US2474851A - Bromine counter - Google Patents

Description

July 5, i949. s. H. LIEBsEN 2,474,851

BROMINE COUNTERS Filed July s, 1947 2 snet's-sneet 1 SIDNEY H. LIEBsoN July 5, i949. s. H. LlEBsEN BROMINE COUNTERS Filed July 3, 1947 2 sheets-sheet 2 BROMINE ARGON BREAK DOWN POTENTIAL O l IOO "lo BROMINE LOW VOLTAGE RANGE SELF QUENCHING RANGE I gwucmlo SDNEY H. LIEBSON July 5, 1949 UITED STATES PATENT OFFICE BROMINE COUNTER Sidney Il. Liebson, Washington, D. C.

Application July 3, 1947, Serial No. 758,972

(Granted under the act of March 3, 1883, as amended y'April 30, 1928; 370 0. G. 757) 13 Claims.

This invention relates to radiation and particle counters. Such counters have been previously used in the form of Geiger counters. Counters of the present invention may operate on the same principles as Geiger counters. On the other hand, the invention comprehends counters which serve the same purposes as Geiger counters, but operate to count through entirely new physical principles. These latter counters oier novel advantages over previously k'nown counters. Further, the new method of operation is not limited to the specific counters herein disclosed, the invention comprehending a broad class of counting devices and novel methods therefor.

The counters of the present invention characteristically employ bromine as a constituent of the gas filling. The gas may be used alone, or in combination with other gases. The other gases are preferably the noble gases lighter than radon. Certain of these have preferable characteristics for various specialized purposes, as will appear in detail hereafter.

In some cases gases other than the bromine and the noble gases may be present without deleterious effect. The necessary cases mentioned should be employed in spectroscopically pure form, however, so that undesired or unlmown constituents are avoided. Combinations of noble gases also may be used.

In the employ-ment of bromine in counters, it has been foundthat the operating characteristics depend on the proportion of bromine present in the active constituents.

Larger amounts of bromine, up to per cent. provide self-quenching counters. The function of the bromine is similar to that of organic vapors previously used in self-quenching Geiger counters. The elementary gas is not destroyed in the gas volume during operation of the tube, how-l ever, as is the complex organic molecule. 'I'he counting lifetime of a tube employing bromine instead of an organic vapor is much longer than previously known self-quenching tubes. Elec- 'trode elements not substantially reactive with the gas under operating conditions should be used, manifestly, but such are readily obtained. Carbon electrodes offer a satisfactory material, for example. Tungsten is also sufficiently nonreactive for practical use.

For self-quenching operation, particular choice of the cathode surface is necessary. With noble gases lighter than krypton, small amounts of bromine aiord low voltage counters. These counters may operate on a-s low as 250 volts. Their sensitivity and counting speed is about 2 equal to previously known high voltage counters. They may be self-quenching, and in part the reduced voltage operating range overlaps the self-quenching range in the proportion of bromine present.

At very low amounts of bromine, reduced voltage operation occurs below the range wherein sulcient bromine is present to eiect selfquenching.

.It is, therefore, apparent that bromine provides a suitable gas filler for counters. With suitable cathode surfaces, the counters are selfquenching when enough bromine is present for such operation'. Operation without self-quenching may be obtained, if desired, depending on the cathode surface employed. As the amount of bromine is increased, operation at reduced voltage is obtained with some noble gases, and very low operating voltages may be used if desired. The range of reduced voltage operation extends below the amounts ,of bromine necessary for self-quenching.

The operation of counters employing bromine is complex. Re-ignition is believed to be dependent upon relation between the cathode and the bromine. Operation of counters employing bromine and noble gases as active constituents is believed to depend upon excitation of the noble gas and transfer of the energy therefrom to the bromine for ionization. While the theory of operation of counters of this invention, and those of more conventional counters, set forth herein, differs in various respects from previously accepted theory; it has been derived from experimental investigations. The theory is offered as the best available and not in limitation of the invention.

As the ranges of bromine employed in selfquenching operation is wide, extending from less than one per cent to per cent, the quenching operation 'may be considered first.

For a counter to be self-quenching, the ionization initiated by radiation or a particle must not be re-ignited after extinguishment by formation of the ion sheath at the anode. The ion sheath formed in the process is drawn to the cathode and neutralized. It is believed that the ions.

upon neutralization, may take up electrons in an` 'ent invention, the work function of the cathode is greater than any metastables state energy 3 level of bromine, and when the bromine is ionized, the ions may therefore be collected without secondary emission to provide self-quenching operation.

