High Background Radiation Rates - Is This Normal?

  • I
  • Thread starter dlgoff
  • Start date
  • Tags
    Radiation
In summary, the background readings on the Ludlum Model 3 Survey Meter are high, and it is unclear what could be causing the high readings.
  • #1
dlgoff
Science Advisor
Gold Member
4,432
3,221
TL;DR Summary
Background radiation question
I'm seeing 2-3K counts/minute background on my Ludlum Model 3 Survey Meter. Are we having above normal rates lately?

Never mind, I was on the X0.1 scale. But that's still high, isn't it?
 
Physics news on Phys.org
  • #2
What kind of probe do you have? GM or scintillator?
 
  • #3
gleem said:
What kind of probe do you have? GM or scintillator?
It's a NaI(Tl) scintillator, 2.5 cm (1 in) dia. x 0.1 cm (0.04 in) thick*

*From this link: https://ludlums.com/products/all-products/product/model-44-3

That link does show that Typical Background is 300cpm, so maybe what I'm seeing is normal.
 
  • #4
Ok. I have used this probe for contamination surveys. It is used primarily for low-energy gamma/Xrays. Being thin its response for high energy gammas is fairly low. 2-3k counts per minute is definitely high. Is this a new instrument for you? Did it come with a check source?
 
  • Informative
Likes berkeman
  • #5
gleem said:
Did it come with a check source?
I do have a check source. Not sure what I should be seeing, but at about 6 inches away, I'm seeing a little less than 1K counts/minute.
 
  • #6
6 inches away from what?
 
  • #7
gleem said:
6 inches away from what?
Away from the check source.
 
  • #8
Duh, sorry about that question. That sounds reasonable. How does it compared to the reading when you first got the instrument?
 
  • #9
gleem said:
Duh, sorry about that question. That sounds reasonable. How does it compared to the reading when you first got the instrument?
Actually, I've only had it for a short time. I purchased it on Ebay and there are no calibration stickers. I guess it needs to be calibrated correctly, which I don't know how to proceed.

BTW: Thanks for your time with this.
 
  • #10
The problem with this particular probe is that it is meant for low-energy gammas. Most sources of radiation that you might be interested in are fairly high. The problem is the sensitivity and therefore the cpm/ millisievert is dependent on the radiation energy.

Here is an energy response curve from Ludlum
Gamma energy response curve.png


As you can see the cpm varies significantly for the energy of the gamma so radiation exposure rate for a given isotope will not be the same for a given cpm reading.
 
  • #11
gleem said:
The problem with this particular probe is that it is meant for low-energy gammas. Most sources of radiation that you might be interested in are fairly high. The problem is the sensitivity and therefore the cpm/ millisievert is dependent on the radiation energy.

Here is an energy response curve from LudlumView attachment 291421

As you can see the cpm varies significantly for the energy of the gamma so radiation exposure rate for a given isotope will not be the same for a given cpm reading.
I understand, but I'm seeing these high readings from just the background.

edit: Still could be a good calibration is needed.
 
  • #12
Then you said you had 1 Kcpm from the check source. Did you get that number by subtracting the background reading from the check source reading? Where was the check source when you initially read the background?
 
  • #13
gleem said:
Then you said you had 1 Kcpm from the check source. Did you get that number by subtracting the background reading from the check source reading? Where was the check source when you initially read the background?
Let me try reading the check source then subtracting the background. I've got someone coming for dinner, so I'll check later and reply here.

The check source was in a lead pig when I looked at the background.

You've been very helpful. Thank You
 
  • #14
Take the meter outside away from any buildings to check the background. If the reading is still high there may be a problem with the meter.
 
  • #15
gleem said:
Take the meter outside away from any buildings to check the background. If the reading is still high there may be a problem with the meter.
Will do. I moved around in my trailer thinking there might be a source somewhere. But outside it is. :)
 
  • #16
@gleem
Okay. Outside, the check source, at about 8 inches away from the detector, was reading 1000 counts/minute and the background varied from 200 to 300 with burst sometimes to 500 counts/minute. So, what does this tell us?

Thanks. Just trying to make sense of this.
 
  • #17
Sounds like the detector is fine. Go back in the house with the detector on and watch the background as you get closer. Does it stay about the same 200-330 cpm?
 
  • #18
gleem said:
Sounds like the detector is fine. Go back in the house with the detector on and watch the background as you get closer. Does it stay about the same 200-330 cpm?
Inside, the background is now showing 200 to 300 counts/minute. So all's good, right?
 
  • #19
Right so why the 2 - 3 Kcpm counts in the OP?
 
  • #20
gleem said:
Right so why the 2 - 3 Kcpm counts in the OP?
I don't know. Looks like the thing fixed itself.
 
  • #21
Check the counter with your check source in the same configuration every time you use it.
 
  • Informative
Likes dlgoff
  • #22
gleem said:
Check the counter with your check source in the same configuration every time you use it.
Okay. Hopefully it stays fixed.

Thanks for all your help
 
  • Like
Likes gleem
  • #23
Maybe what you observed was radiation from a solar flare that is occurring presently.
 
  • #24
gleem said:
Maybe what you observed was radiation from a solar flare that is occurring presently.
That's what I was thinking when I wrote, Are we having above normal rates lately?, in the OP. So there is a solar flare going on? I tried googling about that, but couldn't find a reliable site. Thanks.
 
