This invention relates to a particle detector. More specifically, it relates to a device for simultaneously measuring alpha and/or beta particles with the same detector element.
Conventional detection devices for the measurement of alpha and beta particles employ, in addition to solids detectors, proportional counting tubes which generate pulses having heights proportional to the energy emitted by the particles. The separation between alpha and beta radiation is accomplished in these prior art devices by an energy analysis; that is, the separation process depends on the assumption that at a lower operating voltage only the alpha pulses exceed the counting threshold and at a higher operating voltage both alpha and beta pulses are measured. If two energy channels are provided, alpha and beta radiation can be detected simultaneously and separately. The same result can be realized by the use of a double counter if a separating foil is employed to assure that no alpha radiation enters the second counter.
The acuracy of these methods depends on the assumption that all alpha particles emit more energy than beta particles, or when a "mechanical" foil separation is employed, that the "reach" of all beta particles is greater than that of alpha particles. However, both requirements are met only in part and therefore lead to erroneous measurements.
For example, in large area counters, beta particles emit up to 200 keV of energy. Depending on the counting gas, alpha particles, which have already lost the majority of their energy due to absorption and self-absorption, emit the same amount of energy. Accordingly, under these conditions, alpha particles can not be distinguished from beta particles using the prior art energy analysis method. In practice, this means that a pure alpha radiator simulates an additional beta radiator and, depending on the operating voltage, a pure beta radiator may simulate an additional alpha radiator.
Depending on the operating voltage applied in a "mechanical" separation, the low-energy beta component, which is always present because of the continuous beta spectra, is lost or is recorded as alpha radiation. In difference measurements, alpha and beta particles cannot be distinguished if they both emit the same energy.
These basic errors become evident in measurements with proportional counting tubes and with solids and liquid detectors. The error occurs particularly with large counting tubes, when there is diffuse impingement of radiation and in certain counting gas mixtures such as CH.sub.4
Since prior art detector devices, particularly those employing proportional counting tubes, cannot distinguish alpha particles from beta particles of the same energy if the alpha particles have lost energy before they entered the detector device, it is an object of the present invention to provide a device for detection of alpha and/or beta particles of any energy. That is, the invention is an improvement over prior art devices in which alpha particles which have lost energy within the radiation source cannot be detected or distinguished from beta particles.