Diamonds are forever

When a diamond is irradiated with x-rays it luminesces  (or fluoresces, I never know the difference). This property is used to separate diamonds from gravel in the mining industry. Once the rock is mined, it is crushed to a gravel of small stones and diamonds. The gravel mix is fed into a sorting machine. The first stage is a vibrating bed to spread the gravel evenly on to a short conveyor belt about 4 feet wide (if I remember correctly). At the end of the conveyor, the gravel is launched through an x-ray beam and above this zone are a number of photomultiplier (PM) tubes ranged across the width of the machine to detect the light emissions from the diamonds. When a flash is detected, an air blast is fired at the area associated with a particular PM tube. The blast takes out the diamond and some of the gravel around it. Several machines are arranged in series with the sorted product feeding from one machine to the next in order to reduce the amount of gravel collected with the diamonds.

I was working for a large mining organization which also had multiple support groups. One of the groups was a research facility that had defined the above process, and the machines had been made the same way for many years as per their specifications. The group I was working on was responsible for manufacturing the sorters and now wanted to update the electronics. As is usual the research group looked down on us with some disdain, which wasn't quite as bad as the budgets, which were heavily skewed in their favour.

Since diamonds are the target of theft, the temptation was removed by using "tracers", small cubes that floures... ah, lumines... ah, emit light when x-ray irradiated.  While investigating a problem I and another engineer climbed through a hatch to have a closer look. When we came out we discovered that the x-ray generator had been on the whole time. Even though back then life was cheap in South Africa, we were rushed to a clinic for blood tests and an interlock fitted to the hatch immediately. However, this is merely an aside and a warning to all not to make assumptions.

At any rate, the problem we were having was that an air blast would occur simultaneously on several air "guns" for a single tracer. For all the years that this style of machine was made, no one had ever thought of doing the test in this detail and the problem existed in all production machines to that date. In order to improve performance, we mounted a tracer on a rotatable disk. The disk could be moved along the axle across the width of the sorter.  I measured the reactions of the PM tubes gradually across the breadth and created a profile and it was obvious there was significant overlap between tubes. The solution was to provide collimators for each PM tube and the results improved dramatically.

The best part was where we got to present the results to management and the research group, who were not happy at being shown up.

Aubrey Kagan is a professional engineer with a BSEE from the Technion-Israel Institute of Technology and an MBA from the University of the Witwatersrand. He is engineering manager at Emphatec, a Toronto-based design house of industrial control interfaces and switch-mode power supplies. In addition to writing several articles for Circuit Cellar and having ideas published in EDN and Electronic Design, Aubrey wrote Excel by Example: A Microsoft Excel Cookbook for Electronics Engineers (Newnes, 2004).