Safecast - Kickstarter Open Source Geiger Counter

[T_Warne]

Bump for more info.

What questions should I be asking the makers to get good answers about the calibration and sensitivity of this meter?

 

[Sigurthr]

Ask what isotope is used for the calibration and if proper geometry has been observed so that the sensor is uniformly exposed in the radiation field. Ask for an energy-response compensation chart for non-calibration isotopes. Ask for the energy response range and also what the method of beta shielding employed is. You can't call a device a dosimeter if it is geiger based and does not have beta shielding at least optional.

 

[T_Warne]

Cool. Thanks! I'll ask and see what they say.


This is what I sent:

Mr Bonner,
I have a few questions that I hope you can answer for me.

What isotope is used for the calibration?
Is proper geometry observed so that the sensor is uniformly exposed in the radiation field?
Is there an energy-response compensation chart for non-calibration isotopes?
What is the energy response range?
What the method of beta shielding is employed?

 

[T_Warne]

I definitely will.

My justification for maybe buying this toy ( other than having a neat new tool ) is to be more aware of possible contamination dangers. I live 145 miles northeast of Hanford, Washington and I own a house in Ca that is 10 miles north, as the crow flies, from the San Onofre Nuclear Generating Station.

It might also be interesting to take it next time I visit family in Romania. They are about 250 miles from Chernobyl.

~~~~~~~~~~~~
Here is the response.
Sean Bonner says:

Hi Ted-

Thanks for getting in touch. Let me see if I can answer your questions:

- I believe it's Cesium 137, but you'd need to check with International Medcom as they are the manufacturers.

- Yes, the entire sensor is exposed and International Medcom is a leading geiger counter manufacturer who has been an intricate part of the design process.

- I'm not sure, we use the LND 7317 sensor
LND, Inc. OCEANSIDE NEW YORK - Designers and Manufacturers of Nuclear Radiation Detectors

- We do not shield Beta or Alpha, these devices are designed to take in the full A/B/G measurement.

Thanks!
-sean
~~~~~~~~~~~~~~~~

***

This is from the kickstart page:
We feel that this is important and why we've standardized our measurements and devices around the 2" pancake sensor so that we can detect Alpha, Beta, and Gamma radiation. Medcom's Inspector uses this sensor, and we designed our new device around it as well. As this device is to some extent an update to the Inspector,


***
This is from the Inspector Alert page:
Specifications:

Detector: Halogen-quenched Geiger-Müller tube. Effective diameter 1.75″ (45 mm). Mica window density 1.5-2.0 mg/cm².
Display: 4-digit liquid crystal display with mode indicators
Operating Range:
mR/hr: .001 to 100.0
CPM: 0 to 350,000
Total: 1 to 9,999,000 counts
µSv/hr: .01 to 1,000
CPS: 0 to 5,000

