BAM 1020-9800 Manual Rev W.docx
Page 103
11
THEORY OF OPERATION
14
C (carbon-14) is a naturally occurring isotope of carbon that forms in the atmosphere by the
interaction of cosmic rays with nitrogen. Of the three naturally occurring isotopes of carbon,
14
C is the only to occur in trace amounts. The half-life of
14
C is 5,730 years. It undergoes
beta-decay and is converted into
14
N (nitrogen-14). During the beta decay process high
energy electrons are emitted. Beta radiation through the decay of
14
C is distributed around an
average energy of approximately 49 keV. In air these electrons travel a maximum distance of
around 22 cm before being fully absorbed.
Beta-ray absorption is the principle by which the BAM 1020 makes its measurement.
14
C is
convenient source to use in beta absorption measurements. Its long half life means that the
source will outlast the service life of the instrument. When used in modest amounts (less than
100 microcuries), generally no license is required to possess the equipment.
This beta ray absorption process by matter may be described by the following relationship:
𝐼 = 𝐼
0
𝑒𝑥𝑝 (−
𝜇𝑀
𝑆
)
In the above equation
𝐼
0
is the measured beta ray flux (counts) across clean filter tape,
𝐼
is
the measured flux (counts) across aerosol-laden filter tape,
𝑀
is the aerosol mass deposited
on the filter tape (mg),
𝑆
is the spot area (cm
2
).
𝜇
is the beta ray absorption cross section
(cm
2
/mg). The absorption cross section
𝜇
to a very good approximation depends only on the
mass of the absorbing species and not on its chemical composition. In other words the
absorption cross sections for commonly found species in ambient particulate matter such as
soot, iron oxide, silica, or salt are all approximately the same. It is for this reason that one
does not need to know ahead of time the chemical composition of the aerosol being sampled
in order to perform an accurate mass measurement using the BAM 1020.
During the factory calibration process, the BAM 1020 being calibrated is first challenged with
a membrane whose mass density
(
𝑀
𝑆
)
is known. Repeated measurements of
𝐼
0
and
𝐼
for the
beta gauge under calibration allows us to calculate the absorption cross section
𝜇
as is
shown below:
𝜇 =
𝑆
𝑀
𝑙𝑛 (
𝐼
0
𝐼
)
Tiny variations in the measured absorption cross section
𝜇
will be found from one BAM 1020
to the next. These differences are due to small differences between the activities of the beta
sources and due to small differences in the geometries (i.e., manufacturing tolerances) that
exist between one instrument and the next. This membrane calibration process allows us to
standardize the span response of all BAM 1020 monitors.
The same membrane whose known mass density
(
𝑀
𝑆
)
is used to determine
𝜇
is built into the
instrument and subsequently used to challenge the same BAM 1020 on an hourly basis to
ensure that the original span calibration is being held.
