Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1998-09-28
2001-08-28
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S362000
Reexamination Certificate
active
06281502
ABSTRACT:
DESCRIPTION
1. Technological Field
This invention relates to a device and a method of remotely locating and of displaying sources of &agr; particles, that is to say, particles the energy of which is generally less than 10 MeV.
The invention can be used notably in the field of radioprotection in order to locate radioactive sources on a surface or within a volume.
The device, object of the invention, may be applied in numerous other fields such as the decommissioning of nuclear installations, dismantling and maintenance operations, a radioactive inventory, post accident operations, or as an aid to operators with regard to following a process under operation.
The invention also finds applications in the detection of leaks of radioactive gases and the detection of the presence of a radioactive gas (in particular radon) or radioactive contamination in the form of an aerosol.
In a general way, the invention essentially relates to the detection and location of strongly ionising particulate radiation (with high linear energy transfer).
2. State of the prior art
&agr; radiation is the natural radioactivity most frequently occurring during the disintegration of nuclei whose atomic mass is greater than 200. This takes place through the emission of a particles which are doubly ionised helium atoms.
Particle detectors generally supply information resulting from the interaction of the particles which pass through them with the sensitive part of the latter.
When an &agr; particle passes through a medium, it suffers an energy loss essentially due to the ionisation and the excitation of the medium passed through. The phenomena of diffusion and of Bremstrahlung radiation appear to be negligible.
Ionisation can be understood as the removal of one or more electrons from an initially neutral atom. The electron removed can remain free or may attach itself to another atom and form a negative ion.
Two major phenomena come into play during the creation of these pairs of positive and negative ions: about ⅓ of the ionisations are produced by primary ionisation, that is to say by direct interaction with the &agr; particles, the remaining ⅔ being created by secondary ionisation, produced by fast electrons emitted during the primary ionisation. These fast electrons bear the name “&dgr; rays”.
As previously indicated, the passage of an &agr; particle in the medium can also be expressed by simple excitation of the medium.
An atom is considered to be excited when it passes from a stable energy state to a state of higher energy. The excitation energy is nevertheless insufficient to eject an electron. In general, the energy used up by excitation is subsequently dissipated either in a non-radiative fashion, that is to say in the form of thermal energy of vibration or of translation, or in radiative fashion, that is to say by the emission of photons.
A medium in which a non-negligible fraction of the absorbed energy is released in radiative fashion through the emission of photons is called a scintillator.
Hence known &agr; particle detectors in general, comprise a solid or liquid scintillating medium that allows the “conversion” of the &agr; radiation into photons and a photomultiplier system or a sensitive surface for detecting the photons emitted by the scintillator.
These detectors are not however capable of remotely locating a source of &agr; radiation. In effect, the free path of the &agr; particles in the air is very small and it is necessary to bring the detector to the direct vicinity of the source in order to carry out the measurements.
The scintillators are not necessarily solids or liquids. They may also be in gaseous form.
The scintillation of gases excited by nuclear particles has been known since the beginning of research into radioactivity, but it was only in 1951, thanks to the use of photomultipliers that the study of the mechanism of the emission linked to the passage of a particle within a gas was able to be undertaken by Grün and Schopper. A year later, C. Muehlhause used a gas as a scintillator in nuclear physics.
Numerous studies have been made of the scintillation of the noble gases. In effect, in any gas, the atoms can interact with one another and in so doing transfer a part of their excitation energy. If an atom is in a complex molecule, the energy transfer can take place in the form of thermal energy of vibration or rotation, thereby not creating any light emission. The noble gases on the other hand, due to their highly stable electronic structure, can only transfer the energy of one excited atom to another, at the time of collision.
Studies have shown that air also has scintillation properties and that the spectrum of air, excited by sources of &agr; radiation, is made up of a series of bands identical to those observed in the emission spectrum of nitrogen.
Argon present in air has excellent emission properties but the very low proportion present means that its contribution to the luminescence of air is negligible.
The oxygen in the air does not emit fluorescence but on the contrary acts as a quenching substance. In effect, even a small quantity of oxygen mixed with a gas can obstruct its scintillation properties. By way of example, the presence of 2% oxygen in a mixture of oxygen and xenon causes a reduction of the order of 70% in the scintillation amplitude of the xenon.
Air, because of this quenching phenomenon has a very low scintillation yield. The scintillation yield is only a few photons per &agr; particle.
In pure nitrogen, the number of photons emitted is considerably greater than the number of photons emitted in air. However the scintillation phenomenon remains relatively weak.
Table I below shows that the pressure of the gas is also an important parameter for the scintillation effect of gases and, in particular, nitrogen.
Table I gives an example of the number of photons emitted by an &agr; particle as a function of the pressure of the scintillator gas (nitrogen) for a source of particulate radiation.
TABLE I
Pressure (hpa)
No. of photons emitted by an &agr;
331
1970
350
1310
833
1060
It is apparent that the total number of photons reduces with an increase in the pressure. This is due to the fact that at high pressure, the number of collisions between atoms and molecules of the gas increases. The increasing number of collisions causes a greater loss of non-radiative energy.
Because of the poor scintillation properties of nitrogen and in particular of air, and the high cost of noble gases, the only use of gases as scintillators is confined to experimental counting devices.
Documents 1 and 2, references to which are given at the end of this description give examples of detection installations that use a gas such as, for example, nitrogen, as a scintillator. In these installations, the source and the scintillator are positioned in proximity to a detector such as a photographic chamber or a photomultiplier, so as to carrying out counts or spectroscopic studies of the luminescence of the gas scintillator.
Document 3, the reference to which is also given at the end of the description, states that neither nitrogen nor air is of practical interest as a scintillator. The presence of nitrogen in a scintillation counter using a noble gas is even considered to be undesirable.
STATEMENT OF THE INVENTION
The invention stems from the surprising fact discovered by the inventors that despite the very poor scintillation properties of nitrogen and above all a nitrogen-oxygen mixture such as air, it is not impossible to use these gases as a scintillator for the remote location of radioactive sources emitting heavy charged particles.
By remote location one understands that this occurs at a distance from the source that can be very substantially greater than the path of the &agr; particles in the gas or in air (which is a few centimetres).
In effect the invention relates to a method of remotely locating sources of &agr; particles in an environment in which one uses a gas containing nitrogen to fill the surrounding space, in order to convert a particles emitted by the sources into p
Imbard Gerald
Pineau Jean-Francois
Commissariat A l'Energie Atomique
Hannaher Constantine
Pearne & Gordon LLP
LandOfFree
Remote &agr;source location device and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Remote &agr;source location device and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Remote &agr;source location device and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2454663