EPA-2 Radiospectrometer Combined with the ESG-2.5 Accelerator

L. V. Serykov, A. A. Vasil’ev

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Kinetic studies of the accumulation and loss of paramagnetic particles formed upon the interaction of radiation with the substance under study, as well as studies of short-lived radicals, are most conveniently carried out directly under irradiation. In such irradiation, X-rays, $\gamma$-rays, and likewise neutrons do not create great difficulties in design development, since all these types of radiation do not interact with the magnetic fields of the spectrometer. However, from the standpoint of work efficiency, i.e., having in view the possibility of obtaining significant absorbed-dose values over a short interval of time, it is advisable to supplement accelerators of charged particles—linear accelerators, electrostatic generators, etc. We have prepared an experimental setup satisfying the stated requirements.

A standard EPA-2 instrument, produced by the Angarsk Special Design Bureau, was used as the spectrometer. The source of accelerated electrons was the ESG-2.5 electrostatic generator, installed in the Research Institute of Nuclear Physics, Electronics, and Automation at Tomsk Polytechnic Institute.

The creation of the combined EPA-2–ESG-2.5 installation required solving a number of problems connected with changing the design of the accelerator beamline, stabilizing the electron-beam current, and protecting the operating personnel from the action of ionizing—

…ionizing radiation, etc.

At the same time, it was necessary to solve certain design problems involved in modernizing the EPA-2 radiospectrometer. The standard EPA-2 spectrometer is a narrowly specialized instrument intended for express analysis of preparations under industrial conditions. For this reason, the design of the spectrometer is sufficiently simple and reliable, but it does not make it possible to use it in operation with an electron beam introduced into the resonator. The latter is associated with the small gap between the pole pieces, the insufficient diameter of the pole pieces, the introduction of high-frequency modulation by a loop, and, consequently, the insufficient power of the high-frequency modulator. Elimination of the above shortcomings made it possible to combine the radiospectrometer with the electrostatic generator for operation at the moment when accelerated electrons pass through the sample.

The changes in the design of the spectrometer were as follows:

An electromagnet of a yoke type was calculated and manufactured with the following parameters:

a) induction in the gap — up to 5000 G,

b) diameter of the pole pieces — 160 mm,

c) air gap — 35 mm.

The magnet core is made of steel 3, and the poles and pole pieces are made of “Armco” iron.

To ensure operation of the constructed electromagnet, the power of the stabilized magnet power supply was increased fivefold.

The introduction of new modulation coils entailed an increase in the power of the transformer providing the current required for modulation.

For convenience in operation and in order to ensure better extraction of the electron beam onto the sample, the magnet is made asymmetric, so that the field coils are located on one of the poles.

The electron beam is directed onto the sample along the lines of force of the electromagnet through holes of diameter 6 mm at the center of one of the poles. Operating experience [1] showed that a hole of such diameter at the center of the pole piece has only an insignificant effect on the uniformity of the magnetic field and, for the width of the lines of the radicals usually studied, does not play a substantial role.

The need for quantitative measurements of the concentrations of free radicals, as well as the possibility of operating the spectrometer under an electron beam, impose certain conditions on the choice of resonator, since the method of quantitative measurements is as follows: the signal of the sample under study is recorded simultaneously with a reference signal—a “side” standard—which must be positioned outside the region of action of the electron beam. The side standard serves as an indicator of changes in the sensitivity of the spectrometer during operation as a result of various external factors.

Thus, the requirements on the resonator that ensure normal operation of the spectrometer under an electron beam are as follows:

1. The possibility of introducing the electron beam into the resonator.

2. The creation of a sufficiently uniform high-frequency modulation field throughout the entire volume of the resonator (which would make it possible to work with a “side” standard).

3. The possibility of using a side standard during operation without subjecting it to irradiation.

4. The possibility of placing the resonator in the air gap of the electromagnet, which must be sufficiently narrow to reduce the path of the electrons in air.

On the basis of the requirements listed above, a rectangular resonator with the $H_{104}$ mode of oscillation was chosen, with a system for introducing the high-frequency modulation similar to that used in the commercial RE 1301 spectrometer [2]. The high-frequency circuit is fed through a power amplifier, triggered by...

from the EPA-2 modulator. The power amplifier is assembled on two GU-50 tubes; structurally it is made as a separate unit and is located in the immediate vicinity of the resonator.

The schematic diagram of the power amplifier is shown in Fig. 1.

The resonator was calculated by the generally accepted method [3]; it is made of brass, with subsequent grinding of the inner surface and silver plating. In the narrow wall of the resonator there are two openings for introducing the specimen under study and the side standard.

