Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/20—Measuring radiation intensity with scintillation detectors
  • G01T1/208—Circuits specially adapted for scintillation detectors, e.g. for the photo-multiplier section
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/17—Circuit arrangements not adapted to a particular type of detector
  • G01T1/178—Circuit arrangements not adapted to a particular type of detector for measuring specific activity in the presence of other radioactive substances, e.g. natural, in the air or in liquids such as rain water
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/36—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry
  • G01T1/38—Particle discrimination and measurement of relative mass, e.g. by measurement of loss of energy with distance (dE/dx)

Definitions

• the present invention relates to an instrument for assaying radioactivity. More particularly, the invention relates to a photon detector having a solid scintillator and a single photomultiplier tube.
• the counting of nuclear disintegrations emitted from select specimens is frequently accomplished with scintillator materials, photomultiplier tubes and associated circuitry including a pulse height analy ⁇ zer.
• a specimen of radioactive material is admixed with a liquid scintillator or is placed in the vicinity of a solid scintillator in close proximity to a photomultiplier tube.
• the radioactive material decays, the emitted particles interact with the scintillator producing light pho ⁇ tons which are collected by the photomultiplier tube.
• the tube in turn provides an electric signal output and with suitable intermediate circuitry the pulse height analyzer provides an output count for each photomultiplier tube signal which meets some prese ⁇ lected amplitude requirements.
• An object of the present invention is to pro ⁇ vide radioactivity monitoring using only one photo- multiplier tube. Another object is to be able to discriminate between the electric signals resulting from the noise inherent in a photomultiplier tube and the signals resulting from the interaction of radioactive decay particles with a scintillator material. These goals are achieved by a novel pulse discrimination circuit employing a photon counting technique.
• the present invention is predicted on the acknowledgment that the output pulse shape from the photomultiplier tube monitoring the interaction be ⁇ tween a liquid scintillator and nuclear radiation is identical to the output from the photomultiplier tube due to random noise.
• the invention is also predicted on the recognition that the output signal from a photomultiplier tube monitoring a solid
• BUR O scintillator event is actually a burst or continuum of component pulses extending over an interval of time which is long compared to the time duration of a tube noise pulse.
• Each component pulse is the photomultiplier tube response to a single photon.
• a single photo ⁇ multiplier tube is combined with amplifiers, a photon pulse counter, timers, and a pulse height analyzer forming a new pulse discriminating circuit that discriminates between tube noise pulses and pulses representing genuine nuclear events by recog ⁇ nizing that pulses representing a genuine nuclear event occur in bursts and that tube noise pulses do not.
• this radiation assay in- stru ent includes a solid scintillator, a photo ⁇ multiplier tube whose output is split among a two- path circuit containing along a first path means for providing an integrated pulse signal as input to a pulse height analyzer and containing in a second path means for discrimination which is based on the number of component pulses in the signal and for providing an enable signal to the pulse height analyzer.
• the measurement of radiation with such equipment involves interacting radiation from the specimen with a solid scintillator material, cap ⁇ turing the light consequently emitted from the scintillator with a photomultiplier tube, buffering the resultant electric signal from the photomulti ⁇ plier tube, integrating such buffered signal in the first of two parallel circuits to provide a smooth contour signal as input to a pulse height analyzer, differentiating the buffered signal in the second of the two parallel circuits to reduce each such signal to a series of sequenced component pulses, producing an output count with the pulse height analyzer for those smooth contour signals which fall within a preselected voltage range and contain at least a minimum number of component pulses as dis ⁇ cerned by the burst identifier.
• One of the distinguishing features of the present invention is that the system requires only one photomultiplier tube.
• a solid scintillator is used to detect the radiation and this type scintillator produces a burst of photons which individually occur at a rate far in excess of the rate of any random pulses from the photomulti ⁇ plier tube due to noise.
• the result is an assay instrument with improved counting efficiency as well as minimum detection level which is particu ⁇ larly useful for assaying beta and weak gamma emit- ting radionuclides. Since the instrument requires only one photomultiplier tube, the overall system is correspondingly less complicated and lower cost.
• the system encompassing the invention requires no pre ixing of solvents.
• the radioactive solution passes around the solid scintillator permitting radionuclide decay to activate the solid scintilla ⁇ tor with subsequent bursts of light photons propor ⁇ tionate to the energy of the nuclide.
• the liquid stream containing the radionuclide is completely recovered unchanged. Thus, the sample is easily recovered.
• Fig. 1 is a curve showing the electric charge and duration relationship for a typical electric output signal from a photomultiplier tube due to
• Fig. 2 is a curve showing the electric charge and duration relationship for a typical electric output signal from a photomultiplier tube in re- sponse to the photons produced in a solid scintilla ⁇ tor by a nuclear disintegration;
• Fig. 3 is a simplified block diagram showing the main components of a radiation assay instrument using a single photomultiplier tube in accordance with the present invention.
