Mounting the AmO2 Pellet
Once the end plug has been selected, simply place a tiny drop of instant-setting glue in the center of the top side of the plug and set the pellet, AmO2 side up (usually the flat side), centered on the glue. Let it set for a few minutes before handling.
Making the Target Screen
Cut about 1” (25 mm) by 3/8” (9.5 mm) of stiff, clear plastic, blister-pack cover from a product display card. Crease the two ends of the strip into support legs for the flat which will be eventually instant-glued down to the end plug over the AmO2 source.
Applying the Phosphor
If you are using CRT phosphor, apply a small dot of white Elmer’s or other clear-drying glue about the size of a sesame seed or grain of rice (smaller than a paper match head) to the center of the screen (the undersurface of the target with the legs) and carefully cover it with an equal amount of phosphor.
Using a fine-tip artist’s brush, mix thoroughly and spread it evenly and thinly over the flat screen, smoothing and stippling with the brush until you have a fine, translucent coat like frosted glass which easily lets diffused light through like thin tissue or wax paper, but won’t allow you to clearly see objects through it. Too little is better than too much!
If you chose to use zinc sulfide paint, place a small drop of it in the center of the screen and spread as above. In either case, dry the paste thoroughly before mounting the target; you can hasten this process with a hair dryer.
Spacing the Source from the Target Screen
After the phosphor is dry, place the screen, legs up, alongside the source pellet on the end plug. The spacing between the top of the source and the target should be 3/32” (2.5 mm—the combined thickness of two dimes). Actually, anything between 1/16” (1.5 mm—the thickness of a penny) and 1/8” (3 mm—the combined thickness of a penny and a nickel) should work fine. Any closer than that, the spot is too small and remains too persistently illuminated; farther separated, the flashes are too scattered and unimpressive.
Trim the legs of the target so that they will hold that spacing as they straddle the source, and lightly instant-glue the two legs down to the end plug.
Alternatively, you could glue the source to the end of a screw which has been previously threaded through the end cap, so that you can adjust the pattern to your liking—scattered like a fireworks display, or more concentrated for intensity. But first, place a nut, washer or bushing under the head of the screw to limit its travel so that the source doesn’t scratch the phosphor on the screen.
Selecting the Tubing
The tube may be any material--brass, aluminum, steel, or plastic pipe; there are even suppliers of ornate tubing for crafts like kaleidoscope building. Check an electrical supplier for decorative lamp tubing.
Tubing may be readily found at hardware stores and hobby shops; flea markets and yard sales yield lamp hardware, pocket flashlights, towel bars, toilet-paper spindles, turkey basters, hair curlers, old lipstick tubes, tubular-aluminum lawn furniture, brass pipe and other tubing that can be called into service. Even an opaque plastic pill vial or 35 mm film canister will work in a pinch. You will need to cap or plug both ends of the tube--the top will be drilled for the lens and the bottom will support the source and target.
The finished tubing should be roughly 1-1/2 inches (38 mm) or so in length, and the diameter will be from ½” (12.5 mm) to an inch or so, depending upon the lens system you choose, how you choose to mount the parts, and the way you want your ‘scope to look when you’re done.
When you cut the main tubing, it’s a good idea to make it an inch or so too long; you can cut off the excess after you’ve determined the final, exact length. To prevent marring the surface of the tubing, wrap the cutting area with masking or PVC tape, and use a pipe cutter, not a saw, unless you’re very careful.
Picking the proper Lens
Lenses are available everywhere—the best are compound (dual-lens) eyepieces from telescopes, binoculars and microscopes, but old 8 mm movie cameras and projectors, video cameras and even jeweler’s loupes are sources as well. Don’t waste your time salvaging lenses from old point-and-shoot cameras or slide projectors; their focal lengths are too long.
A single lens with a diameter between 3/8” (9.5 mm) and 5/8” (16 mm), and a focal length between ¼” (6.5 mm) and ½” (12.5 mm) works quite well. Best for minimum distortion is a plano-convex lens (one side flat, the other curved outward) with the flat side toward your eye. If you have to stack two narrow-spaced lenses for enough magnification, try to select plano-convex lenses and position both of them with the flat side toward the eye. A bi-convex lens (both sides bulge) will introduce distortion, making perimeter flashes stretch outward like rays.
