Experimenter's Ionization Chamber for quick, simple experiments. Improved Transistor Circuit drifts much less than the single Darlington transistor circuit. The Polonium Pen for spotting those "hot" drinks. Nuclear War Detector - save yourself a trip to the window. Frankenstein's Dosimeter uses parts from a CDV Ionization Chambers page 2 featuring high-value resistors as feedback elements.
Prophylactic Effect of Second Hand Smoke - possibly? Radon Detector for the Student includes a simple ionization chamber. Prospector's Ionization Chamber Area 50 simulates a Geiger counter with an ion chamber.
When ionizing radiation far ultra-violet light, x-rays, alpha and beta particles, etc. If an electric field is present, the ions Explain the principle of radioactive hookup move apart, each moving in opposite directions along the electric field lines until they encounter the conductors that are producing the electric field. An ion chamber is an extremely simple device that uses this Explain the principle of radioactive hookup to detect ionizing radiation.
The basic chamber is simply a conducting can, usually metal, with a wire electrode at the center, well insulated from the chamber walls. The chamber is most commonly filled with ordinary dry air but other gasses like carbon dioxide or pressurized air can give greater sensitivity. A DC voltage is applied between the outer can and the center electrode to create an electric field that sweeps the ions to the oppositely charged electrodes.
Typically, the outer can has most of the potential with respect to ground so that the circuitry is near ground potential. The center wire is held near zero volts and the resulting current in the center wire is measured. The voltage required to sweep the ions to the conductors before a significant number of them recombine or stick to a neutral molecule is usually under volts and is often just a few volts depending on the dimensions of the chamber.
The resulting current is extremely low and detecting individual particles is difficult, especially with ordinary air at atmospheric pressure. The simple chambers to be discussed respond primarily to beta particles and, if they can get inside, alpha particles. Most other rays don't cause enough ionization to be easily detected. These room-pressure chambers respond to the average level of ionizing radiation and do not provide "clicks" like a Geiger counter tube.
Here is a truly simple experimenter's chamber made from an ordinary cookie tin:.
This experimenter's chamber is made from a 4" 10 cm diameter, 5. A 5-way binding post is mounted in the center of the can and a 4" "Explain the principle of radioactive hookup" cm wire is suspended from the post inside the can.
The wire length is short enough to insure that it doesn't touch the lid. Another all-metal binding post and pin are installed in the bottom of the can, and a sheet of gray insulating plastic is glued into place to keep hastily constructed experiments from contacting the can.
The electrometer circuitry will be extremely sensitive to stray electric fields, so a shield is mandatory. Another can previously containing mints is pressed into service:. It is important to shield the electronics! Please believe that; if you leave the electronics exposed you will not get good results. A pin jack is soldered to the tin so that it can be plugged onto the pin on the chamber, and tape is applied to the lip of the can so that the pin is the only connection point. The inside surfaces of the shield are also insulated with tape to prevent accidental contact with the circuitry.
The method of connecting the shield to the chamber can isn't critical and a clip lead will also work; the main culprits are the ever-present, low frequency, line-related electric field and changing electrostatic fields due to movement near the chamber. The wires from the Explain the principle of radioactive hookup can simply slip between the shield and the chamber, or a small notch may be made in the shield to make a little room for a few conductors.
The opening of the chamber may be covered by the original lid, aluminum foil, or wire screen, depending on the experiment. Leaving the end open will let in too much stray electric field in most environments. The can is connected to the positive battery voltage through a 4. The meter is on the 1 volt scale for most measurements. The resistor can be any value above 1k; it simply limits current in the event of a short circuit.
A little piece of double-sided foam sticky tape holds the battery in position. When an alpha or beta particle passes through the chamber, several atoms are ionized and the positive voltage on the can attracts the electrons. The positively charged atoms wander to the more negative center wire and, upon contact, reclaim their missing electrons.
This process results in a current flow in the base of the transistor which is amplified by a factor near 30, This higher current flows "Explain the principle of radioactive hookup" the 10 megohm resistance of the meter, producing the indicated voltage. As a point of reference, a reading of 10 mV would correspond to roughlyelectrons per second, so even weak radioactive sources produce large numbers of ions. To observe the background and leakage level, the lid is placed on the bottom, the top shield is added and the reading is allowed several minutes to stabilize.
The meter settles to a little over 30 mV and exhibits an occasional jump. A camping lantern mantle known to contain radioactive thorium is place in the lid of the chamber, and the lid is
Explain the principle of radioactive hookup on the open end of the can such that the mantle is inside the chamber.
