Thursday, April 29, 2021

My XRF Setup - Part 1 / How it all works?

 What is XRF? 

XRF stands for "X-Ray Fluorescence" and there are two main types: Energy Dispersive XRF and Wavelength Dispersive XRF. 

I'll focus on the Energy Dispersive, Direct Excitation (2D) Method as this is what I use. 

EDXRF is a Non-Destructive method for material analysis, used to determine the Elemental composition of a material or a chemical compound. It is an extremely useful tool to analyze raw materials, minerals, alloys, etc.

The Physics behind XRF is absolutely fascinating and at the same time relatively  simple to understand.

The description of the whole process can be boiled down to this: exposing a test sample to a beam of X-Ray radiation and detecting the energy of the secondary / characteristic X-Rays emitted by the atoms in the sample and then building a histogram of the energy spectrum in order to identify specific secondary X-Ray peaks.

When a material is irradiated by short-wavelength ionizing radiation like X-Rays or low-energy Gamma Rays, the electrons from the innermost electron shells, the ones closest to the nucleus (K, L, M shells) will become excited and are expelled from the atom. This causes a vacancy in that lower electron shell and it is immediately filled with an electron from a higher-energy shell.

For example, if the electron is ejected from the K-shell, this vacancy will be filled by an electron from the L or M shells. When the electron makes the jump from a higher-energy shell to a lower-energy shell in order to fill such vacancy, it must give off the excess energy and does it so by emitting a photon with energy equivalent to the difference. This secondary photon is again, an X-Ray photon but with a very specific energy to the particular element due to the unique binding energy between the nucleus of each element (with its protons) and the surrounding electron shells. 

This, secondary emitted photon is called a Characteristic X-Ray. By detecting the energy of these characteristic X-Rays we can determine which Element from the Periodic Table the examined atom belongs to. 

There is a number of such characteristic X-Rays emitted, based on which shell, the electron comes from to fill the vacancy of the electron expelled by the primary X-Rays - if the vacancy is in the innermost shell (K-shell) and it is filled from the L-shell it is called Kα energy, if it is filled by the M shell is Kβ energy and so on. Vacancies in the L-shell are filled from the M-shell and are called Lα and when filled from N-shell - Lβ

These characteristic X-rays energy are published in lookup XRF tables.

Overlaps between Kα/Kβ and Lα/Lβ energies for some elements exist so identifying an element often relies on identifying multiple energy peaks in the spectrum, coming from different transition lines.

This is a typical XRF histogram as produced by the MCA software. I used a small sample of pure (99.98%) Cobalt metal for this test. The two blue peaks on the very left are the Kα and  lines of Cobalt. 

The other peaks in the spectrum above are just parasitic peaks coming from the X-ray exciter or the environment - for example the tall green peak on the right of the Cobalt peaks is the Bromine 1-line @ 11.92 keV immediately followed by the Br 1-line @ 13.29 keV, all coming from the plastic clamp I used to hold the small Cobalt metal sample in front of the detector - the Bromine was likely used in manufacturing of the dye or filler of the plastic and even though the clamp was just partially exposed to the detector the Bromine (Br) lines were still detected.

What do I need for XRF?

At a very basic level - three things : X-Ray Source, X-Ray detector and a computer.

The Amptek XRF Kit - Mini-X X-Ray tube and the all-in-one X-123 detector.

X-Ray source: obviously the best source is an X-Ray tube in the 40-60 kV range but these require forced cooling, HV power supply, lots of shielding and a collimator. Such setups are large and very expensive. There are small and portable tubes but they usually have an even higher price tag.

Alternatively a radioactive isotope emitting low-energy gammas / X-Rays can be used as an exciter - Cd-109, Fe-55 or Am-241 just to name a few. The intensity is usually lower than the beam from an X-Ray tube, even when mCi amounts of activity are used so counting times are longer, but it is a very portable and uncomplicated method to produce the primary X-Rays. It is important that the energy of the exciting X-Ray beam is higher than the characteristic energy to be detected.

Needless the say, regardless whether the exciter is an X-Ray tube or a Radioactive Isotope, caution must be exercised at all times dealing with ionizing radiation. 

X-Ray Detector: the X-Ray detector must have very high-resolution (typically 122-200 eV or less) as some peaks are really close to each other and high efficiency in the low-end of the X-ray energy spectrum - typically, efficiency is >25% in the 1 to 25 keV range but my detector covers energies all the way up to 60+ keV at a reduced efficiency.

The required resolution and energy range are normally outside of the capabilities of most Gamma Spectroscopy detectors. Even the thin-crystal GS probes designed for the X-ray region will not have the resolution needed - some limited XRF might still be possible though.

 A specialized semiconductor X-Ray detector device is needed - Si-PIN, SDD or CdTe detector.

These detectors are very expensive, complex and very delicate devices employing a thin and very fragile (0.5 mil or less) Beryllium window and are usually evacuated or filled with low-pressure Helium gas. Inside the detector device are housed many components: the Si-PIN or SDD detector semiconductor chip, an input FET transistor for the preamp, a temperature sensor, a built-in multi-stage thermo-electric cooler (TEC) with a delta of ~85°C which reduces thermal noise in the detector chip and a temperature sensor. The heat pumped out from the chip must be constantly dissipated in the environment thru the mounting stud and the component's back surface thermal interface.

The detector is connected to a charge-sensitive pre-amplifier and the output of the pre-amp is fed into a Digital Pulse Processor (Dpp) which does the pulse detection, pulse shaping, ADC and pulse-sorting as it has a built-in Multi-Channel Analyzer (MCA) (8k channels). 

A Power Supply module generates the bias for the detector, the power to the TEC module and controls the temperature of the detector chip, besides powering the pre-amp and Dpp.

Because of the very low Characteristic X-Ray energies of light elements it can be extremely difficult to detect these elements as their secondary X-rays are easily absorbed even by air - lightest elements emit energies <1keV.

 High-Intensity primary x-ray beam, very thin Be-window, Silicon-Drift Diode (SDD) detector, vacuum chambers and even Helium-filled test chambers are often needed for elements lighter than Potassium to be detected.

Typical Si-PIN detectors work well for elements heavier than Scandium (Z>21) but I am actually able to observe even the Calcium lines - not in great detail but visible in the spectrum.

No comments: