Apparatus for reducing pulse pileup in an elemental analyzer measuring gamma rays arising from neutron capture in bulk substances

• US 4,152,596 A
• Filed: 07/05/1977
• Issued: 05/01/1979
• Est. Priority Date: 07/05/1977
• Status: Expired due to Term

• Current Assignee: MDH Industries Incorporated
• Original Assignee: MDH Industries Incorporated
• Inventors: Marshall, J. Howard III
• Primary Examiner(s): Willis, Davis L.
• Application Number:
  US05/812,769
• Time in Patent Office: 665 Days
• Field of Search: 250/390, 250/391, 250/392, 250/302, 250/303, 250/308, 250/270, 250/362, 250/358 R
• US Class Current: 250/358.1
• CPC Class Codes: G01N 2223/0745 neutron-gamma activation analysis
• CPC Class Codes: G01N 2223/304 electric circuits, signal processing
• CPC Class Codes: G01N 2223/617 ash in coal
• CPC Class Codes: G01N 2223/635 fluids, granulates
• CPC Class Codes: G01N 23/222 using neutron activation analysis [NAA]
• CPC Class Codes: G01T 1/171 Compensation of dead-time counting losses see provisionally also G01T1/17

Abstract

The active reduction of the number of analyzed events with pulse amplitudes which pileup has distorted improves measurement accuracy and response time in an apparatus for neutron-capture-based on-line elemental analysis of bulk substances. Within the apparatus, the analyzed bulk substance is exposed to neutrons, and neutron capture generates prompt gamma rays therefrom. A detector interacts with some of these gamma rays to produce electrical signals used to measure their energy spectrum by pulse-height analysis. Circuits associated with this pulse-height analysis also detect the pileup of the signals of two or more independent gamma rays using one or more of several techniques. These techniques include multiple outputs from a special amplifier-discriminator system, which has been optimized for low pulse-pair resolving time and may have adaptive thresholds, and the requirement that the relative amplitudes of the outputs of slow and fast amplifiers be consistent with a single event producing both outputs. Pulse-width measurements are also included in the pileup detection.

Claims

1. In an apparatus for the on-line analysis of the composition of a bulk substance, wherein said analysis includes the production and capture of neutrons and the detection and measurement of the energy spectrum of the resulting capture gamma rays, an improvement for reducing the deleterious effects of pulse pileup, the improvement comprising, in combination:

(a) a gamma-ray detector operably associated with the bulk substance which is exposed to neutrons to generate capture gamma rays, the detector producing electrical signals indicative of the energies of the gamma rays to provide for the measurement of their energy spectrum;

(b) slow-amplifier means connected to the output of the gamma-ray detector, the slow-amplifier means producing a pulse of sufficient width that satisfactory resolution can be obtained in the energy-spectrum measurement;

(c) fast-amplifier means also connected to the output of the gamma-ray detector, the fast-amplifier means producing a pulse with a width chosen to optimize the detection of pulse pileup; and

(d) amplitude-correlation means connected to the outputs of the slow-amplifier means and of the fast-amplifier means, the amplitude-correlation means detecting an event where the amplitude of the pulse produced by the slow-amplifier means is inconsistent with the amplitude of the pulse produced by the fast-amplifier means,

whereby events with pulse amplitudes which pileup has distorted can be eliminated from the measurement of the energy spectrum of the capture gamma rays.

2. The apparatus of claim 1, above, wherein the amplitude-correlation means comprises first means for pulse-amplitude discrimination, the first means being connected to the output of the fast-amplifier means in order to indicate when the output pulse from the fast-amplifier means exceeds any one of a plurality of amplitude thresholds,

whereby the amplitude of the output pulse from the fast-amplifier means can be known within the limits defined by the thresholds of the first means for pulse-amplitude discrimination.

3. The apparatus of claim 2, above, wherein the amplitude-correlation means further comprises second means for pulse-amplitude discrimination, the second means being connected to the output of the slow-amplifier means in order to indicate when the output pulse from the slow-amplifier means exceeds any one of a plurality of amplitude thresholds,

whereby the amplitude of the output pulse from the slow-amplifier means can be known within the limits defined by the thresholds of the second means for pulse-amplitude discrimination.

4. The apparatus of claim 3, above, wherein the amplitude-correlation means further comprises digital-logic means connected to the outputs of the first and second means for pulse-amplitude discrimination, the digital-logic means indicating those events for which the outputs of the first and second means for pulse-amplitude discrimination are inconsistent with a single output pulse from the gamma-ray detector,

whereby pulse pileup can be detected.

