US2871365A - Apparatus for detecting radioactivity - Google Patents

Description

Jan. 27, 1959 e. M. O'AHEN EI'AL APPARATUS FOR DETECTING RADIOACTIVITY Filed April 29. 1955 2 Sheets-Sheet 1 AAAAA Jan. 27, 1959 ca. M. CAHEN nu. 2,371,365

APPARATUS FOR DETECTING RLDIOACTIVITY Filed-April 29, 1955 2 shun-shut z VENTUBS M #01 Y r rfi/ APPARATUS FOR DETECTING RADIOACTIVITY Gilbert M. Cahen, Paul Bonnet, and Pierre Treilie, Paris, France, assignors to Etat Francais, represented by ecretariat dEtat a la Defense Nationale et aux Forces Armes (Direction ties Etudes et Fabrications dArmement), Paris, France Application April 29, 1955, Serial No. 504,822 Claims priority, application France May 7, 1954 11 Unions. (Cl. 25fi83.6)

The present invention relates to apparatus for detecting radioactivity and which include an ionization chamber or any other device including at least two electrodes between which is maintained a potential difference which varies under the effect of radioactivity, and detecting means responsive to variations in the potential difference between the electrodes of said chamber.

The object of our invention is to provide an apparatus of this kind better adapted to meet the requirements of practice than those used at the present time, and in particular which is of light weight, portable and autonomous.

Our invention consists chiefly in making use, to provide the potential difference necessary between the above mentioned electrodes, of a high insulation capacitor and in incorporating in the apparatus portable means for recharging at intervals of time said capacitor to a poten- States Patent tial higher than the critical potential necessary for operation of the ionization chamber or other equivalent device as above specified.

Other features of our invention will become apparent in the course of the following detailed description of some embodiments thereof with reference to the accompanying drawings, given merely by way of example and in which:

Fig. 1 shows a lay-out of an apparatus according to our invention.

Figs. 2 and 3 are lay-outs of two other embodiments of our invention.

Fig. 4 is a diagrammatic perspective view of the casing of an apparatus according to our invention.

It should be reminded that the construction of apparatus of the above described type involves some difi'iculties, in particular concerning the provision of a source for supplying the desired potential. If, as it was the ease up to now, use is made of batteries, experience teaches that it is diflicult to solve the problems which arise concerning the renewal and maintenance of such batteries which, on the other hand, are generally of prohibitive weight and dimensions, especially for military uses.

On the other hand, it has been found that it is not necessary, for apparatus of this kind, to have a source of constant electric potential. As above stated, these apparatus include a device sensitive to radioactive radiations and including at least two electrodes between which a minimum difference of potential is to be maintained by a source, the variations in the intensity of the radioactive radiation producing a corresponding variation in the current at the output of the device. Now, it has been ascertained that this current is independent of the voltage of the source, provided that this voltage is above the minimum or critical value. As a matter of fact, these output currents are very small since, when the above mentioneddevice consists of a cylindrical ionization chamber of a volume of one cubic decimeter, the saturation current corresponding to an intensity of gamma rays equal to 1 Roentgen per hour averages 1O- amperes.

Our invention is based on this fact and it consists in using as source of potential for the detecting device, a

high insulation capacitor, the apparatus including means.

for periodically recharging said capacitor, for instance a magneto or a dynamo actuated by the operator, an electromagnetic impulse system, or the like.

If the current to be measured is caused to pass through. a high value resistance and if the voltage across said resistance is measured by an apparatus consuming practicaily no current, such as a quadrant electrometer, we obtain a system such that the above mentioned capacitor can be kept charged for a very long time.

The apparatus is therefore autonomous and it can be stated that, in the present state of the art, and even for radioactive intensities amounting to several hundreds of Roentgens per hour, such an apparatus will necessitate recharging by the operator only at relatively long time intervals, for instance averaging one hour or even more. Furthermore, such an apparatus can be made of light weight and .small dimensions.

