Needing more than a spark test?

[rwm]

I think Homebrew is correct that the gamma emission occurs simultaneously with with alpha? Seems like that will be a lot of flux with 6 sources. I suspect the geometry and the efficiency losses will substantially reduce the signal however. The Am 241 gamma has to hit the sample and then the xray photon from the sample has to hit the scintillator. That is a relatively small part of the spherical area. Does any one know the activity or number of sources in a Bruker?
Robert

 

[homebrewed]

Interesting difference in the decays/sec. I guess it just illustrates the fact that you have to take info from the 'net with a grain of salt .

BTW, I just found some LYSO scintillators on Ebay for $12. 4x4x22mm, and they appear to be in decent shape. LYSO should be better than BGO for use in a spectrometer and it isn't hygroscopic, either. The more photons/Kev the better.

 

[rwm]

4x4x22mm ? Small square that is 22mm thick? What do we anticipate the face area of the PMT will be?
Robert

 

[graham-xrf]

BTW, one curie = 3E9 decays/second. So .9 uC = 9E-7*3E9 = 27E2, or 2,700 alphas/gammas per second. Multiply that by the number of Americium "buttons" you've got & there you go.

What is the emission rate of a thorium lamp mantle supposed to be?

Firstly - thanks so much for your replies.
Re: The Thorium Mantles. They are available on eBay from UK, sold as a source for checking Geiger counters, but you have to dig quite far to get any information at all about count rates, and one supplier says the amount of thorium "varies".

One seller does say this ..
"Thorium coated lamp mantles up to 200 counts per second on the Mini Monitor Geiger counter with an EP15 probe that is sensitive and has a thin mica window and detects alpha beta and gamma the PDS -100G scintillation counter 123 counts per second +/- (The PDS-100G is very sensitive,very expensive and only detects gamma that's why the cps are lower)".

They also say "hard to come by now", though you would hardly think so given the number of sellers.
--> Geiger Counter test Source v2, Thorium Mantle. Check source

Another eBay seller claims to have sold ..
.. "to Universities, Schools, Nuclear Medicine Research Centers, Students, Teachers, Professors, Scientists and even to at least one CERN Scientist".

He does ship to the Europe, United States, Japan, Canada, Australia. I don't know if imports radioactive at this low level are a problem.
--> Geiger Counter Check Source v2, Thorium Mantle. test

The thorium, if not for provoking X-Ray responses from metals, is useful as a calibration source.
Old radio valve tubes cathodes are also likely a source of thorium dioxide.

 

[homebrewed]

I'm looking at a 3x3mm or 6x6mm SiPM instead of a PMT. A 3x3 demo board is about $68 from DigiKey, a bit more than some of the used PMT assemblies I've found on ebay; but the SiPM approach would make it easier for other folks to make a similar one. I'm mulling over the idea of a little light pipe to match the 4x4 crystal to the 3x3 SiPM. That shouldn't be too difficult to make. So he sez in total ignorance (so far) <g>.

A PMT with an active area larger than the scintillator is better than the reverse, in terms of collecting as much light from the crystal as possible. Clearly, you want to buy something no larger than necessary, to avoid spending more money than you need to. But if you're shopping ebay you are limited to what's available as well.

 

[rwm]

I though we ruled out the SiPM approach earlier due to noise? This would be really cool if you built one based on the SiPM and Graham built one with a PMT.
I read about LYSO scintillators. Sounds like a good plan. A little less sensitive than CsI but not being hygroscopic is big.
I have to say I got lost on this: "They are using the 16 bit A/D on a USB sound card, so it's limited to a maximum sampling rate of 192KSPS. That's not nearly fast enough to capture the pulses coming out of a PMT, so they are stretching the pulses using a simple 2-pole low pass filter, then amplifying by 100 to get the pulse amplitude back high enough to get decent numbers out of the A/D. "

Robert

 

[RJSakowski]

Firstly - thanks so much for your replies.
Re: The Thorium Mantles. They are available on eBay from UK, sold as a source for checking Geiger counters, but you have to dig quite far to get any information at all about count rates, and one supplier says the amount of thorium "varies".

