XRF Particle Detector

Introduction

As explained in my previous articles, the basic principle of detecting alpha, beta and gamma radiation with a Geiger–Müller tube is the ionisation of a suitable low-pressure gas between electrodes to which a high voltage is applied.

A GM tube can detect the basic types of ionising radiation—α, β and γ—and, with suitable modifications such as a cadmium converter, may also be used for slow neutrons.

Limitations of the Geiger–Müller Tube

The main problem is that a GM tube cannot measure very high radiation fluxes. Its dead time—the recovery interval during which it cannot register another particle—is relatively long, typically around 100 µs. In a purely theoretical estimate, this limits the count rate to roughly ten thousand pulses per second; practical values depend strongly on the tube and circuit.

The dead time can increase with the age of the probe, unsuitable overvoltage and prolonged operation in a strong radiation field. The gas filling is gradually affected by the repeated discharges.

Nitrogen or argon is commonly used as the main gas, with a small quantity of a halogen vapour such as bromine. The halogen quenches the discharge rapidly after detection, while the carrier gas supports the avalanche.

Because of these limitations, a GM tube is not ideal for intense gamma- or X-ray fields, where the flux may exceed the tube's ability to recover between events.

Another important limitation is that the detected pulses have almost constant duration and amplitude. Therefore, they cannot normally be used for advanced pulse-height analysis or spectroscopy.

Scintillation Detection

More advanced particle-detection methods use specialised detectors. Many principles exist, but this article concentrates on scintillation detection.

Scintillation detection is a common and comparatively straightforward method for measuring ionising radiation, especially high fluxes of gamma rays and X-rays. The pulse amplitude depends on the energy deposited by the particle or photon, so this method is widely used for spectral analysis.

The method is based on two essential parts. First, a scintillating material produces tiny flashes of visible or near-visible light when radiation deposits energy in it. These flashes are extremely weak and must therefore be detected, amplified and converted into electrical pulses.

Photomultipliers and Semiconductor Photodetectors

A photomultiplier tube can detect and amplify the light flashes from a scintillator. Using the photoelectric effect, photons release electrons from the photocathode. These electrons are accelerated and multiplied at a series of dynodes, producing an electrical pulse at the output.

Other light-detection methods can also be used, especially semiconductor photodiodes. These may be specialised silicon photomultipliers containing hundreds or thousands of avalanche microcells, or conventional PN photodiodes with high sensitivity in the emission wavelength range of the scintillator and sufficient response speed.

The signal from an ordinary photodiode is extremely weak and must be amplified by precision operational amplifiers. In photovoltaic mode, the measured quantity is the small current generated in the diode. This current may be in the femtoampere to picoampere range. It must first be converted to a voltage by a transimpedance amplifier and then amplified further.

Spectral Properties and Radiation Energy

The spectral properties of the scintillation detector are determined mainly by the scintillator itself. It emits light at a characteristic wavelength compatible with the photodetector, and the light intensity is approximately proportional to the energy deposited by the incident quantum.

Gamma and X-ray radiation have characteristic energies expressed in electronvolts. These energies depend on the source: the isotope in the case of radioactive gamma emission, the accelerating voltage in the case of bremsstrahlung, and the target material in the case of characteristic X-rays.

The material being irradiated can also emit its own characteristic secondary spectrum. This is the principle of X-ray fluorescence analysis (XRF). From the measured line energies, the elements present in a sample can be identified because every element has its own characteristic X-ray energies.

Reason for Building the Detector

After the theoretical work and my earlier GM-tube detectors, I decided to build a scintillation detector intended mainly for X-ray fluorescence analysis. The goal was not simply to count a strong gamma flux, but to analyse the amplitudes of the individual pulses.

The simplest common arrangement uses a photomultiplier and a scintillator. The pulses are amplified and analysed by the open-source Theremino MCA program, which acquires the signal through a sound card.

A dedicated high-speed A/D converter would be technically better, but at the time I had not found suitable software capable of processing the data from a convenient external interface. This remained an idea for a future project.

Choice of Scintillator and Photodiode

A photomultiplier system requires an appropriate scintillation crystal that is optically coupled to the PMT and completely protected from external light. Building a reliable light-tight assembly at home around a small crystal is not very efficient, and suitable crystals for photomultipliers are relatively expensive.

Complete scintillator-and-photomultiplier systems commonly cost approximately CZK 6,000–20,000. The cheaper units are often old and may have reduced detection efficiency. Frequently used NaI(Tl) crystals are strongly hygroscopic and can lose performance if the hermetic enclosure is damaged.

For this reason, I decided to use a special Hamamatsu photodiode supplied with a CsI(Tl) scintillator. CsI(Tl), thallium-doped caesium iodide, is less hygroscopic than NaI(Tl).

New detectors of this kind can cost around CZK 10,000. I was able to obtain a used one from an American seller on eBay at a much more acceptable price.

Low-Current Amplifier

The disadvantage of the photodiode approach is the need to measure an extremely weak current and convert it to a useful voltage.

The current may be only femtoamperes or picoamperes, so a precision FET-input operational amplifier is required. I found a circuit designed specifically for this application. The original design used a precision AD515AL amplifier. I replaced it with newer or more readily available precision components, including an OPA128LM and an AD524 stage.

These amplifiers normally cost thousands of Czech crowns when purchased new, but they can sometimes be found much more cheaply from surplus sellers.

Pulse Shaping and Theremino MCA

The output pulse from the final AD524 amplifier is very short and must be modified before it is connected to the sound-card input.

An RC integrating network lengthens and shapes the pulse, with optional additional gain if necessary.

The Theremino MCA program can then analyse the pulse-height spectrum. It is designed for this purpose and can display the probable element or isotope corresponding to spectral peaks.

Direct Gamma Spectroscopy and XRF

There are two main uses for the detector.

In direct gamma spectroscopy, the peak energies can be used to identify individual radioisotopes by their characteristic gamma spectrum.

In X-ray fluorescence analysis, the detector identifies the elements in a non-radioactive material. XRF is based on the quantum structure of the atom. When a material is irradiated by gamma rays or X-rays, it emits secondary characteristic X-rays. These photons are emitted in many directions and have energies characteristic of the elements in the sample.

This is a non-destructive method for determining material composition. Professional instruments can cost hundreds of thousands of Czech crowns.

My scintillation detector can be used together with a suitable excitation source. The source may be an X-ray tube or a gamma-emitting isotope. For low-energy XRF experiments, even an americium-241 source from an ionisation smoke detector can act as an excitation source, provided that it remains sealed and is handled legally and safely.

Construction Photographs

The original page concludes with several photographs from the construction process. The project was still under development at the time.

• 280 nm / 1 W UV-C LED.
• Excitation of the CsI(Tl) crystal under UV-C light.
• The particle detector with a PN photodiode and CsI(Tl) scintillator.

Project Gallery