This is an overview of various forms of ionizing radiation, and methods/devices used to detect it’s presence and quantify the amount of emissions, and the amount of absorption that may occur.
- ALPHA radiation, a particle composed two protons, two neutrons, very low energy, easily stopped by a piece of typewriter paper. A product of alpha decay with actually decreases the mass of the isotope it is emitted from. Not dangerous unless the emitting substance is ingested or inhaled.
- BETA radiation, high speed electrons or positrons (anti electrons) they are 100X as powerful as alpha particles. The are stopped by thick metal such as thick barss stainless steal, lead, cement, etc. A low level hazard externally, and a high level hazard if ingested or inhaled in sufficient quantity.
- X-RAY/GAMMA-RAY radiation, electromagnetic radiation. X-rays and Gamma rays are essentially the same, with Gamma being high energy. Gamma is produced in interactions with subatomic nuclei, where as X-rays are produced by accelerated electrons striking a mass. Shielding requires heavy mass of a high atomic number.
- NEUTRON radiation, thermal (slow neutrons) are produced by radioactive decay, higher levels are produced with fission or fusion reactions. Neutrons cannot be directly detected easily, detectors elements have to be buried in a moderator substance that will slow neutrons down to thermal levels so they will interact with the detector elements.
- MUON radiation, Muons react very much like electrons, so they can be thought of as super heavy electrons. Muons are usually only produced in exotic nuclear reactions, and by nuclear weapons. Muons are mostly produced by solar flares and CME’s interacting with the earths ionosphere. Muons are also produced by deep space cosmic events. Muons can be detected by any normal radiation detections means (GM tube, Ion-chambers, spark chambers, etc.) Problems with this scheme are that it is difficult to tell difference between pulses caused by Muons and those caused by other forms of ionizing radiation. The simplest way around this is to stack two or more detector devices directly above each other, with the whole array pointing straight up at the sky. Muons are powerfull enough to trigger two or more detectors, where as alpha,beta,gamma will only trigger one tube at a time, and neutrons will either interact with one tube or none at all. Another way would be two connect an oscilliscope to the detector, and look at the energy level of the pulse, a Muon will create a significantly higher pulse over other forms or radiation. See this website for information on construction of a Muon detector, http://www.cosmicrays.org/
Radiations detecting devices can be of several types; Geiger-Muller tubes (Geiger counter, Gm tube), ION chambers (similar to a smoke detectors operation), Scintillation probes, Neutron detectors, muon detectors. Each works on a similar principal, but goes about apply that detection differently.
- GM Tubes, Geiger-Muller tubes are a vacuum tube usually filled with a high molecular weight gas (neon,argon,xenon,etc) sometimes a halogen is used. A high voltage charge is applied to the tube, and variations in current flow through the tubes as radiation particles or rays strike the gas ionizing it are read. GM tubes under most circumstances will not detect neutrons, or alpha particles. GM tubes with a thin MICA window will detect alpha particles, but are rarely used nowadays do to how fragile they are. GM tubes buried in several inches of polyethylene plastic, or paraffin wax salted with Boron can be used to detect approx 10-15% of the neutrons passing through the detector. CDV 700R4 is an excellent example of GM-TUBE Geiger Counter. NOTE: All GM tube type detectors have one flaw! TUBE SATURATION, in the presence of a high radiation field the Geiger tube will display meter will show one short pulse, and quit. The tube is not damaged, but will not respond. The operator should check the probe by placing it next to the operational check source, if no reading on the meter is seen, the operator and any other’s should leave the area immediately, and re-test the GM tube with the test source again. There are modification circuits that add a blinking red LED to the front of the meter to warn of TUBE SATURATION so the operation immediately knows there is a dangerously high radiation field present.
