RADIOlysis by GAMMA RAYS AND PROTONS OF SOLID INORGANIC IONIC SALTS WITH A COMPLEX ANION

A. N. Oblivantsev, V. M. Lykhin, V. V. Boldyrev, L. P. Eremin,
V. F. Komarov

(Submitted by the scientific seminar of the sector of radiation-chemical analysis
of the Research Institute of Nuclear Physics)

Studies of radiation-chemical reactions in condensed media (reactions in solutions, organic compounds) show that their course is to a considerable extent associated with the cage effect and with the inhomogeneity of the deposition of radiation energy in the irradiated medium, with the presence of so-called track effects, that is, large regions of ionization formed along the path of a high-energy particle through a substance.

Chemical transformations in solids with an ionic structure have been studied insufficiently in this direction and concern mainly nitrates of alkali and alkaline-earth metals, for which it has been shown that their sensitivity to gamma irradiation decreases with decreasing atomic weight of the cations. To explain this difference in radiation-chemical stability, Henning, Lies, and Matheson [1], and then Cunningham and Hill [2], and Baverkin [3], introduce and apply an empirical parameter, the so-called “free volume” of the crystal, defined as the difference between the volume of the crystal and the sum of the volumes of its constituent ions. It was found that the radiolysis yield decreases in parallel with a decrease in the free volume. Similar results were obtained in studying the radiation stability of alkali-metal bromates [4].

The presence of such a correlation in the case of nitrates and bromates shows that the dimensions of the “cage” formed by the surrounding ions affect their radiation-chemical stability.

There are also experimental data on the radiation-chemical decomposition of alkali-metal nitrates by charged particles [5—7]. In this connection [5] points to the necessity of taking track effects into account in radiolysis, and substantial importance is attributed to thermal effects in the track.

If one considers that the regularities set forth above were obtained for a small class of compounds, and moreover for compounds decomposing with incomplete destruction of the anion, then the acquisition of new data on the radiation-chemical stability of other classes of inorganic compounds and the use of different types of radiation become necessary and may answer the question of whether the relation found between the radiolysis yield and the magnitude of the free volume is not accidental, and also make it possible to verify the correctness of the hypothesis concerning the significance of thermal effects in the track during chemical transformations in solids.

The paper presents the results we have obtained on the radiation-chemical stability of azides and perchlorates of alkali metals under the action of $\gamma$-radiation, as well as of nitrates, azides, and perchlorates of alkali metals upon irradiation with protons of energy 4.7 MeV. The stability was assessed from the accumulation of radiolysis products with dose, determined by chemical analysis, and from the initial radiation-chemical yield calculated from it.

Experimental Part

Irradiation of samples. For gamma irradiation an isotopic cobalt unit was used. Irradiation with protons was carried out in a particle beam extracted from a cyclotron into the atmosphere. When comparing the radiation stability of compounds under the action of different types of radiation, the correctness of estimating the absorbed dose and expressing the experimental data obtained in comparable absorbed doses is of great importance. We determined the absorbed dose of $\gamma$-radiation for each salt (eV/g) by multiplying the readings of a ferrous-sulfate dosimeter by the ratio of the electron densities of the salt to the electron density of the dosimetric solution. In individual cases, the ratio of mass attenuation coefficients was used for recalculation. The absorbed dose of charged particles was determined by calculation from the known particle flux density and the linear energy loss, calculated by the Bethe–Bloch formula [8].

The dose rate under $\gamma$-irradiation was $2.9 \cdot 10^{16}$ eV/ml·s; under proton irradiation, $(0.5–1.5) \times 10^{19}$ eV/g·s. In all cases, heating of the samples did not exceed the ambient temperature by more than 20°C.

Radiolysis of azides. From the example of sodium azide it is known [9] that alkali-metal azides decompose with complete destruction of the anion and formation of the metal and nitrogen. On this basis, the irradiated azides were analyzed, after dissolution in water, for free metal and for the remaining undecomposed azide ion. The amount of metal was determined by neutralization of the resulting solution with 0.1 N HNO₃; the azide ion was determined by the method of Mayrikh [10], by titration with silver nitrate.

The analyses showed that the degree of stability of azides toward irradiation decreases from sodium to cesium. It should be noted that there was no agreement, especially for rubidium and cesium azides, between the results obtained from the azide ion and from the free metal; this is probably connected with secondary reactions of the radiolysis products with components of the air, as is also indicated by the nitrite ion that we detected in the irradiated azide [11].

