US3028495A - Quenching means and techniques for ionization devices - Google Patents

Description

April 1962 E. E. RANKIN, JR 3,028,495

QUENCHING MEANS AND TECHNIQUES FOR IONIZATION DEVICES Filed March 13, 1959 7 m5 WW 6 IN V EN TOR.

YEW 5 WNW/V, J9. B

United States Patent @fihice 3,028,495 Patented Apr. 3, 1962 3,028,495 QUENCHING MEANS AND TECHNIQUES FOR IONIZATION DEVICES Edward E. Rankin, J12, Houston, Tex., assignor to Hallihurton Company, a corporation of Delaware Filed Mar. 13, 1959, Ser. No. 799,342 14 Claims. (Cl. 250--83.6)

The present invention relates to improved means and techniques for controlling an ionization device of pulse discharge type and particularly a Geiger-Muller'(G.M.) counter used in well logging.

In many types of discharge devices wherein ionization occurs it is desirable to control the ionization process once the same has been initiated. This is particularly true in the use of GM. counters wherein it is desired to shorten considerably the recovery time and thus render the counter capable of counting at higher rates without losing counts. At high counting rates where there is a large probability that counts will fall Within the recovery time, some counts will be lost and thus it is desirable that the recovery time be made as short as possible. Heretofore, many diilerent attempts have been made to provide various expedients for assuring a short recovery time. These involved generally what has been termed in the art as internal quench systems involving the particular choice of gas, or external quench systems involving electrical circuitry which helps the counter to extinguish itself or a combination of the two. The present invention is concerned primarily with circuitry for this purpose.

While the present invention is described in connection with a GM. counter, it is understood that the invention in its broader aspects may be used in conjunction with other similar discharge devices for extinguishing the same once the ionization process has been initiated.

A G.M. counter comprising generally a gas-filled cylinder surrounding a central wire with a voltage applied between the same is a well known device and is considered to be an ion-magnifying device which is sensitive to individual ionizing particles. The resultant flow of charge in discharge devices of this character, except for the socalled proportional counter, is practically independent of the number of ions formed by the original ionizing particle. Thus, in most GM. counters each particle, whether it be an alpha or beta particle or gamma ray, gives rise to pulses of nearly the same size, and each is usually registered as one particle. These counters have now reached a practical state of high development as a means of studying feeble radiations such as those found in cosmic rays and from artificial radioactivity which, for example, may be induced or be present in well bores. While much study has been given in the past to the mechanism of the gaseous discharge in the counters, the exact mechanism, apart from theoretical considerations, does not appear to be too well established.

Briefly, the action of a prior art GM. counter may be described as follows. The electric field immediately around the central wire is high enough so that at the pressure of the gas used any ion entering the space builds up by collision a large number of ions, which in turn build up more ions until the quantity of the charge which finally flows between the inner and outer conductors reaches, for example, the order of coulombs depending, of course, among other quantities, upon the applied potential and until a large ion sheath has developed around the central wire. This charge, collecting on the distributed capacity of the counter, causes the potential across the counter to drop to a point at which the discharge can no longer be maintained, and the charge leaks oil across an external resistance. The circuit then returns to its normally sensitive condition and is ready for a second count and, during this time, the charge which builds up on the capacity causes a drop in potential across the external resistance in the form of a Geiger pulse which is read by a suitable high input impedance means.

In other words, the process of accumulative ionization continues until the potential difierence between the cylinder and the wire'has dropped to a point where ionization by collision can no longer occur. The potential recovers itself according to the time constant R.C. of the circuit, C being usually the distributed capacity and R being the value of the external resistance.

A conventional circuit for measuring the number of counts may involve coupling the external resistance to various kinds of vacuum and gas tubes, which eventually rate, i.'e. the number of pulses per unit time.

The time for recovery of the voltage across the counter,

in the absence of a quenching circuit, may be in the order of, for example, one millisecond. During this time pulses by an ionizing event will be smaller size and thus will possibly not be recorded.

The time during which pulses are recorded, but are of smaller size, is usually termed the recovery time. It is considered by some writers that recovery time starts when the space charge sheath around the central wire has reached a critical radius such that the field at the central electrode hasrecovered to a value equal to that which it would have without space charge and when the threshold voltage is applied to the counter.

Reference has been made to the counting losses at high counting rates. This is also considered to be the result of the ion sheath around the central Wire which limits the Geiger discharge. It appears obvious that if the presence of the ion sheath stops the Geiger discharge from growing, it will also prevent the production of a second avalanche as long as the sheath exists near the center wire. The result is that any ionizing particle in the counter will not be counted if it happens during the existence of the positive ion sheath near the wire. There will be some time during which no Geiger discharges can be produced followed by time during which small discharges will be produced. After all of the ions forming the sheath have been collected, a full-size Geiger discharge is again produced.

