The Kearny meter is essentially a combination of an electroscope and a capacitor. Two metal leaves are suspended in a metal can by means of an insulating thread. Then they are charged by the relatively high voltage generated when hard plastic is rubbed with paper or when a roll of tape is quickly unrolled. Because the leaves receive the same sign of charge (either positive or negative) they spread apart. The process is assisted by the fact that once the leaves are charged the can containing them develops a charge of the opposite sign on its interior surface. Hence each metal leaf is repulsed by the other leaf and attracted by the nearby surface.

When the meter is exposed to ionizing radiation, the air inside the meter is ionized (atoms or molecules in the air gain or lose electrons). The ion pairs separate and drift either to the leaves or to the inside surface of the can. If the charging made the leaves positive and therefore the interior surface negative, the electrons and negative ions (atoms or molecules that have gained an electron) produced by the ionization drift to the leaves, and the positive ions (atoms or molecules that have lost an electron) drift to the inside surface of the can. The accumulation of charge reduces the repulsion between the leaves and the attraction between each leaf and the inside surface.

As a result the leaves move toward each other at a rate that depends on the amount of radiation passing through the can. Kearny and his colleagues have provided a table for converting the rate at which the leaves move together into a radiation rate measured in roentgens per hour. The meter is calibrated in this way if it is built precisely to the specified dimensions, so that the weight of the leaves and the electric field between the leaves and the wall of the can are known quantities. One could test the meter by bringing it near a dentist's X-ray machine or a medical device that employs gamma rays or X rays for therapy. The Oak Ridge National Laboratory has provided patterns and detailed instructions for the Kearny meter, which are cited in the bibliography for this issue [below]. Here I shall describe most of the design and construction procedures.

The meter is meant to measure principally gamma rays, although any ionizing radiation such as beta particles (high-speed electrons and positrons), X rays, neutrons and protons could be detected in the same way. The X rays and gamma rays are high-energy electromagnetic waves identical with waves of visible light except for their much higher frequencies and energies. X rays can be generated when an inner electron has been ejected from an atom. Another electron initially orbiting farther from the nucleus drops into the vacancy, emitting the difference in energy between its initial state and its final one a an X ray. More commonly X rays are the radiation emitted by a charged particle because of its rapid deceleration as it passes through a target.

Gamma rays are generally somewhat higher in frequency and energy than X rays although the two types overlap somewhat. Gamma rays come from changes in the energy state of a nucleus rather than from an electron outside the nucleus. A change in the energy level of a nucleus is greater than a change of energy of an electron, and so when the energy is emitted as an electromagnetic wave (the gamma ray), the wave is more energetic.

When gamma rays pass through the air in the meter, they force the ionization of the air molecules by providing energy for electrons to escape from the molecules. The units for the level of radiation that results in a certain amount of ionization can be confusing, particularly since they have changed over the years. One of the first units, the roentgen (R), was designed solely for X rays and gamma rays. One roentgen of these rays produces 2.08 X 109 ion pairs in a cubic centimeter of air held at a standard temperature and pressure. Since only a fraction of the radiation is actually absorbed by the air molecules, it is not clear whether the roentgen is the amount of radiation available in the beam or the amount actually absorbed. In addition the unit is of limited utility because it does not apply to other types of radiation that have effects on molecules other than just ionization. Since the Kearny meter is primarily meant to measure the radiation levels of gamma rays, however, its readings are in terms of roentgens per hour.

The rad is a unit that is applicable to any type of radiation. It is a measure of the energy absorbed by an irradiated target. One rad is equivalent to the absorption of .01 joule of energy by one kilogram of target. The principal disadvantage of the unit is that it does not take into account the biological effects of the absorption if the target is a human being. Moreover, it does not distinguish between the various types of radiation and their different effects on living tissue. Hence when biological systems are the targets of radiation, the common unit of radiation is the rem ("roentgen equivalent man"). Each of several types of radiation is assigned a quality factor that roughly measures the biological damage caused by that type of radiation compared with the damage caused by X rays or gamma rays. For example, one rad of protons with less than 10 million electron volts of energy per proton results in about the same damage as 10 rads of gamma rays, and so the protons are assigned a quality factor of 10. If a human being is exposed to such protons, the dose in rems is calculated by multiplying the number of rads for the exposure by the quality factor of 10.

By definition the quality factor for X rays and gamma rays is 1, so that one rad of such radiation is the same as one rem. Although the absorption of radiation can vary greatly between bone and soft tissue, one can approximate that a rad of gamma rays is the same as a roentgen. Hence the meter's reading in roentgens per hour for gamma rays can be interpreted as being approximately the number of rems per hour.

