CA1323454C - Phase shifted feedback electrometer - Google Patents

Abstract

PHASE SHIFTED FEEDBACK ELECTROMETER

Abstract of the Disclosure A detector for airborne alpha radiation includes a ion pulse collecting chamber connected to a phase shifted, 100% negative feedback electrometer circuit. The ion pulse chamber includes a probe surrounded by an 80% open grid, with the spacing between the probe and the grid providing no greater than about 50 ms transit time before collection and having a capacitance of less than 1 pf. The field strength between the probe and the grid is less than 10 V/cm.
The ion pulse chamber is contained within a vented cabinet. If it is desired to count only radon, a negative potential with respect to the grid may be applied to the cabinet to collect positvely charged free ions. Alternatively, the cabinet wall can be made positive with respect to the grid to create a potential-well at or near the grid to provide an enhanced daughter detection mode. If the cabinet wall and the grid are the same potential, both radon and daughter alpha radiation are detected.

Description

FTI 001 P2 1 3~3454 PHASE SHIFTED FEEDBAC~ ELECTROMETER

This is a divisional of Canadian Patent Application Serial No. 564,266, filed April 15, 1988.

Background_of the Invention In the past several years, there has been increasing concern over the health hazard of exposure to radon-222, a radioac~ive gas produced in the uranium-238 natural decay series. This is in part due to an improved understanding of the radiobiological effects of radon, but more importantly to the recogni-tion of an exposure hazard to the general population.The exposure hazard ~o uranium miners and mill employees has been recognized for many years, but it has only been recently that radon-222 and its radioactive progeny have been found to pose a potential health hazard in private dwellings. This is in part due to energy conservation measures that lead to nearly airtight structures with little outside air exchange. Radon levels can build up in the~e closed structures by diffusion from underlying rock and soil through cracks 2~ and pores in concrete floors and concrete block founda-tion walls. Because radon is a gas and its decay products are generally found as suspended particulat~s, human exposure is primarily through inhalation. Frac-tions of these radioactive species are retained in ~he lungs and have the potential of pr~ducing lung cancer.
Present monitoring techniques used for radon detection are either of the continuous type which provide actlvity flux level information on a ~real-time~
basis or of ~he integrating type which provide do~e information for a selected time period. Present con-tinuous type monitors are generally based on gaq ' - :
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~TI 001 P2 -2-proportional or scintillation techniques and are sophisticated ins~ruments more suited to the laboratory than for field or private home applications. The most commonly used integrating type monitors use thermo-luminescence detectors (TLD) or solid state nucleartrack detectors (SSNTD) Which are inexpensive passive devices, but only give integrated dose information for periods of from days to months. ~ecause radon levels can vary an order-of-magnitude over a twenty-four hour 10 period, a home owner needs to have continuous monitoring in order to provide economical heating and air condi-tioning and at the same time control radon levels.
Moreover, a radon source tracking, ventilation control, or remedial ac~ion analysis can only be accurately carried out with continuous type detectors. Thus, there is a need for portable, low cost, low power, continuous detection instrumentation for moni~oring airborne alpha radiation.
The use of ionization chambers as a means of 20 detecting nuclear radiation is an adaptation of a very old art, going back to nineteenth-century work on conduction in gases. The passage through a gas of alpha radiation of MeV energies produce, through ion-ization, approximately 10-14 coulomb of charges 25 before coming to rest. It is upon this effect that the class of detection instruments called ionization chambers are based. The ion pairs produced are collected through the use of an electrostatic field gradient imposed on the ionizing volume. If the field 30 gradient is great enough to result in collection at an electrode before recombination, but insufficient to cause secondary collision ioniza~ion, the chamber i8 said to be operating in the ion or linear region. The linear region generally exists below approximately 100 VJcm. If the field gradient is between lO0 and 1000 V/cm, the chamber is said to be operating in the gas proportional region where a controlled ~gain~ in charge carriers is produced through collision ioniz~tion processes, At field gradients greater than approxi-mately 1000 V/cm, the ~Geiger~ region is reached where every primary ionization results in an avalanche dis-10 charge within the chamber.
The Geiger type counter is not suited forairborne radon detection because its sealed-tube dssign precludes unobstructed sampling of the alpha radiation.
Gas proportional type counters are used for radon 15 measurements, but because this type of counter operates with low electron attachment sample gas and high field gradients, it requires costly and high energy consuming power supplies, pumps and sample gas processing equip-ment for its flow-through chamber.
Ion or electron chamber counters operating in the linear or non-multiplication mode possess the requisite low voltage and current characteristics and are both simple in ~esign and can provide real-time information. They can also be operated in either the 25 current or pulsed mode. However, when operated in the current mode, both the ion and electron chamber suffer from base-line drift and do not discriminate between - different radiations. Moreover, current mode ion and electron chambers are highly sensitive to charged 30 particles, such as smoke, and thus require special pre-filtration of sample gases. The electron pulse, sometimes referred to as the fast-pulse ~ode, can only 1 3~3~

