Method and apparatus for the detection of neutrons and gamma rays

A pulse discrimination method for discriminating between pulses having a short decay period and a long decay period, may comprise: Detecting the pulse; integrating a rise portion of the pulse; integrating a decay portion of the pulse; and comparing the integrated rise portion of the pulse with the integrated decay portion of the pulse to distinguish between a pulse having a long decay period and a pulse having a short decay period.

FIELD OF INVENTION

This invention relates to radiation detectors in general and more specifically to a radiation detector for detecting neutrons and gamma rays.

BACKGROUND

Radiation detectors for detecting high energy photons (e.g., gamma (γ) rays and X-rays) are well-known in the art and are used to detect high energy photons produced by any of a wide range of radioactive materials or other types of samples. The detection, identification, and spectroscopy of such energetic photons comprises an integral part of the fields of nuclear and particle physics as well as several fields that make use of radioactivity, including, for example, medicine, forensic science, and industrial inspection applications. Radiation detectors are also used at nuclear power plants and laboratories to monitor and study radiation.

Ionizing radiation detectors, such as gamma (γ) ray detectors, can be classified into one of two types depending on the apparatus that is used to detect the high energy photons. The first type, referred to herein as “gas tube” or simply “gas” detectors utilizes a gas-filled chamber or tube which contains a positively charged wire. When a high energy photon enters the chamber it may ionize a gas atom, causing it to release an electron or electrons in the process. The liberated electron or electrons may in turn ionize additional gas atoms, which liberate yet more electrons. The liberated electrons are collected by the positively charged wire. A detection circuit connected to the wire measures the charged delivered to the wire by the electrons. Generally speaking, the higher the energy of the incoming photon, the more atoms are ionized and the more electrons are liberated. Therefore, the magnitude of the detected charge is generally related to the energy of the incoming photon.

Solid state detectors are similar to gas detectors described above except that the active volume (i.e., the gas) is replaced by a semiconducting material, such as germanium, although other materials may be used. Accordingly, both types of detectors have in common the property that they use the energy of the incoming photon to ionize an atom of some material. Generally speaking, solid state detectors provide superior sensitivity and resolution compared with gas tube detectors, although both types remain in use.

Besides high energy photons, radiation can also comprise high energy particles, such as alpha (α) particles, beta (β) particles, and neutrons (n). Such high energy particle-type radiation is usually detected by other types of detectors. For example, neutrons are typically detected by using a radiator or converter which absorbs incoming neutrons and radiates charged particles. The radiated particles may then be detected by means of an ionizing type radiation detector of the type described above.

While radiation detectors for detecting high energy photons (e.g., gamma rays) and high energy particles (e.g., neutrons) exist and are being used, they are not without their problems. For example, a problem with prior art neutron detectors relates to the sensitivity of the detectors to gamma rays. Consequently, it is difficult for such detectors to discriminate (i.e., differentiate) between gamma rays and neutrons. Since both gamma and neutron radiation must be separately measured in order to accurately measure the radiation field, such neutron detectors are not particularly useful in accurately characterizing the radiation field.

One way to solve the problem of simultaneously measuring both gamma and neutron radiation is to utilize two separate detectors, one optimized for gamma ray detection and the other optimized for neutron detection. While such dual detector systems are known and have been used, they tend to be bulky, heavy, and difficult to carry. In addition, such devices tend to consume a fair amount of electrical power, thus limiting their usefulness, particularly in portable applications. While smaller, more portable detectors exist, they are typically only responsive to one type of radiation. Therefore, a user must carry two separate detectors if it is desired to monitor both gamma radiation and neutron radiation.

SUMMARY OF THE INVENTION

A pulse discrimination method for discriminating between pulses having a short decay period and a long decay period, may comprise: Detecting the pulse; integrating a rise portion of the pulse; integrating a decay portion of the pulse; and comparing the integrated rise portion of the pulse with the integrated decay portion of the pulse to distinguish between a pulse having a long decay period and a pulse having a short decay period.

A radiation detector for discriminating between gamma rays and neutrons may comprise a detector for producing pulses in response to gamma rays and neutrons. A pulse discriminator operatively associated with the detector integrates a rise portion of a pulse and a decay portion of the pulse. The pulse discriminator compares the integrated rise portion of the pulse with the integrated decay portion of the pulse to determine whether the pulse was produced by the detector in response to a gamma ray or a neutron.

