backscatter. However alpha particles are easily attenuated if the active layer is made too thick. The SER is established by measurement using calibrated equipment, normally traceable to a national standard source of radiation. Ratemeters and scalers In radiation protection practice, an instrument which reads a rate of detected events is normally known as a ratemeter, which was first developed by R D Robley Evans in 1939. This mode of operation provides real-time dynamic indication of the radiation rate, and the principle has found widespread application in radiation survey meters used in health physics. An instrument which totalises the events detected over a time period is known as a scaler. This colloquial name comes from the early days of automatic radiation counting, when a pulse-dividing circuit was required to "scale 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 a cascade of "Eccles-Jordan" divide-by-two circuits, today known as flip flops. Early count readings were therefore binary numbers and had to be manually re-calculated into decimal values. Later, with the development of electronic indicators, which started with the introduction of the Dekatron readout tube in the 1950s, and culminating in the modern digital indicator, totalised readings came to be directly indicated in decimal notation. SI Units for radioactive disintegration One becquerel (Bq) is equal to one disintegration per second; 1 becquerel (Bq) is equal to 60 dpm. One curie (Ci) an old non-SI unit is equal to or dps, which is equal to . References Units of radioactivity Units of frequency Radiation protection