Ionizing Radiation Geiger Counter - Neutron Radiation Detector - Principles, Readout, Limitations, Types, Where And What To Purchase - Why hand held radiation detectors cannot be used to detect radiation in foods or drinks
☢ Radioactivity ☢ and How to Use a Geiger Counter -- Part 1
VIDEO: https://youtu.be/ny4Afpl3EZ8 9 min
WHAT IS A RADIATION DETECTOR?
According to Wikipedia; "a Geiger–Müller counter, also called a Geiger counter, is a type of particle detector that measures ionizing radiation. It detects the emission of nuclear radiation — alpha particles, beta particles, or gamma rays — by the ionization produced in a low-pressure gas in a Geiger–Müller tube, which gives its name to the instrument. In wide and prominent use as a hand-held radiation survey instrument, it is perhaps society's best-known radiation instrument.
The original operating principle was discovered in 1908 in early radiation research. Since the subsequent development of the Geiger-Müller tube in 1928 the Geiger Müller counter has been a popular instrument for use in radiation dosimetry, health physics, experimental physics, the nuclear industry, geological exploration and other fields, due to its robust sensing element and relatively low cost. However there are limitations in measuring high radiation rates and in measuring the energy of incident radiation.
PRINCIPAL OF OPERATION
Schematic of a Geiger counter using an "end window" tube for low penetration radiation.
Geiger counter instruments consist of two main elements; the Geiger-Muller tube and the processing and display electronics. The radiation sensing element is an inert gas-filled Geiger-Muller tube (usually containing helium, neon or argon with halogens added) which briefly conducts electrical charge when a particle or photon of radiation makes the gas conductive by ionization.
The tube has the property of being able to amplify each ionization event by means of the avalanche effect and produces an easily measured current pulse which is passed to the processing electronics. The readout can be analogue or digital.
READOUT
There are fundamentally two types of radiation readout; counts or radiation dose. The counts display is the simplest and is the number of ionizing events displayed either as a count rate, commonly "counts per second", or as a total over a set time period (an integrated total).
The counts readout is normally used when alpha or beta particles are being detected. More complex to achieve is a display of radiation dose rate, displayed in a unit such as the sievert. This type of display is normally used for measuring gamma or X-ray dose rates and requires the use of an energy compensated G-M tube, so that the absorbed dose displayed relates to the count events.[1] The electronics will apply known factors to make this conversion.
There is usually an option to produce audible clicks representing the number of ionization events. This is the distinctive sound normally associated with hand held or portable Geiger counters.
The purpose of this is to allow the user to concentrate on manipulation of the instrument whilst retaining auditory feedback on the radiation rate. The electronics also generates the relatively high voltage, typically 400–600 volts, that has to be applied to the Geiger-Muller tube to enable its operation.
LIMITATIONS
There are two main limitations of the Geiger counter. Because the output pulse from a Geiger-Muller tube is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation types.
A further limitation is the inability to measure high radiation rates due to the "dead time" of the tube. This is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count. Typically the dead time will result in an upper count rate limit of 104 to 105 counts per second depending on the characteristic of the tube being used. Therefore ion chamber instruments are used for high radiation rates.
TYPES AND APPLICATIONS
G_M counter with pancake type probe
Laboratory use of a geiger counter with end window probe to measure beta radiation from a radioactive source
A Geiger counter and metal detector combined for security use.
The application and use of a Geiger counter is dictated entirely by the design of the tube.
PARTICLE DETECTION
The first historical uses of the Geiger principle were for the detection of alpha and beta particles, and the instrument is still used for this purpose today. For alpha particles and low energy beta particles the "end window" type of GM tube is used as these particles have a limited range even in free air and are easily stopped by a solid material.
The end window is designed to be thin enough to allow these particles through with minimal attenuation, and normally has a density of about 1.5 - 2.0 mg/cm2. For efficient detection of alpha particles the GM tube window should ideally be within 10mm of the radiation source due to the particle attenuation in free air. However, the G-M tube produces a pulse output which is the same magnitude for all detected radiation, so a Geiger counter with an end window tube cannot distinguish between alpha and beta particles. The "pancake" Geiger-Muller detector is a variant of the end window probe designed with a larger detection area, and is normally used as an alpha/beta contamination monitor.
High energy beta particles can also be detected by a thin walled "windowless" tube; which has no dedicated end window. Although the tube walls have a greater stopping power than an end window, they still allow these more energetic particles to reach the fill gas.
Geiger detectors are still used as a general purpose alpha/beta portable Radioactive contamination measurement and detection instrument, owing to their relatively low cost, robustness and their relatively high detection efficiency; particularly with high energy beta particles.
However for discrimination between alpha and beta particles or provision of particle energy information, scintillation counters or proportional counters must be used. These instrument types can also have much larger detector areas, which means that checking of surfaces for contamination is much quicker.
