Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Transient or portion of cyclic
Reexamination Certificate
1998-09-14
2001-04-03
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Transient or portion of cyclic
C324S076110, C324S111000, C250S370010, C250S370130, C368S113000, C702S190000, C702S193000
Reexamination Certificate
active
06211664
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for measuring the rise-time of signals, which have been disturbed or interfered with by noise, coming from semiconductor detectors used for &ggr; or X radiation.
The invention has applications in the field of single photon spectrometry, namely &ggr; or X radiation spectrometry, when the flux of X radiation is sufficiently low to permit a single photon detection. In particular, the device according to the invention can be integrated into the spectrometric measurement correction device described in French patent application FR-A-2 738 919.
PRIOR ART
Various types of detectors for the detection of or X radiation having a low flux are known to the expert. In particular, detectors based on semiconductors (CdTe or CdZn Te) are well known in the art.
Although said detectors are particularly intended for the detection of &ggr; photons, they also permit the detection of X-rays, in the case where there is a low flux of the latter. Throughout the remainder of the description, only the case of &ggr; radiation will be described, but it is obvious that this can also cover low flux X-radiation.
CdTe or CdZnTe semiconductor detectors have the advantage of directly performing the energy conversion of the &ggr; radiation in the semiconductor material, without involving intermediate stages, such as the emission of visible photons in the case of scintillators. This obviates synonymous efficiency loss coupling problems. The energy necessary for creating an electron-hole pair in a semiconductor is much lower than in a gas or in a scintillator (approximately 4 eV in semiconductors, as opposed to 30 eV in gases and 300 eV in photomultiplier scintillator systems). Thus, the number of free charges created by the detected photon is greater, which makes it possible to obtain better energy resolutions. Moreover, the high density and atomic number of semiconductor materials make it possible to use much smaller detection volumes than those of gas detectors or scintillators, whilst maintaining the same quantum detection efficiency.
The use of these semiconductor materials as X or &ggr; radiation detectors involves the deposition of two electric contacts on the surface of the material, at whose terminals a bias voltage is applied. The charge carriers, i.e. the electron-hole pairs created by the interaction of the &ggr; photon with the material, separate under the action of the electric field, the electrons migrating towards the positive electrode and the holes towards the negative electrode. The capacity of these charge carriers to migrate towards electrodes without being trapped by defects present in the semiconductor material conditions the energy resolution of the measured spectrum. This capacity, also known as the charge carrier transport property, is measured by the mobility and the life of the electrons and holes.
When the product mobility x life is comparable or lower than the migration time of the charge carriers to the electrodes, the energy resolution is mediocre.
Numerous methods have been conceived for limiting the trapping of holes, based either on an electric field effect, or on a geometrical effect, or on an amplitude-rise time correlation measurement, the detected signal then being the sum of the charge of the hole and that of the electron.
French patent application FR-A-2 738 919 describes a process for the exploitation of the signal supplied by a semiconductor detector, which makes it possible to obviate the poor transport properties of the holes, producing a signal representative of the rise time of the electronic component of the signal supplied by the detector, i.e. the component of the total signal corresponding to the collection of electrons resulting from the interaction of each &ggr; photon with the semiconductor material.
This document also describes a device for implementing this exploitation of signals. Said device has various means for treating in analog manner signals from the detector, including a peak detector, to whose output is connected a rise discriminator to the peak of the signal. This discriminator is implemented by means of a transistor, which charges a capacitor to the peak level of the signal. The output of said discriminator is connected to means for determining the end of rise time.
In said device, a high pass filter, positioned upstream of the peak detector, ensures the transmission of the input signal. Thus, said high pass filter must have a cutoff frequency fc lower than ½&pgr; t, in which t is the maximum rise time of the electronic component of the signal.
For a good efficiency of the electronic correction, it is necessary to measure rise times 3 to 10 times lower than the maximum rise time.
The measurement of the rise time with a peak detector is very sensitive to interference signals, which is not disadvantageous when there is a good signal-to-noise ratio, but which can be disturbing in the frequent case where said S/N ratio is mediocre, i.e. when the signals to be measured are disturbed by the electronic noise.
French patent application FR-A-2 738 693 describes a device for processing information resulting from the interaction of a &ggr; particle with a CdTe detector, which comprises an amplitude circuit making it possible to determine the start time of the measurement of the rise time of a pulse and a timing circuit making it possible to detect the end of the rise time of the pulse. However, said device is only effective in the case where the input signal rises relatively slowly and is subject to very little interference.
However, in practice, signals recovered at the output of detectors are very weak (approximately 10
−15
to 10
−19
coulomb) and the background noise is of the same order of magnitude or higher (said noise being reducible by filtering).
DESCRIPTION OF THE INVENTION
The object of the invention is to obviate the disadvantages of the devices described hereinbefore, by proposing a device making it possible to measure more precisely than the prior art devices, the rise time of signals disturbed by electronic noise, whose signal-to-noise ratio is mediocre.
The device according to the invention makes it possible to obtain freedom from background noise as a result of a frequency filtering, followed by an adapted time filtering, i.e. a filtering in which the gain of the amplifier cascade is adapted to the time characteristics of the signal to be measured and which indicates the duration of the rise time of the signal to be measured.
More specifically, the invention relates to a device for measuring the rise time of the electronic component of a signal disturbed by electronic noise, obtained at the output of a semiconductor detector, in response to the interaction of a photon with the semiconductor material of the detector, characterized in that it comprises:
a differentiating circuit having in combination at least one resistor and one capacitor for implementing a high pass filter for filtering the low frequency background noise from the signal obtained from the detector and
a discriminating circuit incorporating a comparator for performing a comparison between the filtered signal from the differentiating circuit and an offset voltage chosen as a function of the noise level interfering with the signal from the detector.
According to an embodiment of the invention, the discriminating circuit comprises an offset voltage source connected to the negative terminal of the comparator, the positive terminal of said comparator receiving the signal obtained at the output of the differentiating circuit.
According to another embodiment of the invention:
the discriminating circuit incorporates a reference voltage source connected in series to a damping resistor, which is itself connected to the negative terminal of the comparator,
the differentiating circuit resulting from the connection, to the negative terminal of the comparator, of the output of a low pass RC circuit and the connection, to the positive terminal of the comparator, of the signal obtained directly from the detector.
RE
Bonnefoy Jean-Paul
Monnet Olivier
Rostaing Jean-Pierre
Burns Doane , Swecker, Mathis LLP
Commissariat A l'Energie Atomique
Deb Anjan
Metjahic Safet
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