GB2255634A - Photomultiplier tube for thickness measurement - Google Patents

Abstract

A photomultiplier tube for use in thickness measurement of hot steel strips in which the dynode voltages are supplied by an independent variable voltage source, 6. The tube may be used with different radiation sources, specifically Americium and Caesium, the choice of source depending on the thickness of the strip, and the dynode voltages being adjusted to produce a high count rate and hence an accurate measurement for the specific source used. <IMAGE>

Description

PHOTOMULTI PLIERS This invention relates to photomultipliers which are commonly used in detection and counting apparatus for radioactivity. In particular the invention relates to the operation of a photomultiplier used in a measurement system with a variety of radioactive sources.

In UK patent specification number 2097915 there is described a means of measuring the thickness of a strip of hot steel as it is rolled, using a source of gamma radiation, a scintillation detector and a photo multiplier. The basic principle of this system is that the thickness of the strip is measured by counting the gamma radiation passing through it from the source. The radiation falls on the scintillation detector which emits photons. The photons are picked up by photomultipliers which emit and amplify electrons to create electrical pulses which can be used in the further electronic detection apparatus. It is obviously an advantage to be able to measure a wide variety of strip thickness with the same equipment, and in order to maintain adequate accuracy during the measurement of greater thicknesses it has been suggested that more energetic radiation sources may be used.For example it has been proposed to use Americium 241 for strip thicknesses up to 6mm and Caesium 137 from 6mm to 17mm. The accuracy of measurement is proportional to the square root of the number of counts and high count rates are necessary if measurements are both to be made quickly and accurately.

However, when the source of Americium 241 is replaced with the more energetic Caesium 137 isotope, despite the higher energy of the gamma rays emitted, the number of counts produced by the scintillation detector/photomultiplier combination was surprisingly low. The normal method of increasing the count rate is to increase the voltage applied to the photomultiplier and it was found that this did not produce an adequate increase in the count rate, and indeed at higher voltages a decrease in the count rate was observed. It is an object of the present invention to provide a photomultiplier that may be adjusted to obtain high count rates in such apparatus with a range of different radioisotopes.

According to a first aspect of the present invention there is provided a photomultiplier including a cathode, anode and a plurality of intermediate dynodes wherein the voltage between the cathode and the first dynode is generated by an independent variable voltage source. Preferably the voltage between first and second dynodes may also be generated by a second independent variable voltage source.Accordingly to a second aspect of the present invention there is provided a method of optimising a photomultiplier for use with a variety of radiation sources in which a photomultiplier having independent variable voltage sources creating the voltages between the cathode and first and second dynodes has the one or both voltage generators set to predetermined settings so as to enable the use of the photomultiplier in a measurement system with comparable accuracy using the various radiation sources.

According to a third aspect of the present invention there is provided a photomultiplier in which a resistive voltage divider chain supplying voltages to the dynode chain of the photomultiplier is optimised for use in a particular environment by choosing resistive values at the cathode end of the chain to hold the dynodes at the cathode end at such voltages as have been determined to optimise the performance of the photomultiplier in such environment.

Said environment includes the nature of the radioisotope being used.

Said resistive values may be chosen by varying variable resistive elements. The selection of the voltages values may be automatically effected by the choice of alternative radiation sources.

The invention will now be described with reference to the accompanying figures of which Figure 1 is a graph of the count rate obtained from Caesium 137 and Americium 241 sources.

Figure 2 is a diagrammatic representation of a conventional photomultiplier.

Figure 3 is a diagrammatic representation of a photomultiplier with variable voltage sources.

and Figure 4 is a circuit diagram of part of a counting circuit incorporating a photomultiplier.

As has been explained before in our UK patent 2097915 a method of measuring the thickness variations in a strip of hot steel using radioactive sources was described. As part of the detection chain a photomultiplier was used so as to convert the photons produced in a scintillation detector into electrons and amplify these pulses to form an input to the electrical counting circuitry. Naturally it is an advantage to be able to use the apparatus with a range of sheet thicknesses, and in order to extend the range of the apparatus beyond a thickness of 6mm with adequate accuracy a more energetic source of gamma rays than the existing Americium - 241 was found in practice to be needed. Such a source is Caesium 137 and this was tried.

Unfortunately the results of the Caesium source are as shown in Figure 1 compared with the results from the Americium 241 source.

The graph in Figure 1 shows the count rate obtained at the photomultiplier output as the potential between the cathode and anode is increased. And it will be seen that for Americium there is a sensibly linear increase in count rate up to rates of about 40 million per second. It will also be seen from the graph that the Caesium source, despite its higher energy gamma rays only produced a maximum of 10 million counts per second at a voltage of 1450, above which voltage the count rate fell. Even the maximum count rate obtained was inadequate for the measurements that were to be made, where because accuracy varies as the square root of the count rate high count rates are needed if fast scanning is used.

This phenomenon of count rate limitation, is not unknown in the field of nuclear measurements, and conventionally the explanation is that the more energetic radiation is producing larger numbers of electrons, and so a space charge effect builds up in the photomultiplier whereby the charge on the electron cloud repels other electrons thereby reducing the number of electrons that travel down the tube. Also there is found to be a blurring of the electron pulses due to the slowing down of the electron velocities because of the repulsion of the electron space cloud.

