7
CPM (or CPS) and timed total counts are the most direct methods
of measurement; mR/hr (or
µ
Sv/hr) is calculated using a conversion
factor optimized for Cesium-137, so this mode is less accurate for
other radionuclides unless you have calibrated the instrument for a
specific radionuclide using an appropriate source. It is more
appropriate to measure alpha and beta activity using CPM than
using mR/hr. Conversion for alpha and beta emitters is calculated
differently, and the Radalert 100’s reading in mR/hr may not be
accurate.
The most immediate indicators of the radiation level are the count
light, the audio beep, and the alert. It takes three seconds before
an increase is shown on the numeric display in the dose rate
modes.
Taking a Timed Total Count
The Radalert 100 can give you a total count for a timed period of
from one minute to 40 hours.
A timed total count is useful for determining the average counts per
minute over a period of time. The number of counts detected by
the Radalert 100 varies from minute to minute due to the random
nature of radioactivity. When a count is taken over a longer
period, the average count per minute is more accurate, and any
small increase is more significant.
Taking an average allows you to detect low-level contamination or
differences in background radiation due to altitude or soil mineral
content, and can be useful for educational purposes. For example,
if one 10-minute average is one count higher than another
10-minute average, the increase may be due to normal variation.
But over 12 hours, a one-count increase over the 12-hour
background average is statistically significant.
To take a timed count, follow these steps:
1. With the Radalert 100 operating, set the Mode switch to
Total
. The display shows
SET
, the hourglass, and the most
recent timing period used. The first time you use the timer,
the setting is 00:01, which means one minute.