Monitoring Radioactive Decay with a Video Camera

Aare Baumer
Executive Director of Tallinn Technology and Science Centre (aare@energiakeskus.ee)

The scintillation of zinc sulfide exposed to a radioactive source was first discovered by William Crookes in 1896. He built a simple device, which he later named the spinthariscope, based on this principle. On 15 May 1903, the apparatus was first shown to the British Royal Society.

Crookes himself described his invention as, "...fit the [zinc sulfide] blende screen at the end of a brass tube with a speck of radium salt in front of it...about a millimeter off...a lens at the other end." (“The Chemical News," 1903).

When particles emitted by the radium hit the screen, they create tiny flashes of light. To see the effect, you should adapt your eyes to darkness for at least 20 minutes.

In the beginning of the 20th century, Ernest Rutherford and his coworkers spent many hours in darkrooms, counting the single particle flashes. All this was done with the human eye as the detector. This article explains how to monitor the alpha particle decay of a radioactive substance with a modern electronic sensor, a video camera with a semiconductor sensor array.

The list of radioactive materials you can obtain from commercial dealers is short. Most of them are just not for sale. But we know that every household should have a smoke alarm. There are various kinds of smoke detectors, but we are interested in the ionization type alarm. These include a small amount of the radioactive isotope Americium 241.

Figure 1. All ionization-type smoke detectors should have a label that identifies the radioactive source. Note "Am-241" (Americium 241) in the text of this label. Also note the icon signifying radioactivity.

The label on the alarm (Fig. 1) should include words to this effect:

Ionization smoke detector.

Approved by (US government or other governmental organization).

Contains maximum 40 kBq of radioactive substance (Am-241).

To get to the isotope, we have to open the alarm and remove the electronic module. On that module should be mounted the isotope covered with aluminum or plastic grating (Fig. 2). Desolder or otherwise remove the grating from the circuit and detach the isotope.

Warning: Americium 241 is a radioactive isotope. It must not be handled or touched! Never leave the Americium 241 radioactive isotope where a child may play with it or even attempt to swallow it! Store the radioactive source in a small box where you can retrieve it for experiments.

Figure 2. The ionization chamber containing the Americium 241 source is behind the aluminum shield of this opened smoke detector. Warning: See warning note in text.

Now that we have a radioactive source, we need a means for detecting the particles it emits, in this case alpha particles. CCD detectors are used by nuclear physicists to detect charged particles. CCD and CMOS solid-state video security cameras are very sensitive devices, some working even in 0.0001 lux photometric levels. We need one for our experiment. We also need a small power pack, video connecting cables and camera dome (see Fig. 3).

Figure 3. The Americium 241 source (foreground) and the video camera with its lens removed (background).

We next have to modify the camera. This is because the alpha particles emitted by the Americium-241 source have such a small amount of energy that even a thin piece of paper or any other medium can stop them. The part number of the camera I used is CS1001 CAM ZWBLA 3. This camera is easily modified as described below. Other cameras may require special treatment.

First, remove the lens or lenses from the camera. Save them, for you may need them for a future project.

The light-sensitive sensitive part of the video camera may be covered with a thin plastic screen for protection from dust that can cause degradation. The screen of the camera that I used (CS1001 CAM ZWBLA 3) was attached by two small screws. Carefully remove the plastic screen. You will need to improvise if the screen is not attached by screws.

Caution: Be very careful when removing the protective screen! Touching the light-sensitive element destroys it immediately!

Secure the camera and radioactive isotope.

Connect the camera cable to the TV and apply power. You should see a foggy image of unfocussed light.

Very carefully move the radioactive isotope holder close to the camera's sensor array. When the Am 241 source is around 3-5 mm from the detector, you should see small flashes on the TV screen. These flashes are caused by alpha particles, which barely penetrate into the light-sensitive semiconductor elements and release small electrical impulses (see Fig. 4) .

Figure 4. The three bright flashes shown here indicate alpha particles from the Americium 241 source that have been detected by the video camera sensor.

The closer the camera matrix and radioactive source are, the more alpha particles that can penetrate the air between. The maximum distance an alpha particle can travel through air is 3 to 4 cm.

The camera I used works at very low light levels (0.0001 lux) and features an automatic light level adjustment that cannot be changed. The high sensitivity can cause the TV screen to show a fog-like background glow. You can use the brightness and contrast controls to obtain better results. Reduce the brightness and increase the contrast

When the experiment is working, you can modify the experiment by moving a magnet around the sensor or by placing a thin, electrically charged screen between the Am 241 source and the camera's sensor.

When you are not experimenting with the sensor and the Am 241 radiation source, it is important to separate the source from the sensor by 5 cm or more. Otherwise, continuous bombardment by the alpha particles will damage the semiconductor and degrade its performance. The sensor that I used was fully destroyed by particles in three months (Fig. 5). Theoretically, this degradation allows the density and energy of the particles to be calculated.

Figure 5. After three months the video camera's detector was so degraded that the monitor showed only a bright cloud of frozen alpha particle hits .

Warning (again): Americium 241 is a radioactive isotope. It must not be handled or touched! Never leave the Americium 241 radioactive isotope where a child may play with it or even attempt to swallow it! Store the radioactive source in a small box where you can retrieve it for experiments.

Going Further

Amateur scientists can do various other experiments with an exposed video camera sensor array. For example, pointing a laser pointer at the exposed camera sensor in a dark room can cause interesting optical effects. These include laser diffraction effects and the visualization of dust particles floating in the air.

References:

Hartmud F. W. Sadrozinsk, "Applications of silicon detectors, " 2000.

Paul W. Frame, "William Crookes and the Turbulent Luminous Sea."