Gamma Spectrometer MSL-GS1
Theremino MCA with NaI(Tl) or LYSO scintillation probe
Introduction
I would like to introduce my new device for gamma spectrometry. The equipment is based on the open-source Theremino multichannel analyser.
This is a basic gamma spectrometer for detecting gamma radiation. It can also be used for X-ray and gamma-fluorescence spectrometry.
Gamma Probe and Scintillator
Correct operation requires a gamma probe fitted with a scintillator. The probe contains a scintillation crystal, usually NaI(Tl) or LYSO.
An important component is the photomultiplier tube. In this design, it uses a bialkali photocathode with an effective area of approximately 15 mm.
A suitable high-voltage divider is required for electron multiplication. The divider consists of a chain of approximately 11 MΩ resistors feeding the dynodes.
Photomultiplier Operation
A gamma ray interacting with the scintillator produces a pulse of light. The photomultiplier tube converts this light pulse into an electrical pulse.
The PMT contains a photocathode, a focusing electrode and ten or more dynodes. Each dynode multiplies the number of electrons, producing a much larger output pulse.
A resistor chain, commonly located in the tube-base assembly, sets the voltages for the photocathode, dynodes and anode.
Signal Processing
The electrical pulses from the probe are extremely short and must be shaped before they can be processed by the sound-card A/D converter.
The high-voltage supply for the photomultiplier dynodes must be stable and have very low noise.
The sound-card signal is processed by multichannel-analyser software, which sorts pulses according to their amplitude and displays the corresponding gamma-ray energy spectrum.
Isotope Identification and XRF
The energy of gamma or X-ray photons can be used to identify a specific radioisotope from its characteristic spectrum.
For XRF analysis, the system can identify the composition of a non-radioactive material by measuring characteristic secondary X-rays emitted after excitation by high-energy X-rays or gamma radiation.
This type of analysis therefore also requires a suitable X-ray or gamma excitation source.
NaI(Tl) and LYSO Comparison
A NaI(Tl) scintillator has an important advantage over LYSO: it does not have the same intrinsic background radiation caused by naturally radioactive lutetium.
NaI(Tl) is therefore suitable for samples with low activity.
LYSO is commonly used for samples with higher activity. Its advantage is good efficiency and useful resolution at higher gamma energies because of its higher density.
Construction and Example Spectra
The original article includes photographs from manufacturing, the photomultiplier probe and example spectra.
The example measurements include:
- Lutetium and americium gamma spectrum.
- Americium-241.
- Caesium-137.
Resources:
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