Data processing: measuring – calibrating – or testing – Calibration or correction system – Circuit tuning
Reexamination Certificate
1999-09-01
2001-09-25
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Circuit tuning
C702S066000, C702S180000, C702S190000, C330S005000, C250S363090
Reexamination Certificate
active
06295508
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system. Specifically, this invention relates to a circuit for an ionizing radiation spectroscopy system which automatically adjusts the pole-zero based upon peak shape.
2. Description of the Related Art
Radiation detection systems generally employ a radiation detector, such as a germanium or a scintillation detector, to detect radiation from a radiation source, such as alpha or gamma rays. The detection of such energy results in a charge pulse having an amplitude proportional to the energy of the incident radiation. The charge pulse is converted to a voltage pulse by a feedback capacitor incorporated in a preamplifier. A resistor is added in parallel with the capacitor in order to discharge the capacitor in a reasonable amount of time to allow for processing of successive pulses. The parallel combination of the capacitor and the resistor defines the time constant of the exponential decay of the trailing edge of the voltage pulse. The voltage pulse is submitted to a high pass filter to shorten the pulse and improve the signal-to-noise ratio. However, the high pass filter, when supplied with an exponentially decaying signal, produces a filtered signal with an undesirable undershoot, i.e., an excursion below baseline voltage. This is a serious problem because the radiation pulses arrive randomly in time and succeeding pulses can occur during the undershoot of a previous pulse. When this occurs, the measurement of the amplitude of the succeeding pulse is distorted. To compensate, a pole-zero cancellation circuit is used to eliminate the undershoot by diverting a portion of the voltage pulse at the high pass filter input around the filter and combining the diverted portion with the high pass filter output.
Various techniques have been used to implement the pole-zero cancellation method. This was originally done manually. (See Nowlin et al., “Elimination of Undesirable Undershoot in the Operation and Testing of Nuclear Pulse Amplifiers”, Rev. Sci. Instr., vol.36, no. 2, December 1965, pp 830-839). However, untrained non-technical personnel, such as at medical clinics, encountered difficulty when compelled to adjust the shunting resistance or other components to null the undershoot and avoid overshoot. Accordingly, an automatic approach was proposed as in U.S. Pat. No. 4,866,400 (the '400 patent) to Britton, Jr. et al., issued on Sep. 12, 1989, entitled Automatic Pole-Zero Adjustment Circuit for an Ionizing Radiation Spectroscopy System, fully incorporated herein by reference. However, even using the automated approach, improvement of the adjustment accuracy is desirable by eliminating errors such as glitches, pedestals, offsets, and temperature drift in the analog automatic pole-zero (APZ) sampling circuit.
U.S. Pat. No. 5,872,363 issued to Bingham et al., discloses an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system which directly measures the over/undershoot of the digital filtered signal. A correction signal is calculated based on the measured over/undershoot value and that signal is applied to a pole-zero adjustment network. Accordingly, Bingham et al., make automatic pole-zero adjustments based on intermediate outputs of the ionizing radiation spectroscopy system.
By making adjustments based upon the final output of the ionizing radiation spectroscopy system, a more accurate and stable solution for making automatic pole-zero adjustments is achieved over the prior art systems.
It is therefore an object of this invention to provide an improved automatic pole-zero adjustment circuit.
Yet another object of this invention is to provide such an improved automatic pole-zero adjustment circuit which is more accurate.
A still further object of this invention is to provide an improved automatic pole-zero adjustment circuit which is more flexible in the selection of sampling and correction circuits.
BRIEF SUMMARY OF THE INVENTION
An automatic pole-zero (APZ) adjustment circuit for an ionizing radiation spectroscopy system is provided. The detected radiation emissions are fed into a preamplifier with a conventional parallel RC feedback circuit and passed to a high pass filter. The high pass filter improves the signal-to-noise ratio but the exponentially decaying output of the high pass filter results in undershoot. Undershoot is canceled by adding a correction signal generated by a pole-zero adjustment network. The correction signal is selected to algebraically cancel the undershoot when summed with the output signal of the high pass filter.
The high pass filter output is delivered to a digital conversion circuit where it is amplified by an amplifier which includes a feedback resistor. A sampling analog-to-digital converter (ADC) samples and converts the amplified analog signal to a digital signal. The digital signal passes through a digital shaping filter that improves the precision of the energy measurement by removing higher frequencies which improves the signal-to-noise ratio and minimizes the effects due to variable rise times and base line errors. The digital shaping filter results in a pulse which has a longer rise time but still represents the energy of the detected emission.
A pulse amplitude sampling circuit samples the peak amplitude of each pulse output from the digital shaping filter. A histogram containing the number of pulses at each different voltage level is recorded by an amplitude histogram circuit. The histogram, which displays one or more peaks identifying the nature of the radiant emission detected, may be viewed in a display.
An automatic pole-zero (APZ) sampling circuit takes the output of the amplitude histogram circuit and uses an analysis of the peak shape of the histogram to drive a control circuit which, in turn, adjusts the pole-zero adjustment network to produce the correction signal. The shape of the histogram peak indicates whether the pole-zero adjustment is correct. A balanced pole-zero network results in a peak having minimum distortion as evidenced by a peak having a minimum width and a near Gaussian distribution, while an unbalanced pole-zero network results in an asymmetric bell-shaped distribution showing low side distortion (undershoot) or high side distortion (overshoot).
The increased accuracy and flexibility of the system are more clearly understandable through a discussion of the operation of the automatic pole-zero sampling circuit and control circuit. A direct way of pole-zero adjustment is to examine the shape of the spectral peaks. If the peak shape shows high side distortion, then the circuit is over-compensated. If the peak shape shows count rate dependent low side distortion, then the circuit is under-compensated. If necessary, adjustments are made until the peak width is minimized.
In an alternate embodiment of the automatic pole-zero, the high pass filter, the pole-zero adjustment network, and the amplifier with feedback resistance are eliminated and the compensation accomplished directly in a programmable digital shaping filter which transforms the exponential pulse shape into the desired pulse shape thus eliminating the undershoot/overshoot.
REFERENCES:
patent: 4866400 (1989-09-01), Britton, Jr. et al.
patent: 4931653 (1990-06-01), Hamm et al.
patent: 5821533 (1998-10-01), Bingham et al.
patent: 5872363 (1999-02-01), Bingham et al.
patent: 5912825 (1999-06-01), Bingham
Nowlin, et al.—Elimination of Undesirable Undershoot in Operation and Testing of Nuclear Pulse Amplifiers Rev. Sci. Instr., vol. 36, No.2, Dec. 1965, pp 1830-1839.
Bingham Russell D.
Gedcke Dale A.
Trammell Rex C.
PerkinElmer Instruments, Inc.
Pitts & Brittian P.C.
Wachsman Hal
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