5. The apparatus of claim 1, above, further comprising means for pulse-width measurement connected to the output of the fast-amplifier means, the width-measuring means producing an electrical signal which is indicative of the width of the pulse at the output of the fast-amplifier means,

whereby events for which pulse pileup causes the pulse from the fast-amplifier means to be lengthened excessively can be detected.

6. The apparatus of claim 1, above, further comprising means for indicating the presence of a plurality of pulses exceeding an amplitude threshold during a known time interval following the first pulse exceeding said threshold, the means for indicating the presence of a plurality of pulses being connected to the output of the fast-amplifier means,

whereby events including more than one pulse during the interval necessary to perform the energy measurement can be detected.

7. The apparatus of claim 5, above, further comprising means for indicating the presence of a plurality of pulses exceeding an amplitude threshold during a known time interval following the first pulse exceeding said threshold, the means for indicating the presence of a plurality of pulses being connected to the output of the fast-amplifier means,

whereby events including more than one pulse during the interval necessary to perform the energy measurement can be detected.

8. In an apparatus for the on-line analysis of the composition of a bulk substance, wherein said analysis includes the production and capture of neutrons and the detection and measurement of the energy spectrum of the resulting capture gamma rays, an improvement for reducing the deleterious effects of pulse pileup, the improvement comprising, in combination;

(a) a gamma-ray detector operably associated with the bulk substance which is exposed to neutrons to generate capture gamma rays, the detector producing electrical signals indicative of the energies of the gamma rays to provide for the measurement of their energy spectrum;

(b) fast-amplitude means connected to the output of the gamma-ray detector, the fast-amplifier means producing a pulse with a width chosen to optimize the detection of pulse pileup; and

(c) means for pulse-width measurement connected to the output of the fast-amplifier means, the width-measuring means producing an electrical signal which is indicative of the width of the pulse at the output of the fast-amplifier means,

whereby events for which pulse pileup causes the pulse from the fast-amplifier means to be lengthened excessively can be rejected in order to reduce in the measurement of the energy spectrum of the capture gamma rays the number of events with amplitudes which pileup has distorted.

9. The apparatus of claim 8, above, further comprising means for determining excessive pulse width connected to the output of the means for pulse-width measurement,

whereby pulse pileup can be detected.

10. The apparatus of claim 9, above, wherein the means for determining excessive pulse width has a variable threshold depending on pulse amplitude.

11. The apparatus of claim 8, above, wherein the means for pulse-width measurement comprises:

(a) a pulse-amplitude discriminator connected to the output of the fast-amplifier means, the discriminator producing at its output one digital state whenever the output pulse from the fast-amplifier means exceeds the discriminator threshold and the other digital state whenever the fast-amplifier output does not exceed the threshold;

(b) gating means with a first input connected to the output of the pulse-amplitude discriminator, the output of the gating means responding to its other inputs only when the pulse-amplitude discriminator indicates that the output pulse from the fast-amplifier means exceeds the discriminator threshold;

(c) clock-pulse-generating means connected to a second input of the gating means, the clock-pulse-generating means producing pulses at a known frequency; and

(d) counting means connected to the output of the gating means, the counting means determining the number of clock pulses which passed through the gating means during the interval when the output pulse from the fast-amplifier means exceeded the discriminator threshold,

whereby a digital signal is generated which is indicative of the width of the output pulse from the fast-amplifier means.

12. The apparatus of claim 11, above, further comprising second gating means connected to the outputs from the counting means, the second gating means producing a digital signal whenever the counting means receives more than a given number of clock pulses,

whereby pulses with widths exceeding a given threshold can be detected.

13. The apparatus of claim 12, above, wherein the second gating means comprises a plurality of thresholds for the number of clock pulses received by the counting means, the apparatus further comprising means for width-threshold selection connected to the second gating means, the means for width-threshold selection selecting the threshold for pileup detection based on the pulse amplitude,

whereby the pulse-width threshold for pileup detection can be optimized at a plurality of pulse amplitudes.

14. The apparatus of claim 8, above, further comprising:

(a) slow-amplifier means connected to the output of the gamma-ray detector, the slow-amplifier means producing a pulse of sufficient width that satisfactory resolution can be obtained in the energy-spectrum measurement; and

(b) amplitude-correlation means connected to the outputs of the slow-amplifier means and the fast-aamplifier means, the amplitude-correlation means detecting an event where the amplitude of the pulse produced by the slow-amplifier means is inconsistent with the amplitude of the pule produced by the fast-amplifier means,

whereby pulse-pileup detection can be improved.

15. The apparatus of claim 8, above, further comprising means for indicating the presence of a plurality of pulses exceeding an amplitude threshold during a known time interval following the first pulse exceeding said threshold, the means for indicating the presence of a plurality of pulses being connected to the output of the fast-amplifier means,

whereby events including more than one pulse during the interval necessary to perform the energy measurement can be detected.

