Don’t eat the glowing fish


After hearing that Fukushima is still leaking radiative waste and coolant into the sea, I wanted to check the fish in Tesco Lotus for radiation. Initially I thought about buying an old school military/academic Geiger counter.

However, after a quick search and having seen some pretty discrete Geiger counters, on eBay, that fit in the microphone socket of the iPhone, and looking up some reviews, in order to gauge their effectiveness, I came across, not only some interesting DIY video tutorials, but also some reviews of more serious geiger counters, scintillation counters, and most interestingly of all, the vintage CDV-700 and the hacks that can be applied to it.

Pictured above is the CDV-700M, which has the capability of being able to detect alpha particles, which the standard CDV-700 can not. This image comes from CD V-700M GM Detector.

See also


DIY Videos

How to make a Geiger Counter

How to build a geiger counter / radiation detector from household materials

The Simplest DIY Gieger Counter

Video Reviews

There are some great videos from antiprotons.

What Geiger Counter to Buy?

Note: A gamma spectrometer will tell you what the radioactive material is, and costs around $1500. A geiger counter can only tell you if the sample is radioactive or not.

Important properties

  • A large surface area of the probe, such as the pancake probe, is good, as this gives more counts per minute, than a smaller probe, such as the probe that comes with the CDV-700. However, the larger the “window”, the more fragile it becomes and can be sensitive to vibrations, knocks and even pressures. Large ,and/or sudden, pressure changes can “blow out” the window, for example, in a airplane, or top of mountain.
  • The ability to sense alpha particles is important – alpha particles, when internal to the body, are very dangerous. They are, in fact, 20 times more than beta and gamma, when internal. Externally, they can not penetrate the skin.

Other points

  • In areas of high radiation, be aware of jamming and saturation –  Where the high concentration of particles causes a continuous circuit in the probe, and which causes the meter to, misleadingly, lower its reading to zero, when in fact there are high levels of radiation. For an example of this, see What Geiger Counter to Buy?@12:33. Older units can suffer for this. Newer models have anti-jamming feature, by recognising that they have hit a high level and provide a visual indicator.
  • Alert level – older units can not give rapid audible alerts – modern models you can set alert levels, which trigger an alarm. The alarm can be set to any level, i.e. 320 Counts Per Minute (CPM).
  • Ability to detect activity of radionuclides – not many handheld geiger counters can do this, it is more common in laboratory geiger counters. For example, the Inspector USB can do this.

Extra considerations

  • An external detector versus an internal detector, i.e. CDV-700 (or Ludlum Model 12) versus Inspector EXP, (or CRM100). Geiger counters with internal detectors are more portable.
  • Rechargeable or replaceable batteries – replaceable batteries are more convenient when out in the field, in an emergency, for example. A rechargeable unit would need to plugged into a charger, and then there would be a delay whilst waiting for the unit to charge.
  • Calibration – Usually the device needs to be sent to manufacturer, requires Caesium 137 with around 370 kBq (10 micro Curies or more). Older units, CDV-700, or modern Ludlum Model 12, come with manuals describing calibration and often have an operational check source in the side.
  • Upgradability – the CDV-700 has a number of upgrades available (i.e speakers, removable probe, different detectors, check source, external power supply, digital counter, etc.)
  • Metal and Plastic – durability. Geiger counters with metal cases are, obviously, much more rugged than plastic cased geiger counters. Plastic cased geiger counters should have rubber protectors fitted.

Display types

  • Digital –
    • Pros:
      • Easy to read
      • Typically have memory. data logging
      • Auto ranging
      • Timed counts
    • Cons:
      • Delay time using average reading
      • Slow response time, not real time
    • Uses
      • Uranium prospecting
      • Security checks
  • Analogue –
    • Pros:
      • Real time, fast response
      • Versatile configuration
    • Cons:
      • Difficult to read
      • Can quickly max out the display, and the multiplier needs to be changed, although some have auto-scale
      • No data logging
      • Can be averaged, if using slow mode (precise)
      • No timed counts
    • Uses:
      • Background check
      • Small samples


Geiger counters are never exact, measurements are approximations, therefore there are two types of units and detectors:

  • Counts per minute/second (CPM, CPS) – This measurement may be simple and basic but this is the most important reading. Factual report, no inference.
  • , see Radiation-related quantities, energy based units, convey  how much radiation absorbed, and its biological effect:
    • micro sievert per hour,
    • Grays, Rems, microröntgen, becquerel
    • Only useful for a particular radioactive source, because these units only make sense when it is known how much energy is being put out.