The opening in the wide wall serves for introducing the electron beam. Owing to the slot in the wide wall of the resonator, the resonator itself constitutes an inductance coil, connected in parallel with the capacitance, and forms an oscillatory circuit that serves as the load of the modulator and is tuned to a frequency of approximately 1 MHz.

The slot and the opening in the wide wall of the resonator have little effect on the operating frequency and the quality factor of the resonator, since for the given wave type \((H_{10})\) the resonator is not critical with respect to its dimensions along the \(y\)-axis.

The dimensions of the lips on the narrow side are chosen so as to suppress radiation through the opening. The magnitude of the lips is given by the condition [4]:

\[ h > \frac{1}{2}\lambda_{\mathrm{cr}}, \]

where

\[ h \]

is the height of the lip,

\[ \lambda_{\mathrm{cr}} \]

is the critical wavelength of the resonator.

To broaden the working range of the accelerator, provision was made for splitting the electron beam into three channels. For reasons of economy, the electromagnet of the spectrometer is installed on one of the side channels.

The beam-splitting system is connected by its upper flange to the vacuum system of the accelerator. The electron beam enters through the ion guide into a chamber where, under the action of an external

Fig. 1. Schematic circuit diagram of the power amplifier of the RF generator.

Visible inscriptions in the schematic include: “AC amplifier unit,” “control unit,” “reserve load,” “antenna,” “~220 V,” “2000 V,” and “−200 V.”

of the magnetic field, is turned and enters one of the lateral channels, depending on the polarity of the turning magnetic field.

In the absence of the turning magnetic field, the electron beam is used by the experimenters in the usual way. The entire vacuum system is assembled with rubber seals. At the exit of the ion ducts, aluminum foil 50 µ thick is installed for the extraction of the accelerated electrons.

As the power source for the turning electromagnet, a serially produced UIP-1 unit was chosen; it provides sufficient stability and magnitude of the operating current of the electromagnet. Defocusing of the electron beam is carried out by magnetic lenses.

In the accelerator used, there is no system for stabilizing the electron energy, as a result of which the electron current after turning is unstable in time.

To increase the stability of the electron current, an automatic-control system was used. As the sensor of the automatic-control (AR) system, a plate device installed at the exit of the turning chamber was used. After the turn, the electron beam passes between the plates of the plate device. When the electron energy changes, the beam, after passing through a constant magnetic field, will be deflected toward the upper or lower plates. A balanced direct-current amplifier, Fig. 2, is connected to the plates of the plate device; the signal from it is fed to the control grid of the UIP (lamp L_{13} in the UIP-1 circuit) and corrects the current of the turning magnet. The automatic-control system used ensures stability of the electron current practically no different from the stability of the current of the straight beam, provided that approximately 3% of the current of the entire beam falls on the plates of the plate device.

For purposes of radiation safety, the magnet and the spectrometer control panel are placed in different rooms. Communication with the resonator is carried out through a waveguide line about twenty meters in total length.

Fig. 2. Schematic diagram of the UPT.

Directly in the zone of action of the radiation are the magnet gap, the resonator, and the modulator power amplifier. Irradiation is begun by opening a shutter located above the resonator, which is controlled remotely.

Thermostating of the sample is carried out by placing it in a jet of gaseous nitrogen blown through the resonator. The required temperature was provided by the evaporation rate of liquid nitrogen. The system used is described in [5] and ensures thermostating of the sample at a temperature of \(-196^\circ\mathrm{C}\) for a long time. Smooth temperature variation was carried out over the temperature range \(-180^\circ\mathrm{C}\) to \(+200^\circ\mathrm{C}\).

The parameters of the combined spectrometer, i.e., its resolution and sensitivity for real samples, do not differ from the rated values for the EPA-2.

The current density of the accelerated electrons can be varied by the defocusing system from zero to \(2~\mu\mathrm{A}/\mathrm{cm}^2\).

The method for determining the radiation-chemical yields of the paramagnetic centers formed is similar to that described in [1].

In conclusion, the authors thank the laboratory staff member N. Ya. Buben for repeated consultations, and S. I. Rukovodov, V. V. Shiryaev, and G. V. Zinger for their assistance in the work.

REFERENCES

1. V. D. Molin et al., PTE, 6, 73, 1960.
2. Description and instructions for installation and operation of the RE 1301 unit, Leningrad, 1963.
3. A. A. Lapiks, Cavity Resonators, Leningrad, 1954.
4. N. D. Shirmay, Waveguides and Cavity Resonators, Svyazizdat, 1959.
5. S. I. Rukovodov, Proceedings of the Tomsk Polytechnic Institute, vol. 199, p. 38, 1969.