CA1091823A - Ionization detector - Google Patents

Abstract

IONIZATION DETECTOR

Abstract of the Disclosure An ionization detecting fire alarm device that comprises a double chamber structure, a source disposed in at least one of the chambers and a vernior adjusting screw electrode protruding into one chamber. The chamber containing the adjustable elec-trode is more open to the atmosphere than the other chamber.
Porting id provided between chambers and detection occurs by sensing the rate of change of ionization current in the chamber structure. The source or sources, one being in each chamber, is a beta source such as a nickel 63 source. A change in ionization current is detected by a unique circuit of this invention which comprises a programmable unijunction transistor oscillator circuit.

1.

Description

~/7; 10~1823 1 The present invention relates, in general, to ionization detectors and is more particularly concerned with a device for detect:ing fires which preferably employs a beta source, although the teachings of this invention may also be applicable to use with other types of sources.
There are numerous different types of ionization fire alarm devices which are known. These detectors typically comprise one or two chambers and one or two radio-active sources. These devices operate on the basic principal of a change in the ioniza-10 tion current within the chamber upon detection of products ofcombustion and aerosols in the atmosphere where the detector is located.
Most of these detectors, including virtually all commercial detectors, employ an alpha source such as Americium 241. While these sensors have gained acceptance and are widely used in ~ire d~ection systems, i~ is well known that alpha particles are very much more hazardous than beta particles. It has been argued that normally the radiation is trapped within the ionization chamber and thus there is no problem. However, there are circumstances 20 which have occurred wherein a detector using alpha particles has become hazardous. For example, situations have arisen after a fire where detectors have been lost in the rubble thus making disposal of the rubble a problem .
Accordingly, to make a safer device, it would be desirable to construct a detector using a low activity beta radiation source.
Even some of the prior art patents such as U.S. Patents 3,573,777;
3,271,756; 3,295,121; and 3,560,737 have mentioned the beta source as a possible radiation source for ionization detectors. However, generally speaking rhere is no detector currently available that 30 uses a beta radiation source. There are many factors that may account forthis lack of a use of the beta source. Generally, beta sources which have been considered were of the high activity type

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1 and thus were not suitable for constructing compact detectors.
Other beta sources, such as Tritium, have a short half-life and present mechanical problems, such as migration. Therefore, these detectors were not suitable for use in ionization detection. In accordance with this invention preferably a low activity beta source is used such as nickel 63.
A further problem in the prior art with the use of beta sources is the extremely low ionization current that is available with these detectors. This usually results in difficulties with 10 the associated electronic circuitry as well as producing problems regarding detection of extraneous noise signals. In accordance with this invention, the design of the chamber structure and the choice o the circuitry greatly reduce the problem of the low ionization current.
Still another problem associated with known ionization detect-ors is that, because the detectors may be used in different environ-ments, it is dificult to prodùce a detector that will operate I suitably in all o these environments without requiring adjustment j in the field. In the past, many o these detectors were subject j 20 to humidity changes and air density changes which affected ¦ the sensitivity of the detector. Also, another problem with known ¦ detectors using radio-active sources is the tolerance of the source itself. While dimensions within the chamber can be held to a -very close tolerance, radiation activity differs from source to source.
For example, U.S. Patents 3,295,121 and 3,271,756 reveal a means for adjusting voltages at the ionization chamber output.
However, these means rely on the alteration of the chamber geom-etry or the adjustment of distance electrodes. This is a complex 30 mechanical adjustment and will not give the degree of control as that provided by the adjustment means o the present invention.
With the adjustable electrode of thepresent invention, detectors

