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Abstract: This experiment exercised the ability to build an ionizing radiation detector through the use of common household items and powered with a nine-volt battery. This was completed by wiring the can with a transistor and electrical wiring so that once the battery was connected, the surface of the can was negatively charged, and the central wire was positively charged. This attracted the freely ionized electrons to the central wire. These ionizations created a negative pulse into the transistor, switching it on, thereby allowing it to pass the full electrical charge from the battery along the circuit into a slight detectable reading on the multimeter. Future experiments would benefit from additional transistors to increase the gain allowing students to detect weaker radiation signals.
The first known incidence of radiation detection coincides with the discovery of radiation. In 1895, when William Roentgen discovered X-rays, he verified his discovery with a fluorescent screen which visualized the radiation (Flakus 31). As the research into radiation continued more materials were noted to interact with radiation in useful ways. Initial products like phosphorescent materials and photographic plates were used as evidence of radiation (Flakus 32). Over the years, various other types of materials were discovered to also interact with radiation like crystalline chemical structures, gaseous chemicals, and even human skin and leukocytes (Flakus 33). The earliest use of an ionization chamber to detect radiation dates back to 1899, when not long after JJ. Thomas discovered that X-rays ionized the air they pass through (Flakus 32). It made the air charged as it passed through. Working on a hunch, Marie Curie used this information and an ionization chamber attached to an electrometer to see if it could detect radiation from uranium (Flakus 32). The electrical read out was proportional to the uranium which means her theory was correct. Ionizing chambers continued to advance over the years. For example, it was found in the early 1900s that filling the detection chamber with gas increased the
efficiency of ionizations and made the detectors more effective (Flakus 32).
The next big break in ionization detectors was in the 1928 with the discovery of the Geiger-Muller counter, which was gas filled, easy to make, easy to use, and inexpensive to produce (Flakus 32).
The basic design of an ionization chamber relies on this concept of the radiation ionizing the air or gas in the detector (Rollo 196). In premise, a positively charged anode (usually a wire) is placed in the center of the detection chamber and a positively charged cathode is applied to the outer edges of the chamber, making the interior of the detection chamber electrically charged (Knoll 136). As radiation ionizes the medium around it, the negatively charged electrons are drawn to the positively charged anode (Rollo 196). An electrometer is connected to the anode which will signify when a negative charge hits it, and this is the signal that radiation is interacting (ionizing) the medium in the chamber. Having that electrical field in the chamber will prevent recombination of the electrons with their original atoms, a process known as recombination (Knoll 137).
In this experiment, an ionization chamber will be built using household supplies and powered by a 9-volt battery which will be capable of detecting strong radiation sources.
MATERIALS AND METHODS
In this experiment we first obtained a large metal cylindrical coffee can: 2 inch diameter, 5.5 inch tall (Classic Roast ground coffee medium. 11.5 oz. 1811 Matthews Township Pkwy, Matthews, NC 28105, USA) and punctured a hole in the center of the back. Into this hole we placed a stripped electrical wire (copper electrical wire. 22 gauge, no manufacturer details available) through the hole and use a soldering iron (Weller Standard Duty. 25 Watts. 750- degree Fahrenheit. Apex Tool Group LLC. 1000 Lufkin Road. Apex, NC 27539 USA) to connect it to the base terminal of a Darlington transistor (ON Semiconductors. NPN polarity. Mfr # BC517-D74Z. Phoenix, AZ, USA) using electrical repair solder (Alpha Fry. Item # 51406. Cookson Electronics Assembly Materials, 109 Corporate Blvd. South Plainfield, NJ 07090 USA). The soldering iron uses a heating element to combine electrical components by melting the solder as a filler material. Solder is a fusible metal alloy that conducts electricity and can be melted at lower temperatures than pure metals. The collector terminal of the transistor is connected to a 9V battery (Amazon Basics alkaline battery. 410 Terry Ave North, Seattle WA, 98109 USA) on the positive terminal. This battery will be the power supply for the circuit. Although 9-volts is relatively low voltage compared to commercial ion chambers, it should provide enough charge to prevent recombination (Knoll 144). The emitter, connected to a negative charge, will pull any charge up from the base, then cause a big current to flow from the emitter to the collector.
The negative terminal on the battery is connected to the resistor (4.7kOhm resistor. KOA Speer Electronics, Inc. Mfr# MOS1CT528R472J) which reduces current flow by 4.7kohms, thus preventing the current from excessively heating or causing
damage to the components. The resistor is then soldered to the exterior of the can. The negative terminal of the battery is also connected to one input on the multimeter (Analog multimeter GMT-312. Gardner Bender Instruments. Milwaukee, WI USA) which is used to detect and monitor the electrical signal output from the circuit. The other multimeter input is connected to the emitter of the transistor. This creates a complete circuit running from the battery and creating a negatively charged anode on the can and a positively charge anode on the central wire connected to the transistor base segment. This in turn causes the negatively charge ions in the can to move toward the cathode on the wire. As ions contact the wire, the transistor will amplify their electrical signal and transmit it to the multimeter. From this point, we can cover the opening of the can with aluminum foil to prevent excess background ionizations and attempt to detect the source: Potassium chloride (No Salt: sodium free alternative. French’s. The French’s Food Company. 445 E. Mustard Way. Springfield, MO 65803- 9416). Figure 1 is an electrical diagram of the described set up. Figure 1. Electrical diagram
4.7 k� resistor
9 V battery
The construction of the can required 3 attempts due to poor soldering technique. Once experienced with the solder iron and connecting wires correctly, we connected all the components as seen in figures 2 – 4.
Figure 2. Top of can showing transistor and related connections.
Figure 3. Internals of can showing cathode
Figure 4. External of can showing battery and multimeter connections
When the circuits is connected, the multimeter background reads approximately
0 volts, with occasional spikes up to 0.1 or 0.2.
When placed over 5 tablespoons of NUSALT, the reading begins to peak up to 0.3~0.5 volts. Although this is not very high, the responsiveness suggests that some activity is being detected from the naturally occurring potassium-40 isotopes in the NUSALT.
Figure 5. multimeter readings
The results of the experiment reflect that it is possible to construct a basic ionization chamber using household supplies powerful enough to detect a radioactive isotope that is also commonly found around the house.
The particular stumbling blocks in this experiment arose from two primary issues. First, the inaccessibility to some of the supplies due to the current quarantine and businesses being shut down. By working with a local hardware store, we were able to special order a few of the electronic components, like the wiring, transistor, and resistor. The other components were already carried at the same hardware store and were purchased. The second issue in completion came from the learning curve in how to properly use a soldering iron. We discovered that too much or improper solder can actually inhibit the current flow between wires.
A higher voltage would potentially be useful to produce a higher detectable signal. We conducted a brief experiment using 3 9V batteries in series, which produced a much higher background signal, but no noticeable spike when over the source. Future experiments could incorporate higher voltages with more resistors and shielding the electrical components to reduce signal noise and background interference. Additionally, adding more transistors in series could help increase the gain and allow for detecting weaker radiation signals.
1. Flakus, F.N. “Radiation Detection: Detecting and Measuring Ionizing Radiation – a Short History.” IAEA BULLETIN, VOL 23, No 4.
2. Knoll, G.F. “Radiation Detection and Measurement. 4th ed.” Wiley Publishing. 2010.
3. Rollo F.D. “Nuclear Medicine Physics, Insturmentation, and Agents.” S.M. Mosby Company. Saint Louis, MO. 1978.