Badan Jadrowych Nuclear Research Institute. Report INR No. 739/XIX/D CERN LIBRARIES, GENEVA CM-P Differential Recombination Chamber

Transcription

1 Badan Jadrowych Nuclear Research Institute Report INR No. 739/XIX/D CERN LIBRARIES, GENEVA CM-P Differential Recombination Chamber by M. Zel'chinskij K. Zharnovetskij Warsaw, June 1966 Translated at CERN by A.T. Sanders and revised by N. Mouravieff (Original: Russian) (CERN Trans. 67-1) Geneva June, 1967

2 International Atomic Energy Agency Symposium on Neutron Dosimetry DIFFERENTIAL RECOMBINATION CHAMBER. M. Zel'chinskij K. Zharnovetskij Nuclear Research Institute, Sverk, Poland *). The shape of the current-voltage curve of an ionization chamber often depends on the linear energy transfer (LET) of the particles producing the ionization. This dependence is used in recombination chambers to determine the quality factor (OF) of mixed penetrating radiation. The quality factor can be measured by the ratio between the current of the chamber nt a given electric field strength and the saturation current [1], or the slope of the current-voltage curve presented as a logarithmic scale [2] An essential requirement is that columnar ionization recombination in the chamber should cover the whole LET range and the whole range of dose rates of the radiation studied. For measurement of the radiation producing a wide LET range the ionization collection efficiency should also depend linearly on the QF [3]. These conditions are ensured by selecting the appropriate electric field strength and also by having a gas mixture of suitable composition and pressure. *) Work partly financed by the International Atomic Energy Agency under contract IAEA No Rb

3 - 2 - The quality factor found is usually used for determining the dose equivalent (DE). It is also necessary to determine the dose (or dose rate) of radiation. The dose rate is measured by the recombination chamber itself, working under conditions close to saturation [4], or another tissue equivalent chamber. Thus, in order to determine the dose equivalent it was necessary to make at least two measurements. In this connection, measurement of the DE by the methods so far used was not carried out continuously and took a considerable time. It seems advisable to design the recombination chamber in such a way that the reading of the dose equivalent can be made directly. As shown by our studies, this can be achieved by suitable connection of the electrodes. Let us look at the system of three flat electrodes shown in Fig. 1. The central electrode is a measuring electrode. The required field strength is provided by the outer electrodes. A sufficiently high voltage is fed to electrode 1, practically creating saturation conditions in the gap between electrodes 1 and 3. The current flowing in this gap is proportional to the dose rate absorbed by the material of the electrodes. Let us consider a tissue-equivalent chamber, i.e. one in which the atomic composition of the electrodes, walls and filling gas corresponds to the atomic composition of soft tissue. The current flowing is also proportional to the mass of gas between electrodes 1 and 3. = e m 1 P (1) W where is the saturation current flowing between electrodes 1 and 3.

4 - 3 - e is the charge of the electron W is the mean energy required for producing one ion pair m 1 is the mass of gas in the saturated part of the chamber Ρ is the dose rate. The voltage applied to electrode 2 produces between electrodes 2 and 3 an electrical field not ensuring saturation. Part of the ions in this gap recombine The voltage is selected so that the reduction in the ionization collection efficiency in the gap between electrodes 2 and 3 with the increase of the quality factor of the radiation takes place linearly: where f = = A - Β QF (2) sat f is the ionization collection efficiency in the gap between electrodes 2 and 3 is the current flowing in the unsaturated part of the chamber sat is the charge of the ions produced in a unit of time between electrodes 2 and 3. A and Β are constants QF is the quality factor of the radiation. If the dose field within the limits of the chamber is homogeneous in space, then e m2 sat W m 2P (3) where m 2 is the mass of gas between electrodes 2 and 3. Taking into account that the direction of the electric field in relation to the measuring electrode is the opposite on either side of this electrode, the current measured by electrometer can be represented in the form of a difference: i = ep (m1 W 1 - Am 2 + Bm 2 QF). (4) If the ratio between the volumes of the saturated and unsaturated

