Spectrometer with charge-carrier-trapping correction

A spectrometer having improved energy resolution by correcting for error introduced by charge carrier trapping. By monitoring the shape of the pulses produced by the detector, a digital filter is adjusted to improve the energy resolution. The adjustment is performed manually by an operator or automatically by an automatic optimizer circuit that modifies the digital filter until the spectral peaks have a width and shape matching the desired characteristics, which are a minimum width and a substantially symmetrical shape. By correcting for the energy loss associated with long rise time events, the charge-trapping correcting spectrometer produces spectral peaks with improved energy resolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

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BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an improved gamma-ray spectrometer. More specifically, the present invention relates to a gamma-ray spectrometer incorporating a method of improving the energy resolution of detectors whose energy resolution is degraded because of charge carrier trapping.

2. Description of the Related Art

The germanium detector is the most commonly used high-resolution gamma-ray detector. When the germanium detector absorbs a gamma-ray photon, free electrons and holes are produced. The number of electrons and holes produced is proportional to the gamma-ray energy. The associated electronics collects the electrons and holes producing a signal proportional to the gamma-ray energy. The energy spectrum of the system is a histogram of the number of measured events versus the recorded energy. The width of the line in the energy spectrum caused by a mono-energetic gamma-ray source is called the energy resolution of the system. The width of the line is usually measured at the point where the number of counts is half of the maximum in the peak and is referred to as the Full Width at Half Maximum (FWHM).

Only the electronic noise and the statistical variation in the collected charge would limit the energy resolution of a perfect detector. In addition to electronic noise and statistical variations, the energy resolution of real detectors is degraded when some of the free charge carriers are trapped before reaching the collecting electrodes. The fraction of the charge signal that is lost depends on the point of interaction of the gamma-ray inside the germanium crystal and thus varies from event to event producing an increase in the FWHM.

U.S. Pat. No. 4,937,452, entitled “Charge Trapping Correction in Photon Detector Systems,” issued to Michael L. Simpson, et al., on Jun. 26, 1990 shows an analog spectrometer that corrects for the trapped charge and improves the energy resolution. Although the energy resolution is improved, the Simpson device requires the manual adjustment of two independent parameters to match the specific detector being used.

BRIEF SUMMARY OF THE INVENTION

A gamma-ray spectrometer incorporating a method of improving the energy resolution of detectors whose energy resolution is degraded because of charge carrier trapping is shown and described. By monitoring the shape of the pulses produced by the detector, a digital filter is adjusted to improve the energy resolution. The adjustment is performed manually by an operator or automatically by an automatic optimizer circuit that modifies the digital filter until the spectral peaks have a width and shape matching the desired characteristics, which are a minimum width and a substantially symmetrical shape. By correcting the width and shape of the spectral peaks, the energy resolution improves. The digital filter, as modified by manually by an operator or automatically by an automatic optimizer circuit, applies a fine gain control to individual pulses corresponding to particular rise-times. By correcting for the energy loss associated with long rise time events, the charge-trapping correcting spectrometer produces spectral peaks with improved energy resolution.

The charge-trapping correcting spectrometer includes a germanium detector that detects gamma-ray photons emitted by a radiation source. An analog processing circuit processes the charge collected by the detector and produces a voltage proportional to the collected charge. Optionally, the analog processing circuit provides amplification and pulse shaping to match the characteristics of the sampling analog-to-digital converter (ADC). The sampling ADC samples and digitizes the analog voltage from the analog processing circuit and produces a series of digital numbers proportional to the instantaneous output of the analog processing circuit. The digital output of the ADC is passed to a digital filter. The digital filter produces an output whose amplitude is proportional to the total charge collected by the germanium detector. The output of the digital filter is used as a pointer into a histogram memory. More specifically, the histogram memory uses the peak amplitude of the output of the digital filter to increment the data memory element corresponding to that amplitude. The result of many such measurements is the spectrum of radiation emitted by the radiation source. The spectrum is displayed for an operator on a display.

The parameters of the digital filter are selected to minimize the effects of electronic noise on the output amplitude consistent with the need to provide high data rates from the system. Additionally, the digital filter is designed to produce an output that is proportional to the total charge collected but independent of the detector rise time. The response of digital filter to a very short rise time step pulse, referred to as the filter weighting function, is usually a trapezoid. The peak amplitude of the trapezoid is the best estimate of the energy of the radiation emitted by the radiation source.