Carbon, brass, tin, copper, stainless steel, tantalum, and zinc have suitable work functions.

In a counter whose filling consists of bromine, or in which bromine carries substantially all the discharge by ionization, such self-quenching operation is obtained.

Where bromine is used in connection with another gas, actively taking part in the operation of the tube, such as a noble gas lighter than radon, the quenching operation of the counter is similar to conventional self-quenching counters if the bromine is present to the extent of fraction of a per cent or more. Bromine may be thus used with krypton and xenon, and acts, with suitable! cathodes, similarly to organic vapor quenching agents. It has the advantage of being nondestructively employed, as mentioned above.

Krypton and xenon counters, having large heavy molecules, are preferable for X-ray counting. With bromine they oiIer a self-quenching counter operating at rather high voltages of the same order of magnitude as organic vapor-inert gas mixtures.

A Bromine may be used with helium, neon, and argon. The operation of such counters is much more complex than those previously described, and although their importance is thought far greater, their description has been postponed to describe the self-quenching operation. This latter is the same in all the counters described.

The counters employing the lighter gases are distinguished from the krypton and xenon counters in that operating potentials below the pure noble gas may be obtained, in some instances as low as 250 volts.

This is believed due to energy interchange between the noble gas and the bromine. These noble gases have metastable states whose energy levels are ofthe same order as the energy required to ionize bromine. The noble gas may be excited by radiation or particles and then act, in releasing its energy, to ionize the bromine.

In some instances. in experimental observa*- tions, ionization has occured, although the energy of the metastable state has been not entirely suicient to obtain ionization of the gas. It is thought that momentum or some local forces due to adjacent atoms has supplied the added energy which was a few tenths of a volt. Thus, the metastable state energy level need not necessarily equal the ionization potential of the other gas.

Upon ionization of a single bromine atom, the electron in its passage to the anode will excite many noble gas atoms which in turn effect further ionization oi other bromine atoms. The operation thereby generates an avalanche. The positive bromine ions then sheath the anode, the electrons being drawn under lower and lower velocity to the anode as the sheath is completed to reduce to field gradient, so that no further excitation of the noble gas or ionization of bromine is taking place. The ions are collected by the cathode and neutralized in energy levels all below the work function of the cathode. No cathode emission occurs. The tube is ready for another count.

It is manifest that the generation of the avalanche differs from that in a conventional Geiger tube. It does not demand a high potential gradient for ionization. The central electrode may therefore be large. Ionization may be initiated anywhere in the tube.

The metastable states of higher elements are generally less increased over their ground levels, than for lighter elements. It is believed that for this reason, krypton and xenon do not have suicient energy to ionize bromine from their metastable states.

In the preferable range of proportions, principally the bromine carries the discharge. In lower proportions, other effects take place in that self-quenching of the counter is not obtaired, although low operating voltages may be use Counters of the type described oHer numerous advantages beyond the low voltage operation.

They are exceptionally rugged. Organic vaporinert gas filled Geiger counters are destroyed by sparking between the electrodes, and tubes with narrow plateaus must be carefully operated to avoid this danger. The counters of the present invention may, however, be subjected to intense arcing without deleterious eects.

The ionization is intense, and particularly with neon, is visible enough to be used as a selfindicating device without more than a small low voltage battery power supply. Individual counts may be observed easily.

The operating plateaus of the low voltage counters are broad in proportion to the voltage required. A 250 volt bromine-argon tube of conventional counter dimensions had a plateau of about volts.

The tubes may easily be lled without demanding extreme precision. Spectroscopically pure gases should be used, but the comparatively broad range of proportions suitable for operation permits mass commercial production. Conveniently suitable mixtures of gases can be supplied for effecting the nlling from a single source. Such mixtures may include more bromine than is required, and partial iilling therewith may be completed and diluted with the desired amount of noble gas.

The objects of the present invention are to count particles and radiation, to provide counters for such particles and radiation, and to provide new methods of counting such particles and radiation.

The counters of the present invention will be further described with reference to exemplary embodiments shown in the drawings, in which:

Fig. 1 shows in section a counter of the present invention,

Fig. 2 shows a section taken on the line 2-2 of Fig. 1,

Fig. 3 shows in section a self-indicating counter,

Fig. 4 shows a section taken on the line 4 4 of Fig. 3,

Fig. 5 shows in section a construction similar iny some respects to the embodiment of Fig. 1, and

Fig. 6 is a chart showing operating voltage with respect to the ratio of active constituents.

