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Meter - A visual indicator of the quantity of radiation detected, as opposed to a radiation detector (typically a cheap model) that lacks a numerical visual display of any type in favor of a general alert level.  The analogy would be an automobile dashboard indicator, for example, an oil pressure gauge versus an "idiot light".  We prefer Geiger counters with meters.

Meter Type - Either an Analog or Digital visual display.  The Analog meter has a needle which moves up and down a numbered scale, allowing you to quantify the radiation being detected by lining up the needle with the number on the scale to which it points, like the Monitor 4.  A Digital meter displays the numerical quantity of radiation on an LCD (Liquid Crystal Display), like the Radalert 100.  The decision between these two types of meters can come down to personal preference, although a Digital meter probably has the edge in measuring lower levels of radiation - it's easier to quantify that level, versus on an Analog meter where the needle tends to hover near the left end of the scale.  On the other hand, if a Digital meter is configured to "average" the radiation count frequently, then the numerical readouts can appear to "jump around".

Range - The maximum operating range in mR/hr (milli-Roentgens per hour).  During the 1950's and 1960's, many Geiger counters were built for very high operating ranges to detect the extreme levels of radioactivity likely produced from a nuclear exchange between the superpowers.  However, such meters were generally poor at detecting low levels of radioactivity.  So, with the end of the Cold War, the trend in Geiger counter design is toward more sensitive, lower ranging models.  Nevertheless, to give you some perspective, 50 mR/hr, a typical maximum operating range on many of our Geiger counter models, would be a radioactivity level detected from a 1" or 2" piece of high-grade uranium ore.

Switchable - Applied to the operating range of an analog metered Geiger counter such as the Monitor 4 or the Detector, whereby the user can switch between, for instance, 1X, 10X, or 100X the scale value to expand the versatility of the instrument, just like with a traditional analog voltmeter.  The switchable operating range is an especially nice feature if you are dealing with low levels of radioactivity - you can set the Geiger counter to the most sensitive operating range so the meter is easier to read, i.e., the needle will "jump" that much farther for the same level of radioactivity.

Anti-Saturation - Electronic circuitry built into a Geiger counter to prevent failure in the event of a reading that "maxes out" the operating range of the instrument.  Some instruments are rated with anti-saturation circuitry to 100 times the operating range, which is derived through an arithmetical calculation.

CPM - Counts Per Minute, i.e., radiation counts per minute, a unit of measurement for a Geiger counter, on many models corresponding directly to the audible beeps or clicks per minute.  CPM is the standard unit of measurement for alpha and beta radiation and is also commonly used to express background radiation in numerical terms.

mR/hr - milli-Roentgens per hour, or 1/1000 of a Roentgen per hour, a standard unit of measurement for radioactivity, popular in the United States and Israel.

µR/hr - micro-Roentgens per hour, or one millionth of a Roentgen per hour, as included with the PRM-8000.  When using these units, the displayed numbers appear 1,000 times larger than in mR/hr, so at background radiation levels, you end up working with clean looking integers instead of readings out to 3 decimal places.

µSv/hr - micro-Sieverts per hour, a standard unit of measurement for radioactivity, popular in Canada and overseas.  If you want a Geiger counter that reads out in µSv/hr, look for this feature, standard on some models, and optional on others.

On/Off - The method used to turn the Geiger counter On and Off, including the "Push and Hold down" button type as in the DX-2, versus the Sliding switch type in other models, such as the Inspector, or the Rotary type as in the Prospector.  With the push button, it must continuously be held down while monitoring, the one advantage being that the instrument cannot be inadvertently left on to drain the battery.  On the other hand, for monitoring over extended periods of time, the sliding or rotary switch is preferred because once it is turned on, it remains on and the instrument can be set down, freeing the operator's hands, continuing to monitor until the user slides the switch or turns the rotary dial to the Off position.

Audio - An audible indicator of radiation detection, typically in the form of beeping or clicking corrersponding to one audible note for each radiation count detected, as in the Monitor 4 (play the sound file).  While all of our models produce an audible output, not all Geiger counters do so.  An audio indicator of radiation is strongly preferred because it frees the user from having to continually stare at the visual display of the meter.

Speaker - An internal loudspeaker for hearing the audio output of a Geiger counter.  All of our models have this, but some of the old civil defense units lacked a speaker, requiring the user to continually wear headphones to hear the audio.

Audio Off - A switch giving the capability to turn off the Audio of the Geiger counter, while leaving the instrument on to continue monitoring radiation, discernable through just the visual display, as in the Inspector or Prospector.  This is a helpful feature for discrete monitoring in two instances.  Security personnel can monitor suspects without "tipping them off".  Secondly, the layperson can turn off the audio to avoid disturbing nearby people such as family members.

LED Count - An LED (Light Emitting Diode) that flashes, typically red, with each radiation count, as in the Radalert 100.  This is yet another indicator of radiation detection on many Geiger counter models, not necessary when an instrument already has a visual display and/or audible indicator, but a nice plus.  A flashing LED can be readily seen from a distance, whereby the visual meter display may not be.