DETAILED DESCRIPTION OF THE INVENTION

Apparatus10for detecting neutrons n and gamma rays γ according to one embodiment of the invention is illustrated in FIG.1and may comprise a detector12, a pulse discriminator system14, and a user interface system16. The detector12detects neutrons n and gamma rays γ and produces an output signal18related thereto. More specifically, the output signal18produced by the detector12comprises a series of short decay period pulses, such as pulse20(FIG.2), and long decay period pulses, such as pulse22(FIG.2). Whether the detector12produces a short decay period pulse20or a long decay period pulse22depends on the particular type of radiation that is sensed or detected by the detector12. For example, in the embodiment shown and described herein, a short decay period pulse20is produced by the detector12in response to a gamma ray γ, whereas a long decay period pulse22is produced by the detector12in response to a neutron n.

The pulse discriminator system14is connected to the detector12and is responsive to the output signal18produced by the detector12. The pulse discriminator system14processes each pulse (e.g., a short decay period pulse20or a long decay period pulse22, whichever type of pulse is being processed) in accordance with a method24(FIG. 3) in order to determine whether the pulse is a short decay period pulse20or a long decay period pulse22. Of course, the ability to determine whether the pulse is a short decay period pulse20or a long decay period pulse22is indicative of the type of radiation detected by the detector12. That is, a short decay period pulse20means the detector12detected a gamma ray γ, whereas a long decay period pulse22means the detector12detected a neutron n. In the embodiment shown and described herein, the user interface system16is used to produce for a user (not shown) an indication of whether the detected pulse was a gamma ray γ (i.e., the detected pulse was a short period pulse20) or a neutron n (i.e., the detected pulse was a long period pulse22). Therefore, in the embodiment shown and described herein, the apparatus10produces for the user an indication of whether the detector12detected a gamma ray γ or a neutron n.

As briefly mentioned above, the pulse discriminator system14may operate in accordance with the method or process24(FIG. 3) in order to determine whether the pulse is a short period pulse20or a long period pulse22, thus whether the detector12detected a gamma ray γ or a neutron n (FIG.1). Referring now toFIGS. 2 and 3simultaneously, a first step26in the method24involves detecting a pulse (e.g., a pulse20or22) that is to be discriminated. In the embodiment shown and described herein, the step26of detecting a pulse involves receiving the output signal18produced by the detector12and detecting a pulse contained in the output signal18. Once the pulse has been detected, the pulse discriminator system14operates to integrate a rise portion of the pulse at step28.

However, before proceeding with the description, it should be noted that the shape or profile of the rise portion of the pulse may vary depending on whether the pulse is a short decay period pulse20or a long decay period pulse22. For example, and with reference now specifically toFIG. 2, a rise portion30of a short decay period pulse20comprises that portion of the short decay period pulse20that extends from about an initiation threshold32of short decay period pulse20to about a peak point34of short decay period pulse20. As will be described in greater detail below, the initiation threshold32is defined by a beginning time36of a short gate signal38, whereas the peak point34is defined by an ending time40of short gate signal38.

In the particular embodiment shown and described herein, the shape or profile of the rise portion42of the long decay period pulse22is somewhat different than that of the rise portion30of short decay period pulse20due to the elongated nature of long decay period pulse22compared with the short decay period pulse20. This is because an initiation threshold44and peak point46of long decay period pulse22are also defined by the beginning time36and ending time40of short gate signal38. Therefore, the peak46of long decay period pulse22may not necessarily coincide with the actual peak46′ (i.e., point of maximum value) of long period pulse22, although it may, depending on the shape or profile of the particular peak being processed.

Proceeding now with the description, the pulse discriminator system14integrates the rise portion of the pulse at step28. As described above, the rise portion of the pulse (i.e., either the rise portion30of short decay period pulse20or the rise portion42of long decay period pulse22, whichever pulse is being processed) is defined by the beginning time36and ending time40of short gate signal38. Thus, the integration of the pulse involves integrating that portion of the pulse between the initiation threshold (i.e., either initiation threshold32of short decay period pulse20or the initiation threshold44of long decay period pulse22) and the peak (i.e., either peak point34of short decay period pulse20or peak point46of long decay period pulse22, as the case may be).

The particular integration algorithm or technique that is used by the pulse discriminator system14to integrate the rise portion of the pulse (i.e., either the rise portion30of pulse20or the rise portion42of pulse22) is not critical to the invention, and any of a wide range of integration algorithms, processes, or devices that are now known in the art or that may be developed in the future may be used to integrate the rise portion of the pulse (i.e., either rise portion30of pulse20or rise portion42of pulse22). In any event, the integration process will yield an integrated rise value86. Integrated rise value86may then be stored for later use in a suitable memory system (not shown) or other such device associated with a data processor70operatively associated with the pulse discriminator system14.