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`` ` 10~18Z3 1 may be constructed ~ith wide variations in sources from one detector to another.
Accordingly, it is one object of the present invention to provide a safe and reliable apparatus for detecting products of combustion and aerosols in a gas or typically the atmosphere.
A further object of the present invention is to provide a detector which is easy to produce and easy to adjust for optimum performance.
Another object of the present invention is to provide an 10 improved ionization detector comprising a double chamber structure with one of the chambers being the basic sensing chamber with porting being provided between the cha~bers to compensate for slow ambient changes. The sensing chamber is preferably ported to both the secondary chamber and the atmosphere outside of the cham-ber structure.
Still a further object o the present invention is to provide a simple means of adjusting the voltages available from the ioniza-tion chamber. Actually, one adjustable electrode can be used in each chamber if it is a two chamber structure.
Another object of the present invention is to provide an ionization chamber structure that comprises baffles for directing the air to be sensed and that further comprises an electrostatic screen for the ionization chamber or chambers.
Still another object of the present invention is to provide a unique electronic circuit which will provide an inexpensive and reliable means for detecting the signal change which occurs in the ionization chamber.
A further object of the present invention is to provide a means for adjusting the decision level of the alarm circuit of 30 this invention to allow for any desired sensitivity setting.
Still a further object of the present invention is to provide means associated with the circuitry for providing a ' ' ~ , ' ~ 1091823 vi~ual indication of the condition of the ionization chamber `~
structure.
Another object of the present invention is to provide a three chamber structure characterized by a built-in feedback path that regeneratively stabilize~ the operating point of the device.
A further object of thi3 invention is to provide a ~pecially de~igned chamber con9truction including electrodes shaped that enhance the efficiency of the chamber and reduce ion recombination.
A construction in accordance with the present invent-ion compri~es an ionization detector having means defining a chamber having port ~eans for receiving gases from outside the chamber. Fixed position electrodes are associated with and at least in part define the chamber and are ~paced from one abother. Means including a radioactive source are disposed in the chamber for e~tablishing an ionization current in the chamber between the electrodes. An adjustable particle captur-ing member is contained in the chamber spaced from the radiation source and movable to alter the ionization current. Means for supporting the particle capturing member with the member having an end extending through one of the electrodes into the chamber ;-are also provided. The area of the end within the chamber i8 variable to vary the number of particles captured thereby finely adjusting the ionization current. Means are also coupled from the electrodes for detecting changes in the ionization current.

- 109i823 In a preferred embodiment of the invention there is provided an ionization detector which comprise~ a three ch~mber atructure which preferably comprises mesh means defining at least two of said chambers with a third partially op6~n chamber defirled by structure within one of said first two chambers. The mesh means, in addition to deining the chambers also define~, respectively, opposite main electrodes of the detector. In the preferred embodiment an adjustable electrode or particle capturing member ~upported from one of said mesh means and may be rotated in the chamber structure to finally adjust the ionization current to its optimum value.
~he structure in this preferred embodiment is also improved in that the main electrodes although insulated from each other are directed towards each other so as to more cloqely follow t~le ion distribution within the chamber. It has been ~ound ~hat with the triple chamber ~he optimum operating point is provided while yet compensating for non-fire con-dition3.
Numerous other objects, features and advantages of the invention will now become apparen~ upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view through one embodi-ment of the detector of this invention:
FIG. 2 is a cross-sectional view through a different embodiment of the detector;
FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2 FIG. ~ is still a further cross-sectional view of a slightly different embodiment of the invention !~s4/7. :

1 FIG. 5 is a somewhat schematic cross-sectional diagram of - -~ another embodiment employing a different adjustable electrode;
¦ FIG. 6 is a circuit diagram associated with the detector of s ¦ this invention;
FIG. 7 is a cross-sectional view of still another embodiment o the present invention which is a preferred form of the invention using a three chamber structure;
FIG. 8 is a cross-sectional view along line 8-8 of the detector shown in FIG. 7; and FIG. 9 is a curve showing the typical distribution for beta radiation with the detector of this invention.