How serious is a certain dose of gamma-ray radiation? The report from Oak Ridge contains the following rough guide. If a healthy person has received 100 roentgens spread over the previous two weeks, an additional 100 roentgens in one day or more will probably not result in illness if there is no further large exposure in the next two weeks. If the exposure is as much as 350 roentgens spread over a few days, illness is likely but death can be avoided if the victim is given proper medical care. If the exposure is 600 roentgens or more within a few days, death will follow some weeks later. Natural radiation from radioactive materials in the earth and buildings and from cosmic rays amounts to an average of 200 millirems per person per year.

The ionization chamber in the Kearny meter is made out of a clean eight-ounce can (with an inside diameter of 29-9/16 inches and a height of 2-7/8 inches) from which the top has been cut to leave a smooth rim. Any other can of the proper inside diameter (such as a beer can) can be substituted, but you will have to cut the height down. Using the pattern in the upper illustration on page 238 as a guide, draw a label for the can to the proper scale and then glue it or tape it to the side of the can so that the lower edge is just above the bottom rim. If the label is a bit too tall, trim the lower end rather than the upper one.

Mount the can on a wood handle that is about equal to the can in diameter. With a sharp nail and a small hammer punch four holes in the can at the points indicated on the cover. With a needle fold back the metal strips surrounding the holes so that the perimeter of each hole is smooth. Then run thread through the holes as is shown in the lower illustration on page 238. Be sure to touch only the ends of the thread, because it must remain absolutely clean. To each end tie a toggle made from a sliver of wood, adjusting the ties so that the thread is tight. Pull the thread tighter by sliding one of the toggles down the side of the can. Tape it into position and add tape over the other toggle and over each hole to maintain the position of the thread and to prevent the diffusion of air into and out of the chamber.

The thread is to be a stop to keep the metal leaves from swinging too far toward the wall of the can when they are charged and when the meter is being moved. If the thread becomes the least bit dirty, it will start to conduct electricity and so will distort the electric field between a leaf and the can. Even the contamination from your fingers is sufficient to ruin the calibration of the instrument, so that you should not touch the thread inside the can. The best type of thread to use is twisted nylon. Other types that will also serve, listed in the order of their quality, are unwaxed lightweight nylon dental floss, silk and polyester. Monofilament nylon is a good insulator, but it is too difficult to work with in the other stages of construction calling for thread.

The electroscope leaves are made out of standard aluminum foil. Cut a flat piece of foil four inches by eight inches and fold it once to make a two-ply sheet four inches by four inches. Fold it again to make a four-ply sheet approximately two inches by four inches. Fold it a final time to make an eight-ply sheet that is approximately two inches by two inches, but make certain the two halves of the second-fold edge are exactly aligned. If you do the folding carefully, you will have an eight-ply sheet with one corner exactly square. Now make another leaf that is identical.

If you have only heavy-duty aluminum foil, you need to make five-ply leaves to ensure the calibration of the meter. Start with a flat piece of foil four inches square. Fold it twice to make a two-inch square and then insert into it another piece of foil that is also two inches square to give a total of five plies of equal size.

Following the illustration at left, draw a pattern with which to cut the leaves to their proper size. Place the pattern on the folded foil, press down to make the leaf as flat as possible and position one corner of the pattern on the corner of the leaf that is exactly square. With a sharp pencil trace along the line on the pattern to mark the place where the thread will run once the leaf is suspended. The indentation made by the pencil should be deep enough to facilitate a neat fold when you bring the short length of foil on one side of the line over to form a hem. Next trace around the perimeter of the pattern. Remove the pattern, cut the leaf to size, fold the hem over once and then flatten it back in its original place.

Mounting the support thread on the leaf requires some dexterity. Make the mounting pattern shown in the illustration on the next page, drawing the pattern to scale on a clean sheet of paper. Cut a length of thread 8-1/2 inches long, being careful not to touch or soil any part of it except the ends. Then cut some pieces of tape to match the size of the rectangles on the pattern marked "Tape here." Put the tape on the rectangles. Stretch the thread between the rectangles and put another piece of tape on each end of the thread to hold it in place. (An extra pair of hands will help.) The reason for the double layer of tape is that eventually the thread is to be removed from the pattern. When it is, the bottom layer of tape will remain on the pattern paper and the top layer of tape will come off easily, along with the thread.

