LUMINOSITY

The subject of the invention is a procedure and a circuit arrangement for luminosity measuring, with minimal cover of 6 orders, advantageously in "TLD" equipment.

In the domain of luminosity measurements photo- multiplier tubes /PMT/ are very often applied in scientific and technical practice.

For their operation, such multiplier tubes require high voltage /500-1500 V/ power supplies. The sensitivity of the said tubes depends considerably on supply voltage but in the case of stable supply voltage the luminosity to current conversion ratio of the multiplier tube - or otherwise its sensitivity - is also stable /if constant temperature is assumed/.

Other well-known devices are available for measur- ng luminosity, for example vacuum photodiodes, solid state photodiodes, phototransistors, but their sensitivity is 3-4 orders lower than the sensitivity of multiplier tubes. Therefore to detect very low level luminosities - for example 10 -12 - 10-13 lumen intensity - only multiplier tubes are suitable and applied at present.

Luminosity levels such as this are required to be process¬ ed in scientific and technical practice, e.g. ther o- luminescent dosemeters /TLD/ or the domain of measuring sig ^' nals, from scintillation detectors. But in these domains not only too small luminosity levels occur, but 8 orders higher, say for example 10 -4 - 10-5 lumen magnitude, also occur. If a multiplier tube is set to sense the lower luminosity levels - which means more sensitivity - it may become saturated in the case of too large light intensity, or it "goes blind".

SUBSTITUTE 3HEE -

Moreover, because of the large output current; we have to apply a high current resistor-network, which means a needless high load for the high voltage power supply in most cases, and this heats the multiplier tube thereby reducing its stability.

The current leaking from the multiplier tube is usually displayed in digital from by means of analog to digital converters, which may be luminosity level sensing, but can also be by summing the luminosity within a ce-rtain time interval, such as the method developed in ^• thermoluminescent dose measurements.

In the case of digital display generally 3-4 digits are required because of the necessary 0.1 % accuracy of readout. For a 3-4 digit display, we need to use a decimal point or also display the exponent, to cover a- measuring range of 6-3 orders. In view of this the opera¬ tion of the signal processing circuit must appropriately be modified. A convenient way of doing this is to change the conversion factor of the analog to digital converter but this method does not eliminate the saturation of the multiplier tube if larger luminosity levels are applied. The present invention seeks to solve the problem in "TLD" measurements thereby eliminating the saturation of multiplier tubes in the case of larger light intensities on the one hand and making possible the extension of orders of measurable light quantities on the other.

Thus the task to be solved by this invention may. be considered by creating a method and circuit arrange¬ ment, to reduce the saturation or sensitivity of the multiplier tube by a well-defined factor rendering it. suitable for expanding the measurable light quantities by some orders.

The invention Is based on the concept that the sensitivity of the multiplier tube may very precisely be set by changing the voltage, and it can be changed over

SUBSTITUTE SHEET ' OMPI

more orders, and in the case of switching from a higher voltage or sensitivity to a lower one, the multiplier tube can rapidly adjust to the lower sensitivity level. For example in thermoluminescent dosimetry the time required for light quantity measurements is about 10-30 seconds. The time required for switching is 10-20 milli¬ seconds. If the worst case is presumed, the error caused by this is less than 0.1 %. For example this error is invisible in the case of three-digit display and on the other hand the probable accuracy of "TL" dosimetry is not better than 0.5 % so an error of 0.1 % is negligible.

The method applied in this invention is the improvemenet of another well-known procedure in which a current is produced that is proportional to the light intensity, and this current is measured, and the luminosity to current conversion is performed by photomultiplier.