be applied in low electron attachmen~ gas environments where electrons survive long enough to be collected and counted. This precludes the use o~ the electron pulse technique in normal air environments due to the high electron attachment coefficient of oxygen and water vapor. The ion pulse or slow-pulse mode is possible in air environments, but because ion mobilities are nominally a thousand times smaller than electron mobil-ities, the pulses are long (>100 ms), irregularly 10 shaped and poorly suited for electronlc counting.
Thus, although the first pulse chamber~ used to detect alpha radiation in ai~ were of the ion pulse type (Greinacher 1927), the fast-pulse type is employed almost exclusively in commercial instruments. Overhoff 15 (U.S. patent 4,262,203) describes a slow-pulse type alpha monitor, but because ~he concept is based on a flow-through ~hamber, relatively large volumes are required for measurements in low level environmental samples. The one liter chamber used by Overhoff 20 requires a high electrostatic potential of 500 volts to maintain ~ield gradients suf~icient to overcome ion recombination, Moreover, a multi-staged pulse recogni-tion, shaping and amplification circuit is required to produce countable output pulses~ Thus, this approach 25 has the ~ame high power consumption and cost disadvan-tages as fast-pulse gas proportional counting.

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Summary o~ the Invention The above-noted parent application describes a -~mall portable instrument for selectively detecting airborne alpha radiation in a ~real time~ mode directly in an air environment based on an ion pulse collection and counting ~echnique.
The above application further describes a probe type ion chamber design with sufficient active ~ample volume to detect environmental concentration levels (0.1 10 pCi~l) of airborne radon radiation while requiring less than 30 volt electrostatic potentials be~ween the collecting electrodes.
The system further provides a means of producing directly wi~hin the electrome~er stage, 15 amplified ion pulses suitably shaped and narrowed tfast rise and short decay wi~h a full width at half maximum of between 10 and 25 ms) for electronic counting.
Thus, eliminating the need for separate power consuming delay, comparator, and correlation s~ages.
Also provided is an ion chamber design capable of selectively sampling either radon alpha activity or the total alpha activity in a gas I mixture containing both radon and its alpha emitting daughters. This is a particularly desirable Peature 25 because radiobiological efficacy i~ greater ~or the alpha emitting daughters polonium-218 and poloniu~-214 than it is for radon-222 and knowledge of the re~ative concentrations is required for accurate hazard assess-ment.
There is described hereafter the unique combination of a phase shifted, negative feedback, ~ield effect transis-tor (FET~ electromete~ with an ultra, low capacitance, 1 3~3~