A pulse shape analyzer produces a digital output proportional to selected parameters of the digitized pulse from the sampling ADC. In the simplest case, the output from the pulse shape analyzer is a number proportional to the rise time of the detector. The gamma-ray spectrometer replaces the conventional digital filter with a digital filter having the capability to increase the amplitude of the output from the digital filter according to a programmed charge-carrier-trapping correction function using an input from the pulse shape analyzer. If the programmed charge-carrier-trapping correction function is correct for the detector in use then the total spectrum has a width close to the theoretically predicted value.

In one embodiment, the programmed charge-carrier-trapping correction function is entered by input from an operator. The operator makes an estimate of the function parameters and observes the resulting spectral shape and width on the display. The operator continues to modify the programmed charge-carrier-trapping correction function until the results are acceptable.

The addition of an automatic optimizer circuit eliminates the need for the operator to observe the output of the spectrometer and manually adjust the digital filter. The automatic optimizer circuit takes inputs from the pulse shape analyzer and examines the output from the digital filter in a region around a selected spectral peak. The automatic optimizer circuit modifies the programmed charge-carrier-trapping correction function in the digital filter to minimize the width of the selected peak. Using a simple algorithm similar to an automatic gain stabilizer, the automatic optimizer circuit examines the output of the digital filter to see if it is in a selected region centered on a specified energy. If the output of the digital filter is less than the center of the selected region, the gain of the digital filter for the observed output from the pulse shape analyzer is incremented by a small amount. If the output of the digital filter is greater than the center of the selected region, the gain of the digital filter for the observed value of the pulse shape analyzer is decreased by a small amount. After many events are processed, the programmed charge collection function includes a gain factor depending on the observed pulse shape. The gain factor tends to place the centroid of the spectrum produced for each pulse shape in the same channel, thus decreasing the effects of charge carrier trapping and improving the energy resolution.

DETAILED DESCRIPTION OF THE INVENTION

A gamma-ray spectrometer incorporating a method of improving the energy resolution of detectors whose energy resolution is degraded because of charge carrier trapping is shown and described at400in the figures. By monitoring the shape of the pulses produced by the detector, a digital filter is adjusted to improve the energy resolution. The adjustment is performed manually by an operator or automatically by an automatic optimizer circuit that modifies the digital filter until the spectral peaks have a width and shape matching the desired characteristics, which are a minimum width and a substantially symmetrical shape. By correcting the width and shape of the spectral peaks, the energy resolution improves. The digital filter, as modified by manually by an operator or automatically by an automatic optimizer circuit, applies a fine gain control to individual pulses corresponding to particular rise-times. By correcting for the energy loss associated with long rise time events, the charge-trapping correcting spectrometer produces spectral peaks with improved energy resolution.

FIG. 1illustrates a conventional gamma-ray spectrometer100. The prior art spectrometer100includes a germanium detector102that detects gamma-ray photons emitted by a radiation source104. An analog processing circuit106processes the charge collected by the detector102and produces a voltage proportional to the collected charge. Optionally, the analog processing circuit106provides amplification and pulse shaping to match the characteristics of the sampling analog-to-digital converter (ADC)108. The sampling ADC108samples and digitizes the analog voltage from the analog processing circuit106and produces a series of digital numbers proportional to the instantaneous output of the analog processing circuit106. The digital output of the ADC108is passed to a digital filter110. The digital filter110produces an output whose amplitude is proportional to the total charge collected by the germanium detector102. The output of the digital filter110is used as a pointer into a histogram memory112. More specifically, the histogram memory112uses the peak amplitude of the output of the digital filter110to increment the data memory element corresponding to that amplitude. Typically, histogram memory112has 16,000 words of memory, each word corresponding to a specific output from the digital filter110, which in turn relates to a specific energy observed by the detector102. The result of many such measurements is the spectrum of radiation emitted by the radiation source104. The spectrum is usually displayed for an operator on a display114.

The parameters of the digital filter110are selected to minimize the effects of electronic noise on the output amplitude consistent with the need to provide high data rates from the system. Additionally, the digital filter110is designed to produce an output that is proportional to the total charge collected but independent of the detector rise time. The response of digital filter110to a very short rise time step pulse, referred to as the filter weighting function, is usually a trapezoid. The peak amplitude of the trapezoid is the best estimate of the energy of the radiation emitted by the radiation source104.

