Survey meter

Survey meters can be ratemeters or scalers

In Radiation Protection, an instrument which reads a rate of detected events is normally known as a ratemeter, which was first developed by N.S.Gingrich et al. in 1936.^[2] This provided a real-time dynamic indication of the radiation rate, and the principle has found widespread use in Health Physics and as radiation Survey meter.

An instrument which totalises the events detected over a time period is known as a scaler. This colloquial name stems from the early days of automatic counting, when a scaling circuit was required to divide down a high count rate to a speed which mechanical counters could register. This technique was developed by C E Wynn-Williams at The Cavendish Laboratory and first published in 1932. The original counters used the "Eccles-Jordan divider" circuit, today known as a flip flop.^[3] This was before the era of electronic indicators, which started with the introduction of the Dekatron tube in the 1950s.^[3][4]

The user must have an awareness of the types of radiation to be encountered so that the correct instrument is used. A further complication is the possible presence of "mixed radiation fields" where more than one form of radiation is present. Many instruments are sensitive to more than one type of radiation; alpha and beta, or beta and gamma, for instance, and the operator must know how to discriminate between these. The necessary skills in using a hand-held instrument are not only to manipulate the instrument, but also to interpret results of the rate of radiation exposure and the type of radiation being detected.

For instance, a Geiger end-window instrument cannot discriminate between alpha and beta, but moving the detector away from the source of radiation will reveal a drop off in alpha as the detector tube must normally be within 10mm of the alpha source to obtain a reasonable counting efficiency. The operator can now deduce that both alpha and beta is present. Likewise for a beta/gamma geiger instrument, the beta may have an effect at a range in the order of metres, depending on the energy of the beta, which may give rise to the false assumption that only gamma is being detected, but if a sliding shield type detector is used, the beta can be shielded out manually, leaving only the gamma reading.

For this reason, an instrument such as the dual phosphor scintillation probe, which will discriminate between alpha and beta, is used where routine checking will come across alpha and beta emitters simultaneously. This type of counter is known as "dual channel" and can discriminate between radiation types and give separate readouts for each.

However, scintillation probes can be affected by high gamma background levels, which must therefore be checked by the skilled operator to allow the instrument to compensate. A common technique is to remove the counter from any proximity to alpha and beta emitters and allow a "background" count of gamma. The instrument can then subtract this in subsequent readings.

In dose survey work Geiger counters are often just used to locate sources of radiation, and an ion chamber instrument is then used to obtain a more accurate measurement owing to their better accuracy and capability of counting higher dose rates.

In summary, there are a variety of instrument features and techniques to help the operator to work correctly, but the use by a skilled operator is necessary to ensure reliable results. The UK Health and Safety Executive has issued a guidance note on selecting the correct instrument for the application concerned, and the care and use of such instruments.^[1]

• Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons, ISBN 0-471-07338-5.

Guidance on the Choice, Use and Maintenance ofHand-held Radiation Monitoring Equipment. - National Radiation Protection Board - UK, May 2001.