Alpha Spectroscopy Techniques

Abstract: This article presents some results obtained using the alpha spectrometer described in the previous post. We acquired the spectra of some alpha radiation-emitting sources. The preparation of an alpha source suitable for the acquisition of a spectrum showing the energy peaks with good resolution and with the correct energy values ​​is an activity that requires technical knowledge, a lot of patience and a certain amount of “imagination”. The main problem is the self-absorption by the source itself. Using some chemical and electrostatic deposition techniques, we were able to obtain more than satisfactory results.

Introduction

For details about the instrument, we refer to the post Improved Alpha Spectroscopy with Red Pitaya where we describe the sensor, the apparatus, and the signal acquisition system consisting of the RedPitaya board. The system also includes MCA software (developed by Pavel Demin), which is not part of the Red Pitaya “suite” but needs to be downloaded and preloaded onto a different SD card. All information can be found at the link https://pavel-demin.github.io/red-pitaya-notes/mcpha/. The MCA application allows you to visualise the energy spectrum of the radiation detected by the sensor and also enables data export in CSV format. For data processing, we used the ROOT software, which is the data analysis framework used by CERN. The sources we examined are as follows:

• Americium source from a smoke detector capsule;
• Radium source from luminescent paint;
• Thin films of Radium and Polonium obtained through chemical deposition on a substrate;
• Radon and Thoron decay products obtained through electrostatic capture on a metallic substrate;
• Thin film of Uranium obtained through chemical deposition on a substrate;
• Thin film of Thorium obtained through chemical deposition on a substrate;
• Beta source of Strontium 90;

Americium 241

The Americium 241 source is derived from the classic smoke detector. For safety reasons, the source is encapsulated in a small metal container (as shown in the image on the side), protected by a thin metal film. The decay chain of the Americium 241 isotope is shown in Fig. 1, where it can be seen that the decay occurs through the emission of alpha particles to the Neptunium 237 isotope. The half-life is 433 years, which means that a minimal amount of Neptunium 237 will also be present in the sample, but this amount is too small to be detected through its alpha radiation emission.

 

Fig.1 – Americium 241 decay chain

Alpha particles lose energy as they pass through the protective metal layer, which is why the alpha emission spectrum shows a broad peak around 4.6 MeV, instead of a narrow peak at the correct energy of 5.48 MeV. The spectrum obtained from our instrument is shown in Fig. 2. The emission peak is fitted with a Gaussian curve that closely matches the experimental data.

Fig.2 – Americium 241 alpha-emission spectrum

Radium

The radioactive source is an old clock hand coated with a luminous radium-based paint (as shown in the image on the side). The radioactive content is very small, but these objects must be handled with care because they can easily release fragments of radioactive material.
The decay scheme of Radium-226 is shown in Fig. 3. In its decay chain, several isotopes emit alpha radiation: besides Radium itself, we have Radon-222 and various Polonium isotopes, among which we expect to find Polonium-218, Polonium-214, and Polonium-210. Other isotopes that undergo alpha decay are present in decay branches with such low probabilities that we do not expect to detect their presence.

Fig.3 – Radium 226 decay chain

The alpha emission spectrum, shown in Fig. 2, displays a series of peaks at energies corresponding to the alpha decay isotopes of radium, with a long tail at lower energies due to self-absorption of the sample.
In the graph, the alpha emission energies of the various isotopes present in the radium decay chain are indicated with dashed red lines. It can be seen that there is a good match between the energy peak detected by the instrument and the correct value of the energy.

Fig.4 – Radium 226 alpha-emission spectrum

Fig.5 – Radium 226 alpha-emission spectrum – Details

Radium and Polonium in aqueous solution

Sample preparation for alpha spectroscopy has always been a critical part of the analysis process. The problem arises because the interaction of alpha particles with the bulk parts of the sample being analysed reduces the energy with which the particles exit the sample and reach the sensor. This produces a distorted spectrum with a long tail starting from the peak corresponding to the maximum energy (uninteracted particles) and extending toward progressively lower energies. In cases where multiple peaks are present, this overlap makes it difficult, if not impossible, to correctly identify the peaks produced by different isotopes.

To achieve good energy resolution, it is necessary to have sources in which the isotopes under examination are present as very thin films deposited on a support. The methods for preparing these sources are all quite elaborate and complex, requiring good knowledge of radiochemistry and fairly sophisticated equipment.

However, there are also simpler methods that, in some selected cases, still allow for good results. For example, Polonium is selectively and efficiently adsorbed by Silver, Nickel, or Copper. This chemical property of Polonium can be exploited to obtain samples of the Polonium-210 isotope present in waters, which are being analysed to assess the level of Polonium contamination.

A similar method can be used to evaluate the amount of Uranium and Radium in aqueous solutions. In this case, the selective absorption properties of these isotopes by some materials are exploited. Radium is captured using Manganese Dioxide, while Uranium is collected with a specific resin.
The image on the side shows the NucFilm discs (a commercial product) used for capturing Radium and Uranium. Meanwhile, the Fig.4 image displays the magnetic stirrer with a heating plate used in the procedure for analysing liquid samples. We used a sample of deionised water acidified with nitric acid, brought into contact with a source of natural Radium (Uranium mineral). The nitric acid dissolves ions, which in our minerals include Radium, Polonium, and Uranium ions. The Radium ions are captured by the NucFilm disc, on which they deposit, forming a thin layer.