GAMMA AND X RAY DETECTION
Geiger counters can be used to detect gamma radiation, and for this the windowless tube is used. Efficiency is only about 1%,[2] due to low interaction of gamma with the tube, and the article on the Geiger-Muller tube carries an account of the technique used to detect photon radiation.
For high energy gamma, this relies on interaction of the photon radiation with the tube wall material, usually 1-2mm of chrome steel to produce electrons which can enter and ionize the fill gas. This is necessary as the gas density in the tube is usually low, and most high energy gamma photons pass through undetected.
For lower energy photons a different technique is used. This is the direct interaction with gas in a long thin-walled tube. This design gives an additional gas volume, and thereby increased chance of particle interaction, but still allows low energy photons to enter the gas through the thin wall. Energy compensation of the tube is normally applied to increase the accuracy over a range of photon energies.
NEUTRON DETECTION
A different kind Geiger tube is used to measure neutrons, where the gas used is boron trifluoride or Helium 3 and a plastic moderator is used to slow the neutrons. This creates an alpha particle inside the detector and thus neutrons can be counted.
Gamma measurement—personnel protection and process control
The term "Geiger counter" is commonly used to mean a hand-held survey type meter, however the Geiger principle is in wide use in installed "area gamma" alarms for personnel protection, and in process measurement and interlock applications. A Geiger tube is still the sensing device, but the processing electronics will have a higher degree of sophistication and reliability than that used in a hand held survey meter.
PHYSICAL DESIGN
For hand-held units there are two fundamental physical configurations: the "integral" unit with both detector and electronics in the same unit, and the "two-piece" design which has a separate detector probe and an electronics module connected by a short cable.
The integral unit allows single-handed operation, so the operator can use the other hand for personal security in challenging monitoring positions, but the two piece design allows easier manipulation of the detector, and is commonly used for alpha and beta surface contamination monitoring where careful manipulation of the probe is required or the weight of the electronics module would make operation unwieldy. A number of different sized detectors are available to suit particular situations, such as placing the probe in small apertures or confined spaces.
Gamma and X-Ray detectors generally use an "integral" design so the Geiger–Müller tube is conveniently within the electronics enclosure. This can easily be achieved because the casing usually has little attentuation, and is employed in ambient gamma measurements where distance from the source of radiation is not a significant factor. However, to facilitate more localised measurements such as "surface dose", the position of the tube in the enclosure is sometimes indicated by targets on the enclosure so an accurate measurement can be made with the tube at the correct orientation and a known distance from the surface.
There is a particular type of gamma instrument known as a "hot spot" detector which has the detector tube on the end of a long pole or flexible conduit. These are used to measure high radiation gamma locations whilst protecting the operator by means of distance shielding.
Particle detection of alpha and beta can used in both integral and two-piece designs. A pancake probe (for alpha/beta) is generally used to increase the area of detection in two-piece instruments whilst being relatively light weight. In integral instruments using an end window tube there is a window in the body of the casing to prevent shielding of particles. There are also hybrid instruments which have a separate probe for particle detection and a gamma detection tube within the electronics module. The detectors are switchable by the operator, depending the radiation type that is being measured.
Guidance on application use
In the United Kingdom the HSE has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned [1]. This covers all radiation instrument technologies, and is a useful comparative guide to the use of GM detectors.
The GM pancake type round tube design used determines the types of radiation detected. This is a thin mica round pancake window type for detecting alpha, beta and gamma radiation.
GM detector with thin mica window, and round pancake detector plugged into a microcontroller data-logger with Class 1 Bluetooth adapter sending radiation levels to a PC using radio signals.
GM tube type detectors (no picture) are much less sensitive, and without a window, so they will not detect alpha radiation.
GM tube type detectors (no picture) are much less sensitive, and without a window, so they will not detect alpha radiation.
CAN A PERSON TEST FOR RADIATION IN FOOD WITH A TUBE TYPE GEIGER COUNTER?
Why cheap radiation detectors cannot be used to detect radiation in food
VIDEO: https://youtu.be/BIKro3s6eU4 11 min
This video featuring W Milberry does a great job of explaining why the cheaper tube type Geiger Counters cannot be used for detecting food radiation, at all. M also goes into why a person could potentially use a pancake style detector as a very crude food radiation testing device, to warn against high levels of radiation.
Realizing that money is a limitation for many people, one idea to set up a very inexpensive testing center in your home is to purchase a very sensitive pancake style detector with a handheld wand on the end of a cable.
Now build or buy either a water or (painted) lead box and put the pancake detector inside of it, with the test sample, for a timed test. Compare this timed test of a food sample inside the lead box or water shielded container, to a timed test of background radiation outside of this shielded container. The result would be a crude food testing lab, built for less money than the more expensive and much more sensitive detectors that ARE specifically designed to test food, such as the ones listed in this article. (Remember that lead is very toxic, so protect it by adding a layer of copper, tin, or aluminum both outside and inside.)
