US2994782A

US2994782A - Device for stabilizing the gain of photomultiplier tubes

Info

Publication number: US2994782A
Application number: US859531A
Authority: US
Inventors: Gouyon Arthur Marcel; Kermagoret Marcel; Pascal Andre
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1958-12-16
Filing date: 1959-12-14
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01
Also published as: GB927757A; BE585528A; CH362713A; FR1222536A; NL246387A; LU38035A1

Description

19671 A. M. GOUYON ETAL 2,994,782

DEVICE FOR STABILIZING THE GAIN OF PHOTOMULTIPLIER mass Filed Dec. 14, 1959 F5 Ty. 2

2 Sheets-Sheet 1 Vn- 7 V0 V0 +7 Vd V I I l 800 1000 1200 1400 V ug- 1, 1961 A. M. GOUYON ETAI. 2,994,782

DEVICE FOR STABILIZING THE GAIN 0F PHOTOMULTIPLIER TUBES Filed Dec. 14, 1959 2 Sheets-Sheet 2 United States Patent 2,994,782 DEVICE FOR STABILIZING THE GAIN 0F PHOTOMULTIPLIER TUBES Arthur Marcel Gouyon, Saint-Egreve, Marcel 'Kermagoret, Paris, and Andre Pascal, Boulogne-sur-Seine, France, assignors to Commissariat a IEnergie Atomique, Paris, France, an organization of France Filed Dec. 14, 1959, Ser. No. 859,531 Claims priority, application France Dec. 16, 1958 5 Claims. (Cl. 250-207) The present invention relates to photomultiplier tubes, that is to say to tubes for transforming incident photons into electrical pulses adapted to be used for measuring the number and/or the energy of these photons.

It is known that a photomultiplier tube comprises, inside an evacuated vessel: a photocathode emitting a bunch of electrons in response to an incident photon striking said photocathode, a series of intermediate electrodes or dynodes every dynode emitting, in response to the primary electrons coming from the preceding electrode (photocathode or dynode) that strike said first mentioned dynode, a great number of secondary electrons (multi plication effect by secondary emission), and a collecting electrode or anode which receives the electrons emitted by the last dynode and supplies the electrical pulses to be used.

In order to permit emission and collection of the electrons and to establish the electrostatic field which ensures ,the displacement of the electrons, use is generally made of a high voltage feed source inserted between the anode and the photocathode and of a voltage divider device also inserted between the anode and the photocathode, the respective dynodes being all fed from said voltage divider so as to be brought to potentials which increase progressively in the order in which they are disposed between the photocathode and the anode.

In view of the fact that the total amplification or gain of a photomultiplier :tube having n amplification stages is substantially equal to the nth power of the amplification per stage and that this last mentioned amplification depends directly upon the potential difference between two successive dynodes, which difference is substantially proportional to the total potential difference, the gain of a photomultiplier tube varies very quickly as a function of the supply or feed high voltage. It is known for instance that variations of 1% of the feed high voltage produce gain variations ranging approximately from 8 to 12% and that it is difficult, in industrial applications of this apparatus, to use high voltage feed sources having a stability corresponding to maximum variations lower than 1%.

The object of the present invention is to achieve a stabilization of the gain of photomultiplier tubes making use of feed sources, even little stabilized (having for instance voltage variations which may be as high as through means which are simple, efiicient, unexpensive and require only an easy adjustment, which is quite necessary for industrial applications of photomultiplier tubes (measurement of X-rays intensity, counting of nuclear particles in combination with scintillators, and so on).

Therefore, the present invention for stabilizing the gain of a photomultiplier tube including n dynodes, n being at least equal to two, located between a photocathode and an anode, said tube being fed from a high voltage source having its two terminals connected respectively to said photocathode and said anode and also from a voltage divider connected between said terminals of said source, consists in feeding only n-l dynodes in the known manner from said voltage divider and feeding the remaining dynode with a stabilizing voltage derived from said high voltage and which is an increasing linear function of said high voltage.

A device according to the present invention for stabilizing the gain of a photomultiplier tube and embodying the above stated method, comprises, for deducing from said high voltage a stabilizing voltage to be fed to said remaining dynode and having a value which is an in= creasing linear function of said high voltage, a network disposed between said terminals of said high voltage source, said network comprising in series at least one first resistor, a tube capable of maintaining a constant differ.- ence potential between its terminals and at least one second resistor, at least one of said resistors being adjustable, and a potentiometer connected in shunt with said last mentioned tube, said potentiometer including a slider connected with said remaining dynode to supply it with its feed voltage.

The adjustment of this device is very simple because the adjustable resistor ensures once and for all the automatic regulation of the gain of the photomultiplier, the potentiometer being only used for adjusting the value of said gain Without disturbing the other characteristics.

In the preferred embodiment of this invention the tube capable of maintaining a constant potential dilference between its terminals is a voltage stabilizing tube of the corona eifect type and the dynode fed from the potentiometer is the fourth, fifth or sixth dynode from the photocathode.

Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

FIGS. 1 to 4 show four curves representing the variations of the gain of a photomultiplier tube as a function of the feed voltage and of the voltage applied to a given dynode, these curves facilitating the explanation of the principle of stabilization of the gain according to the invention.

FIG. 5 is a diagram illustrating the gain stabilization of a photomultiplier tube by the method according to the invention.

FIG. 6 shows a photomultiplier tube cooperating with a device made according to the present invention for regulating the gain.

FIG. 7 shows curves of adjustment of the device of FIG. 6.

Before giving a detailed description of the method and device according to the invention, it is believed advisable to explain, with reference to the curves of FIGS. 1 to 4, how the gain of a photomultiplier tube varies and the chief causes of this variation when the feed high voltage of this tube varies.

When the feed voltage of one of the dynodes of a photomultiplier tube is modified, there is produced a gain variation shown by FIG. 1. On this figure, the gain G is plotted in ordinates (with a logarithmic scale) and the voltage Vd of the dynode in abscissae. This figure shows that the same gain G may be obtained for two values (such as Vn-l and Vn+l) of the voltage Vd which are substantially symmetrical with respect to the normal potential V of the nth dynode.

For this reason, if we represent, as on FIG. 2, a curve of constant gain, as a function on the one hand of the total feed high voltage V of the photomultiplier tube and on the other hand of the potential V of the nth dynode, it is found that such a curve comprises two branches which are substantially rectilinear and therefore comply with an equation of the following type in which parameters a and b are constant factors.

The shape of such curves is essentially determined by the change of the secondary emission coeflicient, the effect of which can be calculated. As a matter of fact, if the potential of the nth dynode is modified, there is obtained k being a constant.

FIG. 3 shows, with the same references as on FIG. 1, the variations of the gain of a photomultiplier tube due to the modification of potential of the nth dynode, resulting exclusively from the variations of the secondary emission coefi'icient; the curve in dotted lines shown on the same figure represents the gain variation that has been experimentally observed.

The difference between these two curves is small and corresponds to a defocusing of the electron beam by modification of the interdynode potential which produces a loss of the electrons.

It is therefore found that actually the effects of the variation of the secondary emission are the most material.

If we show on the same figure (FIG. 4) a number of constant gain curves (the value of the gain being indicated on every curve), still as a .function of this voltage V,, applied to the dynode and of the total voltage V (these voltages being expressed in volts), it is seen that, when the gain changes, the slopes of the lower and upper branches of the curves are maintained. It follows that the ordinate, at the origin, of these straight lines varies by an amount DV proportional to the increase DV of the total voltage. Consequently, if a regulating network serving to feed the nth dynode is given a slope such a network would make it possible to obtain a stabilization of the gain of the photomultiplier tube despite variations of the feed voltage for different values. The method and device according to the invention are based upon this basic fact.

We will first refer to the diagram of FIG. 5 which shows a photomultiplier tube comprising, in an evacuated vessel 1 provided with a transparent window 1a, a photocathode 2, a series of dynodes 3 and a collecting electrode or anode 4. The

electrodes

2, 3 and 4 of this tube are fed from a high voltage source 5, the postitive terminal of which is connected with anode 4, the negative terminal, connected to photocathode 2, being grounded and from a voltage divider 6 (constituted by a number of resistances in series) inserted between photocathode 2 and anode 4, this voltage divider serving to the feed of dynode 3 with the exception of a single one, to wit dynode 3a.

According to the main feature of the present invention, this dynode 3a is fed with a voltage which is an increasing linear function of the voltage delivered by source 5. For this purpose, we insert betwen the terminals of source 5, across the voltage divider, a network including in series a first resistor 7, a tube 8 maintaining a constant potential difference between its terminals and a second resistor 9. The feed voltage of dynode "3a is collected through a potentiometer 10 shunting tube 8, the two terminals of this potentiometer being connected to

points

11 and 12, whereas its slider 13 is connected to dynode 3a.

The characteristics of the arrangement of FIG. 5 do comply with Equation I which represents the upper and lower branches of the constant gain curves.

This is due to the fact that, within the operative range of tube 8, the voltage across the terminals of this tube remaining constant and V being the high voltage of source 5,

V the voltage across the terminals of tube 8 between 11 and 12, a

R the resistance of resistor 7,

R the resistance of resistor 9,

V the potential of dynode 3a,

i the intensity of the current through

resistors

7 and 9, V the ground potential,

V the potential of slider 13 of potentiometer 10,

1+ 2) o and if:

V =V +m-V (with m ranging from 0 to l) we have:

V V V =R i+mV As 2 c v V,

we obtain:

R f RIZ RP 121+ R2) By identifying Equations I and IV, we obtain:

a= and b=V,,(m-a) which shows that the slope of the regulation straight line is maintained when m, that is to say the gain of the tube, is changed.

