http://www.theremino.com/blog/gamma-spectrometry
Note :
This page contains rather old information about troubleshooting noise issue with Rap-47 probe + Theremino PMT Adapter with Theremino MCA version4.7. For hardware noise problem, follow Theremino PMT Adapter document and "Starting Guide" in Theremino MCA Document (accessible from Help menu).
- If you are interested in obtaining Theremino PMT adapter, Alessio can send you a kit (or ready made one, depending on his time and other things). Please contact him for more detail. It's a lot easier if you get one made by him. Highly recommended.
Testing Rap-47 + Theremino PMT Adapter
Noise issue? How to reduce this?
1. Try different USB port, hub --- No difference with 2 hubs, so far.
Will try another computer.
2. Base line test setting?
3. RF shield?
4. MF shield?
Note:
It turned out that too high voltage (780 keV) to the Rap-47 was probably the main cause, combined with some other settings (like sound level, etc).
Using different Base line testing configuration, I'm now getting no noise.
Also, I learned that Max slope may influence the resolution (of higher energy).
Edit:
I guess I'm not done with the noise issue. I'm still trying different base line setting. (For HV voltage and sound level, I'm using 650KeV and recording level of 10, for now).
Here is screen shot of noise pulse.
As we can see, the energy of the pulse that is causing big ringing seems to be very high, around 4MeV or can be more.
So, I tried to increase the max energy in the pulse viewer, but it seems to be maxed at 3200KeV.
Also, I tried different Position parameter from 50 to 400, thinking placing it at 50 would not allow big ringing to go through and caught as a good pulse.
I'm not sure about the result, yet.
Once, I know good value for some parameters, I need to test them with high count situation (with lots of pluse), and also for the resolution.
More tests in this page : Theremino Mca Base Line Test
To be continued...
Edit : 2013-07-04
With The new Theremino Mca version 5.0, I'm seeing virtually no noise caused by the ringing of big pulse!
So, I can relax the "Max Noise" and other parameters and still get similar or better result.
I'm very glad to see yet another improvement by Livio et al.
And my PMT adapter (made by Alessio of the Theremino Team) is very stable and reliable.
Thank you for offering us excellent hardware and softwares!
I'll keep the rest of this page for now, but I will move it to other section, later, as it's no longer very relevant.
Theremino PMT Adapter Sound card plugged into 7 port USB Hub
Theremino PMT Adapter Sound card plugged into 4 port USB Hub
Example of (more or less) normal pulse shape
Spectrum of low energy noise
Back ground (with low energy cut off)
Back ground (without low energy cut off)
Solution(s)?
- Using lower voltage for feeding HV of Rap-47 reduces noise.
- Using lower recording volume setting reduces noise.
- Using lower "max noise %" in "base line testing" reduces noise. Ex. 10 instead of 20
- Using higher "Size (uS)" in "base line testing" reduces noise. Ex. 300 instead of 240
- Using lower "Position (uS)" in "base line testing" reduces noise. Ex. 50 instead of 80
Back ground with these corrections applied. We can still see a bit of noise below 10keV, but it's pretty small.
Trying different settings
Suggestions from Livio
http://www.theremino.com/blog/gamma-spectrometry/hardware-testsBefore all you must discover the noise path.
Try to move connectors, change cables and test:
- the audiocard alone,
- the audiocard + cable,
- the audiocard + cable + pmt adapter,
- the audiocard + cable + pmt adapter + PMT tube
Verify what is producing the noise
Try to make a better shield from the power supply and the preampc
Do not connect ground wires – too much wires creates ground loops.
Do not use external power supply – only the usb 5V \>> audio card \>> PmtAdapter
Result of the test
The noise (measured with DAA) was like this:
Just sound card = 0.3mvpp 0.59 - 0.73mS
With sound cable = 0.3mVpp 0.59 - 0.71mS
With PMT adapter = 0.4 - 0.5mVpp 0.74 - 0.86mS
With BNC cable = 0.5mVpp 0.76 - 0.86mS
With Rap-47 = Hard to see, but it goes down to 0.5mVpp and then go up (with pulse, I suppose).
More testing with different settings
I used pulse viewer to see what is the noise, and it is really a few small pulse before the real pulse (of usually higher energy, above 600KeV).
Making size bigger does help.
Making the Max noise seems to be the key.
Max slope does not influence a lot, but smaller Max slope improves the resolution.
Max noise of 5 cuts nearly all noise.
Testing with lots of input. (Combined Am241 and Eu152)
Max noise of 6 isn't so bad, at all.
Testing with lots of input. (Combined Am241 and Eu152)
Max noise of 7 seems still OK. Max slope of 40%
Max slope of 30%
Max slope of 20% 10sec
Max slope of 20% 60sec
Now, lower Size value of 360 13sec
Size value of 360 100sec
Same setting, 2nd try, to be sure
Back to the Max noise of 5
Max noise of 4. Notice cps value is less than half of Max noise of 7 (which was 444 cps)
Max slop of 20%
Max slop of 10%
Max slop of 5%
Max slop of 2%
Max slop of 1% (Minimum)
Max noise of 3
Same setting, just Eu152
Two images below is written Eu152, but it's Am241, evidently.
Am241 placed about 30cm (a foot) from the detector
Back ground, but still seeing the trace of Am241 and Eu152 in a lead pig about 2 meters away (7 feet).
Same thing as above but longer run.
Testing with different sources.
A bit of U235 in the Cs137 disk source? (Or from the back ground???)
I'm very curious about the strontium disk source. Lots of pulse, but no definite peak.
More about Strontium-90 in Strontium Detection section.