open grid chamber design. The electrode spacing and chamber volume are optimized to provide short ion travel while maintaining a rela~ively large sampling volume. Moreover, because this technique works at low S field strengths (<10 V/cm) and accomplishes pulse shaping and amplification directly within the electrom-eter, low voltage, portable power supplies are possible (<15 uA at 18 V).
Discrimination between radon and its alpha 10 emitting daughters is accomplished by biasing the monitor cabinet negative with respect to the grid electrode. Because the daughters are found in air as either positively charged free ions or particulates, they are swept ~rom the sensing volume and collected on lS the cabinet wall and only radon alpha radiation is de~ected.
An enhanced daughter detection mode is obtained if the cabinet wall is biased positive with respect to the grid. In this mode, an electrostatic potential-well 20 is established around the outside of the grid which attracts and holds positively charged daughter particles and ions at a favorable distance for detecting their emitted alpha radiation.
If the cabinet wall and the grid electrode are 25 at the same potential, both radon and daughter alpha radiation are detected. Best separations are accom-plished if the distance between the grid electrode and the cabinet wall is at least one alpha range for the Aighest energy alpha, polonium-214. This distance is 30 found to be at least 7 cm -- the distance reguired to stop a polonium-214 alpha particle in air.

- 6a -The invention of the present divisional relates to a special circuit for use in the electrometer described above. In particular it relates to an electrometer circuit for amplifying short duration pulses including an FET having a low capacitance input and output, feedback circuit means connected between the input and output of said FET
for providing a negative feedback voltage to said input of nearly 100%, and phase shift circuit means in said feedback circuit means for delaying momentarily the application of said feedback voltage, thereby to allow the output of said FET
to have a momentary power gain of sufficient magnitude and pulse width to drive an external counter circuit.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

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Brief Description of the Drawin~s Fig. 1 is a perspective view, partly broken away, o~ a monitor for airborne alpha radiation con-structed according to this invention.
Fig. 2 iS an elevational view of an ion pulse collecting chamber.
Pig. 3 is an electrical schematic diagram of a phase shifted feedback electrometer circuit.
Pig. 4 is a waveform diagram sho~ing the operation of a 100% negative feedback electrometer.
Fig. 5 is a waveform diagram showing the effect of phase shifting, or delayed application of the negative feedback.
Fig. 6 is a waveform diagram showing a phase shifted negative feedback circuit with diode clamping.

Description of the Preferred Embodiment Referring now to the drawings which illustrate a preferred e~bodiment of the invention, the monitor shown in Fig. 1 is housed in a 6~H x lO~W x 7~D metal instrument cabinet 10 with a horizontal divider 12 separating a lower electronic compartmen~ 14 from an upper ion chamber enclosure 16. The metal cabinet serves as an electromagnetic radiation shield for a sensitive ion probe 20, but has screened openings 30 and 31 for allowin~ free ~low of air to the probe 20.
The exterior of the cabinet 10 is also provided with a commercially available six digit LCD counter and display device 35, a battery test button 37, a battery test beeper 40 for indicating when new batteries are 3n needed, and a radon/enhanced/total alpha switch 45.
The interior of the cabinet 10 includes an electrometer package 50 ~shown in ~ig. 31 and a plurality of batteries 55.

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As shown in Fig. 2, the ion pulse collecting chamber or probe 20 includes a highly open grid cylin-drical wall 21 which serves as one electrode, a center rod 22 which serves as the other electrode of the ion S eollection chamber and a high resistance insulator 23.
The unique features of this chamber are: (1) the elec-trode spacing 24 is set, according to t~e field use~, so that any ion created within the chamber will have no greater than 50 msec transit time before being lQ collected, and (2) the open grid reduces the chamber capacitance and at the same time increases the effective chamber volu~e by counting a por~ion of the alpha particles that originate within one alpha range distance 25 outside the chamber wall 21. The effective chamber boundary is defined in Fig. 2 by the dashed line 26.
A typical coaxial cylindrical chamber of one inch diameter and two inches long with an 80% open area outer wall has a capacitance o~ less than 1 pf and results in a calibration factor of 0.3 cpm per pCi/l radon-222. Although this chamber only has a geometrical volume of 26 cm3, its effective volume for radon-222, pol~nium-218 and polonium-214 are 82, 95 and 141 cm3, respectively.
In its simplest configuration, the electrometer circuit shown in Fig. 3 is comprised of a high power gain (typically 109) field effect transistor Tl, an input resistor Rl, high gain transistors T2 and T3 and biasing/load resistors R2, R3 and R4. This type of one hundred percent negative feedback has been employed since the time of vacuum-tube electrometers to i~prove linearity and stability.

