Notes on Geiger Counters2025-09-24

Disclaimer

I am just some random software developer. I have no actual experience in this field. This information is purely for other hobbyists. I'm also not going to provide detailed explanations about their use or about radiation itself, there are more knowledgeable people who have written in depth about that. If you work around radiation in any professional manner, your workplace almost certainly already provides you with the necessary training and proper equipment. Absolutely never place your life in the hands of these cheap toys.

Preface

I have always wanted a geiger counter since I was young. It popped up every few years but they were always pricey and/or rough. I've recently been watching through The X-Files which occasionally shows them and there was also recently a thread on the front page of HN about a DIY counter using a Flipper. So I started looking into options again.

Extremely Basic Overview of Radiation

• In this context of ionizing radiation, there are five types: neutron, alpha (α), beta (β), gamma (γ), and X-rays.
• They are emitted during the decay process of an atomic nucleus.
• Neutron, alpha, and beta radiation are classed as particle radiation.
• X-rays and gamma radiation are classed as electromagnetic radiation (photons). Although they are produced differently, X-rays are formed from electrical acceleration whereas gamma is a nuclear process.
• Different decay chains emit more of each type than others.
• Different emissions also have varying levels of energy per photon, usually measured in electronvolts.
• Alpha radiation is easily stopped by extremely thin materials such as paper or the very outer layer of your skin.
• Beta radiation can be stopped by thin metal.
• Gamma radiation however requires dense materials such as concrete or lead to be stopped.
• Lastly and most importantly is the inverse-square law. As distance from a emitter is increased, the intensity dramatically drops.
• For example Am-241, commonly used in smoke detectors, primarily emits alpha radiation which is nearly entirely blocked by the housing and a small amount of gamma radiation which can escape. But thanks to the inverse square law, this tiny amount of gamma radiation blurs into that of the background radiation when just inches away and is effectively harmless when they are appropriately installed (eg. on ceilings, feet away from us).

Extremely Basic Overview of Radiation Safety

• Any exposure to ionizing radiation directly damages materials, such as the cells of living creatures. But it is also always around us, yet we still live?
• This is because exposure is measured by: type, energy level, and time.
• Since Alpha radiation is blocked by the very outer layer of skin, it is largely only an issue if you were to ingest, inhale, or put it inside your body some other way. Since once inside, it can then do damage to your living cells.
• Gamma radiation on the other hand will happily punch a bunch of small holes through your body without any care.
• Different decay chains have different energy levels, so some can be extremely energetic. These for example can cause more collateral damage in the surrounding areas than that of lower energy emissions.
• And the more time your cells are exposed to ionizing radiation, the more damage will be done especially if they are more energetic.
• So we need sufficient exposure to damage cells to a point where they cannot correctly or quickly replicate themselves anymore to be an actual health concern.
• In safety systems a common term is ALARA or 'as low as reasonably achievable'. For radiation this is achieved by: minimizing time around them, increasing distance from them, avoiding large emitters, avoiding high energy emissions, and ensuring sufficient shielding around them and yourself if/when possible.