To prepare the leaf for mounting put a minute amount of one-hour epoxy along the seam of the hem and inside the leaf wherever the individual plies have free ends. The epoxy will not only hold the hem in place once it is folded over but also prevent the thread that runs along the seam from slipping and destroying the calibration of the meter. Be careful not to put glue near the ends of the seam; otherwise it will seep out once the hem is finished and then will stiffen the thread emerging from the hem.

If you have no epoxy, strips of Band-Aid tape will serve. Cut out two pieces, each 1/8 inch by one inch, and mount them on the leaf at the angles shown in the illustration on the next page. The sticky side is up except at the ends, which are doubled back so that they stick to the foil. When the leaf is mounted on the thread, the tem will be folded over and the tape strips will secure both the hem and the thread.

To position the leaf on the thread without touching the thread with your fingers lift the midpoint of the thread with a knife and slip the leaf under it. (The thread is of course still taped securely to the pattern.) Maneuver the leaf until the thread falls into the seam of the hem when you lower the thread. Once the thread is in position in the seam hold it down with the point of the knife while you fold over the hem. Make certain that the thread emerges properly at the seam line. Then press the hem down to ensure that the glue or the Band-Aid strips will hold the hem well.

With a ball-point pen make two small marks at the places indicated on the thread. The marks show where the thread is to rest on the rim of the can when you mount the leaf inside. Attach to the leaf five strips of Band-Aid tape, each 1/8 by 1/4 inch, just to hold everything in place. Put three pieces along the hem and two pieces on the edges of the leaf where individual plies can be seen.

Lift the leaf and thread from the paper pattern, keeping the top piece of tape on each end attached to the thread. Mount the thread and leaf inside the can according to the pen marks on the rim of the can and with the tape attached to the outside of the can. Make the support for the other leaf and mount it. The threads should cross the rim of the can at the same place. They can be kept from slipping around the rim if you file small notches at the crossing points. The hems should be opposite each other on the leaves. Add small strips of tape to the thread outside the can and just below the rim to help hold the thread in place. Make the top edges of the strips flush with the rim. Add tape to the rest of the thread running down the outside surface of the can.

Part of the calibration of the meter requires you to view the separation of the leaves from a specific height. Put a one-foot ruler between your eyebrows and a "seat" on the outside of the can. The seat is a short length of pencil taped to the outside of the can so that the top of it is three-quarters of an inch below the top rim of the can. When it is mounted at the place marked on the label, your view will be directly between the leaves.

Moisture will destroy the calibration of the meter, and so a drying agent must be placed inside the can. You can make such a material (an anhydrite) by heating pieces of common gypsum wallboard (Sheetrock). Do not use calcium chloride. Remove the paper and glue from the wallboard by wetting the paper. Break the board into pieces no larger than half an inch. From a sheet of aluminum foil form a small bowl to hold the pieces. Dry the gypsum for one hour at a temperature above 400 degrees Fahrenheit (204 degrees Celsius) in your oven. To ensure that you always have enough dry gypsum you could make several batches simultaneously, fold the foil around the extra batches and store them in a large, sealed jar. The radiation meter takes one batch at a time, loaded in the can.

The can is fitted with a plastic cover to stop the diffusion of air in and out, but the cover must be easily removable so that fresh batches of anhydrite can be added. If you cannot find a suitable cover (from, say, a container of nuts), you can make one. Using polyethylene plastic four mils thick (the kind that serves in a type of storm window) or some other strong clear plastic, cut out a circle about the size indicated in the bottom illustration on the preceding page. Stretch the circle over the top of the can and tuck in the "skirt" on the sides of the can so that there are no wrinkles on top. Hold the plastic in place with a strong rubber band encircling the can. Along the rim add strong tape about a quarter of an inch wide so that the cover will maintain its shape when it is removed and so that the snugness of the fit is increased. Cloth duct tape is recommended, but a double layer of freezer or masking tape will do.

Cut off the skirt of the plastic about an inch below the top rim of the can and add a notch in the skirt where the piece of pencil has been taped to the side of the can. Remove the plastic cover and add short lengths of tape around the trimmed lower edge of the skirt. Adding tape to both the inside and the outside of the skirt edge will make the edge firmer, but be careful not to fold up the tucks so tightly that the cover is difficult to get back on the can.

A millimeter scale is taped on the top of the plastic cover at the position indicated on the pattern. The scale should be perpendicular to the leaves below it so that when you look down through the transparent cover, you can measure the separation of the lower ends of the leaves by viewing them and the millimeter scale simultaneously.

