Proper Geiger counter usage, and radiation monitoring.

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.
www.lndinc.com...

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
www.lndinc.com...

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
www.lndinc.com...

Here is a tube by a different manufacture.
www.imagesco.com...
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.

RAD
en.wikipedia.org...(unit)

REM……… “Roentgen equivalent man”
en.wikipedia.org...


The modern equivalents

Gy “Gray” (equivalent to RAD) (1Gy = 100Rad)
en.wikipedia.org...(unit)

Sv “Sievert” (equivalent to REM) (1Sv = 100rem)
en.wikipedia.org...

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 forth.

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.

en.wikipedia.org...

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.

en.wikipedia.org...

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 be useless.

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.

GM tubes/probes should be covered with a thin plastic film to reduce surface contamination of the tube/probe.

You will see this at the Japanese contamination screening sites.

Every half hour or when you get a continuous reading the film should be changed to detect false reading from radioactive particles sticking to the protective film.

This is faster then having to take the Geiger counter out of service for decontamination

Here is an example of two radiation monitoring networks I have seen posted on the forum.

www.radiationnetwork.com...

www.blackcatsystems.com...


Notice something distinctive about both of them.
They list detected levels in CPM (counts per minute)
They use a wide variety of dissimilar detectors.
They use detectors with a wide variety of gamma, beta, and alpha detection capabilities.
They don’t indicate in any way, the type of detector used in each spot.

WTF!!!!!!!!!!!!!!!

The only way to compare readings from dissimilar detectors is to extrapolate a standard unit based on CPM and detector calibration. That standard unit will be Rad, or Gray.

So…..
We don’t know the extrapolated exposure rate is for each detector.
We don’t know what meter is being used for each spot, so we can’t make the extrapolation our selves.
Even if we did know the meter used, the reading may still be useless, because as far as I can tell, some of the manufactures of those meters don’t even have a clue about their product’s calibration.

Example……..
www.blackcatsystems.com...

Look at the GM 50 column. (Not to mention the others!!!!)
Tube size. 9x0.65
Cobalt 60 sensitivity…7800 CPM/mR/Hr………. Ummmmmmmm……..Not a chance in hell!!!!!!
That meter has to be smoking something strong to be tripping at 7800CPM for one mRad.

The LND tube coming in at 10x0.63 has a calibration of 90CPS/mR/Hr.
I could believe 78CPM/mR/Hr.
They are two decimal places off……. Hm……
Now if the tube was specified in mGy…… then the calibration would be believable.
(ie) 7800CPM/mGy/Hr.

And all that doesn’t take into account how having an un-enclosable alpha/beta window will skew the readings.

So, both of those networks don’t give you one shred of information that would go toward making the numbers on the map usable for anything but numbered dots on a map.

The numbers are basically meaningless. You are suppose to sit there and get all worked up if a number goes above an arbitrary level, even if you have no idea in hell what that number represents.

Now I feel like kicking myself…….. I am even getting units mixed up.

As people have probably noticed, I have been mixing up CPS, and CPM…..

Here is the corrected figures for parts of both my post……………

The model 713 has a gamma sensitivity of 7.5 CPS/mR/Hr.
That is to say that if you get a 15 CPS reading, you would be detecting 2mR/Hr
www.lndinc.com...

The model 719 has a gamma sensitivity of 90 CPS/mR/Hr
That is to say that if you get a 180 CPS reading, you would be detecting 2mR/Hr
www.lndinc.com...

Here is a tube by a different manufacture.
www.imagesco.com...
It has a calibration of 18 CPS/mR/Hr

In regards to this section of my last post…… Just ignore it, my mind wasn’t firing on all cylinders………… It was one of those days…..

Even if we did know the meter used, the reading may still be useless, because as far as I can tell, some of the manufactures of those meters don’t even have a clue about their product’s calibration.

www.blackcatsystems.com...

Look at the GM 50 column. (Not to mention the others!!!!)
Tube size. 9x0.65
Cobalt 60 sensitivity…7800 CPM/mR/Hr………. Ummmmmmmm……..Not a chance in hell!!!!!!
That meter has to be smoking something strong to be tripping at 7800CPM for one mRad.

The LND tube coming in at 10x0.63 has a calibration of 90CPS/mR/Hr.
I could believe 78CPM/mR/Hr.
They are two decimal places off……. Hm……
Now if the tube was specified in mGy…… then the calibration would be believable.
(ie) 7800CPM/mGy/Hr.

reply to post by Mr Tranny


Mr. Tranny!

I just thought of a new layman's analogy that might help people understand ...

Think of having a leak some where in your house. You want to know how much of what is leaking. You know it's dripping a certain number of "drips per day". But, that's pretty-close to meaningless by itself.

You need to know a measurement in volume (liters, gallons, etc.) of the leak - not how many drips. It might also be useful to know what is leaking; cold-water? hot-water? sewage line?.

