Just as a general pointer here about radiation detection/monitoring and mistakes to look out for.
I keep seeing people talk about the idea that a specified CPM will correlate into a specified exposure level. That is exacerbated by the fact that
some amateur radiation monitoring sites just give readouts in CPM for each site.
I will post some product specs to help people understand the “calibration factor” I am talking about.
Here is a list of thin wall Geiger tubes LND makes.
The model 713 has a gamma sensitivity of 7.5 CPM/mR/Hr.
That is to say that if you get a 15 CPM reading, you would be detecting 2mR/Hr
The model 719 has a gamma sensitivity of 90 CPM/mR/Hr
That is to say that if you get a 180 CPM reading, you would be detecting 2mR/Hr
Here is a tube by a different manufacture.
It has a calibration of 18 CPM/mR/Hr
If you have a meter that has a removable probe, and a calibrated readout, then that readout is only valid with a specified probe. The specified
probe(s) in which that calibration is valid will be listed in the owners manual. The owners manual may also specify other probes and correction
factors for that calibration. (2x, 7x, exec…..)
If you use it with any other probe, then that calibrated scale on the meter is meaningless. You will have to look at the CPM, and cross that with the
calibration factor for your probe.
If you do not know the calibration factor for your probe, then the probe is useless for anything more than general toying around and experimenting.
The cost of determining the calibration factor for an unidentified probe/tube is more than the probe/tube is worth.
Like those radiation monitoring sites that just give a readout in CPM. Unless they tell us the calibration factor for those sensors they are using,
then all that data is totally useless, and their whole “radiation monitoring network” is a total waste of time.
In regards to the units.
REM……… “Roentgen equivalent man”
The modern equivalents
Gy “Gray” (equivalent to RAD) (1Gy = 100Rad)
Sv “Sievert” (equivalent to REM) (1Sv = 100rem)
Gy/Rad is based on energy delivered by the particles.
Calibration in Gy/Rad of a non proportional GM tube is only good for one type of particle (usually gamma). A tube detecting a specified number of CPM
from a gamma source will be detecting a known radiation level. That calibration doesn’t holed true when you use that tube to detect beta particles
(or X rays). You may have a different calibration for beta, but usually they don’t have such a calibration for GM tubes. The reason for that
is….. A GM tube gives the same single pulse for a gamma, beta, or alpha particle irrelevant of it’s actual energy level. And the energy level is
what determines the number needed to obtain the specified RAD level.
It takes more alpha particles to get a rad than it does beta particles. It takes more beta particles to a rad than it does gamma…….. so on and so
To get a tube that has a single working calibration all the way from alpha, to beta and gamma, you need what is called a proportional detector.
It gives a different response intensity depending on how energetic the particle is. The counter doesn’t count the number of pulses, it counts the
total cumulative intensity of the pulses. Most hand held GM detectors are not proportional though. For proportional detection, Usually you would want
to go with a Scintillation counter which will give you a fully calibrated reading for a variation of radiation types.
They use a scintillation crystal, and a photomultiplier.
REM/Sievert is the amount of radiation the human(or other) body will absorb based on weighing factors. Those weighing factors are based on radiation
type and other factors.
When taking a calibrated measurement of the radioactivity of an object, the reading should be taken at a specified distance from an object with a
specified detector. Remember. Radiation drops off as you get farther from the source. Measured radiation levels will change depending on how far your
detector is away from the object and the sensitivity of your detector to certain particles. You may see people quoting a big reading for a “hot”
object. But if they are holding the probe right against the object, then the reading is useless. The reading will be spectacularly high, but it will
You only move the probe in close when you are trying to pinpoint a small source, or you are trying to find any contamination at all on a surface.
For external radiation, gamma is the main concern. Alpha and beta particles can not go deep enough to hurt us when they outside our body. They only
time they can hurt us is when we ingest them. That is the whole reason they have mica window GM tubes. To detect alpha and beta emitter contamination
of food, water and other stocks that may be ingested. Or other products that may be used. But their use does not mean that the measurements they take
are calibrated. It means that you detect contamination on the product, or you don’t!!!!!!!!!
If radioactive dust settles on a surface. You can use a pancake GM probe to wave across the surface and verify that there is radioactive dust on the
surface, but the reading on the meter should not be construed as a calibrated reading that you can quote to someone. If you have a
proportional/scintillation detector, and hold that probe over several spots at a fixed distance and take averaged readings, then you can quote the
surface contamination level with some idea that the information may actually be usable.
If you use a mica window GM tube on a normal Geiger counter, and hold it by an alpha or beta source, then you will get wild reading that peg the
meter. But the actual RAD dose that the probe is detecting is a small fraction of the displayed reading. Real dose may be as little as 1% or less of
what the meter is displaying.
When you are taking environmental (background) radiation reading, if you have the beta/alpha window open, then your reading is worthless. The
beta/alpha window must be closed, so that the detector only detects gamma particles (what it is calibrated for).
That is the most common mistake that I see on some of these monitoring websites. They have little Geiger counters that have an open mica window on the
front with no secure way to cover it. That leaves the sensor open to radioactive dust (that comes from earth or outer space) that may settle on the
mica window of the GM tube and greatly skew the reading.
If you are taking a valid background radiation measurement, then your reading shouldn’t vary when you move your probe up or down, front to back, or
left to right. If your reading changes drastically, then you are too close to a radioactive object, and your should change your objective over to
making a calibrated measurement of the radioactive object in question, and finding a better place to locate your background radiation monitoring
station (away from that object).
So, when you see someone holding a mica window probe against an object and saying that “It is really radioactive, and generating over 40mR of
radiation!!!!!!!” Or when they have a mica window probe on a dusty table, and complain about how the radioactive it is in their house because of
that stupid nuke plant close by, you will understand why I just want to cry.
edit on 18-3-2011 by Mr Tranny because: (no reason given)