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~TI 001 P2 -9-Even more important to the present application, however, is the reduction in the effective capaci~ance of the input through the relationship Ce~f - C/~l~G), where G is the current gain of the feedback amplifier and C is the actual capacitance of the input. Thus, with typical feedback circuit gains of 105, the input capacitance becomes insignificant. Because the voltage gain of this ~ype of circuit is essentially unity and the pulse width is dependent on the time-of-flight of the ions, further amplification and pulse ~haping is required before the signals are suitable for nuclear counting The present invention overcomes these diffi-culties, especially limiting in portable applications damanding low power drain, through the use of a phase shifted feedback technique. Circuit element~ C2, R7 and Dl provide the RC network to produce this phase shift function. The remainder of the circuit elements, R8, R9, C3, D2 and T4, provide a transistor switch for incrementing a commercially available large scale integrated circuit (LSI) counter 35 with liquid crystal display (LCD).
The differences between a conventional negative feedback electrometer and the phase shifted feedback electrometer, as well as the operating principles of the latter, can be seen by examining Figs. 4, 5 and 6.
The dotted curve in Fig. 4 represents the voltage-time relationship for an ion chamber electrometer with no feedback and an input RC time constant much greater than the collec~ion times of the the positive and negative ions, t~ and t~, respectively. The solid curves in Fig. 4 represent the collecticn electrode 1 323~5~

voltage, Vl, and output voltage, V3, for the same ion chamber and load resi~tor as above, but with a high percent of negative ~eedback. It is seen that the output pulse rather faith~ully ~ollows the time-of-flight collection time of the ions. However, withpractical chamber capacitances ~>1 pf) and input resis-tor values (<1012 ohms), output pulse heights and widths are in the millivolt and 50 to 100 msec range, respectively. Therefore, further pulse shaping and amplification must be carried out before these signals are suitable for input to a nuclear counter.
The negative going pulse for Vl in Fig. 4 is due to circuit non-idealities such as FET gate capaci-tance, gate-to-drain leakage current, and a collection of other control current losses around the feedback circuit. The greater this negative voltage, the lower the feedback and the longer ~he signal pulse tail. To hold this control voltage to a minimum, the ~ET, load resistor and other components of the feedback circuit must be designed with ultra-low capacitances and current leakage. By phase shifting the feedback, all the above enumerated problems are either eliminated or greatly reduced and the output signal pulse is of sufficient amplitude and narrowed width to directly drive a nuclear counter.
The principles of the phase shifted feedback can be understood by examining Fig. 5. The voltage at the collecting electrode, Vl, the intermediate phase ~hifted voltage, V2, and the output voltagel V3, are all shown as functions of time. As in ~ig. 4, the dotted curve 11 represents the voltage on the collection electrode of the same ch~mber without feedback~ With ., , ~, .

