Radiant energy – Luminophor irradiation
Reexamination Certificate
1999-07-08
2002-01-29
Hannaher, Constantine (Department: 2878)
Radiant energy
Luminophor irradiation
Reexamination Certificate
active
06342701
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention disclosed broadly relates to the field of time-resolved photon emission and more particularly relates to the field of fast single photon detection and counting and timing.
2. Description of the Related Art
The use of single photon counting for both analytical tools and research continues to increase. Time correlated photon counting TCPC) (also known as time correlated single photon counting or photon timing) has been known for many years, as described in the article entitled “Single-Photon Timing Detectors For Fluorescence Lifetime Spectroscopy” by Graham Hungerford and David J. S. Birch on pp.121-135 of Measurement Science Technology 7 (1996), printed in the UK.
FIG. 1
illustrates a conventional TCPC time correlated single photon detection (SPD) counting system
100
. A target
102
to be sampled is placed under a lens
104
. A pulsed optical source
120
emits a short pulse of photons which is focused onto a target
102
. Note that neither the pulsed optical source
120
nor the target
102
is part of the TCPC system. The source
120
and target
102
are present here for the well-known purpose of calibrating and improving the temporal response function of the TCPC system. For this calibration and improvement to be effective, the duration of this photon pulse should be much less than the temporal response function of the TCPC system. A detector
106
reads photon emissions elastically scattered from the target
102
as focused by lens
104
. The detector
106
produces an electrical pulse corresponding to each of the photons read from the target
102
. The pulsed optical source
120
also provides an electrical trigger pulse (synchronous with each optical pulse)
110
to an input
112
of a timing discriminator and time-to-pulse height converter, hereinafter converter
108
. The converter
108
provides to output
114
a series of difference signals each of which is an analog electrical pulse with a respective maximum and whose magnitude is related to the time difference detected by converter
108
between the trigger
110
and the electrical pulses from detector
106
corresponding to a detected photon of the optical pulse associated with the trigger
110
. An analog-to-digital (A/D) converter
116
converts the analog signal output
114
from the converter
108
to a digital signal for use by a 1-dimensional multichannel analyzer 1-D MCA
118
. The 1-D MCA
118
displays the temporal response function of the apparatus to the pulsed optical source
120
.
The principle of operation, performance and application of many of the different types of single-photon timing detectors is described in the article entitled “Single-Photon Timing Detectors For Fluorescence Lifetime Spectroscopy” by Graham Hungerford and David J. S. Birch on pp. 121-135 of Measurement Science Technology 7 (1996), printed in the UK. As described in detail by Hungerford and Birch, for any given photon detected, geometrical effects arising from the physical dimensions of the single photon timing detector SPD and the statistical nature of the electron generation and amplification or generation result in an electrical output pulse which can vary in amplitude and shape.
In order to achieve good a temporal response function of minimum width with TCPC, the detector amplifiers and electronics must be able to accurately determine when each detected photon actually struck the photodetector. However, for SPDs, this determination can be difficult because of the variation in the amplitude and the shape of the electrical pulses associated with detected photons. To achieve optimum time resolutions with a given photon detector, many analog triggering techniques have been applied.
One analog triggering technique is described in U.S. Pat. No. 4,179,664 issued Dec. 18, 1979 to Michael O. Bedwell, entitled “Constant Fraction Signal Shaping Apparatus.” U.S. Pat. No. 4,179,664 describes a trigger pulse, derived, for instance, from a radiation detector, that is applied to an input circuit which splits the pulse into two components. The respective component signals are acted on by two characteristic circuits, one of which attenuates the first signal component and the other of which delays the second signal component. The respective attenuated signal and delayed signal are applied to a passive element, such as a differential transformer to invert one component with respect to the other and to sum the resulting signals. The output signal of the differential transformer is a constant-fraction bipolar timing signal which is correlated with the time of occurrence of the event identified with the trigger pulse. In order to achieve accurate timing, constant fraction triggering requires that the amplitude of the electrical impulse created by each detected photon can vary, but that the shape of the electrical impulses remains constant. However this a-priori assumption that the shape the electrical pulse created by each detected photon stays constant is only approximately valid for only a very limited range of varying pulse amplitudes. Accordingly, a need exists for a method and apparatus to count and time photons when the electrical pulse shape is variable over a wider range of pulse amplitudes and triggering models.
An alternate approach to achieve optimum time resolution with a given SPD is used in the pico-Timing™ discriminator from EG&G Ortec, Model 9307. The pico-Timing™ discriminator employs conventional edge triggering on the rising edge of the electrical pulse from the SPD. The discriminator attempts to compensate for pulse-to-pulse variations through use of an analog “slewing compensation” circuit which presumes that the slew rate of the rising electrical edge output pulses of the SPD is constant. However, this approximation is valid only over a limited range of pulse amplitudes, and pulses outside this range are not accurately timed. Therefore a need exists for a method and apparatus to provide accurate single photon counting when the slew rates of the SPD is not constant.
At best, the assumption of constant pulse shape or constant slew rate is valid only over a limited range of pulse amplitudes, and pulses outside this range are not accurately timed. To maintain the best timing resolution, pulses outside this range must be rejected. On the other hand, many times the photonic light emitted by photoluminescent or electroluminescent targets which are to be measured by a TCPC measurement apparatus is weak. If a significant fraction of the detected photons needs to be rejected to maintain good timing resolution, the time required to perform a TCPC measurement can become unacceptably long. Accordingly, a need exists for a method and apparatus to provide better time resolution than currently used analog signal processing methods, while not rejecting a significant fraction of detected photons so as to enable measurements of weakly emitting targets or sources. An example of where it is usually not acceptable to reject a significant fraction of detected photons in order to maintain optimum time is described in U.S. patent application Ser. No. 08/683,837 for Picosecond Imaging Circuit Analysis (PICA), now U.S. Pat. No. 5,940,545 and commonly assigned herewith to IBM.
SUMMARY OF THE INVENTION
Briefly, in accordance with the present invention, a system for time—correlated photon counting comprises: at least one photon detector for producing electrical pulses corresponding to photons read from a target; at least one converter comprising: a first input coupled to a trigger output from a pulsed optical source; a second input for receiving the electrical pulses; digital delay measurement apparatus to measure the time difference between the trigger output and the electrical pulses; a digitizer for digitizing at least one criterion related to the electrical pulse; an interface to a storage device for storing the digital delay measurements and for storing the digitized criterion; and shifter circuit or algorithms for time-sh
August Casey P.
Fleit Kain Gibbons Gutman & Bongini P.L.
Gagliardi Albert
Gibbons Jon A.
Hannaher Constantine
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