The next step48in the process or method24involves integrating a decay portion of the pulse. In this regard it should be noted that, as was the case for the rise portion of the pulse, the shape or profile of the decay portion of the pulse may vary depending on whether the pulse is a short decay period pulse20or a long decay period pulse22. For example, and with reference again toFIG. 2, a decay portion50of a short decay period pulse20comprises that portion of the short decay period pulse20that extends from about a peak point52of short decay period pulse20to about a cut-off point54of short decay period pulse20. The peak point52is defined by a beginning time56of a long gate signal58, whereas the cut-off point54is defined by an ending time60of long gate signal58. Because the peak point52of short decay period pulse20is defined by the beginning time56of long gate signal58, the peak point52may not necessarily coincide with the actual peak34(i.e., point of maximum value) of the short decay period pulse20, although it may, again depending on the particular shape or profile of the peak being processed.

The shape or profile of the decay portion62of the long decay period pulse22is somewhat different than that of the decay portion50of short decay period pulse20in that a peak point64and cut-off point66of long decay period pulse22are also defined by the beginning time56and ending time60of long gate signal58. That is, due to the elongated nature of long decay period pulse22compared with the short decay period pulse20, the peak point64of long decay period pulse22coincides more closely with the actual peak46′ (i.e., point of maximum value) of long decay period pulse22than was the case for the short decay period pulse20.

The pulse discriminator system14integrates the decay portion of the pulse at step48. As described above, the decay portion of the pulse (i.e., either decay portion50of short decay period pulse20or decay portion62of long decay period pulse22, as the case may be) is defined by the beginning time56and ending time60of long gate signal58. Thus, the integration of the pulse involves integrating that portion of the pulse between the peak point (i.e., either peak point52of short decay period pulse20or the peak point64of long decay period pulse22) and the cut-off point (i.e., either cut-off point54of short decay period pulse20or cut-off point66of long decay period pulse22).

The particular algorithm or technique used by the pulse discriminator system14to integrate the decay portion of the pulse (i.e., either pulse20or22, as the case may be) is not critical to the invention, and any of a wide range of integration algorithms, processes, or devices that are now known in the art or that may be developed in the future may be used to integrate the decay portion of the pulse (i.e., either decay portion50of pulse20or decay portion62of pulse22, as the case may be). In the embodiment shown and described herein, the integration process used to integrate the decay portion of the peak is the same as that used to integrate the rise portion of the peak. The integration process yields an integrated decay value88, which may then be stored in a suitable memory system (not shown) or other such device associated with the data processing system70of the pulse discriminator system14.

In order to determine whether the peak contained in the output signal18from detector12comprises a short decay period peak20or a long decay period peak22, that is, in the example of the particular embodiment shown and described herein, whether the peak was produced as the result of a gamma ray γ or a neutron n, the pulse discriminator system14compares the integrated rise value86with the integrated decay value88at step68(FIG.3). The comparison may be based on the characteristics for the short decay period peak20and the long decay period peak22illustrated in FIG.4and described immediately below.

FIG. 4is a graphical representation of normalized integrated values of the rise portions and decay portions (identified inFIG. 4as the “integrated rise value” and “integrated decay value” axes, respectively) for both the short decay period pulses20(i.e., the locus of points generally forming the line adjacent the designation “gamma rays” inFIG. 4) and the long decay period pulses22(i.e., the locus of points generally forming the line adjacent the designation “neutron” in FIG.4). As can be seen fromFIG. 4, the slope of the line formed by the locus of points for neutrons (i.e., long decay period pulses22) is steeper than the line formed by the locus of points for gamma rays (i.e., short decay period pulses20). Accordingly, by comparing the integrated value86of the rise portion of the pulse (i.e., portion30of short decay period pulse20or portion42of long decay period pulse22) with the integrated value88of the decay portion of the pulse (i.e., portion50of short decay period pulse20or portion62of long decay period pulse22), a determination may be made as to whether the pulse more closely fits the line (i.e., locus of points) associated with the short decay period pulse20(i.e., “gamma ray” line inFIG. 4) or the long decay period pulse22(i.e., the “neutron” line in FIG.4).