Total - A button setting on a Digital Geiger counter that accumulates radiation counts over time, as in the Inspector, versus the standard button setting that displays changing radiation levels at a moment in time.  Thus, the Total button setting is somewhat like a dosimeter.  The Total function is very helpful in establishing your background radiation level.  With that setting, the Geiger counter will count the radiation particle emissions for you.  For example, you can set the unit to Total, and after say 10 minutes, divide the Total Count value by 10 to arrive at your background radiation in CPM.

Alert - An audible indicator that sounds off when the radiation detected exceeds a pre-set level, as in the DX-1 or Radalert 100.  This is a very useful feature.  You can leave the unit on continuously, otherwise ignoring it unless the Alert sounds off, freeing you from monitoring either its visual display or its standard clicking.

Adjustable - The ability to adjust the Alert level, as in the Radalert 100.  Scientists disagree on what constitutes a dangerous level of radiation, so an adjustable alert allows the user to make that determination for his or herself.  The optimal adjustment of the Alert level at the sensitive end is one that is low enough to sound off when the radiation detected exceeds a typical background level, accounting for the randomness of background radiation for any one minute, but not so low as to give false alerts.

Alpha - A weak form of ionizing radiation detectable on some models of Geiger counters, typically those that incorporate a thin mica "window" at one end of the Geiger -Mueller tube.  Alpha radiation consists of positively charged particles emitted from the nucleus of an atom in the process of decay.  These particles are also very dense which, with their strong positive charge, precludes them from penetrating more than an inch of air or a sheet of paper.  Because of this, Alpha particles are not a serious health hazard, except when they are emitted from within the body as a result of ingestion, for instance, when their high energy poses an extreme hazard to sensitive living tissue.

Beta - A relatively weak form of ionizing radiation detectable on many Geiger counters, generally dependent on the thickness of the Geiger-Mueller tube wall or the existence of a "window" at the end of the tube.  Beta radiation consists of negatively charged particles emitted from an atom in the process of decay.  These particles are relatively light and can penetrate somewhat better than an Alpha particle, though still only through a few millimeters of aluminum at best.  If ingested, Beta radiation can be hazardous to living tissue.

Gamma -  A very powerful and potentially very dangerous type of ionizing radiation detectable on virtually all Geiger counters.  Gamma radiation represents one extreme of the electromagnetic spectrum, particularly that radiation with the highest frequency and shortest wavelength.  (That same spectrum also includes the more familiar X-rays, ultraviolet light, visible light, infrared rays, microwaves, and radio waves, listed in order of decreasing frequency and increasing wavelength from Gamma rays.)  Gamma rays can pass through virtually anything and are effectively shielded or absorbed only by materials of high atomic weight such as lead.  Gamma rays are produced naturally by the sun and other bodies in outer space, their transmission to earth being known as "cosmic radiation".  Certain minerals that make up part of the earth containing the radioactive elements Uranium and/or Thorium also emit Gamma rays.  This cosmic radiation, along with these radioactive earth minerals, combine to produce the "background count" of a Geiger counter.  That is, even when a Geiger counter (if sensitive enough) is removed from a specific radioactive object, the meter will still register a background level or count of radioactivity.  This might typically be in the range of 15 to 60 counts per minute but will vary depending upon your location on the earth, your altitude, and the efficiency of the Geiger counter.  The background count should always be factored in or "subtracted" from the overall reading derived from a specific radioactive source. 

X-Rays - Very similar to Gamma rays, but with somewhat lower frequency and longer wavelength, and detectable on virtually all Geiger counters.  They are produced from man-made sources such as X-ray tubes, arcs, and lamps.  Like Gamma radiation, X-rays are very powerful and potentially very dangerous.  They can pass through virtually anything, effectively shielded or absorbed only by materials of high atomic weight such as lead.  Because of their penetrating ability, X-rays are used to see inside the human body, destroy cancer cells in radiation therapy, or analyze the internal structure of rocks and minerals, for instance.

Tube Style - The general makeup and shape of the Geiger-Mueller (GM) tube, typically either of a "Standard" cylindrical shape as in the Monitor 4 or Prospector, or "Pancake" shape as in the Inspector and PRM-9000.  The cylinder is more typical, and many cylindrical shaped GM tubes incorporate an open mica "window" at one end, enabling the detection of alpha and low level beta radiation.  On the other hand, Pancake style GM tubes are designed in their distinctive shape to accommodate that thin mica window across a very broad surface area, allowing detection of very low levels of alpha and beta radiation, especially.  Thus, pancake style GM tubes are preferred for highly sensitive applications, such as checking food for radioactive contamination. 

Window Type - The material from which the window of the GM tube is constructed, typically either metal like brass, as in the DX-1, or mica, as in the Monitor 4.  Mica windows can be made very thin, and because of mica's low density, alpha and beta radiation can pass through it relatively easily for detectability.  Thus, GM tube windows of mica are more sensitive than those with metal windows.

