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hannaconner
USA
2 Posts |
 Posted - 07/07/2025 : 00:44:30
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Hi All
I bought GC-500 last year and yesterday also Radiacode 103G which was about 5x more expensive than GC-500.
I noticed that GC-500 readings in CPM are significantly different (lower) compare to Radiacode 103G which shows about 60x higher! I assume that GC-500 has a bug in calculation of CPM as it actually shows CPS (Counts Per Second) and not CPM (Counts Per Minute). Attached comparison photo. At the beginning I suspected that something is wrong with my GC-500 as it says that it has M4011 tube, but mine has J321? Has anyone noticed this, is my GC-500 I bought on Amazon.de counterfeit product? I attached photos.
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| Reply #1
EmfDev
2341 Posts |
Posted - 07/07/2025 : 13:32:03
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The RadiaCode 103G, like many compact radiation detectors using silicon photomultipliers (SiPMs) or similar solid-state sensors, may indeed show higher CPM (counts per minute) values compared to Geiger-Müller (GM) tube-based devices. But that doesnt necessarily mean its detecting 60× more gamma radiation in reality. Heres why:
Possible Reasons for High CPM Readings in the 103G:
Multiple Small Detectors
The 103G uses an array of small light detectors and scintillator material (e.g., CsI(Tl)). These can be highly sensitive and fast, but if the system lacks proper shielding or background rejection, it may pick up more environmental noise or low-energy photons, inflating the CPM. Or a gamma particle just produces too many photons.
CPM vs µSv/h Conversion Differences
A device showing 10,000 CPM doesn’t automatically imply high dose unless the calibration factor justifies it. The 103G may simply report CPM with more granularity, but its µSv/h reading is what should be compared across devices for actual dose rate.
Physical Limitations
Detecting 60× more gamma in the same physical space would require:
A significantly larger or more efficient scintillator
Better photon collection optics
Extremely low noise electronics
While the 103G may be more efficient than a GM tube in some energy ranges, a true 60× improvement for gamma detection in such a compact device is unlikely without trade-offs in precision, selectivity, or false positives.
Youre right to be skeptical. The CPM count can be misleading if viewed in isolation. Its likely that:
The 103G is very sensitive, possibly to a fault (overreporting background),
It shows more raw events, but not necessarily more meaningful gamma dose,
The dose rate (µSv/h) is a better metric for cross-device comparison.
If youre comparing with another device (e.g., GMC-600+ or RADEX), focus on the µSv/h readings, not just CPM. |
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| Reply #2
Kaninbjerget
Denmark
20 Posts |
Posted - 07/09/2025 : 12:04:43
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Neither is defective. But you have a lot to learn about radioactivity.
Counts (per second, minute etc.) are just that. Whenever radiation interacts with matter in the sensor. Counts say absolutely nothing about the radiation level by itself and unless two sensors are identical you can't even compare them (and even then there's also manufacturing tolerances). The more matter to interact with the more counts. And since solid matter has a lot more mass than a gas then naturally there'll be a lot more interactions in the solid matter for the same amount of volume.
A tubed geiger can't distingish types of radiation and energy levels. That is why they're almost never accurate (but fine in the real world). Sure the counts per something is spot on but coverting counts to radiation level is not as it depends on what isotopes it is set for. And remember it's only valid the tube is only exposed to gamma radition so you need to shield it from that.
The Radiacode being a scintillator is quite another beast as it is almost insensitive to beta by itself (the hardest ones it does register) but more importantly it also registers energy levels. Therefore it can calculate doserate pretty accurate even for unknown random sources. Counts on the Radiacode really doesn't make much sense since it's so huge numbers. And really what you should care about when determing how dangerous an object is being near for a given amount of time it's the doserate you should focus on - in that case forget the counts.
Counts do have a purpose however - especially in scintillators. It's useful when searching for weak objects and contamination. Especially when not shielded against beta. A lot of beta doesn't contribute much to doserate but will have a significant impact on the amount of registered events (that is, counts). And yes, if checking for contamination you ideally should have a geiger with a large pancake tube as well to be able to detect alpha too.
J321 and M4011 are equivalents. Just different manufacturers but no difference in performance other than what individual manufacturing tolerances account for. It's like two "identical" coins but one minted in Denver and the other in Philadelphia. |
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PM bug in GC-500
