Preliminary study of the applicability of the thin gap method on alpha emitters

Highlights

• A new radiotracer method for the measurement of alpha emitters is introduced.

• The applicability of the method is verified.

• A process for the preparation of pure ²¹⁰Po isotope is introduced.

Abstract

The thin gap method as an in-situ radiotracer technique is widely used. This study investigated the applicability of alpha emitters. PIPS and CsI alpha spectrometers were applied in a thin gap cell. A suitable ²¹⁰Po source was prepared by spontaneous deposition, Mylar foil was used to simulate water. A maximum intensity decrement of 7% within 25 μm was observed. Even though this method is suitable for the study of surface phenomena, further investigation is necessary e.g. into water and heat sensitivity.

Introduction

Most of the current techniques for measuring the adsorption of different alpha emitters are based on ex-situ methods. Widely used pieces of equipment are Passivated Implanted Planar Silicon (PIPS) detectors, gas scintillators and liquid scintillation counters. In this case there is no equilibrium between the liquid and solid phases during measurements. Radiotracer techniques are widely used to investigate surface processes at solid–liquid interfaces (Horányi, 1999, Horányi, 2004, Varga et al., 2001a, Varga et al., 2001b, Varga et al., 2006). These methods can be divided into two categories: the in-situ and ex-situ methods. In the case of in-situ techniques adsorption can be measured continuously at the solid–liquid interface without disturbing the equilibrium. All of the in-situ radiotracer methods are based on the thin layer principle of Aniansson (Aniansson, 1951) which states that in the case of proper cell arrangement an increased intensity can be measured which originates from the adsorbed particles. The background noise is low in the case of alpha or soft beta emitters as the range of these forms of radiation is short in liquid phases. Based on this principle there are several techniques, namely the “thin-layer”, the “foil” and the “electrode lowering” or “thin-gap” methods (Horányi, 1999, Horányi, 2004, Varga et al., 2001a, Varga et al., 2006). In our institute over the last two decades two of the above methods, namely the foil and the thin gap, have been applied and developed.

The widely used type of the foil method was introduced by Horányi (Kolics and Horányi, 1996). In this arrangement the bottom of an electrochemical cell is made of a thin foil. The adsorption occurred on the surface of the foil within the cell, while the detector was placed beneath the cell thus it did not come into contact with the media. The foil can be made from different materials to fulfil the requirements of the measurements. In most cases a plastic foil is used, and the top of the foil is covered with a suitable material such as vacuum-deposited metal layers, coatings, paints or even metal or oxide powders. The disadvantage of this method is the foil itself. It has to be thin enough to let the radiation pass through it. On the other hand it has to be thick enough to be able to hold the weight of the liquid phase. The advantages of this method are that different solid phases can be used, and the investigated surface is part of an electrochemical system thus the charge and the mass transport can be measured in parallel.

The electrode-lowering method was developed by Kazarinov (Kazarinov, 1966, Kazarinov et al., 1975). In this arrangement during the measurements, the working electrode is set into two different positions as can be seen in Fig. 1. In the raised position the adsorption of labelled species occurs while the distance between the working electrode and the detector is greater than the range of the radiation in the given media. In this position a constant background noise can be detected originating from the solution phase. In the lowered position the working electrode is lowered onto the detector surface thus only a thin layer of solution remains between the two surfaces and the intensity increment originating from the adsorbed species can be detected with low background noise from the so-called “gap”.

Since 1987 the original method has been improved by various means. First Krauskopf and Wieckowski (Krauskopf et al., 1987) introduced their new cell arrangement in which the original detector was replaced with a well-polished glass scintillator, and a working electrode with smooth surfaces (γ<2) was used. With this improvement the gap between the electrode and the detector can be minimised in the lowered position, thus the sensitivity of the method can be increased by one order of magnitude. Over the last two decades further developments have been introduced by Varga and his co-workers (Buják and Varga, 2008, Hirschberg et al., 1998, Hirschberg et al., 1999, Horváth et al., 2013, Varga et al., 2001b). The most important are: the modifications of the cell design, measurements with soft gamma-emitting radionuclides and the detection of adsorption processes on rough surfaces.

These in-situ techniques play an important role in the investigation of interfacial processes at solid–liquid interfaces with the use of beta or low-energy gamma emitters but none of them are capable of measuring the sorption processes of alpha emitting radionuclides. However, it is important to understand the adsorption–desorption properties of alpha emitters. In the case of nuclear power plants, the presence of actinides can cause several problems. Through the absorption of neutrons these contaminants can disturb the self-sustaining chain reaction. Measurement of the alpha radiation is complicated, thus with the understanding of its mechanism one cannot only measure but calculate the adsorption rate of alpha emitters on different surfaces. The aim of the present work is to develop an in-situ radiotracer method based on Aniansson’s thin-layer principle which is capable of measuring the adsorption and desorption processes of isotopes with alpha radiation on solid surfaces.