Process and apparatus for the simultaneous selective detection o

Details Process and apparatus for the simultaneous selective detection o Process and apparatus for the simultaneous selective detection o

1994-05-09

1996-01-02

Willis, Davis L.

Radiant energy

Invisible radiant energy responsive electric signalling

Neutron responsive means

25037011, G01T 120, G01T 300, G01T 306

Patent

active

054811147

DESCRIPTION:

BRIEF SUMMARY
The invention relates to the simultaneous and selective detection of flows of neutrons and gamma or X photons, more particularly for nuclear diagraphy and measurements of specific burn-ups on irradiated fuel assemblies.
The detection of neutrons in the presence of a large gamma photon flow has always proved difficult. One of the processes used in the prior art consists of using detectors such as .sup.10 BF.sub.3, .sup.3 He proportional counters, intrinsically disturbed by high X or gamma dose rates, whilst surrounding them with a shielding effective with respect to said radiation. Another process consists of using fission chambers insensitive to said radiation, but which have a limited efficiency for neutrons.
It is often necessary to simultaneously detect the emissions of neutrons and X or gamma photons from a radiation source. This is generally accomplished in the prior art by using two detectors, one for the neutrons and the other for the X or gamma photons.
The prior art in this field can e.g. be illustrated by FR-A-2 317 668 or DE-A-1 564 271. Both these documents refer to an apparatus using two detectors, one for the neutrons and the other for the photons, both producing visible photons by scintillation, said two detectors being combined with the sane photomultiplier. The problem of this selective detection consequently consists of using means making it possible, from among the scintillations taken into account by the photomultiplier, to obtain a selection of those which are due to the neutrons and those which are due to the X or gamma photons. In the prior art, the selection of scintillations respectively relative to these two radiation categories is performed at the output of the scintillator by a processing of the signal using special, distinctive characteristics of said sane scintillations as a function of whether they are due to neutrons or photons. Essentially this discrimination either takes place on the pulse rise time (FR-A-2 317 668), or on differences of the decay time of the pulses emitted by the two scintillators (DE-A-1 564 271). In both cases, this signal processing phase leads to expensive, complex spectrometric electronics.
Another document "United States Statutory Invention Registration" No. H590 describes a scintillation detector sensitive both to neutrons and gamma rays as a result of the presence of two scintillators, namely an external scintillator for the thermal neutrons and an internal scintillator for the fast neutrons and gamma photons. However, in the above prior art, discrimination takes place in the electronic output systems by shape measurements with respect to the pulses and the counting of the latter one by one after selection using complex and costly equipment. Therefore, in said detector, the scintillator sensitive to the gamma photons is a plastic scintillator which, by its very nature, is unable to supply photoelectric peaks usable in spectrometry.
The present invention relates to a process and an apparatus for the simultaneous selective detection of neutrons and X or gamma photons making it possible with the aid of means which can be simply realized, to completely separate the two scintillation types using a single energy channel spectrometric measuring system, each of the channels corresponding to one of the two scintillation types to be separated.
The process for the simultaneous selective detection of neutrons and X or gamma photons with the aid of a detector having two scintillators, one sensitive to neutrons and the other sensitive to X or gamma photons, and combined with a photomultiplier-type apparatus, is characterized in that the physical characteristics of the two scintillators and/or a possible wavelength converter positioned between the scintillators and the photomultipler apparatus are chosen in such a way that the peak of the scintillations due to the neutrons and the photoelectric peak due to the X or gamma photons are located in wavelength bands which are clearly separated beth from one another and from the electronic background of the apparatus.


REFERENCES:
patent: H590 (1989-02-01), Chiles et al.
patent: 2994769 (1961-04-01), Sun et al.
patent: 3566118 (1971-02-01), Peters
patent: 3988586 (1976-10-01), Stuart et al.
patent: 4829185 (1989-05-01), Cerff

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