A new design of 238Pu activity sources for use in intercomparison exercises

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Abstract

In the framework of a BIPM 238Pu comparison exercise, a new design for α-radiation counting sources was developed. These sources are constructed with a gold-coated Vyns film stretched over a stainless-steel disk. To improve the crystallization of the radioactive salt, the physical properties of the gold-coated polymer surface which receives the radioactivity deposit is modified. A pad of polystyrene micro-spheres (Estapor® K-007) mixed with a wetting agent (Tween 20®) is directly electrosprayed before the radioactive drop is deposited. After hot air-drying, the source is finally sealed with a thin Vyns film covering the entire radioactive spot. This new design makes it possible to create 238Pu sources that are easy and safe to handle, homogeneous and stable during ageing.

Introduction

The requirements for quantitative point sources used in α absolute counting are mainly low absorption/self-absorption and high homogeneity of radiation emitted by the source. The best homogeneous sources are currently manufactured by electro-precipitation processes in inorganic media (for example sodium sulfate) (Tsoupko-Sitnikov et al., 2000; Becerril-Vilchis et al., 1996) or organic media (for example DMSO) (Truffy, 1991) of α-emitters on stainless-steel disks. These sources are not quantitative because the yield values, which never reach more than 95% in the best cases, are not known with sufficient accuracy (no radioactive tracer added). So, these sources cannot be used for determining absolute activity in fixed geometry counters and are used exclusively for spectroscopic determination of nuclear data (for example emission probability). The only way of preparing quantitative sources is firstly to put a weighed droplet of radioactive solution onto a substrate and secondly, to dry it in a dust-free atmosphere. Unfortunately, this procedure leads to poor homogeneity and favors crown-shaped crystallization which complicates determination of the geometry correction factors applied to raw data counting.

During the drying process, solid crystal formation is governed by nucleation and growth. Nucleation is the formation of a new solid phase, often around an impurity in the solution. Layer-by-layer addition of solute to the nuclei is termed growth. The crystal size distribution depends on the relationship between nucleation and growth. If many nuclei are formed, many small crystals will result. However, if only a few nuclei are formed, large crystals will be formed. Crystallization of the radioactive deposit can be improved by modifying the surface properties of the backing support and/or controlling drying of the radioactive drop (Merritt et al., 1959). Backing material can be treated by seeding agents such as ion exchange particle resins (Lowenthal and Willie, 1973), electrosprayed latex microspheres (Blanchis et al., 1990) or SiO vacuum evaporation (Lachance and Roy, 1972) which supply fine particles that act as nuclei of crystallization, whereas wetting agents, such as Tween 20® (NCRP, 1978), simultaneously reduce the surface tension of the radioactive solution.

Radiological protection is another aspect in the preparation of such sources. In some cases, radioactivity leakage occurs when sources are prepared by direct deposition of drops and drying. This is due to heterogeneous crystallization, when some large crystals of radioactive salts can work loose from the support inducing contamination of the operator or counting devices. This radioactivity leakage could be avoided by controlling crystallization of the radioactive salt (small crystals) and by covering the radioactive spot with a very thin polymer film.

This paper describes the appropriate laboratory conditions for preparation and sealing of the safe, well-crystallized 238Pu sources used recently in the framework of a BIPM comparison exercise (LNHB, 2001).

Section snippets

Overview of the backing support

The source backing support comprises four elements:

a polished stainless-steel disk (diameter=30 mm, thickness=0.5 mm),
a thin Vyns® (Merck-Eurolab-Prolabo F-94126 Fontenay-Sous-bois) film [copolymer of vinyl acetate (15%) and PVC (85%)] (thickness=45 μg/cm2),
a fine gold coating (thickness=130 μg/cm2) and
an electrosprayed pad (diameter=8 mm) of an alcoholic suspension of polystyrene microspheres (Estapor® K-007, Merck-Eurolab-Prolabo F-94126 Fontenay-Sous-bois) mixed with a wetting agent,

Results and discussion

The experimental results are presented in Fig. 3 which shows perfect coherence between the values of derived activity concentration considering the uncertainty. The source morphology is perfectly reproducible and homogeneous (absence of crown-shaped crystallization or asymmetrical profiles in the distribution of radioactivity). Furthermore, the active diameter of the radioactive deposit corresponds to the diameter of the zone treated by electrospraying, which indicates that Tween 20® played its

Conclusion

This new design of α counting sources allowed us to produce 238Pu point sources that were safe to handle, homogeneous and stable and which could be used for the BIPM 238Pu comparison exercise. No corrosion of the stainless-steel backing support by the hydrochloric acid of the radioactive solution was noticed in either the short or long term. Autoradiography on a solid track detector showed a homogeneous distribution of the radioactivity for most of sources. The quality of the source supports

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