More specifically, in the embodiment shown and described herein, an artificial line of separation90is constructed between the “gamma rays” line and the “neutrons line” in the manner illustrated in FIG.4. As will be described in greater detail below, the artificial line of separation90is selected to be the dividing line between points that will be deemed to be associated with short decay period pulses (i.e., pulses resulting from gamma rays) and long decay period pulses (i.e., pulses resulting from neutrons). Graphically, points below the line of separation90will be deemed to be short decay pulses, whereas points above the line of separation90will be deemed to be long decay pulses.

The data processing system70may determine whether the pulse is a short decay period pulse (i.e., below the line of separation90) or a long decay period pulse (i.e., above the line of separation90) by multiplying the integrated value86for the rise portion of the pulse by the slope of the line of separation90. To this product is added the intercept of the line of separation90(i.e., the point where the line of separation90intersects the “integrated decay value” axis) to yield a “calculated decay value.” If the integrated decay value88is less then the calculated decay value, then the detected peak was a short decay period peak. Conversely, if the integrated decay value88is greater than the calculated decay value, then the detected peak was a long decay period peak. Thereafter, processed data72indicative of whether the detected pulse was a short decay period pulse20or a long decay period pulse22may be directed to the user interface16which may provide a suitable indication for a user (not shown) regarding the identification of the detected pulse.

A significant advantage of the method and apparatus for detecting neutrons and gamma rays according to the present invention is that it provides, in a single, highly portable unit, the ability to detect and display whether the radiation detected by the detector12comprises gamma radiation or neutrons. Consequently, the present invention dispenses with the need for the user to carry multiple radiation detectors.

Another advantage of the present invention is that it is capable of distinguishing between short decay period pulses20and long decay period pulses22produced by the detector12for gamma rays γ and neutrons n of low energy levels without having to increase the gain of the detector at such low energy levels. For example, in one preferred embodiment, the present invention is able to reliably distinguish between gamma rays and neutrons having energies as low as 50 kilo-electron-volts equivalent (keVee). Traditional pulse discrimination techniques are typically unable to resolve gamma rays and neutrons below about 300 keVee without raising the gain of the detector.

Having briefly described one embodiment of the method and apparatus of the present invention, as well as some of its more significant features and advantages, the various embodiments of the method and apparatus for the detection of neutrons and gamma rays according to the present invention will now be described in detail.

Referring back now toFIG. 1, one embodiment of apparatus10for detecting neutrons n and gamma rays γ may comprise a detector12, a pulse discriminator system14, and a user interface system16. The detector12may comprise any of a wide range of radiation detectors now known in the art or that may be developed in the future suitable for detecting the types of radiation desired to be detected and that produces as an output signal18pulses having short decay periods and pulses having long decay periods, depending on the type of radiation detected. By way of example, in one preferred embodiment, the detector12comprises a xylene liquid scintillation detector available from Saint-Gobain Crystals & Detectors (formerly Bicron, Inc.) of Valley Forge, Pa. as model number BC501. As mentioned above, this liquid scintillation detector detects neutrons n and gamma rays γ and produces an output signal18related thereto. More specifically, a short decay period pulse20is produced by the detector12in response to a gamma ray γ, whereas a long decay period pulse22is produced by the detector12in response to a neutron n.

The pulse discriminator system14is connected to the detector12and is responsive to the output signal18produced by the detector12. In the embodiment shown and described herein, the pulse discriminator system14comprises a linear fan-out or signal splitter74that receives the output signal18from the detector and distributes the signal to a constant fraction discriminator76, a first integrator78, and a second integrator80. The linear fan-out74may comprise any of a wide range of devices now known in the art or that may be developed in the future that are or would be suitable for receiving the output signal18from the detector12and distributing the signal to the various devices in the manner described herein. Consequently, the present invention should not be regarded as limited to any particular device. However, by way of example, in one preferred embodiment, the linear fan-out comprises a Quad Linear Fan-In/Fan-Out available from Phillips Scientific of Ramsey, N.J., as model no. 740.

