The experiment on the penetrating power of radiation (Figure 62) could lead you to believe that alpha radiation is weak because it is so easily absorbed. However, the exact opposite is true. Alpha particles are so massive that they lose their energy by ripping atoms to pieces as they fly through matter. Eventually, alpha particles lose all their energy and stop harmlessly.
A simple but interesting experiment to demonstrate this point is shown in Figure 66. This is a homemade spark counter, in which a thin wire is placed at high voltage referenced to a nearby ground plane. Without a source of ionizing radiation nearby, some current will leak from the wire through the corona effect, producing a hissing noise. However, when a source of alpha radiation is brought nearby, the alpha particles passing close to the wire leave behind a trail of ions that render the air conductive, causing a spark to jump between the wire and the ground plane.
Figure 66 In our homemade spark counter, high voltage is placed between a thin wire and a large ground plane. Alpha particles ionize the air between the wire and the ground plane, causing sparks to jump between the two. Beta and gamma radiation from sources with the same activity do not ionize the air sufficiently to cause sparking.
Our spark counter follows a design by Klein et al.21 The ground plane is a sheet of aluminum 25-cm wide × 10-cm tall. We used thin, bare copper wire for the center electrode. The resistor and capacitor are rated at 5 kV. We found that placing the aluminum sheet on an incline at a distance of around 2 to 3 mm away from the wire causes a steady corona current of around 10 μA, which we measure using a cheap analog multimeter. We had to dampen vibrations on the wire with a few beads of putty to prevent sparks caused by alpha particles from triggering continuous sparking due to shaking of the wire. Loud sparks are produced when we bring a source of alpha particles (e.g., StaticMaster cartridge containing 210Po) close to the wire. We used the high-voltage power supply that we built to energize our CRTs (Figure 39).
Go ahead—build one yourself and try it out with different sources of radiation. Just remember to exercise care! The setup uses high voltage, and will retain some charge even after you turn the power supply off. A 3,000-V charge produces a nasty jolt, so make sure that you use a dry wooden dowel or plastic rod to bring your radioactive sources close to the charged wire. Look back at the diagram of the GM tube of Figure 56. Do you see a similarity to the spark counter you just built?
As an interesting aside, ionization-type smoke detectors take advantage of the ionizing power of alpha particles by using a steady ionic current as a detection mechanism. As shown in Figure 67, air inside a sensing chamber is ionized by a 1 μCi 241 Am source. Whenever smoke particles enter the sensing chamber, the ionic current is reduced. This drop in current is sensed by a specialized circuit that triggers an alarm. Many semiconductor companies offer very inexpensive, one-chip solutions for ionization smoke detectors. Examples include the Freescale MC14468, Microchip RE46C114, and Allegro A5367 integrated circuits.
Figure 67 Ionization-type smoke detectors contain a 1 μCi source of 241Am. (a) The americium source is a small button placed inside the ionization chamber. (b) Alpha particles from the 241Am ionize the air within the chamber, allowing a current to flow between its two electrodes. (c) When smoke particles enter the chamber, they cling to the ions, causing a marked drop in current flow, which triggers the alarm. Please note that it is not legal in the United States (and some other countries) to remove the radioactive source from a smoke detector.
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