Gamma Sensitivity: 3500 CPM/mR/hr referenced to Cs-137
Smallest detectable level for I-125 is .02 µCi at contact
Efficiency: For 4 pi geometry at contact
Beta
C-14 (49 keV avg. 156 keV max.): 5.3%
Bi-210 (390 keV avg. 1.2 MeV max.): 32%
Sr-90 (546 keV and 2.3 MeV): 38%
P-32 (693 keV avg. 1.7 MeV max.): 33%
Alpha
Am-241 (5.5 MeV): 18%

~~~~~~~~~~~~~~

I just emailed International Medcom in the hopes of finding more information.

 

[Seoul_lasers]

Actually sigarthur is correct.
The gm counter needs calibration to a known
source. Cs137 is generally the industry standard.
However, price consideration is the wide area
tube which makes it possible to detect low
Energy events. 3.6x the surface area of an LND 712.

 

[Sigurthr]

So far the only thing I don't like is that it can display the readings in RADS (mR/hr) [or maybe they meant Roentgens, either way my point is valid...] without having a Beta shield.

When Alpha, Beta, and Gamma are lumped together the only unit of measurement that is correct to use is CPM - Counts Per Minute.

If you can filter out Alpha and Beta you can correctly display readings in Rads/Rems/Roentgen or Sieverts.

So, assuming Medcom correctly calibrates these to Cs137 while shielding the tube from BETA radiation, which Cs137 also emits, but is normally filtered out, these geiger counters pass the muster so to speak as long as one uses the CPM scale without an external beta shield (good for checking for contamination where isotope is unknown), or adds an external beta shield and then uses the mR/hr or mSv/hr scales (dosimetry). The buyer MUST be aware that readings in mR/hr or mSv/hr for the unit as it comes ARE NOT CORRECT. Agsin; BETA shielding MUST be used to measure Rads/Rem/Roentgen or Sieverts!

 

[T_Warne]

This is the response so far from Medcom

Hi Ted,

Thank you for your interests in supporting the Kickstart project. I have forwarded this to our engineer Lee Henderson who is working on the instrument. You should hear from him before the said noted date. Please contact us with any further questions you might have.

Kind Regards,
Molly Lynch
Customer Care
International Medcom


@ Sigurthr: When I hear back from Lee Henderson, do you have any questions I should ask him?

Also, can you tell me more about Beta shields? What are they made out of? Can you show me what they look like? Do you think I could DIY one for this device?

I saw this on the kickstart page: ~ embedded hall-effect sensor (to detect attachments, e.g. for occluding alpha or beta radiation)

 

[Sigurthr]

You most certainly can DIY a beta shield, the most simple form is a sheet of aluminium exactly 3mm thick. The trick is getting it to cover the entire sensitive surface of the sensor, which for a pancake tube like the one used, is not hard at all actually. It's really only difficult for "hot dog" style cyllindrical sensors and end window sensors.

A Beta shield can also be made of high density plastic (like polycarbonate). I don't remember the exact needed thickness but 1.5cm comes to mind.

Basically beta shields have to be dense enough to stop the betas without being so dense as to cause Bremsstrahlung (secondary braking radiation from abruptly stopping betas/electrons). Any material can be used to stop betas, but if it is too low of density you need a large thickness or too high of density and you cause more radiation to be emitted.

No other questions really, other than why would a trained RAD engineer make a product which can display dosimetric rates (rad/rem/roentgen) when the sensor used is unfiltered. There is no single sensor in creation which can give an accurate dose rate when two or more radiation types are capable of being detected, so you must always filter all but one out if you are going to display dose rates.

 

[Seoul_lasers]

You most certainly can DIY a beta shield, the most simple form is a sheet of aluminium exactly 3mm thick. The trick is getting it to cover the entire sensitive surface of the sensor, which for a pancake tube like the one used, is not hard at all actually. It's really only difficult for "hot dog" style cyllindrical sensors and end window sensors.

A Beta shield can also be made of high density plastic (like polycarbonate). I don't remember the exact needed thickness but 1.5cm comes to mind.

Basically beta shields have to be dense enough to stop the betas without being so dense as to cause Bremsstrahlung (secondary braking radiation from abruptly stopping betas/electrons). Any material can be used to stop betas, but if it is too low of density you need a large thickness or too high of density and you cause more radiation to be emitted.

No other questions really, other than why would a trained RAD engineer make a product which can display dosimetric rates (rad/rem/roentgen) when the sensor used is unfiltered. There is no single sensor in creation which can give an accurate dose rate when two or more radiation types are capable of being detected, so you must always filter all but one out if you are going to display dose rates.

Absolutely correct, Sigurthr.
GM tubes are unfiltered therefore requiring shielding from unwanted "lower energy" radiation that create over saturation. Am241 is a great example of an isotope that easily over saturates wide area GM tubes and simular mica window tubes like the LND712. Hence, Sig's explanation regarding the filters "sliding cover filters" made from Aluminum
or 50mm lead shielding for mica window GM tubes being an absolute must.

---- just to clear up measurement stuff about ionizing radiation------
