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Chapter 4 Using the RAD7
4.6
Managing Background
High flow rates are useful for thoron
measurement and for fast response RAD AQUA
monitoring of radon and thoron in water. Low
flow rates are useful for continuous soil gas
monitoring and for sampling gases, such as stack
gases, that need significant conditioning.
A major concern in radon testing is background.
The RAD7 has a number of features that help to
keep short and long-term background under
control. These are discussed in Chapter 3.11.
Following a few simple rules will help to keep
background to a minimum.
4.7.1
The maximum recommended airflow rate is 2.5L/
minute. Beyond that rate the RAD7’s sensitivity
will eventually drop, because the very fast
movement of air interferes with the electrostatic
precipitation process. Airflow rates approaching
the 2.5L/min limit are suitable for measuring
thoron, which must be brought into the RAD7
swiftly due to its rapid decay.
Short term background is activity left in the
detector after the air sample has been flushed
from the measurement chamber. The higher the
radon concentration and the longer the sample is
held in the cell, the more daughter activity it
leaves behind. So, to avoid background, when
you see high radon readings, finish your
measurement, and purge the sample cell promptly.
Take the instrument somewhere with little radon,
such as outdoors. Make sure the drying tube is
connected, and select >Test Purge. Let the
RAD7 purge for 5 to 10 minutes, or longer if the
sample was exceptionally “hot”.
If the RAD7 has been specifically calibrated for
thoron however, the specified thoron sensitivity
will remain in effect only when the RAD7 is
configured exactly as instructed on the Thoron
Calibration Certificate. Generally this entails
operating the RAD7 in Thoron Protocol, causing
its internal pump to run continuously, producing
an airflow rate of about 800mL/min.
The two alpha peaks decay at different rates. The
polonium-218 peak, in window A, decays with a
3.05 minute half-life. So in 10 minutes it will be
down to about one-tenth of its original count rate.
The peak in window C, however, will take over
two hours to get down to one-tenth its count rate.
4.7.2
Minimum Airflow Rate
If the RAD7’s cycle time exceeds 5 minutes and
its internal pump is set to Auto, the pump
normally runs continuously to dry out the RAD7
until the RH drops below 10%. After that the
pump runs for 5 minutes at the start of every cycle
to put an entirely fresh sample into the
measurement chamber, and thereafter for one
minute in every five, to keep the air sample fresh.
Therefore the typical average flow rate is less
than 0.2 L/min. For certain applications lower
airflow rates may be preferred. To determine the
minimum acceptable airflow rate, consider the
following parameters:
Rather than wait around for hours, you can start
the next radon test in SNIFF mode, which ignores
window C. In fact, the preset, one and two-day,
monitoring protocols, in the RAD7, use AUTO
mode, which starts a measurement in SNIFF
mode and automatically changes to NORMAL
mode after three hours. This takes care of all but
extreme exposure to very high radon.
You can always measure the short-term
background, with 5-minute SNIFF mode tests.
Run a few to see that the background is low.
4.7
Maximum Airflow Rate
a) The distance from the sample source to the
RAD7 (very slow flow rates may allow
significant radioactive decay of the radon
before it reaches the RAD7).
Airflow Rate Limits
When the RAD7’s pump is set to OFF (Setup,
Pump, Off [ENTER]), it is permissible to use
an external pump device, such as the DRYSTIK,
which may provide a higher or lower airflow rate
than the RAD7’s built-in pump. However certain
flow rate limits should be observed.
b) Whether thoron is being measured (with a oneminute half life, sample decay during
acquisition is significant).
c) The required response speed of the RAD7 (a
low flow rate may cause an unacceptably long
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