Each time a gamma-ray photon is absorbed by the germanium detector102, one count is added to the location in the histogram memory112corresponding to the measured amplitude of the charge signal. After many such events have been recorded, the histogram memory112contains the spectrum of the gamma-ray radiation seen by the germanium detector102. Individual lines in the spectrum correspond to discrete gamma-ray energies in the radiation field. The spectral lines have a finite width even when the gamma-ray photons are mono-energetic. The width of the spectral lines is determined by electronic noise, statistical variation in the number of electrons and holes produced in the crystal, and charge carrier trapping. The degradation of energy resolution caused by charge carrier trapping varies from detector to detector, primarily because of differences in crystal quality and purity.

FIG. 2shows the variation in the spectra produced by a prior art spectrometer having a radiation detector exhibiting charge carrier trapping when exposed to a mono-energetic radiation source104having an energy E1. In many detectors, the charge collected decreases as the rise time increases because charge carriers are held up from reaching an electrode and arrive too late to be taken into account by the electronics of the spectrometer. A first spectrum200corresponds to a short rise time from detector102. The maximum amplitude of the first spectrum200is correctly recorded at energy E1. A second spectrum202is produced by a somewhat longer rise time. The amplitude is somewhat reduced from the correct value E1. Similarly, a third spectrum204and a fourth spectrum206show further loss of amplitude as the rise time increases. In a detector free from charge carrier trapping, the spectral peak would be at E1for all rise times. The amount of deviation from the correct value E1varies from detector to detector depending on the crystal properties.

FIG. 3shows the spectral degradation seen by a prior art spectrometer resulting from the charge carrier trapping depicted inFIG. 2. The spectra200,202,204,206ofFIG. 2at various rise-times and exhibiting offsets due to charge carrier trapping are graphed to relate number of counts to the energy observed. The sum of the spectra200,202,204,206for all possible rise times weighted by the volume of the detector having that rise time defines the total spectrum300. In the absence of charge carrier trapping, the total spectrum300would have a Gaussian shape. However, as illustrated inFIG. 3, the total spectrum300has the often-observed low-side tailing and is broader than predicted by the electronic noise and charge carrier generation statistics.

FIG. 4shows a charge-trapping correcting spectrometer400in which the effects of charge carrier trapping are reduced. The charge-trapping correcting spectrometer400adds a pulse shape analyzer402to the conventional spectrometer100ofFIG. 1. The pulse shape analyzer402produces a digital output proportional to selected parameters of the digitized pulse from the sampling ADC108. In the simplest case, the output from the pulse shape analyzer402is a number proportional to the rise time of the detector102. The gamma-ray spectrometer400replaces the conventional digital filter110with a digital filter404having an input for the usual pulse amplitude information from the sampling ADC108and an additional input for pulse shape information from the pulse shape analyzer402. The digital filter404has the capability to increase the amplitude of the output from the digital filter404according to a programmed charge-carrier-trapping correction function using an input from the pulse shape analyzer402. The output of the digital filter404depends on both the amplitude and pulse shape information to minimize the effects of charge carrier trapping and improve energy resolution. It is often the case that the amount of charge carrier trapping for a specific pulse correlates with the shape of the pulse. For example, longer rise time pulses might indicate that the carriers traveled a longer distance and were thus more likely to be trapped. If the programmed charge-carrier-trapping correction function is correct for the detector102in use then the total spectrum300has a width close to the theoretically predicted value.

In one embodiment, the programmed charge-carrier-trapping correction function of the digital filter404is entered by an operator. The operator makes an estimate of the function parameters and observes the resulting spectral shape and width on the display114. The operator continues to modify the programmed charge-carrier-trapping correction function of the digital filter404until the results are acceptable.

FIG. 5shows an alternate embodiment of the charge-trapping correcting spectrometer500. The charge-trapping correcting spectrometer500adds an automatic optimizer circuit502to the embodiment ofFIG. 4. The automatic optimizer circuit502that adjusts the parameters of the digital filter504to match the germanium detector102thus producing improved resolution with little effort by the operator. The digital filter504is updated to utilize information from the automatic optimizer circuit502. The automatic optimizer circuit502takes inputs from the pulse shape analyzer402and examines the output from the digital filter504in a region around a selected spectral peak. The automatic optimizer circuit502modifies the programmed charge-carrier-trapping correction function in the digital filter504to minimize the width of the selected peak.