16. The apparatus of claim 14, above, further comprising means for indicating the presence of a plurality of pulses exceeding an amplitude threshold during a known time interval following the first pulse exceeding said threshold, the means for indicating the presence of a plurality of pulses being connected to the output of the fast-amplifier means,

whereby events including more than one pulse during the interval necessary to perform the energy measurement can be detected.

17. In an apparatus for the on-line analysis of the composition of a bulk substance, wherein said analysis includes the production and capture of neutrons and the detection and measurement of the energy spectrum of the resulting capture gamma rays, an improvement for reducing the deleterious effects of pulse pileup, the improvement comprising, in combination:

(a) a gamma-ray detector operably associated with the bulk substance which is exposed to neutrons to generate capture gamma rays, the detector producing electrical signals indicative of the energies of the gamma rays to provide for the measurement of their energy spectrum;

(b) fast-amplifier means connected to the output of the gamma-ray detector, the fast-amplifier means producing a pulse with a width chosen to optimize the detection of pulse pileup; and

(c) means for indicating the presence of a plurality of pulses exceeding an amplitude threshold during a known time interval following the first pulse exceeding said threshold, wherein the threshold depends upon the measured pulse amplitude, the means for indicating the presence of a plurality of pulses being connected to the output of the fast-amplifier means,

whereby events including more than one pulse during the interval necessary to perform the energy measurement can be detected in order to reduce in the measurement of the energy spectrum of the capture gamma rays the number of events with amplitudes which pileup has distorted.

18. The apparatus of claim 17, above, wherein the means for indicating the presence of a plurality of pulses exceeding a variable amplitude threshold comprises:

(a) first means for pulse-amplitude discrimination connected to the output of the fast-amplifier means, the first means producing a plurality of output signals whenever the output pulse from the fast-amplifier means exceeds a plurality of amplitude thresholds;

(b) multiple-pulse-detection means connected to the plurality of outputs of the first means for pulse-amplitude discrimination, the multiple-pulse-detection means producing a separate output signal for each output from the first means for pulse-amplitude discrimination which has two or more pulses occuring within a known time after the first detected pulse;

(c) means operably associated with the gamma-ray detector for determining the amplitude of the signal at the detector output; and

(d) detection-inhibiting means connected to the outputs from the multiple-pulse-detection means and to the output of the means for determining the amplitude of the detector signal, the detection-inhibiting means causing multiple pulses corresponding to specific amplitude thresholds depending on the amplitude of the signal at the detector output to be ignored,

whereby the amplitude threshold for multiple-pulse detection can be changed based on the measured signal amplitude.

19. The apparatus of claim 18, above, wherein the means for determining the amplitude of the signal at the detector output comprises:

(a) slow-amplifier means connected to the output of the gamma-ray detector, the slow-amplifier means producing a pulse of sufficient width that satisfactory resolution can be obtained in the energy-spectrum measurement; and

(b) second means for pulse-amplitude discrimination connected to the output of the slow-amplifier means, the second means indicating when the output pulse from the slow-amplifier means exceeds any one of a plurality of amplitude thresholds,

whereby the amplitude of the output signal from the detector can be known within the limits defined by the thresholds of the second means for pulse-amplitude discrimination.

20. The apparatus of claim 17, above, wherein the amplitude threshold for multiple-pulse detection changes after a signal from the gamma-ray detector has been registered.

21. The apparatus of claim 17, above, further comprising:

(a) slow-amplifier means connected to the output of the gamma-ray detector, the slow-amplifier means producing a pulse of sufficient width that satisfactory resolution can be obtained in the energy-spectrum measurement; and

(b) amplitude-correlation means connected to the outputs of the slow-amplifier means and of the fast-amplifier means, the amplitude-correlation means detecting an event where the amplitude of the pulse produced by the slow-amplifier means is inconsistent with the amplitude of the pulse produced by the fast-amplifier means,

whereby pulse-pileup detection can be improved.

22. The apparatus of claim 17, above, further comprising means for pulse-width measurement connected to the output of the fast-amplifier means, the width-measuring means producing an electrical signal which is indicative of the width of the pulse at the output of the fast-amplifier means,

whereby events for which pulse pileup causes the pulse from the fast-amplifier means to be lengthened excessively can be detected.

23. The apparatus of claim 21, above, further comprising means for pulse-width measurement connected to the output of the fast-amplifier means, the width-measuring means producing an electrical signal which is indicative of the width of the pulse at the output of the fast-amplifier means,

whereby events for which pulse-pileup causes the pulse from the fast-amplifier means to be lengthened excessively can be detected.