[ RadSafe ] Field determination of radon progeny

[Michael McNaughton]

Stewart

According to the Bateman equations, the combination of decay products 
decays with an effective half life equal to the sum of the individual 
half lives, which in this case is 3+27+20 = 50 minutes. With 
clothing, the effective half life may be shorted, perhaps as a result 
of decay products detaching from the clothing. However, in the case 
of radon decay products washed to the ground by rain or snow, I 
observe about 45 to 50 minute half lives.

I have hundreds of observations recorded by the LANL NEWNET system, 
http://newnet.lanl.gov/ . In New Mexico, it rains heavily for 10 
minutes and then stops suddenly. During these storms, the background 
increases rapidly; sometimes it doubles. After these storms, the 
gamma background follows the theoretical exponential decay curve very 
accurately. I have confirmed the 45- to 50-minute half life many times.

mike

At 12:08 PM 4/13/2010, Stewart Farber wrote:
>In terms of the effective half-life of radon daughters on clothing, 
>I believe the good working value is just about 30 minutes. If 
>clothing measured contamination drops by a factor of 2 in 30 min, 
>and 4 in 1 hour, it is almost certainly Rn daughter contamination.
>
>Bi-214 has a half-life of about 20 min, and Pb-214 about 27 min, 
>with a rough overall effective half life for the Rn-daughter mix of 30 min.
>
>Another interesting source of Rn-daughter contamination  that might 
>show up at a nuclear plant is that due to rainfall.  I was involved 
>with gathering and reviewing High Pressure Ionization Chamber 
>measurements of exposure rate  near an operating nuke plant 30 years 
>ago. Measurements were being made with a custom built instrument 
>[like the Reuter Stokes
>  RSS-111, which had a 10" diameter sealed sphere, pressurized to 25 
> atm with Ar, so it was quite sensitive to ionization from x rays
>  and gamma rays]. I had a custom unit built because I needed to 
> record data to a tape unit for long-term exposure rate measurements 
> at the site boundary a BWR nuclear plant [only 700' from the 
> turbine]. The State of VT had set an annual limit at the site 
> boundary of only 5 mR/year  [0.05 milli-Sv/hr] from plant 
> operations, with turbine shine being the major factor at only 700'.
>
>I had plotted hourly average radiation exposure rates. I also had 
>gathered [for another purpose] concurrent data of hourly rainfall 
>rates in mm/hr.  Putting the exposure rate data together with the 
>rainfall rate in mm/hr some very interesting things became evident.
>
>  During the start of initial hours of high rainfall, the exposure 
> rate increased from 8 uR/hr [0.08 uGy/hr] to
>  about 14 micro-R/hr [0.14 microGy/hr], an increase in background 
> exposure rate of about 75%, due to ground-plane deposition of 
> Rn-daughters around the HPIC system located at the site boundary.
>
>Subsequently, after a few hours the rainfall rate went to almost 
>zero. The exposure increment began to drop with about a half hour 
>half life, returning toward 8 uR/hr. Then a few hours of increase 
>and drop of rainfall rate,  and the total exposure rate went up and 
>down concurrent with the rainfal rate affecting the deposition of Rn 
>daughters. After about 10 hours the rainfall ended, and the 6 uR/hr 
>[0.06 uSv/hr]  increase in background due to Rn-daughters decayed 
>away, and the total exposure rate returned [with about a 30 minute 
>half-life] to the background of about 8 uR/hr [0.08 uGy/hr] for that 
>location. Of note, the plant was not operating at the time, so there 
>was no contribution from N-16 turbine shine, and no plant releases occurring.
>
>This 75% increase in background measured at the time was due to rain 
>washing ["scrubbing"] Rn-daughter particulates from the air column 
>and depositing them to the ground where they caused an increase in 
>exposure rate measured
>  by the fixed HPIC.
>
>So, it is worth being aware that background exposure rates at or 
>near a nuclear facility [which might be monitored by HPIC or 
>detectors able to measure slight increases in background] can 
>increase by 75% [or who knows how much more depending on rainfall 
>rate and average Rn-daughter concentration in the air mass at the 
>location being measured at that time]. Then the increase exposure 
>rate above background, not due to plant releases, will decay away 
>with about a half hour half-life.
>
>Stewart Farber, MSPH
>
>Farber Medical Solutions, LLC
>
>Bridgeport, CT 06604

Mike McNaughton
Los Alamos National Lab.
email: mcnaught at LANL.gov or mcnaughton at LANL.gov
phone: 505-667-6130; page: 505-664-7733