In one embodiment, the chamber structure of the present invention is constructed in two separate sections and is preferably provided having three separated fixed electrodes or -~
¦ plates. In addition to the fixed electrodes, one of the chambers also has extending thereinto an adjustable electrode or particle capturing member which may be in the form of a vernier adjusting 20 screw or an adjusting plate.
FIG. 1 shows one embodiment for the chamber structure which comprises an insulated cylinder 10, a top conductive plate 12, a bottom conductive plate 14, and an intermediate conductive plate 16. The cylinder 10 is suitably supported in a printed circuit board 18 having an opening therethrough of appropriate size to receive the cylinder 10. The printed circuit board 18 has terminals for receiving connections from the chamber structure.
The plates and cylinder define a bottom chamber 20 and a top chamber 22. The cylinder at its bottom end has a plurality of 30 slots 24 so that the chamber 20 is virtually open to the outside environment allowing for free movement of air through the chamber 20. The chamber 22, on the other hand, contains one or more , 7.

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1 orifices 26 wllicll permits any slow changes in the outside environ-ment to be communicate~ to chamber 22. Passages also exist in plate 16 so that any changes in the environment in chamber 2~ are commutated to chamber 22. In this way slow variations are not detected by the chamber structure of this invention.
Preferably, there is one source 28 in chamber 20 and one source 30 in chamber 22. Alternatively, if only one source is used, preferably source 28, which is disposed in chamber 20, is used. Preferably, the source is used in the chamber that also containers the adjustable electrode.
The chamber structure may be supported by an insulated base 32 having a mesh screen or shield 34 supported therefrom about the cylinder lO. This shielding prevents r.f. and static pickup. In the embodiment shown in FIG. l, it is noted that the plate or electrode 14 is conductiv~ly coupled to the shield 34.
FIG. l also shows the baffle 36 which is suitably secured to su~port base 32. This baffle 36 directs the air stream and yet limits the air stream passing to the detector. The detector is supported by means of support posts 38 and 40 both of which may be hollow. These support posts support the printed circuit board 18 at opposite ends from a main support frame 42. The posts 38 ancl 40 may have wires running therethrough so that connections can Ibe provided from the chamber structure to the circuitry discussed later in FIG. 6.
IAs previously mentioned, one problem with detectors that ¦use radioactive sources is the tolerance of the source. While the dimensions within the chamber can be held to a very close tolerance, radiation activity differs characteristically from source to source. In accordance with this invention adjustin~
means are provided to enable the detectors to be constructed with a wide variation in the source that is employed. To achieve this an extra adjustable electrode 44 is employed. This ,,.
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~J~ 091823 t 1 electrode has a screw thread that is received by a threaded nut suitably supported in the wall defining the cylinder 10. The electrode may be electrically connected to any of the collector plates 12, 14 or 16 or may even be connected to a separate ref-erence voltage. In the preferred embodiment, the electrode 44 is connected to either plate 12 or plate 14. In FIG. 1 it is noted that tlle electrode couples to plate 14 and is also shown being conductively tied to a point on the printed circuit board 18.
The electrode 44 extends into the ionization chamber 20 a predetermined distance. In this way the electrons are captured by this adjustable electrode and the voltage between the plates 14 and 16 is consequently increased. As previously mentioned the electrode can simply be an adjusting screw which is adjusted to protrude into the chamber to varying depths. The further that the ~lectrode protrudes into the chamber the more electrons are cap-tured and the voltage between the plates 14 and 16 is increased.
With this adjustable electrode it is thus possible to vernier-adjust the voltage level between the plates 14 and 16 to an optimum level which is preferably about one half the supply voltage.
In FIGS. 2-6, reference characters are used like those shown in FIG. 1 to identify like parts. Thu~, for example, FIG. 4 shows the printed circuit board 18, insulating cylinder 10, plates 12, 14 and 16, and chambers 20 and 22. Chamber 20 has a series of elongated slots 24. In this embodiment there are two sources 28 and 30 disposed respectively in chambers 20 and 22. The adjust-ing electrode 44 is like that shown in FIG. 1 and the basic chamber structure is also like the chamber structure shown in FIG. 1. ~lowever, in FIG. 4 the bottom plate 14 terminates in deflector ends 46 and 48 each having perforations therein. The structure shown in FIG. 4 and in the other drawings is basically of cylindrical shape as is the outer collar 50. The collar 50 ~, .