The meter reading climbs to over mV:. Placing the item to be tested inside the chamber in this manner gives the ultimate sensitivity, but care must be taken to avoid touching the center wire. This very simple detector demonstrates how easily an effective radiation sensor may be made with a minimum of effort.
Below is another way to build this simple circuit with even less effort.
First, solder the 4" wire directly onto the Explain the principle of radioactive hookup of the transistor:. Drill a hole in the can right in the center of the bottom and epoxy the transistor, face-down, such that the wire protrudes into the can without touching the sides. Make sure that no epoxy touches the center lead of the transistor base lead.
The epoxy is too conductive! Connect two wires to the collector and emitter leads. The picture shows a length of solid copper telephone cable used for the connections. The two blue conductors are connected to the transistor legs and the two orange wires are connected to the can. The blue wires are given a little slack so that the cable pulls on the can connection and not on the transistor legs.
The resistor is connected on the other end of the orange wires. A mint tin is tacked to the coffee can in a couple of places to act as a shield and a wiring reminder is written on the back of the mint tin. A little optional notch is seen near the bottom of the mint tin that allows the phone cable to exit the tin without being pinched. The opening of the coffee can is covered with ordinary kitchen aluminum foil held in place by an elastic band.
Without a radioactive source, the meter reading settles to 50 mV and placing the lantern mantle on the foil gives a reading of just under mV. The reading is lower because the foil is blocking a significant amount of the radiation from Explain the principle of radioactive hookup mantle.
This simple circuit has serious limitations. It is extremely sensitive to the ambient temperature and a slight warming will cause a large increase in the zero reading and the gain. A modern small-signal transistor should be chosen since older or larger die transistors will probably exhibit too much leakage current. Despite these limitations, the simple circuit can be used for sensing unusually high radiation levels, observing sudden changes as when bringing a radioactive item near the chamber window.
The single Darlington circuit could use more "Explain the principle of radioactive hookup" for detecting weaker sources, but simply adding another transistor exacerbates the temperature drift problem. By building a differential amplifier with one side not connected, a temperature-compensated circuit results: Explain the principle of radioactive hookup the room temperature increases a little, both sides leak more current but the voltage across the meter stays about the same.
The 10k resistors are not necessary but are included to protect the transistors from inadvertent shorts during testing. Another 10k resistor between the battery voltage and the outer can is probably a good idea in the event the can is shorted to the base of the transistor. The schematic shows 12 volts but a 9 volt battery works fairly well. The higher voltage sweeps the ions out of the chamber faster, before they have a chance to recombine so the meter reading will be a little higher at 12 volts.
The sensitivity is good; the photo shows the response to a radioactive mantle held about an inch away from an aluminum foil window on the end of the chamber. Construction of the chamber is similar to the earlier devices except the metal end cap from a spool of resistors is used for the shield. This end cap accommodates a multi-pin header that is used to hold the components and also acts as a feedthru. The ion chamber sense wire is soldered directly onto the base of the transistor and is suspended in air by the transistor without additional support.
It might be a good idea to use a plastic bead to hold the wire more securely but this unit is well-behaved as-is. The meter should read upscale slightly but, if it doesn't, try connecting the sense wire to the other base and reverse the meter leads.
If it still doesn't read a little above zero, clean the transistors with acetone. If that doesn't work, try new Darlington transistors since you probably have a leaky one. Hold a radiation source near the aluminum foil window and the reading should quickly climb. The meter can be a 1 volt digital panel meter or even a digital multimeter set to the 1 volt scale.
Since the original construction of this meter, a couple of useful modifications have been made. A wire shelf was added to allow items to be inserted into the chamber through a small hole in the foil without fear the item will touch the center pin.
The foil is peeled back to reveal the shelf in the photo to the left. Later I realized it's better to put the "shelf" at the bottom so that the inserted item rests on the can with the screen simply acting as an electrostatic shield and physical guard. Additionally, a k potentiometer was connected across the battery and the wiper was connected through a 1 megohm resistor to the base of the Darlington that was originally not connected.
This Explain the principle of radioactive hookup becomes an effective zero control. The meter was zeroed with the new potentiometer before inserting the rod. The reading climbs quickly to over half-scale and drops slowly back to zero when the rod is removed.
Before leaving the darlington transistor approach behind, here are a couple of projects purely for entertainment value:. Recent events point to the need for a simple device for testing cocktails and beers for excessive quantities of polonium.
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