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The RadAlert GEM 2 Radiation Monitor

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Ionising Radiation Meter - the RadAlert GEM 2

The RadAlert GEM 2 is a handy and accurate ionising radiation monitor that detects gamma-rays, x-rays and to a limited extent alpha and beta radiation in your environment. It is made with state-of-the-art technology and is the result of over 15 years of development and testing. It is ideal for use by the general public and business communities wishing to make a simple and quick assessment of the levels of their background ionising radiation.

We are no longer stocking the RadAlert Gem II monitors as we are concentrating on our own instrument range. You can still purchase them from the manufacturer - Perspective Instruments .

Further information on the RadAlert GEM 2 and background radiation

Technical specifications

The Display

The RadAlert GEM 2 has a vertical array of 7 leds, each denoting a rate of exposure from 0 to greater than 7.5 micro-sieverts per hour. A Sievert is a very large unit of radiation dose ("dose equivalent"), so it is often expressed in units of one-millionths (micro = µ) or one-thousandths (milli = m). The 7 lights are grouped in 3 ranges of exposure from green to red, and these lights and ranges indicate:

  • RED 7.5 µSv/hour The instantaneous dose rate which defines a Radiation Controlled Area (IR Regs 1985).
  • RED 5.0 µSv/h
  • YELLOW 2.5 µSv/h The instantaneous dose rate which defines a Radiation Supervised Area (IRR 1985).
  • YELLOW 1.0 µSv/h Levels of dose rate which might be encountered in areas of enhanced natural background.
  • YELLOW 0.5 µSv/h
  • GREEN 0.3 µSv/h Overall levels of radiation which are roughly equivalent to normal background variations in most of the UK
  • GREEN 0.l µSv/h Typical low level background gamma radiation in the UK

  • Detector: miniature GM tube LND Inc. Model 712 stainless steel cathode Mica window effective dia. 9.14mm gas filling neon halogen

  • Sensitivity (primarily to Gamma and x-ray radiation)
      » 0.l µSv/h = very low level background radiation
      » 1 count per second from GM tube = 0.7 µSv/hour
      » Dose rate response to Cs137 is nearly linear over the range 0.1 µSv/h with confirmation from a reference point at 5.4 µSv/h using Ra226. The energy response of the Geiger Muller tube peaks at 60keV but is flat over the range 200keV to >1MeV to within +10% -20% at the side window and +/- 5% along the top side of the case.
  • Overall accuracy: typically around ±10% on 10 minute period monitoring
  • Power source: 1 x PP3/MN1604 9 volt battery (> 24 hours continuous monitoring; approximately 2 years with 15 minutes use per week)
  • Current draw: < 1 uA on standby; 25 mA in use
  • Size (mm): 100 x 60 x 30(LxWxD) nominal
  • Weight: 150g, including battery
  • Operating Environment: Temperature: 0°C to +40°C : Relative Humidity <95%

Factory-set Visual ALARM.

Every second that your RadAlert is operating, whether in 10-SEC or 10-MIN mode, it is also checking for a sudden burst of radiation that calls for immediate action by you. If in any one second period RadAlert detects a radiation dose over 15 microsieverts per hour, RadAlert will stop counting and alert you with a unique display of flashing lights using the two MODE lights and the top red light. To turn off this alarm display, simply press the button until RadAlert turns off, then turn it on again.

The UK-made RadAlert GEM 2 ionising radiation monitor has a good reputation as a practical, accurate and simple to use monitor for gamma radiation and x-rays. It also detects alpha and beta decays. Note: The GEM is a PASSIVE Instrument and is not itself a source of radiation.

Many building materials contain low degrees of natural radioactivity and radon gas seeps from the ground into all buildings, so the largest exposure is to naturally occurring radiation in homes and workplaces. In the UK, Public Health England has calculated that on average people are exposed to about 2.7 millisieverts (mSv) of radiation a year (equivalent to an average of about 0.3 µSv per hour) coming from a number of sources which include significant contributions from naturally occurring radioactivity in food and from medical exposures.

There are 3 main types of ionising radiation:

Alpha and beta radiation take the form of particles that are emitted from radioactive material whereas gamma radiation is a form of electromagnetic radiation that is emitted as a wave of energy. This electromagnetic spectrum includes light, ultraviolet rays and X-rays.