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-` 10918Z3 `i~/71' 1 also has o~e or more apertures 52 for causing an equilization inany slow changes between the outside environment and the environ ment inside of tlle collar 50. The deflector ends are essentially arranged concentrically around the chamber. The arrangement incluclin~ the downwardly extending wall 51 of the collar 50 prevents direct horizontal or vertical air movement into the chamber 20.
~ 'IGS. 2 and 3 show still another embodiment of the present invention. This embodiment is supported by the printed circuit board 18 and comprises base plate 14 and associated source 28;
intermediate plate 16 and associated source 30 and caps 55 and 56 -The plate 16 has at least one port p~,ssing therethrough for com-munication between the chamber 20 and 22. Insulating ring 58 separates the plate 16 from the printed circuit board section 59 A ring 62 extends below the board 18 and supports a wire mesh 64 betwe~n the ring 62 and the support base 14. An annular sliding ring 66 fits within the base 14 and has an aperture 67 which may be aligned with the aperture 69 (see FIG. 3) to permit access inside of the chamber 20 for cleaning or replacing the Z0 source 28 contained therein.
The cap 55 may be constructed of a solid metal or a metal mesh. The cap is secured to the section 59 of the printed circuit board by soldering. Cap 56 is preferably a metal mesh having three bottom tabs 60 fitting into holes in the printed circuit board 18. The ring 62 mates with the tabs 60, as shown, to electrically connect the cap 56 and ring 62 (also mesh 64).
The top of ring 62 extending above board 18 is soldered to board 18.
In the embodiment shown in FIGS. 2 and 3 there is not dis-closed any adjustable electrode. However, this electrode couldsimply be supported for insertion into the chamber 20 through the mesh 64.

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--^ 1091~23 ;;~/,-1 Referring now to FIG. 5, there is shown a partial cross-sectional and schematic diagram disclosing a structure quite similar to that shown inFIGS. 2 and 3. In this arrangement there is provided a lower mesh 64 that is open and provides quite free acce.ss into the chamber 20. Mesh 64 connects at its top and at a number of points to cap 56 as shown in FIG. 5.
The board 18 has a like number of passages for receiving the tab of cap 56 and top end of mesh 64. The caps 55 an~ 56 are con-structed of a mesh that is quite closed with quite small aperture as schematically depicted in FIG. 5. A port 65 is provided above the plate 14 so that there is access to the source 28 for clean-ing this source. The source 30 may be cleaned by removing the caps 55 and 56.
The embodiment of FIG. 5 differs from that shown in FIGS.
2 and 3 primarily because of the adjusting screw 44 which has a vane 45 disposed along its length. As the screw is rotated, the surface area presented to the ionization path varies thus altering the current within the chamber. With this structure, the adjust-ing screw can provide an adequate range of adjustment through one revolution of the screw or less.
FIG. 6 shows a preferred circuit for connection to the ionization chamber for generating an alarm condition upon detec-tion o~ smoke. The detection chamber shown in FIG. 6 may be of the type disclosed and previously discussed with refe~ence to FIG. 1. In this construction, there are provided the two chambers 20 and 22 each respectively housing beta sources 28 and 30. The plate 13 couples by way of protect-ion circuit 70 to the positive voltage supply and plate 14 along with adjusting screw 44 couples ~ the negative voltage supply. The adjusting screw 44 is prefer-ably adjus~ed so that the voltage at plate 16 is at the desiredoptimum level which is typically one half the positive supply voltage.

