How to calibrate a NaI(Tl) scintillator?

[consuli]

Hello!

I've got an gamma/x-ray NaI(Tl) Scintillator. It has a positive going signal design (Hamamatsu photomultiplier tube plus preamplifier).

I want to acquire its raw signal by a 250 MHZ digital oscilloscope and to process the signal myself using scientific programming.

What are the options for callibration? Please advice different approaches, cheap and more sophisticated ones.

Thanks in advance for your help.

 

[gleem]

What will be the source of your signal and how do you want to process it?

 

[consuli]

I need one or more known and pure frequency signals for the calibration, so that I can map the electrical output signal of scintillator to them. The calibration signals could be either be gamma or x-ray. Ideally I need one gamma ray and one x-ray signal as calibration signals.

 

[gleem]

I assume by your use of the term pure frequency you mean a gamma/x-ray of known energy. What range of energies do you want?

 

[consuli]

Low gamma and medium x-ray frequences. The exact frequence does not matter, as far they are pure and unmixed.

 

[gleem]

X-rays and gamma ray are not customarily characterized by frequency but by energy in electron volts,i.e., the energy an electron gains transiting a potential difference of one volt. Gamma rays and x-rays are physically the same and only differ by their origin. BTW what frequency in Hz do you think a medium gamma has?

 

[consuli]

As notation in frequency does not seem to be self-explicating, I did some research for Geiger check sources myself.

Which of these geiger check sources on Ebay do you recommend for the calibration of my NaI(Tl) scintillator?

 

[gleem]

None. Thorium or Uranium ck sources might be useful if you are prospecting for Uranium or Thorium. You are evading the question of what you specifically mean by low to medium gamma/x-rays. This can be subjective. In medical x-ray applications low would be less than 50 keV and above 50 keV to maybe 100 keV might be considered medium. In nuclear medicine applications low might be less than 100 keV and medium above that to 511 keV. In radiation therapy low might be less than 30 keV and medium might be 150 to 600 keV.

Typical calibration sources used in medical applications include Co57, Ba133, and Cs137 covering a range of energies from 32 keV to 662 keV. Sources need be no more than 1uCi and in the US such sources do no require a license for possession. See http://www.laurussystems.com/Isotope_disc_sources.htm

 

[consuli]

Thank you gleem!

However, the law situation for radioactive experimental source is terrrible in my country. Effectively it seems like everyone has a different opinion of what is allowed and what is not allowed. Last but not experimental source are much more expensive in Europe.

Is there a possibility to use an electrical generated x-ray for calibration, e.g. from an x-ray tube?

 

[gleem]

Is there a possibility to use an electrical generated x-ray for calibration, e.g. from an x-ray tube?

No and Yes . The x-rays produced by and x-ray machine are mostly continuous in energy with the maximum energy equal to the Voltage used to produce them. That voltage can be well controlled but the yield of those x-rays is very low compared to the lower energy x-rays which will swamp the detector. So using an xray machine directly is out.

When elements are exposed to to x-rays of high enough energy the radiation excites atomic electron to higher energy levels which then decay producing what are know as characteristic x-rays which are monoenergetic. These have well known energies. An elemental analysis technique known as X-ray Fluorescence Spectrometry detects various heavy elements by the presence of their characteristic x-rays. A sample of material is irradiated with a source of radiation whose energy is greater than that of the characteristic radiation being detected. Check this article for a much better idea of how this might be used to calibrate your detector https://xrf-spectroscopy.com/

 

[consuli]

This xrf-setup sounds pretty perfect to me. Among others I have got a little 99,99% palladium bar, which I could use for it. The wavelength of Pd is statet 0.05859 nm in https://en.wikipedia.org/wiki/X-ray_fluorescence.

So are there special requirements for an xrf-tube?
Are there complete xrf-boxes, that shield the x-ray and in which I can mount my scintillator for calibration?

 

[gleem]

You usually like to have at least two calibration points. The energy of the Pd x-rays is about 21 keV pretty low but I do not know for what you will be using your detector. The highest energy X-rays and yield from Lead are about 88 keV. To excite the characteristic radiation you need your x-ray unit set higher than the characteristic radiation and the higher the better up to a point because you will also be producing more extraneous radiation that will obscure the radiation you desire to see.

Another point is that NaI detectors are typically used for higher energy radiation which means that they may be encased with more material usually aluminum. which will absorb some of the low energy radiation.

Are there complete xrf-boxes, that shield the x-ray and in which I can mount my scintillator for calibration?

I do not know but using the link I gave you should be able to make one fairly easily. Important points to remember are shield the X-ray source you are using extremely well to avoid its radiation getting into you detector directly, collimate the x-ray beam well so that the radiation only strikes the sample you are irradiating.I still am not sure how you will be analyzing the signals from the detector. Could you supply more information? This might be very important.

 

[Vanadium 50]

I've done XRF and it's quite noisy - most of the x-rays are not from fluorescence. That makes it not very good for calibration. Also, NaI is typically used for 10-100x these energies which means a) your resolution is terrible (30% or so) and b) any non-linearity will be difficult to see.

I still am not sure how you will be analyzing the signals from the detector. Could you supply more information? This might be very important.

+1

 

[geoelectronics]

Home made XRF from Lanthanum on sodium iodide (Tl)

George Dowell