The constant fraction discriminator76receives the output signal18from the detector12via the linear fan-out74and generates the short gate signal38and long gate signal58illustrated in FIG.2. The constant fraction discriminator76allows the user to select certain parameters for the short gate signal38and long gate signal58. As briefly described above, the short gate signal38defines the rise portion of the pulse (i.e., either the rise portion30of short decay period pulse20or the rise portion42of long decay period pulse22, whichever pulse is being processed). Therefore, the selection of the beginning time36and the ending time40of the short gate signal38defines the rise portion of the pulse that will later be integrated. Consequently, the user will select the beginning time36and ending time40of the short gate signal38to define the proper rise portions for the types of pulses to be integrated. By way of example, in one preferred embodiment, the time between the beginning time36and ending time40of short gate signal38is selected to be in the range of about 8 nanoseconds to about 30 nanoseconds (10 nanoseconds preferred). However, other times may be used depending on the nature and duration of the particular pulses that are to be discriminated.

Before proceeding with the description, it should be noted that the actual duration of the short gate signal38may actually be longer depending on the particular operational characteristics of the various devices comprising the pulse discriminator system14. For example, in the embodiment shown and described herein, the integrators78and80require a “lead” time of about 20 nanoseconds before they start integrating. Therefore, the actual durations of the short gate signal38and long gate signals58are increased by this amount. However, because this first 20 nanoseconds is only required to provide the required advance or lead time for the integrators78and80, the lead times do not extend the rise portion or the decay portion of the pulse. Accordingly, the lead time that is built into the short gate pulse38is not shown in FIG.2. This additional lead time (e.g., 20 nanoseconds) may be compensated for by the delay circuit84in the manner to be described below. That is, the delay circuit84may be made to delay the pulse signals by an additional 20 nanoseconds so that the proper portions of the pulses are made to coincide (on a time basis) with the short and long gate signals38and58, as best seen in FIG.2.

Continuing now with the description, the constant fraction discriminator74may also be programmed to initiate the beginning time36at some suitable threshold level for the pulse being detected. Generally speaking, the threshold level at which the constant fraction discriminator74sets the beginning time36should be above the expected level of noise contained in the output signal18from the detector12in order to avoid processing false pulses. However, because the particular threshold level will vary depending on the particular components utilized in a given application, and could be easily determined by persons having ordinary skill in the art after having become familiar with the teachings provided herein, the present invention should not be regarded as limited to any particular threshold level.

The long gate signal58is also produced by the constant fraction discriminator74. Because the long gate signal58defines the decay portion of the pulse (i.e., either the decay portion50of short decay period pulse20or the decay portion62of long decay period pulse22), the selection of the beginning time56and the ending time60of the long gate signal58defines, in part, the decay portion of the pulse that will later be integrated. As will be described below, the decay portion of the pulse is also defined by the time by which the long gate signal58lags the short gate signal38. Consequently, the user will select the interval between the beginning time56and ending time60of the long gate signal58to define the proper decay portions for the types of pulses to be integrated. By way of example, in one preferred embodiment, the time or interval between the beginning time56and ending time60of long gate signal58is selected to be in the range of about 100 nanoseconds to about 300 nanoseconds (200 nanoseconds preferred). However, other times may be used depending on the nature and duration of the particular pulses that are to be discriminated. As was the case for the short gate signal38, the duration of the long gate signal58is extended by the additional lead time (e.g., 20 nanoseconds) required by the second integrator80in order to begin integrating. However, this additional lead time is not shown in the drawings.

In the embodiment shown and described herein the time or interval between the beginning time36of the short gate signal38and the beginning time56of the long gate signal58is controlled by a delay circuit82. Thus, the amount of delay imposed by the delay circuit82is also important in defining the decay portion of the pulse that will later be integrated. Consequently, the user will also select the delay or interval between the beginning time36of the short gate signal38and the beginning time56of the long gate signal58to complete the definition of the proper decay portion for the types of pulses to be integrated. By way of example, in one preferred embodiment, the delay or interval between the beginning time36of the short gate signal38and the beginning time56of the long gate signal58is selected to be in the range of about 15 nanoseconds to about 40 nanoseconds (20 nanoseconds preferred). However, other times may be used depending on the nature and duration of the particular pulses that are to be discriminated.

The delay circuit82may comprise any of a wide range of delay circuits now known in the art or that may be developed in the future that are or would be suitable for use in the particular application. In addition, because such a delay circuit82could be easily provided by persons having ordinary skill in the art after having become familiar with the teachings provided herein, the delay circuit82utilized in one preferred embodiment will not be described in further detail herein.