  • Alpha radiation can be the most harmful if breathed in or eaten. However, it can only travel a few centimetres in air and can be stopped by a thin layer of material such as paper, clothing or the skin so is less harmful if it is outside the body.
  • Beta radiation is less harmful than alpha if breathed in or eaten, but can travel further in air and is more penetrating through material. It passes through paper and needs a few millimetres of aluminium or a thin piece of lead to stop it. Similarly it can pass through the skin from outside the body to affect the internal organs.
  • Gamma radiation is high-energy electromagnetic radiation and can travel a long way through air and is extremely penetrating: several centimetres of lead or metres of concrete are required to stop it. This means that it can easily pass through the skin into the body and affect internal organs. However, it gets weaker as you move away from the source. It includes x-rays and cosmic rays.
  • Operation

    With a suitable 9-volt battery (PP3, MN1604, etc) installed, simply press the button once and the RadAlert's display lights will illuminate sequentially to indicate that it is functioning properly and the battery has sufficient charge. The "10-sec" light will begin to flash every 2 seconds, indicating that it is counting for 10 seconds and will provide a new scan result every 10 seconds thereafter. The 10-second scan period enables you to take action to avoid a high dose in an extreme radiation environment. At any time, you may press the button a second time to start the 10-minute counting period (10-MIN Mode) for measurements of greater statistical accuracy (especially at low dose rates), or press the button a third time to switch your GEM off.

     » Press once = ON, in 10 second mode

     » Press again = ON, in 10 minute mode

     » Press a third time = OFF

    While gamma and x-ray radiation penetrate RadAlert's plastic case unimpeded, a round "window" on the left side of RadAlert is provided for the detection of alpha and beta radiation, which are not capable of penetrating RadAlert's case and only travel a few centimetres in the air. So if you wish to be sure to include alpha and/or beta when testing a source, orient this window towards the source and as close to it as possible. Do not immerse the RadAlert in liquids ... it is neither water resistant nor waterproof! (Firemen use RadAlerts in simple poly bags to provide basic protection from moisture).

    Note: Because of the way in which ionising radiation is detected, placing the GEM-2 very close to strong magnetic fields can produce erroneous readings. Moving GEM a few inches away from these sources will eliminate any possibility of interference.

    Low Battery Warning
    If the energy in the battery becomes too low, the RadAlert GEM 2 will flash lights 2 and 6 (of the array of 7) and cannot be used until the battery is replaced.

    uk rad

    A comparison of doses from sources of exposure

    Source of exposure Dose Average Dose Rate µSv/h
    Dental x-ray0.005 mSv 
    Chest x-ray0.014 mSv 
    Transatlantic flight0.08 mSv 
    Nuclear worker0.18 mSv/yr  0.02 µSv/hr
    UK avg Radon exp1.3 mSv/yr  0.15 µSv/hr
    single CT scan of the head1.4 mSv 
    UK avg rad dose2.7 mSv/yr  0.3 µSv/hr
    USA avg rad dose6.2 mSv/yr  0.7 µSv/hr
    single CT chest scan6.6 mSv  0.75 µSv/hr
    Cornish Radon exp7.8 mSv/yr  0.9 µSv/hr
    single CT spine scan10 mSv 
    UK Nuclear worker limit20 mSv/yr  0.9 µSv/hr
    Blood cell damage100 mSv 
    50% of people die in 30 days5000 mSv 

    High readings in the yellow or red bands with 10-minute readings:
    If you are outdoors and this increased 'background' persists, go indoors, shut the windows and doors and take another measurement - radioactive particles are carried by the winds and rain, so going indoors should provide the most protection. If you are indoors, go outdoors and take a reading to determine if the problem is specific to your home, workplace, or general to the local environment. If you suspect and object, move away from it and take further readings.

    IMPORTANT - Changing the Battery
    The RadAlert GEM 2's size is a useful advantage over most products on the market today, but it means that the battery is in a very confined space and will require an extra minute to change. Slide off the battery compartment cover and, using a small-screwdriver, gently lever out the battery connector and battery being careful not to short circuit the battery contacts. Insert the replacement battery so that the battery's "+"terminal is positioned closest to the bottom of the RadAlert. Clip on the battery connector and position the battery wires on the back side of the connector. Replace the sliding battery compartment cover.

    Each RadAlert GEM 2 is guaranteed to be free from defects in materials and workmanship for a period of one year from the date of purchase. This warranty extends to the original purchaser only and does not apply to batteries or any parts subject to misuse, neglect, accident, unauthorised service or abnormal conditions of operation. In the event of an instrument failure, EMFields Solutions Ltd will repair or replace the faulty RadAlert GEM 2, once it has been returned to us and inspection has revealed the unit to be defective within the warranty conditions. Other than the battery compartment, do not try to open the bonded case of your RadAlert. There are high voltages within and it will irreparably damage the case. It will also void your 12-month warranty.

    Calibration Check (usually with 21 Day turnaround)
    Depending upon the intended use of your RadAlert and the regulations and laws which may apply, you may wish to have your GEM calibrated every few years, though this is not normally necessary for personal use. We can arrange a calibration check of your RadAlert GEM 2 for a charge, currently £75 but subject to change.
    Important: Before sending your RadAlert for warranty service or calibration, please contact us first.

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