Window Size - The diameter of the mica window.  Larger diameter mica windows, such as those found on pancake style GM tubes like that of the Inspector or Detector, offer a broad surface area for detection, and are therefore more sensitive or efficient.  Given the same radioactive source, a large, windowed Geiger counter will produce a higher reading than a standard size windowed counter.  There is one tradeoff in window size of which to be aware.  While larger windows of mica are far more sensitive, they are more prone to shattering in the face of a change in air pressure as could be found at very high altitude, such as in an un-pressurized cargo hold of a high-flying jetliner, or on a high mountain.

Sensitivity - This is an overall, somewhat subjective rating of each Geiger counter's sensitivity to low levels of radiation.  What does it really mean to say that one Geiger counter is more sensitive than another?  Here are some examples:  A model of standard sensitivity will not detect any radiation from a small sample of weakly radioactive uranium ore, while a Geiger counter of high sensitivity would; or a stronger uranium sample would produce a reading of 8 mR/hr on a standard Geiger counter, versus a reading of 17 mR/hr on a highly sensitive model; or a standard model would have to be positioned no more than 3" away from a particular uranium sample to get a reading, whereby a highly sensitive Geiger counter would begin to detect radiation at a distance of 9", and so on.

Timer - A built-in timer that counts the radiation particles over user adjustable time periods, as in the Inspector.  A timer is an excellent feature when taking a background count to establish a baseline, then followed by a contamination check over the same period.  Either a Timer or a Total function (see above) are a very desirable feature on a Geiger counter - they do the counting for you, an advantage when dealing with lower levels of radiation, especially.

Data Logging - A feature whereby the Geiger counter logs the data collected to internal memory, typically at a remote location, for later upload to a computer for analysis, such as in the PRM-8000 or Inspector USB.  This is a nice feature to have because otherwise one would need to also carry some sort of software loaded computer to log the same data for permanent storage.

Data Port - An electronic and physical port enabling the Geiger counter output to a data logger or computer via a data cable, as in the Radalert 100 or Inspector.  A data port would be a necessity if the Geiger counter lacked any audible output or visual display.  Otherwise, it is an added feature that accommodates items described under Software below.

Software - Available for Digital Geiger counters with data ports, as in the GeigerGraph software usable with the Radalert 100 and the Inspector.   The software graphs the radiation counts over time, logs the data for a permanent record, and provides an alert for radiation above a user set level.  The software comes with a data cable that hooks the Geiger counter to the computer.

Audio Port - An electronic and physical port that sends the Geiger counter audio through an optional earphone (or amplifier or tape recorder), as in the Radalert 100.  This enables discrete detection, bypassing the instrument's internal speaker so that persons nearby do not hear the audible beeps.  Also, headphones can be used to filter out background sound in a high noise environment, as in the Prospector.

Power - The electrical source off which the Geiger counter operates.  For portable models, that means battery power, typically long lasting, easily replaceable 9-Volt batteries like those readily found in hardware stores.  Certain older Geiger counters from the 1950's used lead acid batteries that are not easily replaceable.  Some larger probe style units of today, such as the Prospector and Detector, use separate battery power for the internal speaker, so that if the instrument is inadvertently left on, only those batteries will run down, leaving independent battery power to still run the instrument as a whole, along with its visual display.

Battery Check - An indicator of battery strength, either an LED that loses brightness with battery wear, as in the DX1, or a battery check position at the right end of the analog meter, as in the Monitor 4, or a low battery indicator on the LCD display of a digital unit like the Radalert.

A/C Adapter - While most portable Geiger counters operate on batteries, some units such as the PRM-9000 incorporate a power supply port for plugging in an optional A/C Adapter.  This can be preferable especially when monitoring continuously, to avoid battery changes.

General/Hobby - Suitable for general use in detecting radioactivity, as a hobby, or in educational demonstrations, as in the DX-1 or Monitor 4 Kit.  Meets the basic requirement of detecting radioactivity, but generally lacking in specialized features or in high sensitivity.

1st Response - As applied to emergency personnel, for making an initial determination of the presence of radiation, to be further investigated by HazMat personnel with more specialized instruments.  First response instruments are typically pocket or handheld models for easy portability, often wearable on the body around a belt, and of sufficient sensitivity for initial determinations of potentially dangerous levels of radiation.  Ideally, these units incorporate an adjustable audible Alert, as in the Radalert 100.

HazMat - Suitable for detecting and quantifying dangerous, including low levels of radiation.  These instruments typically incorporate highly sensitive pancake GM tubes, ideally configured in external probes for safer detection at a distance, with less chance of contamination on hands and body, as in the Inspector EXP or Detector.

Check Food - If one is going to use a Geiger counter for checking food for contamination, certain models are more effective because of their highly sensitive and efficient pancake style GM tubes, with thin mica end window allowing the detection of alpha as well, as in the Inspector or PRM-9000, along with a Timed Count or Timed Measurement feature.  Follow this link on checking Food Contamination for more details.

X-Ray Leaks - Recommended for checking X-Ray machines for leaks, because of a specialized design.  For example, the Monitor 4EC has an energy compensated GM tube that offers a more accurate linear reading above 40 keV, the energy level for Gamma and X-rays.

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