Radiolysis of perchlorates. Chemical analysis of irradiated potassium, rubidium, and cesium perchlorates showed that these compounds decompose mainly with formation of chloride, chlorate, and oxygen, as well as small amounts of hypochlorite and chlorite [12]. The analyses showed that the stability of perchlorates toward irradiation decreases from potassium to cesium and that, under proton irradiation, this difference decreases.

Radiolysis of nitrates. Alkali-metal nitrates irradiated with protons were analyzed for nitrite ion. For nitrates irradiated with protons, an increase in sensitivity to irradiation is observed in going from sodium to rubidium, cesium, and potassium.

Discussion of Results

From the obtained dependences of the amount of decomposed substance on dose, the values of the initial radiation-chemical ...

yields of azides, perchlorates, and nitrates irradiated with γ-rays or protons. Figure 1 shows the dependence, obtained for γ-irradiation, of the yields on the free volume of the unit cell of the crystal; the data on nitrates and bromates are taken from other works [2, 4]. The observed decrease in radiation stability with increasing free volume of the unit cell of the crystal indicates that, in a series of salts of the same type, the cage effect, the degree of whose manifestation depends on the magnitude of the

Fig. 1. Dependence of yield on free volume during radiolysis by γ-rays of alkali-metal compounds:
1—nitrates [15]; 2—bromates [4]; 3—perchlorates; 4—azides.

free volume, is one of the factors affecting the radiation stability of salts. However, it cannot be regarded as the only one. Thus Baberkin showed [3] that the radiation stability of nitrates with cations having an 18-electron outer shell is somewhat lower than that of nitrates with cations of comparable size but with an eight-electron outer shell.

Comparison also shows that, for salts with the same cation but different anions, one can indicate salts having close values of the free volumes but differing greatly in the value of the radiation-chemical yield

\[ \left( \frac{G\,\mathrm{NaBrO_3}}{G\,\mathrm{NaNO_3}} \sim 4; \qquad \frac{G\,\mathrm{RbClO_4}}{G\,\mathrm{RbBrO_3}} \sim 2 \right), \]

that is, the influence of the anion proves in this case to be decisive for the radiation-chemical yield. Here, evidently, it is necessary to take into account the energy of the chemical bond in the complex anion, the difference in radiolysis mechanisms, and the competing influence of reverse reactions.

In addition, there is a correlation between the decrease in radiation stability and the decrease in the energy of the crystal lattice. It is possible that the energy of the crystal lattice determines its resistance to internal pressures created by the radiolysis products within its volume.

When considering the factors affecting the radiation stability of inorganic ionic salts, it is also necessary to take into account the influence of impurities. The influence of impurities may manifest itself through a decrease in

the cell effect through the appearance of a large number of vacancies, through changes in the rates of reverse reactions, and also as a result of the participation of impurities as electron donors or acceptors in the electronic stages of radiation-chemical decomposition [13].

In studying the radiation decomposition of salts by heavy charged particles, track effects must also introduce a complicating influence. Data from various authors [5, 14, 15] are available on the dependence of nitrate yields on free volume for gamma, X-ray, and α-particle irradiation, as well as our data on proton irradiation [19], which show that, with increasing linear energy transfer, an increase in the yield of nitrate radiolysis products is observed (Fig. 2). The dependence of the yield on free volume is retained in this case, but becomes weaker.

Fig. 2. Dependence of the yield on free volume in the radiolysis of alkali-metal nitrates: 1—gamma rays [15]; 2—X-rays [14]; 3—protons; 4—alpha particles [5].

When considering the influence of track effects on radiolysis, a number of works [5, 16] indicate that thermal effects in the track are of substantial importance in radiolysis. As evidence for this, reference is made to yields during the decomposition of alkali-metal nitrates [5] and sodium chlorate by α-particles [16] that are higher than under γ-irradiation, to the absence of an effect of irradiation temperature on the radiolysis of sodium nitrate, and also to the absence of post-radiation thermal annealing of sodium chlorate irradiated likewise with α-particles. Let us consider the applicability of these propositions to other compounds.

A comparison of yields under gamma and proton irradiation for azides and perchlorates of alkali metals (Figs. 3, 4) shows that alkali-metal perchlorates prove to be less radiation-chemically stable under proton irradiation, while alkali-metal azides are less stable under γ-irradiation.

If one takes into account that, according to Hill’s data [9], when the temperature of γ-irradiation of sodium azide is raised to 200°C the yield increases by more than a factor of 4, then, if only thermal effects had a decisive influence on the processes in the track of charged particles, the yield of radiolysis products of this compound should be greater under proton irradiation than under the action of γ-rays. But this is not confirmed by experiment.