To test a lens or eyepiece for adequate magnification and minimum distortion, mark two dots, ¼” (6.5 mm) apart, on a piece of paper. With the lens held very close to your eye, bring the dots to focus; they should be at the left and right edges of your field of view, and not blurred. If the dots are so close together that you can see well to the left and right of them, the magnification is probably not enough to resolve the glow into impressive, distinct flashes.
You can also test the suitability of a lens before mounting it by viewing your alpha-illuminated screen with it. Set the target screen, spaced 3/32” (2.5 mm) over the source and, with your eyes dark adapted, hold the lens by its edges between your thumb and index finger and focus in on the glowing field. If the flashes look good to you, it’s good enough!
You can support the lens in a sandwich of two plastic, rubber or fiber washers, or you can simply drill a hole in the upper end cap just large enough to carefully instant-glue the lens to the rim of the hole. If you use a commercially-made compound eyepiece from a telescope or microscope, simply slide it into a suitable tube.
Setting the focus
Final focus is extremely critical and must be done in a dark room with the eyes dark-adapted. You can use very dim background lighting while you make the adjustments. Properly spaced, the sparkles will appear tiny, bright and sharp, not fuzzy. You could provide a focus adjustment to accommodate the varying eyesight requirements of those few folks who are actually interested enough to see what a spinthariscope does!
NOTE: The radioactive sources may be removed from the smoke detectors for experimentation, but may not be taken off the premises or sold without a license, which allows the manufacturer to distribute a device which contains a radioactive source. The source within the device is not licensed separately; once you remove the source from the device, you are, in effect, violating the license for that product.
Americium 241 A NRC or state license is required to purchase this isotope. The maximum activity in a sealed holder is 2.5 microcuries per square mm.
Polonium 209 (half life 102 years) is available domestically (Po-210 has a half life of only 138 days)
Thorium 232 (natural) is another good alpha emitter
Lumilux zinc sulfide: Honeywell Chemical Specialties, Effect Green N FG Art No. 50060 (high initial brightness, quick excitation by light)
Smoke detectors and americium oxide
Smoke detectors monitor a tiny level of electrical current across and air gap as a radioactive pellet of americium oxide ionizes (electrically charges) normal air circulating through a chamber. If smoke particles enter the chamber, they will be ionized instead, reducing the electrical current normally present in the chamber, and triggering the alarm.
A typical smoke detector contains 0.0002 grams (two ten-thousandths of a gram) of americium oxide (Am-241) in a gold matrix with an activity of 0.9 microcurie, producing 3.7x104 alpha particles per second (37 kBq). Pressed against a Geiger counter probe, a typical specimen shows approximately 35,000 counts per minute (CPM), or 35 milliroentgens (mR). This level is not considered hazardous in its encapsulated form and, even if swallowed, the pellet would not be digested, but expelled from the bowel.
For practical purposes, consider the roentgen, rad, and the rem to be equal with gamma or x rays (thus 1 mR/hr is equivalent to 1 mrem/hr.)
Other Sources of Radioactivity in the Home
Short-lived radon 222 decays into “daughter products”-- polonium-214 and 218, lead-214, and bismuth-214. These isotopes attach themselves to dust particles which are attracted to your electrostatically-charged cathode ray tube (CRT) TV screen.
If you lightly dampen a small spot on a tissue with isopropyl (rubbing) alcohol and clean off the screen, you will be surprised at how much response a Geiger counter will give to that spot! Fortunately, these radioactive isotopes are very short-lived, losing virtually all of their activity within a few hours.
Red-orange pottery of the 1930s like Homer Laughlin’s popular “Red Fiesta,” and yellow-green “Vaseline glass” contain uranium oxide. Their weak radioactivity can be detected with a Geiger counter.
