CONGRATULATIONS and thank you for purchasing the RadAlert GEM II
GEM II
is the result of over 15 years of development and testing, and is made with state-of-the-art technology in order to provide the precision
and reliability you deserve. It is calibrated for the measurement of gamma rays and will detect the presence of x-rays and of alpha and beta
contamination, and is ideal for uses among the general public and industrial and business communities wishing to make a simple assessment of
their radiation background.
Operation
With a suitable 9-volt battery (PP3, MN1604, EN22) installed, simply press the button once and GEM’s display lights
will illuminate sequentially to indicate that it is functioning properly and the battery has sufficient charge. The “10-
SEC” light will begin to flash every 2 seconds, indicating that it is counting for 10 seconds and will provide a new scan
result every 10 seconds thereafter. The 10-second scan period enables you to take action to avoid a high dose in an
extreme radiation environment. At any time, you may press the button a second time to start the 10-minute counting
period (10-MIN Mode) for measurements of greater statistical accuracy (especially at low dose rates), or press the
button a third time to switch your GEM off.
Press once
= ON, in 10 second mode
Press again
= ON, in 10 minute mode
Press a third time
= OFF
General Techniques
At normal “background” levels, you may find that in any one counting period, not even the first green light will illuminate, but in the next period
the first three lights may illuminate. This is because radiation is a random phenomenon and 10 seconds is a relatively short time to accumulate
a representative sample of counts on which to base a measurement. So if you have reason to believe your immediate safety is at stake, we
recommend the use of the 10-SEC mode. If time and safety permit and you need greater accuracy, the 10-MIN mode is suggested, as it takes a
much larger sample from which a more accurate measurement can be derived. For checking the radiation levels in your workplace or home
(first take a measurement outdoors to determine the area’s “normal background”), we suggest the use of the 10-MIN mode.
Note:
Because of the way in which ionising radiation is detected, placing GEM very close to strong magnetic fields such as those within an inch
of TV and Computer screens can produce erroneous readings. Moving GEM a few inches away from these sources will eliminate any possibility
of interference.
The Display
GEM has a vertical array of 7 lights, each denoting a rate of exposure from 0 to greater than 7.5 micro-sieverts per hour. A Sievert is a very
large unit of radiation dose (“dose equivalent”), so it is often expressed in units of one-millionths (micro = u) or one-thousandths (milli = m).
The 7 lights are grouped in 3 ranges of exposure from green to red, and these lights and ranges indicate:
RED
7.5
µ
Sv/h The instantaneous dose rate which defines a Controlled Area
(IR Regs 1985). Access to such an area is normally limited to
classified radiation workers.
RED
5.0
µ
Sv/h
YELLOW
2.5
µ
Sv/h The instantaneous dose rate which defines a Supervised Area
(IRR 1985). Access is normally limited to named persons who
need not be classified radiation workers.
YELLOW
1.0
µ
Sv/h Levels of dose rate which might be encountered in areas of
enhanced natural background.
YELLOW
0.5
µ
Sv/h
GREEN
0.3
µ
Sv/h Levels of radiation which are roughly equivalent to normal
background variations.
GREEN
0.1
µ
Sv/h Low level background radiation.
Factory-set Visual ALARM
Every second that your GEM is operating, whether in 10-SEC or 10-MIN mode, it is also checking for a sudden burst of radiation that calls for
immediate action by you.
If in any
one second period
GEM detects a radiation dose
over 15 microsieverts per hour
(for GEM, this is approximately 21 counts per
second), GEM will stop counting and
alert you with a unique display of flashing lights
using the two MODE lights and the top red light. To
turn off this alarm display, simply press the red button until GEM turns off, then re-start.
Alpha and Beta
While gamma and x-ray radiation penetrate GEM’s plastic case unimpeded, a “window” on the left side of GEM is provided for the detection of
alpha and beta radiation, which are not capable of penetrating GEM’s case and only travel a few centimetres in the air. So if you wish to be sure
to include alpha and/or beta when testing a source, orient this window towards the source and as close to it as possible.
