Radiant energy – Invisible radiant energy responsive electric signalling – Neutron responsive means
Reexamination Certificate
1999-07-15
2002-03-26
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Neutron responsive means
C250S390030, C250S370050
Reexamination Certificate
active
06362485
ABSTRACT:
This invention is concerned with improvements in and relating to instruments and methods of measuring, particularly, but not exclusively with regard to neutron radiation.
Accurate measurement of neutron dosimetry is important in ensuring that environments intended for operations involving personnel access are accurately surveyed and/or that personnel operating in environments where significant radiation exists receive a dose suitably below the limit permitted.
In most cases the dose is made up of neutrons of different energies forming a spectrum. The spread of this spectrum and the proportions of the dose in each part are significant because of the differing level of biological hazard they present.
In general in existing instruments the neutrons are detected and the results presented as a total dose equivalent. The relative hazard of different energies varies, however, and so a weighting is applied to the results or the instrument is designed to obtain an approximate weighting by physical means, in obtaining this equivalence. A typical indication of the weighting given to the different energy levels is indicated in FIG.
1
. Prior art devices face a number of problems including the following.
Firstly many prior art instruments are limited in terms of the range of neutron energies they can monitor. Problems particularly occur at the high energy range. Such instruments are typified by a polyethylene sphere with a central detector. The volume of polyethylene necessary to attenuate high energy neutrons is prohibitive in terms of weight and size. Such devices, therefore, detect neutrons in part of the energy range with low efficiency.
Secondly the sensitivity of prior art instruments unfortunately varies across the energy range of interest. Thus an instrument may pick up a value of counts
Sv a factor of 6-10 different (peak to trough) at different neutron energies across the range. Such a sensitivity profile is illustrated in
FIG. 2
for a prior art device. Thus if the device is calibrated at high energy levels the reading in the presence of a lower energy source can be quite erroneous. This can lead to access problems being identified for areas which are not actually subjected to high radiological dose or to unduly low values being determined. The use of supplemental spectroscopic measurements to establish the spectrum of neutrons detected and so apply an absolute correction factor is time consuming, expensive and impracticable.
These inaccuracies, and others, have been tolerated in the past as to an extent it was thought the worst deviations occurred in parts of the neutron energy range which were not of the greatest importance or where little dose was received. In recent times however it has been realised that the neutron risk factor is greater than previously accepted (a weighting of 20 for instance, rather than 10 relative to gamma photons is now applied at some energies) As a result increased sensitivity with less deviation throughout a wide energy range is sought.
Additionally as a third problem, the weighting factors applied to different parts of the spectra are periodically updated. The nature of many prior art instruments means that such changes would require wide scale reconfiguration in the device, even necessitating hardware replacement in some cases or acceptance of larger deviations. A more versatile instrument is thus also desirable.
The device should desirably be readily portable in terms of size and weight. Hand held applications are common and the operator is frequently called upon to hold the device at arms length to avoid interactions caused by the operator's body.
According to a first aspect of the invention we provide an instrument for detecting radiation, the instrument comprising an inner neutron detector and an outer neutron detector, the inner and outer detectors being separated by an inner layer of neutron attenuating material, an outer layer of neutron moderating material being provided around the outer detector.
The instrument is preferably a survey instrument. The instrument is preferably used away from the operator. A separation of at least 50 cm, more preferably at least 1 m and ideally far greater may be employed.
Attenuation and/or moderation for the neutrons detecting is preferably only provided by the instrument, for instance when considered relative to ambient ions in the environment.
Preferably the inner layer of neutron attenuating material substantially or completely surrounds the inner detector. Preferably the inside of the inner layer conforms to the outside of the inner detector.
Preferably the outer layer of neutron moderating material substantially or completely surrounds the inner layer and/or outer neutron detector. Preferably the inside of the outer layer conforms to the outside of the inner layer and/or outer neutron detector.
The outer neutron detectors may be provided in or on a continuous or substantially continuous neutron detector carrier layer. The neutron detector carrier layer may be sandwiched between the inner and outer layers.
Preferably the inner layer is spherical. Preferably the inner surface of the inner layer is defined by a radius of between 1 and 8 cm, more preferably 2 to 5 cm. Preferably the outer surface of the inner layer is defined by a radius of between 3 and 13 cm, most preferably 4 to 8 cm.
Preferably the outer layer is spherical. Preferably the inner surface of the outer layer is defined by a radius of between 3 and 13.5 cm. The outer detector carrier layer maybe between 0.05 and 0.5 cm. Preferably the outer surface of the outer layer is defined by a radius of between 7 and 25 cm.
The inner detector may be cylindrical of cross sectional radius between 2 and 5 cm and of length between 5 and 15 cm.
The inner layer may be defined as a hollow cylinder with or without hollow hemispherical ends.
The inner surface of the cylindrical portion may be defined by a radius of between 1 and 5 cm. The outer surface of the cylindrical portion maybe defined by a radius of between 4 and 7.5 cm. The hemispherical end's inner surface maybe defined by a radius of between 1 and 5 cm. The hemispherical end's outer surface may be defined by a radius of between 4 and 7.5 cm.
The inner surface of the inner layer may be defined by a right cylinder of total length between 5 and 15 cm and/or of a radius between 1 and 5 cm. The outer surface of the inner layer may be defined by a right cylinder of between 10 and 25 cm in length and/or of radius between 4 and 7.5 cm.
The outer layer may be defined as a hollow cylinder with or without hollow hemispherical ends.
The inner surface of the cylindrical portion may be defined by a radius of between and 4 and 8 cm. The outer surface of the cylindrical portion may be defined by a radius of between 8 and 16 cm. The hemispherical end's inner surface maybe defined by a radius of between 4 and 8 cm. The hemispherical end's outer surface maybe defined by a radius of between 8 and 16 cm.
Alternatively the hemispherical end's inner surface may be defined by a right cylinder of radius between 4 and 7.5 cm and/or with a length into the hemisphere of between 4 and 9 cm.
Preferably the inner layer comprises a plurality of layers. The layers may be of different materials and/or be of different properties. Preferably the inner layer comprises a first and second layer. Preferably the inner surface of the second layer conforms to the outer surface of the first layer. The first and second inner layers may be spherical or present as a hollow cylinder with or without hemispherical ends or as a right cylinder. The second layer may be thicker than the first and outside the first. The inner layer may be of composite form. The layers may both be of substantially constant thickness throughout. The thickness of the two layers may be the same or different to one another.
Preferably the outer layer is formed from a hydrogen containing material. Preferably the material is a plastic, such as polythene. The use of high density polythene is particularly preferred, for instance having a densit
Bartlett David Thomas
Jones David Glyndwr
Joyce Malcolm John
More Brian Robert
Tanner Richard John
Beyer Weaver & Thomas LLP
British Nuclear Fuels PLC
Gagliardi Albert
Hannaher Constantine
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