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I What's going on Cs-137

  1. Jul 3, 2017 #1
    Hey guys, im currently running an experiment with Cs-137 multiple sources ranging from 1 to 10 uCi, i have a lead pig with a Cs-137 source at the top and a 1/4" small tunnel drilled through the 2" thick slab inside the pig is the Gieger counter, Using various heavy metals as a shield i seem to only get the radiation shielded down to certain level, even with 2mm of lead the level doesn't drop any lower then .2mm of lead. Is this radiation causing other reactions on the other side of the shield to cause a consistent transmission level?
  2. jcsd
  3. Jul 3, 2017 #2


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    Do you have a sketch of the setup?
    What is the rate you measure?
    What happens if you remove the source?
  4. Jul 3, 2017 #3
    What kind(s) of radiation does Cs-137 produce, and how much lead do you need to shield them?
  5. Jul 3, 2017 #4
    Pending the requested diagram let me note that in comparing count rates between 0.2mm and 2mm of lead shielding the difference may be too small for you to reliably notice a difference if you are trying to read the rate meter because it may be too low with too much fluctuation. The fraction transmitted through 0.2mm slab Is expected to be 0.98 and through the 2mm 0.84. If you have a scalar you should collect enough data to get a statistically significant difference between the different reading.. BTW the half value layer for Cs-137 is 8 .mm of lead.
  6. Jul 3, 2017 #5
    After checking the exposure rate constant for Cs-137 which is 0.33mR/mCi.hr at 1m. a 10uCi source would produce an exposure of only 0.052 mR/hr at 25 cm which would only be measureable on the lowest scale of most GM counters and subject to large fluctuations. So you need a stronger source to get a reasonable steady reading.
    Last edited: Jul 3, 2017
  7. Jul 5, 2017 #6
    The source is about 3-4 in from the detector, without a sample in the direct beam the reading is apprx 5 mR/hr with my 5uCi source. When i remove the source the reading drops to less than .004 mR/hr [​IMG][/IMG] [​IMG]

    Attached Files:

  8. Jul 5, 2017 #7
    With no source the reading is 0.004 hr/hr and with a 5uCi sources the reading is 5 mR/hr.

    However at 3 inches (7.5 cm) the expected reading should be 0.34 mR/hr for a 5 uCi source. So why the high reading? From your diagram I would expect backscatter scatter from the shielding behind the detector from the Compton effect from the Cs gammas that pass through. Also the 662 kV gamma from Cs will interact with the lead via the photoelectric effect to emit 70 -80 KeV x-rays which in your case are entering the GM counter mostly from the rear. You have what we call in the business "bad geometry" . You must reduce the scattered gammas and characteristic xray. One way is to move the lead behind the detector farther away using the inverse square law to reduce these radiations. You could also add some low atomic number material like aluminum. With sufficient thickness it will reduce the backscatter and characteristic xrays from the lead while not contributing as much as the lead would.
  9. Jul 5, 2017 #8
    Okay so when i removed the rear lead wall my reading was unchanged, still about 5 mR/hr. To cut down on any interactions from the aperture I ran another test with a free floating source and no lead anywhere in the path my reading jumped to 23 mR/hr @ 7.5cm. Both tests were run without a shield in the path of the beam.
  10. Jul 5, 2017 #9
    The reason that you have a larger reading without any shielding is because you are exposing the whole GM tube. The GM counter is calibrated by exposing the whole tube to a radiation field that is constant over its volume. I do not know why you are reading 5mR/hr since the source is not capable of emitting that much radiation assuming the counter is calibrated correctly.

    Question, what are the dimensions of the GM tube.

