Abstract:
A microphonics noise cancellation system and method for improving the energy resolution for mechanically cooled high-purity Germanium (HPGe) detector systems. A classical adaptive noise canceling digital processing system using an adaptive predictor is used in an MCA to attenuate the microphonics noise source making the system more deployable.

Description:
CLAIM OF PRIORITY IN PROVISIONAL APPLICATION 
       [0001]    This application claims priority in provisional application filed on Nov. 17, 2009, entitled “Active Noise Canceling System for Mechanically Cooled Germanium Radiation Detectors” application No. 61/262,062 by Karl E. Nelson et al, and incorporated by reference herein. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    A. Technical Field 
         [0004]    The present invention relates to noise cancellation techniques, and more particularly to a method and system for actively canceling microphonics noise in radiation detectors using a multi-channel analyzer with an adaptive predictor for adaptive signal processing. 
         [0005]    B. Description of the Related Art 
         [0006]    High-purity Germanium (HPGe) detection systems are used to measure gamma ray spectroscopy to detect and identify radiological sources in the field. They do so by using highly sensitive preamplifiers which convert the electrons from each gamma ray detection event into an electrical signal. These detectors must operate at cryogenic temperatures and thus field operations require mechanical cooling solutions. Unfortunately, mechanical vibrations from the cooler are also converted to electrical signals by inducing changes in capacitance which are translated by the preamplifier into microphonic noise. To address the microphonics phenomenon, often a series of mechanical dampers and electrical filters are used to attenuate and cancel microphonic noise. 
         [0007]    In the field of adaptive signal processing, adaptive filters (and adaptive processors generally) adapt their performance based on an input signal by self-adjusting their transfer functions according to an optimization algorithm. Such adaptive filters are often digital filters in a digital processing system for use in such example applications as speech-processing and noise cancellation, both of which involve continuous time processing where there are no periods in which the adaptation is switched between training and predicting. 
         [0008]    In radiation detection applications and nuclear physics, however, adaptive filters and processers are not used in any of the four traditional configurations of prediction, system identification (modeling), equalization (deconvolving, inverse filtering, or inverse modeling) or interference canceling. There is a disconnect between the two fields of adaptive signal processing and nuclear physics because the signals in nuclear physics are random pulses for which only the timing, energy and occasionally the decay constant are considered useful. This is quite different from signal processing in which phase and magnitude of discrete versions of continuous time signals are considered. 
       SUMMARY OF THE INVENTION 
       [0009]    One aspect of the present invention includes a standalone multichannel analyzer (MCA) for collecting and analyzing radiation data, comprising: a preamplifier for converting a charge pulse of a radiation detection event into an analog signal characterized by a step rise from an external preamplifier; an anti-aliasing filter for bandlimiting the analog signal and having means for adjusting gain and offset; an analog to digital converter (ADC) which converts the analog signal from the anti-aliasing filter to a digital signal; and a digital processor for processing the digital signal to produce an energy and time associated with the arrival of each radiation pulse, said digital processor including a pulse shaping filter for converting the step rise of the charge pulse into a unipolar shaped pulse, and an adaptive predictive filter for removing a predicted signal from the output of the pulse shaping filter. 
         [0010]    Another aspect of the present invention includes a radiation detection system, comprising: a data collection system comprising: a semiconductor radiation detector material; a mechanical cooler for cooling the radiation detector material; a high voltage power supply; a preamplifier for converting a charge pulse of a radiation detection event into an analog signal characterized by a step rise from an external preamplifier; an anti-aliasing filter for bandlimiting the analog signal and having means for adjusting gain and offset; a digital sampling system for sampling a detector output channel at a regular time period; and an analog to digital converter (ADC) which converts the analog signal from the anti-aliasing filter to a digital signal; and a digital processor for processing the digital signal to produce an energy and time associated with the arrival of each radiation pulse, said digital processor including a pulse shaping filter for converting the step rise of the charge pulse into a unipolar shaped pulse, and an adaptive predictive filter for removing a predicted signal from the output of the pulse shaping filter. 
