Extended Range Radiation Dose-Rate Monitor

FIG. 1 is a schematic circuit diagram of an extended-range radiation dose-rate detection system according to the present invention.

FIG. 2 is a semilog plot of measured dose-rate (R/h) versus percent of full scale deflection comparing the response range for a conventional counter (Curve A) and the extending range counter as in FIG. 1 (Curve B). A 100 mm.sup.3 Si solid state detecting element was used for this illustration.

Referring now to FIG. 1, there is shown an extended range radiation dose-rate monitor according to the present invention based on a solid state radiation detection element, such as a surface barrier diode detector 7, formed of intrinsic silicon. The diode used in the illustrated device is an ORTEC model #CB-20-100-1000 supplied by the ORTEC Co., Oak Ridge, Tenn. This detector is a charged particle surface barrier diode with a surface area of 1 cm.sup.2 and a thickness of 1 mm. Other detecting elements, such as inexpensive PIN silicon diodes or proportional counters, may be used as the radiation detecting element. The diode 7 is connected in a reverse bias configuration by connecting the anode to ground and the cathode through a current limiting resistor 9 to the positive terminal of a high voltage supply (typically 270 V).

The current pulse signals generated upon the detection of ionizing radiation by the diode 7 are coupled to the input of a charge sensitive preamplifier 11 by means of a dc voltage blocking capacitor 13 connected between the cathode of diode 7 and the preamplifier 11 input. The output of the preamplifier 11 is connected to the input of a pulse-shaping amplifier 15 which generates voltage pulses in response to the detector current pulses from the preamplifier 11 having an amplitude proportional to the total charge deposited by the ionizing radiation detected by the detector and a time width, .tau., in the 2-20 microsecond range corresponding to the induced detector current pulses in the 20-200 nanosecond duration range, depending upon the detector 7 thickness and bias voltage.

The output of the pulse-shaping amplifier 15 is connected through a coupling capacitor 17 to the negative input of a comparator (discriminator) 19. The positive input of comparator 19 is connected to receive a feedback voltage V.sub.f which varies the threshold of the comparator as will be explained hereinbelow. Each pulse from the amplifier 15 whose amplitude exceeds the comparator threshold level (V.sub.f) produces a comparator output pulse. Thus, by connecting the output of comparator 19, which is connected to ground through a load resistor 21, to a frequency meter 23, the radiation count rate may be read directly in digital form on the calibrated meter 23.

The output of the comparator is further connected to the input of an integrator 25 whose output is connected through a resistor 27 to an ammeter 29. The ammeter 29 is calibrated with an expanded scale for direct readout of the radiation dose-rate over the range of from 0 to at least 1000 R/h.

This wide range is obtained by feeding back a portion of the integrator output signal to vary the comparator 19 threshold voltage V.sub.f such that the threshold voltage increases as the dose-rate increases, thereby delaying the approach of the integrator output signal to the meter full scale value and extending the dynamic range of the device. The feedback is obtained by connecting the output of integrator 25 to ground through a potentiometer 31. The adjustable arm of the potentiometer 31 is connected through a resistor 33 to the positive input of the comparator 19 to form a negative feedback arrangement with respect to the comparator output. Thus, at low nuclear interaction rates the comparator is operated at a fixed low discrimination level set by the adjustment of the potentiometer 31 and at higher interaction rates the threshold level of the comparator 19 is increased and only triggers on the pulses which exceed the higher threshold level, as will be explained in more detail in the following description of the operation of system. Additional adjustment of the comparator threshold is provided by connecting the negative input of comparator 19 through a resistor 35 to the adjustable arm of a second potentiometer 37 connected between the system power supply line and ground potential. The potentiometer 37 is adjusted to provide a zero output on the meter 29 when the detector is subjected to only the normal background radiation level and to prevent detection of noise pulses generated by the detector and input circuitry from triggering the comparator 19.

In operation it is to be assumed that the circuitry, with exception of the detecting diode circuit, operates from dual supplies of +V and -V which are of equal magnitude. Through proper choice of integrated circuits, all of the circuitry shown in FIG. 1 can be operated with 1 milliampere of supply current and supply voltages of .+-.3.6 volts. The circuit has a band width of about 50 to 150 kHz.

