Choosing a radiation monitor


When you think about it buying a radiation monitor is a rather odd thing to do. For most of us radioactivity or ionising radiation is something we rarely think about, except when it hits the headlines, following a serious incident, leak or explosion, and that’s when the trouble begins.


The media revels in a good scare story: all of the old clichés are rolled out, a lot of people are unnecessarily worried and think they need a Geiger Counter for their personal safety or to prevent them from growing two heads (the chances of developing super powers after radiation exposure have yet to be proved scientifically...).


The fact is, the only people who actually need radiation monitoring equipment for health and safety reasons are those involved in the handling of radioactive materials, or can reasonably expect to encounter potenially hazardous radioactive sources in the course of their work. The devices they use are usually highly specialised and supported by trained experts and technicians who know how to interpret the readings they provide. At the risk of losing sales we have to say that this is not what we are about here at anythingradioactive..


We sell radiation detection and monitoring equipment for everyday use and - dare we say it - for fun, leisure and education; they are the sort of thing that's of interest to geologists and rock hounds, antique collectors, students, 'doomdsday' preppers and of course to satisfy the curiosity of science nuts and gadget freaks, like us. The point is none of the instruments that we are involved with are intended for safety-critical or health and safety related applications, So please don't waste your money if you think a Geiger Counter is going to protect you against radioactive fallout, poisoning or atomic mutation. Of course, that's not to say if you have one and encounter a an unexpectedly lively source of radiation it is prudent to move smartly away from it and report it to the appropriate authorities, such as the police, the UK Health protection Agency, or your local health, safety or environmental agency.


By the way, it's important to distinguish between ionising and non-ionising radiation. The latter are electromagnetic fields coming from from the likes of mobile phones and radio transmitters and none of our instruments are designed to respond to it. The jury is still out on the health effects of electromagnetic (EM) radiation and we're not about to get into that debate. You should also be wary of electromagnetic detectors being wrongly described as Geiger Counters.


Let’s take a look at the different types of radiation monitor that we sell, starting with the classic Geiger Counter that everyone has heard of and seen in the movies . A lot of devices are mistakenly or misleadingly referred to as Geiger Counters so let’s sort that one out straight away. To qualify as a pukka Geiger Counter an instrument should employ a device called a Geiger Müller tube, which detects radioactive particles or waves and, by some means or other, is able to count or log the ‘clicks’ generated by the tube when it detects radioactivity.


The counter part is important as this allows measurements to be directly observed or recorded so they can be compared with readings taken over time or in other locations. The displays on Geiger Counters vary enormously.  For example, our Mini Monitor 900 (left) and  CDV-700 (top of the page) show relative levels of radioactivity in clicks or counts per second or per minute (cp/s or cp/m) on an analogue meter. This is really easy to understand; the higher the reading the greater the level of radioactivity and if properly calibrated, the readings can be very accurate indeed. In contrast Geiger counters like the popular JD-3001 (right) and popular models made by GQ Electronics (GMC series), Soeks, Radex, Gamma Master and so on have digital readouts, opften with cps/cpm readouts or so-called 'dose rates' typically shown as microsieverts per hour (uSv/h). This is a more sophisticated way of measuring the harmful effects of - compared with counting clicks - as it normally only relates to Gamma radiation. This is generally reckoned to be the most dangerous type due to its penetrating power (it can pass through most materials) and the damage it can do to living cells. This type of measurement provides a more accurate representation of the received level of activity or dosage and its potential health effects, but the general idea is the same, the higher the reading the more radioactivity there is. The bottom line, though, is that most instruments are sensitive enough to register the small amounts of natural background radioactivity that we are exposed to all of the time - since the beginning of time in fact -- and not considered to be harmful to health. This makes it is asy to establish a benchmark reading, so higher or elevated readings will be immediately obvious.


The next class of instrument is the dosimeter, and the models that we sell or have sold, like GeigerActive Personal Plus, K8 Nuke (right) and GeigerActiv K, also employ Geiger Muller tubes for detection. However, these do not count or have displays as such and respond to radioactivity by clicking, bleeping and/or flashing an LED. By observing the clicks and flashes it is possible to gauge the relative level of activity. This makes it useful for identifying localised sources of radioactivity, prospecting and so on. Some models, like the K8 and DSRB-01 have a built in ‘alarm’ facility that sounds an alert or illuminates an LED when the level exceeds a preset threshold, so it can act as a very basic warning device. Additionally, our GeigerActiv Pro has a facility to connect to a PC, and with the software supplied it is possible to display and keep a record of the readings, effectively turning it into a simple Geiger Counter.  


The dosimeter family also includes things like film badges and ‘Pen’ type dosimeters. These are passive devices that record radioactive dosage over time. They cannot be used for detecting or as warning devices as the measurements they take are analysed at preset intervals, hours or days after exposure.


The last category of instrument is the CDV-715 Survey Meter; unfortunately now out of stock. Like the CDV-700 they were produced in the US in the 1960s and 70s, at the height of the Cold War.


These are not Geiger Counters – they have no Geiger tube – instead they use a device called an Ion Chamber for detecting very high levels of radioactivity that would be present in the aftermath of a nuclear war. In short they are totally useless for detecting the sort of radiation levels you are ever likely to encounter in the normal course of events. In fact, if you ever see the needle move in anger you have probably just survived a nuclear attack and you really shouldn’t be booking any holidays…


A lot of CDV-715s turn up on ebay, sometimes at absurdly inflated prices,  (we have seen them priced at over £1000...) and misleadingly labelled as Geiger Counters. Do not be misled! As a matter of interest they can be modified to make them a little more sensitive and we will happily direct you to the instructions but it’s a complicated job and they are still hopelessly insensitive for doing anything interesting or useful, other than as doorstops! Nevertheless, they are superbly well-made and a good source of vintage electronic components. They're interesting examples of cold war memorabilia and conversation pieces...


Last but not least is another class of radiation detecting or measuring instrument that technically are not Geiger Counters as they do not use Geiger Müller detection tubes, yet often have readouts scaled in counts per second or minute, as well as microsieverts. Instruments in this category include Mini Monitors with Types D, E, 42 and 44 probes. A number of devices and technologies are in use but they mostly into rely on a process called scintillation. The idea is certain organic (e.g. Sodium Iodide) and plastics (Polyvinyltoluene) in crystalline form emit brief flashes of light when struck by Gamma radiation (and X-Rays). The flashes of light are extremely weak and generally not visible to the naked eye, so the crystals are mounted onto devices like photomultipliers and solid state light detectors that amplify to flashes so they can be counted or measured. In the case of Sodium Iodide detection crystals the brightness and duration of the flashes are an indication of the type of radiation and can be used to identify the radioactive isotopes it is coming from. These types of probe can also be many times more sensitive to (gamma) radiation that other detectors and some models, like our Mini Monitor 900 with a type 44A probe can pick up the tiny amounts of gamma radiation coming from such everyday things as household smoke detectors (ionisation type).  


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