Introduction: Measuring the Half-life of Radon-220 With a Simple Ionization Chamber
Radioactivity surrounds us constantly. Either in its natural form by radioactive rock or in the form of nuclear power plants. Unfortunately, also in the form of nuclear bombs. We distinguish natural and artificial radioactivity. Natural radioactivity refers to the spontaneous decay of radioactive substances in three different ways. The alpha, beta and gamma decay. In alpha decay, the radioactive atom emits a helium nucleus. In beta decay, a neutron or proton is converted into a proton + electron or neutron + positron. In gamma decay, the nucleus only goes into a lower-energy state and emits electromagnetic radiation (so to say light) of very high energy.
In artificial radioactivity, the nuclear transformation is stimulated from the outside, for example by neutron bombardment. As a result of this bombardment, the atomic nucleus splits into two lighter fragments and further neutrons. These can in turn cause further nuclear fissions. This is how the famous chain reaction arises.
But radioactive substances are also used in medicine. These are injected into the body, for example during the examination of the thyroid gland. Here are radioactive substances with consistently low half-life used. Half-life is understood to be the time until which half of the original starting atoms have decayed. The half-life of the 99mTc used in scintigraphy is about 6 hours. Therefore, these short-lived radioactive atoms must be generated locally. Uranium 238 has a very long half-life of 4.5 * 10 ^ 9 years. For example, uranium-235 can be artificially split by neutron bombardment, as already mentioned.
In physics and mathematics lessons, so-called exponential functions play a very important role. This is a function that either increases more and more (eg world population or number of bacteria) or decreases less and less (radioactive decay). The variable (usually the time t) is not in the base, but in the exponent. That's why the name exponential function. The typical function for radioactive decay is:
N (t) = N0 * exp (-k * t)
The factor k contains the half-life of the radioactive element. If this has a short half-life, it decays quickly and the coefficient k is large. Conversely, with a long half-life, the element decays more slowly and k must therefore be smaller.
Understandably, only radioactive substances with a short half-life are suitable for teaching physics. No student wants to wait a million years for half of the radioactive atoms to decay. Thorium is perfect for this. Thorium-232 breaks down into radon-220 along the so-called thorium decay series. And this Radon 220 has a half-life of only about a minute.
But how do we notice or measure these decays? Radon-220 decays due to alpha decay. The released helium nuclei have a large (kinetic) energy, which means they move very fast. On their way through the air, they collide with air molecules and ionize them. These ions can migrate in an electric field (generated by an electrical voltage) and cause a very small current. This is the principle of the so-called ionization chamber. Put simply, there is an electrical voltage between a wire in the middle of the chamber and the housing. When the alpha rays now ionize the air, a small current flows between the wire and the case, which can directly be measured or converted into a measurable voltage.
If a certain amount of radioactive atoms are trapped in the chamber, many atoms decay at the beginning and later less and less. As a result, the electric current or the voltage is greater at the beginning and decreases exponentially with the respective half-life. If we measure the output voltage U as a function of the time t, we obtain a decay curve with a half-life of about 1 minute.
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Step 1: Parts
For a simple ionization chamber, not many components are needed. The
required parts are:
* a 9V battery
* 2-3 gas lantern mantles coated with Thorium (look on ebay...)
* a multimeter for voltage measurement
* 3 pieces 10 kOhm resistors
* 2 pieces 2.2 kOhm resistors
* 1 piece of 1 Mohm resistor
* a 100 kOhm potentiometer for the offset
* 2 pieces of pnp BC516 transistors or similar
* 2 pieces npn BC517 transistors or similar
* a metal box with a lid
* a glass with a screwed lid
* a thin, stiff wire
* a syringe
All electronics come outside on the bottom of the case. To protect against interference, the cover of the housing is placed over the electronics. The 4 cables for the power supply and the voltmeter are led through a small gap to the outside. You have to drill a hole in the bottom of the case. Solder the long, stiff wire to the base of the BC517-transistor. Then lead the wire through the hole. The wire must not touch the case! Therefore bend the base-leg of the transistor by 90° and glue it above the hole. Then connect the other two legs with the rest of the components.
Step 2: Measurements
The measurement process:
First set the voltage to 0V with the potentiometer without a radioactive source. Then you suck with a syringe air from a filled with thorium mantle glass vessel. This air is then injected through a small hole in the interior of the ionization chamber. The ionisation chamber must be placed on a table with the opening facing downwards, so that no air exchange with the environment can take place. Now you will notice that the voltage on the voltmeter is increasing. If it is maximum, the time is stopped with a clock. Afterwards one reads, for example every 10 seconds the voltage and put it in a table. Finally, these values are used to draw the graph U (t). One would have to get a decreasing exponential function. Using the graph you can now read off the half-life. That is the time after which the electrical voltage has dropped to half of the maximum value. This should be about 56 seconds.
Step 3: Conclusions
With this simple ionization chamber, the important physical topic of
radioactivity can be treated impressively. The costs of this simple but effective experiment are only around $ 10 without the multimeter. Therefore, one could tinker several ionization chambers for a whole class, as I did.
Have fun copying this experiment and Eureka.
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