Dosimeter with a thimble ionization chamber for ultrahard radiation from betatrons and synchrotrons
Abstract Generated abstract
This paper describes the design and performance of a dosimeter intended for point dose measurements in superhard bremsstrahlung fields from betatrons and synchrotrons, where conventional large-volume ionization chambers are unsuitable. The instrument combines a 1 cubic centimeter graphite thimble ionization chamber with a remote first amplifier stage using an electrometric tube, allowing chamber movement, chamber replacement, reduced interference, and measurement in phantoms including water. The reported sensitivity permits measurement of ionization currents on the order of 10 to the minus 14 amperes, corresponding to dose rates beginning at about 0.00326 roentgen per minute. Operational experience over five years is presented as evidence of stability, accuracy, and practical usefulness in betatron laboratories and related high-voltage measurements.
Full Text
A Dosimeter with a Thimble Ionization Chamber for Superhard Radiation from Betatrons and Synchrotrons
V. A. Moskalev
(Presented by the scientific seminar of the Faculty of Physics and Technology)
When working with generators of superhard bremsstrahlung—betatrons and synchrotrons intended for clinical use, as well as for industrial flaw detection—it is necessary to carry out a large amount of dosimetric work to check protective devices, determine the distribution of isodoses in body tissue by means of phantoms, determine the distribution of radiation in the space around the emitter, monitor the required radiation dosage, etc. For these purposes ionization chambers operating with DC electronic amplifiers of various systems and designs are widely used. The standard instruments produced, of the USDD-6, UP, “Mak,” “Kaktus,” and other types, are designed for the use of ionization chambers with a relatively large working volume (on the order of liters). Therefore the indicated instruments are applicable only in measurements where it is not necessary to determine the value of the radiation dose at a given point; for example, as control instruments when working with radiation, for estimating the average value of the radiation dose generated by the installation, etc. The design of the instruments does not permit replacement of the large ionization chamber by a so-called “thimble” chamber for technical reasons. In addition, the presence of a large volume of gas in the remote unit of these instruments makes the use of a thimble chamber impossible in principle, owing to the occurrence of an inadmissible error in the measurements.
In measurements connected with determining the distribution of isodoses in body tissue, and in a number of other tasks, it is necessary to have an instrument with a small ionization chamber that would make it possible to measure the dose in a small volume of space. In our opinion, such an instrument must satisfy the following requirements:
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The possibility of measuring the amount of absorbed radiation energy in a volume on the order of \(1\ \mathrm{cm}^3\).
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The ionization chamber must be capable of free movement within several meters of the recording instrument (microammeter).
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The chamber must be made of “tissue-equivalent” material in order to reduce the error when measuring dose in a phantom.
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The design of the instrument must permit rapid replacement of the ionization chambers.
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The effect of interference on the amplifier must be minimal.
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The response of the ionization-chamber holder and of other amplifier components to radiation must be reduced to a minimum.
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The sensitivity of the instrument must make it possible to measure ionization currents of the order of \(10^{-13}\) a and less.
In accordance with the above, an instrument was constructed, the schematic diagram of which is shown in Fig. 1.
Fig. 1
As the input tube of the dosimeter, an experimental “finger” electrometric tetrode of type 1E1P, of domestic manufacture, was used. In the second amplification stage a pentode of type 2Zh2M was installed; it has the same filament current as the 1E1P tube, which makes it possible to connect the cathodes of these tubes in series. The input resistance of the amplifier circuit is \(10^{11}\) ohm. The amplifier is supplied from a stabilized voltage source. The tube cathodes are heated by rectified current, and the absence of storage batteries in the filament circuit makes the instrument very convenient in use.
The radiation detector is a graphite ionization chamber with a volume of \(1\ \mathrm{cm}^3\) and a wall thickness of \(3\ \mathrm{mm}\) ¹). The chamber body is connected to a source of constant voltage of 150 V, and the collecting electrode is connected directly to the control (anode) grid of the output tube.
The dependence of the current at the amplifier output on the potential at the control grid of the electrometric tube has the form of a straight line. When the grid potential is changed by 1 V, the output current changes by \(550\ \mu\mathrm{A}\). This means that \(1\ \mu\mathrm{A}\) at the amplifier output corresponds to an input current (if the input resistance is \(10^{11}\ \Omega\)) of \(1.82 \cdot 10^{-14}\) a.
In CGSE units this current will be:
\[ I_{\mathrm{in}} = 1.82 \cdot 10^{-14} \cdot 3 \cdot 10^{-9} = 5.45 \cdot 10^{-5}. \]
¹) The chamber wall thickness may be changed up to \(3.8\ \mathrm{cm}\) in \(5\ \mathrm{mm}\) steps by means of special graphite caps.
Dividing \(I_{v.k.}\) by the ionization volume of the chamber, we find the radiation intensity in \(r/\mathrm{s}\) per 1 division of the instrument:
\[ \frac{I_{v.k.}}{V}=\frac{5.45\cdot 10^{-5}}{1}=5.45\cdot 10^{-5}\ r/\mathrm{s}, \]
which corresponds to \(0.326\cdot 10^{-2}\ r/\mathrm{min}\), i.e., the instrument makes it possible to measure radiation doses starting from \(0.326\cdot 10^{-2}\ r/\mathrm{min}\). Such sensitivity is quite sufficient for most practical work carried out with betatron radiation.
Structurally, the instrument is made in the form of a remote unit and a direct-current amplifier. The remote unit combines the ionization chamber and the first stage of the amplifier. The input resistance is also installed in the remote unit. The part of the circuit placed in the unit is shown in the diagram within the dashed frame, and to the right in Fig. 1 the connection diagram in the unit is shown. The arrangement of the parts in the unit is shown in Fig. 2. The unit is connected to the amplifier by an armored cable. The holder of the ionization chamber
Fig. 2
(together with the chamber) can easily be replaced by another one, of greater or lesser length, depending on the conditions under which the measurements are performed. The hermetic connection of the ionization chamber to the holder makes it possible to measure the distribution of isodoses in a water phantom.
If strong magnetic fields are present near the instrument, a light screen made of sheet iron, placed over the housing of the remote unit, completely eliminates interference caused by these fields.
The proposed dosimeter has been in operation for 5 years, has shown good stability and accuracy in operation, and can be recommended for various kinds of experiments requiring dose measurements in small volumes.
The simplicity of the instrument circuit makes it possible to manufacture and adjust it in a short period of time. Assembly of the circuit presents no difficulties. It should be especially noted that the remote unit must be made very carefully; before assembly all parts are washed with alcohol and thoroughly dried, and after assembly the interior of the unit is filled with pure paraffin. If this condition is fulfilled, stable and reliable operation of the dosimeter can be guaranteed.
At present the instrument has come into use in a number of betatron laboratories of the Soviet Union (Moscow, Tbilisi, and others), as well as in high-voltage laboratories for measuring leakage currents in dielectrics in studies of the electrical breakdown of the latter (Tomsk).