Turning now to Figure 2 there is shown diagrammatically a photomultiplier. Photons in this impinge upon the cathode 1 which is photocathode which is of a material that will emit electrons when photons fall on it. The space between the cathode and the anode 2 is occupied by a series of additional electrodes known as dynodes (3a-3i) and these are held at voltages intermediate to, the cathode and anode by a resistor chain, 4, conventionally linear in resistance. As the electrons released by the photons leave the cathode 1 they are attracted to the first dynode 3a by the positive voltage of that dynode with respect to the cathode; the electrons accelerate towards the first dynode, impinge upon it and cause the first dynode to emit a greater number of electrons owing to the kinetic energy of the impacting electron which has been acquired through its increase in velocity. These further electrons are again accelerated towards the second dynode 3b by the greater positive potential of this, and accelerate, collide and emit an even greater number of electrons. The process is repeated down the dynode chain until a very large number of electrons is collected at the anode for each impinging photon at the cathode. It is believed that for certain radioisotopes the photons arriving at the cathode have sufficient energy to generate a very large number of electrons towards the anode end of the photomultiplier, and that the space charge referred to previously builds up to both reduce the kinetic energy imparted to electrons and to slow down the rate of travel of the electrons so as to blur the individual pulses as detected at the anode.It has been suggested that this space cloud can be removed by selectively increasing the voltage between dynodes at the anode end of the photomultiplier; this appears in practice not to be the case, and it has been further suggested that the increased voltages merely impart higher velocities to the electrons which then, on impact with the next dynode in the chain, produce even more electrons by the impact. Thus the problem did not appear to be easily solvable.

Turning now to Figure 3 there is shown a photomultiplier tube as in Figure 2 with a photo cathode 1. A dynode chain 3a to 3i is provided connected to a resistive voltage divider chain 4. So far this is similar to Figure 2. However, in Figure 3 there is shown a variable voltage generator 4 in between the cathode and the first dynode 3a. Between the first and second dynodes there is a second variable voltage generator 6. These voltage sources can be adjusted as required, and Table 1 shows the voltages required to produce 30 million counts per second from the radioactive sources Americium 241 and Caesium 137 in the apparatus we use to measure the thickness of steel strip.

It will be seen from Table 1 that the voltage between the dynodes 3a (D1) and 3b (D2) is satisfactory set at 180 volts for either of the sources, for the Caesium source a lower voltage of 50 volts is used for the 3a (D1) - cathode potential but 120 volts for the 3a (D1) cathode potential was suitable for the Americium one.

Clearly the voltages required can be preset by using a resistor of appropriate value in the resistive divider chain, although this may not work as satisfactorily except at a given overall voltage between anode and cathode because the voltages are set by the ratio of the resistors rather than any absolute value of voltage. Also, either of the voltages between the cathode and the first dynode and between the first and second dynodes could be set with a resistor while leaving the other to be controlled by a voltage generator. It will be seen from table 1 that the voltage between the cathode and the first dynode is most critical.

The explanation for the increased count rate with this modification of the dynode potentials at the cathode end of the photomultiplier is suggested as being by limiting the number of electrons produced in the early stages and thus preventing the saturation of the photomultiplier at any point along its length.

The arrangement of Figure 3 may be adapted for automatic changing of voltages between source. It has been suggested for example that when thick strip is to be measured the measuring apparatus should automatically select a radioactive source of Caesium while shuttering over the Americium source, and should start scanning using this. At the same time as shown in Figure 4 a relay 10 activated by a trip switch 11 is arranged to operate in the input control circuit of the voltage generator 4 fed by a DC/DC convertor 12 which varies the output voltage to the appropriate chosen voltage selected for the the source that is being used. In the example given this would be 50 volts if a Caesium source is being used and 120 volts if the Americium source is being used. The generator 6 of figure 3 can be left set at 180 volts. In this way the photomultiplier can be adjusted electronically and automatically to give an output usable whichever of the sources has been selected. Clearly therefore a range of steel sheet thicknesses up to 17mm rather than just 6mm, can be measured satisfactorily. With variations of the voltages, other thickness ranges can be measured using further gamma emitting radioisotopes or x-ray generators.

TABLE 1 Am 241 Cs 137 Anode - D2 1750 1750 D2 - D1 180 180 D1 - Cathode 120 50

Claims (8)

1. CLAIMS 1. A photomultiplier including a cathode, anode and a plurality of intermediate dynodes characterised in that the voltage between the cathode and the first dynode is generated by an independent variable voltage source.
2. 2. A photomultiplier as claimed in claim 1 characterised in that the voltage between the first and second dynodes is also generated by a second independent variable voltage source.
3. 3. A method of optimising a photomultiplier for use with a variety of radiation sources in which a photomultiplier having independent variable voltage sources creating the voltages between the cathode and first and dynode and first and second dynodes characterised in that one or both voltage generators are set to predetermined settings so as to enable the use of the photomultiplier in a measurement system with comparable accuracy in respect of the various radiation sources.
4. 4. A photomultiplier in which a resistive voltage divider chain supplies the voltages to the dynode chain of the photomultiplier characterised in that it is optimised for use in a particular environment by choosing resistive values at the cathode end of the chain to hold the dynodes at the cathode end at such voltages as have been determined by experiment to optimise the performance of the photomultiplier in such an environment.
5. 5. A photomultiplier as claimed in claim 3 in which the environment includes the nature of the radio isotope in use.
6. 6. A photomultiplier as claimed in either claim 4 or 5 in which the resistive values are chosen by varying variable resistive elements.
7. 7. A photomultiplier as claimed in any preceding claim in which the selection of the voltage values is automatically effected by the choice of alternative radiation sources.
8. 8. A photomultiplier substantially as described herein with reference to the accompanying drawings.