Fig.6 – Setup for NucFilm disks analysis

In the graphs of Fig.7 and Fig.8, we report the alpha spectra of the samples we obtained: the peaks of Radium and Radon are visible, and to a lesser extent, those of two Polonium isotopes. The energy resolution is good, and the tail caused by self-absorption is quite limited, indicating that the adsorption of ions occurred on a thin surface layer.

Fig.7 – Radium alpha-emission spectrum

Fig.8 – Radium alpha-emission spectrum – Details

Using the same nitric acidified solution and a nickel sheet, we tried to capture Polonium ions. The results are shown in Fig.9 and Fig.10. For the isotope Po-218, we obtained a quite high and very narrow energy peak, while for Radium and Radon, the alpha peaks have much lower intensity.

Fig.9 – Polonium alpha-emission spectrum

Fig.10 – Polonium alpha-emission spectrum – Details

Radon and Thoron from the electrostatic trap

To capture the progeny of radon and thoron, an “electrostatic ion trap” was used to capture and concentrate the positive ions produced by decay onto a metal plate. The ion trap consists of a semi-spherical metallic container that is positively charged relative to a metal plate placed inside in a central position. Inside, radon gas is introduced (or generated directly within), which decays to produce positively ionised isotopes that are repelled by the positively charged outer walls and attracted toward the central metal plate. The images in Fig.11 show the device.

Fig.11 – Setup for using the electrostatic trap to capture radioactive ions

The results obtained by inserting a Radon source, composed of a Uranium mineral, into our “trap” are shown in Figures 12 and 13. We refer to the decay chain of Radium illustrated in the previous diagram in Fig.3. The alpha spectrum shows three prominent peaks with excellent energy resolution. The highest energy peak is attributed to the isotope Polonium-214, while the intermediate energy peak has been assigned to the isotope Polonium-216, although this attribution is uncertain; in fact, this isotope is not part of the decay chain of Radon-222 but of Radon-220 (Thoron), which is instead present in the Thorium chain. The lower energy peak could be due to the same Radon-220. The presence of these isotopes, assuming their attribution is correct, could be due to contamination of the sample.

Fig.12 – Radon progeny alpha-emission spectrum

Fig.13 – Radon progeny alpha-emission spectrum – Details

For the progeny examination of Thoron, we used a toriated net, which is known to be a weak source of Thoron. The toriated net was inserted directly into the electrostatic trap. In Fig.14, we report the decay chain of Thorium 232, which includes Radon 220 (Thoron) and its products, among which we expect to find Polonium 212 and Bismuth 212.

Fig.14 – Thorium 232 decay chain

The alpha spectra obtained are shown in Fig.15. The peaks corresponding to the isotopes Polonium 212 and Bismuth 212 are present, both with excellent energy resolution. The measured activity ratio between the two isotopes also matches the expected one: 36% Bismuth and 64% Polonium.

Fig.15 – Thoron progeny alpha-emission spectrum

Uranium in aqueous solution

Using the technique of deposition on a substrate from an aqueous solution, described above, we attempted to obtain alpha samples with Uranium isotopes. We used NucFilm discs, which have specific absorption properties for Uranium, and in Fig.16, we show the decay chain associated with it.

Fig.16 – Uranium 238 decay chain

The alpha spectrum is shown in Fig.17. The alpha emissions corresponding to the isotopes Uranium-238 and Uranium-234 are visible.

Fig.17 – Alpha-emission spectrum from Uranium adsorbed on NucFilm base

Thorium in aqueous solution

We also tested ionic capture for thorium from an aqueous solution. The material used for capture was nickel, and the solution was obtained by treating monazite (sand containing thorium) with nitric acid. The alpha peaks corresponding to Polonium-212 and Bismuth-212 are correctly present, as shown in Fig.18 and Fig.19, although with much reduced intensity.

Fig.18 – Alpha-emission spectrum from Thorium plated upon nickel base

Fig.19 – Alpha-emission spectrum from Thorium plated upon nickel base – Details

Beta emission of Strontium

Our detector is also sensitive to beta radiation. It’s therefore interesting to perform spectrometry tests with beta emitters. We know that the emission from a beta source is not monoenergetic but spans a range from near zero up to a maximum value, which corresponds to the case where all the energy produced by the decay is transferred to the emitted electron. In all other intermediate cases, part of the energy is given to the electron and part to the neutrino.

Fig.20 – Strontium 90 decay chain

In Fig.20, we show the decay scheme of Strontium-90, and in Fig.21, the resulting energy spectrum. You can notice that there are two distinct emissions: one at a lower energy (from Strontium-90) and another at a higher energy (from Yttrium-90). The energy ranges correspond to what is expected.

Fig.21 – Strontium 90 beta-emission spectrum

Conclusions

In this post, we have described some alpha spectrometry measurements made with the instrument presented in the post Improved Alpha Spectroscopy with Red Pitaya. The results are satisfactory and demonstrate the possibility of measuring samples with low or very low activity to estimate the content of some radioactive isotopes (Uranium, Radium, Radon, Polonium) present in liquid or gaseous matrices.

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