The counter shown in Fig. 1 is similar in structure to conventional counters in that it employs a pair of coaxial electrodes within a glass envelope. 'I'he electrodes shown are of carbon applied in a liquid suspension and allowed to dry. This material provides the requisite work function for the cathode and is substantially non- The tube comprises a cylindrical envelope I closed at each end. 'I'he central electrode is carried by rod 6 sealed coaxially with the envelope. The envelope structure may be evacuated and filled before nal sealing at tube I0.

The outer electrode 2 is applied to the inner face of the envelope and is terminated short .of end 3 of the tube structure. A lead 4 is introduced through the end and deflected to contact the electrode surface to supply one of the terminals of the counter.

The central electrode 'I is applied to the surface of rod E.' This electrode may be a metallic conducting coating sputtered or chemically deposited on the surface, or a carbon coating. Lead 8 is brought through the end 3 of the counter tube and is deflected to contact the central electrode.

The counter of Fig. 1 may be operated by supplying a positive potential to the central electrode from source 9 through resistor 3l'. While for normal counting operations, a resistor of the order of a megohm would beused, tubes of the types described have operated satisfactorily with resistances as low as 1000 ohms. Condenser 32 is shown connected between lead 8 and ground. In most cases this will be constituted by the distributed capacitance of the tube electrodes.

A variation of the construction of Fig. 1 is shown in Fig. 5. The inner coated electrode 'I is replaced by a wire 28 as in conventional counter manufacture. Tungsten may be used. The external electrode is formed by cylinder 26 positioned in the envelope I by spacers 21. Wire 28 may be extended through the ends of the tube, as was rod 6, to form the anode lead, and lead 4 may be brought into contact with cylinder 26.

Cylinder 26 is preferably nonreactive metal or carbon. In the latter case, careful out-gassin?,r may be necessary to obtain long life of the tube, since carbon is highly absorptive.

The counter shown in Fig. 3 constitutes a selfindicating device, and for this purpose the external electrode is apertured to permit efficient visual observation of the counting ionization within the tube.

The envelope I5 is generally spherical, and carries anode I6 positioned transversely in the drawing. Cathode I'I is positioned coaxially around the anode by supporting lead 20. The cathode, as shown, is of cylindrical shape with an interrupted sector to provide for viewing the discharge.

The tube is positioned by mounts 32 in a visually opaque wooden box 33 transparent to the radiations to be detected. Battery 34 may be of the dry type such as used in radiosondes. Resistor 35, of the order of a megohm is connected .in series with the tube. The enclosure 33 is apertured for eye piece 36, and the aperture I8 in the outer electrode of the tube is aligned therewith for elcient observation.

As a specic example of the counters of the type disclosed, an argon-bromine counter employing a illing of centimeters pressure, consisting of roughly .05% bromine was constructed with a mil tungsten anode and a 3A inch diameter cylindrical carbon cathode. The operating threshold was 250 volts, and the plateau was 75 volts. The efficiency, counting rates, and pulse magnitudes were about the same as those of organic vapor counters.

It is apparent from the discussion above that different proportions of bromine could be employed to obtain other characteristics where desired. The use of other noble gases may be substituted in the example.

The following examples, for instance. illustrate some of the variety oi' llings taught by the invention. The partial pressure percentage of active gases are shown. The last 3 cases illustrate operative counters containing additional gases, the third mentioned gas not taking part in the operation of the tube.

Bro- K'ryp- Helim, e Xenon ton Argon Neon um Nitrogen 50 The counters 2 through 32, employing high work function cathodes, are self-quenching. Counters 19-21; 24-26; and 27-32 are low voltage counters. The xenon krypton counters are high voltage counters with the exception of number 32, in vwhich the xenon is inactive. Countr ers using less bromine than those exampled may fall in the range of low voltage non-self-quenching counters when the bromine amounts to less than about .01%.

Total pressures of the same order as in conventional Geiger counters may be used. This is normally about 10 cm., but operative counters can be made over a very wide range of pressures.

The lower pressure ratios given above are somewhat difficult to arrive at experimentally without highly sensitive manometers. The following procedure may be used, however. The

, envelope is evacuated. One half centimeter of bromine, as measured by an oil manometer, iS introduced. Argon is then added to a total pressure of 10.5 cm. Hg. After diiusion, the tube is pumped down to 5.25 cm. and then argon is again added up to a total pressure of 10.25 cm. By progressive steps, in this way any desired amount of bromine may be obtained.