One seller does say this ..
"Thorium coated lamp mantles up to 200 counts per second on the Mini Monitor Geiger counter with an EP15 probe that is sensitive and has a thin mica window and detects alpha beta and gamma the PDS -100G scintillation counter 123 counts per second +/- (The PDS-100G is very sensitive,very expensive and only detects gamma that's why the cps are lower)".

They also say "hard to come by now", though you would hardly think so given the number of sellers.
--> Geiger Counter test Source v2, Thorium Mantle. Check source

Another eBay seller claims to have sold ..
.. "to Universities, Schools, Nuclear Medicine Research Centers, Students, Teachers, Professors, Scientists and even to at least one CERN Scientist".

He does ship to the Europe, United States, Japan, Canada, Australia. I don't know if imports radioactive at this low level are a problem.
--> Geiger Counter Check Source v2, Thorium Mantle. test

The thorium, if not for provoking X-Ray responses from metals, is useful as a calibration source.
Old radio valve tubes cathodes are also likely a source of thorium dioxide.

Also some TIG electrodes are thoriated.

 

[graham-xrf]

I'm a little behind in the discussion since H-M was down for a bit. Anyway, to comment on some of the recent discussion points....

1. A/D resolution and speed. My comment here is based on reading through the Theremino gamma spectrometer information. They are using the 16 bit A/D on a USB sound card, so it's limited to a maximum sampling rate of 192KSPS. That's not nearly fast enough to capture the pulses coming out of a PMT, so they are stretching the pulses using a simple 2-pole low pass filter, then amplifying by 100 to get the pulse amplitude back high enough to get decent numbers out of the A/D. This is a "nice" approach because it permits the use of _very_ inexpensive A/D's; and the drivers are generally included with the computer OS (the Teensy approach would use the provided audio library to accomplish the same thing). The LP filters also improve the SNR which helps improve the resolution of the multi channel analyzer.

The cheap A/D kit from PJRC can be found here

The Theremino group also has some information regarding the energy resolution (in FWHM) vs A/D bits. You really do want at least 14 bits. I believe more bits is better due to the method they use to find the pulse peak in the presence of noise and the inevitable misalignment of sample time relative to the peak -- it is highly unlikely you will actually sample the peak, so some form of curve fit or interpolation is needed. The energy resolution is completely dependent on how accurately the peak voltage can be measured. You'd then conclude that one of the fancier audio A/D's that can get to 24 bits would be better yet: but in fact those A/D's really don't give you true 24 bit resolution. From what I've read on the web about this, the lower 4 bits are basically noise. So 16 bits is the sweet spot in terms of "real" system performance.

I had mentioned in post #125 using a low-pass filter to "stretch" the pulse event, and I called it "smearing". I was maybe too hasty. It could be that all it loses is mostly noise, and leaves an easier pulse to capture.

Thanks for the link to the PJRC A/D converters kit.
Forgive that I still miss on this, but the $2.65 PT8211 Audio Kit the link leads to is a DAC kit for turning number streams into sound waveforms. I am thinking this is not what we mean.

16 Bits.
I have been looking through A/D converters, considering 14-bit and 16-bit candidates at rates up to 40MSPS.
These would be Texas Instrument, Analog Devices, and Linear Technology.
I have decided it has to be 16 bits. Also, I go for the highest sample rate possible for about £20 for the device alone.

For the 16 bits, at sample rates like 2MSPS or 5MSPS or even 10MSPS or so, come in at near the cost I was aiming for. Many can be had on a evaluation board, or evaluation kit. That costs a bit more, but I see this as a part as vital as the scintillator, or PMT. Going on the information from @RJSakowski in post #140, I am thinking the response scintillations may come at perhaps some kHz.

Other thoughts
Some A/D chips are 2-channel simultaneous sampling. I thought having two samples is a way to a fast first average.

If we use the known fastest rise time of the scintillator crystal, and the frequency response of the detector PMT or SiPM, then there is no harm in setting an input low-pass filter to something slightly above that, because we know what it cuts is noise. A low noise gain stage as part of the filter is a good idea, to set the A/D count range suitably near maximum.

