15
Radiation Measurement Units
Different units are used to measure radiation, exposure to radiation, and dosage.
A
roentgen
is the amount of X-radiation or gamma radiation that produces one
electrostatic unit of charge in one cc of dry air at 0
°
C and 760 mm of mercury
atmospheric pressure. The Digital Radiation Monitor displays readings in
milliroentgens per hour (mR/hr). A milliroentgen is one one-thousandth of a
roentgen.
A
rad
is the unit of exposure to ionizing radiation equal to an energy of 100 ergs per
gram of irradiated material. This is approximately equal to 1.07 roentgen.
A
rem
is the dosage received from exposure to a rad. It is the number of rads
multiplied by the quality factor of the particular source of radiation. The rem and
millirem (one one-thousandth of a rem) are the most commonly-used measurement
units of radiation dose in the U.S. One rem is generally considered to equal one rad.
A
sievert
is the standard international measurement of dose. One sievert is
equivalent to one hundred rems. A microsievert (
μ
Sv) is one millionth of a sievert.
A
curie
is the amount of radioactive material that decays at the rate of 37 billion
disintegrations per second, approximately the decay rate of one gram of radium.
Microcuries (millionths of a curie) and picocuries (trillionths of a curie) are also
often used as units of measurement.
A
bequerel
(Bq) is equivalent to one disintegration per second.
Higher Than Normal Readings
Due to the random nature of radioactivity, the Digital Radiation Monitor reading
varies from minute to minute. In one location with only background radiation, the
reading in mR/hr might vary from 0.007 to 0.018 in ten minutes and from 0.004 to
0.021 in an hour. Averages for both periods would be very close.
Normal radiation levels in different locations can vary greatly due to soil
composition, altitude, and other factors. For example, normal background at
10,000 feet might be double that at sea level. On an airplane, the radiation at 35,000
to 40,000 feet may be as much as 30 to 50 times the normal level on the ground.
When monitoring radiation levels in one location, it is useful to determine the
highest reading you can normally expect to see in that location.
16
Appendix A Technical Specifications
Sensor:
Halogen-quenched Geiger-Mueller tube with mica
end window (LND 712 or equivalent). Mica
window density 1.5-2.0 mg/cm
2
. Side wall is .012”
#446 stainless steel.
Display:
4-digit liquid crystal display with mode indicators
Operating Range:
mR/hr: 0.001 to 110
CPM: 0 to 350,000
Total: 1 to 9,999,000 counts
μ
Sv/hr: 0.01 to 1100
CPS: 1 to 3,500
Energy
Sensitivity:
1000 CPM/mR/hr referenced to Cs-137
Accuracy:
±
10% typical,
±
15% max. (mR/hr and
μ
Sv/hr
modes)
Count light:
Red LED flashes with each count
Beeper:
Chirps for each count (operational in audio mode
only – can be muted)
Ports:
Dual miniature jack sends counts to CMOS-
compatible devices, including computers, data
loggers, earphones, and educational data collection
systems. Sub-mini jack provides calibration input.
Output:
Dual 3.5 mm jack provides output pulse for each
count for interface to computers, data loggers,
earphones, and educational data collection
systems. 0-9 V, 1 kOhm impedance.
Calibration Input:
2.5 mm mono jack provides calibration input.
0–3.3 V, > 5
μ
s width, rising edge triggered.
Anti-Saturation:
“Jam” protection allows readout to hold at full
scale in high radiation fields
Temperature
Range:
–20
°
to +50
°
C , –4
°
to +122
°
F
Power:
One 9 volt alkaline battery; battery life is average
2160
hours at normal background, average
625 hours at 1mR/hr with beeper off
Size:
150 x 80 x 30 mm (5.9" x 3.2" x 1.2")
Weight:
225 grams (8 oz) including battery
CE Certifications:
Emissions: EN 55011:98 + A2 (Class B emissions
limits); EN 61326: 98 (Class B) RF Emissions
Immunity: EN 61326: 98 (Annex C) Portable Test
and Measurement Equipment; EN 61000-4-2: 95
(ESD); EN 61000-4-3: 97 (EM); ENV 50204: 95
