| T O P I C R E V I E W |
| tedssled |
Posted - 03/08/2023 : 13:37:17
I've been trying to find a source of current radiation information for my area so I can see if either of the G-M counters I have are in the neighborhood in terms of accuracy.
In that search I came across the RadNet site run by the U.S. EPA. When I run a query for my area, the result has values for the time period, the dose equivalent rate (in nSv/H) and 8 Gamma Channel Ranges. Each of the channels have significantly different rates (in CPM) and I can't find out why that is or what these channels represent. There is one comment in the FAQ that says it helps scientists with accuracy. Wow. That helped explain everything!
Furthermore, the CPM for any of them is higher (in some cases MUCH higher) than anything I'm getting from these two counters.
The counters I have are a brand new 320S and an antique HDER G-01 (also known as the AN/PDR27-5). The CPM average for the 320S is around 20-25 CPM and for the G-01 it's about double that at 45-50 CPM. The range of values for each of the 8 channels and for the last hour of today is from a low of 48 and a high of 3338 (Austin Texas monitor location).
I've been measuring at my house which is located in a rural area at the edge of the Austin metro area and the 320S is showing (via GeigerLog) a range of 5-35 CPM with an average of 20, and the average for the G-01 is about 50 CPM.
I'm so confused. Why are these numbers so different? Furthermore, why is there a 2X difference between the two GC's I have that are sitting literally right next to either?
One other inconsistency: According to RadNet, the dose equivalent rate has been around 42 nSv/hr. According to the 320S it is around 0.14 uSv/hr. If I got the decimal places right, 0.14 uSv/hr is the same as 1400 nSv/hr, right? That's a massive difference!
Thanks for any enlightenment! |
| 7 L A T E S T R E P L I E S (Newest First) |
| tedssled |
Posted - 03/10/2023 : 07:14:51
Brilliant! I was laying awake last night and came up with the EXACT same analogy!! In my imaginings, I had 100 1cm X 1cm tubes plus a single 10cm X 10 cm tube scattered around the backyard. Same surface area but completely different quantities of rain. I also considered the size of the drops. Clearly a 1mm droplet of rain is less water than a 5mm drop. I got so interested, I looked up the distance it takes for a raindrop to reach terminal velocity and found that it isn't very far. It is drop size dependent but generally a rain drop has reached terminal velocity in a matter of meters. Now that kinda messes up our nice analogy cuz the energy of same size drops would be the same at the ground since they would all be at terminal velocity by then. But still it helps me get my arms around this whole thing.
Thanks for writing such a helpful reply!! |
| ihab17 |
Posted - 03/10/2023 : 00:33:55
quote:
Originally posted by tedssled
So CPM (or CPS) is a pretty useless number it seems.
If I have a pile of radioactive waste in the backyard it is gonna radiate alpha, beta and gamma at a certain rate. If I have a GC with a small tube it tells me 200 CPM. Another GC with a much larger tube tells me 2000 CPM. If I use one number, I might think I'm safe. If I use the other I might pack and move! I totally made this scenario up, but the point is that CPM doesn't really tell me much of anything, right?
From what I'm learning, it's the Sv/Hr that matters. It takes into account the biological damage done by different particles at different energies.
If I understand what you wrote, the dose equivalent rate is related to CPM but how many CPMs it takes to indicate a specific Sv/Hr level is also dependent of the size and sensitivity of the specific GM tube used. If I got all that wrong please correct me.
So one more question: Is it possible to calculate the DER in Sv/Hr with nothing more than a CPM? The old GC I have just clicks. That's about it. It does have a meter that displays Roentgen but on the most granular setting the smallest amount it can display is 0.001mR. That's just one single hash mark from zero on the meter. So it's pretty much useless measuring background levels. The needle on the meter rarely goes past the second hash mark and I've got to get right on top of a sample to get it to start getting into the middle range of the meter still on the highest sensitivity setting. Clearly it was designed and built for a post event where levels are sky high.
However, I've rigged up a way to use some audio recording software (Reaper) to measure CPM (or CPS) accurately and over as long a period of time as I have hard drive space!
So...is there a way to take the CPM info and figure out what the dose equivalent rate so then any comparisons between the old one and my shiny new 320S are valid?
Well, I also struggled with what you are struggling with, but doing some research and asking many questions, especially with the help of Ullix, I now understand things better.