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~09i823 l The protection circuit 70 comprises dio~e Dl, resistors Rl and Rll, and capacitor C6. This circuit provides line conduct-cd r.f. interference protection. The basic voltage nlaintained across the detection chamber is established by the Zener diode Zl.
'l'his d;iode or a like voltage regulator may be used to insure a stable voltage supply for the ionization chamber and the associated circuitry. Capacitor CI is preferably of a relatively low value such as .01 microfarad. These two parallel arranged capacitors provide transient and r.f. protection to the chambers and the associated circuitry.
Transistor Tl is a field effect transistor having its gate electrode coupled from the plate 16 of the detection chamber. The drain electrode of the transistor couples to the positive supply line and the source electrode of the transistor couples by way of resistors R2 and R3 to the minus voltage line 72. The transistor 'rl is pre~ferably contained within the shield as clearly indicated in ~IG. 1. This transistor is a source ollower which converts the extremely high impedance at its input gate electrode to a more manageable value at the source electrode of the transistor. The resistors R2 and R3 form the load for the field effect transistor.
Capacitor Cf is a relatively low value bootstrap capacitor connected between the node of resistors R2 and R3 and the gate electrode of the transistor. The purpose of this capacitor is to minimize the influence of r.f. radiation and transient signals that may occur at the node of resistors R2 and R3. The voltage at the node 74 is coupled to two separate but like circuits one of which is relaxation oscillator 75. This oscillator comprises resistors R4, R9, R10, and Rll, capacitor C3, light emitting diode (LED) 76, and programmable unijunction transistor 78. The reference voltage for the oscillator 75 is established by resistors R10 and Rll. The node between these resistors couples to the gate electrode of the transistor 78. The values of : . ?

:10918Z3 . t/711;

1 resistor R4 ~nd capacitor C3 are chosen so that there is a relative ly long pulse rate of, for example, one pulse every ~ive seconds for illuminating LED 76. The purpose of the oscillator 75 is to sùper~rise the conditioll of the ionization chamber. The resistors K10 and Rll are preselected so that the voltage at the node therebetween is lower than the source voltage of transistor Tl i~
the chamber is functioning properly. Under these conditions, the oscillator 75 is operating and the LED 76 produces a periodic light pulse to indicate the operative condition of the chamber.
The resistors R10 and Rll may be adjusted so that the voltage at the node therebetween is, for example, ~5 volts. This voltage mi~ht correspond to a source voltage at transistor Tl of, for exàmple, ~8 volts.
The node 74 also couples by way of resistor R5 to a simi~ar type relaxation oscillator circuit 80. Gircuit 80 comprises resistors R5, R6, R7 and R8, variable resistor VRl, capacitor C5 and programmable unijunction transistor 82. The reference voltage at the gate of transistor 82 is set by means of the variable resistor. This voltage is set at a higher voltage than the voltage at the gate of transistor 78. This voltage set by variable resistor VRl is set above the quiescent (no alarm) voltage at the node 74 by an amount dependent upon the sensitiv-ity required. Thus, the voltage at the node 74 must rise by a predetermined amount before there is an output from the cathode electrode of transistor 82. The output from the transistor 82 may be connected directly to an alarm system or via a gating circuit to provide isolation from other sensors. Alternatively, this out-put can be connected to a suitable device such as an SCR or relay.
The resistor R5 and capacitor C5 are chosen to give the proper delay which may be on the order of five seconds. This delay insures insensitivity to transient conditions that occur in the circuitry or that are induced extraneously.

