*** Update! ****
I have calculated the activity:
My original calibrated Cs137 source (cal. vs. NIST tracible source, source ID SRS:80899-854, at 95% accuracy) was 3737 Bq.
I accounted for decay of the source:
3737*e^-((ln(2)/10979)*173) = 3696.4059560683608390980241545539265887454856828520474 Bq
For an ROI of the same size for both calibrated sample and rain water sample, I ran tests and determined counts per second:
Calibrated Source 91.2633 c/s
Rain Water Sample: 0.01439814814814814814814814814815 c/s
Now, I divided the detected calibrated sample c/s into the expected c/s to determine ratio of emission vs detection for the energies around 661.66 keV. (3696Bq * 0.851 [intensity for gamma from Ba137m])/2 = 1572.648. The division by two is because I entirely detected one side of the thin sample disk. so… 91.2633 / 1572.648 = 0.05803161292291727074335769987944
My detector is only about 5.8% efficient for such energies. (lower than my 12% “ball park by half”)
Now, merely divide the counted detection from rain by the efficiency and you have about the correct result.
(311counts/21600) /0.05803142216185694446564011781403 =
=0.01439814814814814814814814814815 / 0.05803142216185694446564011781403 = 0.24810951742643665986093914169915
Or 0.248 Bq/liter
(that is zero point two four eight Becquerels per liter)