Extremely Basic Overview of Geiger Counters

• edit: I've made a handful of corrections regarding scintillators thanks to u/Physix_R_Cool and some adjustments thanks to u/PhoenixAF. Their more detailed posts are both linked.
• A geiger counter has become a generic term to refer to radiation measuring devices.
• But the most well known type of geiger counter is one that uses a Geiger-Müller tube.
• Geiger-Müller (GM) tubes are usually metal or glass tubes filled with an inert gas and have a high-voltage electrical charge applied across them. When ionizing radiation passes through, the conductivity of the gas is altered which can be measured by the device to form a reading.
• More recently scintillation counters have become more common. These have two parts: a scintillator and a photosensor. A scintillator is a material that emits light when ionizing radiation passes through it. And then a photosensor (ie. a camera) measures the pattern of light emitted amount of light, which is proportional to the energy impacting the scintillator, which the device uses to form a reading. These have the benefit that different isotopes cause different patterns the actual energy level can be measured, which the device can use to identify the isotope.
• GM tubes notably have a "dead time", this is a period where it cannot detect events due to the matter in the tube physically changing state. Scintillators on the other hand emit light, which well... travels at the speed of light, so they effectively do not have a "dead time" do still have a similar decay time but this is a much shorter period, which makes them able to measure & respond to events much quicker.
• GM tubes can be sensitive to both alpha, beta, and gamma radiation. But depending on the model of tube or how the tube is encased by the counter they may not be able to measure alpha or beta radiation. Some counters will have a physical sliding sheath that allows the tube to be "toggled" between detecting alpha & beta or not.
• The scintillators commonly used in cheap detectors cannot detect alpha or beta radiation but are very sensitive to gamma radiation. All scintillators are similary sensitive as GM tubes, but in most cheap counters they are shielded by the casing enough in order for the photosensor to not be exposed to outside light to be very poor at detecting alpha and beta radiation.
• Size matters: a larger GM tube or a larger scintillator & photosensor area will allow for greater sensitivity. Increased photosensor area generally only improves uncertainty of energy measurements.
• Counters are usually calibrated to a specific source such as Cs-137. This means the energy readings from other emitters will not be accurate, unless the counter provides energy compensation and most importantly is accurately calibrated to the actual expected energy ranges. GM tubes can perform compensation by having physical sheaths that block out certain low or high energy levels to narrow the exposure. Scintillators can try and identify the isotope and perform compensation directly calculate the dose from the observed energy level in software. In cheap counters, both approaches are likely just rough approximations.
• Radiation measuring devices usually have two goals: showing dose rate (current amount) and/or dose (exposed amount).
• Some devices will only show dose rate, these are typically called radiation or survey meters.
• Some devices will only show dose, these are typically called dosimeters. These can also be passive, but I'm only covering active (ie. electronic) ones.
• But either device can often do both, it just may not be its primary function or accurate.
• Ionizing radiation is typically measured in grays (Gy). And in this context of human body exposure, dose and dose rate are usually measured in sieverts (Sv) and its respective hourly rate (Sv/hr). In the past some regions roentgens (R) and their human body exposure roentgen equivalent man (rem) units are still common were common, but the definition of them has changed over time leading to it being less accepted. Grays are also a replacement for rads. Different units are used in different contexts or regions.
• A sievert is the typical base unit, but numbers are typically displayed as microsieverts (µSv) or millisieverts (mSv), which are one-millionth and one-thousandth of a sievert respectively.
• Devices can usually display in clicks per second or per minute in addition to eg. sieverts. A click corresponds to a detected event.
• Randall Munroe of XKCD has a fun (not necessarily scientific) scale chart of various real world dose examples here.
• Geiger counters do not detect measure non-ionizing radiation.
• Geiger counters may provide false readings if exposed to UV-C light if they use a glass GM tube.
• Geiger counters may provide false readings if exposed to high power radio interference. This is in the same way our PC speakers used to buzz moments before a 2G/3G phone call came in.

Additional Notes About Radiation

• edit: Corrections about contrast agents thanks to u/Altruistic_Tonight18
• Non-ionizing radiation cannot directly cause harm, but at high enough power levels it can cause rapid localized heating which can do damage.
• Devices such as radios, cell phones, Wi-Fi access points, Bluetooth dongles, and microwave ovens all use non-ionizing radiation. They also all have strict power limits and/or shielding requirements.
• That little mesh on the front of your microwave oven is literally small enough to physically block the large microwaves from escaping.
• Standing in the sun will expose you to substantially more radiation than that of a cell phone. Which is why you should wear hats, sunglassess, and sunscreen.
• CRT displays can actually leak small amounts of X-rays, especially if the brightness is high or the power supply is poorly regulated.
• Being exposed to radiation does not directly make you radioactive, you must be contaminated somehow first.
• MRI scans without contrast themself will not expose you to radiation.
• X-ray, CT, and PET scans without contrast themself will expose you to radiation, but you will not become radioactive.
• Scans with contrast radiopharmaceuticals will both expose you to radiation, and make you temporarily radioactive. The modern isotopes used usually have short half-lifes and can be expelled by the body within a week or two.
• Radiocontrast agents themself are not radioactive, and are distinct from radiopharmaceuticals.
• You may encounter a close friend or family member that is radioactive from medical tests: if they've already talked to you about such procedure, then they may be curious to visualize it on a geiger. Otherwise respect their privacy.
• You may encounter complete strangers that are radioactive from medical tests, don't harass them or scare them, just ignore them like you do other strangers.
• If you're out and about and your geiger goes off, there is near-zero reason to start scaring people around you or call emergency services.

