Although they’ve been on the shelves of many online retailers for quite a few years now, Geiger-Tubes have been conspicuously absent from hobby electronics magazines/websites. It has to be said that building Geiger Counter circuits, especially if we set ourselves a certain level of choice, is far from simple — or rather, was far from simple, until these recently available Geiger –Tubes came along to help us out!
Geiger counter, also known as Geiger–Müller (G-M) counter, detects ionizing radiation such as alpha particles, beta particles and gamma rays using the ionization effect produced in a Geiger-Tube. The Geiger-Tube (GMT or GT) is filled with an inert gas such as helium, neon, or argon at low pressure, to which a high voltage is applied.
The tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization. The ionization is considerably amplified within the tube by the Townsend Discharge effect to produce an easily measured detection pulse, which is fed to the processing and display segments. This large pulse from the tube makes design of the Geiger-Counter relatively easy, as the subsequent process is greatly simplified.
Thanks to the new Geiger-Tubes marketed by many prominent vendors. It’s now possible not just to build a Geiger Counter, but also to give it a level of performance that would make most commercial products green with envy. Intended to gradually supersede earlier Geiger-Tubes, these new devices enable a leap forward in terms of quality and performance. In figure 2, you can see the historic GeigerMüller tube made in 1932 by Hans Geiger for laboratory use (left), and today’s compact Geiger-Müller tube type SBM-20 available for makers and hobbyists (right).
SBM-20 is a low-priced, quintessential Russian tube, more sensitive to beta and gamma than most. Other similar/near-similar Geiger-Müller tubes in this class are SI-3BG, SI-1G, and LND-712. The type LND-712 is infact an End Window-Alpha-Beta-Gamma Detector from LND, INC.
A Geiger counter circuit offers an excellent option for obtaining concrete values for the radiation load. In principle, it uses the ionizing properties of nuclear radiation, from which it generates a measurement of the radiation load. As described above, Geiger counter detects nuclear radiation by means of a metal counter tube, which is at the same time a cathode, and inside the tube is a wire that functions as the anode. The counter tube is filled with argon or xenon, noble gases that cannot form anions.
When ionizing radiation strikes a noble gas atom, an electron is knocked out of its orbit and immediately moves toward the anode. In the process, it is able to knock additional electrons out from the noble gas atoms, and the noble gas atom ions ultimately move toward the cathode, where they take up electrons. As a result of this impact ionization, which essentially serves as amplification, a little, measurable current arises in the Geiger counter that is proportional to the strength of the radiation. The larger the voltage difference between the cathode and the anode, the greater the energy in the electron, and the number of impact ionizations increases.
Now we know that a GM tube provides pulses that correspond to “events” in the tube that occur from it’s interaction with ionizing radiation gamma rays, beta, and alpha particles (for some tubes). The events are counted by the Geiger Counter over a time period and eventually result in counts per minute (CPM). If we know what the CPM are for “normal background” with our tube, the difference measured will give us a good sense of the radiation intensity. However, different models of GM tubes vary greatly in their detection sensitivity. Note that, correct amount of the high-voltage (HV) supply, and right value of the anode resistor is crucial for all GM tubes.
The suggested values are usually given in the datasheet for the tube. For example, 4.7M resistor seems good for SBM-20 and 10M resistor for LND-712 as anode resistor. Similarly, recommended operating voltage (HV) for LND-712 is 500 volts; in 450-650 range, and about 400 volts for SBM-20.
Usually the datasheet indicates maximum anode current of the tube in uA. This would make the load resistor (anode resistor) calculation much easier. For example, the specified anode current of SBM-20 is 18.2 uA at 400V. Although this has been translated to a resistance value of 22M, the original circuit diagram in the datasheet shows a 4.7M resistor which is more reasonable according to the manufacturer.
Yes, it’s not too hard to build a Geiger-Counter. A Geiger-Counter obviously consists of two distinct circuit sections: The high-voltage (HV) generator, and the Geiger–Müller (GM) tube interface. First of all, select the right GM tube, and then gear up appropriate circuits using electronic components.
Schematics around GM tubes are mainly related to the precision high voltage generation. Beside the generation of the high voltage, a computing and display circuit for processing tube-clicks and displaying the final data is needed. Fortunately, great examples are widely spread in the internet. Figure 7 shows the basic system diagram of a full-fledged Geiger-Counter. Some kit builders have advertised about layman’s do-it-yourself Geiger-Counter kits to expedite the process from concept to completion. Construction is extremely simple, easily within the capacities of any amateur who know how to hold a soldering iron (by the right end)!
Do-it-yourself Geiger Counter kits are usually powered by a dc supply in the 3v-5v range, catered by common dry cells, Ni-Cd, Ni-MH, or Li-Ion cells. The high-voltage generator converts this low voltage dc input to a high-voltage dc output using traditional voltage booster (dc-dc boost converter) circuits. This HV output is fed to the anode of the GM tube through the anode resistor. Click signals generated by this high-voltage section (during a detection process) is fed to the rest of the system through a buffer circuitry for further processing by the associated microcontroller.
Finally, a readout is available through the display unit. Figure 8 is an open-hardware design of a diy Geiger Counter kit introduced by MightyOhm (http://mightyohm.com). Note that, only front-end of the entire circuit is reproduced here. Remaining portion holds an Attiny 2313 microcontroller-based beeper unit. Pulse output from the pulse detector section is routed to one Interrupt input (INT0) of this AVR microcontroller.
Regarding conversion and calibration, this is a little messy. The main point is that the datasheets for most GM tubes define the CPM (or CPS) that are equivalent to some dose unit (usually mR/hr). Since GM tubes also have varying sensitivity to different isotopes, there may be several values listed, so have a keen look at a part of the datasheet to learn how to process the raw data (cpm/cps). When it comes to calibration, there are variations within the same model of GM tube, based on factors like the age of the tube, and the bias voltage used. Moreover, to truly calibrate your counter you must use a standardized check source of the isotope you want to calibrate to and use procedures which include such things as distance from the source, tube geometry, etc. to fine tune your ratio of CPM to the dose unit you want to use.
If you are an electronics hobbyist never used a Geiger Counter before, or have already been disappointed by certain commercial models, I have not the slightest hesitation in recommending a diy project which makes no compromises in terms of performance. Given the cost of the Geiger-Tube, however, it’s wise to carry out a few simple checks before powering up your finished project!
Reference Courtesy: wikipedia.com, mightyohm.com, gstube.com, lndinc.com, geigercounter.org
Disclaimer: The included circuit (fig. 8) is one that I have not tested/used myself. It’s only for information, and I’m not suggesting or recommending any particular circuit/component/diy kit/source!