         [0011]    Generally, the present invention is a new design for a multi-channel analyzer (MCA) and a radiation detector system using such MCA, which uses classical adaptive filtering techniques (i.e. adaptive predictor) to attenuate microphonic noise from mechanical vibrations and thus improve the energy resolution of mechanically cooled radiation detector instruments, such as high-purity Germanium (HPGe) detector systems. Radiation detection signals are very confined in time, i.e. impulse, but very large. Their large nature is such that even coincidental correlations with the noise in the adaptation will cause the coefficients of the adaptation feedback to be grossly distorted in the presence of the signal. However, because radiation detection signals are so well confined in time, the present invention can add new functions to the adaptive processor by utilizing knowledge of other available information in the system to avoid the difficulties from direct application of traditional signal processing. The system of the present invention has periods in which the adaptation is switched between training and predicting. Deltas (delays to the input signal) are inserted such that we insure that the predictive filter is not using any information around the arrival of the signal not just to insure lack of correlation in training. 
         [0012]    As a microphonic noise-canceling radiation detector system, the present invention generally includes two main subsystems: (1) a data collection system, and (2) a computer or other digital processing system with an adaptive predictor for processing digital signals received from the data collection system to produce energy and time values associated with gamma ray detection events. The data collection system includes (1) a semiconductor radiation detector typically which further includes a HPGe detector mounted in a cryostat, a mechanical cooler such as a cryogenic cooler, a preamplifier which converts a charge pulse into a voltage or a buffer which takes a step rise from an external preamplifier, an anti-aliasing filter, an ADC, and a high voltage supply; and (2) a digital sampling system which samples the detector output channel at a regular time period. And the digital processing system includes a shaper (i.e. pulse shaping filter/amplifier) which converts the step rise into a unipolar shaped pulse, and an adaptive filter for removing a predicted signal from the output of the pulse shaping filter. And as an MCA, the present invention may be characterized as including the preamplifier, anti-aliasing filter, ADC, and digital processing system, without the front end detection and sampling components or functionality. 
         [0013]    The digital processing system performs the following actions. First the digital signal from detector&#39;s ADC is shaped with a digital pulse shaping filter, or simply “shaper”. This shaper is typically implemented, for example, with a trapezoidal filter with the transfer function: 
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         [0000]    And the shaper often includes a pole-zero compensation system, characterized by: 
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         [0000]    In addition, downsampling may be performed after the shaping filter if required. 
         [0014]    The digital processing system also includes an adaptive noise removing circuit which implements an adaptive filter arranged as an adaptive predictor (and having either a finite impulse response (FIR) or infinite impulse response (IIR) structure) to output the non-predictive portion e(t) (i.e. error signal or cost function) where: 
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         [0000]    and where y(t) is the undelayed input signal, d(t) is the delayed input signal, with w i (t) is the filter&#39;s transfer function, and 
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         [0000]    is the predicted signal value. 
         [0015]    The digital processing system also implements a system to detect if the signal is over a threshold value and locks the system into pure predictive mode such that no adaptation occurs during the gamma ray event. In other words, if y(t)&gt;K, w is not updated and 
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         [0000]    Following the end of the shaped event d(t) is allowed to update, but w i (t) is not until all elements of d(t) are clear of the event time period. And an algorithm finds the maximum of e(t) during the event time period to record the energy of the pulse. 
         [0016]    The microphonic noise canceling technique/algorithm described above has been tested in simulation with noise having similar characteristics to the real mechanically-cooled radiation detection system, and showed a considerable improvement in performance such that the microphone noise was reduced to the same magnitude as the sum of all other noise sources. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated into and form a part of the disclosure, are as follows: 
           [0018]      FIG. 1  is a signal processing diagram of an example embodiment of the MCA of the present invention. 