On the other hand, the large increase in the yield of radiolysis products of potassium perchlorate under proton irradiation compared with γ-irradiation cannot be explained by the indicated hypothesis, since, according to Prince and Johnson [17], the yield under γ-irradiation of potassium perchlorate is practically independent of temperature. It is also known [17] that irradiation of potassium perchlorate at elevated temperatures with γ-rays—

by gamma rays, or its subsequent isothermal annealing, leads to the disappearance of thermally unstable radiolysis products of the hypochlorite and chlorite type. However, testing potassium perchlorate irradiated with protons for its content of chlorite and hypochlorite indicates the presence in it of these products in amounts comparable with γ-irradiation. It becomes unclear how these products could have been preserved under the considerable thermal heating in the track.

Fig. 3. Dependence of the yield on free volume in the radiolysis of alkali-metal azides: 1 — gamma rays; 2 — protons

Fig. 4. Dependence of the yield on free volume in the radiolysis of alkali-metal perchlorates: 1 — gamma rays; 2 — protons

The experimental results obtained by us, as well as the results of work on the radiolysis of lithium bromate under the action of α-particles and tritons [18], lead to the conclusion that the proposed connection, for nitrates, between the mechanism of radiolysis and thermal heating in the track of an ionizing particle requires further verification.

Comparison of the dependence, observed under γ-irradiation, of the radiation-chemical yield on free volume with analogous results under proton irradiation shows that this dependence decreases and is practically not observed in the case of alkali-metal perchlorates. For example, for perchlorates irradiated with protons the difference in yields is about 25%, whereas under γ-irradiation they differ by almost a factor of 2.

Conclusions

1. It has been shown that, under gamma irradiation, the radiation-chemical stability of inorganic ionic salts that decompose upon irradiation both with complete destruction of the anion (azides) and with partial destruction of the anion (for example, perchlorates) depends on such lattice parameters as free volume and the energy of the crystal lattice.

2. Under the action of different types of radiation on alkali-metal nitrates, an influence of linear energy transfer on the radiation-chemical yield is observed. Comparison of the yields under gamma and proton irradiation for alkali-metal azides and perchlorates shows that the change in yields under the action of radiations with high LET cannot be explained by thermal heating in the track.

1. A comparison of the dependence of the radiation-chemical yield on the free volume under gamma and proton irradiation shows that, under proton irradiation, this dependence decreases for alkali-metal azides and nitrates and is practically not observed for alkali-metal perchlorates.

REFERENCES

1. G. Hennig, R. Lees, M. Mathesen. J. Chem. Phys., 21, 664 (1953).
2. J. Cunningham, H. Heal. Nature, 179, 1021 (1957).
3. A. S. Baberkin. Zh. fiz. khimii, 35, 373 (1961).
4. G. Boyd, E. Graham, O. Larson. J. Phys. Chem., 66, 330 (1962).
5. C. I. Hochanadel. Rad. Res. 16, 286 (1962).
6. S. R. Logan, W. I. Moore. J. Phys. Chem., 67, 1042 (1963).
7. S. R. Logan. Nature, 199, 4891 (1963).
8. Experimental Nuclear Physics, ed. by E. Segrè, Vol. 1, IL, Moscow, 1955.
9. H. G. Heal. Trans. Faraday Soc., 53, 410, 210–217 (1957).
10. G. Kast, L. Meshch. Chemical Investigations of Explosives. Moscow–Leningrad, 1934.
11. V. V. Boldyrev, A. N. Oblivantsev, V. M. Lykhin. Doklady AN SSSR, 159, No. 5, 1113 (1964).
12. A. N. Oblivantsev, V. M. Lykhin, V. V. Boldyrev. Zhurnal VKhO, 5, 598 (1965).
13. V. V. Boldyrev, V. M. Lykhin, A. N. Oblivantsev, K. Salikhov. Kinetics and Catalysis, 7, 432 (1966).
14. J. Cunningham, H. G. Heal. Trans. Far. Soc., 54, 1355 (1958).
15. J. Cunningham. J. Phys. Chem., 65, 628 (1961).
16. C. I. Hochanadel. J. Phys. Chem., 67, 2229 (1963).
17. L. A. Prince, E. R. Johnson. J. Phys. Chem., 69, 359 (1965).
18. G. E. Boud, T. G. Ward. J. Phys. Chem., 68, 3809 (1964).
19. L. P. Eremin, V. M. Lykhin, A. N. Oblivantsev, V. V. Boldyrev. ZhFKh, 41, 1180 (1967).