A radiation dosimeter is comparable to the odometer of a car, whereas a dose rate meter is like the speedometer. Any of those familiar with the popular Kearny Fallout Meter (or KFM for short) know that it is a set of plans on paper to make a practical radiation dosimeter using common local resources. There is also need for a radiation dose rate meter that can be similarly home made with easily scavenged parts. I consider this a type of "Salvage Technology"; a fall-back method to provide needed tools after society (and supply) breaks down in time of disaster.
A dosimeter will register the total accumulated dose received (R or mR) and a rate meter will indicate strength of the radiation field (R or mR per hour). The dosimeter shows exposure after it has occurred and the dose rate meter indicates an immediate danger warning ahead of time. Of the two, the rate meter gives survivors the most versatile and best protection; thus the reason for this project.
This project is a work in progress not completed as yet. I just today became aware of a VERY simple design that may not require a gigohm resistor at all, but its not yet a certain thing. The following was my previous explanation of making one that DOES use such a high value resistor. I will leave the writeup below as it was for the time being, but I wanted to mention this in case we go that route.
The main parts to be assembled are a dedicated multimeter that displays the readings, a JFET transistor, a dual op amp, a 9V battery, and a standardized size and type of metal can to make the ion chamber. There is also a high value (gigohm) resistor needed to bias the JFET. Common multimeters cannot measure this high. A fairly simple but elegant JFET megohmmeter circuit is constructed and then used to measure and create the correct gigohm resistor. Sources for the resistor are still to be determined. Most promising is a salvaged focus control pot from a high voltage (flyback) transformer used in a TV set or computer monitor. Removal of control requires some surgery with a small saw.
Other possibilities are to use a stable resistance material or conductive ink on substrate to "roll your own". Key to this is the megohmmeter circuit to measure/adjust results. Things to remember are that such high resistance components are very touchy indeed. Insulating material and fingerprints can be relatively conductive at these resistance levels. Teflon and some non-static plastics are suitable. Also moisture (humidity in the air) can be absorbed into or condensed onto materials thus changing resistance and meter calibration. Humid climates are more critical in this regard.
Commercial gigohm resistors are encased in a glass tube sealed against moisture at the ends. Moisture can be driven out in an oven at boiling (212F or 100C) before sealing glass tube or container with epoxy. Epoxy itself is somewhat conductive, so it must not bridge both leads. This post is to familiarize readers with basic aspects, and is not all inclusive. More to come, but in the meantime if you want more insight into ion chambers and such, try this link: http://www.techlib.com/science/ion.html#Super%20Sensitive