Radiation sources reportedly found in the natural and man-made environment
Uranium minerals: pitchblende, uraninite, autunite, uranophane, carnotite, coffinite
Phosphates and Fertilizers
Water (radon in mineral springs, health spas, wells)
Radon Removal Systems
Air Filters, Ionizers, and Electrostatic Precipitators
Radiation in Food, Tobacco, and Health Care Products
Low Sodium Salt and Potassium Compounds
Health Care Products
Radioactivity Essential to Product Function
Quackery and Questionable Medical Products
Radium Therapy (nuclear medicine)
X-ray Fluorescence Analyzers
Instruments, Gauges, Markers, etc.
Radioluminescent Watches and Clocks
Ion Producing Devices
Irradiated Spark Gaps and plugs
Density, Moisture and Thickness Gauges
Smoke and Aerosol Detectors (americium 241)
Static Eliminators (polonium 210; half-life, 138 days)
Radioactivity Incidental to Product Function
Uranium (Containers, Munitions, Frizzens and Flints, Photographic Prints, Slides and Negatives, Vaseline glass
Neon Signs and bulbs
Thorium (Welding Rods, Arc Lamps, Electric Lights, Gas Lantern Mantles, Thorotrast, Neutron Dosimeters,
Optical Coatings and glass)
Uranium and Thorium in Ceramics
Rare Earths in Glass and Magnets
RADIATION MEASURING UNITS
The Becquerel is a unit used to measure a radioactivity. One Becquerel is that quantity of a radioactive material that will have 1 transformations in one second. Often radioactivity is expressed in larger units like: thousands (kBq), one millions (MBq) or even billions (GBq) of a becquerels. As a result of having one Becquerel being equal to one transformation per second, there are 3.7 x 1010 Bq in one curie.
Note : A common measurement of the presence of radon in a structure is given in Becquerels per cubic meter (Bq/m3). Measuring becquerels per cubic meter with a Geiger counter is not possible because the counter only measures the radiation the probe encounters rather than the radiation existing in a cubic meter of air.
Counts per Minute (CPM)
This rather arbitrary unit was assigned to the rate of clicks (ionizing events) heard from a Geiger counter. Since the clicks could be caused by alpha, beta, gamma, Xrays, and cosmic rays, there is no dosage equivalent.
The curie is a unit used to measure a radioactivity. One curie is that quantity of a radioactive material that will have 37,000,000,000 transformations (decays) in one second. Often radioactivity is expressed in smaller units like: thousandths (mCi), one millionths (uCi) or even billionths (nCi) of a curie. The relationship between becquerels and curies is: 3.7 x 1010 Bq in one curie.
The gray is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. The unit gray can be used for any type of radiation, but it does not describe the biological effects of the different radiations. Absorbed dose is often expressed in terms of hundredths of a gray, or centi-grays. One gray is equivalent to 100 rads.
Rad (radiation absorbed dose)
The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of radiation.
Rem (roentgen equivalent man)
The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem, or mrem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of radiation.
The roentgen is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. It is a measure of the ionizations of the molecules in a mass of air. The main advantage of this unit is that it is easy to measure directly, but it is limited because it is only for deposition in air, and only for gamma and x rays.
The sievert is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of millionths of a sievert, or micro-sievert. To determine equivalent dose (Sv), you multiply absorbed dose (Gy) by a quality factor (Q) that is unique to the type of radiation. One sievert is equivalent to100 rem.
Radiation dose examples:
6000 mSv Fatal if received all at once
1000 mSv May cause symptoms of a radiation sickness (e.g. tiredness and nausea) if received within 24 hours
100 mSv The highest permitted dose for a radiation worker over a period of five years
4 mSv The average annual radiation dose for Finns caused by indoor radon, X-ray examinations, etc
2 mSv The annual dose of cosmic radiation received by a person working in an airplane
0.1 mSv The radiation dose received by a patient having a lung X-ray
0.01 mSv The radiation dose received by a patient having teeth X-rayed
Common readout units are roentgens per hour (R/hr), milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour (mrem/hr), and counts per minute (cpm). For practical purposes, consider the roentgen, rad, and the rem to be equal with gamma or x rays. So, 1 mR/hr is equivalent to 1 mrem/hr.)
