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.

A simple ionization chamber.

The ionization chamber is a simple detector for ionizing radiation. It shares some similarities to a Geiger-Müller counter ,except it usually operates at a lower voltage and can be less sensitive. This post details the construction of a simple ionization chamber.

The ion chamber consists of three main parts, the sensing tube, the circuit and the readout. The sensing tube consists of a copper pipe with a insulating cap at one end (made from a rubber) , a wire penetrates the end cap and goes into the chamber, terminating just short of the opening. The wire should be kept centered. The multimeter is set to 20 volts DC. Before a source is brought near the chamber opening the meter shold read zero, it might fluctuate to 0.01 or 0.02 and that’s fine. When a source is brought near the chamber it ionizes the air and the free electrons are accelerated towards the walls of the chamber and the nuclei to the central cathode. The feeble ion current is multiplied by the Darlington pair and read on the multimeter.

I find that this chamber is pretty blind to gamma and X-ray, and only really reacts to beta and alpha. The best results are obtained from an Am 241 source.

Let’s look at it mathematically. The energy lost by a nonrelativistic electron  when passing through a medium is given by the Bethe formula:

When the ions are formed in a gas subject to an electric field, as happens here, they move under the influence of the field. The average velocity, w, in the direction of the field , is proportional to the field with a constant of proportionality called the mobility, denoted by μ. We also see that the particles have a mean free path ,λ , and an average random velocity,u ,which may be equal to the velocity of thermal agitation but is not necessarily so. In time τ the electron drifts in the z direction of the electric field.

I can’t add equations because typing them is impossible. I’ve tried.


Uranium extraction

The subject of this blog is to make Uranium peroxide (a yellowcake compound) from uranium ore.

Yellowcake uranium is a title given to a broad range of uranium compounds, some of which are not even yellow! uranium comes in four main isotopes, 235 (fissile) ,238 (fertile) , 234 and 232. To make the yellowcake we will be carrying out an acid leach of uranium ore and processing the leachate.The first material needed is uranium ore, 900 grams of high grade ore should suffice (the lower the grade the more ore required). There are many varieties of uranium ore but we shall be using pitchblende and they “duller” ores. The ore can be purchased online or found in nature. A Geiger counter was used to select some good, active pieces.

Required materials and equipment for step one.

  1. 900g of high grade ore (crushed into 1-2 cm wide pieces)
  2. 1 liter of hydrochloric acid.
  3. Glass or plastic vessel to hold the ore and acid.

Place the ore in the vessel, a large beaker or small bucket. Now add the hydrochloric acid. This will form a slurry and begin dissolving the uranium, carbonates , other metals and various other constituents of the rock. The acid will dissolve uranium that’s in the VI oxidation state, but more on that later. After about a day the solution will have evolved significant carbon dioxide and become light green, possibly fluorescing under UV light, mine did. The solution is left for about a week before step 2. It is periodically mixed about during the week. It registers as rather hot on a Geiger counter (exact activity depends on the ore used).