How did the Curies Measure Radioactivity?

Musée Curie

The Curie Method

One eye on the ruler, one hand holding a chronometer
This is often how we see Marie Curie in photos of her measuring radioactivity using the "Curie method." But what was she doing exactly? To gain a proper understanding, we will take a look at the experiment first conducted by Marie and Pierre Curie in 1898, as well as the various instruments that were used in this experiment.
The Principle of measurement
Radioactive rays electrify the air. The objective of the Curie method is to measure the number of electric charges produced, which is proportional to the radioactive emissions of the sample. The apparatus used by the Curies for their experiments included an ionization chamber, a quadrant electrometer, and a piezoelectric quartz.

The unique feature of the method established by Marie and Pierre Curie was the introduction of the piezoelectric quartz as a measurement standard.

The Ionization chamber
As with all matter, air consists of electrically neutral atoms. Atoms are composed of particles with electric charges. These are called negative electrons and positive nuclei and they offset each other. The radioactive rays extract electrons from the air atom, thus creating an electric charge. The air in this case is said to be "ionized."

To collect the electrons produced by the radioactive sample, the sample is placed inside the ionization chamber. The two plates of the chamber are electrically charged. The negative electrons move to the positive plate, creating an electric current within the chamber.

The Quadrant electrometer
Once set in motion, the electric charges produced by the radioactive source move towards the quadrant electrometer. The electrometer then measures the electric charge.

This electrometer is composed of 8 quadrants between which a mobile electrode, or needle, is suspended (this image shows the view from the side). A small mirror is attached to this needle...

The quadrants are connected to batteries in order to generate an electric field. The electrons travel from the ionization chamber to the electrometer, where they cause the needle to rotate gently in the electric field. The more electrons there are, the more the needle rotates.

As it rotates, the needle drives the small mirror. The movement of the mirror, as with that of the needle, is therefore proportional to the number of electrons produced in the ionization chamber.

A light beam is sent to the small mirror, which reflects the beam onto a graduated ruler. When the needle rotates, the mirror rotates also, causing the beam of light to move along the ruler.

The Piezoelectric quartz
According to Marie Curie's laboratory notes, radioactivity measurements obtained using just the electrometer and ionization chamber were not reliable. In order to obtain more precise measurements, Marie and Pierre added a fourth instrument to their experiment apparatus—the piezoelectric quartz.

The piezoelectric quartz electrometer was developed by Pierre Curie and his brother Jacques long before the discovery of radioactivity. In 1880, the two brothers had discovered that certain crystals emit small amounts of electricity when expanded or compressed. This is the case with quartz.

"The two ends of the blade [of quartz] are secured between a set of jaws, one of which allows the blade to be suspended...

... the lower jaw is fitted with a plate on which weights can be placed. (...) This traction has the effect of releasing quantities of electricity on both sides of the blade (...) which are then collected."
Marie Curie, July 16, 1921

Marie and Pierre Curie transmitted the known charge of quartz to the electrometer and then compared it with the charge produced by the radioactive sample that they wanted to measure. This piece of ingenuity allowed them to obtain accurate measurements even though the currents involved were very weak (10^-13 amperes).

One eye on the ruler, one hand holding a chronometer
Let's go back to the photos of Marie Curie taking a radioactivity measurement...

At the beginning of the measurement, the electric charges produced by the radioactive sample begins to move the small light along the ruler.

Marie Curie then delicately raises the weight positioned under the piezoelectric quartz, which emits an electric charge. This charge will offset the opposite charge produced by the radioactive sample for a certain period time.

The chronometer allowed Marie Curie to measure the length of time during which the charge emitted by the piezoelectric quartz compensated the charge produced by the radioactive sample. The shorter the time, the greater the amount of charge, and the greater the radioactivity of the sample.

"[Marie Curie] sits in front of the apparatus, conducting measurements in the semi-darkness of an unheated room to avoid fluctuations in temperature. The series of tasks involved—opening the device, starting the chronometer, lifting the weight, etc.—are performed by Madame Curie with an admirable sense of discipline and harmony. She moves her hands with a precision that not even a piano virtuoso could match. It is a perfect technique, which has the effect of reducing the margin of human error to zero. After her calculations, which Madame Curie completes eagerly in order to compare the results, we see her sincere, unconcealed joy, because the differences are well below the permissible limit, which ensures the accuracy of her measurements."

Catherine Chamié, Marie Curie's colleague

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