The 1,4000,000-volt X-ray Machine at the High Voltage Laboratory at the National Bureau of Standards, ca, 1940s

In 1940, the U.S. National Bureau of Standards (NBS) built this 1,400,000-volt X-ray generator, the most powerful of its kind at the time. The machine was designed to deliver X-rays at an extremely stable voltage, a necessary attribute for the development of standard radiation dosage measurements as well as for research and testing of equipment to protect against X-ray radiation.

It was used to conduct basic research such as x-ray crystallography. The two lefthand vertical columns contained a 10 stage voltage multiplier in which 10 capacitors were charged in parallel to a voltage of 140 kV using high voltage kenotron diode vacuum tubes, then discharged in series, creating a high DC voltage on the top terminal. The righthand column is the x-ray tube, which accelerates electrons to 1.4 Mev before they collide with a metal target at bottom. The 9 horizontal conductors linking the corona rings on the separate columns equalize the voltage drops across each insulator, ensuring that the electric field is distributed equally along the columns to prevent arcs. A high value resistor extending down the lefthand column with 10 equal taps divides the voltage equally. The high voltage electrodes have smooth, rounded shapes with no sharp edges to prevent leakage of the charge into the air by corona discharge.

While X-rays were known to be dangerous much earlier than 1940, there wasn’t a good understanding of exactly how much exposure was bad for humans, or what kinds of materials offered effective amounts of protection. At the time, for example, concrete was commonly used to absorb and scatter X-rays, but nobody knew exactly how well it worked. Testing at NBS revealed that concrete responded differently depending on if the radiation was narrow beam or broad beam, and the results helped establish guidelines on radiation-resistant construction. The agency also established the first U.S. standards for X-ray exposure.