Neutron sources

My most recent project concerns neutrons. Specifically their generation. The project is to build an αBe ,  or alpha-beryllium source. These sources were first developed by Chadwick for his discovery of the neutron. They work by striking a beryllium nucleus with an alpha particle, which it absorbs. This forms an atom of carbon-12 and a fast neutron. About 30 neutrons are produced for every million alpha particles.

9  Be + 4 He  →  12 C +  n

4          2               6

Sorry for the strange way the equation is typed, as I can’t find how to do subscript or superscript on this.

Speaking in more practical terms, the source would consist of a lead puck with the alpha emitter imbedded in a small hole on it’s surface. Over the hole a thin piece of beryllium would be placed. Most of the other radiation emitted from the sample would make it through the beryllium, and some alpha might, so this is in no way a pure beam of neutrons. My lead block is approximately 5cm high and 2cm wide, cast from lead roofing sheets in a wooden mould. The beryllium is a 99.9 % pure sputtering target, in the form of a small disk, about 1cm wide and 1.5mm thick.

Beryllium is an interesting element, as it is the second lightest metal on the periodic table. Apparently beryllium has a sweet taste ( which is unfortunate because it is extremely toxic). One scientist who noted this was Robert Bunsen, or burner and spectroscope fame. One day, as worked with a small amount of beryllium compound, a fly was attracted by it’s sweetness. The fly flew over and promptly ate the precious sample. Bunsen chased the fly until he caught it, killed it, and burned it to ashes. From the ashes of the fly he obtained his beryllium back.  Beryllium is transparent to X-rays, leading to it’s use in X-ray windows. It is also a good neutron reflector.

More in instalment two.


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