           [0019]      FIG. 2  is a signal processing diagram of an example embodiment of a detailed adaptive filter with control of the present invention. 
           [0020]      FIG. 3  is a signal processing diagram of an example embodiment of a single-input adaptive transversal filter of the present invention. 
           [0021]      FIG. 4  is a signal processing diagram of an example embodiment of an adaptive predictor of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Turning now to the drawings,  FIG. 1  shows a first exemplary embodiment of an improved MCA of the present invention, generally indicated at  10 . The MCA  10  includes a preamplifier  11  which takes a current pulse from the detector and transforms it into a step function as well as providing a very large amplification. This step function has a decay time associated with it. The amplification serves to increase the intensity of the signal such that electric noise of all further steps is insignificant. While the preamplifier is sometimes characterized as a separate function from the operations of the MCA, the present invention incorporates the preamplifier as part of the MCA since the adaptive filtering in the MCA requires knowledge of this preamplifier function. In any case, the preamplifier output is provided to the anti-aliasing filter  12 , shown having DAC offset control  13 , and DAC gain control  14 . Output from the anti-aliasing filter/amplifier  12  is then outputted to the ADC  15  which converts the analog signal of the radiation detection event to a digital signal. 
         [0023]    The digital signal from the ADC  15  is then processed by the digital processing system, generally indicated at  16 . In particular, the digital signal is processed by a shaper and pole/zero compensation filter  17  for pulse shaping/amplifying and pole/zero cancellation. The shaper  17  converses the step response back into a pulse with a specific time length called the shaping time, shown controlled at  18 . As the step function from the preamplifier is not perfect, a pole-zero cancellation filter (or simply “PZC”) is used to remove or cancel the pole from the preamplifier and introduce an ideal pole which would produce an ideal step function. The zero in the PZC must match the preamplifier and thus be tunable, shown by control  19 . The result is to make the preamplifier appear to be perfect. Although the shaper and PZC are linear components, the range signals must be finite and thus the order of these two elements is generally fixed with the shaper first and the PZC second. Analog designs often blur these functions thus the pole cancellation (the tunable zero) may appear first, while the pole portion may be last. The shaping filter may be a trapezoidal shaper which accounts for variations in pulse shape. 
         [0024]    The digital processing system  10  also includes an adaptive noise removing circuit shown at  22  which implements an adaptive FIR or IIR filter  52  (shown in  FIG. 4  arranged in an adaptive predictor  50 ) to output the non-predictive portion e(t) as discussed in the Summary. The adaptive noise removal circuit  22  can be switched between training and predictive modes. When in predictive mode, the adaptive filter substitutes predicted values for all samples which are corrupted by the assumed shape resulting from a pulse determined by the peak detector. The training decision may have a stretcher (for example  36 ) to implement hysteresis to prevent training on rising edge of pulse. Control  20  is based on the undelayed input signal from the ADC, with a lower level discriminator  21  as input. As shown in  FIG. 3 , the adaptive filter may be a FIR filter implemented as a “single-input transversal filter.” To represent the low frequencies of the microphonics noise, the taps  42 - 46  were spaced with a delay greater than 1 (typically 5-10), but the exact spacing depends on the noise characteristics and sampling rate. Non-regular taps seem like they may have an advantage as they could potentially capture a greater range of frequencies. 
         [0025]    The adaptive filter  39  in  FIG. 2  operates in a predictive fashion, i.e. as an adaptive predictor. A range of lag values is possible from one to several times the tap delay without. The update routine was a Newton algorithm such that: 
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         [0026]    It is possible for control  1  and control  2  to be the same signal (of control  20  in  FIG. 1 ), but this can lead to instability in the event than multiple pulses arrive in a short time frame. Control  1  locks the adaptive predictor into prediction mode during the arrival of the pulse to ensure that we do not subtract away a portion of the energy from the radiation pulse. It must last at least until the maximum is recorded. Because this must switch prior to the arrival of the radiation signal at the adaptive filter, either a separate shaper or the shaper output must be delayed by at least one half the shaping as shown by shapers  31  and  35  in  FIG. 2 . Control  2  prevents the adaptive filter from updating while the radiation pulse is being processed and in addition while some substantial fraction of the states in the adaptive filter hold predicted as opposed to observed values. Because of this additional requirement, the functionality of the control signals are split. Constants τ and κ are chosen on by optimization of the performance on training samples. 
