The spinthariscope

Class practical

This is a simple toy radiation counter that enables students to see the bright flashes of light, scintillations, produced by alpha particles when they strike a fluorescent screen.

Apparatus and Materials

• Spinthariscopes with 0.02 μCi sources, (740Bq)
• Spinthariscopes with 0.5μCi sources, (18500Bq)
• United Nuclear, an American supplier, markets a Super spinthariscope designed so that the distance from the source to screen can be varied, eliminating the need for two different source activities.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern spinthariscopes do not use radium nitrate or bromide. They are more likely to contain a small speck of thorium ore. Moreover, the source is not removable.

Crookes spinthariscope: A = ZnS screen; B = radium nitrate source; C = eyepiece lens.

Procedure

1. The eye must be light-adapted to see the scintillations, so it is not possible to use spinthariscopes in daylight. Use a blacked-out room with a very limited amount of artificial light. It is then relatively easy for the pupils to see scintillations.
2. Pass the spinthariscopes around the room so that each of the students can see the scintillations for themselves.
3. Two strengths are recommended: one gives a shower, the other makes it easier to see the random nature of the process. The students should see both if possible.

Teaching Notes

• The spinthariscope was the first radiation detector and was the forerunner of scintillation counters. A modern scintillation counter employs a photomultiplier tube as a light sensitive component in an electronic circuit to count the flashes in the fluorescent material.
• Geiger and Marsden used a type of spinthariscope to count the number of alpha particles being deflected into each angle in the gold foil experiment. They had to sit in a darkened room counting these flashes of light; no wonder Rutherford encouraged Geiger to develop the Geiger tube!
• The spinthariscope was developed by William Crookes in 1903 after an accidental discovery. He was looking at the fluorescent glow produced by radium bromide on a zinc sulphide screen. He spilt some of the radium salt onto the fluorescent screen and, given its huge expense at the time, started to pick up all the specks of dust. He needed a magnifying glass to locate each speck. He then noticed the individual flashes, or scintillations, produced around each speck on the screen. Each flash corresponded to a single alpha particle. This was the first radiation detector. Crookes made it into a self-contained scientific toy to impress his lecture audiences.
Thanks to Paul Dolk, Haarlam, Netherlands, for suggesting the super spinthariscope.

This experiment was safety-tested in March 2006