Badan Jadrowych Nuclear Research Institute Report INR No ...
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C E R N LIBRARIES, G E N E V A
CM-P00100517
Badan Jadrowych Nuclear Research Institute
Report INR No. 739/XIX/D
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
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. 392 - Rb
- 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 1P (1) = W m 1P (1)
where is the saturation current flowing between electrodes
1 and 3.
- 3 -
e is the charge of the electron
W is the mean energy required for producing one ion pair
m1 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:
f = = A - Β · Q F (2) f = sat
= A - Β · Q F (2)
where
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
s a t e m2P (3)
s a t W m2P (3)
where m2 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 - Am2 + Bm 2QF). (4) i = W (m1 - Am2 + Bm 2QF). (4)
If the ratio between the volumes of the saturated and unsaturated
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part of the chamber is so selected that m1 = Am2 (5)
then
i = em2 Β
P· QF (6) 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).
- 5 -
A schematic diagram of the differential recombination
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 them
selves, 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)
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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 - tissue-equivalent
conducting plastic; thickness of chamber walls - 1mm
duralumin + 1.5 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
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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 60Co gamma-radiation source (QF = 1) and a 210Po - 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 (U2) can be
normalized at the point U2 = U1 (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 un
saturated 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
- 8 -
quantity of 222Rn 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 U2 = -U1 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 - 10-14 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; maximum
dose rate of the radiation measured - 10 rad/h (Fig. 5 ) ; 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
- 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 plane of
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
- 10 -
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
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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 applica
tion as an individual dosimeter for mixed radiation.
The drawbacks of the differential chamber are the require
ment 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.
- 12 -
References
1. M. Zielczynski, Neutron Dosimetry, (IAEA, 1963) Vol. II, P· 397.
2. A.H. Sullivan and J. Baarli, report CERN 63-17 (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.
- 13 -
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: U1, U2 - the supply electrodes ensuring satura
tion 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.
FIG.
1
FIG
. 2
FIG. 3
FIG. 4
FIG. 5