    Also another aspect of bad geometry is that the absorber is next to the tube and might be producing its own scatter/xrays that might have otherwise not have passed through the tube. Move the absorber closer to the source until the reading of the GM counter reaches a minimum. This is the optimal position.
  11. Jul 5, 2017 #10
    Pancake Gieger 5x2.5cm calibrated not more then 6 months ago. Just tested the same experiment with another detector and received a much more believable reading, .35mR/hr it appears my main Gieger is out of calibration i'll be sending that out soon for re-calibration in the mean time my back up Gieger doesn't fit in my chamber nor does it communicate with a computer :/

    I figured that and to reduce the chance of other reactions we reduced the amount of the counter that is exposed by having a smaller aperture. Hence the lower count.
  12. Jul 5, 2017 #11
    The pancake chamber is a thin window chamber an sensitive to electrons and almost 100% efficient as comparted to gammas The electrons which will be knocked out of the lead absorber by the Cs gamma (Compton recoil electrons) could account for the high readings. To check this hypothesis put a few sheets of Aluminum foil between the absorber and the pancake chamber and see if this helps.
  13. Jul 5, 2017 #12
    Even with the aluminum sheets (3) i'm still having the high counts in the 4-5 mR/hr range, which shouldn't be happening with a 5uCi source...
  14. Jul 5, 2017 #13
    I should have checked the range of the electrons .It's more the 0.5mm Al. not a few sheets of Al foil. sorry.
  15. Jul 5, 2017 #14
    Okay, I have some Cs-137 calibrated sources i'm going re-cal the GM tomorrow morning and try the Al tests again with thicker layer. i'll post the results. Thank you
  16. Jul 6, 2017 #15
    I've calibrated the detector and repeated the recoil electron hypothesis, with apprx .8mm of Al @ 7.5cm im reading a measure of .9 - 1.6 mR/hr open source.
    Much lower reading but still hot for a 5uCi source.
  17. Jul 6, 2017 #16
    You said previously that the GM counter was connected to a computer. If the GM counter a stand alone instrument with an output to a computer or what?
    Tell me about you calibration procedure.
  18. Jul 6, 2017 #17
    The GM is a SEI Inspector EXP usb, it is capable of connecting to a computer to store and print a graph read out of exposure or you can use it by itself as a handheld with a probe. I receive the same reading either way.

    I called SEI directly and they put me in touch with their calibration lab. Using a 1 uCi calibrated source they had me put the source directly on the detector screen while connected to the computer, and change the "Sensitivity" to a higher setting. Until the detector was reading an average of 35 mR/hr with the source directly on the screen.
  19. Jul 6, 2017 #18
    Not what I would consider a standard calibration procedure. First do you know if your source has a calibration certificate? If not how do you know if it is a 1 uCi source and not a 0.8 uCi source. Is the source a button source or a rod?
  20. Jul 6, 2017 #19
    It is a button source, we don't no the exact Ci, but the paperwork that came with the source say its +,- 10%, as far as the tech on the phone i dont know what kind of cert's his source has. TBH the whole experience seemed a little weird as far as "calibration" goes, just by changing a sensitivity to calibrate seems iffy.
    Most everything i've read about calibrating this particular unit says u need to have the probe at a distance that corresponds to 50mR/hr and record the data for 20 consecutive readings.(which also doesn't make sense cause its a continuous readout).
  21. Jul 6, 2017 #20
    So you know it is between .9 and 1.1 uCs. If SEI is a certified calibration lab. it will have a source traceable to the NIST (National Institute for Science and Technology formerly the National Bureau of Standards). A typical calibration procedure would be to use a relative strong source say 100 mCi and expose the probe at a distance a meter or more from the source where the radiation field is fairly uniform over the volume of the detector. They know the exposure rate at this point form the accepted value of the Gamma Ray exposure constant which is 0.38 mR/hr/mCi. They may repeat this for different scales by using different distances.
    Now having done that they can place a 1uCi source a specific way on the detector and note the reading which of course will be different. If you repeat this procedure at a future date and the reading is the same they you should be reasonably confident that your instrument is still in calibration. Of course you have only check the reading on one scale.

    Your instrument has built into it a set of calibration factors for different radiation sources since each source emits different amounts of radiation per mCi. The probe only measure the number of interactions per minute so it does not have any idea of the type of energy of the radiation that it detects.
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