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PTI 001 P2 1 3~ 4 54 phase shifted feedback, however, Vl increases negatively along the non-feedback curve until a ~control point- is reached at time, tc. At this time, the voltage drop across Rl is sufficient to main~ain a current through Rl equal to the ion collec~ion curren~. At tc, the output voltage, V3t peaks and starts down while V2 continues to rise until V2 equals V3, and then V~ falls of f also.
If no damping is provided, the signals will ~ring-, as shown in Fig. 5. A diode, Dl, is thus placed across R7 to clamp the circuit and minimize the undershoot of V3, as illustrated in Fig. 6. This clamping of ~he tail of the pulse rearms the electrom-eter to be ready to accept another pulse so that dead time is reduced. The circuit must be ~tuned~ so that the time constant, (R7xC2~, is large enough to give the desired ampli~ication of V3, but small enough to operate within the ion collection times, t+ and t-.
The feedback phase shift circuit has the effect of amplifying the output pulse by leading edge overshoot and results in an increase in the output of more than an order-of-magnitude in signal-to-noise.
The phase shifted feedback circuit tends to discriminate against all noise frequeneies with rate of rise times different from the ion pulses the circuit was designed for. ~dditional high frequency filtering is provided by the filter network formed by R~ and Cl. The follow-ing values are typical for these components and lead to output pulses of over 200 mV from the unphase shifted and over 1.5 V from phase shifted circuits, ~hen count-ing 5 MeV alpha particles.

Rl loll ohm R2 1 meg ohm R-~ 120 K ohm R4 1 meg ohm R5 lO x ohm R6 220 K ohm R7 470 K ohm R8 100 K ohm R9 330 K ohm Tl MFE 823 Cl 0.001 mfd C~ 0.1 mfd C3 0.1 mfd Dl IN34 D2 INgl4 Bl, B2 18 V Battery The circuit shown in Fig. 3, without the phase shifted feedback, has a signal-to-noise ratio of approximately 20 whereas the phase shifted version, as shown, has a signal-to-noise of over 200. Because pulse wi~ths are narrowed to approximately 10 msec with 25 the phase shifted feedback ~ircuit, pulse pile-up is not a problem for count rates below 50 counts per :::
second.
The combination of the small, low capacitance open grid chamber with the phase shifted feedback 30 electrometer leads to an ion pulse detector capable of operating in an air environment, drawing only micro ~ .

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~TI 001 P2 -13-amperes of current and ea~ily capable of det~cting environmental levels (approximately 0.1 pCi/l) of radon and/or its daughters. Wi~h batteries $n~talled, the monitor weigh~ less ~han f$ve pound~ ~nd can easily be carried by a handle on top of th~ cabinet. T~i~ counter module i8 ~elf~contained ~nd operate~ for up to four years on two 1.5 V alkal~ne ~N~ cell~. The remainder of the power requirement~ ~re 8Uppl~ ed by readily and convenien~ly obtainable 9 V transistor radio batterie~
housed in the lower compartment o~ the monitor enclo-sure. secause the electrometer draw~ le~s than 15 uA
current, a standard carbon zinc ba~tery glves oYer two years of continuous operation -- alkaline o~ other greater capacity batteries will provide commensura~ely longer service. secause there i~ essentially no current drain on the biasing batterie , their service life in this application is essentially the ~el -life o~ the battery.
While the orm of appara~us herein described constitutes a preferred embodiment of this invention, it is to be understood ~hat the inven~ion is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended cl~ims.
The embodiments of the invention in ~hich an exclusive property or privilege is claimed are defined as follows:

Claims

CLAIMS:
1. An electrometer circuit for amplifying short duration pulses including an FET having a low capacitance input and output, feedback circuit means connected between the input and output of said FET for providing a negative feedback voltage to said input of nearly 100%, and phase shift circuit means in said feedback circuit means for delaying momentarily the application of said feedback voltage, thereby to allow the output of said FET to have a momentary power gain of sufficient magnitude and pulse width to drive an external counter circuit, 2. The electrometer of claim 1 wherein said phase shift circuit means includes a resistor-capacitor circuit for providing an output pulse width which is adjusted to permit the electrometer output pulses to be generated within the expected pulse repetition rate of the pulses being measured.

3 . The electrometer of claim 2 wherein said resistor-capacitor circuit is adjusted to provide an output pulse width in the order of 10 milliseconds.

4 . The electrometer circuit of claim 1 further including a clamping diode connected in said output circuit for preventing the output voltage from reversing polarity.