Data processing: measuring – calibrating – or testing – Measurement system – Measured signal processing
Reexamination Certificate
2000-08-21
2003-07-01
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Measured signal processing
C702S057000, C702S085000
Reexamination Certificate
active
06587814
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to signal processing systems, and more particularly to processing the step-like output signals generated by non-ideal, nominally single-pole (“N-1P”) devices responding to possibly time-varying, pulse-like input signals of finite duration, wherein the goal is to recover the integrated areas of the input signals.
The specific embodiments described relate to processing step-like signals generated by detector systems in response to absorbed radiation or particles and, more particularly, to digitally processing such step-like signals in high resolution, high rate gamma ray (&ggr;-ray) spectrometers with resistive feedback preamplifiers connected to large volume germanium detectors. The application of measuring the step-like output signals from &ggr;-ray detector preamplifiers to measure the &ggr;-rays' energies is just a specific example, and is described because this was the area in which the method was first developed.
The techniques that we have developed solve this problem generally, and therefore should not be construed as being limited to this specific application. Any detection system, for example, that produces output current signals that are integrated by charge sensitive preamplifiers could be treated by these techniques, whether the detected quantities are light pulses, x-rays, nuclear particles, chemical reactions, or otherwise. The techniques, in fact, is not limited to “detector systems” per se, but are, in fact, general purpose signal processing techniques which may be broadly applied, once understood. The outputs from superconducting bolometers, for example, produce step-like signals that are readily treated by the invention. The field of gamma spectroscopy, where 0.1% or less makes the difference between a bad and a good detector, however, provides particularly stringent tests of our techniques.
The term “step-like signal” also requires some discussion. The output of an ideal single-pole (“1P”) device to an ideal impulse (delta) function input, is an infinitely fast rise time followed by an exponential decay whose time constant &tgr;
d
is characteristic of the pole. Viewed on a time scale short compared to &tgr;
d
, this output will look like a pure step, while, when viewed on a time scale long compared to &tgr;
d
, it will look like a pulse. A real 1P device output, however, will have a finite risetime, &tgr;
r
, whose duration will be determined both by the nature of the device and, particularly, by the duration of its real input signal. Provided that &tgr;
r
is significantly shorter than &tgr;
d
, a real 1P device output signal, viewed on a time scale comparable to &tgr;
d
, will then show a risetime region, whose shape may be difficult to describe mathematically, followed, after a period comparable to &tgr;
r
, by an exponential decay with time constant &tgr;
d
. The output of a N-1P device will be similar, with additional distortions. We will refer to such signals, viewed on this time scale, as “step-like”.
Gamma-ray (&ggr;-ray) Detection Requirements
The detection and measurement of &ggr;-ray energies is a well-established discipline whose primary goal is to accurately determine both the number and energies of &ggr;-rays emitted from some target source. The requirements of good energy resolution and high count rate capability usually conflict, however, since count rates are enhanced by increasing detector volume, which increases output signal distortion and so degrades energy resolution. High count rates also degrade energy resolution directly due to practical problems in preamplifier design.
Description of the Problems
The field of &ggr;-ray detection is highly developed. A fairly comprehensive introduction to the state of the art may be found in the volume “Radiation Detection and Measurement, 2nd Ed.” by Glenn F. Knoll [KNOLL-1989]. Below we note only the issues relevant to the present invention. In the first section, we discuss how pole/zero cancellation errors introduce a second pole, spoiling the preamplifier's single pole response. In the second section, we examine how the finite input signal duration, in this case due to charge collection, distorts the preamplifier's output from the ideal.
Pole/zero Cancellation Errors
FIG. 1A
shows a typical solid state &ggr;-ray spectrometer comprising a semiconductor detector diode
7
biased by a voltage supply
8
and connected to a preamplifier
10
comprising an amplifier
13
with a feedback capacitor C
15
and resistor R
17
. As drawn, preamplifier
10
is a single pole circuit whose response to an impulse (delta function) input is A exp(−t/&tgr;
2
), where &tgr;
2
=RC and A is the area under the impulse. Because &tgr;
2
is typically of order 1 ms, which is too long for the following circuits, a pole/zero (P/Z) network
20
cancels the pole at 1/&tgr;
2
and replaces it with a pole at 1/&tgr;
1
, where &tgr;
1
typically is 50 &mgr;s. Gain stage
22
then amplifies and buffers the preamplifier's output signal for shaping amplifier
23
which feeds multichannel analyzer (MCA)
24
.
If the time duration of the current pulse arising from the charge deposited in detector
7
by a &ggr;-ray absorption is very short compared to &tgr;
1
, the output of stage
22
will be an exponentially decaying step whose amplitude is the pulse integral and proportional to the deposited charge. &ggr;-ray spectrometers are therefore designed to measure these step amplitudes to measure the charge deposited by the absorbed &ggr;-ray. Other forms of radiation, including neutrons, alpha and beta particles, and x-rays behave similarly and their energies are measured the same way.
Commonly, however, both the input's finite duration and the pole-zero circuit's imperfections distort the preamplifier's response, destroying the proportionality between the output step's amplitude and the deposited charge and so degrading the system's energy resolution. Imperfections in P/Z network
20
arise from difficulties in precisely canceling the &tgr;
2
component, leaving a small residual fraction, of order 1-2%, in the output signal.
FIG. 1B
shows a 5% residual &tgr;
2
component for ease of viewing: an exponential decay signal
25
with time constant &tgr;
2
, input to the P/Z network
20
, produces either output signal
27
or
29
, depending upon whether the residual &tgr;
2
term is positive or negative.
These &tgr;
2
residuals are particularly bothersome at high counting rates, where each signal step rides upon a &tgr;
2
background from all preceding steps. As these arrive randomly, the resulting baseline bias also fluctuates randomly in time, which the spectroscopy amplifier's baseline restoration circuit cannot track well. These terms, which may only be a few tenths of 1%, become a significant resolution degradation at 1 MeV where 0.05% energy resolution is desired.
Signal Risetime Fluctuations and Ballistic Deficit
FIG. 2
shows a preamplifier
10
front end with a cross sectional view of the detector
7
of
FIG. 1
, for the common coaxial geometry, The dashed lines show electric field line within the detector body
30
, which vary considerably with local geometry. Two factors cause charge collection time variations within the detector and thus risetime variations in the preamplifier's
10
signal output: 1) the difference between carrier velocities; and 2) the existence of different path lengths within the detector. RAUDORPH-1982 describes these issues. These risetime variations produce ballistic deficit by two paths, one direct, one indirect. The direct effect is well understood, per GOULDING-1988: the output filter's response varies with the time dependent shape of the charge arrival, being the convolution of the two. A trapezoidal filter greatly reduces this effect in the absence of exponential decay.
The indirect effect source of ballistic deficit is due to fluctuations in charge loss through the feedback resistor with differing risetime, as seen in
FIG. 3A
with two risetime
Momayezi Michael
Warburton William K.
Barbee Manuel L.
Hoff Marc S.
Townsend and Townsend / and Crew LLP
Warburton William K.
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