Bottom line, Geiger Counters are not designed to test radiation in food, but it is possible to do a very crude test with a pancake style detector, which will not provide any information about what kind of radioactive substance is actually being detected.. The benefit of the more expensive food radiation testing machines is that they actually tell you what kind of radiation is being detected, via a chart, graph, computer screen or printout.
antiprotons WOW! All I can say is, Perfect! Your analysis of food testing with a Geiger counter is dead on and well stated. Interestingly, when it comes to gamma energy, most Geiger counters, barely 1-5% efficient.
antiprotons WOW! All I can say is, Perfect! Your analysis of food testing with a Geiger counter is dead on and well stated. Interestingly, when it comes to gamma energy, most Geiger counters, barely 1-5% efficient.
HISTORY
In 1908 Hans Geiger, under the supervision of Ernest Rutherford at the Victoria University of Manchester (now the University of Manchester), developed an experimental technique for detecting alpha particles that would later be used in the Geiger-muller tube.[3] This counter was only capable of detecting alpha particles and was part of a larger experimental apparatus. The fundamental ionization mechanism used was discovered by John Sealy Townsend by his work between 1897 and 1901, and is known as the Townsend discharge, which is the ionization of molecules by ion impact.
It was not until 1928 that Geiger and Walther Müller (a PhD student of Geiger) developed the sealed Geiger-Müller tube which could detect more types of ionizing radiation[4][5] and it became a practical radiation sensor. Once this was available, Geiger counter instruments could be produced relatively cheaply because the large output pulse required little electronic processing to give a count rate reading, which was a distinct advantage in the thermionic valve era due to valve cost and power consumption.
Modern versions of the Geiger counter use the halogen tube invented in 1947 by Sidney H. Liebson.[6] It superseded the earlier Geiger tube because of its much longer life and lower operating voltage, typically 400-600 volts.[7]"
Source: http://en.wikipedia.org/wiki/Geiger_counter
WHY DIFFERENT KINDS OF RADIATION DETECTORS CAN SHOW WILDLY DIFFERENT RADIATION READINGS
Vital1August 5, 2015 "Various live radiation monitoring station networks operate Geiger Counters or other detection equipment, that can have different sensitivities. Meaning, two different Geiger counter models using different Geiger counter tubes placed at the same location would show different counts per minute (CPM).
One could show 12 cpm, while an another 120 cpm. If they used a scintillator for detection purposes, which is even a more sensitive Gamma radiation detector, it could be in the thousands of counts per minute. Monitoring station counts per minute measurement all depends on the type of detection equipment the monitoring station is using.
uSv/hr or uRem/hr measurements are a way of defining human exposure dose rates independent of Counts Per Minute (CPM) equipment sensitivities.
You can only compare detections of a monitoring station against it's average background for that location. If it usually averaged around 12 cpm, 0.10 uSv/hr or 10uRem/hr at that particular location, this would be considered the average background level at that location. It may not be unusual to get a very brief detection three times this. If it stayed 3x or more background for any length of time, then something more serious is possibly happening.
GEIGER COUNTERS ARE ONLY CALIBRATED FOR ONE SPECIFIC ISOTOPE
In most Geiger counters they use a simple CPM formula to convert the CPM into a uSv/hr or uRem/hr dose amount.
Even dose amounts are misleading, as the dose only accurately applies to the Cesium-137 or Cobalt-60 reference, which was used to calibrate the Geiger counter or dosimeter. So if you are exposed to something other than these reference isotopes, the dose reading is not accurate."
WHERE TO SHOP FOR A GEIGER COUNTER
If you don't have a radiation detector, click on the link below. This brand is recommended by AGRP and has the round pancake style of detector head, which is much more sensitive than the cheaper tube types of detectors. Cheaper brands and many more choices are available at the links below.
Shop and price compare for Pancake Style Geiger Counters
(more expensive but much more sensitive as well, much higher quality, can be used to detect lower dose radiation in some objects, dried foods, etc)
(more expensive but much more sensitive as well, much higher quality, can be used to detect lower dose radiation in some objects, dried foods, etc)
Shop and price compare for cheaper Russian Or Chinese tube type Geiger Counters
(less sensitive, cannot be used at all for testing food, useful for major nuclear disasters, radiation plumes, hot spots, generally not sensitive enough for any other purpose)
(less sensitive, cannot be used at all for testing food, useful for major nuclear disasters, radiation plumes, hot spots, generally not sensitive enough for any other purpose)
Stay safe out there, AGRP has your back.
OPTIONS FOR TESTING FOOD AND OTHER OBJECTS, WITH ABILITY TO TELL WHAT SPECIFIC RADIOACTIVE SUBSTANCE(S) ARE PRESENT
Food Radiation Testing Machine Choices, Reasons Why, And How To Set Up A Community Or Store Based Testing Center; via @AGreenRoad
Scintillators capable of testing food or other items and being able to tell what type of radioactive element is present range in cost from approximately $20000 to $20000. The article above goes into detail about these detectors, and details them side by side, so you can compare which one would work the best for your needs.
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