My Usecase and Requirements

• I have two primary uses:
  • be able to make sure nothing in my environment at home & work is unexpected
  • be able to be mindful of unusual objects when I'm out traveling in weird & fun places such as (hypothetically)
    • is that new scanning archway at the exit of the big box store actually X-ray or just cameras?
    • a spicy rock or boulder out on a trail
    • your friend wearing a "5G protection" pendant which turns out to be a chunk of Thorium
    • that posh restaurant which thought it'd be cool to serve food on Uranium glazed fiestaware
    • the poorly maintained X-ray machine at the dentist right next door to the coffee shop in the strip mall
• My requirements are:
  • can actually detect radiation, but no need for real accuracy
  • pocketable
  • battery that can last a few days
  • affordable, ie. not $3,000

Available Contenders

My Choice

I ultimately ended up purchasing the Radiacode 102 for its higher sensitivity and broad featureset. I strongly considered the S2-Mini, but the RC102 was more polished for only slightly higher (relative) cost. The S2-Mini would've been my first choice if the firmware or hardware was open-source. The others I considered too brick like to be comfortably pocketable. I may still pick up a HFS-P3 for its small size and a GMS-800 to try the Rad Pro firmware in the future.

Hardware and Menu Interface

It is what you expect for build quality. It has no real form of water or shock resistance. The menus are quite intuitive and functional and feature handy quick shortcuts. I think the loop on the case could be a bit thicker.

Testing

I first tested it against the only known radioactive source I had, a smoke detector with Am-241, and it expectedly tripped the default alarm threshold when in direct contact. Then I did the fun part of checking everything in my home, of which I did not find anything of concern. Although the granite in my bathroom clicks slightly higher than the granite in my kitchen. I was particularly interested in checking some suspicious reflective hats I had gotten a few years ago, but those too were seemingly fine.

Battery

I charged my RC102 four and a half days ago as of this writing, yet it still has 48% battery left, which is on par with their claims. I should note I do toggle the Bluetooth function as needed, since it otherwise constantly broadcasts and has no passcode. Edit: It is now six and a half days and at 28%.

Radiacode App

I do not recommend using the iPhone app since it phones home to Yandex and iOS provides no real mechanism to block internet access. They provide direct APK downloads for the Android app and it works fine, excluding the map function, on GrapheneOS with internet permission revoked. Even the firmware updates are bundled in the app so that works too. There is also thankfully no account function/requirement. So this is a very cool bonus feature for seeing rate and spectrum graphs as well as identified isotopes. Data access is also possible via USB.

Every Day Carry (EDC)

While the RC102 easily fits in a pocket, I opted for the $10 silicone protective case with carabiner and wore it clipped to my jeans pant loop. It dangles around, but is light enough to not bother me, although you must be mindful to not sit on it or have it eg. horizontal when closing your car door. The acid green color I chose also adds a tiny splash of color to any outfit. I spent a few hours in a particularly large concrete building and it was neat to see the background radiation nearly 2x higher indoors compared to outside of it. Otherwise I've yet to come across anything actually unusual. Do I actually plan to EDC a geiger long term? Unlikely.

Conclusion

Did I satisfy my childhood wants of a geiger? Yes. Should you EDC a geiger? Probably not. Should you own one? If you take some time to learn a basic understanding of them, I definitely think there is some benefit in having one handy.

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