The improvement, namely the said invention is that at the same time or parallely with current measuring we compare the current, and if the current is higher than a given level, we decrease the sensitivity of the photomultiplier, advantageously by one order by changing the supply voltage, and this change in order is stored, for example, by a register, and measurement and comparison are then continued. In the sense of the invention it is practical to compare the current to be measured with a linear circuit. In other words it is practical to transform the current to a digital signal, and then to measure and compare this digital signal. The circuit arrangement applied in this invention . is an improvement of a well-known circuit arrangement which contains a photomultiplier tube with a high voltage power supply and level sensor, the input of the photo¬ multiplier tube being connected to the output of the high voltage power supply, and the output of the tube

SUBSTITUTE SHEET

being connected to the input of the level sensor.

The improvement, namely the invention, is that this circuit arrangement also has a control unit whose input is connected to the output of the level sensor through the control line, and its output is connected to the input of the high voltage power supply through the reference line.

In the sense of the invention it is practical if the level sensor has an analog to digital converter, a digital to analog converter, a comparator and digital counter, connected in series in such a way that the count input of the digital counter is connected to the output of the analog to digital converter through the impulse line, and the order-select input of this counter is connected to the output of the comparator through the control line.

Namely it is practical if the level sensor has an analog to digital converter, decimal counter and control register, each connected in series in a way that the control register output is connected to the preset input of the decimal counter through the preset line.

Moreover it is practical if the control unit has a reference source and a calibrating divisor the inputs of the said calibrating divider being connected to the output of the reference source through the stable voltage line and also to the control line. The output of the calibrating divider is connected to the reference line.

"Moreover it is-also practical, if the calibrating divider has three resistors designated in the following as first, second and third, each resistor being connected to the reference line by one pole, and the other pole of the first resistor being connected to the stable voltage line, the other pole of the second resistor being connect¬ ed to -ground and the other pole of the third resistor being connected to the control line.

SUBSTITUTE SHEET

The invention is presented in more detail by the following five figures:

Figure 1 illustrating the timing of the method applied in the invention Figure 2 illustrating an example showing the realization of the circuit arrangement applied in the invention Figure 3 showing a possible realization of a level sensor applied in the invention Figure 4 showing another possible realization of level sensor Figure 5 showing a possible configuration of the control units applied in the invention.

In the diagram of Figure 1 we introduce the summing method of thermoluminescent dosimetry based on the principle of sensitivity modification. If a thermo¬ luminescent dosemeter is evenly heated it will radiate light after a certain time. The increase of temperature or time /which coincides with the ' ' axis/ causes an increase in the radiated light /axis 'g'/, and later this light decreases with increasing temperature, If a light- intensity diagram is plotted it shows a bell-shaped curve which is known as the glow curve. The area below this curve or the total light quantity is proportional to the dose and the purpose of TLD measurement is to determine this area. The g °max lig°ht-current is the saturation current of the multiplier tube and in the normal case the tube cannot follow the "glow" curve so the supply voltage of the multiplier tube is decreased with the stated procedure. As an example, the sensitivity of the multiplier tube is decreased by an order, and then we continue the measuring of light quantity and the order-change is dis¬ played on the decimal display. The total light summing time T is generally 10-30 seconds, and the switching tim T,

SUBST-TU" ✓" ^{" 4}

/

is less than 10 milliseconds so the uncertainty of the summing caused by the switching time is negligible, as may be seen in the figure.

In Figure 2 one possible circuit arrangement is shown according to the invention. This circuit arrange¬ ment has a photomultiplier tube 11, high voltage power supply 13 and level sensor 12. The input of the photo¬ multiplier tube 11 is connected to the output of the high voltage power supply 13 through the voltage line f, the output of the tube is connected to the input of the level sensor 12 through the current line m. The circuit arrangement also has a control unit 14. The input of the control unit 14 is connected to the output of the level sensor 12 through the control line v, and the output of the control unit is connected to the input of the high voltage power supply 13 through the reference line r.