If the tube is operated at each dilution stage, the operating conditions are easily found andreproduced. The curve of Fig. 6 was obtained in this way. Curves for helium and neon are similar in configuration and have minima at approximately the same ratios.

In all cases, the percentages refer to the ratios of the partial pressures of the constituents.

Voltage breakdown characteristics of a specic pair of gases in diierent proportions is shown qualitatively in Fig. 6. It should be emphasized that the gases employed in these counters should 7 be of spectroscopic purity, and that the minor proportions of impurity in commercial grades of gases will wholly prevent the desired operation.

The combination of Fig. 6 comprises bromine and argon. I'he tube used employed a 30 mil tungsten wire centered in a .75 inch diameter carbon cathode. The breakdown potential for pure argon is shown, having a value of approximately about 850 volts for counter dimensions. Ten centimeters total pressure was used. The breakdown potential of the combined gases is plotted against the percentage of bromine on an exponential scale. It will be noted that below 1 per cent the optimum operating range occurred, in which operating potentials from 150 to 250 volts are obtained. As the amount of bromine increases over 1 per cent, the operating potential rises until around 2.5. per cent it exceeds the initial value for pure argon, and gradually tends to rise towards the breakdown value of pure bromine. At zero bromine, the curve rises to the pure argon breakdown potential. y

Neon and helium give low voltage breakdown curves similar to Fig. 6. Xenon and krypton do not, and the breakdown potential lies between the values of bromine and the noble gas values.

From the discussion above, it will be appreciated that self-quenching counters are obtained in the range above the minimum voltage operation proportions, but that smaller percentages of bromine are operative in low voltage counters.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

It has been found that very useful counters may be obtained through the use of spectroscopically pure noble gases lighter than radon, and chlorine, or-iodine. Small amounts of iodine and chlorine of the same order as bromine may be used for similar operating characteristics as in the bromine counters described. The advantages of these counters are believed due to the ionization potentials and energy levels of the metastable status of the gases involved. The same electrode materials should be used. Chlorine is par- 'ticularly usefull for radiation detection at low temperatures.

What is claimed is:

i. An ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a gaseous filling therefor consisting essentially of a noble gas lighter than radon and bromine up to about 2.5 percent thereof.

2. An' ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a gaseous filling therefor consisting essentially of neon and the bromine up to about 2.5 percent thereof.

3. An ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a gaseous iilling therefor consisting essentially of argon and bromine up to about 2.5 percent thereof.

4. An ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a gaseous filling therefor consisting essentially of helium and bromine up to about 2.5 percent thereof.

5. A low voltage ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a filling therefor consisting essentially of a noble gas lighter than krypton and up to 2.5 per cent bromine.

6. A low voltage ionization pulse counter tube com'prisingl an envelope, a pair of electrodes therein, and a lling therefor consisting eSsen tially of argon and up to 2.5 per cent bromine.

7. A low voltage ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a filling therefor consisting essentially of neon and up to 2.5 per cent bromine.

8. A low voltage ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a filling therefor consisting essentially of helium and up to 2.5 per cent bromine.

9. A self-indicating ionization pulse counter tube comprising an envelope, a pair of electrodes therein, and a gaseous lling therefor consisting essentially of neon and bromine in an amount less than 1% of the neon.

10. An ionization pulse counter tube comprising an envelope, a pair of electrodes therein, a filling consisting essentially of a noble gas lighter than radon and bromine in an amount up to 2.5 percent thereof, and a cathode having a surface work function greater than any metastable state energy level of bromine.

1l. The counter as defined in claim 10 wherein both electrodes are substantially nonreactive with bromine.

12. A gas mixture for a low voltage discharge tube consisting essentially of a noble gas lighter than radon and bromine in an amount less than 2.5% thereof.

13. A self-indicating radiation detector of the Geiger-Mueller type comprising a tube having an envelope, a pair of electrodes therein, one of the electrodes being a cylinder having an interrupted portion and enclosing the other, a radiation responsive ionizable gas mixture filling in the envelope, means for energizing the electrodes, a visually opaque radiation transparent enclosure for the tube, and aperture means in the enclosure in facing relation with the interrupted electrode portion for viewing the discharge.

SIDNEY H. LIEBSON.

REFERENCES CITED 'I'he following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,481,422 Hout et al June 22, 1924 2,401,735 Janes et al June 1l, 1946 2,427,663 Mateosian et al. Sept. 23, 1947

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