Fig. 1 shows the lay-out of an apparatus according to our invention. This apparatus includes the following elements:

An ionization chamber 1, with its central electrode 2, the necessary difference of potential between the electrodes 1 and 2 of this chamber being supplied by a high insulation capacitor I, of a capacitance averaging for instance one microfarad. A load resistor 3 of several thousands of meghoms is inserted in the circuit of this capacitor and the terminals of this resistor are connectedto those of an electrometer 4;

A charging system constituted for instance by a magneto.-

And switch means to separate the active portion of the apparatus, that is to say capacitor I from said charging system, except when this capacitor is being charged. Said means include for instance a double contact switch 8 controlled by a relay 9 fed from a winding 10 mounted on the charging circuit. Of course, said charging system- With a lay-out such as illustrated by Fig. 1, the tential U of value, that is c pocapacitor I can be maintained at a suitable to say above the critical for a relatively long time, and therefore does not call" for frequent rechargings.

However, it is eter as illustrated tivity for low voltages to be measured, which may be aserious drawback in the present case.

It is therefore advantageous to apply on the electrom-.

auxiliary capacitor C, stant sensitivity electrometer.

We will now descri modifications of the sponding respectively 2) and to an heterostatic electrometer (Fig. 3).

The construction 1, but includes two Patented Jan. 27, 1959.

3 of obtaining both charging of the main capacitor I at the suitable potential U and charging of the reference capacitor C at a suitable potential E. A switch 8 is used also in this case but it includes three contacts.

The reference capacitor C which is very strongly insulated (in particular insulated by means of a material such as styrofiex) has for instance one of its terminals connected with one of the quadrants of electrometer 4; and the other terminal connected with the terminal of resistor 3 which is common with the main capacitor, whereas the other end of sa d resistor is connected with the external electrode of chamber 1. The central electrode of this chamber is connected with the corresponding terminal of the main capacitor.

This apparatus works as follows:

It will be supposed that the apparatus is charged and that the voltage U of the main capacitor has been brought we value higher than the critical potential, which depends only upon the dimensions of the ionization chamber (potential corresponding to the above mentioned saturation current).

In the absence of any radiation. the potential E of the reference capacitor (which potential remains constant for a very long time owing to the good insulation of said capacitor) is the only potential applied to the electrometer. Its indication is then at the zero of the graduation of said electrometer (in roentgens per hour).

As soon as a radiation reaches the ionization chamher, there is produced therein a current which causes a drop of potential V. As the capacitor C maintains its charge, this drop of potential V is wholly maintained across the electrometer.

We thus obtain an angular deflection which theoretically' is proportional to the square of the difference E V when the quadrants and the pointer are of the conventional circular shape. high sensitivity corresponding to a small intensity of radiations.

But we may, while keeping a circular movable electrode (which permits an easy balancing), make the fixed quadrant of the electrometer of a shape such as to obtain any desired response.

- Advantageously, according to another feature which is applicable in a general manner to the apparatus according to our invention, we further provide means for checking at any time the state of charge of the capacitor or capacitors and also the state of operativeness of the various elements of the apparatus, these means being in particular arranged in such manner that they permit, through suitable switchings, of using the electrometer to give successively, on the one hand the value of the charging potentials supplied by the electricity generator (magneto, etc.) and on the other hand, indications from the capacitors.

For instance, as shown by Fig, 2, the above mentioned means are constituted by an auxiliary resistor 3, which may be inserted so as to short-circuit the main capacitor I, possibly through resistor 3, said auxiliary resistor being in particular interposed between one of the terminals of the main capacitor and the external electrode of chamber 1. The insertion of this auxiliary resistor is controlled by means of a switch 13, for instance of the push-button type, having two positions, to wit a normal position, with the push-button released, where the auxiliary resistor 3 is out of circuit, and a measurement position, with the push-button depressed, where resistor 3 is inserted in this circuit and thus insulates the ionization chamber.

The checking operations are for instance as follows:

a. When charging apparatus G is operated without depressing the switch push-button 11, the charging voltage at the terminals of capacitor .C is transmitted to the terminals of the electromete'r (through resistor 3 for one of said terminals).

It is thus possible to check up the zero position of the apparatus and also the charging conditions. This is true 4 even in a field of radiation (since resistor '3 is so to speak short-circuited) b. When the charging apparatus is actuated while depressing the switch push-button 11, charging is effected through resistors 3 and 3, which supplies across the terminals of the electrometer a well defined potential, independent of that of the main capacitor 1 since this capacitor discharges but very slowly through said resistors.

0. Finally if the switch push-button 11 is depressed without operating the charging apparatus, a slow discharge of the main capacitor is produced through the above mentioned resistors, which gives rise to another potential across the terminals of the electrometer, which, if the charge is sutficient, must exceed a given value which has been previously marked on the dial of the electromet'er.