The high voltage power supply 13 feeds the multiplier tube 11 which in turn senses light, for example, that emitted by the TLD, and the light-current of the multiplier tube 11 reaches the level sensor 12 through the current line . At a certain current level, in practice slightly lower than the saturation current g _maχ of the multiplier tube 11, the level sensor gives a switch¬ ing signal to the control unit 14 through the control line v.and control unit 14 then determines the output voltage of the high voltage power supply 13 through reference line r. The effect of the switching signal causes the control unit 14 to decrease the output voltage of the high voltage power supply 13 through the reference line r. It Is practical to set the decrease to be rounded entire orders lower than the original value.

In Figure 3 a possible realization of the level sensor according to the invention is shown. The level sensor 12 has an analog to digital converter 21, a digital to analog converter 22, a comparator 23 and a

SUBSTITUTE UH T

digital counter 24 all of which are connected in series. The count input of the digital counter 24 is connected to the output of the analog to digital converter 21 through the impulse line i, the order-select input of the counter is connected to the output of the comparator 23 through control line v.

The current of the multiplier tube 11 is convert¬ ed to impulses by the analog to digital converter 21, an the frequency of these impulses is directly propor- tional to the current of the multiplier tube. The impulses reach the digital to analog converter 22 and the digital counter 24 count input through the impulse line i. The digital to analog converter 22 gives a voltage to the analog line d that is proportional to the frequency, and this line is connected t ^' o the input of the comparator unit 23. The comparator unit 23 is set so that its threshold is slightly lower than the saturation current of the multiplier tube. At this said level it gives a switching signal to the control unit 14 through the control line v, as well as to the order-select input of the digital counter 24.

In Figure 4 another example of a circuit arrange¬ ment of level sensor according to the invention is shown. According to this the level sensor 12 has an analog to digital converter 21, decimal counter 25 and control register 26, connected in series. The output of the control register 26 is connected to the preset input of the decimal counter 25 through the preset line b. The impulses of the analog to digital converter 21 reach the decimal counter 25- through the impulse line i. The overflow output of the decimal counter 25. gives a carry impulse to the input of the control register 26 through the carry line c. The control register 26 gives a switching signal to the order-select input of the decimal counter 25 through the preset line b, as well as switching the control unit 14 through the control line v.

SUBSTITUTE SHEET

In Figure 5 a possible realization of the control unit according to the invention is shown. Thus, the control unit 14 has a reference source 27, and a calibrating divider 28. The inputs of the calibrating divider 28 are connected o the reference source 27 through the stable voltage line s and to the control line v, its output is connected to the reference line r. The calibrating divider according to the inven¬ tion is also shown In Figure 5. The calibrating divider 28 has three resistors /hereafter designated Rl, R2, R3/, each connected to the reference line r by one pole. The other pole of the first resistor Rl is connected to the stable voltage line s; the other pole of the second resistor R2 is connected to the ground; the other pole of ^' the third resistor R3 is connected to the control line v.

The most general solution of the high voltage power supply 13 is that its output voltage is compared with a reference voltage by an internal circuit. The output voltage is in a closed connection with the reference voltage which appears on the reference line r in our figure. In this realization the voltage of the reference source 27 is divided to the desired value by the calibrating divider 28. The calibration occurs so that the voltage divided by the Rl and R2 resistors adjusts the high voltage power supply 13 to the original value through the reference line r. The level obtained through the control line v, for example 0 V, switches the R3 resistor into the division. The voltage of the reference line r Is thereby varied and so the high voltage is also changed. It is practical to adjust the value of resistors Rl, R2, R3 so that the varied high voltage would make the sensitivity of the multiplier tube 11 decrease by entire orders. The advantages of the method and circuit arrange-

f O PI

ment according to the invention may be summarized as follows:

- it prevents the saturation of the multiplier tube thereby eliminating the non-linearity error of luminosity to current conversion

- the measuring range of the photosensitive system is increased by more orders

- the divisor circuit of the multiplier tube may be of low current type thereby the load and the noise of the high voltage power supply is smaller

- the electrical power dissipation is lower, which means smaller heating for the tube and thus the temperature of the tube decreases and the stability of the tube increases.

O PI

W. WIPO <