The present invention generally relates to apparatus and methods for detection of ionizing radiation, particularly but not exclusively X-rays, and is usable in a variety of fields including e.g. medical radiology, computerized tomography (CT), microscopy, and non-destructive testing.
Gaseous avalanche detectors, in general, are very attractive for detection of ionizing radiation. The main advantages of these detectors are that they are cheap to manufacture compared to e.g. solid state detectors, and that they can employ gas multiplication to strongly (on many orders of magnitude) amplify the signal amplitudes.
In a typical gaseous detector, an incident photon interacts with a gas atom, which as a result thereof emits a primary ionization electron, which electron in turn produces electron-ion pairs that are further multiplied in a gas avalanche, and this avalanche is detected in a position sensitive readout arrangement by means of integrating the charge induced by the avalanche, or by simply counting the avalanche. By such approach, single-photon detection can be performed.
One commonly employed detector of this kind is the multiwire proportional chamber, see e.g. S. E. Baru et al., Multiwire proportional chamber for a digital radiographic installation, Nuclear Instruments and Methods in Physics Research A, Vol. 283 (Nov. 10, 1989), pp. 431-435. In its basic configuration, the multiwire proportional chamber includes of a set of thin anode wires stretched between, and parallel with, two cathode planes. Application of a voltage between the anode wires and the cathode planes creates an electric field within the chamber. Electrons emitted in the gas by ionization of gas atoms, caused by incident radiation photons, drift towards the anode wires, and when approaching the thin wires they enter a strong electric field and experience ionizing interactions with gas molecules. The ensuing avalanche multiplication of electrons provides a noiseless amplification of the charge signal, by a factor as large as 105 or more.
In another similar approach the charge induced by the avalanche electrons is not detected, but visible light emitted as a result of interaction between the avalanche-multiplied electrons and the gas atoms is detected by means of a standard light detection arrangement.
Such light polling method is described in G. D. Bogomolov, Yu.V. Dubrovskii and V. D. Peskov, Photoelectric polling of multiwire gas counters, Institute of Physics Problems, Academy of Sciences of the USSR, Moscow, Plenum Publishing Corporation, 1978, translated from Pribory i Tekhnika Eksperimenta, No. 3, pp. 84-86, May-June, 1978, wherein weak visible radiation of the avalanches near the anode wires is registered. Gas mixtures of He and Xe or Ar and N2 with small amounts of quenching additives such as toluene were used, the purpose of the additives being to suppress secondary processes in the gas mixtures to thereby provide for position sensitive measurements.
Another similar approach is disclosed in G. Charpak, Light-emitting projection chambers for the study of rare processes in particle physics, Nuclear Instruments and Methods in Physics Research, A310, pp. 47-56, 1991. The use of TEA (triethylamine)
Drawbacks of such light polling technique include the need of quenching additives to achieve a position sensitive detection and the need of light emitting additives to provide high yield of light. These additives are chemically aggressive and cause damage to the instrument, especially when ionized by radiation. Furthermore, they emit light preferably in the UV and VUV region which is why the exit window needs to be made of quartz of UV-transmitting crystals which makes the instrument expensive and complicated. Furthermore, the light detector needs to be sensitive to UV-light.
Further, the need for providing sufficient space for wire mounting and high voltage isolation results in losses of X-ray detection efficiency.
Still further, the use of radial wires to solve the parallax problem results in a position resolution limited by the smallest practical anode wire pitch of about 1 mm. Yet further, the use of thin wires (typically less than 100 xcexcm in diameter) in multiwire proportional chambers makes them difficult to construct, and reduces reliability, since one broken wire disables operation of the whole detector.
A particular problem using multiwire proportional chambers for medical imaging is the space charge effect that degrades the detector performance at high X-ray fluxes above 10 kHz/mm2.
Accordingly, it is an object of the present invention to provide an apparatus and method for detection of ionizing radiation, particularly X-rays, which use avalanche amplification and which avoid the above-mentioned problems associated with the prior art.
In this respect there is a particular object of the invention to provide for very high spatial resolution without the need of adding quenching additives to the avalanche amplification substance.
A further object of the invention to provide for light emission without the need of adding chemically aggressive additives to the avalanche amplification substance.
A further object of the invention is to provide such detection apparatus and method, which provide for high signal-to-noise ratios and high sensitivity.
Still a further object of the invention is to provide such detection apparatus and method, wherein isotropically emitted light from the electron avalanches is forced to illuminate a small area of the detector arrangement only to further improve the spatial resolution.
Yet a further object of the present invention is to provide such detection apparatus and method, which are effective, fast, accurate, reliable, and of low cost.
Still a further object of the invention is to provide a device for use in planar beam radiography, which includes a plurality of detection apparatus that attain the above-said objects.
These objects among others are, according to the present invention, attained by apparatus, devices and methods as claimed in the appended claims.
By detecting light emitted from interactions between accelerated electrons and a scintillating substance, which may be the same substance as the one used for ionization and optionally for used for electron avalanche amplification, or may be a separate substance, in geometrically limited regions to obtain geometrical discrimination of light emitted in unwanted directions, an improved spatial resolution is achieved.
The scintillating substance is preferably a noble gas, nitrogen or a combination thereof. These gases are not aggressive to the detector. They emit light preferably in the UV region, and therefore the detector can optionally be equipped with a wavelength shifter at the exit window to convert the UV light to visible light to simplify the construction.
The geometrically limited regions may be an array of separately located scintillating elements, wherein each scintillating element is separated from the other ones by means of a light impermeable wall such that light emitted in anyone of said scintillating elements is prevented from reaching any other ones of the scintillating elements.
Alternatively, the scintillating substance has an extension in the direction of the accelerated electrons shorter than the absorption length of the photons in the substance, typically less than 1 mm, such that a major fraction of light emitted in the scintillating substance is prevented from being re-absorbed in said scintillating substance, and to thereby cause a further photon to be emitted and affect the spatial resolution of said position sensitive light detection arrangement adversely.
The light detecting arrangement, e.g. a lens and a CCD camera or a light sensitive TFT, includes an array of light detecting elements, i.e. pixels, wherein each pixel preferably is arranged to detect light emitted from a respective one of the array of separately located scintillating elements.
By such arrangement a low-cost position-readout system having a large number of channels to obtain high resolution is achieved.
By geometrical discrimination to avoid feedback of scintillating light, the detector may be used at higher amplification levels than the detectors described in the prior art.
Further, the detector apparatus may be provided as a modular system, wherein all portions but the light detecting arrangement is provided as a first module, an amplification and conversion module, whereas the light detecting arrangement constitutes a second module, where the modules can be releasably attached and aligned prior to use. Such solution is very flexible and provides for the use of particular light detecting arrangements, e.g. including a fiber coupler, a magnification lens, light sensitive TFT, or an image intensifier, in some applications.