5 - 4 - part of the chamber is so selected that m 1 = Am 2 (5) then em 2 Β i = W P QF (6) The differential current is proportional to the product of the dose rate and the quality factor, i.e. proportional to the equivalent dose rate of the mixed radiation measured. A theoretical calculation [6] based on the Jaffé theory of columnar recombination, gives values of A = 1 and Β = 1/27. However, within reasonable limits these values can be varied by varying the electric field strength within limits which do not interfere with the linearity of f(qf). The value of the coefficient A is also affected by the existence of a preferred ionization recombination, and this makes possible some variation of the coefficient A with the gas pressure in the chamber. Thus, condition (5) can be finally satisfied not by varying one of the volumes of the chamber (which after mechanical fixing of the electrodes may prove difficult) but by varying the pressure and field strength in the chamber. Formula (6), which points to the possibility of using the differential chamber for measuring the DE, is derived on the assumption that the dose absorbed in both parts of the chamber is constant. This condition is generally not fulfilled, which is particularly noticeable in the immediate vicinity of sources of radiation, where the gradient of the dose rate is steep. The error due to the gradient can be considerably reduced by symmetrically alternating the saturated and unsaturated sections in a multi electrode chamber (Fig. 2a).

6 - 5 - A schematic diagram of thedifferentialrecombination chamber constructed by us on the basis of this principle is given in Fig. 3. Each electrode of the chamber is connected to one of the six pins brought out through teflon isolators. Spacer sleeves on the pins ensure that the electrodes are mechanically fixed at an equal distance from each other. The external connection of the pins can be carried out using two different circuits according to the use to which the chamber is to be put. Fig. 3 shows the circuit allowing simultaneous measurement of the dose and dose equivalent, which makes it possible to estimate the quality factor of mixed radiation, whose intensity is not constant in time. In the case considered, the chamber is connected to two electrometers. If the absorbed dose and the quality factor are not of interest in themselves, then one of the electrodes may be disconnected. In this case it is advisable to connect the electrodes in such a way that all sections of the chamber (and not two out of three sections as in the previous case) work under differential operating conditions. For this purpose the booster electrodes, which in Fig. 3 are connected to terminal 2, should be connected to terminal 5, and vice versa, and the terminals of the measuring electrodes 3 and 4 should be connected together. The chamber with the electrodes connected up in this way is used for measuring the dose equivalent. Of course, both terminals of the supply electrodes can be supplied with voltage of the same sign, ensuring saturation in all sections of the chamber - in this case the current in the chamber is proportional to the dose rate. Thus the quality factor can also be determined with exclusively differential connection of the electrodes. For this purpose it is necessary to carry out alternate (and not simultaneous, as in the case of the differentially saturated chamber)

7 - 6 - measurements of the dose and dose equivalent. The circuit ensuring only differential connection of the electrodes will be considered later. The constructional data for the chamber described are as follows: quantity of electrodes - 25; thickness of electrodes - 3mm; spacing between electrodes - 7mm; ratio between the volume of the saturated and unsaturated sections - 1:1; electrode material - conducting plastic; thickness of chamber walls - 1mm duralumin mm mylar. The chamber is filled with a tissue-equivalent gas mixture up to a pressure of about 7 atm. The choice of pressure is a compromise. The greater the pressure applied the wider the range of dose rates for which the recombination chamber may be used. The lower limit of the range of dose rates is set by the sensitivity of the chamber and the upper by the volume recombination of the ions. On the other hand, the increase in the pressure is limited by the mechanical strength of the chamber walls, which should not be too thick, in view of the attenuation of the weakly penetrating radiation. When the pressure is raised it is also difficult to fulfil the saturation condition. This would mean applying too high a voltage to the chamber electrodes. It is true that it is not essential to have full saturation in the whole QF range. Linear correction can be introduced for the absence of saturation [7]. However, correction should not be too great - otherwise the error due to the actual non-linearity of the correction is no longer negligible and the sensitivity of the chamber is also considerably reduced. For the calibration of the chamber at least two radiation sources are required producing a different known LET range. Then a curve is plotted of the dependence of the current of the chamber