The short gate signal38and long gate signal58are then fed to respective first and second integrators78and80. The first and second integrators78and80are also connected to the linear fan-out74and receive the output signal18from the detector12via the linear fan-out74. In the embodiment shown and described herein, there is a delay associated with the operation of the constant fraction discriminator76in producing the short and long gate signals38and58. There is also a 20 nanosecond delay associated with the operation of the first and second integrators78and80, as described above. Consequently, a second delay circuit84is positioned between the linear fan-out74and first and second integrators78and80. The second delay circuit84delays the pulses contained in the output signal18by an amount identical to the delay imposed by the constant fraction discriminator76so that the pulse (i.e., short decay period pulse20or long decay period pulse22), and any other delays (e.g., the 20 nanosecond lead time requirement associated with the integrators78and80) is properly temporally aligned with the short and long gate signals38and58, in the manner best seen in FIG.2. Because the amount of delay required to be imposed by the second delay circuit84is dependent on the amount of delay imposed by the constant fraction discriminator76in producing the short and long gate signals38and58, the lead time required by the first and second integrators78and80, and any other devices, persons having ordinary skill in the art will be able to readily ascertain the amount of delay required to be imposed by the second delay circuit84after considering the particulars of the specific implementation of the invention. Consequently, the present invention should not be regarded as limited to any particular delay.

The second delay circuit84may also comprise any of a wide range of delay circuits that are now known in the art or that may be developed in the future. Consequently, the present invention should not be regarded as limited to any particular type of delay circuit. In addition, because such a delay circuit84may be readily provided by persons having ordinary skill in the art after having become familiar with the teachings provided herein, the delay circuit84utilized in one preferred embodiment will not be described in further detail herein.

The first integrator78receives the short gate signal38and delayed pulse from the second delay circuit84. The first integrator78uses the short gate signal38as the trigger to begin and end the integration process. That is, the integrator78will begin integrating the pulse at the beginning time36and stop integrating at the ending time40. This corresponds to integrating the rise portion of the pulse between the initiation threshold and the peak point. Actually, as mentioned above, the first integrator78requires a 20 nanosecond lead time before starting the integration process, so the actual portion of the short gate signal38that triggers the integration process is actually selected to be about 20 nanoseconds earlier than the beginning time36. However, so long as any lead time required by the integrator78is accounted for, the integrator78will integrate the rise portion of the pulse illustrated in FIG.2. Accordingly, if the particular pulse being integrated is a short decay portion pulse20, the first integrator78will integrate the rise portion30of pulse20between the initiation threshold32and the peak point34. Conversely, if the particular pulse being integrated is a long decay portion pulse22, the first integrator78will integrate the rise portion42of pulse22between the initiation threshold44and the peak point46. See FIG.2. The output of integrator78comprises the integrated rise value86.

The first integrator78may comprise any of a wide variety of systems or devices suitable for integrating the pulse in the manner described herein. Consequently, the present invention should not be regarded as limited to any particular type of integrator78. However, by way of example, in one preferred embodiment, the first integrator78comprises a charge integrating analog-to-digital converter available from LeCory of Chestnut Ridge, N.Y. as model no. 2249.

The second integrator80receives the long gate signal58and delayed pulse from the second delay circuit84. The second integrator80uses the long gate signal58as the trigger to begin and end the integration process. That is, the integrator80will begin integrating the pulse at the beginning time56and stop integrating at the ending time60. This corresponds to integrating the decay portion of the pulse between the peak point and the cut-off point. Actually, as mentioned above, the second integrator80also requires a 20 nanosecond lead time before starting the integration process, so the actual portion of the long gate signal58that triggers the integration process is actually selected to be about 20 nanoseconds earlier than the beginning time56. However, so long as any lead time required by the second integrator80is properly accounted for, the second integrator80will integrate the decay portion of the pulse illustrated in FIG.2. Accordingly, if the particular pulse being integrated is a short decay portion pulse20, the second integrator80will integrate the decay portion50of pulse20between the peak point52and the cut-off point54. Conversely, if the particular pulse being integrated is a long decay portion pulse22, the second integrator80will integrate the decay portion62of pulse22between the peak point64and the cut-off point66. See FIG.2. The output of second integrator80comprises an integrated decay value88.

The second integrator80may comprise any of a wide variety of systems or devices suitable for integrating the pulse in the manner described herein. Consequently, the present invention should not be regarded as limited to any particular type of integrator80. However, by way of example, in one preferred embodiment, the second integrator80also comprises a charge integrating analog-to-digital converter available from LeCory of Chestnut Ridge, N.Y., as model no. 2249.

The first and second integrators78and80are operatively connected to a data processor70which receives the integrated rise and decay values86and88and processes them in accordance with the teachings provided herein. Data processor70then produces output data72that are indicative of whether the pulse being processed comprises a short decay period pulse20or a long decay period pulse22.

