Low Background Steel — So Hot Right Now

The nuclear age changed steel, and for decades we had to pay the price for it. The first tests of the atomic bomb were a milestone in many ways, and have left a mark in history and in the surface of the Earth. The level of background radiation in the air increased, and this had an effect on the production of steel, so that steel produced since 1945 has had elevated levels of radioactivity. This can be a problem for sensitive instruments, so there was a demand for steel called low background steel, which was made before the trinity tests.

The Bessemer process pumps air through the iron to remove impurities. shropshirehistory.com

The production of steel is done with the Bessemer process, which takes the molten pig iron and blasts air through it. By pumping air through the steel, the oxygen reacts with impurities and oxidizes, and the impurities are drawn out either as gas or slag, which is then skimmed off. The problem is that the atmospheric air has radioactive impurities of its own, which are deposited into the steel, yielding a slightly radioactive material. Since the late 1960s steel production uses a slightly modified technique called the BOS, or Basic Oxygen Steelmaking, in which pure oxygen is pumped through the iron. This is better, but radioactive material can still slip through. In particular, we’re interested in cobalt, which dissolves very easily in steel, so it isn’t as affected by the Bessemer or BOS methods. Sometimes cobalt is intentionally added to steel, though not the radioactive isotope, and only for very specialized purposes.

Recycling is another reason that modern steel stays radioactive. We’ve been great about recycling steel, but the downside is that some of those impurities stick around.

Why Do We Need Low Background Steel?

Imagine you have a sensor that needs to be extremely sensitive to low levels of radiation. This could be Geiger counters, medical devices, or vehicles destined for space exploration. If they have a container that is slightly radioactive it creates an unacceptable noise floor. That’s where Low Background Steel comes in.

A person is placed into a low background steel container with sensitive equipment to measure the radioactivity of the body, which may be near the background level. Photo from orau.org

So where do you get steel, which is a man-made material, that was made before 1945? Primarily from the ocean, in sunken ships from WWII. They weren’t exposed to the atomic age air when they were made, and haven’t been recycled and mixed with newer radioactive steel. We literally cut the ships apart underwater, scrape off the barnacles, and reuse the steel.

Fortunately, this is a problem that’s going away on its own, so the headline is really only appropriate as a great reference to a popular movie. After 1975, testing moved underground, reducing, but not eliminating, the amount of radiation pumped into the air. Since various treaties ending the testing of nuclear weapons, and thanks to the short half-life of some of the radioactive isotopes, the background radiation in the air has been decreasing. Cobalt-60 has a half-life of 5.26 years, which means that steel is getting less and less radioactive on its own (Cobalt-60 from 1945 would now be at .008% of original levels). The newer BOS technique exposes the steel to fewer impurities from the air, too. Eventually the need for special low background steel will be just a memory.

Oddly enough, steel isn’t the only thing that we’ve dragged from the bottom of the ocean. Ancient Roman lead has also had a part in modern sensing.

Explaining The Operation Of The 74181 ALU

You will all no doubt be familiar with the 74 series logic integrated circuits, they provide the glue logic for countless projects. If you look back through old listings of the series you’ll find alongside the familiar simple gates a host of now obsolete chips that reveal their roots in the pre-microprocessor computer industry of the late 1960s, implementing entire functions that would now be integrated.

One of the more famous of these devices is the 74181, a cascadable 4-bit arithmetic logic unit, or ALU. An ALU is the heart of a microprocessor, performing its operations. The 74181 appeared in many late-60s and early-70s minicomputers, will be familiar to generations of EE and CS students as the device they were taught about ALUs on, and can now be found in some home-built retrocomputers.

[Ken Shirriff], doyen of the integrated circuit teardown, has published a piece taking a look at the 74181, in particular at its logic functions and the reason for some of them that are rather surprising. As well as the normal logic functions, for example the chip can do “(A + B) PLUS AB“. Why on earth you might think would an ALU need to do that?

The answer lies in the way it performs carrying while adding, a significant speed-up can be achieved over ripple carrying along a chain of adders if it can be ascertained whether a bit addition might generate a carry bit. He explains the function required to perform this operation, and suddenly the unusual extra function makes sense. Addition is transformed from a serial process to a parallel one, with a consequent speed increase.

It’s one of those moments in which you have to salute those logic designers from an era when on-chip real-estate was costly and every ounce of speed had to be teased from their designs. Give it a read, and have a go at the interactive 74181 simulator further down [Ken]’s page. We learned something from the article, and so may you.

We brought you the first part of [Ken]’s 74181 investigations earlier in the year. If you would like to see a 74181 in action, take a look at this 4-bit 74 logic single board computer.

Boost Converter Functionality At Rock-Bottom Prices

Linear voltage regulators are pretty easy to throw into a project if something in it needs a specific voltage that’s lower than the supply. If it needs a higher voltage, it’s almost just as easy to grab a boost converter of some sort to satisfy the power requirements. But if you’re on a mission to save some money for a large production run, or you just like the challenge of building something as simply as possible, there are ways of getting voltages greater than the supply voltage without using anything as non-minimalistic as a boost converter. [Josh] shows us exactly how this can be done using a circuit known as a charge pump to drive a blue LED.

One of the cool things about AVR microcontrollers is that they can run easily on a coin cell battery and source enough current to drive LEDs directly from the output pins. Obviously enough, if the LED voltage is greater than the voltage of the power supply, this won’t work. That is, unless you have a spare diode and capacitor around to build a charge pump.

The negative charge pump works by charging up a capacitor that is connected to an AVR pin, with the other side between the LED and a garden-variety diode to ground. That results in a roughly (VCC – 0.7) volt difference across the capacitor’s plates. When the AVR pin goes low, the other side of the capacitor goes negative by this same amount, and this makes the voltage across the LED high enough to light up. Not only is this simpler than a boost converter, but it doesn’t need any bulky inductors to work properly.

Will this work for any load? Am I going to start any fires by overdriving the LED? Luckily, [josh] answers all of these questions and more on the project page, and goes into some detail on the circuit theory as well. Granted, the charge pump doesn’t have the fine control over the power supply that you can get out of a buck or boost converter (or any switch-mode power supply). But it does have good bang-for-the-buck.