Note: Biological effect units are only useful in highly calibrated laboratory conditions, where the geiger counter has been calibrated for a particular material, so that it is known how much energy is put out, and what type of energy. This is because different materials put out different energies.

It is possible to use Energy compensating covers, or energy compensation filter, for the pancake detector. These covers/filters flatten the energy curve. Or get an energy compensated geiger counter. Japanese geiger counters favour the biological/energy units, but make sure that the geiger counter can also measure counts per seconds/minute.

For accuracy, with respect to energy, you need an energy sensitive scintillation counter – which is more expensive – but not a geiger counter.

What to avoid

  • Be aware of devices that aren’t geiger counters:
    • CDV717/715/720 Ionisation detector
    • Electromagnetic field/Electromagnetic radiation detector
    • Must detect gamma radiation
  • Old units – i.e. LM Model 3 with a 44/9 (pancake) detector, beware of an old tube. Repairs may be required (no more difficult that an old analogue TV) – Make sure you get one with a warranty, certification of repair/refurbishment, calibration. If you pay only it $20/30, the geiger counter may break soon, or upon arrival, and will cost you more in the long run, once you have had it professionally repaired (unless you can fix it yourself).

Food testing

  • Gross contamination – in a nuclear accident, fallout, highly contamination
    • Geiger counter can detect
  • Trace contamination – On the FDA limits, microscopic amount, 200-300 becquerel/kg
    • Geiger counter will not detect, or if it does you will not know if it is natural potassium (A gamma spectrometer is required to achieve this).

Models featured in the video

  • Ludlum Measurements, Inc. Model 12 survey meter
  • Ludlum (LM) Model 3
  • Inspector EXP+
  • International Medcom CRM100
  • Victoreen instrument Co., CDV-700 Model 6A – This geiger is blind to Alpha particles, due to the probe not having a mica window (but can be modified, see CDV-700M – Alpha capable below)
  • Polimaster Scintillator
  • Probes:
    • CVD-700 probe
    • Ludlum Measurements big pancake probe (44/9 detector)
    • Ludlum 44-2 gamma scintillation tube

Buying a Geiger Counter 101

All geiger counters have a Geiger Müller  tube, the bigger the better.


  • Mazur Systems PRM8000
  • Atomic Dave
  • Geiger counter kit
  • International Medcom CRM100
    • Similar to International Medcom RadAlert 100 (has an alarm) and RadAlert 50
  • CDV-700: Sub type 6A, or 6B
  • Inspector – get the rubber housing $500
  • SE International inspector EXP+ $700 – lab version of Inspector, with a separate probe.
  • Mazur Systems PRM9000 – data logging, same detector $700
  • Ludlum with a 44/9 Probe

My New CDV-700 Victoreen Model 6A Geiger Counter!

Inspector USB Geiger Counter – Review!

Geiger Counter Myths

Testing food is not particularly useful, at @9:01, basically. See also What Geiger Counter to Buy?@36:35.

Three Geiger Counters Compared

The three models compared are:

  • CRM 100
  • CDV-700 Model 6A
  • SE International inspector EXP+ $700 – lab version of Inspector, with a separate probe.

Buying a Geiger Counter

For sources of the CDV-700, see CDV

Other Geiger counters

Differences between similar Ludlum and CDV-700 models

Difference between the Ludlum geiger models 2, 3, 12, and 14

  • Model 3 – Survey meter, Supports GM & Scintillation Type Detectors
  • Model 12 – Rate meter, Supports GM, Proportional & Scintillation Type Detectors
  • Model 14C – Survey meter, with internal GM tube and external GM or Scintillator Detector

Connectors: BNC, C, and MVH


See also CDV, for extensive information about the CDV-700.


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