The time during which no Geiger pulses are produced is indicated in FIGURE 1 and is commonly called the dead time and the time required before another fullsize pulse can be produced is called the recovery time. At high counting rates where there is a large probability that counts will fall within the recovery time, some counts will be lost. In fact, all of the counts that fall within the dead time will be lost. Some which fall Within the recovery time will be lost unless the sensitivity of the detector is such that it will count pulses considerably smaller than the full-size Geiger pulses.

The action of the GM. counter may be explained also as follows. in the absence of any ionizing particle or ray, the voltage gradient near the central small wire is large then, and when gamma radiation first ionizes the gas with which the counter is filled, the initial ionized particles cause further ionization in a multiplying action. This action has been termed an avalanche. The lighter negative particles migrate very rapidly to the center wire, forming a resulting positive ion sheath around the wire. The heavier positive particles proceed more slowly to the cylinder and their capture by the cylinder walls occurs some time after the capture of the negative particles by the wire. Upon striking the walls of the counter, the positive particles will liberate photons, which are capable of initiating another avalanche. This tendency for a second avalanche to begin can be eliminated, however, if the potential of the wire is reduced to the point Where no appreciable ionization can occur. This process is called quenching and is accomplished using novel means and techniques in accordance with the present invention.

While the invention is described in connection with a Geiger counter in which a single wire extends centrally of a cylinder, it will be appreciated that the same is applicable also to other Geiger tubes in which several Wires are suspended through a cylinder for purposes of increasing etficiency by having more than one region of high potential gradient in which radiation may strike to cause ionization.

An object of the present invention is, therefore, to provide improved means and techniques for controlling the ionization process in a discharge device in which the ionization process may be initiated either by an ionization particle or in some cases by an electrical signal as in the case of, for example, a thyratron tube.

A specific object of the present invention is to provide improved means and techniques whereby the recovery time of a Geiger-Muller counter may be shortened to allow high counting rates.

Another object of the present invention is to provide improved means and techniques whereby a Geiger-Muller counter may be quenched in a relatively short period of time without requiring the use of large-valued series impedances.

Another object of the present invention is to provide improved means and techniques whereby the stability of a Geiger-Muller counter is not appreciably disturbed as a result of changes in potential supplied to these counters.

Another object of the present invention is to provide improved means and techniques whereby the recovery time of a Geiger-Muller counter is influenced to a relatively small degree by the capacitance of the counter or by a series impedance or by the effects of changes in supply potential.

Another object of the present invention is to provide improved means and techniques whereby a Geiger-Muller counter may be quenched without requiring the use of a high voltage, high current-type power supply.

Another object of the present invention is to provide improved means and techniques whereby transistors and vacuum tubes of low voltage ratings may be used in quenching the counter.

Another object of the present invention is to provide an improved quench circuit which allows the use of a high voltage, low current-type power supply.

Another object of the present invention is to provide an improved quench circuit which allows the use of circuit components having low voltage ratings.

Another object of the present invention is to provide an improved quench circuit in which the quench action is obtained by injection of a voltage across a series impedance of reverse polarity derived from a second supply.

Another object of the present invention is to provide an improved quench circuit which incorporates a lowvalue series impedance to accomplish a faster recovery time.

Another object of the present invention is to provide an improved quench circuit in which the quenching action results from a change in current flowing to the counter in contrast to prior art arrangements which depend essentially on the magnitude of current flow through a series resistance.

Another object of the present invention is to provide an improved quench circuit of this character in which the cylinder of the counter may be grounded, thereby rendering unnecessary shielding and insulation.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. This invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 illustrates generally the time variation of the voltage across a G.M. counter as a result of its ionization by an ionizing particle in accordance with prior art operation.

FIGURE 2 illustrates a G.M.. counter embodying features of the present invention lowered in a well bore.

FIGURE 3 illustrates in schematic form a generalized quench circuit embodying features of the present inven tion, specific forms thereof being illustrated in schematic forms in FIGURES 4 and 5 which show respectively the use of a transistor and the use of a tube for controlling a secondary current for quenching purposes.

The present invention is particularly useful in radioactive well logging, and in such case the self-quenched G.M. counter with an associated pulse amplifier is mounted in a well logging tool 5 which is lowered in a well bore as indicated in FIGURE 2. The Geiger pulses are transferred to the upper surface through cable 6 where the same may be further amplified in amplifier 7 before being recorded or indicated on indicating means 3.

The particular pulse amplifier and indicating means are considered conventional and reference is made to the same in the form of blocks in the drawings. In one form a high input impedance input amplifier may be used to trigger a multivibrator circuit having its output connected to an integrator circuit which in turn has its output coupled to a recorder as indicated in FIGURE 2.

