X-ray spectrum Analyzer

Scintillator and PHA?

John

what energy , what source?

Steve

Well I was thinking PHA, the source would be a glass tube with Tungsten target ~80kV. I could go with a beryllium window as well. Theres two books I see on Amazon, I'll be using them as references.

There are a few things I don?t quite understand in the pulse amplifier, but I its just part of the learning phase.

Happy New Year

"Martin Riddle" ha scritto:

I think that X-Ray spectrum depends mainly on target material and energy (kV). Once the material is chosen, the spectrum should not change. In which case do we use a spectrum analyzer?

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I believe the OP is talking about a x-ray fluorescence spectrum analyzer where high energy x-rays from a source excite fluorescence x-rays from a sample. The energies of the fluorescence x-rays allow measurement of which elements are in the sample and the elemental composition.

Bret Cannon

"Bret Cannon" ha scritto:

Opss... Thank you for the explanation.

I work with X-rays in electro-medical devices, and in mind I was thinking about the energy spectrum of the X-ray beam...

Fabio G.

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Using a scintillator lets you use a PMT to do the low level amplification of the signal, but you lose energy resolution. For covering large areas, scintillators are much cheaper than semiconductor ionizing radiation detectors, which is why they are used in radiation portal monitors at border crossings. For this application, a properly packaged scintillator and a PMT is probably more expensive than a silicon detector and preamp. The most common inorganic scintillator is thallium doped sodium iodide (NaI:Tl), which is hygroscopic and the toxicity of thallium is also a concern. Plastic scintillators are cheaper but they don't offer any energy resolution.

Energy resolution is a significant difference between scintillator and semiconductor x-ray detectors. A good scintillator generates about one scintillation photon per 20 eV of absorbed x-ray energy, but with light collection efficiency and quantum efficiency the signal "current" (ignoring PMT amplification) is at best about one electron per 400 eV of x-ray photon energy. This signal has full shot noise plus the excess statistical noise of the PMT, so the energy resolution of characteristic x-ray lines in severely degraded.

In contrast, using a silicon diode detector gives 1 electron-hole pair per

3.76 eV, they can all be collected, and this signal has much less than full shot-noise. This means that the energy resolution the measurement will be determined by the electronics used, especially the charge integrating pre-amp. The sub-shot noise behavior of semiconductor radiation detectors is quantified by the Fano factor, which is the ratio of the observed variance of the signal to the average number of electrons in pulses due to a fixed x-ray energy. In room temperature silicon detectors the Fano factor has been measured to be less than 0.1.

Makers of radiation detection electronics have application notes on pre-amps for radiation detectors including AMETEK/Ortec and Cremat,

Bret Cannon

Classically, semiconductor detectors like lithium-drifted silicon (Si(Li), pronounced "silly") and Ge(Li) (pronounced "jelly") are used in conjunction with a pulse-height analyzer (multichannel analyzer) that does single-slope conversion on the integrated pulses (this is called a Wilkinson D/A converter).

That only works with a slow photon count rate, 100 kHz or so.

There are also systems which scan a monochromator and measure X-ray flux with ionization chambers, but that requires a VERY HOT collinear X-ray beam, which means a relativistic synchrocyclotron...

"Bret Cannon" wrote in message news:GSa7l.2646$ snipped-for-privacy@nwrddc01.gnilink.net... | | "John Larkin" wrote in message | news: snipped-for-privacy@4ax.com... | > On Thu, 01 Jan 2009 02:58:33 GMT, "Martin Riddle" | > wrote: | >

| >>Anyone have any tips. Good reference links. | >>I'm toying with the idea, seems like a nice hardware/software project. | >>

| >>Cheers | >

| > Scintillator and PHA? | >

| > John | >

| Using a scintillator lets you use a PMT to do the low level amplification of | the signal, but you lose energy resolution. For covering large areas, | scintillators are much cheaper than semiconductor ionizing radiation | detectors, which is why they are used in radiation portal monitors at border | crossings. For this application, a properly packaged scintillator and a PMT | is probably more expensive than a silicon detector and preamp. The most | common inorganic scintillator is thallium doped sodium iodide (NaI:Tl), | which is hygroscopic and the toxicity of thallium is also a concern. | Plastic scintillators are cheaper but they don't offer any energy | resolution. | | | Energy resolution is a significant difference between scintillator and | semiconductor x-ray detectors. A good scintillator generates about one | scintillation photon per 20 eV of absorbed x-ray energy, but with light | collection efficiency and quantum efficiency the signal "current" (ignoring | PMT amplification) is at best about one electron per 400 eV of x-ray photon | energy. This signal has full shot noise plus the excess statistical noise | of the PMT, so the energy resolution of characteristic x-ray lines in | severely degraded. | | In contrast, using a silicon diode detector gives 1 electron-hole pair per | 3.76 eV, they can all be collected, and this signal has much less than full | shot-noise. This means that the energy resolution the measurement will be | determined by the electronics used, especially the charge integrating | pre-amp. The sub-shot noise behavior of semiconductor radiation detectors | is quantified by the Fano factor, which is the ratio of the observed | variance of the signal to the average number of electrons in pulses due to a | fixed x-ray energy. In room temperature silicon detectors the Fano factor | has been measured to be less than 0.1. | | Makers of radiation detection electronics have application notes on pre-amps | for radiation detectors including AMETEK/Ortec and Cremat, | | Bret Cannon | | |

Bret,

Thanks, Yes this would be for Identifying various elements in a particular sample. I was planning on a Si-diode detector, Hamamatsu or some other. I did find the Amptek site which has similar info to Cremat and the others. Right now I am collecting information, particularly on the PA.

Cheers

Quite correct, although the "classic" Wilkinson ADC is no longer a reasonable option. Directly digitizing the detector signal and subsequent DSP-ing is what has been on offer last 5-10 years. Better resolution, better throughput, easier to make (a lot less analog stuff, especially for those who can make both the fast and slow channels digitally (most can't).

And the Ge detectors have been without Li for quite some time - they are called HPGe (high-purity), unlike the GeLi they may be warmed up and will work fine after cooling. There is also a relatively new brand of Si detectors, which work at much higher than LN2 temperatures (apr. 0C), just Peltier cooled, which do deliver very good resolution at 1uS or so "shaping" times. But they are not too large - not the greatest efficiency, I think (have demonstrated our MCA with those at customers, but was not curious enough to find out what they do with them.... :-) ).

Dimiter

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Original message:

How about a spectrometer?

Good Luck! Rich