Let us forget radiation for a while and talk about something easier to understand. Suppose that it is raining outside and you want to measure the amount of water from the rainstorm. So what do you do? You take a bucket and put the bucket under the rain for a specific amount of time (say 60 minutes), then take the bucket inside and you measure the volume of water you collected. Suppose you have a cubic container with a 10x10x10 cm (1 Liter container), so the upper area where rain can enter the container is 10x10 = 100 square cm and you collected 200 ml of water, then how would you interpret these numbers?
Now I (your neighbor) perform the same exact experiment, same day, same place, same rainstorm, but I have a larger bucket, say 20x20x20 cubic container, so I have a surface area (where water can enter the cube) of 20x20 = 400 square cm, that is 4 times larger than your bucket area. When I measure the amount of water I got, I see I have 800 ml of water. Now you got 200 ml and I got 800 ml, what should we conclude about the storm? Was it abundant? A heavy rain shower or a mild storm? Should we be worried or alerted for floods?
Assuming rain intensity is constant during the storm, and the experiment has been performed on the same day, place, and time, how do we interpret these results? You got 200 ml but I got 800 ml, who is right here? In this case we should calibrate our instruments to give us the correct reading per unit area per time. After all, I had a larger bucket (larger sensor, or larger tube) and yours was small, one fourth of my sensor, yet if you try to determine how much volume of water we got per unit area for that specific amount of time, we both get the same results! You got 200 ml per 100 square cm in 60 minutes, I got more water, 800 ml per 400 square cm in 60 minutes which means that for 100 square cm in my bucket, I got 200 ml which is also what you got.
Hence, every Geiger counter has got its own specs and its own calibration factor. Radiation in reality is not easy to calculate because even if we use the same counter we might get slightly different results, especially for short times, because not all tubes are equal. Even tubes from the same manufacturer differ slightly, hence we should take the average of many tubes over a long period of time
The same concept of the rain, more or less applies here. What your bucket is doing is counting the number of droplets falling from the sky, but it doesn't and it can't tell you the amount of energy each droplet is carrying and delivering. Suppose the rain droplets formed at 3 km altitude, thus it will accelerate for 3 km distance towards the ground until it hits the bucket. Clearly if the droplets were formed 10 km away from the ground, then the same droplet had 10 km of distance to accelerate, and clearly they have more energy to deliver, but your bucket had no idea of the amount of energy the droplet had, it simply counted the numbers! So if a droplet formed 10 cm from the bucket and delivered mild energy when it entered, or came as fast as a bullet and delivered high energy, to a Geiger counter this is the same, no distinction, just a count!
So CPM or CPS are meaningless numbers without introducing the sensor size, and the correct calibration. CPM and CPS can be interpreted as the amount of droplets you collected in your bucket, those numbers have to be calibrated considering your sensor size, sensitivity, and other factors to tell you the correct dose in units of uSv/h |
| tedssled |
Posted - 03/09/2023 : 11:19:48
So CPM (or CPS) is a pretty useless number it seems.
If I have a pile of radioactive waste in the backyard it is gonna radiate alpha, beta and gamma at a certain rate. If I have a GC with a small tube it tells me 200 CPM. Another GC with a much larger tube tells me 2000 CPM. If I use one number, I might think I'm safe. If I use the other I might pack and move! I totally made this scenario up, but the point is that CPM doesn't really tell me much of anything, right?
From what I'm learning, it's the Sv/Hr that matters. It takes into account the biological damage done by different particles at different energies.
If I understand what you wrote, the dose equivalent rate is related to CPM but how many CPMs it takes to indicate a specific Sv/Hr level is also dependent of the size and sensitivity of the specific GM tube used. If I got all that wrong please correct me.
So one more question: Is it possible to calculate the DER in Sv/Hr with nothing more than a CPM? The old GC I have just clicks. That's about it. It does have a meter that displays Roentgen but on the most granular setting the smallest amount it can display is 0.001mR. That's just one single hash mark from zero on the meter. So it's pretty much useless measuring background levels. The needle on the meter rarely goes past the second hash mark and I've got to get right on top of a sample to get it to start getting into the middle range of the meter still on the highest sensitivity setting. Clearly it was designed and built for a post event where levels are sky high.
However, I've rigged up a way to use some audio recording software (Reaper) to measure CPM (or CPS) accurately and over as long a period of time as I have hard drive space!
So...is there a way to take the CPM info and figure out what the dose equivalent rate so then any comparisons between the old one and my shiny new 320S are valid? |
| Damien68 |
Posted - 03/09/2023 : 09:59:40
in your counter there is a Geiger-Muller tube, the counter therefore counts the number of particles that pass through the tube.
if for example you consider another counter with a tube twice as big, it will detect twice as many particles for the same level of radiation and therefore its CPM will be twice as high for the same level of radiation as with your first one counter.