It will be seen that through these measurements a, b and c, it is possible to verify:

a. If the potential applied during the charge to capacitor C is correct and if the zero of the electrometer has not varied,

b. If resistor 3 and the antagonistic torque of the electrometer have not varied,

c. If it is necessary to recharge the system, that is to say if potential U has not dropped below the critical value.

It should be noted that, even if measurement a does not give a satisfactory result, that is to say if there is a displacement of the zero, it is still possible to use the apparatus since the deflection depends upon the amount EV and V depends upon the current that flows through the ionization chamber under the influence of the incident radiation. Everything takes place as if the radiation had been modified by a parasitic radiation of a value corresponding to the displacement of the zero. It is therefore easy to correct the readings so as to take this displacement into account.

Other disturbances checked by the above mentioned operation must on the contrary involve a revision of the apparatus.

According to the construction of Fig. 3 which will now be described, we use an electrometer 4 of the heterostatic type which, as it is well known, is constituted essentially by at least two systems of fixed conductors F and F and at least one movable conductor M.

One of the two fixed systems, for instance F is brought to a fixed potential E with respect to M, whereas the potential to be measured x is applied between F, and.

F The angular displacement of M depends upon this potential x. When this voltage is zero, said displacement is therefore itself zero whatever he the value of the reference potential E. V v

In the application of such an electrometer to the apparatus according to our invention, we apply for instance on conductors F the potential of one of the electrodes of the ionization chamber 1, in particular of the ex ternal electrode. The high insulation capacitor C, similar to that of Fig. 2 (and which is charged from an intermediate terminal 7 of stabilizer 7), is in this case interposed between conductors F and the movable conductor M, whereas the main capacitor I is inserted between conductors F and the other electrode 2 of the ionization chamber. Finally the resistor 3 which produces thepotential x to be measured is interposed between the systems of fixed conductors F and F Such a system works in a manner analogous to that above described, the current which passes through the ionization chamber producing an angular displacement of movable conductor M. However, the arrangement of gig. 3 may have the following advantages over that of If the apparatus is not struck by any radiation, there is a certainty that the movable conductor M does not movewhatever be the value of the reference potential E, The apparatus being supposed to be charged in" in- -poteiitial undergoes slight variations due to the losses of capacitor C and'due alsoto the fact that in the periods when the, apparatus is subjected to a radiation, this capacitor supplies the small ionization current which flows through the electrometer, due to the unavoidable presence of air inside this instrument. Therefore the potential of the zero is quite fixed, whereas in the preceding arrangement there was a slight variation of the position of the zero.

The disadvantage is that the displacement of the pointer for a given intensity of radiation depends upon the potential E to which it is most often roughly proportional. However, if the variation of potential E is only of some percent, the measurements will not be seriously disturbed since this degree of precision is quite sufficient in most cases.

Of course, by giving suitable shapes to the fixed and movable conductors, it is possible to obtain either a linear scale of reading or a logarithmic one, or any other kind of scale as may be desired.

In this case also, other means may be provided for checking up the operation of the apparatus, which means will consist for instance in an auxiliary resistor 3 capable of short-circuiting, during the checking up operations, resistor 3 by means of a switch 12.

This resistor 3" being of relatively low resistance so that the highest possible radiation intensities produce when passing therethrough but a negligible drop of potential, it is for instance possible to perform the check up operations as follows:

Concerning first the checking up or" the charging voltages, we may for instance, after having closed contact 12, operate the charging apparatus, thus applying, on the system of conductors P with respect to the system of conductors F a potential which is but a portion of the potential normally applied on capacitor 1. This reduction or division of the potential will be obtained for instance by making use of a bridge of resistors one branch of which is constituted by the auxiliary resistor 3" introduced by closing switch 12 and the other branch of which is formed by a resistor 3" of a value of the same order of magnitude as resistor 3". if the voltage thus introduced and that applied to capacitor C have the desired correct values, the displacement of the movable conductor M is of a value which characterizes the good operation of the apparatus.

It should be noted that resistor 3", due to its low value with respect to resistor 3, does not prevent the obtainrnent across the terminals of capacitor 1 of the full charging voltage of generator G when contact 12 is open and the magneto is operated.

It may be interesting, according to still another feature of the invention, to provide means for modifying at will the sensitivity of the apparatus. These means consist in the provision, for cooperating with the ionization chamber, of at least two high resistors 3 and 3 of different values and which may be switched for each other by means of a reversing switch 13. This feature is visible on the drawing, both on Fig. 2 and on Fig. 3.

