Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-05-17
2001-10-09
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370040, C250S336100, C250S337000
Reexamination Certificate
active
06300635
ABSTRACT:
TECHNICAL FIELD
This invention relates to an X-gamma dosimeter responsive to low energy.
The term “low energy” designates energy comprised in the range from about 10 keV to about 50 keV.
The invention is in particular implemented in the field of radiation protection.
BACKGROUND OF THE INVENTION
Most known electronic dosimeters, in particular portable dosimeters, are limited in the low energy range for reasons related to measuring the dose absorbed by an appropriate material, which will be very dependent on this material under about 50 keV.
In fact, dosimetry consists in measuring, from a measure obtained in general by means of a silicon based detector, the dose theoretically absorbed by human tissues.
At high energy, i.e. at energy exceeding about 100 keV, the dose left in the human tissues and the response of such a detector are simply linked together by a proportionality factor which is hardly energy dependent.
Consequently, compensation is easy.
On the other hand, as soon as energy is less than 100 keV, this factor begins to be energy dependent.
This makes it necessary to fine-tune the dosimeter's configuration allowing, among others, for the addition of absorbent materials in front of the detector itself.
However, the mitigation in such absorbent materials is all the more important since the energy of incident X and gamma radiation is low.
Consequently, the dosimeter is essentially no longer responsive to radiation with energy values of less than 50 keV.
And yet, in the field of radiation protection, a low energy radiation involves a risk, especially for the first millimeter of skin where a very large superficial dose can be left.
Therefore, in the field of radiation protection, there is a strong demand for the development of dosimeters with a low energy limit on the order of 10 keV, or even less.
Of course, this demand has increased due to the emergence of new European standards as far as radiation protection is concerned.
In particular, in the case of portable dosimeters, where the X and gamma radiation detector used is a standard silicon based photodiode, there is no simple technical solution available regarding the above-mentioned limitation.
As mentioned above, the 50 keV energy limitation is substantially due to the fact that filters have to be used for compensating the detector's response in terms of dose measuring.
Unfortunately, these filters function as very efficient absorbents at low energy, so that most known dosimeters are not responsive to low energy radiation.
SUMMARY OF THE INVENTION
It is an objective of the present invention to remedy the preceding disadvantages.
With the invention it is possible to compensate the dose response absorbed at low energy, not by adding filters in front of a radiation detector, but by using two radiation detectors responding differently in terms of doses.
One of the detectors overestimates the dose absorbed at low energy with regard to the human tissues.
On the contrary, the other detector underestimates this absorbed dose.
By electronically summing these two responses according to an optimized linear combination it is possible to return perfectly (i.e. within less than ±30%, which is the uncertainty tolerated by current standards) the actual dose absorbed by human tissues, and this is true for a large energy range from about 10 keV up to several MeV.
More generally speaking, the invention combines, in a specific way, the signals provided respectively by the detectors so as to return in the best possible way a dosimetric quantity to be measured.
The present invention provides an X-gamma dosimeter comprising:
a means for detecting X and gamma radiation, such detection means being provided for supplying signals that are functions of the dose absorbed by said detection means, and
an electronic means for processing such signals, said electronic processing means being provided for determining a dosimetric quantity from the signals,
this dosimeter being characterized in that the detection means comprises a first X and gamma radiation detector and a second X and gamma radiation detector, said first and second detectors responding differently in relation to the dose absorbed at low energy, the first detector overestimating the dose absorbed at low energy with regard to human tissues, the second detector underestimating the dose absorbed at low energy with regard to human tissues, and in that the electronic processing means is provided for building a combination of signals respectively supplied by the first and second detectors, said combination being optimized so as to report accurately the dosimetric quantity within a large energy range.
Said combination can be a linear combination, with the coefficients of this linear combination being optimized so as to report accurately the dosimetric quantity to be measured.
It is also possible to use a non-linear combination of signals respectively supplied by the first and second detectors for measuring this dosimetric quantity.
The latter can be the dose absorbed by human tissues.
According to a first particular embodiment of the inventive dosimeter, the electronic processing means is provided for reporting accurately the actual dose globally absorbed by human tissues.
According to a second particular embodiment, the electronic processing means is provided for reporting accurately the actual dose absorbed by human tissues for a specific tissue depth.
In this case, the electronic processing means is for instance provided for reporting accurately the equivalent of the superficial individual dose, labeled as HP(0.07), or the equivalent of the deep individual dose, labeled as HP(10).
Said dosimetric quantity can also be the equivalent of the ambient dose, the dosimeter then being used as a radiation meter.
The first detector is preferably a silicon based photodiode.
The second detector is preferably a diamond based detector.
It should be noted that the diamond offers the advantage of having an atomic number (Z=6) lower than the average atomic number of human tissues, so that a diamond based detector can underestimate the dosimetric response.
According to a first particular embodiment, the electronic processing means comprises, for each of the first and second detectors, a measuring channel called first measuring channel and comprising a current preamplifier followed by an analog-to-digital converter, with a view to measuring large doses.
According to a second particular embodiment, the electronic processing means comprises, for each of the first and second detectors, a measuring channel called second measuring channel and comprising a charge preamplifier followed by a discriminator, in turn followed by a pulse counter, with a view to measuring small doses.
These two particular embodiments can be combined together.
The electronic processing means then further comprises a switch provided for sending the signals supplied by the first and second detectors, either to the corresponding first measuring channels or the corresponding second measuring channels, with a view to measuring either large doses or small doses.
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“Evaluation of the dosimetric characteristics of a diamond detector” Rustgi et al 2389 Medical Physics; 22(1995) May; No. 5; Woodbury NY US; pp. 567-570.
Brambilla Andrea
Pochet Thierry
Commissariat A l'Energie Atomique
Gabor Otilia
Hannaher Constantine
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
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