Dolby-style noise removal (nonlinear gain to expand the low levels, filter, then compress back is not necessary if one has the sample numbers. If the genuine peaks are bouncing up and down from noise, or living in the middle of independent scatter noise counts, then replacing some least significant bits with a single set at some threshold level strips out those not making a valid contribution to the peaks. A vicious, crude, but effective digital noise filter.

I am still deciding the line between maximizing the effectiveness if the gadget, and the cost.
I still try for something that uses a USB link to a phone app, with some tiny thing like a Teesy 4.0, Arduino, PIC, or a Raspberry Pi Compute Module (like a Teensy, but 64-bit Coretex-A53, 1.2GHz, 1GB SDRAM, and 32GB Flash).
That last one is £30.90 in UK

 

[homebrewed]

I though we ruled out the SiPM approach earlier due to noise? This would be really cool if you built one based on the SiPM and Graham built one with a PMT.
I read about LYSO scintillators. Sounds like a good plan. A little less sensitive than CsI but not being hygroscopic is big.
I have to say I got lost on this: "They are using the 16 bit A/D on a USB sound card, so it's limited to a maximum sampling rate of 192KSPS. That's not nearly fast enough to capture the pulses coming out of a PMT, so they are stretching the pulses using a simple 2-pole low pass filter, then amplifying by 100 to get the pulse amplitude back high enough to get decent numbers out of the A/D. "

Robert

Apparently the filter cutoff frequency attenuates the pulse by quite a bit so that's why the Theremino H/W has a gain stage after the filter. I've been thinking about that filter some myself. A two-stage passive filter has pretty crappy filtering characteristics compared to what's possible with active filters. I'm thinking that it actually is acting more like a simple charge amplifier -- basically a type of integrator that outputs a signal whose peak is proportional to the total energy in the input pulse. A low pass filter should attenuate the crap out of a fast pulse (and in the process lose important information about the pulse), but the Theremino approach seems to work. There MIGHT be an opportunity to improve things a bit more if we understand exactly what that filter really is doing for the overall system resolution.

 

[graham-xrf]

I though we ruled out the SiPM approach earlier due to noise? This would be really cool if you built one based on the SiPM and Graham built one with a PMT.
I read about LYSO scintillators. Sounds like a good plan. A little less sensitive than CsI but not being hygroscopic is big.
I have to say I got lost on this: "They are using the 16 bit A/D on a USB sound card, so it's limited to a maximum sampling rate of 192KSPS. That's not nearly fast enough to capture the pulses coming out of a PMT, so they are stretching the pulses using a simple 2-pole low pass filter, then amplifying by 100 to get the pulse amplitude back high enough to get decent numbers out of the A/D. "

Robert

Re: A low-pass filter
I was not thinking about a passive filter. Let the first stage be a very low noise high bandwidth gain stage. Something that can amplify and reproduce even the noise waveform, while adding very little noise of it's own. Then an active low-pass filter with a roll-off starting beyond the frequency that a good scintillation has much meaningful information. Capture this with a an A/D that is fast enough to reasonably capture waveform. The shape might be one pulse, or perhaps the decay of one added to the rise of another, or all lifted by other scatter noise.

Later, we all get to figure out various strategies to dig out the trace of alloy metals in the data we get. and work on in the subsequent software. One approach is to run it for long enough, rejecting all but those that have a single stand-alone characteristic, and which repeat themselves enough to be considered valid, each with a height of their own.

There may be other schemes we can dream up that uses the data more efficiently. Once we get some real kit experiments running, and hopefully showing some promise, we may get some help from HM members who are really hot with software.

I think the pulse height lets us find a place on the X-Axis to put them, because the higher the energy, the shorter the wavelength that provoked it. The number of them having that height provides is Y-Axis histogram plot.The more there are, the higher the spike. This hopefully indicates the proportion of the alloy metal in the sample.

By then, we may have enough to know the steel. The simple combination of certain metals reveals what it is, even if the proportions indicated are totally screwed. Unfortunately, I am not sure this kit will ever see carbon well, or at all.

For now, I am concentrating on capturing the scintillations as cleanly and with as high quality information as possible, with the lowest cost stuff I can find.