The Geiger tube 10 of conventional construction in cludes a central wire 11 of positive potential forming an anode and a metal drum 12 forming a cathode. The anode 11 is connected through the low-resistance primary winding 13 of transformer 14 to the positive terminal of the high voltage source 15 which has its grounded negative terminal connected to cathode 10. The transformer 14 includes two other windings 17A and 17B which are interconnected at junction point 17C. The other terminals of windings 17A and 17B are connected to a second current-producing means indicated generally by the block 18 in FIGURE 3; and specific forms of such means illustrated generally in FIGURE 3 are illustrated in FIGURES 4 and 5.

In FIGURE 4 the junction point is connected to the negative ungrounded terminal of source 16. The other terminal of winding 17B is coupled to the base 22 of transistor 20. This base 22 is returned to ground through resistor 23. The emitter 24 is grounded; and the collector 19 is connected to the other terminal of winding 17A. The transistor 20 is of the PNP type as illustrated and it will be observed that this transistor 20 is normally non-conducting since no voltage normally appears between the base and emitter electrodes. In order to render the transistor conductive, the base is made negative with re spect to the emitter; and this is accomplished when the Geiger tube 10 is rendered conductive by an ionizing particle. In such case there is a surge of current through the primary winding 13 which induces a voltage in the winding 17B of such polarity that a current flows in the series circuit which extends from the junction point 17C, through source 16, through resistance 23 and condenser 21 to the other terminal of winding 17B, thus producing a voltage drop across resistance 23 of such polarity as to render the base electrode 22 negative with respect to the emitter. Thus, the transistor 20 is rendered conductive and a current then flows through the series circuit which extends from the positive terminal of source 16, through the emitter 24, through the collector 19 and winding 17A to the negative terminal of source 16. This current flowing through winding 17A induces a voltage in the primary winding 13 of such polarity as to resist further change in the primary current flowing in winding 13.

It will be seen that this action has the effect of in creasing the inductance of the primary winding 13 in that effectively a higher (ii it exists in the primary circuit which includes the winding 13. The term L has reference to inductance and the term di/dr has reference to the rate of change of current in the primary winding with respect to time.

In the absence of the secondary windings 17A and 178, the current flow in the primary series circuit which comprises the voltage source 15, primary winding 13 and Geiger tube It may be represented, as is well known, by

the following expression E-tr-i L where E is the voltage of source 15, i is the current flowing in the series circuit, r is the resistance in the circuit and L is the inductance of winding 13.

The inductance of a winding is sometimes expressed in terms of the flux i-nterlinkages per unit current in the winding. In the present the current flowing through the winding 17A produces additional flux interlinkages in winding 13 in response to the original current flowing in winding 13 and thus it can be stated that the above-described operation results in increasing effectively the inductance of winding 13.

Stated in other language, the counter or back electromotive force developed in winding 13 is increased by the voltage induced from winding 17A into winding 13. This, of course, results in a lower available voltage across the counter tube and thus shortens its recovery time. The resulting pulse developed in unit 18 is transferred through coupling condenser 30 to a suitable high impedance input circuit of an amplifier where the same is amplified and suitably shaped, if desired, prior to recording. In the present instance, the condenser 30 may have one of its terminals connected to the central wire 11 and the other one of its terminals connected tothe primary winding 31 of an impedance-matching transformer 32 which matches the impedance of the GM. counter to the inner conductor 33 of cable 6. The secondary winding 34 and primary winding 31 each have one of their terminals connected to the grounded sheath of cable 6.

It will also be seen that the transistor 20 is, in response to the original current pulse in winding 13, driven from its normally non-conducting condition towards a saturated condition. This is so since flux produced by winding 17A induces a voltage in winding 17B which renders the base 22 of the transistor more negative and hence renders the transistor 20 more conductive. In other words, there is a regenerative action which continues until the transistor 20 is driven into a saturated condition, after which the current through winding 17A no longer changes and is no longer effective to induce a voltage in windings 13 and 178. In order to allow the flux thus built up in the core of the transformer to decay without the likelihood of an oscillating condition existing, dampening means in the form of a diode 26 is connected across the winding 13.

