Data processing: measuring – calibrating – or testing – Calibration or correction system – Zeroing
Reexamination Certificate
2001-06-04
2003-08-19
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Zeroing
C702S022000
Reexamination Certificate
active
06609075
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to systems for detecting, counting and measuring step-like signal events produced by preamplifiers connected to radiation detectors used to detect x-rays, gamma-rays, nuclear particles, and the like. More particularly, it relates to increasing the accuracy with which the amplitudes of these step-like events are measured by increasing the accuracy of determining the event energy filter's baseline output at times when no events are being processed. The specific embodiment described relates to a spectrometer used with a solid state detector, but the same techniques can readily be applied a to radiation spectrometers operating with other detectors as well, since their operating principles are nearly identical.
A Synopsis of Current Spectrometer Art
FIG. 1
is a schematic diagram of a prior art radiation spectroscopy system employed with a solid state detector diode
7
. Similar systems are used for measuring x-ray, gamma-ray and alpha and beta particle radiations, differing primarily in the physical form of the detector diode
7
, which might also be replaced with a proportional counter or other detector. All of these detectors
7
share the common property that, when biased by a voltage supply
8
, they produce an output current pulse when detecting an absorption event and the total charge Q
E
in this pulse is approximately proportional to the energy E of the absorbed ray or particle. This current flows into a preamplifier
10
having an amplifier
12
and a feedback capacitor
13
. The current is integrated onto feedback capacitor
13
by an amplifier
12
, whose output is then a step-like pulse of amplitude A
e
=Q
E
/C
f
, where C
f
is the capacitance of feedback capacitor
13
. Other types of preamplifiers may process the current pulses to provide other types of output pulses. For example, a transimpedance amplifier would produce an output voltage pulse proportional to the input current pulse. As a matter of nomenclature, pulses produced by the preamplifier in response to events will sometimes be referred to as event pulses.
A spectroscopy amplifier
15
is then used to measure A
E
. Within modern spectroscopy amplifiers
15
the output of preamplifier
10
is typically sent to both a “slow” energy filter circuit
17
and a “fast” pileup inspection circuit
18
. The energy filter circuit filters the A
e
step to produce a low-noise shaped output pulse whose peak height A
E
is proportional to A
e
. The pileup inspection circuit applies a filter and discriminator to the preamplifier output to inspect for the presence of A
e
signal steps (events) and signals the filter peak capture circuit
20
to capture the amplitudes A
E
of shaped pulses from the energy filter circuit
17
which are sufficiently separated in time so that they do not interfere with each other's amplitudes (i.e., do not “pile up”). The distinction between the “fast and “slow” filters is relative, based on the particular application, but the “fast” filter's time constant is typically an order of magnitude shorter than the “slow” energy filter's time constant. A typical x-ray spectrometer, for instance, might use a 200 ns fast filter and a 4 &mgr;s energy filter. The inspection circuit
18
also determines when events are sufficiently separated so that the output of the energy filter circuit has returned to its DC value and signals a baseline capture circuit
22
to capture these values so that they may be subtracted from captured peak values by a subtraction circuit
23
. These differences are then passed to a multichannel analyzer (MCA) or digital signal processor (DSP)
25
, for binning to form a spectral representation (spectrum) of the energy values present in the incident radiation.
It should also be noted that, although the most commonly implemented spectrometers capture the value of the shaped pulse's amplitude A
E
as an estimate of the energy, other designs measure other values that characterize the shaped pulse. For example, some designs capture the value of the shaped pulse's area, or capture several points across the shaped pulse and then fit a mathematical function to the captured points. As a matter of nomenclature, we shall refer to these various measured values as “characteristic values” of the shaped pulse. Because the energy filter is a linear filter, these characteristic values are all proportional to one another, although which has the best signal to noise properties will depend upon the nature of the energy filter and the noise spectrum of the detector-preamplifier combination. Therefore, although we shall primarily refer to the shaped pulse's amplitude A
E
in the remainder of this specification, since it is the most commonly measured characteristic value of the shaped pulse, the reader should bear in mind that it is, in fact, only a representative of all the various characteristic values that could be measured and that our invention applies to them all as well.
The Need for Baseline Correction
FIG. 2A
shows the need for baseline correction in radiation spectrometers of the type described above. Trace
30
represents the output from the preamplifier
10
, with the step
34
occurring in response to detected radiation. As drawn, trace
30
has a slight slope, which physically may result from such causes as detector leakage current, a non-ideal amplifier
12
, or noise pickup. Trace
32
represents the output of energy filter
17
, which produces the shaped pulse
35
in response to input step
34
. The actual amplitude A
37
of the pulse is seen to be the sum of the height of pulse
35
and the baseline (DC) offset B
38
of the signal, where this baseline offset results from the reaction of the filters in the energy filter circuit
17
to the slope in the preamplifier output signal
30
. Thus, in the spectroscopy amplifier
15
shown in
FIG. 1
, the function of the peak capture circuit
20
is to capture the values A
37
in
FIG. 2A
, while the baseline capture circuit
22
measures baseline values B
38
, and the subtraction circuit
23
outputs the difference A-B which represents the energy of the radiation absorbed in the detector
7
.
The art of building spectroscopy amplifiers is relatively mature and many variations, using both analog and digital electronics, exist on the basic circuit shown in FIG.
1
. The reference book by Knoll [KNOLL 1989] provides a good introduction to the subject. See, for example, Chapter 16, Section III, “Pulse Shaping.”
In some designs the filter peak capture circuit senses and captures peaks autonomously and the job of the pileup inspector is to discriminate between valid and invalid captures and only allow valid values to pass on to the subtraction circuit
23
. Indeed, even the order of the components shown may be altered to achieve the same ends. Thus, in analog circuits the baseline capture circuit is commonly a switched capacitor which is tied to the output of the energy filter circuit
17
as long as the baseline is valid and disconnected whenever the pileup inspection circuit detects an event. The time constant of this circuit is long enough to filter baseline noise. The subtraction circuit
23
is then typically an operational amplifier with the capacitor voltage applied to its negative input and the peak capture circuit
20
applied to its positive input. In some cases, however, the order of circuits
20
and
23
are reversed, so the offset is removed from the energy filter circuit's output before peak amplitudes are captured. In traditional MCA's, in fact, the peak capture capability is included in the MCA
25
and removed from the spectroscopy amplifier
15
. The net result is the same, however, and the basic functions presented in
FIG. 1
capture the essence of the operation of these spectrometers as a class.
In digital spectrometers, single baseline values B
38
are captured by the baseline capture circuit
22
, and, having the same noise as the amplitude values a
37
must therefore be averaged so that
Grudberg Peter M.
Harris Jackson T.
Warburton William K.
Barlow John
Lau Tung
Townsend and Townsend / and Crew LLP
Warburton William K.
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