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

- 4 -

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)

- 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 - 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

- 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 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

- 11 -

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