  • #27
Count rate depends on how the pulse-height discriminator is set. What electronics are you using on the Ludlum detector? I have not seen anyone here talk about pulse-height. The graph that Gleem posted is not counts per minute; it is energy of the pulses, which is proportional to pulse-height.

The detector you purchased is more of an x-ray detector than a gamma detector, because it is only 1 mm thick. You should use Am-241 from a smoke detector as a check source. There is both a gamma and x-ray from Am-241 to use to calibrate how to set your discriminator level. You can look up the eV; the gamma is near 59KeV if I remember correctly. So, you should see the two peaks and then you know what you are looking at. Set the discriminator to see only the larger peak (the gamma) on a pulse height analyzer. If you do not have a pulse height analyzer it is very difficult to make sense of these counts.

If you do not have the ability to set the pulse-height on a discriminator, you will not make any sense from these counts; you might be looking at noise. The detector might be bad. These thin detectors go bad from water leaking in over time. From reading only the gamma pulses, then you can calculate from the 1 uCi source and the distance from it the expected count rate and compare to your experimental rate. The half-life of Am-241 is so long that it should be close to what is marked on the smoke detector (~1 uCi). You will need to tear the smoke detector apart to get the radioisotope close to your detector.
 
  • Informative
Likes dlgoff
  • #28
The instrument that is used with the probe is the Ludlum model 3 which is just a pulse counter/HV supply which can also be used with GM tubes. The graph which I posted is to indicate the sensitivity to various energies of his probe. The threshold setting is fixed.

As far as check source is concerned any source that can produce a response is sufficient as long as the count rate is adequate, the counting geometry remains the same for the checks and the source is sufficiently long lives.

The OP was alerted to the problem of the meaning of the reading.
 
  • Like
Likes dlgoff
  • #29
Eric Reiter said:
You should use Am-241 from a smoke detector as a check source.
Thank you for this. It detects it nicely. Makes a great check test.
 
  • #30
dlgoff said:
Okay. Hopefully it stays fixed.

Thanks for all your help
gleem said:
Check the counter with your check source in the same configuration every time you use it.
This counter is still showing the correct background reading, even inside.:partytime:
 
  • Like
Likes gleem
  • #31
gleem said:
Right so why the 2 - 3 Kcpm counts in the OP?

Don gave an answer to that in his first post, he has it on the wrong setting ...

Never mind, I was on the X0.1 scale. But that's still high, isn't it?
Ohhh and auroral activity isn't going to make a detector read anything extra... The aurora is produced by
mostly electrons and some protons being trapped in the Earth's magnetic field

And fortunately, because of our atmosphere's protection, reading a burst of x-rays, when they arrive, several days
before the auroral activity begins is unlikely to occur at ground level either.
 
  • Like
Likes dlgoff
  • #32
davenn said:
Don gave an answer to that in his first post, he has it on the wrong setting ...
Looks like the half-life of my memory is about 1 post. :sorry:

davenn said:
Ohhh and auroral activity isn't going to make a detector read anything extra... The aurora is produced by
mostly electrons and some protons being trapped in the Earth's magnetic field
Admittedly that was a bit of a reach but x-rays and gamma rays are a small component of the radiation, and gammas from neutron activation have been observed and Don's NaI detector is quite sensitive to low-energy x-rays.
 
  • Like
Likes davenn and dlgoff

1. What is considered a high background radiation rate?

A high background radiation rate is typically defined as any level of radiation that exceeds the average natural background radiation level of 2.4 millisieverts (mSv) per year. This can vary depending on location and can also be affected by human activities such as nuclear power plants or medical procedures.

2. What are the potential health risks associated with high background radiation rates?

Exposure to high levels of radiation can increase the risk of developing certain types of cancer, such as leukemia, thyroid cancer, and breast cancer. It can also lead to other health issues such as cataracts and genetic mutations.

3. What factors can contribute to high background radiation rates?

Natural sources of radiation, such as radon gas and cosmic rays, can contribute to high background radiation rates. Human activities, such as nuclear power plants, mining, and medical procedures, can also increase radiation levels in certain areas.

4. How can I protect myself from high background radiation rates?

There are several ways to protect yourself from high background radiation rates, including limiting your exposure to sources of radiation, such as avoiding unnecessary medical procedures and using protective gear when working in areas with high radiation levels. It is also important to be aware of your surroundings and follow safety protocols when visiting areas with high background radiation rates.

5. Is it normal to experience fluctuations in background radiation rates?

Yes, it is normal to experience fluctuations in background radiation rates. Natural sources of radiation, such as radon gas and cosmic rays, can vary depending on location and time. Additionally, human activities can also cause fluctuations in radiation levels. However, if there is a significant and sustained increase in background radiation rates, it is important to investigate the cause and take necessary precautions.

Similar threads

  • High Energy, Nuclear, Particle Physics
Replies
6
Views
1K
  • Electrical Engineering
Replies
12
Views
1K
  • High Energy, Nuclear, Particle Physics
Replies
24
Views
2K
Replies
26
Views
830
  • High Energy, Nuclear, Particle Physics
Replies
6
Views
4K
Replies
13
Views
2K
Replies
29
Views
2K
  • High Energy, Nuclear, Particle Physics
Replies
7
Views
2K
  • High Energy, Nuclear, Particle Physics
Replies
2
Views
2K
  • Special and General Relativity
Replies
13
Views
1K
Back
Top