• Fig. 4 is a simplified high pressure flow cell containing solid scintillator material in accordance with the present invention.
• the present invention allows a radiation moni ⁇ tor to operate with the simplicity of one photomul ⁇ tiplier tube and avoids the problems of -timing the leading edge to zero crossing of pulses as is re ⁇ quired in the Colmenares et al and Landis et al articles.
• Random pulses from a photomultiplier tube are commonly referred to as tube noise and appear as electric pulses approximately ten nanoseconds in duration such as is shown in Fig. 1. While the pulses do occur randomly, the time between pulses is typically in the neighborhood of one hundred thousand nanoseconds.
• the photomultiplier tube output due to a photon burst from the scintillator is typically as is shown in Fig. 2.
• the photon pulse burst is characteristically spread over an interval of sev ⁇ eral microseconds. This characteristic is used to advantage in accordance with the present invention in a system having a solid scintillator material in the form of fine granules in a high pressure flow
• a flow cell 10 having suitable reflector means and packed with solid scintillator material is lo ⁇ cated adjacent to a photomultiplier tube 12 which feeds a buffer 14.
• the scintillator produces opti ⁇ cal pulses 16 which impinge on the photomultiplier 12 and result in an electric charge output signal 18 which is converted to a voltage signal with en ⁇ hanced drive capability by the buffer 14 and be ⁇ comes buffered signal 20.
• the buffered signal is split in two and follows a first parallel path 22 which comprises a differentiator 24 a first ampli- fier 26, a threshold detector 28 and a burst iden ⁇ tifier 30 and a second parallel path 32 which com ⁇ prises an integrator 34, a lead network 36 and a second amplifier 38.
• the differentiator looks at the buffered sig- nal and breaks it down into component or photon response pulses 40 which are amplified and input to the threshold detector. Pulses that exceed a certain threshold voltage emerge as digital output pulses 42. The burst identifier then interrogates the threshold detector output and each time an ar ⁇ bitrarily preselected number of pulses occurs with ⁇ in a somewhat arbitrarily preselected time duration, an enable signal 44 is passed to a pulse height analyzer 46. In accordance with a preferred em- bodiment of the present invention three pulses oc ⁇ curring within a seven hundred fifty nanosecond duration will precipitate an enable signal indi ⁇ cating the occurrence of a genuine nuclear event. This preselected time interval is not absolute and may be increased or decreased in duration by a few hundred nanoseconds with no functional deviation
• OM from the present invention.
• two pulses or four or more pulses occurring during the prescribed interval will work although not as well.
• interrogation of the threshold detector may result in a continuous or near continuous signal which is longer than the response of the threshold detector to a single pho- ton pulse and is caused by overlapping photon pulses.
• the continuous signal may have a duration of approximately two hundred to four hundred nanoseconds.
• the burst identifier will provide an enable signal to the pulse analyzer.
• the integrator looks at the buffered signal and by an integration process produces a smooth contoured integrated pulse 48.
• the inte ⁇ grated pulse is then shaped into a bipolar pulse by lead network 36 and after being amplified by the amplifier 38 emerges as an amplified bipolar pulse 50 which is fed to the pulse height analyzer 46.
• the pulse analyzer receives an enable signal 44 and a corresponding bipolar pulse 50 which satisfies the pulse height criteria set in the ana ⁇ lyzer an output count 52 is produced.
• Each output count corresponds to a nuclear disintegration of interest which caused an optical pulse 16 from the scintillator and is transmitted to the microproces- sor circuitry for proper analysis of the data.
• a flow cell 54 for use with the present inven ⁇ tion is shown in Fig. 4.
• the cell comprises a glass-walled centerpiece 56 having a filter element 58 at each end as is filled with a fine mesh solid scintillator material 60 such as calcium fluoride
• Inlet tube 62 and outlet tube 64 deliver and carry away respectively the fluid containing the radioactive material of interest.
• Typical dimensions for the cell are two inches in length, eight millimeters outside diameter and a two to five millimeter in ⁇ side diameter.
• the tubes are metal with chroma- tographic grade end fittings and filters and the scintillator particles are smaller than one hundred eighty mesh. With this apparatus a flow rate of one quarter to two milliliters per minute is main ⁇ tained and results in a pressure drop of fifty to three hundred pounds per square inch between the filter elements.
• the flow cell is used in co junc- tion with a high efficiency reflector to maximize the photon flux which reaches the photomultiplier tube.
• the system may also be utilized to measure radioactivity in gas streams.
• the invention has been described in terms of calcium fluoride and glass scintillator although the concept is workable with sodium iodide or any other long decay time scintillator material solid or liquid, i.e., one having an output photon burst which is long in comparison with the pulse interval of photomultiplier tube noise.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• General Physics & Mathematics (AREA)
• High Energy & Nuclear Physics (AREA)
• Molecular Biology (AREA)
• Measurement Of Radiation (AREA)
• Nuclear Medicine (AREA)

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• 1983
  • 1983-09-27 WO PCT/US1983/001479 patent/WO1985001584A1/fr not_active Application Discontinuation
  • 1983-09-27 DE DE1983903332 patent/DE156801T1/de active Pending
  • 1983-09-27 EP EP19830903332 patent/EP0156801A4/fr not_active Withdrawn
• 1985
  • 1985-05-24 FI FI852092A patent/FI852092L/fi not_active Application Discontinuation

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