Of course, the electrometer to be used may be of any suitable type, for instance of the quadrant type, of. the pointer type, of the fibre type, etc;, and it may include a suspension by means of a pivot, a twisted wire, etc.

The detection apparatus according to our invention is enclosed in a casing 14 of any suitable shape in which all the parts can be easily reached, and which is provided with a crank 15 for operating the charging device, with -switch controls 11, 13 (or 12, 13), the quadrant being visible at 16 on Fig. 4.

Such a system has many advantages over the existing apparatus, and in particular its dimensions and its weight are small so that it is easily portable.

Furthermore, it is fully autonomous since it will suflice to turn the crank from time to time so as to recharge the capacitors by means of the charging apparatus.

It can be adapted to measure all kinds of radiations, for instance ranging from 0.1 to 500 Roentgens per hour.

In a general manner, while we have, in the above description, disclosed what we deem to be practical and efiicient embodiments of our invention, it should be well understood that we do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

What we claim is:

1. A radiation measuring apparatus comprising detec-' tor means for producing, in the presence of radiation under a predetermined minimum potential dilference, a current, said detector means including a circuit for the flow of said current, a resistance inserted in said circuit, whereby a potential difference is created between the ends of said resistance when such a currentis produced, a principal condenser connected to said circuit for impressing at least said minimum potential difference on said detector means, an electrometer mounted in shunt across the terminals of said resistance, and an auxiliary condenser connected to said electrometer for supplying a reference potential thereto, and means for charging said condensers at the desired voltages.

2. In an apparatus as claimed in claim 1, stabilizing means connected at least to said auxiliary condenser for applying a constant charge thereto.

3. A device as claimed in claim 2 in which said stabilizing means comprises discharge tubes.

4. A radiation measuring apparatus comprising detector means for producing, in the presence of radiation under a predetermined minimum potential difference, a current, said detector means including a circuit for the flow of said current, a resistance inserted in said circuit, whereby a potential dilterence is created between the ends of said resistance when such a current is produced, a principal condenser connected to said circuit for impressing at least said minimum potential dilference on said detector means, an electrometer mounted in shunt across the terminals of said resistance, and an auxiliary means comprising an alternating current source, and between said source and said condensers, a bridge one of whose branches is constituted by discharge tubes acting as a stabilizer while the other three branches are constituted by resistances balanced with the internal resistance of said tubes.

5. A device as claimed in claim 1 having switch means to disconnect the charging means from the condensers when they have been charged.

6. In a device as claimed in claim 1, stabilizing means connected at least to said auxiliary condenser for applying a constant charge thereto, and switch means to disconnect the charging means from the condensers when they have been charged.

7. A device as claimed in claim 1 having switch means to disconnect the charging means from the condensers when they have been charged, and switch means for disconnecting the electrometer so as to measure the charge.

8. A device as claimed in claim 1 in which said auxiliary condenser is connected in series with said resistance.

9. A device as claimed in claim 1 having switch means to disconnect the charging means from the condensers when they have been charged, and means to shunt said resistance by a second resistance of different value for making measurements during charging.

10. A device as claimed in claim 1, in which the principal condenser is in series with the resistance and the detector means, and the auxiliary condenser is in series aernsea with the resistance and the electrometer, and said electrometer is an idiostatic electrorneter.

11. Radiation measuring apparatus comprising detector means for producing, in the presence of radiation under a predetermined minimum potential difference, a current, said detector means inciuding a circuit for the flow of said current, a resistance inserted in said circuit whereby a potential difierence is created between the ends of said resistance when such a current is produced, principal condenser inserted in said circuit in series With said resistance for impressing at least said minimum potential difference on said detector means, a heterostatic electrometer having at least two fixed electrodes and one movable electrode, one of said fixed electrodes being denser and one end of said resistance in series between the other of said fixed electrodes and said movable electrode, means connecting said last mentioned fixed electrode with the other end of said resistance and means to charge both said condensers at the desired voltages.

References Cited in the file of this patent UNITED STATES PATENTS 2,645,722 Chaminade July 14, 1953 2,666,857 McLaren et al.'- Jan. 19, 1954 2,675,484 Hepp Apr. 13, 1954 2,676,270 Lahti Apr. 20, 1954 2,735,947 Molloy Feb. 21, 1956

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