5
Mode Switch (5)
mR/hr
μ
Sv/hr.
The LCD shows the current radiation level in milliroentgens per
hour from 0.001 to 110 or, when SI units are used, in microsieverts per hour
from 0.01 to 1100.
CPM CPS.
The LCD shows the current radiation level in counts per minute
from 0 to 350,000 or, when SI units are used, in counts per second from 0 to
3,500. When
X1000
is shown, multiply the numeric reading by 1000 to get the
complete reading.
Total/Timer.
The display shows the accumulated total of counts starting when
the switch is turned to this position, from 0 to 9,999,000. When
X1000
is
shown, multiply the numeric reading by 1000 to get the complete reading.
+, -, and SET Buttons (7) (on the end panel)
+ and – Buttons:
These are used to adjust the numeric display for timed counts
and calibration settings, and to scroll through the Utility Menu.
Set Button:
This is used to set the correct calibration reading and to select items
in the Utility Menu.
For more information, see “Taking a Timed Count” and “The Utility Menu” in
Chapter 3.
The Detector
The Digital Radiation Monitor uses a Geiger tube to detect radiation. Alpha radiation
does not penetrate most solid materials, so this Geiger tube has a thin disk of mica,
which alpha radiation can penetrate, on its end. The screened opening at the top of
the instrument is called the
alpha window
(
8
). It allows alpha and low-energy beta
and gamma radiation to penetrate the mica end of the tube.
CAUTION:
The mica end surface of the Geiger tube is fragile. Be careful not to let
anything penetrate the screen.
The Ports
There are two ports on the left side of the Digital Radiation Monitor.
The
calibration input port (2)
is used for factory calibrating electronically using a
pulse generator.
The
output port
(3)
below the calibration input port allows you to connect the
Digital Radiation Monitor to a computer, data logger, earphones, or other device
using a 3.5 mm stereo plug. For more information, see “Interfacing to an External
Device” in Chapter 3.
6
3 Operation
The guidelines in this chapter describe how to use the Digital Radiation Monitor.
Units of Measurement
The Digital Radiation Monitor is designed both for users of conventional units
(milliroentgens per hour and counts per minute) and for users of SI units
(microsieverts per hour and counts per second). To switch between conventional and
SI units, use the Utility Menu. See “The Utility Menu” in this chapter.
Starting the Digital Radiation Monitor
Be sure that a standard 9-volt alkaline battery is installed in the battery compartment
in the lower rear of the Digital Radiation Monitor.
Note:
When installing the battery,
place the battery wires along the side of the battery and not under it.
To start the Digital Radiation Monitor, set the top switch to the mode you want, and
set the bottom switch to
On
or
Audio
. The Digital Radiation Monitor then does a
four-second system check, displaying all the indicators and numbers.
After the system check, the radiation level is displayed in the selected mode. One
minute after you start the Digital Radiation Monitor, a short beep indicates that
enough information has been collected to ensure statistical validity.
Operating Modes
When the mode switch is set to
mR/hr
or
CPM
, the numeric display is updated
every three seconds. At low count rates, significant changes in the radiation level
displayed can take up to one minute to stabilize. See “Operating Ranges and
Response Times” in this chapter for more information.
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 Digital
Radiation Monitor’s reading in mR/hr may not be accurate.
The most immediate indicators of the radiation level are the count light and the audio
beep. It takes three seconds before an increase is shown on the numeric display in
the dose rate modes.
Taking a Timed Total Count
The Digital Radiation Monitor 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 Digital Radiation Monitor
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.