RADIOACTIVE PARTICLES AND RADIONUCLIDES
Alpha Particles: An alpha particle consists of two neutrons and two protons imparting a positive charge of two and a relatively large mass. The alpha particle has a very short range of about 1 to 2 inches in air. A single piece of paper or even the outer layer of your skin is enough shielding to stop an alpha particle. Therefore, external alpha particles are not considered to be dangerous. There are, however, serious health hazards if inhaled, ingested, or absorption through an open wound. Some alpha sources include uranium-238, radium-226, and thorium-232.
Beta Particles: Beta particles have a charge of negative one and are very small in size, 1/3600 the mass of a proton or neutron. Negative beta particles are also known as electrons. Compared to an alpha particle, a beta particle’s range is much greater. The range a beta particle can travel depends upon the energy level of the specific beta emitter. Beta particles can penetrate soft tissues like your eyes and the outer layer of skin. Generally, beta particles can be shielded with plastic, glass, aluminum, or wood. Beta-emitting sources include tritium, lead-214, cesium-137, strontium-90 and potassium-40.
Tritium (H3): Tritium is a radioactive isotope of hydrogen. It consists of one proton and two neutrons, and has a half-life of 12.3 years. Beta radiation from H3 has a very low energy and can travel just a few inches in air. However, because tritium is a pure beta emitter, it can easily be absorbed through a person’s skin. Proper shielding, such as glass or wood, should be used to prevent this absorption.
Gamma-Emitting Radionuclides: Gamma radiation originates in the nucleus of the atom and travels by way of electromagnetic waves, similar to light. Gamma rays have neither charge nor mass. Gamma rays are produced succeeding spontaneous decay of radioactive substances such as cobalt-60 or cesium-137. A gamma ray can travel through the air at long ranges, as much as several hundred feet. Furthermore, gamma rays such as cobalt-60 can penetrate deep into the human body and can therefore be used as the radiation in cancer treatment. Thick shielding is needed for gamma ray protection, for example: lead, steel, or concrete.
Uranium-238 (U-238): Naturally occurring uranium generally consists of 99.3% uranium-238. All isotopes of uranium are radioactive. U-238 decays by way of alpha emission and has a very long half-life of 4.5 billion years. This extremely long half-life has been useful in estimating the age of igneous rocks. The most important aspect of uranium is its potential as a nuclear fuel. U-238 itself is not useful, but it can be converted into Plutonium-239, which is fissionable by means of a breeder reactor. Daughters of U-238 include Th-234, Ra-226, Pb-214, and Bi-214.
Uranium-235 (U-235): Naturally occurring Uranium-235 is only about 0.7%. U-235 has a half-life of approximately one billion years and it decays by alpha and gamma emission. U-235’s significance comes from its ability to easily fission. U-235 is used to fuel nuclear power reactors for the generation of electricity. The average human intake of uranium is due primarily to food ingestion.
Radium-226 (Ra-226): Radium is known to have sixteen isotopes. Radium-226 is the most common of these isotopes. Radium-226 is found in the uranium series, which has a decay chain that begins with U-238 and ends with Pb-206. Ra-226 has a half-life of 1,602 years and decays by way of alpha and gamma emission.
BI-214: Bismuth-214 is also a daughter nuclide of RA-226. The half-life of BI-214 is approximately 20 minutes. BI-214 decays by beta emission at 1000 keV (23%), 1510 keV (40%), and 3260 keV (19%). Three main gamma energy peaks for BI-214 are at 609 keV, 1120 keV, and 1764 keV.
Thorium-232 (Th-232): Thorium is thought to be even more abundant than uranium. Twelve isotopes of thorium are known. Thorium-232 occurs naturally. It has a half-life of 141 billion years and decays by alpha and gamma emission. Thorium-232 is at the top of the thorium series, which ends with the stable isotope Pb-208. The mass numbers of the isotopes in the thorium series are exactly divisible by 4. This implies that all members of that series decay by alpha emission.
