SOME ASPECTS OF THE GAMMA-ACTIVATION METHOD OF ANALYSIS USING A HIGH-CURRENT ELECTRON ACCELERATOR AS A RADIATION SOURCE

R. P. Meshcheryakov, B. M. Yakovlev

In recent years, various methods of elemental and isotopic analysis of matter have found broad practical application; these methods are based on the analysis of $\beta$- or $\gamma$-radiation produced during the decay of induced activity in the samples under study, which are irradiated by fluxes of neutrons, charged particles, or high-energy photons. These methods include, above all: a) the neutron-activation method (thermal and fast neutrons from neutron generators, nuclear reactors, $Po—Be$, and other neutron sources); b) radioactivation methods based on the use of charged particles: protons, neutrons, tritons, $He^3$ and $He^4$, obtained with cyclotrons and other heavy-particle accelerators; c) the gamma-activation method. It should be noted that this latter method of determining material composition, using radioactive isotopes as a gamma-radiation source, was developed as early as the prewar period [1], and especially in the postwar years both in the Soviet Union and in a number of foreign countries. As a gamma-radiation source for activation-analysis purposes, among other isotopes the radioactive isotope of antimony, $Sb^{124}$, which emits gamma quanta with an energy of 2.09 MeV and has a half-life of 60.1 days, has found the widest application. It is important to emphasize that radioactive gamma-radiation sources have low energy and relatively low intensity; therefore their use for activation analysis is very limited. In practice they can be used only for photoneutron determination of deuterium and beryllium, for which the thresholds of the $(\gamma, n)$ reaction are 2.23 and 1.667 MeV, respectively, and also for excitation of isomeric levels in a number of elements.

The creation of new powerful gamma-radiation sources (betatron, linear electron accelerator, microtron), as well as modern apparatus for recording radiation, has significantly expanded the possibilities of this method of analysis and has made it possible to determine, with high accuracy and sensitivity, the content of trace impurities of various elements in multicomponent compositions in the liquid or solid phases.

In our work, as a source of high-energy bremsstrahlung, a high-current betatron of the Nuclear Physics Research Institute at TPI was used, with a maximum radiation energy of 25 MeV. The accelerator has a high bremsstrahlung yield: at an energy of 25 MeV the power ...

the radiation dose rate is no less than 1500–1700 R/min·m. The accelerator unit is equipped with a circuit for monitoring the maximum energy of the bremsstrahlung radiation [2], with the aid of which the radiation energy required in carrying out experiments can be maintained with an accuracy of no worse than 0.1%, and with a device for measuring the radiation dose, making it possible to vary the time constant of the \(RC\) circuit so as to eliminate fluctuations in the induced activity caused by instability of the radiation intensity in time.

It should be noted that increasing the dose rate of the betatron bremsstrahlung, while simultaneously increasing the absolute sensitivity of the gamma-activation method, leads to a sharp increase in the flux of fast and thermal neutrons in the radiation beam incident on the sample. This may lead to a substantial decrease in sensitivity at low contents of the elements being studied in the specimen, owing to interfering activities. Since we usually used the \((\gamma,n)\) reaction, which leads to the formation of \(\beta^+\)-radioactive nuclei, in order to reduce the background in measurements of the induced activity during isotope identification, a \(\gamma\)–\(\gamma\) coincidence scheme was used, based on NaI(Tl) crystals of size \(70 \times 70\) mm and FEU-52 photomultipliers, analogous to that described earlier in [3] and differing only in the channel width in the amplitude-selection circuit.

The absolute sensitivity was determined by bringing the sample activity to saturation, and the measurements were carried out in the time interval for which the signal-to-background ratio was optimal. Since the sensitivity limit was taken to be the quantity of substance whose activity is equal to three statistical errors of the background count, the value of this limit depends, although only weakly, on the parameters of the counting setup. Therefore the sensitivity was determined both for the counting setup mentioned above and for a scintillation spectrometer with an NaI(Tl) crystal of size \(40 \times 40\) mm, with a resolution of 7.5% on the cesium-137 line, and by the method of absolute counting of beta activity.