The data processor70may comprise any of a wide range of data processors that are now known in the art or that may be developed in the future that are or would be suitable for processing the integrated rise and decay values86and88in accordance with the teachings provided herein. However, because such data processors, such as data processor70, are well-known in the art and could be readily provided by persons having ordinary skill in the art after having become familiar with the teachings provided herein, the data processor70that may be utilized in one preferred embodiment will not be described in further detail herein.

The user interface16is operatively associated with the pulse discriminator system14and receives the output data72from the data processor70. The user interface16may comprise any of a wide variety of systems and devices suitable for providing for the user an indication of the identity of the detected pulse. The user interface16may provide a visual indication of the identity of the detected pulse, an aural indication of the identity of the detected pulse, or a tactile indication of the identity of the detected pulse. Alternatively, some combination of these indications could be provided for the user. Examples of visual indications include flashing lights or lights of different colors. Examples of aural indications include tones having different durations or of different frequencies. Tactile indications include vibrations of different durations or of different frequencies. However, because persons having ordinary skill in the art could readily provide such a user interface after having become familiar with the teachings of the present invention, the particular user interface16utilized in one preferred embodiment will not be described in further detail herein.

As briefly mentioned above, the various components comprising the pulse discriminator system14may operate in accordance with the method or process24(FIG. 3) in order to determine whether the pulse is a short period pulse20or a long period pulse22. Referring now toFIGS. 2 and 3simultaneously, a first step26in the method24involves detecting a pulse (e.g., a pulse20or22) that is to be discriminated. In the embodiment shown and described herein, the step26of detecting a pulse involves receiving the output signal18produced by the detector12and detecting a pulse contained in the output signal18. As mentioned above, the constant fraction discriminator76detects a pulse by comparing the output signal18of detector12with a user-selected threshold value. If the output signal18exceeds the threshold value, then a pulse is deemed detected. Once the pulse has been detected, the pulse discriminator system14operates to integrate a rise portion of the pulse at step28.

As previously noted, the shape or profile of the rise portion of the pulse may vary depending on whether the pulse is a short decay period pulse20or a long decay period pulse22. For example, and with reference now specifically toFIG. 2, the rise portion30of the short decay period pulse20comprises that portion of the short decay period pulse20that extends from about the initiation threshold32of short decay period pulse20to about the peak point34of short decay period pulse20. The initiation threshold32is defined by the beginning time36of the short gate signal38produced by the constant fraction discriminator76, whereas the peak point34is defined by the ending time40of short gate38.

As noted, the shape or profile of the rise portion42of the long decay period pulse22is somewhat different than that of the rise portion30of short decay period pulse20in that the initiation threshold44and peak point46of long decay period pulse22are also defined by the beginning time36and ending time40of the short gate signal38. That is, due to the elongated nature of long decay period pulse22compared with the short decay period pulse20, the peak46of long decay period pulse22does not coincide with the actual peak46′ (i.e., point of maximum value) of long period pulse22.

The pulse discriminator system14integrates the rise portion of the pulse at step28. This is done by the first integrator78. That is, the first integrator78receives the short gate signal38and the delayed output signal18from the detector12via linear fan-out74and second delay circuit84. As described above, the rise portion of the pulse (i.e., either the rise portion30of short decay period pulse20or the rise portion42of long decay period pulse22) is defined by the beginning time36and ending time40of short gate signal38produced by the constant fraction discriminator76. Thus, the integration of the pulse involves integrating that portion of the pulse between the initiation threshold (i.e., either initiation threshold32of short decay period pulse20or the initiation threshold44of long decay period pulse22) and the peak (i.e., either peak point34of short decay period pulse20or peak point46of long decay period pulse22). First integrator78produces an integrated rise value86that is then directed to data processor70.

The next step48in the process or method24involves integrating a decay portion of the pulse. As was the case for the rise portion of the pulse, the shape or profile of the decay portion of the pulse may vary depending on whether the pulse is a short decay period pulse20or a long decay period pulse22. For example, with reference again toFIG. 2, a decay portion50of a short decay period pulse20comprises that portion of the short decay period pulse20that extends from about a peak point52of short decay period pulse20to about a cut-off point54of short decay period pulse20. As will be described in greater detail below, the peak point52is defined by a beginning time56of a long gate signal58, whereas the cut-off point54is defined by an ending time60of long gate signal58. Because the peak point52of short decay period pulse20is defined by the beginning time56of long gate signal58, the peak point52may not necessarily coincide with the actual peak34(i.e., point of maximum value) of the short decay period pulse20.