         [0027]    The optimization required to determine the training constants and the delays, it given by sampling noise and asserting a radiation pulse has occurred at some regular frequency. No actual radiation signal is required, but instead the standard deviation of the predicted minus the observed is measured. The better this baseline is predicted the better the noise performance of the system will be. 
         [0028]    It is appreciated that the delay ( 32  in  FIG. 2 ) and the energy shaper ( 35 ) can be reverse and with a sufficiently long delay, the shaper ( 35 ) and short shaper ( 31 ) could in principle be the same element. However, they are drawn separately, because there may be times in which the short shaper may benefit from a different design. Also the short shaper ( 31 ) does not need pole zero compensation. In addition having the short shaper have a significantly shorter shaping time allows the system the possibility of identifying piled up pulses where two radiation signals arrive in a short period of time. 
         [0029]    The actions of the pulse stretchers  36  and  37  in  FIG. 2  operates as follows. When the short shaper triggers it is known that a radiation pulse is arriving but do not know its center yet. Thus the blanking is started assuming that this was the peak of signal, that is we must blank for a count of the length of the shaping time. However, if short shaper  31  output stays high we know that the peak (determined by peak detector  33 ) was actually later in time and thus the time remaining needs to increase by only one half of a sample of every additional time period that short shaper  31  is over the threshold. For the second pulse stretcher  37 , it must wait until the predictive values have largely cleared the adaptive filter  39 . Failure to do so will mean that error in prediction will be incorporated into the weights of the adaptive filter. If a second or third radiation pulse arrives before this portion of the incorporated error is cleared, the predicted values can become increasingly unstable resulting in undesirable behavior. Thus the second pulse stretcher is used to avoid it. 
         [0030]    After adaptive noise removal is performed at  22  in  FIG. 1 , two subsequent functions are performed with the shaped signal. The first is a peak detect function performed at  23  which determines if the shaped signal has exceeded a specified value. The output of this trigger will be delayed and is then used to trigger the ADC and decide when the system will be locked out from a second pulse which may appear while the system is recovering from the previous pulse. The energy and time (via timer  26 ) may be recorded into memory (not shown). This recovery time results in a system “dead time.” A second continuous circuit called a track &amp; hold circuit shown at  25 . Traditionally this is implemented as a diode and a rectifier which serve to grab and hold the maximum signal which occurs in the time window around the trigger. Alternately a timed sample and hold circuit could serve this function. 
         [0031]    Though not shown in  FIG. 1 , it is appreciated that the system  10  also includes the supporting circuitry to enable operation, such as for example: the pulse pileup detector which looks for multiple pulses in a given time window, timers to estimate the count rate or dead time, a timer which time stamps the pulse (for list mode application), a histogramming circuit which bins the energy to form a spectrum, a gain control feedback system which locks onto a peak in the spectrum and dynamically adjusts the gain of the shaper to compensate for system drift, etc. These function are would all be considered an enhancements of the basic operation of an MCA. 
         [0032]    Note that this filter may help with not only microphonics by potentially parallel noise in the detector. Thus we will need to introduce both concepts in the background information. This primarily benefits a HPGe system, but we should not include any description of the detector in the claims. We cannot anticipate what further detector developments will bring and thus should not restrict ourselves. 
         [0033]    While particular operational sequences, materials, temperatures, parameters, and particular embodiments have been described and or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.