CPM levels are therefore highly dependent on the type of sensor (tube) used.
Certainly in the metro there are several sensors and larger sensors which would explain why they obtain very much higher CPM levels, but this sensor configuration don't change the uSv/h value.
In the metro, it is buried, the levels of radiation are certainly very much lower than the level of radiation on the surface which explains that they have only a few dozen of nSv/h and that at home you have a few hundred of nSv/h.
I think your datas are consistent. |
| tedssled |
Posted - 03/09/2023 : 08:36:41
Here's an example of a few hours output from a day or so ago from the RadNet site for the Austin metro area.
I understand (I think) what you are saying. Different energy levels for different particles and that might be what these Gamma channels are.
Do those CPM measurements look reasonable? Cuz I'm not getting anything greater than about 40 CPM with the 320S. Seems like it would be much higher especially if the 320S doesn't discriminate between energy levels. It would count ALL events equally within its range of energy detection. Right?
Does the Dose Equivalent Rate look right? Cuz once again, it doesn't really match up with what the 320S is saying. It's been running continuously for nearly 24 hours now and it's showing a Dose Equivalent Rate of 0.12 uSv/Hr. I always mess up the darn decimal point, but if I have it right, the 40 nSv/Hr we see in the RadNet report is the same as 0.04 uSv/Hr or about a third of what the 320S is reporting.
If that's correct, that means the 320S has a 3X difference and that's gotta be significant. Is this just the nature of these economy priced GCs? They only offer a relative value, not an accurate one?
So should I ignore the actual value it presents and look instead for substantial variations from the norm (norm according to the 320S)?
Clearly this is pretty new for me and I'm struggling to figure out if I'm getting accurate data or just relative data. If I ever had to use this during some post apocalyptic event, I'd really want to know the actual radiation levels. In this case what the 320S reports could be three times higher than it actually is. I'd rather have it report too high than too low, though. Tell me it's safe and it's not is kind of an issue! |
| Damien68 |
Posted - 03/09/2023 : 00:27:11
Hi tedssled
CPM and uSv/h are two things that have a relationship but are very different, which should not be confused:
1. CPM is the number of detections a counter will take in one minute (Count Per Minute)
2. uSv/h is a radioactive energy flow.
each radioactive radiation particle has an energy that can vary greatly from one particle to another.
Alpha radiation is a very high speed helium molecule that has been ejected from a nucleus.
beta radiation is a single electron always at very high speed which has been ejected from a nucleus.
an X ray or gamma ray (although there is a wave-particle duality) we will say that it is pure energy exactly like light. it is also ejected from a nucleus (for gamma rays) or from a high-speed collision between an electron and an atom (for X-rays).
Energy of an X-ray particle is from 1 keV to 100keV
Energy of a gamma particle is from 100keV to 100 MeV and more
for reference: energy of visible light is from 1.5 eV to 3.5eV:
1keV is 1 "killo-electron-volts"
1eV is the energy gained or lost by an electron that jumps a 1 volt gap.
1eV = 1.60217663e-19 joules
we therefore see that when we detect a particle, the energy of the detected particle can range from simple to 100,000 times more
a Geiger-Muller counter can only count the number of detections but cannot estimate the energy of a particle it has detected. the Geiger counter will simply can say for exemple I counted 15 particles.
also, now, if we want to convert to uSv/h, as the unit (Sv/h) does not consider the number of particles detected but only considers the energy of the radioactive particle field (number of particles multiplied by the energy level of each of them), we cannot do it from of measurements made by a Geiger counter.
that said, we do it anyway by using a calibration coefficient which will give a very approximate but still significant result. let's say that if we make a measurement, we can convert it, and obtaine a result but which will remain subject to interpretation (that an expert will be able to interpret).
To measure the energy of a particle or a field, we cannot do it with a Geiger-Muller counter, but we can do it for example with a scintillation chamber, and we can then obtain more judicious values in uSv/h.
EDIT: So gamma chanel range must be a classification according to the energy level of the gamma particle (from about 1keV to 100Mev or more). gamma and X-ray can be considered together as gamma. |
| EmfDev |
Posted - 03/08/2023 : 16:51:14
Hi tedssled, when measuring radiation, the CPM is kind of represents the sensitivity of a counter. These CPM values are then converted to the real radiation rate which is usually measured in µSv/h. Please check the µSv/h values when comparing different detectors. Also some counters may be able to detect alpha. The GMC-320S detects beta gamma and xray and background radiation is usually around 5-30 CPM. Also .14 µSv/h = 140 nSv/h not 1400. |
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