TOPIC 1: Types of Survey Meters

Let’s explore Types of Survey Meters.

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If given the choice between finding a biological hazard, a chemical hazard or a radiological hazard, which would be the easiest to find?

To find the biological hazard would require petri dishes, incubators and several days to grow the germs. Then they have to be viewed under a microscope and then someone with expertise would have to be able to recognize the bacteria to identify it.

To find the chemical hazard requires Draeger tubes, gas chromatographs, mass spectrometers, test tubes, shaker tables, centrifuges and a battery of other tests. Each reaction to a chemical would add another piece to the puzzle in locating and then identifying a chemical.

To find the radiological hazard simply requires a survey meter.

As with any hazard we deal with at work or in our private lives, the first task is to identify the hazard.  Next is to quantify the hazard.  We want to identify and then quantify so we know how to protect ourselves, others and the environment.

Different types of detectors have been invented to answer these questions. But each one operates on the exact same principle.  Radiation is made up of energy.  This energy can be detected.

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We know that Radiation is the decay of an atom releasing some form of energy.  This release of energy ionizes air. If there were a way to measure the interactions in a volume of air (or some other gas), the amount of radiation in the location could be determined.

This was the first technique used to measure radiation.

During the height of the Cold War, when Fallout Shelters were built in backyards, even Sears produced a survey meter for public use. Today there are not many manufacturers of professional quality survey meters. However, the basic functions and technology of the survey meter have remained the same. Computerization has improved the interpretation of results. Digital displays have made the interpretation of the measurements more presentable. But, again, the basic function remains the same.

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The survey meter consists of three main parts. The first part is the Detector.  The Detector is placed in the radiation field.  The interactions of the radiation on the Detector exit as electrical impulses which are then sent to the Electrical Current Measuring Device.  The electrical impulses are interpreted.  The electrical impulses generate a voltage which manipulates the Display.

The simplest type of display is a needle moving over a range of numbers to indicate the radiation field.  Through calibration, the Display can provide a fairly accurate indicator of the quantity of radiation in the area of the Detector.   There are only a few varieties of Detectors.

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The original Detector was the Ion Chamber.

This lead to the invention of the Geiger Mueller (GM) Detectors

Then technology created the Scintillation Detectors

Let’s look at each one in detail.

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The Ion Chamber, originally open to the air, was wrapped with an electrical conductor (cathode).  An electrical wire (anode) was placed in the middle of the chamber. A voltage was applied between the outer cathode and the inner anode. The voltage was only strong enough to pull the ionized electrons to the positively charged anode and allow the positive ionized atom to be pulled to the negatively charged cathode.

The number of electrons between the anode and cathode has changed.  To replace the electron taken by the ionized atom, the deposited electron travels from the anode to the cathode. On its way to the cathode, it passes through the Electrical Current Measuring Device.

Ion chambers are the simplest of all detectors. They collect charges created by direct ionization of air. Hence, they can operate in a current or a pulse (count) mode. They are used to measure gamma and X-Rays.

Because air is not that dense and the amount of ionizations in that volume of air had to be great to be measured, this early innovation was only effective in high radiation fields.

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Since the amount of atoms in the air determined how much ionization could be produced, an improvement to the open air ion chamber was to seal the chamber and put in more atoms of air or some other gas, such as helium.

In addition, the voltage was increased to between 600V and 1200V. At this voltage, when radiation ionizes a gas atom, the electron still travels to the anode to be collected. But the remainder of the atom, with its positive charge and with the greater voltage pulling it to the cathode, can actually ionize other gas atoms along the way.

The end result is that less radiation can create more electrons to go through the Electrical Current Measuring Device. In this way, a small quantity of radiation can be detected, because it creates a larger current. This concept was introduced by Geiger and Mueller in 1928, which is why these type detectors are still referred to as GM detectors or simply Geiger counters.

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Science soon learned that Ionizing radiation also affects the molecules within certain solid crystals such as Sodium-Iodide. The result is a very small, virtually undetectable, flash of visible light.

In the survey meter, the light enters a device called the “photocathode”. This device generates electrons when struck by a photon generated by the crystal. Thus, it turns light into electrical energy.

The electrons then go through a series of plates (dynodes). These plates are loaded with electrons and are in a condition called Avalanche.  As the electrons strike a plate, the Dynode is overwhelmed and the electrons Avalanche releasing many more electrons. Each successive plate releases more and more electrons.

This combined unit made of a photocathode and dynodes is called the Photomultiplier Tube. As the large population of electrons leaves the Photomultiplier Tube, they go through the Electric Current Measuring Device to be counted.

A solid crystal detector is very sensitive to extremely low levels of radioactivity and low-energy gamma rays that are generated from naturally-occurring radioactive materials.

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Just like the gamma rays, measuring Alpha particles is accomplished in the same way using the photomultiplier tube.  The Sodium-Iodide crystal detects gamma radiation.  It is the Zinc-Sulfide crystal that detects Alpha particles by emitting a bright photon of light.  This photon is converted to an electron and is amplified in the photomultiplier tube.

One down side is the ZnS is also sensitive to light.  So the ZnS must be shielded with a light-tight seal but it must be thin enough to allow the alpha through.  A thin piece of foil is placed over the ZnS crystal.