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4/7~ 109~823 1 ~lany existing circuits employ comparators ~or detection or voltage variations at the ionization chamber. However, in accord-ance ~ith this invention, it has been found that the use of progranlmable unijunction transistors for supervising tlle voltage levels has distinct advantages over comparators. For one thing, these comparator circuits are generally more expansive and the circuitry is more complex especially if a time delay and trigger circuit are to be combined with the comparator. On the other hand, a programmable unijunction transistor circuit in accordance with this invention provides a delay, voltage sensing and an adjustable trigger level while also providing excellent noise immunity.
Additionally, the capacitor of the circuit is fully discharged at tlle end of each cycle, thereby providing a known datum from which a charge cycle can be determined. This is especially useful wh~never theoutput is connected to a pulse counting circuit ~or alarm purposes. Another major advantage to the circuit of this invention is that the stored charge in the capacitor C3 is used to illuminate the light emitting diode, thus removing the necessity of a relatively large intermittent load being applied to the power supply.
~ hen the ionization chamber detects the presence of smoke, the impedance between the plates 14 and 16 increases, and thus the source voltage of transistor Tl also increases. This voltage increase is coupled by way of resistor R5 from node 74 and after a delay period determined by resistor R5 and capacitor C5 the transistor 82 conducts. When this occurs, an alarm condition is generated from a signal at the cathode of transistor 82. With the chamber structure of this invention, atmospheric changes over a relatively long period of time are not detected, as the chamber structure provides for equalization of the environment in this condition. ~loweverJ when a change in atmosphere occurs relatively rapidly, as when smoke is present, this smoke enters the chamber 14.
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~091823 1 20 relatively rapidly and causes an almost immediate detection.
FIGS. 7-8 show a preferred form of the present invention, employing a three cllamber structure, which has been found to provide the optimum working point for a detector while compensat- -ing for non-~ire conditions. This arrangement preferably comprises the use of Beta sources which permit a closer inter-electrode spacing. This is because of the low energy and the attendant short range of these nickel 63 sources. With this embodiment in order to operate at the proper operating point, a third compensat-ing chamber is used. This, in effect, "pads" the detection chamber with a high impedance that results from current flowing between the center electrode and the outer electrode/screen.
With regard to the embodiment shown in FIGS. 7 and 8, the current derived in the third chamber is a unction of the current in the re~erence chamber, as ionization o~ both chambers is caused by the same source due to the relatively large opening be-tween these chambers. Therefore, a feedback condition exists where the reduction in current in the detection chamber causes a decrease in voltage across the reference chamber and an increase in voltage across the third chamber. This increase in voltage will cause a greater proportion of ions generated by the reference chamber source to be captured by the third chamber electrode and results in the reduction of the effective impedance of the third chamber. This, in effect, stabilizes the operating point of the detector. Also, for slow ambient changes, current changes occur in all three chambers, thereby compensating for these slow ambient changes without the need for requiring a gas communication between chambers.
In accordance with the present invention, the spacing between the main electrodes is optimized. FIG. 9 shows or a particular source a distance at which maximum ionization occurs. By provid-ing the main electrode spacing at or about ~lis distance, optimum j ~/ 71~ 1091823 1 conditions exist for detection. I~hen the electrodes are spaced too closely, if the gas density increases, the number of molecules within the path of the particle increases and theionization current will increase, thus causing an imbalance. If the air density decreases, the reverse will occur, with an imbalance in the oppo-site direction. But, on the other hand, where the electrode spacing is too large, re-combination effects are larger and will tend to increase with an increase in ear pressure. By optimizing the spacing, ionization is complete for low values of air density and, as the density increases, it will not result in an increase in the total ionization, thus resulting in a more stable operating point.
FIGS. 7 and 8 show a preferred form of the present invention which comprises a screen mesh ~ap 88, a second screen mesh cap 90, electrode g2, and common electrode 94. The screen mesh cap 88 has a circular base 89 for holding this screen mesh in a supported position. Similarly, the screen mesh 90 has a circular base 91 or supporting the mesh cap 90. One of the main electrodes 93 is shown in the form of a plate supported at the bottom of the cap 90 in the position shown in FIG. 7. The plate 93 accommodates one of the radioactive Beta sources 95. Access to the radioactive source may be provided by means of an opening 99 in the mesh cap 90. This enables cleaning or replacement of the radioactive source.
Both of the caps 88 and 90 have a plurality of tabs~l00 which secure both of the caps to a printed circuit board 102. The tabs 100 may connect to runs on the printed circuit board, and, in addition to securing the caps to the printed circuit board, func-tion as connections for dielectrically tying the two caps together so that they are maintained at the same potential.
- One of the other main electrodes 92 ls also secured to the printed circuit board 102 and has bottom legs extending thereto as 16.