Although the minimum distance from the target to the irradiated object in our experiments could be taken as approximately 40 cm, the figures given below characterize the absolute sensitivity at a distance of 50 cm from the target. Taking into account that it is planned subsequently to extract the electron beam from the betatron chamber, the minimum quantity of substance that can be detected by the gamma-activation method using this accelerator unit should be reduced by a factor of 50–100 in comparison with that given in Table 1.

Table 1

As irradiated samples in the analysis for oxygen, we used $\mathrm{Al_2O_3}$ of KhCh grade; in the analysis for nitrogen, $\mathrm{Ca(NO_3)_2}$ of KhCh grade; for carbon, samples made of polystyrene; and for fluorine, samples made of Teflon.

We determined the content of oxygen, nitrogen, carbon, and fluorine in complex mixed compositions at concentrations of the indicated elements ranging from 0.1 to 25–30%. Measurements of the concentrations of the elements were carried out by two methods.

According to the first method, the investigated samples and comparison standards weighing 100–200 mg were irradiated for 30 minutes in the bremsstrahlung beam of the betatron at maximum energy. Measurements of the decay of the activities induced in the sample and in the standard were carried out over four half-lives of the longest-lived isotope formed in the given mixture; in our case, this longest-lived isotope proved to be the chlorine isotope $\mathrm{Cl}^{34}$, with a half-life of 32.5 min.

The activities of the radioactive isotopes formed at the initial time $t_0$ can be calculated by solving a system of equations by the method of least squares, using an M-20 electronic computer.

To check the accuracy of the measurements, simultaneously with the content of the given element, the measurement error was also determined (likewise by the method of least squares) and compared with the deviations obtained in the analysis of a standard mixture of known composition.

At element concentrations ranging from 5 to 30%, the accuracy of the measurements was better than 3% relative, and at concentrations from 0.1 to 5%, better than 5%. Similar results were also obtained when using, for analytical purposes, another method, according to which, in determining nitrogen and fluorine, irradiation of the samples was carried out with bremsstrahlung of energy lower than the threshold energy of the $(\gamma,n)$ reaction on the oxygen isotope $\mathrm{O}^{16}$ ($E_{\mathrm{th}} = 15.67$ MeV). In determining the content of carbon and oxygen in the investigated samples, the energy of the betatron bremsstrahlung was 25 MeV, but the irradiation time was varied from 2 min to 30 min for oxygen $\mathrm{O}^{16}$ and carbon $\mathrm{C}^{12}$, respectively.

An analogous method was used by us in determining the content of nitrogen and oxygen in petroleum [4]. Petroleum is—in the absence of mechanical impurities in it—an object ideal in elemental composition for carrying out such investigations. We analyzed samples of primary Siberian petroleum, which is characterized by a low concentration of emulsified water, weak salinity of the latter, and an almost complete absence of mechanical impurities. The accuracy of a single determination of the nitrogen content in petroleum is approximately 3–4% relative, which is three times higher than the accuracy obtained by the most advanced chemical methods, at a comparable cost for these analyses.

REFERENCES

1. B. S. Aidarkin, G. V. Gorshkov, A. G. Grammakov, V. S. Zhardin, A. G. Kolchina. On a method for determining beryllium in ores by photoneutrons. Proceedings of the Radium Institute, Academy of Sciences of the USSR, 1957, 5, issue 2, 89.

2. R. P. Meshcheryakov, Yu. A. Panov, G. A. Tsyba, B. M. Yakovlev. A scheme for monitoring the maximum energy of betatron bremsstrahlung. Certificate of registration No. 46435 of June 27, 1964.

3. R. P. Meshcheryakov et al. Activation analysis with deuterons. In: Proceedings of the Republican Conference on Nuclear Physics and Elementary-Particle Physics, Uzhgorod, 1967 (in press).

4. R. P. Meshcheryakov, B. M. Yakovlev, G. I. Tronov. Determination of nitrogen content in petroleum by the gamma-activation method. See the present collection, p. 25.