The shape or profile of the decay portion62of the long decay period pulse22is somewhat different than that of the decay portion50of short decay period pulse20in that a peak point64and cut-off point66of long decay period pulse22are also defined by the beginning time56and ending time60of long gate signal58. That is, due to the elongated nature of long decay period pulse22compared with the short decay period pulse20, the peak point64of long decay period pulse22coincides more closely with the actual peak46′ (i.e., point of maximum value) of long decay period pulse22than was the case for the short decay period pulse20.

The pulse discriminator system14integrates the decay portion of the pulse at step48. This is done by the second integrator80. The second integrator80receives the long gate signal58and the delayed output signal18from the detector12via linear fan-out74and second delay circuit84. As described above, the decay portion of the pulse (i.e., either decay portion50of short decay period pulse20or decay portion62of long decay period pulse22, as the case may be) is defined by the beginning time56and ending time60of long gate signal58. Thus, the integration of the pulse involves integrating that portion of the pulse between the peak point (i.e., either peak point52of short decay period pulse20or the peak point64of long decay period pulse22) and the cut-off point (i.e., either cut-off point54of short decay period pulse20or cut-off point66of long decay period pulse22). The second integrator80produces an integrated decay value88that is then directed to data processor70.

In order to determine whether the peak contained in the output signal18from detector12comprises a short decay period peak20or a long decay period peak22, that is, in the example of the particular embodiment shown and described herein, whether the peak was produced as the result of a gamma ray γ or a neutron n, the data processor70compares the integrated rise value86with the integrated decay value88at step68(FIG. 3) in accordance with the characteristics for the short decay period peak20and the long decay period peak22illustrated in FIG.4.

With reference now toFIG. 4, the slope of the line formed by the locus of points for neutrons (i.e., long decay period pulses22) is steeper than the line formed by the locus of points for gamma rays (i.e., short decay period pulses20). Accordingly, by comparing the integrated value86of the rise portion of the pulse (i.e., portion30of short decay period pulse20or portion42of long decay period pulse22) with the integrated value88of the decay portion of the pulse (i.e., portion50of short decay period pulse20or portion62of long decay period pulse22), a determination may be made as to whether the pulse more closely fits the line (i.e., locus of points) associated with the short decay period pulse20(i.e., “gamma ray” line inFIG. 4) or the long decay period pulse22(i.e., the “neutron” line in FIG.4).

In the embodiment shown and described herein, an artificial line of separation90is constructed between the “gamma rays” line and the “neutrons line” in the manner illustrated in FIG.4. The artificial line of separation90is selected to be the dividing line between points that will be deemed to be associated with short decay period pulses (i.e., pulses resulting from gamma rays) and long decay period pulses (i.e., pulses resulting from neutrons). Graphically, points below the line of separation90will be deemed to be short decay pulses, whereas points above the line of separation90will be deemed to be long decay pulses. The precise location of the artificial line of separation90(i.e., the slope and intercept of the artificial line of separation90) may be determined in advance to be the best balance between the locus of points identified as “gamma rays” in FIG.4and the locus of points identified as “neutrons” in FIG.4. That is the artificial line of separation90may be placed substantially between the “gamma rays” line and the “neutron” line in the manner illustrated in FIG.4. Alternatively, other methods, such as statistical methods, may be used to locate the artificial line of separation90.

As mention above, in one preferred embodiment, the data processing system70may determine whether the pulse is a short decay period pulse (i.e., below the line of separation90) or a long decay period pulse (i.e., above the line of separation90) by multiplying the integrated value86for the rise portion of the pulse by the slope of the line of separation90. To this product is added the intercept of the line of separation90(i.e., the point where the line of separation90intersects the “integrated decay value” axis) to yield a “calculated decay value.” If the integrated decay value88is less then the calculated decay value, then the detected peak was a short decay period peak. Conversely, if the integrated decay value88is greater than the calculated decay value, then the detected peak was a long decay period peak. Thereafter, processed data72indicative of whether the detected pulse was a short decay period pulse20or a long decay period pulse22may be directed to the user interface16which may provide a suitable indication for a user (not shown) regarding the identification of the detected pulse in the manner already described.

It is contemplated that the inventive concepts herein described may be variously otherwise embodied and it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.