In FIGURE 5 the auxiliary current-producing means i1- lustrated generally as 18 in FIGURE 3 comprises a triode 40. The cathode 41 of tube 40 is grounded. The control grid 42 is returned to ground through resistance 43 and the bias source 44 having its positive terminal grounded whereby the tube 40 is rendered normally nonconducting. The junction point 170 of windings 17A and 17B is connected to the positive ungrounded terminal of source 46, the terminal of winding 17B is connected to the anode 47 of tube 40, and the other terminal of winding 17A is coupled to the control grid 42 through coupling condenser 48. The action of the circuit shown in FIGURE 5 is like that described above in that a current change in the primary winding 13 (in response to ionization of the Geiger tube 10) results in a voltage induced in winding 17A which in turn results in rendering the tube 40 conducting. junction point 17C, through winding 17A, through coupling condenser 48, resistance 43, and sources 44'and 46 to render the conrtol grid 42 more positive with respect to its cathode 41. This causes a current to flow through the following series circuit which includes the positive terminal of source 46, the winding 17B, the tube 40 and the ground terminal of source 46. This change in current in winding 17B causes a voltage to be induced in both windings 13 and 17A, the voltage induced in winding 13 being in such direction as to oppose further change.

in current through the winding 13, and the voltage induced in windings 17A being regenerative in the sense that it produces further increase in current through winding 17B. This regenerative process continues until the tube 40 is driven towards and in a saturated condition wherein there is no substantial further increase in current in winding 17B. Here again, in order to avoid the possibility of any oscillating condition existing as a result of the decay in the flux built up in the core of the transformer, dampening means 26 in the form of a diode is shunted across the winding 13.

The transistor 20 may, for example, be of the 2N366 type manufactured by Texas Instrument and in some cases may be of the PNP type. I

It will be seen that in FIGURES 4 and 5 a quenching action is obtained by injecting a voltage of reversepolan'ty into the primary winding 13 derived from the means 18. This voltage thus injected is of sufiicient magnitude to partially or completely cancel the voltage supplied from the source 15 and this cancellation of voltage aids the Geiger tube 10 to quench itself.

It will thus also be seen that the above recited objects of the present invention are accomplished in a simple and expeditious manner which does not require the use of a high-value series resistance and without requiring the use of a high vlotage, high current type supply for the Geiger tube.

While the particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. Apparatus of the type described including, an externally actuated ionization device of electron avalanche type, a high voltage direct current source, a transformer having a first, a second and a third winding, said first winding, said ionization device and said source being in series connection, a current control element including a control means, an actuating means including said secondwinding in connection with said control means of said control element for initiating current flow through said control element in response to increasing current flow said first winding and said ionization device, low voltage current means including said third winding connected in series with a low voltage energizing source and said control element for causing said actuating means to regeneratively increase the current flow through said control element while inductively reducing the current flow through said first winding, whereby the voltage potential across said ionization device is quenched to below discharge level immediately following initiation of current through said ionization device.

2. The apparatus of claim 1 in which said ionization device is a Geiger Muller counter.

3. The apparatus of claim 1 in which said control element is an electron tube.

4. The apparatus of claim 1 in which said control element is a transistor. v

5. The apparatus ofclaim 4 in which said ionization device is a Geiger Muller tube.

In such case, a current flows from the i 6. The apparatus of claim 4 in which said control element is driven to current saturation.

7. Current control apparatus including, an ionization device of pulse discharge type, a high voltage energizing source, an inductive means, said ionization device, said high voltage source and said inductive means being in series connection, a current control element including a control means actuating means comprised of said control means in connection with said inductive means for causing current flow through said control element in response to an increasing current flow through said inductive means and said ionization device, means including a low voltage energizing source in connection with said control element, said actuating means and said inductive means for causing said actuating means to regeneratively increase the current flow to saturation through said control element and concurrently to inductively reduce the current flow through said inductive means and said ionization device in response to increasing current flow through said control element, whereby the voltage potential across said ionization device is quenched to below discharge level immediately following initiation of current through said ionization device.

8. The apparatus of claim 7 in which said control element is a transistor.

9. The apparatus of claim 7 in which said control element is an electron tube.

10. The apparatus of claim 7 in which said ionization device is a Geiger Muller tube.

11. The apparatus of claim 7 in which said first inductive means is a transformer having a first, a second, and

a third winding with said first winding in said series connection.

12. The apparatus of claim 11 in which said actuating means includes said second Winding connected with a control means of said control element.

13. The apparatus of claim 11 in which said third means includes said third winding connected in series with a low voltage energizing source and said control element.

14. The apparatus of claim 11 in which a diode is connected in shunted relation across said first winding and in opposite polarity to said energizing source.

References Cited in the file of this patent UNITED STATES PATENTS 2,465,938 Shonka Mar. 29, 1949 2,536,617 Weller Jan. 2, 1951 2,605,429 Herndon et a1 July 29, 1952 2,617,043 Hepp Nov. 4, 1952 2,789,233 Gross Apr. 16, 1957 2,866,100 Leaver Dec. 23, 1958 FOREIGN PATENTS 691,730 Great Britain May 20, 1953 1,164,321 France May 12, 1958 302,397 Italy Oct. 26, 1932 OTHER REFERENCES Spear: Transistorized Radiation Survey Instruments, Nucleonics, June 1957, pp. 100, 102.

Images