how to test a dosimeter with easily available materials...

[unclejed613]

i've had one of those CDV-777-2 civil defense radiation monitor kits for a few years. it came with an ion chamber meter and the dosimiter pens and charger. it also came with a test source embedded in a plastic disk, but because of the relatively short half-life of the test source, you couldn't really test anything with it. the kit is from at least the early 1960s, as the transistors for the high voltage supply are in painted TO-5 cans. i haven't yet taken either instrument completely apart yet, but my guess is that the only component date codes to be found will be on the transistors. most of the CDV-715 (the model number of the ion chamber meter in the kit) meters were made between 1960 and 1964, and 1964 was the final production year. the test source that came with the meter had a half-life of about 5.5 years, so it's current gamma output is about 1/512 what it was in 1964. so, what to test one of these meters with? first of all, the CDV-715 is a "gamma only" meter. it can read beta particles indirectly (from a beta particle hitting the metal case and emitting an x-ray or gamma photon). so, looking around the web, i looked for radioactive sources that are common in everyday items (and some less common, but easily available, or likely to be in a tinkerer's junk box), or sources that would at least be available enough to do a verification the instrument is working.
1) X-ray machine. this would be a verification source, and you would need to get permission to use it (some local doctors and/or chiropractors, or dentists have X-ray equipment), or have the doctor operate it for you. i don't think they would charge you except for their time. because of the possibility of people giving themselves hazardous doses of radiation most X-ray equipment does not get scrapped like other electronics. if you have some large transmitter tubes that can handle the voltage, a 25kV or higher power supply operating any vacuum tube will produce small amounts of X-rays. an old tube type color TV using a 1B3 rectifier generally would have an anode supply between 25 and 35kV, and the 1B3 itself was an emitter of X-rays.
2) welsbach mantle. many kerosene lanterns use a mantle that contains thorium. thorium itself is an alpha emitter, but thorium's decay products have gamma and beta emitters among them.
3) smoke detectors. smoke detectors use a small amount of americium to generate a charged stream of particles. americium, is also an alpha emitter, and most of it's early decay products are also alpha emitters. an older smoke detector may contain small amounts of decay products that are also gama or beta emitters.
4) granite. most granites contain thorium and uranium both of these elements are alpha emitters, but in their natural form contain significant amounts of their decay products, some of which are gamma and beta emitters also.
5) thoriated tungsten vacuum tube filaments. these are usually in larger transmitter tubes, which are often very expensive, even in non-working condition. there may be sufficient decay products in older tubes to produce measurable gamma radiation.
6) thoriated tungsten welding rods. these are readily available, and contain 4% or less thorium. again, the decay products of thorium will be the source of any beta or gamma radiation.
7) active antistatic brushes. there are antistatic brushes that use polonium, a strong alpha emitter to ionize the air and discharge static charges on photographic film and vinyl records. polonium decays to lead, so there are no gamma or beta emitters in it's decay chain. not a good choice as a test source.

ok, so, of all the sources listed above, i found a package of thoriated tungsten welding rods from a friend who gave them to me for an experiment i never got the chance to try (casting lead rifle bullets with a tungsten core). when testing the CDV-715 meter by holding the package of welding rods up to the meter (with the meter set to it's most sensitive scale of 0.5R), i got a meter deflection of 1/2 of a small division, or somewhere around 0.005R, not much, but measurable and repeatable. the rods are 2% by weight thorium. if the meter wasn't sitting on a table, but i was out walking around, i would never notice such a small reading. so to get some idea how much the welding rods radiate, i've got two of the dosimeter pens charged up. one (the control) is just sitting on the shelf near my computer, and the other (the test unit) is sitting on top of the package of welding rods. i will check them daily to see if there's any indication on the test unit that it's detecting anything significant (it should, these welding rods are about 30 years old, and there should be some thorium decay products present by now). in the meantime i will also keep my eyes open for other sources, like discarded chunks of granite, or discarded smoke detectors. the electrometer tubes in these meters are not made anymore, and i have been seeing ion chamber meters made with low leakage jfets and mosfets. i'm sure that running a tube filament is the largest drain on the battery when the meter is on. the electrometer circuit and the bias supply don't require much current. i'll draw out the schematic for this meter if anyone is interested. it operates off of a single "D" cell (1.5V), and the plate supply is 10V, and the ion chamber bias is 50V, both supplies derived from a single transistor DC-DC converter.

 

[large_ghostman]

Thorium in welding rods is really low. 2-3 years ago we used one at school in a cloud chamber to look at radiation particles. You suspend a rod in a upside down fishtank, the bottom tray has dry ice and above the rod you put ethanol soaked cloth like a tent above it, this is to saturate the air with ultra cold ethanol vapor. Look on you tube for similar experiments.

You see a few Beta particle trails but mainly Alpha. These rods were fairly new but to be honest it wasnt much of a spectator sport if you see what i mean. Try old yellow glass (uranium) Or Blue glow in the dark old watch hands, not the green ones but the Blue type, i cant remember what they are made of, but they sent the schools detector into a frenzy

 

[unclejed613]

i've been letting the test pen sit on top of the welding rods for about 4 days now, and the indicator has moved a little bit, maybe about 2R worth. the control pen hasn't moved at all. so this would be a dose rate of 20mR/hr. not a lot, but i wouldn't want to be carrying TIG welding rods in my pocket for weeks at a time.

usually, the radioactive stuff encountered in everyday objects contain thorium and uranium in small amounts, and since they are mild alpha emitters, there's not much of a hazard. it's the decay products that are more dangerous, because they are beta and gamma emitters. beta and gamma emissions are usually happening at the same time, because what's happening is a neutron decays into a proton and an electron. the gamma emission comes from the neutron splitting into a proton and electron, and the electron is the beta particle. i find it interesting that this process was never very well explained in chemistry or physics class in high school, as well as a few other things, like the assumption that solids are "incompressible" (they really are not incompressible, but we were taught that incompressibility was one property of solids). another thing not explained well in physics class was that isotopes that have very long half-lives are only mildly radioactive, but isotopes that have short half-lives are very radioactive. just that one point alone would silence a lot of the media hype about the question of what to do with spent nuclear fuel from nuclear power plants.

 

[large_ghostman]

All i remember of the chamber thing we did was some particles leave fast straight lines in the vapor clouds (Beta particles?) these are not many, and then you get a fair few lazy all over the place short traces that we were told are Alpha.

But what gets me is welders use a open mask with these rods!! In Devon some houses have Radon Gas, not something i want to be around, although we get granite up here that can send a meter clicking away. Radiation isnt my thing at all, i liked the chamber but its not a spectator sport.

 

[unclejed613]

beta particles will usually leave straight trails, unless a strong magnetic field is applied. alpha particles usually follow a slightly curved track. if a strong magnetic field is applied, the beta particles will curve one direction, alphas in the other direction. alpha particles also are more likely to impact oxygen or nitrogen nuclei in the cloud chamber, and kind of zig-zag through the cloud chamber

 

[large_ghostman]

I am strange enough without playing with radiation