I need to stress that mR/hr reading applies to only hard penetrating radiation such as X-rays
or Gamma rays. It is not to be used for Beta and Alpha particles.
uSv/hr is a dosage (biological effect) rate. 1Sv/hr = 100rem (older dosage unit)

CPM is counts per minute and can be used for any radiation detection. How many ionizations per minute in the GM tube are occurring... Also used in Scintillation detection as well. light flashes (scintillations) per minute. Same thing..


The only sensors known to be radiation specific are Scintillation detectors and ZnSe
detectors.
These are commonly used for Gamma Spectroscopy. There are alpha and beta Scintillation
detectors around too. Commonly used in around nuclear reactors (sometimes called "friskers")

 

[T_Warne]

Time is running out and I'm a bit disappointed that I have not heard back from Mr Henderson yet.

Can someone please tell me if my understanding of the LND 7317 sensor correct? I think it is directional in its ability to sense radiation. It only "sees" radiation coming in from the direction of it's window?

(_____) <--- sensor with window on bottom
///||\\\ <--- radiation being "seen"

 

[Sigurthr]

It will sense high energy gamma radiation from all directions as it is not a shielded probe. It will sense beta and alpha from below the sensor only. Alpha only travels a few cm in air, and beta only travels a few inches (unless very high energy or a very large field).

Keep in mind pancake style probes are EXCEPTIONALLY FRAGILE. Being in a car and having the door slam shut is enough to rupture one. If a blade of grass pokes through the screen over the sensor, it will rupture it. You must be vigilant in guarding the fragile surface of the sensor.

 

[T_Warne]

Is proper geometry observed so that the sensor is uniformly exposed in the radiation field?
[Lee] The pancake tube is oriented in a similar manner to our Inspector. This has proved itself in calibration at the lab in uniform radiation fields many times.

Is there an energy-response compensation chart for non-calibration isotopes?
[Lee] This information is available from the tube manufacture, LND. See Attached

What is the energy response range?
[Lee] There is a graph showing this on the LND web site. See Attached

LND 7317 LND, Inc. OCEANSIDE NEW YORK - Designers and Manufacturers of Nuclear Radiation Detectors

 

[Sigurthr]

Pretty linear between 200keV and 1MeV and very close matching between window and side lines, this means that for Cs137 the orientation of the tube has a minimal impact. Huge spike (3-5x) in sensitivity centered at 60keV - this is the range medical/dental Xrays are commonly in. Expected 2-3x response for <40keV range. Below 100keV there is a large descrepancy between window and side wall readings, so for lower energy gamma/xray sources orientation is very important, for example at 70keV the window is 2x more sensitive than the sides.

 

[T_Warne]

Thank you for explaining that. I had no idea what the chart was showing.

Weird that it is so sensitive to medical Xrays.

The window being more sensitive makes sense. Wouldn't this make it easier to pinpoint an area of contamination?

 

[Sigurthr]

No problem, haha I had a feeling you might not know how to interpret the chart - it takes experience, they all look bewildering at first.

Generally pancake tubes like this are used for frisking - sweeping over a larger area looking for contamination. You want a generic "hot or not" answer, so you would use the window as the primary sensory location nearly always. It would depend on the isotope and field size whether you would get erroneous directional readings or not. Generally it is a non issue as you would never want to be in a field large enough to encompass the sensor on both sides. That would only come to play in a nuclear disaster type scenario, and GM tubes don't fare well in those scenarios anyway (the tube can actually report that no radiation is present when there is actually a lethal field because it overwhelms the sensor).

Anyway, I digress, for pin-pointing contamination once it has been discovered one would usually use an end window tube as they are very small and insensitive perpendicular to their axis so it is like looking down a tunnel - lots of side rejection.

Pancake tubes have two main features;
1) senses Alpha Radiation
2) large sensor area (increased overall sensitivity and more efficient at frisking)

So, all in all this unit looks to be pretty decent if you don't mind making a beta shield to go along with it. Just remember to only use the CPM function if you have the beta shield removed - as S_L and I have explained mR/hr and uSv/hr are only to be used WITH a beta shield.

 

[T_Warne]

CPM, got it.

I'm sure eventually there will be lots of accessories ( or DIY instructions ) available. I'm interested to see what people will do as far as linking it to GPS and for data storage. I hope someone will make an easy way to plug the data into Google maps.


Beta shield: I'm having trouble finding any info on a DIY beta shield. To give you an idea just how little is out there, this thread shows up on the 3rd page of google results.

I was able to find this set...



Shielding

Plexiglas (lucite) is the best shield for beta particles from P-32. When more than 1 millicurie of P-32 is handled, a sufficient number of x-rays (bremmstrahlung) may be formed to require Lead foil to be added to the exterior of the shield. The beta particles travel a maximum of 3.1 mm. in glass, 6.7 mm. in lucite, and 8 mm. in tissue.