In one embodiment, the automatic optimizer circuit502uses a simple algorithm similar to an automatic gain stabilizer. The output of the digital filter504is examined to see if it is in a selected region centered on a specified energy. If the output of the digital filter504is less than the center of the selected region, the gain of the digital filter504for the observed output from the pulse shape analyzer402is incremented by a small amount. If the output of the digital filter504is greater than the center of the selected region, the gain of the digital filter504for the observed value of the pulse shape analyzer402is decreased by a small amount. After many events are processed, the programmed charge collection function includes a gain factor depending on the observed pulse shape. The gain factor tends to place the centroid of the spectrum produced for each pulse shape in the same channel, thus decreasing the effects of charge carrier trapping and improving the energy resolution.

As previously discussed, in a perfect spectrometer each spectral peak measured by the charge-trapping correcting spectrometer would be centered about the appropriate energy and would have the minimum width and a symmetric shape. Real world spectrometers are plagued by noise and statistical variations causing the spectral peaks to be broader and not symmetric. For spectrometers using germanium-based and other similar detectors, charge-carrier trapping contributes to the increased width and non-symmetric shape of the spectral peaks.

FIG. 6is a flow chart of the method600by which the charge-carrier-trapping correction function of the digital filter is modified to improve the energy resolution of the charge-trapping correcting spectrometer. Initially, the histogram is analyzed to identify the location of a spectral peak representing a radioactive element found in the source, thereby identifying the responsible element in step602. For example, a radiation source containing Cesium-137 (137 Cs) produces pulses centered around 662 keV. Because of charge-carrier trapping and noise, there some pulses show up at energy values close to but less than or greater than 662 keV. The energy of interest is determined to be the known energy value corresponding to the identified element identified by the clustered pulses. Next, a window around the energy of interest is selected in step604. For example, a 4 keV window centered around 662 keV (662±2 keV) is selected to capture pulses produces by 137 Cs but offset due to charge-carrier trapping and noise. The width of the window is selected based upon criteria such as the proximity of spectral peak produced by other elements and the magnitude of the offsets. The output of the digital filter504is a digital representation of an output pulse collected by the detector102and the output of the pulse shape analyzer402and is obtained in step606. If the output lies within the selected energy window then the output pulse is used to modify the charge-carrier-trapping correction function in step608. When the output peaks at an energy value greater than the energy of interest in step610, the value of the charge-carrier-trapping correction function corresponding to the output of the pulse shape analyzer402is decremented, which shifts the output pulse back towards the correct energy, in step612. Similarly, when the output pulse peaks at an energy value less than the energy of interest in step614, the value of charge-carrier-trapping correction function corresponding to the output of the pulse shape analyzer402for that energy is incremented, which shifts the output pulse towards the correct energy in step616. The method is repeated using additional output pulses until the charge-carrier-trapping correction function produces a spectral peak having a minimum width and a substantially symmetrical shape in step618. Once the charge-carrier-trapping correction function has been adjusted, the calibration is complete in step620for the detector and future measurements made by the charge-trapping correcting spectrometer500with the calibrated detector will be corrected to reduce the effects of charge carrier trapping. The charge-carrier-trapping correction function remains associated with the detector used to generate it.

For the embodiment of the charge-trapping correcting spectrometer500including the automatic optimization circuit502, adjustments to the charge-carrier-trapping correction function is performed by the automatic optimization circuit502. In the manual version of the charge-trapping correcting spectrometer400, adjustments to the charge carrier trapping function are made by the operator.

A spectrometer having improved energy resolution by correcting for error introduced by charge carrier trapping has been shown and described. The spectrometer includes a pulse shape analyzer producing an output utilized by a digital filter to reduce or eliminate the error introduced by charge carrier trapping. The digital filter of charge trapping correcting spectrometer is adjusted until the spectral peaks have a width and shape matching-the desired characteristics, which are a minimum width and a substantially symmetrical shape. By correcting for the energy loss associated with long rise time events, the charge-trapping correcting spectrometer produces spectral peaks with improved energy resolution. Adding an automatic optimizer that monitors the output of the digital filter and applying a fine gain control to individual pulses corresponding to particular rise-times results in improved energy resolution and eliminates the need for manual adjustment of the digital filter by an operator.

The examples described herein make reference to the specific example of gamma-ray spectrometers using germanium detectors. It will be recognized by those skilled in the art that the method of charge-trapping correction applies to other types of detectors in which the output response depends on the pulse shape including gamma ray spectrometers using other types of detectors.