Next you must make the wire with which you will charge the leaves. Doorbell wire (outside diameter 1/16 inch) or any other wire of comparable size will do. Both ends of the wire must be stripped bare. Around the midpoint wrap a small strip of Band-Aid tape held in place with a drop of glue. Wrap more tape near the top end in order to attach about five inches of thread near the midpoint of the thread. The threads will serve to maneuver the wire close to the leaves when you are charging them and then to move it out of the way while you make measurements. Leave one end of the thread bare. Run the other end between two strips of tape. One strip is just large enough to cover the thread. The other piece is larger. On it fix even more tape or a section of folded paper large enough for you to hold with two fingers. You must also arrange for part of the sticky side of the tape to be exposed there.

The tape and paper at this end of the thread can be pulled so that the charging wire is positioned properly and can then be fastened to the side of the can, thereby freeing both of your hands to produce the charge for the leaves. Mount the charging wire through a small hole pricked into the plastic cover on the can at the position indicated on the pattern for the cover. Slip the wire down until the tape at its midpoint stops it.

To charge the leaves bring the lower end of the charging wire to a position about 1/16 inch above the top of the leaves and tape the paper and tape holder of the wire to the side of the can. Fold one or more pieces of paper into 20 layers and then rub the paper over a piece of hard plastic that you hold by one end. With many types of hard plastic the contact of the paper and the plastic will result in a charge separation that endows the plastic with a net positive or negative charge (which type does not matter). The multiple layers of paper are necessary to prevent the plastic from being discharged through your hand. The discharge through the hand that is holding one end of the plastic is insignificant because the hand touches only a small section of the plastic.

When the plastic has been charged, bring it near the upper end of the charging wire. Small sparks will leap between the two and between the lower end of the wire and the leaves. (The sparks will be visible if you turn off the room lights.) As the charge builds up on the leaves the charging wire may be forced away from them by the electrostatic repulsion. To charge the leaves sufficiently you may have to hold the wire in place by pulling on one of its threads.

You can also develop a large electrostatic charge by quickly unrolling certain kinds of tape. The Oak Ridge group recommends Scotch Brand Magic Transparent Tape and PVC electrical tape, although others may work. Rapidly unroll several inches, face the stick side toward the upper end of the charging wire about a quarter of an inch from it and pass the sticky side past the wire slowly to allow the charge transfer t occur. Once the transfer is finished roll the tape back up so that you can use it for the next charge.

If the meter is to be used in a very humid environment, it must be placed inside a container made out of a large bucket topped with more of the thick transparent plastic from which the top of the meter was made. Fasten the plastic to the bucket, cut holes in the plastic for your hands and then attach cutoff ~: bread wrappers to the holes so that you can work with the meter without exposing it to the outside air. Put anhydrite in the bucket. Readings could be taken with the meter still in the bucket as long as you keep the proper distance between the meter and your eyebrows. The charging materials (hard plastic, tape and paper) could also be put in the bucket to keep them dry.

To measure thc radiation level with your meter charge the leaves and then move the charging wire out of the way. Charge the leaves enough to make them separate more than seven millimeters (as measured with the scale on top) but not so much that either leaf touches its stop thread. (The deflections of the leaves are unlikely to be equal. The discrepancy has no effect on your reading of the instrument, since only the change in the leaf separation matters.)

Put the meter on a horizontal surface about three feet off the ground or floor. Position the foot ruler between its seat and your eyebrows so that you peer straight down between the leaves. The apparent separation of the lower ends of the leaves is measured with respect to the scale, which is also in your field of view. The calibration of the instrument depends critically on your success in getting your eyes at the correct height above the scale and the leaves.

If you have no idea what amount of radiation may be present, expose the meter for about a minute. A proper reading can be made if the final separation between the leaves is no smaller than five millimeters and the change in the separation between readings is at least two millimeters. Measure the change and then use the table on the side of the can to convert the change in leaf separation into radiation rates in unit of roentgens per hour. To increase the accuracy of the measurement you could make multiple readings.

Bibliography

THE EFFECTS OF NUCLEAR WEAPONS. Edited by Samuel Glasstone and Philip J. Dolan. U.S. Department of Defense and U.S. Department of Energy. Government Printing Office, 1977.

THE KFM: A HOMEMADE YET ACCURATE AND DEPENDABLE FALLOUT METER. Cresson H. Kearny, Paul R. Barnes, Conrad V. Chester and Margaret W. Cortner. Oak Ridge National Laboratory. National Technical Information Service, U.S. Department of Commerce; January, 1978.

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