8 - 7 - on the voltage fed to the electrodes which are to produce the recombination operating conditions. At the same time a constant rather high voltage of opposite sign is fed to the other supply electrodes. The working point is located at the intersection of the sensitivity curves (Fig. 4). The X-coordinate of the point of intersection determines the voltage ensuring the required recombination conditions, the y-coordinate the sensitivity of the chamber. As reference sources we used a 60 Co gamma-radiation source (QF = 1) and a 210 Po - Be neutron source (QF = 7.3). In order to determine the sensitivity of the chamber it is necessary to know relatively accurately the activity of only one source. For the other sources the dependence i (U 2 ) can be normalized at the point U 2 = U 1 (allowing for error due to incomplete saturation), since the ratio between the saturation current and the absorbed dose rate can be considered independent of the type and spectrum of radiation. The ratio between the current of the differential chamber at the point of intersection of the curves (Fig. 4) and the current corresponding to saturation in all sections is about 0.02 for gamma-radiation, i.e. when the volumes of the saturated and unsaturated sections are equal, the ionization collection efficiency in the latter is about 96%. As follows from the above-mentioned investigations, this figure corresponds to the ionization collection efficiency when it is linearly dependent on the QF [4], [6]. Intersection of the curves at another point would show the absence of linearity. This can be checked directly by using additional calibrating radiation, producing a LET range different from the previous ones. For instance, one can use alpha radiation from a small

9 - 8 - quantity of 222 Rn added to the gas filling the chamber during calibration. Under conditions of linearity all three curves intersect at one point. When the ratio between the volumes of the saturated and unsaturated sections is incorrect, the intersection will not take place at one point. In that case correction of the volumes should be carried out. When the differential current is equal to zero for U 2 = -U 1 in a uniform radiation field, this shows that under these conditions the volumes of the sections are equal. But for other voltages, the ratio between the effective volumes can be different. The variation of the effective volume with the voltage may be due to mechanical deformation of the electrodes owing to electrostatic forces, and also to a possible variation of the configuration of the electrical field at the edges of the electrodes. In the chamber described, both effects have been reduced to a minimum, owing to rigid construction and the use of guard rings for each measuring electrode. The basic data for the operation of the chamber are as follows: sensitivity A/mremh -1 ; composition and range of radiation measured - arbitrary; average depth of tissue for which the dose equivalent is determined - of the order of 2 cm; dose rate of the radiation measured - 10 rad/h (Fig. 5); maximum accuracy of determining the dose equivalent of mixed radiation of unknown composition and range - of the order of 20% (without taking into account errors introduced by the instability of the sources of supply and the electrometer). The error in measuring the DE is of almost the same order as the indeterminacy of the formulation in the recommendations of the International Radiation Protection Committee of the dependence

10 - 9 - of the QF on the LET. The basic factors affecting the error are: the ambiguity of the dependence of the ionization collection efficiency on the QF in the LET range < 3.5. kev/μm, and > 175 kev/μm, the deviation from linearity, the dependence of the ionization collection efficiency on the distribution of delta-electrons and on the direction of the particle tracks in relation to the planeof the electrodes, the inaccuracy of calibration, the inconstancy of the energy required for the production of one ion pair, the incomplete tissue-equivalence of the components of the chamber, the relatively large geometrical dimensions of the chamber, the inconstancy of the ratio between the volumes of the sections of the chamber, the presence of volume recombination ionization, the variation of the dose rate in space through the volume of the chamber. The effect of the latter factor in a twin non-differential chamber (Fig. 2B) can be counteracted by repeated measurement carried out after mutual interchange of voltages connected to the supply electrodes of the chamber. In the case of a differential chamber such switching is possible only if the saturated and unsaturated sections are of exactly similar volume. In other cases the error due to the gradient of the dose field can be reduced by averaging the results of the measurements made with different positions of the axis of the chamber. If the arrangement of the sections in the differential chamber is symmetric (Fig. 2a) it is usually not necessary to make measurements for different positions of the chamber, since the effect of the gradient in this case is automatically counteracted and the error in measurement is only influenced by the inhomogeneity in space of the gradient. Apart from the factors characteristic of a recombination chamber, the error in determining the DE is also affected by the