. : . . . -~ ogi823 1 shown in FIG. 7. The electrode 9Z is of cap shape but has an opening 97 which is preferred in accordance with the teachings ~-of this invention.
FIGS. 7 and 8 show components such as transistor 103 connect-ed also to the printed circuit board 102. A support 104 which is constructed out of insulating material holds the common electrode 94. FIG. 8 shows one terminal of the transistor 103 connecting by ~
means of lead 105 to the main common electrode 94. A second radio- --action source 106 is supported from the electrode plate 94. The device, including the caps 88 and 90 and the printed circuit board 102, is supported by suitable means such as studs 108 from a support structure 110 which may be constructed of an insulating ~;
plastic material.
The top end o mesh cap 88 has an inwardly threaded bushing 110 for supporting the adjusting electrode 112. In this embodi-mcnt, the electrode 112 is constructed of a dielectric insulating material. The electrode 9Z also includes an aperture opening that may be threaded for receiving and guiding the adjustable elec-trode 112. The electrode 112 operates analogously to the electrode 44, for example, as shown in FIG. 1. But even though the electrode in this embodiment is constructed out of insulating material, it also functions as a means for capturing electrodes and varying the ionization current through the chamber.
With the embodiments shown in FIGS. 7 and 8, a circuit like the one shown in FIG. 6 may be used. In this case, the electrode 92 may be connected to a positive voltage supply; the electrode 93 to a ground or a negative voltage supply, and the signals are coupled to transistor 103 from the common electrode 94.
As previously mentioned, it is preferred that both of the caps 88 and 90 be connected together, and connected also to the same potential, which may, for example, be ground voltage or a negative voltage. It is also preferred that the electrode 92 be 17.
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:;4/,1 ~ 10~1823 1 open to permit somc ionization current to flow to the outer electrode represellted by the mesh cap 88. This structure thus modifies the current which ~lows from the electrode 94 to the electrode 92 with the outer mesh electrode 88 capturing some of the ionization current. This arrangement functions as a high resistance shunt between electrode plates 94 and 93, it being recalled that electrode 93 and mesh caps 88 and 90 are all at the same potential. When smoke enters the area between elec-trodes 93 and 94, the current is reduced, but, as smoke cannot enter the upper two chambers, there is a modifying effect which allows the detection chamber to operate at a desired part of its characteristic with regenerative stabilization.
The distribution of ions within the chamber structure tends to be cone-shaped, especially when the source is an Alpha radio-active source. Thus, it has been realized in accordance with the present invention that the electrodes can provide increase collection of ions by restructuring the shape of the electrodes.
More particularly, it is desirable to decrease the path length and thus the electrodes such as electrode 92 extend downwardly as shown in FIG. 7 toward the common electrode 94. Also, the caps 88 and 90 terminate near the printed circuit board, also creating smaller path lengths for the particles between these electrodes and the common electrode 94.

This application is a division of application Serial No. 284,655 filed August 10"1~77.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An ionization detector comprising:
means defining a chamber having port means for receiving gases from outside of the chamber, fixed position electrodes associated with and at least in part defining the chamber and spaced from one another, means including a radio-active source disposed in the chamber for establishing an ionization current in the chamber between the electrodes, an adjustable particle capturing member contained in the chamber but spaced from the radioactive source and movable to alter the ionization current, means for supporting the particle capturing member with said member having an end extending through one of said electrodes into the chamber, the area of said end within the chamber being variable to vary the number of particles captured thereby finely adjust-ing the ionization current, and means coupled from the electrodes for detecting changes in the ionization current.

2. An ionization detector as set forth in claim 1 including at least two chambers having a common electrode therebetween, said particle capturing electrode extending toward said common electrode.

3. An ionization detector as set forth in claim 2 wherein one of said electrodes is disposed within one of the two chambers to thereby define a third chamber therein.

4. An ionization detector as set forth in claim 3 wherein said one electrode defining the third chamber is at least partially open.

5. An ionization detector as set forth in claim 4 wherein said one electrode is cap-shaped having an opening for receiving the particle capturing member.