However, it is necessary to make a suitable choice of:

(a) tube 8 so that it permits a relatively large modification of voltage for the regulated dynode, without exceeding 1,000 volts; preferably a corona efiect tube is used; we might also make use of a neon tube but there are two important drawbacks inherent in this kind of tubesz-its low regulation voltage volts) and its high consumption.

(11) the regulating dynode 3a; the current of this dynode must not be too high which would exclude the possibility of using the last dynodes; the fourth, fifth or sixth dynode is quite suitable.

Account being taken of the above remarks, in the embodiment illustrated by FIG. 6 (which concerns a photo multiplier tube 1 having ten dynodes, nine of which, designated by reference numeral 3, are fed from a set 6 of resistors 6a in series, whereas the remaining dynode 3a, which is the fourth dynode is fed from a network according to the invention), the voltage stabilization tube 8 is constituted by a corona effect tube and resistor 7 is divided into two portions, to wit a fixed resistor 7a and an adjustable resistor 7b (consisting for instance of a rheostat); in a likewise manner, potentiometer 10 is advantageously replaced by a series arrangement including, in addition to a potentiometer 10b, two fixed resistors 10a and 100, which makes it possible'to use a potentiometer 1% adapted for a narrower resistance band.

Consequently, in the arrangement of FIG. 6, R will include resistors 7a and 7b and m is no longer variable between 0 and l, but between two values m m such as m 0 and m' 1.

Calculation of the values of the resistances to be used in the gain stabilizing arrangement according to the invention may be effected in the following manner.

Starting from the constant gain curves (FIG. 4) of the photomultiplier tube, we choose a point of operation as a function of V and V,,; this point of operation must be necessarily chosen on a rectilinear portion of the constant gain curve such that the feed voltage V remains, despite possible variations, on the rectilinear portion of this curve. For instance, if a point of operation is chosen for a voltage V=1,700 volts with possible variation of the constant gain curve must be rectilinear between 1,530 and 1,870 volts.

Knowing V and V (voltages across the terminals of corona tube 8), R +R is deduced therefrom. Then, starting from the main current i and from the mean cur- Resistance of resistor a=17 M ohm. Resistance of resistor 10c=2 M ohm. Resistance of potentiometer 10b=12 M ohm.

The adjustment of the gain may be performed very quickly in the following manner:

We trace a curve (FIG. 7) which represents the amplitude A in volts (which can be read 'for instance on a voltmeter located at 14, FIG. 6, at the extremities of a not shown resistor) obtained in response to a constant luminous phenomenon, as a function of the feed voltage V ap plied to the photomultiplier tube. If the correction is too great (curve C the slope of the regulating system is too small and the value of R must be increased by acting upon rheostat 7b (FIG. 6); on the contrary, if the correction is not suflicient (curve C3), the value of R is reduced so as to obtain finally a curve comprising a horizontal portion (curve 0,).

The adjustment being finished, it suflices to actuate slider 13 of potentiometer 10b ('FIG. 6) to obtain the desired gain.

We thus obtain excellent results since for a variation of the feed voltage of -10% the relative variation of gain is about 1%.

We thus achieve in very good conditions the desired stabilization of the gain of a photomultiplier tube, which makes it possible to use such tubes without either stabilized feed sources, which are very expensive and of very great dimension, or to known gain stabilizing arrangements requiring long and delicate adjustments (which cannot be made in industrial utilisations) due to the dispersion of the characteristics of the photomultiplier tubes and of the regulating elements associated therewith and due to the fact that adjustment of the stabilization reacts upon the gain and conversely, which requires an adjustment by successive approximations.

In a general manner, while we have, in the above description, disclosed what we deem to be practical and eflicient 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. For use with a photomultiplier tube including a photocathode, n dynodes located behind one another after said photocathode and an anode located behind said dynodes, a high voltage source having its two terminals connected respectively to said photocathode and said anode and a voltage divider device connected between said source terminals, a device for stabilizing the gain of said photomultiplier tube which comprises conventional means for feeding n-l of said dynodes from said voltage divider and means for supplying the remaining dynode with a stabilizing voltage derived :from said high voltage and the value of which is an increasing linear function of said high voltage, said last mentioned means including a network disposed between said terminals of said high voltage source, said network comprising in series at least one first resistor, a tube capable of maintaining a constant potential difference between its terminals and at least one second resistor, at least one of said resistors being adjustable, and a potentiometer connected in shunt with said last mentioned tube, said potentiometer including a slider connected with said remaining dynode to supply it with its feed voltage.

2. A device according to claim 1 in which said tube maintaining a constant potential difference between its terminals is a corona efiect tube.

3. A device according to claim 1 in which n is an integer close to ten, said remaining dynode being the fourth starting from said cathode.

4. A device according to claim 1 in which n is an integer close to ten, said remaining dynode being the starting from said cathode.

5. A device according to claim 1 in which n is an integer close to ten, said remaining dynode being the sixth starting from said cathode.

No references cited.