ON THE EFFECT OF PRELIMINARY NEUTRON IRRADIATION ON THE ATTENUATION COEFFICIENT OF β-RADIATION IN ALKALI-HALIDE CRYSTALS

V. A. CHERNYSHEV, V. A. POPOV

The purpose of the present investigation was to determine the possible effect of preliminary neutron irradiation on the attenuation coefficient of β-radiation in alkali-halide crystals of the potassium series \((\mathrm{KCl}, \mathrm{KBr}, \mathrm{KJ})\).

Statement of the Problem

Radiation damage in a solid leads to a significant change in its physical and chemical characteristics. In the present case we consider the action of radiation on crystals of alkali-halide salts. If, under the action of ionizing radiation, such properties of materials as the elastic modulus, the coefficient of thermal expansion, and optical ability change, then the question is naturally posed: does irradiation affect the attenuation coefficient of electrons in crystals of alkali-halide salts, and if it does, to what degree?

In [1] it was found that the attenuation coefficient of β-radiation changes after preliminary neutron irradiation. At the same time, as a preliminary estimate of the number of defects produced by large neutron fluxes shows, a change in the attenuation coefficient of β-radiation can hardly be expected.

Indeed, let us consider the question of the influence of the crystal lattice of a substance on the energy losses (scattering) by electrons in crystals. It is obvious that such an influence manifests itself through electron diffraction. Electron diffraction is observed in the case when the de Broglie wavelength for the electrons is commensurate with the distance between lattice sites, i.e., up to energies of the order of \(10\ \mathrm{keV}\). As the electron energy increases, the losses are determined by the number of atomic nuclei and electrons per unit volume, as well as by the binding energy of the electrons in the atomic shells. The penetration of electrons into matter in the present case is governed by ionization losses and losses due to excitation of atoms of the medium, elastic collisions, and bremsstrahlung. Losses due to bremsstrahlung and elastic collisions depend on the number of nuclei of charge \(Z\) per unit volume. Preliminary

irradiation does not change the charge of the nucleus, if nuclear reactions make a small contribution; otherwise the chemical composition of the substance changes, and, consequently, this type of loss does not change.

Losses due to ionization and excitation of the atoms of the medium are best considered with the aid of the Bethe--Bloch formula

\[ \left(\frac{dT}{dx}\right)= \frac{2\pi e^{4}n}{m_{0}v^{2}} \left[ \ln\frac{m_{0}v^{2}T}{2I^{2}(1-\beta)^{2}}-\beta^{2} \right]. \tag{1} \]

In condition (1) the slowing medium is characterized by two parameters: \(n\)—the number of electrons in \(1\ \text{cm}^{3}\); \(I\)—the mean potential of the electron shells of the atoms.

Change in electron density

If we consider an irradiated specimen with Schottky defects of the order of \(10^{18}\ \frac{1}{\text{cm}^{3}}\), then the change in electron density (we take the worst case, for example, for KCl) is \(0.04\%\).

Change in the ionization potential

Under the action of irradiation, the chemical bonding of atoms in a substance may change; i.e., the ionization potential of the valence shells of the atoms may change. Since the energy of a chemical bond is of the order of \(5\)--\(10\ \text{eV}\), a change of it by \(20\%\) amounts to \(1\)--\(2\ \text{eV}\). In view of the fact that a penetrating electron interacts with all the shells of the atom, the mean ionization potential of the electron shells is considered. For example, for Cl the mean ionization potential is about \(150\ \text{eV}\). It is clear that a change in the binding energy of the valence electrons has very little effect on the ionization potential. Moreover, in equation (1) \(I\) stands under the logarithm sign, which gives a resulting change of the order of \(0.2\%\). Thus, the change in the energy loss of electrons can, in the best case, reach \(0.3\%\).

Measurement procedure and results

Measurements of the attenuation coefficient of \(\beta\)-radiation in alkali-halide crystals were carried out on a standard B-2 setup with a lead housing. The accuracy of the measurement was of the order of \(2\%\). The crystals were irradiated in a paraffin block from two sources with intensities \(6\cdot10^{5}\) and \(3.14\cdot10^{6}\ \frac{\text{neutr.}}{\text{cm}^{2}\text{sec.}}\). The integral dose received by the specimens was of the order of \(10^{10}\ \text{neutrons}/\text{cm}^{2}\). A Tl\(^{204}\) preparation with maximum energy \(E=0.766\ \text{MeV}\) [4] served as the source of \(\beta\)-rays. The attenuation coefficient of \(\beta\)-radiation was determined on the basis that, for not too thick absorber layers (in comparison with the electron range), the curve describing absorption can, with good accuracy, be replaced by an exponential [2,3]. Thus, the intensities of \(\beta\)-radiation as a function of absorber thickness have the form

\[ I_{x}=I_{0}e^{-\mu x}, \tag{2} \]

where \(\mu\) is the attenuation coefficient of \(\beta\)-radiation, and \(x\) is the absorber thickness.

The results of measurements of the attenuation coefficient of \(\beta\)-radiation in alkali-halide crystals are presented in Fig. 1.

As is seen from Fig. 1, irradiation of alkali-halide crystals with neutrons of energy \(10\) MeV at a dose of \(10^{10}\) neutrons/\(\text{cm}^2\) does not change the attenuation coefficient of \(\beta\)-radiation; or, if it does change it, the given

Fig. 1. Dependence of the logarithm of the count of \(\beta\)-particles on the thickness of the samples

KCl □ □ □ unirradiated,
  ■ ■ ■ irradiated,

KBr × × × unirradiated?
  ♦ ♦ ♦ irradiated?

KJ ○ ○ ○ unirradiated?
  ● ● ● irradiated.

method for measuring the attenuation coefficient of \(\beta\)-radiation does not make it possible to detect this change. Consequently, the change in the attenuation coefficient of \(\beta\)-radiation obtained in work [1] was not confirmed by us.

REFERENCES

1. A. K. Berzin, S. L. Kashchuk. In the collection The Effect of Radiations on the Properties of Materials. Issue 2, Novosibirsk, 73, 1963.

2. S. M. Raiskii, V. F. Smirnov. Physical Foundations of the Method of Radioactive Measurements. Gosatomizdat, 1959, p. 30.

3. G. M. Estulin. Radioactive Radiations, p. 91, Fizmatgiz, Moscow, 1962.

4. G. M. Fradkin. Sources of \(\alpha\)—\(\beta\) and Neutron Radiations. Gosatomizdat, 1962.