Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
2001-10-03
2004-05-11
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S374000, C250S375000
Reexamination Certificate
active
06734433
ABSTRACT:
TECHNICAL FIELD
The present invention concerns an ionization chamber, an activity measuring channel for a &bgr; radiation gas emitter, which can for example be a tritium detection chamber, and an implementation process for this.
STATE OF PREVIOUS TECHNIQUES
An ionization chamber tritium channel such as described in the document in reference [1] at the end of the description, is used to measure the activity of a &bgr; radiation gas emitter in a given gaseous environment, for example in a glove box, a laboratory ventilation circuit or furthermore a stack control in a nuclear building. The ionization chamber which is immersed directly in the environment to be controlled, provides a current in proportion to the activity to be quantified. Process electronics facilitate the measurement of amperage between 10
−14
and 10
−8
A.
Such a measuring channel is made up of an ionization chamber, which plays the role of detector, a pre-amplifier, signal processing electronics, and a link cable between the pre-amplifier and the processing electronics.
The ionization chambers of today's technology are of two types:
the massif anode and cathode chambers—a chamber of this type illustrated in
FIG. 1
comprises a central electrode
10
in nickel brass forming an anode, a shell
11
in brass forming a cathode, a filler hole
12
, an end
13
for adapting a pre-amplifier, a guard ring
14
in nickel brass, insulators
15
in polystyrene and O-rings
16
.
Such chambers are used for the measurement of low activity, for example below 5,000 Admissible Contamination Limit. These are generally of large size, for example about 10,000 cm
3
.
Anode and/or light cathode chambers—a chamber of this type illustrated in
FIG. 2
comprises removable electrodes
20
and
21
, in practice a central anode
20
and a cathode
21
formed by a brass, copper or aluminium grid and is fitted with a filler hole
22
. In this case it is set out in a shell
23
in steel, with two openings
24
and
25
respectively for filling and draining. Furthermore it comprises an O-ring
26
, a guard ring
27
, insulators
28
in polystyrene, an outlet
29
to a high voltage and an outlet
30
to a pre-amplifier.
Such chambers are dedicated to measuring higher activities (glove box, or even certain manufacturing processes). They are generally small-volume chambers, for example 100 cm
3
. The use of light electrodes for the measurement of intense activity is aimed at limiting the surface contamination of electrodes which generates considerable background noise.
In the channels of today's technology, a pre-amplifier assembled directly on an ionization chamber facilitates amplifying the current which can be very low up to a satisfactory level so that it can be transmitted through the connector cable to the processing electronics.
Two methods are currently used for the signal process electronics to quantify the current due to ions produced by the &bgr; radiation and picked up on the electrodes of the ionization chamber:
capacitor load (load quantifier)
high resistance voltage read on the terminals in which the generated current circulates.
Electronics of the first type are acknowledged to be more accurate because drift over a period of time is lower. Electronics of the second type are less complicated to implement and less expensive.
What is more the signal processing electronics must have considerable measuring scope (from 10
−14
A to 10
−8
A, or from 10
−12
A to 10
−6
A), which means an automatic rating change-over.
Such measuring channels of today's technology have numerous drawbacks:
the prohibitive cost of electronic systems allowing measurement of very low currents,
the important background noise of light electrode chambers,
in certain cases these chambers are inoperative after one single measurement,
the impossibility of decontamination by baking (400° C.) under vacuum of existing ionization chambers,
the lack of heater systems in all chambers, in particular those of less than 100 cm
3
in volume—measurements are in fact more stable when the atmosphere in the vicinity of the chamber can be heated (stabilization at a given temperature, convection stirring, impact on hygrometry, etc.),
the need to replace a complete chamber+base+connector unit in case of breakdown or after important contamination which is very expensive, as today's chambers are in fact welded to their connection systems and to their mechanical base so as to limit noise generated by the contact resistors,
the need to have recourse to guard rings in order to correctly delimit lines of electric fields in the ionization chamber, as certain pieces of the framework of today's chambers (excluding cathode, anode and connector)are in fact made in conductive material,
the absence of leaktightness of the mechanical base which is the interface between the ionization chamber and the connector,
generation of a not inconsiderable weight (today about 1 kilo) of contaminated waste at the time of replacing a complete detector (chamber+base+connector),
the multiplication of measuring channels (250 measurement lines constantly operational in a base nuclear installation treating tritium) which involve important installation costs—a complete measuring channel (detector+process electronics+cable) costs between 72 kF (about 14 kF for a detector of 100 cm
3
and about 58 kF for the electronics) plus about 126 kF (68 kF for a detector of 10,000 cm
3
and about 58 kF for the electronics).
The purpose of the present invention is to overcome the shortcomings of first generation devices.
ACCOUNT OF THE INVENTION
The present invention concerns an ionization device cylindrical in shape comprising an ionization chamber with an anode made up of a central rod in current-carrying material, and a cathode in current-carrying material around the said anode, both connected to two elements of a mechanical base of the said chamber, two cylindrical end shields in non-magnetic and insulating material, centred on the anode and arranged at right angles to this at both ends.
As the cathode is made up of a spooled wire on the outer rim of these two end pieces, typified in that the base is removable, the lower extremity of the anode and the two ends of the wire making up the cathode connected to pin plugs arranged on the lower end plate, being suitable for insertion in the sockets arranged on a contact holder unit on the base, and in that the end shields are provided with openings.
In a convenient realization method, the base is fitted in its two extremities—upper and lower respectively—with a contact holder unit and connector, the said unit comprising the sockets arranged on the lower end shield and connected by conducting wires to connector lugs. A cylindrical protective shield for the chamber is fixed on the upper part of the base. Conveniently the anode is in stainless steel, the two end shields are in Teflon (PTFE), in ceramics or a mixed material (ceramics+Teflon), and the cathode comprises a platinum wire, for example 0.05 mm in diameter.
The present invention equally concerns an activity measuring channel for a &bgr; radiation gas emitter, comprising such an ionization chamber, a pre-amplification unit assembled just behind the ionization chamber, offset signal process electronics and a connector cable between the pre-amplification unit and the process electronics. Conveniently the pre-amplification unit makes an analog to digital conversion.
The measuring channel can, for example, serve as a tritium measuring channel.
The present invention also concerns an implementation process for this ionization chamber such as provided by one of the three following variants, in which a heat current is circulated in the cathode:
during the measurement so as to create movements of convection of the gaseous mixture to be measured,
during the measurement so as to stabilize both the sensor temperature and impact on the hygrometry of the gaseous mixture,
at the time of decontamination vacuum baking, as the temperature
Bachelet Franck
Le Serrec Christian
Meunier Gilles
Commissariat a l'Energie Atomique
Hannaher Constantine
Moran Timothy
Thelen Reid & Priest LLP
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