Radioactivity: Background

Story: Chernobyl

In April 1986, a serious accident occurred at the nuclear power station at Chernobyl in Russia. A week later, radiation detectors (Geiger counters) in Britain recorded higher than usual levels of radiation. Britain is more than 1000 miles from Chernobyl!

Explain what reached the Geiger counters in Britain to make them record extra counts.

Misconceptions:

Many students confuse ‘radiation’ and ‘radioactive material’.
After the Chernobyl accident, many newspaper articles referred to a “cloud of radiation” and drinking water contaminated with “radiation”.
Many students appear to interpret the idea that “radiation is absorbed” differently from the scientific interpretation. They believe that objects that have been irradiated will themselves become radioactive – that they can re-emit the radiation some time later.
The underlying idea here is that they seem to think that radiation is somehow “conserved”.
An inability to distinguish between the ideas of irradiation and contamination
An inability to interpret the ideas of activity and dose

What is Radioactive?

Problem:

Protons are positive, and repel. What holds the nucleus together?

Enter stage left: The Strong Force.

It never seems to occur to students up until this point — what holds those repelling protons together? – Turns out there’s a sub-atomic “glue” that only works over tiny ranges holding it all together. (Why/How? — might as well ask why anything, at some point we have to deal with the fact that the universe is beautiful, amazing, and weird)

We call this glue “the strong force” and the interplay between the strong and the electromagnetic force is critical to the stability of atoms…

Unstable Trends in the Periodic Table

Some Deep History:

History:

Roentgen was exploring the path of electrical rays passing from an induction coil through a partially evacuated glass tube. Although the tube was covered in black paper and the room was completely dark, he noticed that a screen covered in fluorescent material was illuminated by the rays. He later realised that a number of objects could be penetrated by these rays, and that the projected image of his own hand showed a contrast between the opaque bones and the translucent flesh. He later used a photographic plate instead of a screen, and an image was captured. In this way an extraordinary discovery had been made: that the internal structures of the body could be made visible without the necessity of surgery.

By 1896 an x-ray department had been set up at the Glasgow Royal Infirmary, one of the first radiology departments in the world. The head of the department, Dr John Macintyre, produced a number of remarkable x-rays: the first x-ray of a kidney stone; an x-ray showing a penny in the throat of a child, and an image of a frog’s legs in motion. In the same year Dr Hall-Edwards became one of the first people to use an x-ray to make a diagnosis – he discovered a needle embedded in a woman’s hand. In the first twenty years following Roentgen’s discovery, x-rays were used to treat soldiers fighting in the Boer war and those fighting in WWI, finding bone fractures and imbedded bullets. Much excitement surrounded the new technology, and x-ray machines started to appear as a wondrous curiosity in theatrical shows.

It was eventually recognised that frequent exposure to x-rays could be harmful, and today special measures are taken to protect the patient and doctor. By the early 1900s the damaging qualities of x-rays were shown to be very powerful in fighting cancers and skin diseases.

•1896 — Henri Becquerel discovers that rocks that contain uranium emit radiation

In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26th-27th of February, when his experiment “failed” because it was overcast in Paris. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

Becquerel used an apparatus similar to that displayed below to show that the radiation he discovered could not be x-rays. X-rays are neutral and cannot be bent in a magnetic field. The new radiation was bent by the magnetic field so that the radiation must be charged and different than x-rays. When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral.

The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon recently discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium. It took four more years of processing tons of ore to isolate enough of each element to determine their chemical properties.

Ernest Rutherford, who did many experiments studying the properties of radioactive decay, named these alpha, beta, and gamma particles, and classified them by their ability to penetrate matter. Rutherford used an apparatus similar to that depicted in Fig. 3-7. When the air from the chamber was removed, the alpha source made a spot on the photographic plate. When air was added, the spot disappeared. Thus, only a few centimeters of air were enough to stop the alpha radiation.

Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting–several centimeters of lead or a meter of concrete is needed to stop most gamma rays.

•1908/9 — Marie Curie discovers radium, polonium and thorium — good biog here: https://www.nobelprize.org/prizes/physics/1903/marie-curie/biographical/

•1903 — Becquerel, and the Curies get Nobel Prize

Page updated

Google Sites

Report abuse