Method and Apparatus for Extending a Scintillation Counter's Dynamic Range

• US 20110284753A1
• Filed: 03/15/2011
• Published: 11/24/2011
• Est. Priority Date: 05/21/2010
• Status: Active Grant

• Current Assignee: Lewis Ronald Carroll
• Original Assignee: Lewis Ronald Carroll
• Inventors: Carroll, Lewis Ronald
• Granted Patent: US 9,081,102 B2
• Time in Patent Office: Days
• US Class Current: 250/369
• CPC Class Codes: G01T 1/17 Circuit arrangements not adapted to a particular type of detector pulse-selection circuits H03K, G01R

Abstract

A semiconductor diode scintillation detector probe, in conjunction with a base-line-stabilized, wide-bandwidth first amplifying circuit DC-coupled to a constrained-bandwidth second amplifying circuit DC-coupled, in turn, to a novel analog threshold discriminator circuit, suppresses base-line fluctuation and noise at low input count-rates, while providing a linear rate-meter response for time-random input pulse rates far in excess of what would otherwise—as in the prior art—be 100% saturation.

Claims

1. ) A scintillation counter with extended dynamic range;

said scintillation counter having a radiation detection probe, a post-amplifier, and an analog threshold discriminator with an adjustable threshold;

said radiation detection probe incorporating a radiation detector transducer element converting energy from impacts of x-ray or gamma-ray photons absorbed in said transducer element into pulses of electronic charge;

said transducer element being typically combined with a pre-amplifier and associated electronic circuitry to convert pulses of electronic charge into voltage or current signal pulses;

said signal pulses being typically combined with naturally occurring, but unwanted, electronic noise also generated within said transducer and associated pre-amplifier and electronic circuitry;

said post-amplifier comprising a wide-bandwidth first amplifying circuit followed by a constrained-bandwidth second amplifying circuit;

the signal-pulse output of said radiation detection probe being connected to the input of said wide-bandwidth first amplifying circuit, the output of said first amplifying circuit having a first baseline voltage which is initially set to a fixed, pre-determined level while radiation-induced signal pulses are not present;

the amplified signal pulses at the output of said wide-bandwidth first amplifying circuit being preferably unipolar relative to said first base-line voltage, said amplified pulses having a temporally fast rise-time and subsequent return to base-line generally commensurate with the underlying physical energy-conversion processes characteristic of the transducer element within said radiation detector probe;

the amplitudes of said amplified signal pulses being nominally proportional to the energy absorbed from corresponding photon impacts in said transducer element;

said wide-bandwidth amplified signal pulses being superimposed upon said unwanted, but concurrently-amplified, electronic noise;

said wide-bandwidth first amplifying circuit incorporating a base-line-holder and feedback-control means to maintain said first base-line voltage fixed with respect to a pre-set reference value;

the output of said wide-bandwidth first amplifying circuit being DC-coupled to the input of said second amplifying circuit, the bandwidth of said second amplifying circuit being constrained so as to attenuate said unwanted electronic noise relative to said radiation-induced signal pulses;

the output of said constrained-bandwidth second amplifying circuit being DC-coupled to the input of an analog threshold discriminator circuit;

the amplitude of radiation-induced signal pulses at the input of said analog threshold discriminator circuit and the threshold setting of said threshold discriminator circuit both being expressed relative to a second base-line voltage which is measured at the input of said analog threshold discriminator circuit while radiation-induced signal pulses are not present;

said second base-line voltage being conveniently, but not necessarily, set to zero volts by appropriate initial setting of said first base-line voltage;

said threshold voltage being pre-set to a value which is purposely less than the peak amplitude of radiation-induced signal pulses, but preferably greater than the peak amplitude of unwanted residual noise observed at the input of said analog threshold discriminator circuit while radiation-induced signal pulses are not present;

said analog threshold discriminator circuit thereby yielding an output signal current only when the voltage at the input to said analog threshold discriminator circuit exceeds said threshold setting;

the input-output transfer characteristic of said analog threshold discriminator circuit being described by a plot-line on a graph;

the abscissa and ordinate values on said graph representing, respectively, the voltage at the input of said analog threshold discriminator circuit measured with respect to said second base-line voltage, and the corresponding output current of said discriminator circuit;

said ordinate value being equal to zero when the abscissa value is less than the pre-set threshold;

said ordinate value being equal to the product of the abscissa value multiplied by a pre-determined numerical constant when said abscissa value exceeds the pre-set threshold.

2. ) The scintillation counter of claim 1, in which said analog threshold discriminator circuit incorporates a first N-channel junction field effect transistor and a second N-channel junction field effect transistor;

said field effect transistors each having respective gate, drain, and source terminals;

said analog threshold discriminator circuit also incorporating a first PNP bipolar transistor and a second PNP bipolar transistor, said transistors each having respective emitter, base, and collector terminals;

said analog threshold discriminator circuit having a signal input terminal, a threshold-voltage setting means, a power-supply terminal, a signal output terminal, and a common, or ground terminal;

the source terminal of the first junction field effect transistor being connected to the source terminal of the second field effect transistor, and further connected through a first resistor to common, or ground potential;

the gate of said first field effect transistor being designated the signal input terminal, the gate of said second field effect transistor being connected to said threshold-voltage setting means;

the drain of said first field effect transistor being connected to the base and collector of said first PNP bipolar transistor, being further connected to the base of said second PNP bipolar transistor, and further connected through a second resistor to said power supply terminal;

the emitter of said first PNP bipolar transistor being connected through a third resistor to said power supply terminal;

the drain of said second field effect transistor being connected to the emitter of said second PNP bipolar transistor and further connected through a fourth resistor to said power supply terminal;

the collector of said second PNP bipolar transistor being connected through a fifth resistor to common, or ground potential, and further connected to said output terminal;

the respective resistance values of each of said resistors, and the device characteristics of said field effect transistors being chosen by design to produce the desired input-output signal transfer characteristic of the analog threshold discriminator of claim 1.

3. ) The scintillation counter of claim 1 in which said baseline holder and feedback control means employs a semiconductor junction diode as a baseline sensing element.

4. ) The scintillation counter of claim 1 in which the radiation detection transducer element is a scintillation detector employing a scintillating crystal optically coupled to an electro-optical transducer such as a photo-multiplier tube or a semiconductor photo-diode.

5. ) The scintillation counter of claim 1 in which the radiation detection transducer element is a semiconductor device within which radiation is detected directly, without a scintillating crystal.