- ION chambers, These devices originally were vary insensitive to radiation, but vary rugged and durable. Their only use was after an actually nuclear accident or nuclear detonation occurred. The original units from the 60’s through the 80’s only started reading at 5 mR or greater (that’s about 3000 Counts per minutes CPM). The average background radiation around the USA is about 20-30 CPM. Newer ION chamber units are far more sensitive thanks to modern JFET transistors, and super sensitive amplifier IC’s. ION chamber work like a GM tube, except there is not any gas inside, just dry air, and the chamber is sealed to keep dust and smoke out which would alter the readings. ION chambers will only detect BETA, GAMMA/X-RAY emissions. An open ION chamber would detect alpha particles as well, but would but very prone to dust, smoke, moisture (think about false alarms and your smoke detector). An example is a CDV-717 ion chamber detector
- SCINTILLATION probes, Interesting fusion of technology. A screen made of Sodium Iodide doped with Thallium is placed with a light proof seal over a photo-multiplier tube. The photo multiplier tube amplifies any light striking it by one million times. The screen made of Sodium/iodide/thallium will react to alpha, beta, gamma emissions by creating extremely faint bursts of light, the photo multiplier reacts to this by producing an electrical charge. Scintillation probes are very sensitive, and used to trace down radioactive contamination on the surface or inside the human body. They are not as fragile as MICA window GM-tubes, but must be kept light tight, or room light entering the photomultiplier tube will destroy it almost instantly.A type of scintillation probe
- NEUTRON detectors, sometimes an ion chamber, but usually a GM_TUBE buried in something dense enough to slow neutrons down to thermal neutrons so that they can interact with the GM-TUBE. The substance is usually BORON, but can also be HE3, or an exotic Florine compound. With all that said, most neutron detectors only detect approx. 10-15% of the neutrons striking them. A type of neutron detector. There are newer technologies being used that can detect neutrons much more easily, A Boron-Carbide diode has been successfully used as a detector element.
- ELECTROSTATIC detectors, These are commonly used as DOSIMETERS. They are very reliable, and resistant to damage, not affected adversely by EM pulses, etc. They work on the principal of a high voltage charge is injected into them, moving a quartz fiber to the opposite end of a graduated scale. The electrostatic field holds the fiber there, and as BETA, and X-rays/Gamma-rays strike the device, the radiation depletes the charge causing the fiber to creep the other direction. They are surprisingly accurate devices to have, and easy to maintain. They do have to be checked periodically, and recharged when being used, do to the eventual leakage of electricity will render the fiber at the opposite end. The most common was the CDV-742 (1-200 Rads). They used a simple “D-Cell” high voltage charger to charge and read the device. The charger was the CDV-750, a small yellow box, built just as tough as the other civil defense equipment, and very readily obtainable today.
For anyone interested in purchase this equipment, now would be a good time. The government disbanded the civil defense corps awhile back, and but FEMA in charge of everything. Many states have dismantled their civil defense shelters, and auctioned their equipment off on E-BAY. I was able to pick up three CDV-700R4’s + One CDV-750 Dosimeter charger + 10 CDV-742 dosimeters for around $50 with shipping. Two of the three meters worked, but needed cleaning, and some adjustment. The most common thing wrong with them I have found is that there is a 400 uF 3 V (part C1 in the original diagram) capacitor across the meter, these capacitors were poorly made, and all have proven faulty, from voltage leaks, to being way out of their design specs (try 680, and 800 uF). I replaced these capacitors with a modern 16Volt axial 330 uF electrolyte type, and all works well. I used the third non working unit for case parts, and a better working meter to fix the other two. Six of the CDV-742 dosimeters were working, the others were useless, and the CDV-750 charger was fine with just a battery installed. I will be switching the probe on one of the CDV-700R4’s to a scintillation probe eventually. I also purchased a CDV-717 ION chamber, these were so rarely used, that all I had to do was open the case up (toolbox type latches) and place it in an oven at 300 Deg F. for 2 hours to perform a “BAKE OUT” as described by the user manual to drive out moisture from the ION chamber, and the HIGH Q electronics. I put a couple of fresh bags of desiccant in the unit when it was baked out, and sealed it back up with a fresh D-Cell Duracell battery, it works just fine, and I am going to construct and x-ray generator so I can fully test it later on.
There are a number of websites devoted to this equipment;