Further, putting the drips-per-day for your house, on the same chart as another house (your vacation house, your neighbor's house, etc.) doesn't provide any meaningful comparison. The pipes in each house are different, and each "drip" might be a different volume from house to house, and may be of a different type (cold, hot, sewage, etc.).

So, think of "drips-per-day" as "counts-per-minute". Cold-water, hot-water and sewage as Alpha, Beta and Gamma radiation. You need some quantifiable measurement (preferably over time for comparison) to be useful. Basically, "how many gallons of sewage are dripping into my house?"

Knowing it's dripping/clicking may not even be abnormal, without the ability to compare measurements.

Does that make sense?

That works to some extent, but the better correlation would be with light measurement. The measurements used in lighting correlate better with radiation than anything else. They both have the same line of sight behavior too.

There is three types of measurements in radiation.

Old REM/new Sievert
Total cumulative exposure to a specified object.

Old RAD/new Gray
Total amount of energy going through a specified volume.

Old curie/new Becquerel
Total number of particles being emitted by the materiel in question irrelevant of energy or direction.

Think of it in the same terms as you do measurements of light. Lumens, lux, lumens per square meter, radian, steradian……. All that.

When you talk about radiation absorbed by an object, then you talk about the REM/Sievert. The amount absorbed from a specified field will depend on the geometry of an object. Same as the total heat absorbed by an object (in Watts per meter squared) will depend on it’s color, and the color of the impinging light.

If you are measuring background radiation, you use the unit RAD/Gray. That is because background radiation should not change levels (appreciably) depending on where you move the detector. Background radiation is the homogeneous radiation field that surrounds us on all sides. If the level of detected radiation changes when you move the detector, then by definition, you are not detecting the background radiation, you are detecting the radiation from an object close to you, and you have to shift to a different measurement type. It is like measuring the ambient light levels in lumens per square meter.

When you are measuring the radioactivity of an object, is when you really get into the light measurement mode of things. To measure the lumens produced by a light, you don’t hold the light meter right against the light. You hold it at a specified distance from the light. You find out how many lumens per square meter is hitting the meter, then you extrapolate how many lumens per steradian that would be. Then the total number of lumens based upon the number of steradian the light source emits from. (Usually isotropic/360 in all directions for a simple light bulb.)

When you do that for radiation, you do the exact same thing. You measure the amount of radiation (as you would for lumens with a light source) going through a specified volume (rads), at a specified distance. That allows you to extrapolate the total number of radiation particles emanating from that object in all directions. That allows you to assign a quantitative number to the radioactivity of an object. AKA, in curies, or Becquerels

If, the geometry of the object causes it to have a non uniform radiation field, to the point that you can not make an estimation of it’s true value in curies, or becquerels, then you should measure the highest reading in rads, at a specified distance. That allows you to produce a rough idea of the radiation per solid angle coming from the object, where you can extrapolate what the reading would be closer in, or farther away from the object. Same as you would for measuring light in lumens per square meter, or lumens per steradian in front of a non uniform light source.

So, when you are measuring background radiation, or radiation in a living space, you should use rads, or grays.

When you are measuring radioactivity of an object, then you measure in curies, or Becquerel’s. If the situation doesn’t allow you to do a true approximation of the radioactivity of an object based upon your readings, then make a measurement at a specified distance, then record the reading and distance which it was taken.

The total number of RADs or REMs something will receive from an object with a specified activity in curies will depend on the type and energy distribution of the particles being emitted.
……………………………………………..

On a side note. Most radon decay products produce beta emissions. The decay products from reactors produce gamma emissions.

If you have two detectors side by side, one that can detect gamma and beta, and the other one can just detect gamma.

When it rains, if you see a spike in the reading from the one that can detect beta and gamma, but not from the one that can only detect gamma, then you are detecting natural radon decay products. If you get a spike in the reading from both detectors, then you are possibly detecting fallout from a fission device (bomb or reactor)

If you are looking at a readout from a station that gives a breakdown based on gamma, and beta, and you see a spike in beta, but not gamma, then it’s radon, if you see a spike in gamma, but not so much beta, then you are seeing fission decay products.

Basically, if you can’t detect a spike in radiation with the alpha/beta window closed, when it rains… Then you are not detecting any man made radioactive particles. If you have to open up that alpha/beta window to detect the spike, then it is most likely naturally occurring radioactive materials in the rain.

And……… The CPM of a detector for a specified exposure level will depend on the volume of the detector and it’s efficiency. The rad is based on the number of particles going through a specified volume. So if the volume of the detector increases, then it will detect more particles for a specified exposure level.

Perhaps I'm missing something here. I thought the title of this thread was "Proper Geiger counter usage, and radiation monitoring" what I seem to be reading here is a lot of things NOT to do. I seem to have missed the actual how....

So lets say one is now worried enough that one wants to monitor their environment. Lets start with the basics, what meter should we get? A lot of people seem to like the Radiation alert inspector but if I'm understanding what you've posted correctly this would be a poor choice in your opinion?