11 accuracy of measurement of the current of the chamber, depending on such factors as the spurious volumes of the connecting structures, the instability of the electrometer, and the drift of the supply voltage. As regards the two latter factors, here also one can see the advantage of a differential chamber. The required stability of the electrometers in the case of a differential chamber can be lower by one order of magnitude than the stability required in the case of a non-differential chamber, since in the latter the DE depends on the difference between the two currents measured. The error due to the drift of the voltage, applied to the electrodes of the differential chamber can be partially counteracted owing to the different polarity of the voltages. Maximum compensation is attained when the moduli of the product UC are identical for sections of the differential chamber with a positive and a negative supply (U = the voltage, C = the electric capacitance between the supply and measuring electrode). This can be achieved when the proportion between the gaps between electrodes is appropriate, using different gas pressure in the saturated and unsaturated parts of the chamber. To sum up the advantages of a differential recombination chamber: 1) The chamber gives the possibility of direct measurement of the dose equivalent of mixed penetrating radiation of any spectrum and composition. 2) It ensures continuity of measurement 3) It makes it possible to carry out measurements in fields varying qualitatively and spatially in time. 4) In order to make measurements by means of a differential

12 recombination chamber it is sufficient to have available only one electrometer, without strict requirements as to accuracy and stability, and a source of voltage supply consuming practically no current. This makes it possible to use the differential recombination chamber not only for laboratory measurements but also as a field instrument for dosimetric monitoring. It should also be anticipated that after overcoming a series of technical difficulties the modified differential chamber of suitably small dimensions will find an application as an individual dosimeter for mixed radiation. The drawbacks of the differential chamber are the requirement for a constant ratio between the volumes of the saturated and unsaturated sections of the chamber, relatively low sensitivity, and the difficulty of making measurements in dose fields with considerable inhomogeneity in space. In spite of the above drawbacks, the differential recombination chamber can be successfully used for determining the degree of radiation hazard in fields of mixed radiation near accelerators, reactors and other atomic plant.

13 References 1. M. Zielczynski, Neutron Dosimetry, (IAEA, 1963) Vol. II, P A.H. Sullivan and J. Baarli, report CERN (1963). 3. M. Zielczynski, Nukleonika, 7 (3), 175 (1962). 4. M.Zel'chinskij, V.N. Lebedev and M.I. Salatskaya, Pribory Tekh. Eksperim. No. 6, 73 (1964). 5. J. Baarli and A.H. Sullivan, Health Physics 11, 353 (1965). 6. M. Zel'chinskij, Radiobiologiya, 5 (2), 161 (1965). 7. M. Zel'chinskij, Determining the dose equivalent of mixed radiation by means of detectors, whose sensitivity depends on the LET. IAEA symposium, Report Sm-76/39.

14 Figure captions Fig. 1 : Block diagram of the differential recombination chamber 1,2 - supply electrodes; 3 - measuring electrode; 4 - supply source ensuring saturation conditions; 5 - electrometer; 6 - source of supply for the unsaturated region of the chamber. Fig. 2 : Arrangement of electrodes in recombination chambers a) Differential chamber; b) Twin differentially saturated chamber; c) Twin non-differential chamber. Symbols: U 1, U 2 - the supply electrodes ensuring saturation and recombination conditions respectively; i - the measuring electrodes of the saturation current, the current for recombination conditions and the differential current. The saturated region is shaded. Fig. 3 : Schematic drawing of the chamber. 1 - manometer; 2 - terminal for the application of the voltage ensuring recombination conditions; 4 - differential current terminal; 5 - terminal for application of voltage ensuring recombination conditions; 6 - valve; 7 - grounded guard rings; 8 - booster electrodes. Fig. 4 : Sensitivity of the differential chamber. The dotted curve corresponds to the sensitivity to neutrons in units of pa/mrad h -1. Fig. 5 : Dependence of the differential current on the equivalent dose rate.

15 FIG. 1

16 FIG. 2

17 FIG. 3

18 FIG. 4

19 FIG. 5