| T O P I C R E V I E W |
| luke.simon |
Posted - 10/10/2020 : 16:17:15
When did the GMC-300E Plus change from using the M4011 Geiger-Muller tube to the J305? I tested this tube and it is not triggered by visible light, but it is triggered by beta radiation from my 1940s ceramic bathroom tiles.
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| 8 L A T E S T R E P L I E S (Newest First) |
| Damien68 |
Posted - 10/12/2020 : 09:06:58
LND-7317 require 500v
Is expensive largly over 100USD you can find/buy it at LND websit: https://www.lndinc.com/products/geiger-mueller-tubes/7317/
I don't know if you can hack it into GMC-300e.
you can also ask GQ what he thinks about his sensitivity at 30KeV |
| luke.simon |
Posted - 10/12/2020 : 07:34:34
CRT high voltage is proportional to the size of the CRT. I have 32-inch CRTs that use 30 kilovolts, and my small 13-inch CRTs use 22 kilovolts. So the LND-7317 GB tube is sensitive down to 10keV x-rays. Impressive!
Can the LND-7317 GM tube be hacked into the GMC-300e Plus? Where can the LND-7317 GM tube be purchased?
I'd prefer not to have to buy an entirely new unit if I can just buy the tube and hack it into my existing chassis. I don't need wifi and the other features in the GMC-600 Plus. |
| Damien68 |
Posted - 10/12/2020 : 06:18:16
what could be cool, would be that GQ makes a small gamma spectrometer for cheap |
| Damien68 |
Posted - 10/12/2020 : 06:04:36
I find spec for radex 1503+ that use SBM-20-1 tube inside:
Sentitive for:
Gamma: 0.1 - 1.25 MeV
Beta: 0.25 - 3.5 MeV
So SBM-20-1 Gamma sensitivity starting is the same like for M4011 or J305
GMC-600+ is specified like this by GQ: Minimum Detectable Energy: ~0.1MeV(Gamma), ~2.0MeV(Alpha), ~10KeV(X-Ray)
GMC-600 is specified like this by GQ: Minimum Detectable Energy: ~0.2MeV(Gamma,Beta), ~3.0MeV(Alpha)
So following specifications, LND-7317 of 600+ should be the only one suitable for detecting X-Ray from 10KeV
But in my opinion we will not detect much,
It will be necessary that the X-rays have an energy greater than 10 KeV, and also that they cross the CRT tube glass plate.
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| Damien68 |
Posted - 10/12/2020 : 05:12:03
THT power supply for CRT tubes is generally between 3000v and 10,000v, may be that it has larger tubes.
so in principle it shouldn't emit X-rays beyond 10keV, are you sure of your 30keV?
the main nuisance of CRT tubes is more the possible creation of ozone when the insulation of the high voltage cable is defective. there are people who feel it right away, others never (even witout covid)  |
| ullix |
Posted - 10/12/2020 : 02:00:45
I agree with your ideas on cost cutting. It is just that it leaves a bad taste when changes are made affecting the specs of the device, yet pretending nothing has changed.
Case in point: the so called "calibration". It is the old one. Here the readout from an old GMC300E+ with a M4011 tube. This is standard on all GMC counters with a M4011 tube (readout done with GeigerLog):
Device Calibration Points:
Calibration Points: CPM = µSv/h CPM / (µSv/h) µSv/h / CPM
Calibration Point 1: 60 = 0.39 153.8 0.0065
Calibration Point 2: 240 = 1.56 153.8 0.0065
Calibration Point 3: 1000 = 6.50 153.8 0.0065
Calc: e.g. divide 60 by 0.39= 153.8 = 1 / 0.0065; pay attention to the units!
This calibration is no longer valid for the new tube. (Not that we know it had ever been valid for the M4011 - to my knowledge a calibration has never been done. Convince me otherwise by showing data.)
It gives an idea about the hand-waving nature of this "calibration".
And, to repeat it one more time: the calibration is meaningless when you have beta rays!
Re tubes for low-energy x-rays: 30keV is really on the low side.
When looking for a low-cost tube, compare them by areal-thickness of the tube walls: glass tubes (M4011) have 50±10cg/cm2 or 500 mg/cm2, while stainless steel tubes (SBM20) have 40 mg/cm2, i.e. only a tenth! Not sure a SBM20 suffices for 30keV, but surely a better chance.
Bigger tubes are also better. (SBM20 is marginally bigger than M4011).
Thinner walls are offered only by tubes with Mica windows, i.e. alpha sensitive tubes. I wouldn't be surprised if you need them for such low energy gammas, but I have no own experience.
This site https://www.imagesco.com/geiger/geiger-counter-tubes.html claims a window tube sensitive down to 7 keV, a glass tube only 36keV. M4011 specs (of obscure origin) claim 100keV.
The next challenge then may be to make sure that you measure in an environment with no beta radiation!
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| luke.simon |
Posted - 10/11/2020 : 10:14:07
Cutting costs to maintain a $75 (USD) price point is reasonable given that there is an annual inflation of around 2% per year. I care more about my tube not being triggered by harmless visible light, which apparently is an issue with the M4011. While the GMC-300E Plus is not a truly scientific lab grade device, I am impressed with what it offers at such a low price... and the device was clearly designed to be easily modified by the consumer, which is also nice.
Here are the calibration settings that came with my device...
- Calibrate 1: CPM=60, uSv=0.39
- Calibrate 2: CPM=240, uSv=1.56
- Calibrate 3: CPM=1000, uSv=6.5
...what is the calibration that comes with the M4011 tubes?
I mainly care about detecting low energy x-rays emitted by the cathode ray tubes that I collect. I read on the eevblog forums that the GMC-300E Plus can detect low energy x-rays, when the plastic back cover is removed, and in fact one of the board members tested it with a lab x-ray machine
What GM tube do you recommend for measuring low energy x-rays around 30keV? I'd prefer an inexpensive tube that let's me know if miscalibration of the cathode ray tube's anode voltages is causing the CRT to emit low energy x-rays. |
| ullix |
Posted - 10/11/2020 : 00:41:00
Oops, how did that happen?
This is the same tube as used in the low-cost CAJOE counter, which I reviewed here https://sourceforge.net/projects/geigerlog/files/Articles/GeigerLog-Radiation-v1.1%28CAJOE%29-Support-v1.0.pdf/download.
Low-cost seems to be the key word, as some browsing reveals that the J305 tubes tend to be a few bucks cheaper than the M4011 tubes!
But they are also a little bit shorter. Measuring from outer-end to outer-end the J305 is 86.5mm, while the M4011 is 89.0 mm.
This also implies that the volume of the J305 as well as its inner surface is a little bit smaller than that of a M4011, effectively making it less sensitive. Great ad: For the same price you get less!
So, my question: has the internal Calibration Factor also been reduced from the M4011 value of 0.0065 µSv/h down to a lower value?
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