Radiation monitoring | Device roundup
EcoGamma-G fixed gamma radiation detector20 April 2012
Canberra’s new fixed EcoGamma-g Geiger-Mueller detector collects gamma radiation data with intensities from 10 nSv/hr to 10 Sv/hr (+/- 15%) in the 30 keV to 3.0 MeV operating range.
Temperature data is also automatically recorded. The device consists of two Ne+halogen gas Geiger Mueller detectors, whose crossover point for increasing doses is 500 mR/hr, and on decreasing doses is 300 mR/hr. It connects to a computer via either its single ethernet (10Base-T) port or its USB port. Either connection also powers the device. A combination of Flash memory and non-volatile RAM memory allows 180 days of data storage at one-minute intervals. It also records total integrated dose (maximum: 100 krad). The EcoGamma-g is controlled through a web-based computer interface, with additional user tools such as interactive data displays and statistical tools.
Like other Canberra detectors, the EcoGamma-g uses a ‘time to count’ methodology. Readings of the radiation rate are a function of the number of pulses (counts) produced by the detector per unit time. Conventionally, a GM detector operated with a fixed DC voltage continuously applied is characterized by ‘dead-time’—increasing non-linearity as the field intensity increases—and saturation, Canberra said. Canberra claims that the ‘time-to-count’ technique eliminates these effects by raising and lowering the device voltage hundreds of times a second. A low DC bias voltage is abruptly raised to 500 V DC, carrying the detector into its operating region. The rise time of this voltage is less than 0.2 microseconds. At the same time as this rapid increase in voltage is applied, a crystal-controlled, 1 megacycle oscillator (in other words, a clock) is gated on. From then, time, in the form of 1 microsecond cycles, starts being counted. Time counting continues until a detector pulse is obtained. At that point, time counting is stopped and the accumulated time is recorded. At the same time, the anode voltage is reduced to the low bias level. The anode voltage is maintained at the low bias level for 1.5 to 2 milliseconds, a time period which is long compared to the dead time and recovery time of the detector. Voltage at anode is applied again when the GM detector is fully recovered. Only one GM detector pulse can occur in any one ‘on’ time. Since the detector is fully recovered between ‘on’ times, the pulses produced by the detector are full-size. The process is repeated many times to obtain a statistically reliable average time-to-count. Thus, the radiation field intensity is proportional to the reciprocal of the time required to obtain a GM count.