Beryllium-7 (Be-7): Beryllium is one of the lightest of all metals. It is utilized as an alloying agent in generating beryllium copper. Beryllium itself is toxic and should be handled accordingly. Be-7 is a radionuclide that comes from natural background radiation that is created in the upper atmosphere, mostly in the stratosphere, by cosmic rays spoliation of carbon-12, nitrogen-14, and oxygen-16. Therefore, it will show up in most all samples analyzed by the Gamma Spectroscopy System. Be-7 has a half-life of 53.44 days and decays by electron capture.
Cesium-137 (Cs-137): The metal cesium is used in photoelectric cells. The detection of the radioactive isotope Cs-137 in samples is due to past nuclear activity. This nuclear activity is primarily due to nuclear fall-out and testing. Over time, however, the Cs-137 levels have returned to “normal” levels. Cs-137 has a half-life of 30 years and is a beta emitter.
Potassium-40 (K-40): Potassium is the seventh most abundant metal. It makes up about 2.4% of the earth’s crust. K-40, a radioactive isotope, occurs naturally but “presents no appreciable hazard” (Weast B-25.) Potassium is found in most soils and is essential for plant growth. The half-life of K-40 is 1.27 billion years. K-40 decays by way of beta and gamma emission, which accounts for the positive correlation between high K-40 activity levels and high beta activity levels in most every soil sample.
Lead (Pb): Radioactive isotopes of lead include Pb-212 and Pb-214, which decay both by beta emission and gamma emission. Lead isotopes in general have relatively short half-lives compared to other radioactive isotopes. The half-life of Pb-214 for example is about 27 minutes. Stable lead isotopes are the end products of each of the series of radioactive decay processes.
Soil: Gamma-emitting radionuclides include uranium-238, Radium-226, Thorium-232, Uranium-235, Beryllium-7, Cesium-137, and Potassium-40.
Water: Radionuclides of concern are Radium-226, Lead-214, Bismuth-214, and Potassium-40.
In a three-color cathode ray tube (CRT), the traditional phosphors used are (1) zinc-sulfide doped with copper, aluminum and sometimes gold for the green color; (2) zinc-sulfide doped with silver for the blue color; and (3) yttrium-oxysulfide doped with europium for the red color. The zinc-sulfide based green and blue phosphors are about 20% efficient in light-energy transmission (i.e., conversion of energy from the electron beam to energy illuminated by the excited phosphor.
This product consists of a very uniform deposit of blue-emitting P22 type silver activated zinc sulfide phosphor applied to one side of clear polyester plastic sheet. This is specifically intended for alpha particle detection, and the phosphor layer density is sufficient to completely absorb common alpha particles such as those from Am-241.
The phosphor layer is smooth and sufficiently robust to stand up to normal handling. The sheets are quite flexible and can be easily cut with a scissors or paper cutter. The EJ-440 material has often been used to successfully replace the Product #ASP3 ZnS:Ag sheets formerly manufactured by the Wm. B. Johnson Company. EJ-440 is available in full-size sheets measuring 216mm x 279mm ( 8·1/2" x 11" ) and is also supplied in 49mm dia. ( 1.94" ) precut discs. This material is very stable, and its shelf life at room temperature is at least two years.
Eljen Technology EJ-444 ZnS:Ag coating on thin plastic scintillator sheets for dual alpha-beta detection. The water-proof adhesive is soluble in alcohol and other organic solvents.
Light Output: Anthracene 300%
Wavelength of maximum emission, 450 nm
Decay time: 200 nanoseconds
Phosphor Density: 3.25 mg/sq.cm
Phosphor Density range: +/-0.25 mg/sq.cm
Thickness of polyester film: 0.25 mm
Density of polyester film: 36 mg/sq.cm
Density: 1. 032 g/cc
SOLID-STATE NUCLEAR PARTICLE DETECTORS
The most recent class of detector developed is the solid state detector. These detectors convert the incident photons directly into electrical pulses. Germanium detectors have the best resolution, but require liquid nitrogen cooling. Silicon needs no cooling, but is inefficient in detecting photons with energies greater than a few tens of keV (kilo electron volts). Detectors fabricated from high Z semiconductor materials operate at room temperature and have high efficiency. Detectors made from cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), and mercuric iodide are routinely used.