During operation, X- and/or gamma rays interact with the intrinsic region of the diode detector 7 and create charge carriers which produce a current pulse of 20--200 nanosecond duration. The charge-sensitive preamplifier 11 responds to the resulting detector current pulse by generating a 50 to 100 microsecond tail-pulse having an amplitude proportional to the total charge deposited by the ionizing radiation. This pulse is then amplified and shaped by the pulse-shaping amplifier 15 and applied to the inverting (negative) input of comparator 19. If the adjustable arm of the feedback adjustment potentiometer 31 is set at ground, the circuit operates as a conventional charge-sensitive pulse counting system. That is, at sufficiently low count rates, less than about 0.1 R/h, the comparator generates one logic pulse each time a photon deposits more than the threshold energy level in the detector. These pulses are normally counted by the frequency meter 23 to display the dose-rate. At these dose rates, the system produces a count rate which is approximately proportional to the radiation dose rate. Beyond 0.1 R/h, the count rate begins to saturate at a frequency f.sub.o determined principally by the pulse shaping amplifier 15, as follows: ##EQU1## where .omega.=2.pi.f and T(.omega.) is the amplifier 15 transfer function.

For most counting applications, this low threshold level system is considered useless at dose rates above 0.1 R/h because most users want a count rate output that is linearly proportional to the nuclear event rate. The conventional system approaches f.sub.o rapidly such that at a fixed threshold level of about 140 milovolts the system has little sensitivity beyond 1 R/h. It has been discovered that the system is still responsive at higher count rates if the threshold level of the comparator 19 is increased. However, fixing the threshold level at an increased value causes the system to be nonresponsive at low count rates.

Thus, in accordance with this invention a means has been provided to automatically extend the counting range of a dose rate meter by varying the threshold level of the discriminator by means of a feedback system arrangement which feeds back a portion of the integrated pulse rate signal provided at the output of the integrator 25. The integrator 25 integrates the comparator 19 output pulses to produce a current signal proportional to the dose rate. This signal is fed to meter 29 which is calibrated to read the dose rate in R/h over the extended dynamic detecting range of the system. The integrator time constant .tau..sub.f is selected to be long in comparison to the pulse rate at frequencies well below f.sub.o (i.e., .tau..sub.f =R.sub.f C.sub.f >>1/f.sub.o) so that its response to an isolated pulse from the comparator 19 produces an insignificant change in the integrator output. But as the comparator trigger rate increases toward f.sub.o, the integrator 25 output increases toward +V/2. By feeding back a portion of the integrator output signal through proper adjustment of the feedback potentiometer 31, the threshold voltage V.sub.f applied to the positive, or reference, input to the comparator 19 is varied in direct proportion to the integrator output to raise the threshold as the integrator output increases and vice versa. The potentiometer 31 is typically set during calibration so that the meter indicates 100 percent full scale for the maximum dose rate which the system is designed to detect. Calibration is performed in a conventional manner by exposing the detector element to radiation from a calibrated source, such as the Civil Defense Model V-794 radiation detector calibrator.

Referring now to FIG. 2, there is shown a comparison between the output signal from the integrator 25, in percent of full scale of the meter 29 for a fixed comparator threshold system (Curve A) and the variable threshold system of the present invention (Curve B) for dose rates ranging from 0.1 m R/h to 1000 R/h corresponding to photon interaction rates of 101 to 108 events/seconds. As shown, the variable threshold system extends the detection range automatically by feeding back a portion of the integrator 25 to vary the comparator 19 threshold reference voltage. The dynamic counting range is extended by at least one decade by delaying the asymptotic approach to 100 percent full scale of the meter deflection.

As will be noted in FIG. 3, the conventional linear counting range extends from 0 to about 104 events/second, corresponding to 0 to 0.1 R/h. This linear portion may be preserved by clamping the adjustable arm of potentiometer 33 to ground by means of a conventional diode clamp circuit (not shown), until the integrator output exceeds the level corresponding to a dose-rate of 0.1 R/h. With this circuit addition, the lower end of the calibrated scale of the meter 29 would be linear with respect to the dose-rate for values below 0.1 R/h.

Thus, it will be seen that an instrument has been provided which allows extended range detection of radiation dose-rates of ionizing radiation, such as x-rays or gamma rays, without the need for range switching. Since the measurement is based on dynamic detector response, it is insensitive to temperature-sensitive leakage currents which are typically much larger than the signal current.

Although the invention has been illustrated by means of a specific embodiment, it will be apparent to those skilled in the art that various modifications and changes may be made therein without departing from the spirit and scope of the following claims attached hereto and forming a part of this specification.